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Photosystem II (PSII) uses light energy to split water into protons, electrons, and oxygen, ultimately sustaining heterotrophic life on Earth. The major light harvesting complex in plants (LHCII) is packed with chlorophylls and carotenoids and is the main supplier of excitation energy to PSII reaction centers. The protein scaffold acts as a programmed solvent for the pigments in LHCII, tuning their orientations while at the same time impeding concentration quenching to ensure efficient storage of excitation energy by chlorophylls. However, under stress, the very fuel of PSII, solar photons, can damage its delicate inner components and hamper photosynthesis. In a crucial regulatory strategy in plants, LHCII evolved a flexible design that allows it to switch between light-harvesting and dissipative conformations, thereby safely releasing the excess energy that is absorbed into heat. Several mechanisms have been proposed to explain chlorophyll de-excitation pathways in LHCII, such as chlorophyll–chlorophyll charge transfer states, resonance energy transfer from chlorophylls to a carotenoid S1 state, and chlorophyll–carotenoid reductive energy transfer. This Perspective critically assesses the listed proposals, addressing both the physical mechanism of quenching and the nature of the quenching pigment. These hypotheses are then discussed in the context of state-of-the-art biochemical, physiological, and genetic knowledge to scrutinize their likeliness to occur in the native thylakoid membranes. [ABSTRACT FROM AUTHOR]

Photosynthetic pigment–protein complexes control local chlorophyll (Chl) transition frequencies through a variety of electrostatic and steric forces. Site-directed mutations can modify this local spectroscopic tuning, providing critical insight into native photosynthetic functions and offering the tantalizing prospect of creating rationally designed Chl proteins with customized optical properties. Unfortunately, at present, no proven methods exist for reliably predicting mutation-induced frequency shifts in advance, limiting the method's utility for quantitative applications. Here, we address this challenge by constructing a series of point mutants in the water-soluble chlorophyll protein of Lepidium virginicum and using them to test the reliability of a simple computational protocol for mutation-induced site energy shifts. The protocol uses molecular dynamics to prepare mutant protein structures and the charge density coupling model of Adolphs et al. [Photosynth. Res. 95, 197–209 (2008)] for site energy prediction; a graphical interface that implements the protocol automatically is published online at http://nanohub.org/tools/pigmenthunter. With the exception of a single outlier (presumably due to unexpected structural changes), we find that the calculated frequency shifts match the experiment remarkably well, with an average error of 1.6 nm over a 9 nm spread in wavelengths. We anticipate that the accuracy of the method can be improved in the future with more advanced sampling of mutant protein structures. [ABSTRACT FROM AUTHOR]

Molecular electronic or vibrational states can be superimposed temporarily in an extremely short laser pulse, and the superposition-state transients formed therein receive much attention, owing to the extensive interest in molecular fundamentals and the potential applications in quantum information processing. Using the crossed-beam ion velocity map imaging technique, we disentangle two distinctly different pathways leading to the forward-scattered N2+ yields in the large impact-parameter charge transfer from low-energy Ar+ to N2. Besides the ground-state (X2Σg+) N2+ produced in the energy-resonant charge transfer, a few slower N2+ ions are proposed to be in the superpositions of the X2Σg+-A2Πu and A2Πu-B2Σu+ states on the basis of the accidental degeneracy or energetic closeness of the vibrational states around the X2Σg+-A2Πu and A2Πu-B2Σu+ crossings in the non-Franck–Condon region. This finding potentially shows a brand-new way to prepare the superposition-state molecular ion. [ABSTRACT FROM AUTHOR]

Photodissociation of [Ar–N2]+ induced by a near-IR (800 nm) femtosecond laser pulse is investigated using ion-trap time-of-flight mass spectrometry. The intra-complex charge transfer proceeding in the course of the decomposition of the electronically excited Ar+(2P3/2)⋯N2(X1Σg+), prepared by the photoexcitation of the electronic ground Ar(1S0)⋯N2+(X2Σg+), is probed by the ion yields of Ar+ and N2+. The yield ratio γ of N2+ with respect to the sum of the yields of Ar+ and N2+ is determined to be γ = 0.62, which is much larger than γ ∼ 0.2 determined before when the photodissociation is induced by a nano-second laser pulse in the shorter wavelength region between 270 and 650 nm. This enhancement of γ at 800 nm and the dependence of γ on the excitation wavelength are interpreted by numerical simulations, in which the adiabatic population transfer from Ar+(2P3/2)⋯N2(X1Σg+) to Ar(1S0)⋯N2+(X2Σg+) at the avoided crossings is accompanied by the vibrational excitation in the N2+(X2Σg+) moiety followed by the intra-complex vibrational energy transfer from the N2+(X2Σg+) moiety to the intra-complex vibrational mode leading to the dissociation. [ABSTRACT FROM AUTHOR]

