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Protein ubiquitin in its +7 charge state microhydrated by 5 and 10 water molecules has been interrogated in the gas phase by cold ion UV/IR spectroscopy. The complexes were formed either by condensing water onto the unfolded bare proteins in a temperature-controlled ion trap or by incomplete dehydration of the folded proteins. In the case of cryogenic condensation, the UV spectra of the complexes exhibit a resolved vibrational structure, which looks similar to the spectrum of bare unfolded ubiquitin. The spectra become, however, broad-band with no structure when complexes of the same size are produced by incomplete dehydration under soft conditions of electrospray ionization. We attribute this spectroscopic dissimilarity to the structural difference of the protein: condensing a few water molecules cannot refold the gas-phase structure of the bare ubiquitin, while the retained water preserves its solution-like folded motif through evaporative cooling. This assessment is firmly confirmed by IR spectroscopy, which reveals the presence of free NH and carboxylic OH stretching vibrations only in the complexes with condensed water.

Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidence. Recently, electron paramagnetic resonance (EPR) has been used to study deeply supercooled water in a rapidly frozen polycrystalline ice. The same technique can also be used to probe the presence of LW in polycrystalline ice that has formed through a more conventional, slow cooling one. In this context, the present study aims to emphasize that in case of an external probe involving techniques such as EPR, the results are influenced by the binary phase (BP) diagram of the probe-water system, which also predicts the existence of LW domains in ice, up to the eutectic point. Here we report the results of our such EPR spin-probe studies on water, which demonstrate that smaller the concentration of the probe stronger is the EPR evidence of liquid domains in polycrystalline ice. We used computer simulations based on stochastic Liouville theory to analyze the lineshapes of the EPR spectra. We show that the presence of the spin probe modifies the BP diagram of water, at very low concentrations of the spin probe. The spin probe thus acts, not like a passive reporter of the behavior of the solvent and its environment, but as an active impurity to influence the solvent. We show that there exists a lower critical concentration, below which BP diagram needs to be modified, by incorporating the effect of confinement of the spin probe. With this approach, we demonstrate that the observed EPR evidence of LW domains in ice can be accounted for by the modified BP diagram of the probe-water system. The present work highlights the importance of taking cognizance of the possibility of spin probes affecting the host systems, when interpreting the EPR (or any other probe based spectroscopic) results of phase transitions of host, as its ignorance may lead to serious misinterpretations. [ABSTRACT FROM AUTHOR]

Protonated ethylenediamine monomer, dimer, and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C2H8N2, en) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm-1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a gauche conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm-1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol-1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer's NH3+ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm-1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within 2 kcal mol-1 of the experimentally observed structure. The spectrum of the trimer is rather diffuse, and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum. [ABSTRACT FROM AUTHOR]

Herein, we report on the theoretical-experimental analysis of the two photon absorption and circular dichroism spectra of 1-(2-pyridyl)-4-methoxy-carbo[6]helicene derivative (P6). The primary outcomes of our investigation on this particular helicene derivative with a donor-acceptor motif on one end led to two important conclusions: (1) the lengthening of the π-electron delocalization within the helical core of P6 predominantly increases the contribution of the magnetic dipole transition moment to the two-photon circular dichroism (TPCD) signal; and (2) the electric quadrupole transition moment contribution to the TPCD signal is enhanced by the intramolecular charge transfer (ICT) produced by the donor-acceptor combination on one end of the molecule. To corroborate our results, we performed a comparative theoretical analysis of the effect of the energy gap and ICT on TPCD on a series of P6-like helicenes with different donor-acceptor combinations. Two-photon absorption and TPCD spectra were obtained using the double L-scan technique over a broad spectral range (400-900 nm) using 90 fs pulses at a low repetition rate (2-50 Hz) produced by an amplified femtosecond system. The theoretical simulations were calculated using time dependent density functional theory at the CAM-B3LYP/6-311++G(d,p) level of theory. [ABSTRACT FROM AUTHOR]

Peptide deamidation of asparaginyl residues is a spontaneous post-translational modification that is believed to play a role in aging and several diseases. It is also a well-known small-molecule loss channel in the MS/MS spectra of protonated peptides. Here we investigate the deamidation reaction, as well as other decomposition pathways, of the protonated dipeptide asparagine-valine ([AsnVal + H]+) upon low-energy activation in a mass spectrometer. Using a combination of infrared ion spectroscopy, guided ion beam tandem mass spectrometry, and theoretical calculations, we have been able to identify product ion structures and determine the energetics and mechanisms for decomposition. Deamidation proceeds via ammonia loss from the asparagine side chain, initiated by a nucleophilic attack of the peptide bond oxygen on the γ-carbon of the Asn side chain. This leads to the formation of a furanone ring containing product ion characterized by a threshold energy of 129 ± 5 kJ/mol (15 kJ/mol higher in energy than dehydration of [AsnVal + H]+, the lowest energy dissociation channel available to the system). Competing formation of a succinimide ring containing product, as has been observed for protonated asparagine-glycine ([AsnGly + H]+) and asparagine-alanine ([AsnAla + H]+), was not observed here. Quantum-chemical modeling of the reaction pathways confirms these subtle differences in dissociation behavior. Measured reaction thresholds are in agreement with predicted theoretical reaction energies computed at several levels of theory [ABSTRACT FROM AUTHOR]

Time-resolved and two-dimensional sum frequency generation (TR-SFG and 2D-SFG) spectroscopies are promising tools in the experimental study of molecular dynamics, specifically at interfaces. Most implementations of TR/2D-SFG spectroscopy rely on a pump-probe scheme, where an excitation pulse excites a fraction of interfacial molecules into the first excited state of a specific vibrational mode, and a subsequent SFG probe pair detects the time-dependent changes of the surface vibrational response. In steady state SFG spectroscopy, phase-resolved detection (also known as heterodyne-detection), as opposed to SFG intensity measurements, has been shown to be useful in unraveling the steady-state response of interfacial vibrations. Here, we explore the merits of phase-resolved TR/2D-SFG spectroscopy. This purely theoretical and numerical study reveals that, for a typical response from aqueous interfaces, the intensity 2D-SFG measurements contain the same information content as phase-resolved 2D-SFG measurements. We specifically analyze the frequency-dependence of the bleach lifetime (reflecting vibrational relaxation), and the time-dependent slope of the on-diagonal features observed in a 2D spectra. We show that for different systems, the intensity-based and phase-resolved 2D-SFG measurements provide the same information and are quantitatively very similar. We investigate the effect of different lineshapes, anharmonicity, and nonresonant signal contributions, and show that none of these effects substantially change the conclusion that intensity-based and phase-resolved 2D-SFG measurements provide equivalent information. [ABSTRACT FROM AUTHOR]

