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A full-dimensional analytical potential energy surface (PES) is developed for the Cl + CH3CN reaction following our previous work on the benchmark ab initio characterization of the stationary points. The spin–orbit-corrected PES is constructed using the Robosurfer program and a fifth-order permutationally invariant polynomial method for fitting the high-accuracy energy points determined by a ManyHF-based coupled-cluster/triple-zeta-quality composite method. Quasi-classical trajectory simulations are performed at six collision energies between 10 and 60 kcal mol−1. Multiple low-probability product channels are found, including isomerization to isonitrile (CH3NC), but out of the eight possible channels, only the H-abstraction has significant reaction probability; thus, detailed dynamics studies are carried out only for this reaction. The cross sections and opacity functions show that the probability of the H-abstraction reaction increases with increasing collision energy (Ecoll). Scattering angle, initial attack angle, and product relative translational energy distributions indicate that the mechanism changes with the collision energy from indirect/rebound to direct stripping. The distribution of initial attack angles shows a clear preference for methyl group attack but with different angles at different Ecoll values. Post-reaction energy distributions show that the energy transfer is biased toward the products' relative translational energy instead of their internal energy. Rotational and vibrational energy have about the same amount of contribution to the internal energy in the case of both products (HCl and CH2CN), i.e., both of them are formed with high rotational excitations. HCl is produced mostly in the ground vibrational state, while a notable fraction of CH2CN is formed with vibrational excitation. [ABSTRACT FROM AUTHOR]

In this study, peptides designed using fragments of an antifreeze protein (AFP) from the freeze-tolerant insect Tenebrio molitor, TmAFP, were evaluated as inhibitors of clathrate hydrate formation. It was found that these peptides exhibit inhibitory effects by both direct and indirect mechanisms. The direct mechanism involves the displacement of methane molecules by hydrophobic methyl groups from threonine residues, preventing their diffusion to the hydrate surface. The indirect mechanism is characterized by the formation of cylindrical gas bubbles, the morphology of which reduces the pressure difference at the bubble interface, thereby slowing methane transport. The transfer of methane to the hydrate interface is primarily dominated by gas bubbles in the presence of antifreeze peptides. Spherical bubbles facilitate methane migration and potentially accelerate hydrate formation; conversely, the promotion of a cylindrical bubble morphology by two of the designed systems was found to mitigate this effect, leading to slower methane transport and reduced hydrate growth. These findings provide valuable guidance for the design of effective peptide-based inhibitors of natural-gas hydrate formation with potential applications in the energy and environmental sectors. [ABSTRACT FROM AUTHOR]

The lifetime of a hydrogen bond between water and dimethyl sulfoxide (DMSO) is found to be considerably longer than that between two water molecules in neat water. This is counter-intuitive because the charge on the oxygen in DMSO is considerably less than that in water. Additionally, the strength of the water–dimethyl sulfoxide (w–D) hydrogen bond is found to be strongly composition dependent; the lifetime of the hydrogen bond is ten times larger at 30% than at very low concentrations. Using computer simulations, we perform microscopic structural and dynamic analysis to find that these anomalies arise at least partly from an "action-at-a-distance" effect where the attraction between the hydrophobic methyl groups results in the self-aggregation of DMSO molecules that "cages" both the rotational and linear motions of the molecules involved. This is reflected in the observed strong correlation of the lifetime with the local coordination number of the associated methyl groups. The elongated w–D h-bond lifetime causes a slowdown of collective dynamics and affects the lifetime of the w–w h-bond. This nonlinear feedback mechanism explains the strong composition dependence of viscosity and is anticipated to play a dominant role in many self-assemblies. Furthermore, the w–D hydrogen bond breaking mechanism changes from low to high DMSO concentration, a phenomenon not anticipated a priori. We introduce a new order parameter-based free energy surface of the bond breaking pathway. A two-dimensional transition state rate theory calculation is performed for the lifetime of the w–D h-bond that is found to be semi-quantitatively accurate. [ABSTRACT FROM AUTHOR]