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reactionCH3+ +CH3CCCH3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C5H7+, stemming from H2 loss from the addition complex. This is the only observed condensation product with the formation of new C [ABSTRACT FROM AUTHOR]

The collisional dynamics of N2+(2Σg+) cations with Ar atoms is studied using quasi-classical simulations. N2+-Ar is a proxy to study cooling of molecular ions and interesting in its own right for molecule-to-atom charge transfer reactions. An accurate potential energy surface (PES) is constructed from a reproducing kernel Hilbert space (RKHS) interpolation based on high-level ab initio data. The global PES including the asymptotics is fully treated within the realm of RKHS. From several ten thousand trajectories, the final state distribution of the rotational quantum number of N2+ after collision with Ar is determined. Contrary to the interpretation of previous experiments which indicate that up to 98% of collisions are elastic and conserve the quantum state, the present simulations find a considerably larger number of inelastic collisions which supports more recent findings. [ABSTRACT FROM AUTHOR]

We derive an analytical equation of state for the hard-sphere fluid that is within 0.01% of computer simulations for the whole range of the stable fluid phase. In contrast, the commonly used Carnahan- Starling equation of state deviates by up to 0.3% from simulations. The derivation uses the functional form of the isothermal compressibility from the Percus-Yevick closure of the Ornstein-Zernike relation as a starting point. Two additional degrees of freedom are introduced, which are constrained by requiring the equation of state to (i) recover the exact fourth virial coefficient B4 and (ii) involve only integer coefficients on the level of the ideal gas, while providing best possible agreement with the numerical result for B5. Virial coefficients B6 to B10 obtained from the equation of state are within 0.5% of numerical computations, and coefficients B11 and B12 are within the error of numerical results. We conjecture that even higher virial coefficients are reliably predicted. [ABSTRACT FROM AUTHOR]

We report resonance-enhanced multiphoton ionization spectra of the isotopologues fluorobenzene h5 and fluorobenzene-d5. By making use of quantum chemical calculations, the changes in the wavenumber of the vibrational modes upon deuteration are examined. Additionally, the mixing of vibrational modes both between isotopologues and also between the two electronic states is discussed. The isotopic shifts lead to dramatic changes in the appearance of the spectrum as vibrations shift in and out of Fermi resonance. Assignments of the majority of the fluorobenzene-d5 observed bands are provided, aided by previous results on fluorobenzene-h5. [ABSTRACT FROM AUTHOR]

We have observed that an assembly of the bacteriochloropyll a molecules into B850 and B875 groups of cyclic bacterial light-harvesting complexes LH2 and LH1, respectively, results an almost total loss of the intra-molecular vibronic structure in the fluorescence spectrum, and simultaneously, an essential enhancement of its phonon sideband due to electron-phonon coupling. While the sup-pression of the vibronic coupling in delocalized (excitonic) molecular systems is predictable, as also confirmed by our model calculations, a boost of the electron-phonon coupling is rather un-expected. The latter phenomenon is explained by exciton self-trapping, promoted by mixing the molecular exciton states with charge transfer states between the adjacent chromophores in the tightly packed B850 and B875 arrangements. Similar, although less dramatic trends were noted for the light-harvesting complexes containing chlorophyll pigments. [ABSTRACT FROM AUTHOR]

A modified Padé approximant is used to construct an equation of state, which has the same large-density asymptotic behavior as the model fluid being described, while still retaining the low-density behavior of the virial equation of state (virial series). Within this framework, all sequences of rational functions that are analytic in the physical domain converge to the correct behavior at the same rate, eliminating the ambiguity of choosing the correct form of Padé approximant. The method is applied to fluids composed of 'soft' spherical particles with separation distance r interacting through an inverse-power pair potential, [lowercase_phi_synonym] = [variant_greek_epsilon](σ/r)n, where [variant_greek_epsilon] and σ are model parameters and n is the 'hardness' of the spheres. For n < 9, the approximants provide a significant improvement over the 8-term virial series, when compared against molecular simulation data. For n >= 9, both the approximants and the 8-term virial series give an accurate description of the fluid behavior, when compared with simulation data. When taking the limit as n → ∞, an equation of state for hard spheres is obtained, which is closer to simulation data than the 10-term virial series for hard spheres, and is comparable in accuracy to other recently proposed equations of state. By applying a least square fit to the approximants, we obtain a general and accurate soft-sphere equation of state as a function of n, valid over the full range of density in the fluid phase. [ABSTRACT FROM AUTHOR]