We revealed the chloride ion pumping mechanism in halorhodopsin from Natronobacterium pharaonis (pHR) by exploring sequential structural changes in the retinal chromophore during its photocycle using time-resolved resonance Raman (RR) spectroscopy on the nanosecond to millisecond time scales. A series of RR spectra of the retinal chromophore in the unphotolyzed state and of the three intermediates of pHR were obtained. Using singular value decomposition analysis of the C═C and C-C stretch bands in the time-resolved RR spectra, we identified the spectra of the K, L, and N intermediates. We focused on structural markers of the RR bands to explore the structure of the retinal chromophore. In the unphotolyzed state, the retinal chromophore is in the planar all-trans, 15-anti geometry. The bound ion affects the polyene chain but does not interact with the protonated Schiff base. In the observed intermediates, the chromophore is in the 13-cis configuration. The chromophore in the K intermediate is distorted due to the photoisomerization of retinal. The hydrogen bond is weak in the unphotolyzed state and in the K intermediate, resulting in exchange of the hydrogen-bond acceptor to a water molecule in the K-to-L transition, relaxation of the polyene chain distortion, and generation of an alternative distortion near the Schiff base. The bound halide ion interacts with the protonated Schiff base through the water molecule bound to the protonated Schiff base. In the L-to-N transition, the hydrogen acceptor of the protonated Schiff base switches from the water molecule to another species, although the strong hydrogen bond of the protonated Schiff base remains. This paper reports the first observation of sequential changes in the RR spectra in the pHR photocycle, provides information on the structural evolution of the retinal chromophore, and proposes a model for chloride ion translocation in pHR. [ABSTRACT FROM AUTHOR]

Highly correlated ab initio calculations have been performed to describe the potential energy curves (PECs) and the spectroscopic properties of the X2Π state and of the first excited state of the CI radical. Multi Reference configuration interaction calculations with Davidson correction (MRCI+Q) and relativistic effective core potential for the iodine atom have been performed. It is found that the two lowest electronic states, the X2Π and the a4Σ- states, are stable against dissociation and well separated from the other electronic states. Spectroscopic constants of these two states have been evaluated using their calculated PECs. Because of the presence of the iodine atom in this molecular system, spin-orbit (SO) interactions are playing an important role in the molecular and in the dissociation regions. The excitation energy of the a4Σ- state is calculated 1.67 eV (MRCI) above the X2Π ground state and 1.70/1.62 eV (MRCI with SO correction) for the Ω = 1/2 and 3/2 transitions, respectively. The dissociation energy D0 of the X2Π ground state is evaluated 2.66 eV (MRCI calculation) without SO correction and 2.46/2.36 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. The dissociation energy D0 of the a4Σ- state is evaluated 0.99 eV (MRCI calculation) without SO correction and 0.83/0.72 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. This work should help for the identification of this radical in laboratory and in atmospheric media. [ABSTRACT FROM AUTHOR]

In this study, we propose a new approach to generate two-dimension spectra to enhance the intensity of cross peaks relevant to intermolecular interaction. We investigate intermolecular interaction between two solutes (denoted as P and Q, where P has a characteristic peak at XP) dissolved in the same solvent via the near diagonal cross peaks around the coordinate (XP, XP) in a two-dimensional (2D) asynchronous spectrum of generalized spectroscopy. Because of physical constrains in many cases, the variation ranges of the initial concentrations of P or Q must be kept very narrow, leading to very weak cross peak intensities. The weak cross peaks vulnerable to noise bring about difficulty in the investigation of subtle intermolecular interaction. Herein, we propose a new of way constructing a 2D asynchronous spectrum without the subtraction of the average spectrum often used as a reference spectrum. Mathematical analysis and computer simulation demonstrate that the near diagonal cross peaks around the coordinate (XP, XP) in the 2D asynchronous spectrum using the new approach possess two characteristics: (1) they can still reflect an intermolecular interaction reliably; 2) the absolute intensities of the cross peaks are significantly stronger than those generated by the conventional method. We incorporate the novel method with the DAOSD (double asynchronous orthogonal sample design scheme) approach and applied the modified DAOSD approach to study hydrogen bonding behavior in diethyl either/methanol/THF system. The new approach made the weak cross peaks, which are not observable in 2D asynchronous spectrum generated using conventional approach, become observable. The appearance of the cross peak demonstrate that When a small amount of THF is introduced into diethyl solution containing low amount of methanol, THF breaks the methanol-diethyl ether complex and forms methanol-THF complex via new hydrogen bond. This process takes place in spite of the fact that the content of diethyl ether is overwhelmingly larger than that of THF. The above result demonstrates that the new approach described in this article is applicable to enhance intensity of cross peaks in real chemical systems. [ABSTRACT FROM AUTHOR]

The intrinsic structures of biomolecules in the gas phase may not reflect their native solution geometries. Microsolvation of the molecules bridges the two environments, enabling a tracking of molecular structural changes upon hydration at the atomistic level. We employ density functional calculations to compute a large pool of structures and vibrational spectra for a gas-phase complex, in which a doubly protonated decapeptide, gramicidin S, is solvated by two water molecules. Though most vibrations of this large complex are treated in a harmonic approximation, the water molecules and the vibrations of the host ion coupled to them are locally described by a quantum mechanical vibrational self-consistent field theory with second-order perturbation correction (VSCF-PT2). Guided and validated by the available cold ion spectroscopy data, the computational analysis identifies structures of the three experimentally observed conformers of the complex. They, mainly, differ by the hydration sites, of which the one at the Orn side chain is the most important for reshaping the peptide toward its native structure. The study demonstrates the ability of a quantum chemistry approach that intelligently combines the semiempirical and ab initio computations to disentangle a complex interplay of intra- and intermolecular hydrogen bonds in large molecular systems. [ABSTRACT FROM AUTHOR]