We investigated the rotational spectrum of 2,5-dimethylfluorobenzene containing coupled large amplitude motions of two methyl groups in the frequency range from 2 to 26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The internal rotation of two inequivalent methyl groups with low torsional barriers (around 16 and 226 cm−1) causes splittings of all rotational transitions into quintets with separations of up to hundreds of MHz between the torsional components. Spectral analysis and modeling of the observed splittings were performed using the programs XIAM and BELGI-Cs-2Tops, whereby the latter achieved measurement accuracy. The methyl internal rotation can be used to examine the electronic and steric environments around the methyl group because they affect the methyl torsional barrier. Electronic properties play a particularly important role in aromatic molecules in the presence of a π-conjugated double bond system. The experimental results were compared with those of quantum chemistry. Benchmark calculations resulted in the conclusion that the B3LYP-D3BJ/6-311++G(d,p) level of theory can be recommended for predicting rotational constants to guide the microwave spectral assignment of dimethylfluorobenzenes in particular and toluene derivatives in general. [ABSTRACT FROM AUTHOR]

In this study, the total ion yield near-edge x-ray absorption fine structure spectra of four similar peptoid molecules, which differ in the numbers and positions of methyl groups, were investigated experimentally and theoretically. At each excitation energy, the intensity and branching ratio of each ionic product were measured. At a few resonant excitation energies, a specific dissociation of the C–CO bond at the nitrogen and oxygen K-edges and of the N–CO bond at the carbon K-edge was dominant, which correlated well with the predicted destination antibonding orbitals of the core electron excitation. These specific dissociation mechanisms of small peptoid molecules could provide insights into similar phenomena that occur in peptide molecules. [ABSTRACT FROM AUTHOR]

Samples of propyne trapped in solid parahydrogen show multiple peak structures in their infrared spectra. These structures are attributed to molecules in two distinct kinds of matrix sites. The most intense lines are assigned to propyne molecules executing a slightly hindered methyl rotation, as was extensively studied in our earlier publication from our two groups, and the other set of peaks to propyne trapped in a secondary site where the methyl rotation is quenched and replaced by methyl torsion within the matrix site. The assignment of the various rovibrational transitions is made possible by the observation of nuclear spin conversion (NSC) within the methyl group at long timescales. The NSC rate depends on the site and is much slower in the sites where the methyl rotation is quenched. [ABSTRACT FROM AUTHOR]

Terahertz molecular motions are often probed by high-frequency molecular oscillators in different types of non-linear vibrational spectroscopy. Recently developed two-dimensional terahertz-infrared-visible spectroscopy allows direct measuring of this coupling and, thus, obtaining site-specific terahertz vibrational spectrum. However, these data are affected by the intensity and phase of the employed laser pulses. In this work, we develop a method of extracting sample response—representing solely physical properties of a material—from experimental spectra. Using dimethyl sulfoxide as a model molecule to verify this method, we measure the coupling between C–H stretch vibration of its methyl groups and terahertz intramolecular twist and wagging modes. [ABSTRACT FROM AUTHOR]

Thermal atomic layer etching (ALE) is one promising method to achieve atomic level precision and high conformality over three-dimensional structures that can further enable the manufacturing of gate-all-around devices. Initially, an ALE process using CF4/NH3/Ar remote plasma exposure followed by infrared (IR) annealing was studied on SiCO films. The process showed self-limiting behavior and achieved an etch per cycle (EPC) of 0.2 nm/cycle. To increase the EPC, an O2 remote plasma exposure step was added before the CF4/NH3/Ar plasma exposure step in the ALE cycle. The process achieved an EPC of 1.0 nm/cycle. Measurements of the EPC of the SiCO film showed self-limiting behavior in both the O2 and CF4/NH3/Ar steps. X-ray photoelectron spectroscopy results showed an increase in atomic concentration (AC) of oxygen while the AC of carbon decreased following the exposure of the film to an O2 remote plasma. The results indicate that methyl groups (-CH3) in the top layers of the film are being replaced by hydroxyl (-OH) groups and Si-O-Si bonding. The N1s spectrum showed the formation of an ammonium fluorosilicate (NH4)2SiF6-based surface-modified layer following exposure to a CF4/NH3/Ar remote plasma. IR annealing of the film showed desorption of the ammonium fluorosilicate surface-modified layer and the return to an as grown SiCO film surface composition. [ABSTRACT FROM AUTHOR]