We have developed an ion-molecule reaction apparatus for state-selected absolute total cross section measurements by implementing a high-resolution molecular beam vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) ion source to a double-quadrupole double-octopole ion-guide mass spectrometer. Using the total cross section measurement of the state-selected N2+(v+, N+) + Ar charge transfer (CT) reaction as an example, we describe in detail the design of the VUV laser PFI-PI ion source used, which has made possible the preparation of reactant N2+(X 2Σg+, v+ = 0-2, N+ = 0-9) PFI-PIs with high quantum state purity, high intensity, and high kinetic energy resolution. The PFI-PIs and prompt ions produced in the ion source are shown to have different kinetic energies, allowing the clean rejection of prompt ions from the PFI-PI beam by applying a retarding potential barrier upstream of the PFI-PI source. By optimizing the width and amplitude of the pulsed electric fields employed to the VUV-PFI-PI source, we show that the reactant N2+ PFI-PI beam can be formed with a laboratory kinetic energy resolution of ΔElab = ± 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (Ecm's) down to thermal energies. Absolute total rovibrationally selected cross sections σ(v+ = 0-2, N+ = 0-9) for the N2+(X 2Σg+; v+ = 0-2, N+ = 0-9) + Ar CT reaction have been measured in the Ecm range of 0.04-10.0 eV, revealing strong vibrational enhancements and Ecm-dependencies of σ(v+ = 0-2, N+ = 0-9). The thermochemical threshold at Ecm = 0.179 eV for the formation of Ar+ from N2+(X; v+ = 0, N+) + Ar was observed by the measured σ(v+ = 0), confirming the narrow ΔEcm spread achieved in the present study. The σ(v+ = 0-2; N+) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stückelberg formulism are found to be in fair agreement with the present measured σ(v+ = 1 or 2; N+). Taking into account of the experimental uncertainties, the measured σ(v+ = 1 or 2, N+) for N+ = 0-9 at Ecm = 0.04-10.0 eV are found to be independent of N+. [ABSTRACT FROM AUTHOR]

Codeposition of a Ne:N2:CO = 200:1:1 mixture at 4.3 K with a beam of very pure neon atoms excited to their energy levels between 16.6 and 16.85 eV leads to stabilization in the resulting solid of sufficient NNCO+ for detection of its NN- and CO-stretching vibration fundamentals. Detailed isotopic substitution studies and density functional calculations for the various isotopologues support the identification of NNCO+ and permit estimation of the positions of two of its low-frequency fundamentals. A sufficient concentration of NOCN is also stabilized in the neon matrix for detection of its NO-stretching vibrational fundamental [ABSTRACT FROM AUTHOR]

The question of whether the known virial coefficients are enough to determine the packing fraction η∞ at which the fluid equation of state of a hard-sphere fluid diverges is addressed. It is found that the information derived from the direct Padé approximants to the compressibility factor constructed with the virial coefficients is inconclusive. An alternative approach is proposed which makes use of the same virial coefficients and of the equation of state in a form where the packing fraction is explicitly given as a function of the pressure. The results of this approach both for hard-disk and hard-sphere fluids, which can straightforwardly accommodate higher virial coefficients when available, lends support to the conjecture that η∞ is equal to the maximum packing fraction corresponding to an ordered crystalline structure. [ABSTRACT FROM AUTHOR]

Using the first seven known virial coefficients and forcing it to possess two branch-point singularities, a new equation of state for the hard-sphere fluid is proposed. This equation of state predicts accurate values of the higher virial coefficients, a radius of convergence smaller than the close-packing value, and it is as accurate as the rescaled virial expansion and better than the Padé [3/3] equations of state. Consequences regarding the convergence properties of the virial series and the use of similar equations of state for hard-core fluids in d dimensions are also pointed out. [ABSTRACT FROM AUTHOR]

The structural and energetic properties of the H2S2 molecule have been studied using density functional theory, second-order Mo\ller–Plesset method, and coupled cluster theory with several basis sets. In order to extend previous work on intra- and intermolecular dynamics of the chirality changing modes for H2O2 and its derivatives, our focus has been on the torsion around the S–S bond, along with an extensive characterization of the intermolecular potentials of H2S2 with the rare gases (He, Ne, Ar, and Kr). Use is made of previously defined coordinates and expansion formulas for the potentials which allow for a faithful representation of geometrical and symmetry properties of these systems that involve the interaction of an atom with a floppy molecule. The potential energy surfaces obtained in this work are useful for classical and quantum mechanical simulations of molecular collisions responsible for chirality changing processes of possible interest in the modeling of prebiotic phenomena. [ABSTRACT FROM AUTHOR]