The FCH2CN-BCl3 and ClCH2CN-BCl3 complexes were investigated by quantum-chemical computations and low-temperature, matrix-isolation-IR spectroscopy. Theory predicts two stable equilibrium structures, with distinctly different B-N distances, for both complexes. One set of structures, which correspond to the global energy minima, exhibit B-N distances of 1.610 and 1.604 Å for FCH2CN-BCl3 and ClCH2CN-BCl3, respectively (via M06-2X/aug-cc-pVTZ). The corresponding binding energies are 5.3 and 6.3 kcal/mol. For the metastable structures, the B-N distances are 2.870 and 2.865 Å for FCH2CN-BCl3 and ClCH2CN-BCl3, respectively, and the corresponding binding energies are 3.2 and 3.3 kcal/mol. Also, the barriers between these structures on the B-N distance potentials are 2.5 and 2.8 kcal/mol, respectively, relative to the secondary, long-bond minima. In addition, several IR bands of both FCH2CN-BCl3 and ClCH2CN-BCl3 were observed in nitrogen matrices, but the assigned bands are consistent with M06-2X predictions for the short-bond, minimum-energy structures. None of the observed IR bands could be assigned to the metastable, long-bond structures. [ABSTRACT FROM AUTHOR]

Two-quantum two-dimensional electronic spectroscopy (2Q 2D ES) may provide a measure of electron-correlation energies in molecules. Attempts to obtain this profound but elusive signal have relied on experimental implementations using femtosecond laser pulses, which induce an overwhelming background signal of nonresonant response. Here we explore theoretically the signatures of electron correlation in coherent 2Q 2D ES measurements that use spectrally incoherent light, I(4) 2Q 2D ES. One can use such fields to suppress nonresonant response, and therefore this method may better isolate the desired signature of electron correlation. Using an appropriate treatment of the multilevel Bloch electronic system, we find that I(4) 2Q 2D ES presents an opportunity to measure electron-correlation energies in molecules. [ABSTRACT FROM AUTHOR]

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states. [ABSTRACT FROM AUTHOR]

This work investigates the spectroscopic parameters, vibrational levels, and transition probabilities of 12 low-lying states, which are generated from the first dissociation limit, Br(2Pu) + O-(2Pu), of the BrO- anion. The 12 states are X1Σ+, 21Σ+, 11Σ-, 11Π, 21Π, 11Δ, a3Π, 13Σ+, 23Σ+, 13Σ-, 23Π, and 13Δ. The potential energy curves are calculated with the complete active-space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with Davidson modification. The dissociation energy D0 of X1Σ+ state is determined to be approximately 26876.44 cm-1, which agrees well with the experimental one of 26494.50 cm-1. Of these 12 states, the 21Σ+, 11Σ-, 21Π, 11Δ, 13Σ+, 23Σ+, 23Π, and 13Δ states are very weakly bound states, whose well depths are only several-hundred cm-1. The a3Π, 23Π, and 13Δ states are inverted and account for the spin-orbit coupling effect. No states are repulsive regardless of whether the spin-orbit coupling effect is included. The spectroscopic parameters and vibrational levels are determined. The transition dipole moments of 12-pair electronic states are calculated. Franck-Condon factors of a number of transitions of more than 20-pair electronic states are evaluated. The electronic transitions are discussed. The spin-orbit coupling effect on the spectroscopic parameters and vibrational properties is profound for all the states except for X1Σ+, a3Π, and 11Π. The spectroscopic parameters and transition probabilities obtained in this paper can provide some powerful guidelines for observing these states in a proper spectroscopy experiment, in particular the states that have very shallow potential wells. [ABSTRACT FROM AUTHOR]

The behavior of noise in asynchronous spectrum in generalized two-dimensional (2D) correlation spectroscopy is investigated. Mathematical analysis on the noise of 2D spectra and computer simulation on a model system show that the fluctuation of noise in a 2D asynchronous spectrum can be characterized by the standard deviation of noise in 1D spectra. Furthermore, a new approach to improve a signal-to-noise ratio of 2D asynchronous spectrum by a Butterworth filter is developed. A strategy to determine the optimal conditions is proposed. Computer simulation on a model system indicates that the noise of 2D asynchronous spectrum can be significantly suppressed using the Butterworth filtering. In addition, we have tested the approach to a real chemical system where interaction between berberine and ß-cyclodextrin is investigated using 2D UV-vis asynchronous spectra. When artificial noise is added, cross peaks that reflect intermolecular interaction between berberine and ß-cyclodextrin are completely masked by noise. After the method described in this article is utilized, noise is effectively suppressed, and cross peaks are faithfully recovered. The above result demonstrates that the approach described in this article is applicable in real chemical systems. [ABSTRACT FROM AUTHOR]

The anharmonic force fields and spectroscopic constants of the electronic ground state (X¹A') for the HAsO molecule are reported employing the MP2, B3LYP, B3P86, and B3PW91 methods with cc-pVQZ and cc-pV5Z basis sets. The calculated molecular geometries, rotational constants, vibrational frequencies, and anharmonic constants of the HAsO molecule are compared with the experimental data. It is found that the best agreement between the calculated results and experiment data is at the B3LYP/cc-pV5Z theoretical level. The predicted cubic and quartic force fields, vibration-rotation interaction constants, quartic and sextic centrifugal distortion constants, and Coriolis coupling constants of the HAsO molecule at the B3LYP/cc-pV5Z theoretical level are expected to be reliable. [ABSTRACT FROM AUTHOR]

The interaction of atmospheric aerosols with radiation remains a significant source of uncertainty in modeling radiative forcing. Laboratory measurements of the microphysical properties of atmospherically relevant particles is one approach to reduce this uncertainty. We report a new method to investigate light absorption by a single aerosol particle, inferring changes in the imaginary part of the refractive index with a change in environmental conditions (e.g., relative humidity) and inferring the size dependence of the optical extinction cross section. More specifically, we present measurements of the response of single aerosol particles to near-infrared (NIR) laser-induced heating at a wavelength of 1520 nm. Particles were composed of aqueous NaCl or (NH4)2SO4 and were studied over ranges in relative humidity (40-85%), particle radius (1-2.2 μm), and NIR laser power. The ensuing size change and real component of the refractive index were extracted from measurements of the angular variation in elastically scattered light. From the heating-induced size change at varying NIR beam intensities, we retrieved the change in the imaginary component of the refractive index. In addition, cavity ring-down spectroscopy measurements monitored the change in extinction cross section with modulation of the heating laser power. [ABSTRACT FROM AUTHOR]