Scalable methods for improving the performance and stability of a field-effect transistor (FET) based on two-dimensional materials are crucial for its real applications. A scalable method of encapsulating the exfoliated MoS2 on the SiO2/Si substrate by hexamethyldisilazane (HMDS) is explored here for reducing the influence of interface traps and ambient contaminants. This leads to 25 times reduction in trap density, three times decrease in subthreshold swing, three times increase in the peak field-effect mobility, and a drastic reduction in hysteresis. This performance remains nearly the same after several weeks of ambient exposure of the device. This is attributed to the superhydrophobic nature of HMDS and the SiO2 surface hydrophobization by the formation of covalent bonds between the methyl groups of HMDS and silanol groups of SiO2. [ABSTRACT FROM AUTHOR]

Nitroaromatic compounds can photorelease nitric oxide after UV absorption. The efficiency of the photoreaction depends on the molecular structure, and two features have been pointed out as particularly important for the yield of the process: the presence of methyl groups at the ortho position with respect to the nitro group and the degree of conjugation of the molecule. In this paper, we provide a theoretical characterization at the CASPT2//CASSCF (complete active space second-order perturbation theory//complete active space self-consistent field) level of theory of the photorelease of NO for four molecules derived from nitrobenzene through the addition of ortho methyl groups and/or the elongation of the conjugation. Our previously described mechanism obtained for the photorelease of NO in nitrobenzene has been adopted as a model for the process. According to this model, the process proceeds through a reactive singlet–triplet crossing (STC) region that the system can reach from the triplet 3(πOπ*) minimum. The energy barrier that must be surmounted in order to populate the reactive STC can be associated with the efficiency of the photoreaction. Here, the obtained results display clear differences in the efficiency of the photoreaction in the studied systems and can be correlated with experimental results. Thus, the model proves its ability to highlight the differences in the photoreaction efficiency for the nitroaromatic compounds studied here. [ABSTRACT FROM AUTHOR]

Methyl groups in organic solids generally behave as uniaxial quantum rotors. The existing nuclear magnetic resonance (NMR) theory appears to be complete, capable of describing even the finest details of the temperature-dependent spectra of such objects. However, the once reported temperature effects in the carbon spectra of the C 13 -labeled methyl group in a single crystal of acetylsalicylic acid have still not been explained. As the temperature decreases, in the quartet corresponding to the rapid motional averaging regime, the inner lines first begin to broaden, but then, they narrow again, so that at 6 K, a pattern similar to that at room temperature was observed. In the present work, these puzzling effects are explained quantitatively by invoking the molecular Zeeman (MZ) interaction. Like the spin–rotation (SR) interaction long known to occur in methyl groups, it engages the magnetic moments generated by their torsional motions. However, it has not been considered in NMR spectroscopy until now. This is a surprising situation because in the magnetic fields currently used in NMR spectroscopy, the MZ interaction is orders of magnitude stronger than the (magnetic field independent) SR effects. [ABSTRACT FROM AUTHOR]

Beryllium ammonia complexes Be(NH3)4 are known to bear two diffuse electrons in the periphery of a Be(NH3)42+ skeleton. The replacement of one ammonia with a methyl group forms CH3Be(NH3)3 with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH3)4. Two CH3Be(NH3)3 monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH3)3Be(CH2)nBe(NH3)3, n = 1–6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H2 in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H2. The biradical character of the system is analyzed, and the singlet–triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH3)4 complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic. [ABSTRACT FROM AUTHOR]

The effect of trimethylamine-N-oxide (TMAO) on hydrophobic solvation and hydrophobic interactions of methane has been studied with Molecular Dynamics simulations in the temperature range between 280 and 370 K at 1 bar ambient pressure. We observe a temperature transition in the effect of TMAO on the aqueous solubility of methane. At low temperature (280 K), methane is preferentially hydrated, causing TMAO to reduce its solubility in water, while above 320 K, methane preferentially interacts with TMAO, causing TMAO to promote its solubility in water. Based on a statistical-mechanical analysis of the excess chemical potential of methane, we find that the reversible work of creating a repulsive methane cavity opposes the solubility of methane in TMAO/water solution more than in pure water. Below 320 K, this solvent-excluded volume effect overcompensates the contribution of methane–TMAO van der Waals interactions, which promote the solvation of methane and are observed at all temperatures. These van der Waals interactions with the methyl groups of TMAO tip the balance above 320 K where the effect of TMAO on solvent-excluded volume is smaller. We furthermore find that the effective attraction between dissolved methane solutes increases with the increasing TMAO concentration. This observation correlates with a reduction in the methane solubility below 320 K but with an increase in methane solubility at higher temperatures. [ABSTRACT FROM AUTHOR]