Padé approximants have long been used to predict virial series coefficients and to provide equations of state for low and high density materials. However, some justified criticism has appeared about this procedure. Although we agree to impose several restrictions on the use of Padé approximants, we indicate that the Padé approximant is still an excellent way to predict the first unknown virial series coefficients. As an example, we report a calculation of the B11=128.6 and B12=155 virial coefficients of the three dimensional hard sphere model that are in excellent agreement with the two most recent estimates. We also consider that the commonly used method to choose among Padé approximants is not completely reliable for this specific application and suggest an alternative new method. [ABSTRACT FROM AUTHOR]

A search is conducted for the calculation of potential energy curves (PECs), spectroscopic constants, and dipole moment functions for excited and Rydberg states of imidogen radical NH, with a particular emphasis on the Rydberg states arising from 3s configuration of nitrogen and 2s and 2p configurations of hydrogen. A range of about 11 eV above the electronic ground state X 3Σ- atomic separation limit which corresponds to the first eight asymptotes of dissociation is spanned. Computations are carried out at the internally contracted multireference singles plus doubles configuration interaction level of theory, including the Davidson correction to account for quadruple excitations. The Gaussian basis set used has been modified from a standard basis to give a balanced description of valence-Rydberg interactions. States of 1Σ-, 1Π, 1Δ, 3Σ-, 3Π, 3Δ, and 5Σ- symmetries are computed accurately in the range of energy investigated. PECs of the three lowest 5Π states are obtained for the first time. Our spectroscopic constants show good agreement with experimental data in comparison with other theoretical studies reported in the literature. A discussion on the variations of dipole moment functions helps to understand the strong interactions between excited and Rydberg states as well as the avoided crossings. The present study may be of great practical interest for investigations in astrophysical research as well as in laboratory experiments. [ABSTRACT FROM AUTHOR]

We have studied the potentially ionospherically significant reaction between N22+ with O2 using position-sensitive coincidence spectroscopy. We observe both nondissociative and dissociative electron transfer reactions as well as two channels involving the formation of NO+. The NO+ product is formed together with either N+ and O in one bond-forming channel or O+ and N in the other bond-forming channel. Using the scattering diagrams derived from the coincidence data, it seems clear that both bond-forming reactions proceed via a collision complex [N2O2]2+. This collision complex then decays by loss of a neutral atom to form a daughter dication (NO22+ or N2O2+), which then decays by charge separation to yield the observed products. [ABSTRACT FROM AUTHOR]

A theoretical investigation of the intermolecular interaction, operative in collision complexes of He*(2 3S1), He*(2 1S0), and Ne*(3P2,0) with N2O, is carried out to explain the main results of the experimental study reported in the preceding paper. The analysis is carried out by means of a semiempirical method based on the identification, modeling, and combination of the leading interaction components, including the effect of the selective polarization of the more external electronic cloud of the metastable atom in the intermolecular electric field. These and other crucial aspects of our approach have been quantitatively verified by ab initio calculations. The proposed method permits to evaluate the interaction at any configuration of the complexes and provides a useful and inexpensive representation of the intermolecular potential energy for dynamics studies. The main experimental findings can be rationalized taking into account the critical balancing between molecular orientation effects in the intermolecular interaction field and the ionization probability. These orientation effects tend to become less pronounced with increasing collision energy. [ABSTRACT FROM AUTHOR]