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer's and Parkinson's diseases) were investigated. The two conformers of cytisine, whose presence in the S0 state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S1(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, kF = 7.4 × 105 and 3.0 × 105 s-1. Their lifetimes in the S1(n,π*) state are relatively long, τS1 = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕF - 10-3. The long-wavelength band in the cytisine absorption originates from the excitation to the S2(π,π*) state, while the S1(n,π*) state is not observed in this spectrum. Thus, the excited state S2(π,π*) is manifested only in the absorption spectrum, while the excited state S1(n,π*) is seen only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n,π*) and (π,π*) excited singlet states. [ABSTRACT FROM AUTHOR]

The infrared predissociation spectra of the mass-selected electrocatalytic water oxidation intermediate [Ru(tpy)(bpy)(OH)]2+(H2O)0-4 are reported. The [Ru(tpy)(bpy)(OH)]2+ species is generated by passing a solution of [Ru(tpy)(bpy)(H2O)](ClO4)2 through an electrochemical flow cell held at 1.2 V and is immediately introduced into the gas phase via electrospray ionization (ESI). The microsolvated clusters are formed by reconstructing the water network in a cryogenic ion trap. Details of the hydrogen bonding network in these clusters are revealed by the infrared predissociation spectra in the OH stretch region. This improved method for capturing microsolvated clusters yielded colder complexes with much better resolved IR features than previous studies. The analysis of these spectra, supported by electronic structure calculations and compared to previous results on [Ru(tpy)(bpy)(H2O)]2+(H2O)0-4 clusters, reveals the nature of the Ru-OH bond and the effect of hydrogen bonding on facilitating the subsequent oxidation to [Ru(tpy)(bpy)(O)]2+ in the proposed catalytic cycle. Particularly, the hydrogen bonding interaction in [Ru(tpy)(bpy)(OH)]2+(H2O)1 is much weaker than that in the corresponding [Ru(tpy)(bpy)(H2O)]2+(H2O)1 and thus is less effective at activating the hydroxyl ligand for further oxidation via proton coupled electron transfer (PCET). Furthermore, the results here reveal that the Ru-OH bond, though formally described as an Ru3+/OH- interaction, has more covalent bond character than ionic bond character. [ABSTRACT FROM AUTHOR]

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C≡C-Phenyl-X)2, where X = diphenylamino (DPA), NH2, OCH3, t-Bu, CH3, H, F, benzothiazole (BTH), CF3, CN, and NO2. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S1 state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S1 state. The values are small Δμ < 1.0 D for neutral substituents such as CH3, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO2. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation (Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014, 118, 3749−3759) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation. [ABSTRACT FROM AUTHOR]

A quantum theory of atoms in molecules (QTAIM) and stress tensor analysis was applied to analyze intramolecular interactions influencing the photoisomerization dynamics of a light-driven rotary molecular motor. For selected nonadiabatic molecular dynamics trajectories characterized by markedly different S1 state lifetimes, the electron densities were obtained using the ensemble density functional theory method. The analysis revealed that torsional motion of the molecular motor blades from the Franck-Condon point to the S1 energy minimum and the S1/S0 conical intersection is controlled by two factors: greater numbers of intramolecular bonds before the hop-time and unusually strongly coupled bonds between the atoms of the rotor and the stator blades. This results in the effective stalling of the progress along the torsional path for an extended period of time. This finding suggests a possibility of chemical tuning of the speed of photoisomerization of molecular motors and related molecular switches by reshaping their molecular backbones to decrease or increase the degree of coupling and numbers of intramolecular bond critical points as revealed by the QTAIM/stress tensor analysis of the electron density. Additionally, the stress tensor scalar and vector analysis was found to provide new methods to follow the trajectories, and from this, new insight was gained into the behavior of the S1 state in the vicinity of the conical intersection. [ABSTRACT FROM AUTHOR]

A combined ab initio and quantum dynamical study characterizes a family of bent neutral carbon dioxide molecules in terms of their vibrational levels, electric dipole moment surfaces, and infrared spectra in the gas phase. The considered isomers include the dioxiranylidene form of CO2 with the equilibrium valence angle of 72°, belonging to the ground electronic state, and four open structures with the valence angles of 118°/119° (they belong to the singlet and triplet electronic states 2¹A' and 1³A', respectively) and 127°/128° (states 1¹A" and 1³A", respectively). All studied bent structures possess permanent dipole moments. For all isomers, the antisymmetric stretch fundamental is the strongest infrared transition. Individual bent molecules can be distinguished on the basis of strong absorption bands in the frequency window 1100-1800 cm-1 as well as isotopic shifts in the progression of antisymmetric stretch mode. Excitation of bent neutral carbon dioxide near a perfect metal surface is also briefly discussed. It is argued that the excitation energy from the linear ground state exhibits a substantial red shift depending on the molecule--metal distance. [ABSTRACT FROM AUTHOR]

There is a surge in the literature on the development of exchange-correlation density functionals for a wide variety of physical and chemical properties. As a recent endeavor toward the systematic and nonempirical design of density functional approximations, optimally tuned range-separated hybrid (OT-RSH) models have been introduced. In this work, we propose novel OT-RSH density functionals for predicting the oxidation potentials of organic compounds from different categories. In this regard, detailed analysis of the role of nonempirical optimization of the range separation parameter and importance of short- and long-range exact-like exchange in OT-RSH calculations of the oxidation potential has also been done. It is shown that the newly developed OT-RSH approximations not only perform better than other standard long-range corrected functionals but also in many cases outperform other conventional hybrid functionals with a fixed amount of exact-like exchange. Plus, we find that the proposed functionals describe well the oxidation potentials of compounds for which the tuning of the range separation parameter was not performed. From a different perspective, accountability of the computed frontier orbital energies from the OT-RSH density functionals for estimation of oxidation potentials has also been evaluated. Our results reveal that the negative highest occupied molecular orbital energies of molecules and the negative lowest unoccupied molecular orbital energies of their cations correlate remarkably with the observed oxidation potentials. Admittedly, with more efforts along this line, modern OT-RSH functionals with broader applicability can be released for computational electrochemistry. [ABSTRACT FROM AUTHOR]