We have investigated, both theoretically and experimentally, the reactions of naphthylium C10H7+ and d-naphthylium C10D7+ ions with H2 and D2. Cross sections as functions of the collision energy have been measured for a variety of reaction channels. Theoretical calculations have been carried out at the density functional theory level which utilizes the hybrid functional B3LYP and the split-valence 6-31G* basis set. The key features of the potential energy surfaces and the relevant thermochemical parameters have been calculated and they provide insights on the reaction mechanisms. The bimolecular reactivity of C10H7+ with H2 is dominated by the production of naphthalene cation C10H8+. The reaction is not a direct atom-abstraction process, but instead it proceeds via the formation of a stable intermediate complex C10H9+ of σ type geometry, with a significant mobility of hydrogen along the ring. This mobility allows the scrambling of the hydrogen atoms and causes the successive statistical fragmentation of the complex into a variety of product channels. Elimination of one H(D) atom appears to be favored over elimination of one H2 or HD molecule. Alternatively, the intermediate complex can be stabilized either by collision with a third body or by emission of a photon. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of the product channels forming OCF++H++HF and HCF2++H++O following the collisions of CF22+ with H2O have been investigated with a new position-sensitive coincidence experiment at a center-of-mass collision energy of 5.6 eV. The results show the formation of OCF+ occurs via the formation of a doubly charged collision complex [H2O-CF2]2+ which subsequently undergoes a charge separating dissociation to form H+ and HOCF2+. The HOCF2+ monocation subsequently fragments to form HF+OCF+. The lifetimes of the collision complex and the HOCF2+ ion are at least of the order of their rotational period. The kinetic energy release in this reaction indicates that it involves the ground state of CF22+ and forms the ground electronic states of OCF+ and HF. The mechanism for forming HCF2+ involves the direct and rapid abstraction of a hydride ion from H2O by CF22+. The resulting OH+ ion subsequently fragments to H++O, on a time scale at least comparable with its rotational period. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate ab initio calculations have been performed in order to investigate both the stable isomers and the reactivity of the N2H2+ cation. In addition to the trans-HNNH+ isomer already observed in the photoelectron studies, a formaldehyde type (isodiazene cation) and H2O2-like isomers are found. At the coupled cluster level of theory, the isodiazene cation is calculated to be as stable as trans-HNNH+. We have also studied the reactivity of N2H2+ and its implication on the reactive processes involving N2/N2+ and H2+/H2, H/H+ and HN2+/HN2, and HN and HN+ by performing suitable one-dimensional cuts of the six-dimensional potential energy functions of the lowest electronic states of H2N2+. We have pointed out the crucial role of this tetratomic intermediate cation and the importance of the short range internuclear distances during these processes. In the case of N2/N2+ and H2+/H2 reactions, we have shown that the initial orientation of the reactants may influence the N2H2+ tetratomic intermediate: One can expect to form the trans isomer preferentially if the internuclear axes of the H2/H2+ and the N2+/N2 molecules are parallel to each other when these diatoms are colliding and after intramolecular isomerization process. However, if the internuclear axes of the diatomics are perpendicular to each other, the isodiazene cation is formed preferentially. Different branching ratios are expected for each collision scheme. These reactive processes are found to involve vibronic, Renner-Teller and spin-orbit couplings between the electronic states of N2H2+. These interactions mix these electronic states, leading to the formation of atomic, diatomic, and triatomic species via the decomposition of the N2H2+ intermediate complex. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The present study describes the characterization of energy and structure of HBr2+ in its low-lying electronic states, achieved through an extension of a new empirical method [Chem. Phys. Lett. 379, 139 (2003)] recently introduced to evaluate the interatomic interaction in the HX2+ (X=F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck–Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reaction of phenylium and d-phenylium ions with molecular deuterium has been investigated both experimentally and theoretically. Theoretical calculations have been carried out at the B3LYP/6-31G[sup *] level of theory. The key features of the potential energy surfaces and all the relevant thermochemical parameters have been calculated. These results are in good agreement with previous theoretical studies concerning the unimolecular dissociation of benzenium ions. Theoretical calculations provide a clear insight on the reaction mechanisms for the system under study. Experimental studies have been carried out by means of a guided beam tandem mass spectrometer. We have measured the reaction cross section for four different reaction channels, as a function of the collision energy, in the range from thermal energies up to about 1 eV. The bimolecular reactivity of C[sub 6]H[sub 5][sup +] is dominated by the substitution of one or two hydrogen atoms by deuterium, leading to the formation of C[sub 6]H[sub 4]D[sup +] and C[sub 6]H[sub 3]D[sub 2][sup +] products. The reaction cross section is nearly the same for the two reaction channels. This fact is interpreted as evidence of a statistical behavior, as expected by assuming a fast shift of hydrogen (deuterium) within the aromatic ring. Another very weak reaction channel leading to the formation of the benzene ion has been measured. The cross section for such a process is lower than about 10[sup -2] Å[sup 2] over the entire energy range explored in our experiment. A structure observed at collision energies around about 0.1 eV has been attributed to the reaction of phenylium ions in their triplet state. When the density of the gas in the scattering cell is increased, the formation of collisionally stabilized benzenium ions is observed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In the last few years, astronomical spectra have revealed the presence of aromatic and polyaromatic molecules in extraterrestrial environments, near carbon stars, in molecular clouds and meteorites. Moreover, the recent observation of benzene in interstellar space has noticeably increased the interest in the entire class of molecules and in their chemical behavior. In this work, we have investigated the reaction between the benzene molecule and the atomic nitrogen cation and, in particular, the mechanisms by which the reactants are converted into cationic products containing at least one C-N bond, according to the general scheme C[sub 6]H[sub 6] + N[sup +] → H[sub m]C[sub n]N[sup +] + C[sub x]H[sub y]. We have measured the energy dependence of the cross section in a guided ion beam tandem mass spectrometer. Relevant stationary points of the potential energy surface have been studied by using the density functional theory hybrid functional B3LYP with the 6-31G[sup *] basis set. Thermochemical calculations, and the comparison with experimental results, allow us to distinguish between exoergic and endoergic processes and to obtain a detailed description of the reaction mechanisms. We show that aromatic hydrocarbons may be converted into organic-nitrogen compounds via the insertion of N[sup +] into the benzene ring and the formation of C-N bonds from C-C ring reactants. [ABSTRACT FROM AUTHOR]