Extensive ab initio investigations have been performed to characterize the stable conformers of hydrated arginine (Arg-H2O). Many new low-energy canonical Arg-H2O conformers were identified and they are more stable than previous results. The large energy differences (more than 5.00 kcal mol-1) between the canonical and zwitterionic Arg-H2O isomers calculated by the composite CBS-QB3 method confirmed the dominance of the zwitterions. The micro effects of corrections of the zero-point energy and the basis set superposition error on the stability of hydrated isomers were carefully examined for the first time. The infrared (IR) spectra were simulated at a recommended temperature and the notable spectral differences enable the unambiguous identification of the different hydrated forms. Further transition state calculations revealed that the canonical Arg-H2O can be transformed to the zwitterions using the amino group as a bridge. Our study thus shows valuable insights into the hydration of large flexible molecules and provides solid theoretical evidence for the canonical-to-zwitterionic structure transition of hydrated arginine. [ABSTRACT FROM AUTHOR]

Through a combined experimental and theoretical investigation we determine that the fundamental modes of three quinacridones fall in the terahertz spectral range (1-10 THz, ~30-300 cm-1). In each spectrum the terahertz resonances correspond to wagging, rocking, or twisting of the quinacridone rings, with the most intense absorption being an in-plane rocking vibration of the carbonyl oxygens. In spite of these spectral similarities, we demonstrate that terahertz measurements readily differentiate β-quinacridone, γ-quinacridone, and 2,9-dimethylquinacridone. The spectrum of β-quinacridone has a group of closely spaced modes at ~4 THz, whereas in contrast the spectrum of γ-quinacridone displays a widely spaced series of modes spread over the range ~1-5 THz. Both of these have the strongest mode at ~9 THz, whereas in contrast 2,9-dimethylquinacridone exhibits the strongest mode at ~7 THz. Because quinacridones are the basis of widely used synthetic pigments of relatively recent origin, our findings offer promising applications in the identification and dating of modern art. [ABSTRACT FROM AUTHOR]

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X˜1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time. [ABSTRACT FROM AUTHOR]

The infrared spectrum of H+(H2O)4 recently observed in a wide spectral range has shown a series of bands in a range of 1700-2500 cm-1, which can not be understood by the standard harmonic normal mode analysis. Here, we theoretically investigate the origin of these bands with a focus on (1) the possibility of coexistence of multiple isomers in the Eigen [H3O+(H2O)3] and Zundel [H5O2+(H2O)2] forms and (2) the effect of anharmonic coupling that gives rise to nonzero intensities for overtones and combination bands. Anharmonic vibrational calculations are carried out for the Eigen and Zundel clusters by the second-order vibrational quasi-degenerate perturbation theory (VQDPT2) based on optimized coordinates. The anharmonic potential energy surface and the dipole moment surfaces are generated by a multiresolution approach combining one-dimensional (1D) grid potential functions derived from CCSD(T)-F12, 2D and 3D grid potential functions derived from B3LYP for important coupling terms, and a quartic force field derived from B3LYP for less important terms. The spectrum calculated for the Eigen cluster is in excellent agreement with the experiment, assigning the bands in the range of 1700-2500 cm-1 to overtones and combination bands of a H3O+ moiety in line with recent reports [ J. Phys. Chem. A 2015, 119, 9425; Science 2016, 354, 1131]. On the other hand, characteristic OH stretching bands of the Zundel cluster is found to be absent in the experimental spectrum. We therefore conclude that the experimental spectrum originates solely from the Eigen cluster. Nonetheless, the present calculation for the Eigen cluster poorly reproduces a band observed at 1765 cm-1. A possible nature of this band is discussed. [ABSTRACT FROM AUTHOR]

Fundamental and non-fundamental vibrational modes, first overtones, and binary combination modes of selected aliphatic ethers (di-n-propylether, di-iso-propylether, n-butylmethyl ether, n-butylethyl ether, di-n-butyl ether, tert-buytlmethyl ether, and tert-amylmethyl ether) were modeled in a fully anharmonic generalized second-order vibrational perturbation theory (GVPT2) approach on the DFT-B2PLYP/SNST level. The modeling procedure of theoretical line shapes took into account conformational isomers of studied molecules. The calculated spectra of the above ethers were compared to the corresponding experimental spectra in the infrared (IR) region (4000-560 cm-1) of the absorption index k(ν) derived from the neat liquid thin-film transmission data. It was found that IR spectra of aliphatic ethers are heavily influenced by the bands originating from non-fundamental modes, particularly from the combination modes in the C-H stretching region (3200-2800 cm-1). Because of the effects of vibrational resonances, the intensities of overtones and combination bands originating from methyl and methylene deformation modes increase sufficiently to influence the experimental line shape in this region. Less significant contributions from non-fundamental vibrational modes were noticed in the lower IR region (1600-560 cm-1), particularly in the vicinity of the C-O stretching band. The 2700-1600 cm-1 region, which is rich in weak bands due to non-fundamental vibrations, was reproduced accurately as well. It was concluded that a fully anharmonic approach allows significantly more accurate reproduction of the complex IR line shapes, particularly in the C-H stretching region of aliphatic ethers. On the basis of the achieved agreement between the experimental and calculated spectra, it may be concluded that the anharmonic GVPT2 method can adequately reproduce the anharmonic effects and vibrational resonances in particular, influencing the IR spectra of aliphatic ethers. The results obtained in this study show that the non-fundamental modes may play a significant role in shaping the IR spectra of aliphatic ethers and similar molecules in the neat liquid phase. [ABSTRACT FROM AUTHOR]

Although an isolation of elusive tricyanomethane HC(CN)3 was recently reported, the existence of other HC4N3 species has yet to be confirmed. In this work, the relative stabilities, spectroscopic features, and rearrangements of tricyanomethane and its four isomers are examined using single-(CCSD(T), CCSD(T)-F12) and multireference (MCSCF, MRPT2) methods. Tricyanomethane and dicyanoketenimine (NC)2CCNH, which are found to be the two most stable HC4N3 isomers lying within 9 kcal/mol, can be discriminated by their spectroscopic parameters. The predicted stepwise interconversion path relating HC(CN)3 and (NC)2CCNH features the HC4N3 species comprising the C-C-N ring moiety, with the largest barrier being associated with the initial H migration to one of the CN carbons. Adding a water molecule reduces the H migration barrier strongly and makes it possible to interconvert tricyanomethane to dicyanoketenimine in a "concerted" way. [ABSTRACT FROM AUTHOR]

Excimer formation plays a significant role in trapping excitons within organic molecular solids. Covalent dimers of perylene-3,4:9,10-bis(dicarboximide) (PDI) are useful model systems for studying these processes as their intermolecular geometries can be precisely tuned. Using femtosecond visible-pump infrared-probe (fsIR) spectroscopy in the shortwave- and mid-infrared regions, we characterize two PDI dimers with a cofacial and a slip-stacked geometry that are coupled through a triptycene bridge. In the mid-infrared region, fsIR spectra for the strongly coupled dimers are highly blue-shifted compared to spectra for monomeric 1*PDI. The perylene core stretching modes provide a directly observable probe of excimer relaxation, as they are particularly sensitive to this process, which is associated with a small blue shift of these modes in both dimers. The broad Frenkel-to-CT state electronic transition of the excimer, the edge of which has previously been detected in the NIR region, is now fully resolved to be much broader and to extend well into the shortwave infrared region for both dimers and is likely a generic feature of π-extended aromatic excimers. [ABSTRACT FROM AUTHOR]

Infrared (IR) spectra of methanol clusters in helium nanodroplets are reported in the OH stretching region for the first time. A simple series of intense bands are seen which almost perfectly match previous gas phase studies of these clusters and which are consistent with cyclic structures for the trimer and larger clusters. This finding differs from an earlier report of (CH3OH)n clusters in helium nanodroplets, which focused on the CO stretching region and concluded that while the trimer was cyclic, the tetramer and pentamer adopted branched structures based on a cyclic trimer core. We also present preliminary data for small (CH3OH)n(H2O) clusters, and in particular, we report the first IR spectra for (CH3OH)2(H2O) and (CH3OH)3(H2O). Supporting ab initio calculations suggest that, like the pure methanol clusters, cyclic structures are adopted by these mixed solvent clusters in helium droplets. [ABSTRACT FROM AUTHOR]

Hydrogenase enzymes enable organisms to use H2 as an energy source, having evolved extremely efficient biological catalysts for the reversible oxidation of molecular hydrogen. Small-molecule mimics of these enzymes provide both simplified models of the catalysis reactions and potential artificial catalysts that might be used to facilitate a hydrogen economy. We have studied two diiron hydrogenase mimics, μ-pdt-[Fe(CO)3]2 and μ-edt-[Fe(CO)3]2 (pdt = propanedithiolate, edt = ethanedithiolate), in a series of alkane solvents and have observed significant ultrafast spectral dynamics using two-dimensional infrared (2D-IR) spectroscopy. Since solvent fluctuations in nonpolar alkanes do not lead to substantial electrostatic modulations in a solute's vibrational mode frequencies, we attribute the spectral diffusion dynamics to intramolecular flexibility. The intramolecular origin is supported by the absence of any measurable solvent viscosity dependence, indicating that the frequency fluctuations are not coupled to the solvent motional dynamics. Quantum chemical calculations reveal a pronounced coupling between the low-frequency torsional rotation of the carbonyl ligands and the terminal CO stretching vibrations. The flexibility of the CO ligands has been proposed to play a central role in the catalytic reaction mechanism, and our results highlight that the CO ligands are highly flexible on a picosecond time scale. [ABSTRACT FROM AUTHOR]

Efficient acquisition of high-quality ultra-wideline (UW) solid-state NMR powder patterns in short experimental time frames is challenging. UW NMR powder patterns often possess inherently low signal-to-noise (S/N) and usually overlap for samples containing two or more magnetically distinct nuclides, which obscures spectral features and drastically lowers the spectral resolution. Currently, there is no reliable method for resolving overlapping powder patterns originating from unreceptive nuclei affected by large anisotropic NMR interactions. Herein, we discuss new methods for resolving individual UW NMR spectra associated with magnetically distinct nuclei by exploiting their different relaxation characteristics using 2D relaxation-assisted separation (RAS) experiments. These experiments use a non-negative Tikhonov fitting (NNTF) routine to process high-quality T 1 and T 2eff relaxation data sets to produce high-resolution, 2D spin-relaxation correlation spectra for both spin-1/2 and quadrupolar nuclei in organic and organometallic solids under static ( i.e ., stationary) conditions. It is found that (i) T 2eff RAS data sets can be acquired in a fraction of the time required for analogous T1 RAS data sets, because a time-incremented 2D data set is not required for the former, and (ii) Tikhonov regularization is superior to conventional non-negative least-squares fitting, as it more reliably and robustly results in cleaner separation of patterns based on relaxation time constants. [ABSTRACT FROM AUTHOR]

The propargyl radical is considered to be of key importance in the formation of the first aromatic ring in combustion processes. Here we study the bimolecular (self-) reactions of propargyl in a high-temperature pyrolysis flow reactor. The aromatic reaction products are identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as mid-infrared source. This technique combines mass selectivity with structural sensitivity. We identified several aromatic reaction products based on their infrared spectra, among them benzene, naphthalene, phenanthrene, indene, biphenyl, and surprisingly a number of aromatic compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that propargyl is involved in the formation of the first aromatic ring. The observation of compounds with acetylenic side chains shows that, in addition to a propargyl- and phenyl-based mechanism, the HACA (hydrogen abstraction C2H2 addition) mechanism of polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a reactant. On the basis of the experimental results we suggest a mechanism that connects the two pathways. [ABSTRACT FROM AUTHOR]

The pure rotational spectrum of diketene has been studied in the millimeter-wave region from ∼240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (ν24, 2ν24, 3ν24, 4ν24, and ν16), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10 000 distinct rotational transitions. In each case, the transitions were fit to single-state, complete or near-complete sextic centrifugally distorted rotor models to near experimental error limits using Kisiel's ASFIT. Additionally, we obtained less satisfactory least-squares fits to single-state centrifugally distorted rotor models for three additional vibrational states: ν24 + ν16, ν23, and 5ν24. The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (αi) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These αi values were helpful in identifying the previously unreported ν16 and ν23 fundamental states. We obtained a single-crystal X-ray structure of diketene at −173 °C. The bond distances are increased in precision by more than an order of magnitude compared to those in the 1958 X-ray crystal structure. The improved accuracy of the crystal structure geometry resolves the discrepancy between previous computational and experimental structures. The rotational transition frequencies provided herein should be useful for a millimeter-wave or terahertz search for diketene in the interstellar medium. [ABSTRACT FROM AUTHOR]