Ground-state stationary points on the potential energy surface of the reaction CF[SUP2+,SUB+] + H[SUB2]O → OCF[SUP+] + HF + H[SUP+] were calculated using the density-functional theory hybrid method B3LYP and the ab initio coupled cluster singles and doubles with perturbative triples [CCSD(T)] algorithm. The calculations reveal a reaction mechanism involving two transition states. The first transition state involves the migration of one hydrogen within the primary collision complex and the second corresponds to the loss of a proton. The neutral HF molecular product is formed in its stable ground [SUP1]Σ state. Comparison of activation energies for the reactions of CF[SUP2+,SUB2] with H[SUB2]O and with D[SUB2]O, calculated from Becke three parameter Lee-Yang-Parr (B3LYP) zero-point energies, slightly favor the H[SUB2]O pathway by 0.04 and 0.07 eV for the first and second activations, respectively. Rate constant calculations using Rice-Ramsperger-Kassel-Marcus/quasiequilibrium theory also kinetically favor the H[SUB2]O pathway in comparison with the D[SUB2]O pathway. However, the magnitudes of the calculated rate constants are so large (10[SUP12] -10[SUP14] s[SUP-1]) that the differences between the rates of reaction of CF[SUP2+,SUB+] with H[SUB2]O and with D[SUB2]O should not be distinguished by a crossed-beam time-of-flight mass spectrometer experiment. Indeed, the ion yields reported in this paper from new collision experiments between CF[SUP2+,SUB+] and D[SUB2]O showed no isotope effect when compared with previous data from collisions of CF[SUP2+,SUB+] with H[SUB2]O. [ABSTRACT FROM AUTHOR]

The large number of elastic and inelastic collisions which take place during supersonic gaseous expansions produce not only acceleration and internal cooling of molecules, but also their alignment or orientation. The collisional alignment of the rotational angular momentum, corresponding to the orientation of the benzene molecular plane, in supersonic seeded expansions with lighter carrier gases is demonstrated via two complementary experiments: one interrogating benzene via polarized laser light IR absorption the other one probing its orientation via molecular beam scattering on rare gas targets. Typical seeding gases are helium, neon and their mixtures, and molecular hydrogen. Total stagnation pressures are of the order of ∼1 bar and ∼0.1 mm nozzle. A propensity is demonstrated for benzene molecules in seeded molecular beams to fly with the molecular plane preferentially oriented parallel to the molecular beam propagation direction. The analysis of the experimental results has been carried out using a phenomenological model which provides the fraction of molecules traveling in such a "frisbee" mode. A frisbee propensity function is defined and found to range between 0.71 and 0.85, corresponding to IR and to scattering experiments, respectively. These values are significantly higher than the value 2/3, expected for a random distribution of molecular plane orientations. The trend in the measured values is discussed in terms of different angular cones of the supersonic expansion sampled in the two experiments and evidence is provided that the orientation is higher the narrower is the sampled cone aperture. [ABSTRACT FROM AUTHOR]

We present cross sections as a function of the collision energy for the bond-forming reactions of argon dications with oxygen molecules, producing ArO[SUP2+] + O and ArO[SUP+] + O[SUP+], respectively. Both the reactions are exothermic, and have cross sections much smaller than those of the competitive charge-transfer processes. The low-lying electronic states of ArO[SUP2+] have been calculated at the MR-AQCC/cc-pV5Z level. In contrast to previous results we found that the ground [SUP3]Σ[SUP-] state has a local minimum. However, the estimated lifetime of this metastable state is too short to be detected in our setup. Thus the observed ArO[SUP2+] ions must be in the higher-lying [SUP1]Δ or [SUP1]Σ[SUP+] states. [ABSTRACT FROM AUTHOR]

Intramolecular isotope effects in the bond forming reactions following collisions of both CO&sup2+;[sub 2] and CF&sup2+;[sub 3] with HD have been investigated experimentally. For the CO&sup2+;[sub 2] + HD system the bond-forming pathway forming XCO&sup+; (X = H, D) exhibits a strong intramolecular isotope effect favoring the formation of DCO&sup+; at low collision energies. For the CF&sup2+;[sub 3] + HD system the bond-forming pathway forming XCF&sup+;[sub 2] also exhibits a strong intramolecular isotope effect favoring the formation of DCF&sup+;[sub 2] at low collision energies. However, in the CF&sup2+;[sub 3] + HD system a weak, and previously unobserved, channel, forming XF&sup+; exhibits no intramolecular isotope effect over the collision energy regime (0.2–0.5 eV) investigated. The absence of an intramolecular isotope effect in the formation of XF&sup+; casts doubt on the previous explanation of such isotope effects as resulting from orientation effects in the approach of the dication to the HD molecule. Using a recently proposed mechanism for the reaction of CO&sup2+;[sub 2] with H[sub 2], an analysis of the statistical and zero-point factors affecting the competition between the bond-forming channels is presented. This analysis shows that such factors can readily explain the intramolecular isotope effects observed in these reactive systems. [ABSTRACT FROM AUTHOR]

Reports on the guided ion beam investigation of the reaction of carbon ions with neutral carbon. Kinetic energy dependence of the integral cross section; Measurements of the product isotopic ratio; Re-evaluation of the dissociation energy of carbon.

Investigates the spectroscopy and photodissociation dynamics of the NCN radical using fast beam photofragment translational spectroscopy. Major dissociation products; Resolved progression shown by photofragment translational energy distributions; Rotational distributions of molecular fragments.

Examines the vibrational spectroscopy and intramolecular energy transfer in isocyanic acid (HNCO). Room temperature spectra; Strong intramolecular couplings that control the early stages of intramolecular vibrational energy redistribution; Consistency between the couplings and zero-order state separations with ab initio calculations of East, Johnson and Allen.

Reports on the results of a detailed quantum dynamical investigation of energy flow in benzene. Examination of benzene overtone by propagating an initial bright state in an active space of dimension; Factors influencing the rate and extent of energy flow in benzene chromophore.

Reports on a quantum mechanical study of the role of out-of-plane vibrations in benzene in the intramolecular vibrational energy redistribution (IVR). Symmetrized description of benzene vibrations and Hamiltonian; Out-of-plane vibrations in benzene; Symmetrized artificial intelligence search algorithm.

Examines the results of quantum mechanical calculations of the benzene CH(v=2) overtone spectrum. Use of the recursive residue generation method for intensities and eigenvalue computations; Structure of the Hamiltonian matrix; Details on lineshape function and survival probabilities computation.

Examines planar benzene overtone relaxation through quassiclassical trajectories computation. Description of the coordinate system, Hamiltonian and force field models; Conditions of trajectory simulation; Results of energy flow and survival probabilities calculation.

Examines the level mixing at the second CH stretch overtone state in benzene. Analysis of dynamic intramolecular vibrational redistribution characteristics; Involvement of vibrational level density in the vibrational mixing; Degree of localization of the CH stretch overtone vibrational system.

Presents a quantum mechanical description of vibrational motion in benzene in terms of vibrational coordinates and basis wavefunctions. Analysis of vibrational level mixing and intramolecular vibrational energy redistribution; Application of symmetrized local mode formalism; Treatment of remaining benzene vibrations as normal modes.

Examines the dressed states of molecules and microwave-infrared double-resonance spectroscopic techniques employing an electric quadrupole focusing field. Single-photon measurements; Spectroscopy and state-focusing properties of dressed states; Microwave power requirements for observing double-resonance signals.

Reports on the study of the first overtone fluorobenzene (C[sub 6]H[sub 5]F). Production of the supersonic beam; Opto-acoustic spectrum; Opto-thermal spectrum; Theoretical approach used to explain the experimental results of the transition in fluorobenzene; Construction of algebraic operators to describe anharmonic interactions.

Focuses on the complete vibrational spectroscopy of pyrrole within the one-dimensional algebraic framework. Study of the carbon-hydrogen/nitrogen-hydrogen stretching levels including their possible anharmonic resonance mechanisms with other ring modes; Possible action of vibrational energy processes mechanisms on the molecules.

Discusses the eigenstate resolved infrared-infrared double-resonance study of intramolecular vibrational relaxation in benzene. Dynamics of benzene; Density of coupled states for the central portions of R and Q-branch; Direct correspondence between large gaps between the nodes of a tree and presence of a distinct time scales in the spectrum.

Fluorescence-dip infrared spectroscopy (FDIRS) is employed to record the infrared spectra of the isolated, jet-cooled tropolone molecule (TrOH) and its singly deuterated isotopomer TrOD in the O-H and C-H stretch regions. The ability of the method to monitor a single ground-state level enables the acquisition of spectra out of the lower and upper levels of the zero-point tunneling doublet free from interference from one another. The high power of the optical parametric oscillator used for infrared generation produces FDIR spectra with good signal-to-noise despite the weak intensity of the C-H and O-H stretch transitions in tropolone. The expectation that both spectra will exhibit two OH stretch transitions separated by the OH(ν = 1) tunneling splitting is only partially verified in the present study. The spectra of TrOH are compared with those from deuterated tropolone (TrOD) to assign transitions due to C-H and O-H, which are in close proximity in TrOH. The appearance of the spectra out of lower (a[SUB1] symmetry) and upper (b[SUB2] symmetry) tunneling levels are surprisingly similar. Two sharp transitions at 3134.9 cm[SUP-1] (out of the a[SUB1] tunneling level) and 3133.9 cm[SUP-1] (out of the b[SUB2] tunneling level) are separated by the ground-state tunneling splitting (0.99 cm[SUP-1]), and thereby terminate in the same upper state tunneling level. Their similar intensities relative to the C-H stretch transitions indicate that the y- and z-polarized transitions are of comparable intensity, as predicted by ab initio calculations. The corresponding transitions to the other member of the upper state tunneling doublet are not clearly assigned by the present study, but the broad absorptions centered about 12 cm[SUP-1] below the assigned transitions are suggested as the most likely possibility for the missing transitions. [ABSTRACT FROM AUTHOR]