The 2-butenal oxide Criegee intermediate [(CH 3 CH═CH)CHOO], an isomer of the four-carbon unsaturated Criegee intermediates derived from isoprene ozonolysis, is characterized on its first π* ← π electronic transition and by the resultant dissociation dynamics to O ( 1 D) + 2-butenal [(CH 3 CH═CH)CHO] products. The electronic spectrum of 2-butenal oxide under jet-cooled conditions is observed to be broad and unstructured with peak absorption at 373 nm, spanning to half maxima at 320 and 420 nm, and in good accord with the computed vertical excitation energies and absorption spectra obtained for its lowest energy conformers. The distribution of total kinetic energy released to products is ascertained through velocity map imaging of the O ( 1 D) products. About half of the available energy, deduced from the theoretically computed asymptotic energy, is accommodated as internal excitation of the 2-butenal fragment. A reduced impulsive model is introduced to interpret the photodissociation dynamics, which accounts for the geometric changes between 2-butenal oxide and the 2-butenal fragment, and vibrational activation of associated modes in the 2-butenal product. Application of the reduced impulsive model to the photodissociation of isomeric methyl vinyl ketone oxide reveals greater internal activation of the methyl vinyl ketone product arising from methyl internal rotation and rock, which is distinctly different from the dissociation dynamics of 2-butenal oxide or methacrolein oxide.

Heteronuclear transition metal carbonyl cluster cations FeM(CO)8+ (M = Co, Ni and Cu) and MCu(CO)7+ (M = Co and Ni) are produced via a laser vaporization supersonic cluster ion source in the gas phase, which are each mass-selected and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region. Their geometric and electronic structures are established by comparison of the experimental spectra with those derived from density functional theoretical calculations. The FeM(CO)8+ (M = Co, Ni, Cu) complexes are determined to have eclipsed (CO)5Fe-M(CO)3+ structures, and the MCu(CO)7+ (M = Co, Ni) ions are characterized to have staggered (CO)4M-Cu(CO)3+ structures. Natural bonding orbital analysis indicates that the positive charge is mainly distributed on the M(CO)3 fragment. The metal-metal interaction involves an σ-type (OC)4,5M→M(CO)3+ dative bonding. [ABSTRACT FROM AUTHOR]

The structures, properties, and spectroscopic signatures of oxidized water clusters,(H2O)+n=6-21, are examined in this work, to provide fundamental insight into renewable energy and radiological processes. Computational quantum chemistry approaches are employed to sample cluster morphologies, yielding hundreds of low-lying isomers with low barriers to interconversion. The ion-radical pair-separation trend, however, which was observed in previous computational studies and in small-cluster spectroscopy experiments, is shown to continue in this larger cluster size regime. The source of this trend is preferential solvation of the hydronium ion by water, including effects beyond the first solvation shell. The fundamental conclusion of this work, therefore, is that the initially formed ion-radical dimer, which has served as a prototypical model of oxidized water, is a nascent species in large, oxidized water clusters and, very likely, bulk water. [ABSTRACT FROM AUTHOR]

The absorption cross section of an overtone transition of OH radicals at 7028.831 cm-1 has been measured using an improved experimental setup coupling laser photolysis to three individual time-resolved detection techniques. Time-resolved relative OH radical profiles were measured by laser-induced fluorescence (LIF), and their absolute profiles have been obtained by cw-cavity ring-down spectroscopy (cw-CRDS). HO2 radicals were quantified simultaneously at the well-characterized absorption line at 6638.21 cm-1 by a second cw-CRDS absorption path. Initial OH concentrations and thus their absorption cross sections have been deduced from experiments of 248 nm photolysis of H2O2: OH and HO2 profiles have been fitted to a simple kinetic model using well-known rate constants. The rate constant of the reaction between OH and HO2 radicals turned out to be sensitive to the deduction of the initial OH concentration and has been revisited in this work: OH decays have been observed in the presence of varying excess HO2 concentrations. A rate constant of (1.02 ± 0.06) × 10-10 cm³ s-1 has been obtained, in good agreement with previous measurements and recent recommendations. An absorption cross section of σOH = (1.54 ± 0.1) × 10-19 cm² at a total pressure of 50 Torr helium has been obtained from consistent fitting of OH and HO2 profiles in a large range of concentrations. [ABSTRACT FROM AUTHOR]

The low-lying electronic states of platinum ions (Pt+) and platinum monohalides (PtX; X = F, Cl, Br, and I) are calculated using the multireference configuration interaction method with relativistic effective core potentials. The spin-orbit coupling is taken into account through the perturbative state-interaction approach. For the Ω states of PtX below 35000 cm-1, the potential energy curves and the corresponding spectroscopic constants are reported. It is found that the lowest Ω = 3/2 state is the ground one for the four species of PtX. Overall, the theoretical results are in reasonable agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Borylenes, RB, are elusive reactive intermediates. Still not much is known about their excited states from spectroscopic experiments, and existing knowledge is limited to diatomic borylenes only. The electronic structure and geometry of borylenes with diverse substituents on boron (where R = H, F, Cl, CH3, CF3, tBu, NH2, Ph, and SiMe3) were studied by means of computational chemistry. For this purpose, geometries of borylenes in their lowest singlet and triplet states were optimized at the B3LYP/def2-TZVP level of theory. Additionally, the influence of substitution on the energies of frontier molecular orbitals, HOMO-LUMO energy gaps, singlet-triplet energy splittings, and excitation energies was investigated. Two lowest vertical singlet-singlet excitations were computed using EOM-CCSD and TD-DFT (using hybrid B3LYP, and long-range separated CAM-B3LYP and ωB97X functionals) in combination with the aug-cc-pVTZ basis set. The electronic transitions involve excitations from nonbonding sp boron orbital (HOMO) mainly to empty p(B) orbitals and partially to the orbitals of the substituent, and are of n → π* type. The results can facilitate prospective identification of borylenes, e.g., in UV-vis matrix isolation or time-resolved spectroscopy experiments. [ABSTRACT FROM AUTHOR]

On the grounds of a hybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of a variational-perturbational method, the IR spectrum of dicyanoacetylene is revisited in the region 100-4800 cm-1, comparing our results with previous experimental data. A position and intensity analysis of our theoretical results allows us to assign fundamentals, combinations, and overtone bands in the aforementioned range of frequencies. Moreover, the outcomes show a good agreement with the most reliable experimental values and predict several unobserved or unassigned overtones and combinations in the mid-infrared region. [ABSTRACT FROM AUTHOR]