We investigate the influence of an internal barrier on an exothermic adiabatic reaction model between diatomic ions and molecules. Reaction cross-sections are calculated from quasi-classical trajectories for different initial vibrational and rotational states of the reactants and for relative collision energies in the range from 0.01 to 3 eV. It is shown that the height of a late internal barrier strongly influences both the characteristics of the state-selected cross-sections and the energy distributions of the products. In contrast to complex formation in the entrance region according to the Langevin model our analysis emphasizes the role of the full potential energy surface for an understanding of the dynamics of ion–molecule reactions. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Overtone spectroscopy in the gas phase by vibrationally assisted dissociation and photofragment ionization (OSVADPI) has been coupled with mass spectrometric detection of fragments enabling us to separate congested spectra into components arising from several naturally occurring isotopomers. The new technique of isotope selective overtone spectroscopy (ISOS) has been applied to the 41 component of the CH chromophore absorption near 11 385 cm-1 in CHCl3 at room temperature and in supersonic jet expansions. These spectra allow us to assign a close local resonance with a CCl3 frame mode indicating vibrational energy redistribution within some ps. This is to be compared with the ultrafast (ca. 50 fs) redistribution between the CH stretching and bending modes established previously. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this series, the characteristics of intramolecular vibrational-energy redistribution (IVR) present in the CH overtones of CHF3 are investigated. Particular attention is given to the multiple time scales and thus mechanisms present in the IVR dynamics. In Part I, a 9-dimensional ab initio potential energy surface is developed to adequately account for the vibrational couplings of all modes. Furthermore, all-mode vibrational state calculations, of large primitive space dimension, are performed using a recently developed wave operator sorting algorithm in tandem with the recursive residue generation method. All fundamentals, first overtones, and bimodal combination states with up to 3 quanta are presented. Also, the A1 and E-symmetry CH polyads are determined through the second overtone. Equilibrium geometry, rotational constants, and vibrational properties agree quantitatively with experiment in most cases. The error is systematic in origin and largely due to the error in the ab initio harmonic frequencies. New vibrational constants and resonance interactions are reported for the background modes. In contrast to the prominent CH stretch–bend Fermi resonance structure, responsible for ultrafast (t<50 fs) energy transfer, the CH polyads also exhibit vibrational fine structure of order 1 to 10 cm-1 due to background-mode coupling. This secondary coupling results in IVR on the picosecond time scale. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The combination of infrared-optical double resonance excitation with infrared laser assisted photofragment spectroscopy (IRLAPS) detection permits the measurement of rotationally resolved vibrational overtone spectra of jet-cooled CF3H. The IRLAPS detection technique is sufficiently selective to be able to discriminate against the dissociation of molecules in the ground state and the vCH=1 level used as an intermediate in the double resonance scheme. The combination of this selective detection technique with both supersonic cooling and double resonance produces J state resolved overtone spectra of the 42 vibrational overtone band. These spectra put a limit on the homogeneous linewidths of the rovibrational transitions of ∼0.5 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This work is part of a series aimed to investigate the fundamental and overtone transitions of the CH stretching modes in partially F-substituted benzenes. In this article we report the study of the fundamental transitions of fluorobenzene (C6H5F) in the wave numbers range from about 3000 to 3150 cm-1. The absorption spectrum has been measured through both a low-temperature opto-acoustic cell and a supersonic-beam opto-thermal spectrometer. The opto-thermal spectrometer enabled us to measure the detailed vibrational structure, providing the basis for an algebraic analysis of the fundamental CH transitions. We show, in particular, how benzene algebraic procedures must be modified for taking into account the effects induced by the substitution of one hydrogen atom with fluorine, i.e., (i) the reduction of symmetry and (ii) the frequency (chemical) shift due to the change of electron distribution. Moreover we include the contribution of anharmonic resonances between unperturbed CH stretching modes and overtone or combination bands of lower energy molecular vibrations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]