Noble-gas hydrides have been extensively studied in noble gas matrices. However, little is known on their stability and properties in molecular hosts. Here, HXeBr in the N2 environment is modeled at the B3LYP-D level of theory in a complete single shell of 22 N2 molecules. The system is compared to similar models of HXeBr in CO2 and Xe clusters. The optimized structure of (HXeBr)@(N2)22 is of low symmetry and is highly anisotropic. None of the N2 molecules are freely rotating, and the host molecules are not symmetrically positioned with respect to the HXeBr axis. The axes of the N2 molecules are nonuniformly distributed. The computed anharmonic H-Xe stretching frequency of HXeBr in the N2 cluster is in good accord with the experimental value. The soft-mode frequencies of the cluster including both intermolecular vibrations and librations, have a broad distribution that ranges from 8.7 to 107 cm-1. It is expected that these findings and specifically, the single-shell model, may shed light also on the local structure and vibrations of other impurities in a molecular media. [ABSTRACT FROM AUTHOR]

Penning ionization reactions in merged beams with precisely controlled collision energies have been shown to accurately probe quantum mechanical effects in reactive collisions. A complete microscopic understanding of the reaction is, however, faced with two major challenges the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful. [ABSTRACT FROM AUTHOR]

Quantum coherence is proclaimed to promote efficient energy collection by light-harvesting complexes and prototype organic photovoltaics. However, supporting spectroscopic studies are hindered by the problem of distinguishing between the excited state and ground state origin of coherent spectral transients. Coherence amplitude maps, which systematically represent quantum beats observable in two-dimensional (2D) spectroscopy, are currently the prevalent tool for making this distinction. In this article, we present coherence amplitude maps of a molecular dimer, which have become significantly distorted as a result of the finite laser bandwidth used to record the 2D spectra. We argue that under standard spectroscopic conditions similar distortions are to be expected for compounds absorbing over a spectral range similar to, or exceeding, that of the dimer. These include virtually all photovoltaic polymers and certain photosynthetic complexes. With the distortion of coherence amplitude maps, alternative ways to identify quantum coherence are called for. Here, we use numerical simulations that reproduce the essential photophysics of the dimer to unambiguously determine the excited state origin of prominent quantum beats observed in the 2D spectral measurements. This approach is proposed as a dependable method for coherence identification. [ABSTRACT FROM AUTHOR]

We have determined absorption of the near UV light (290-345 nm) by nitric acid (HNO3) deposition on sapphire window surfaces as a function of the HNO3 pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO3 surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10-19 and (0.29 ± 0.03) × 10-19 cm2/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO3 adsorption was reported, a new absorption band appeared in the 320-345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO3-) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO3 formed by HNO3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5-7 min. Background transmission changes in the 320-350 nm region after exposing clean sapphire surfaces with many repeated HNO3 deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320-350 nm range. We also modeled photolysis rates of HNO3/NO3- on urban grimes. Atmospheric implications of the results are discussed. This paper was published ASAP on May 2, 2016, with an incorrect Figure 6 and errors in Figure 7. The corrected version was reposted on May 3, 2016. [ABSTRACT FROM AUTHOR]

This study aims to reliably assign the vibrational sum frequency generation (SFG) spectrum of a-pinene at the vapor/solid interface using a method involving deuteration of various methyl groups. The synthesis of five deuterated isotopologues of a-pinene is presented to determine the impact that removing contributions from methyl group C-H oscillators has on its SFG response. 0.6 cm-1resolution SFG spectra of these isotopologues show varying degrees of differences in the C-H stretching region when compared to the SFG response of unlabeled a-pinene. The largest spectral changes were observed for the isotopologue containing a fully deuterated vinyl methyl group. Noticeable losses in signal intensities allow us to reliably assign the 2860 cm-1peak to the vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl methyl group entirely by synthesizing apopinene, the steric influence of the unlabeled C9H14fragment on the SFG response of a-pinene SFG can be readily observed. The work presented here brings us one step closer to understanding the vibrational spectroscopy of a-pinene. [ABSTRACT FROM AUTHOR]

The ultrafast deactivation processes in the excited state of biomolecules, such as the most stable tautomers of guanine, forbid any state-of-the-art gas phase spectroscopic studies on these species with nanosecond lasers. This drawback can be overcome by grafting a chromophore having a long-lived excited state to the molecule of interest, allowing thus a mass-selective detection by nanosecond R2PI and therefore double resonance IR/UV conformer-selective spectroscopic studies. The principle is presently demonstrated on the keto form of a modified 9-methylguanine, for which the IR/UV double resonance spectrum in the C=O stretch region, reported for the first time, provides evidence for extensive vibrational couplings within the guanine moiety. Such a successful strategy opens up a route to mass-selective IR/UV spectroscopic investigations on molecules exhibiting natural chromophores having ultrashort-lived excited states, such as DNA bases, their complexes as well as peptides containing short-lived aromatic residues. [ABSTRACT FROM AUTHOR]

The Raman optical activity spectra of the epimers β-d-glucose and β-d-galactose, two monosaccharides of biological importance, have been calculated using molecular dynamics combined with a quantum mechanics/molecular mechanics approach. Good agreement between theoretical and experimental spectra is observed for both monosaccharides. Full band assignments have been carried out, which has not previously been possible for carbohydrate epimers. For the regions where the spectral features are opposite in sign, the differences in the vibrational modes have been noted and ascribed to the band sign changes. [ABSTRACT FROM AUTHOR]

The effectiveness of solar energy capture and conversion materials derives from their ability to absorb light and to transform the excitation energy into energy stored in free carriers or chemical bonds. The Thomas-Reiche-Kuhn (TRK) sum rule mandates that the integrated (electronic) oscillator strength of an absorber equals the total number of electrons in the structure. Typical molecular chromophores place only about 1% of their oscillator strength in the UV-vis window, so individual chromophores operate at about 1% of their theoretical limit. We explore the distribution of oscillator strength as a function of excitation energy to understand this circumstance. To this aim, we use familiar independent-electron model Hamiltonians as well as first-principles electronic structure methods. While model Hamiltonians capture the qualitative electronic spectra associated with π electron chromophores, these Hamiltonians mistakenly focus the oscillator strength in the fewest low-energy transitions. Advanced electronic structure methods, in contrast, spread the oscillator strength over a very wide excitation energy range, including transitions to Rydberg and continuum states, consistent with experiment. Our analysis rationalizes the low oscillator strength in the UV-vis spectral region in molecules, a step toward the goal of oscillator strength manipulation and focusing. [ABSTRACT FROM AUTHOR]