The amphiphilicity and denaturation efficiency of urea can be tuned via alkylation. Although the interaction of alkylureas with water and proteins has been studied in detail, hydration of 1-methylurea has remained elusive, precluding the isolation of the effect of an individual methyl group. Here, we study water dynamics in the hydration shell of 1-methylurea (1-MU) using infrared absorption and ultrafast infrared spectroscopies. We find that 1-MU hardly affects the hydrogen-bond distribution of water as probed by the OD stretching vibration of HOD molecules. Polarization resolved infrared pump–probe experiments reveal that 1-MU slows down the rotational dynamics of up to 3 water molecules in its hydration shell. A comparison to earlier results for other alkylureas suggests that further alkylation does not necessarily slow down the rotational dynamics of additional water molecules. Two-dimensional infrared experiments show that 1-MU markedly slows down the hydrogen-bond fluctuation dynamics of water, yet similar to what has been found for urea and dimethylureas. Remarkably, (alkyl-)ureas that share a similar effect on water's hydrogen-bond fluctuation dynamics have a similar (modest) protein denaturation tendency. As such, not only the hydrophobicity but also hydration of hydrophilic fragments of alkylureas may be relevant to explain their function toward biomolecules. [ABSTRACT FROM AUTHOR]

A pure rotational spectrum of methyl p-tolyl sulfoxide (MTSO) was studied using chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 18–26 GHz. A single conformer was unambiguously observed in the supersonic jet expansion, which is consistent with the conformational analysis performed using quantum-chemical calculations. Rotational transitions were split into two components of A and E symmetries due to the low-barrier internal rotation of the ring methyl group [V3 = 11.0178(23) cm−1]. The low energy barrier for the methyl top internal rotation implies an electron-withdrawing effect of the group at the opposite side of the phenyl ring, in comparison with other para-substituted toluenes. The effective ground state (r0) geometry was derived using the rotational constants from the parent species and the 34S and eight 13C singly substituted isotopologues. Compared to two other sulfoxides, methyl phenyl sulfoxide and methyl 4-nitrophenyl sulfoxide, the sulfoxide group in MTSO is slightly more twisted with respect to the plane of the phenyl ring, which could be attributed to the moderate electron-donating effect of the p-methyl group. Furthermore, the pyramidal inversion that interconverts the handedness at the sulfur stereogenic center was explored in the electronic ground (S0) and excited (S1) states with nudged elastic band and time-dependent density functional theory methods. It was found that the pyramidal inversion in S1 is easier than in S0, showing that optical excitation to S1 will facilitate an effectively barrier-free inversion. [ABSTRACT FROM AUTHOR]

In the rotational spectroscopy community, several popular codes are available to treat multiple internal rotors in a molecule. In terms of the pros and cons of each code, it is often a difficult task to decide which program to apply to a specific internal rotation problem. We faced this issue when dealing with the spectroscopic fingerprint of 4-methylacetophenone (4MAP), recently investigated in the microwave region, which we here extended into the millimeterwave region. The methyl group attached to the phenyl ring in 4MAP undergoes internal rotation with a very low barrier of only 22 cm−1. The acetyl methyl group features a much higher barrier of about 580 cm−1. The performances of a program using the so-called "local" approach in terms of Herschbach's perturbative treatment, SPFIT, as well as three programs XIAM, ERHAM, and ntop, representing "global" fits, were tested. The results aim at helping spectroscopists in the decision on how to tackle their own internal rotation problems. [ABSTRACT FROM AUTHOR]

Microsecond coherence times are predicted in spin dynamics calculations of a donor–acceptor electron spin pair P+A1A− created upon light excitation of photosystem I (PSI). The effects of nuclear spin diffusion (NSD) due to the anisotropic protein environment on the predicted coherence times TM are studied. Closely positioned pairs and triples of protons located 5–8 Å from the electron spin are shown to largely control TM. Knowledge of the crystal structure of PSI allows for spin dynamics calculations where specific cofactors and amino acid residues are removed or replaced and the identification of anisotropic environmental features controlling electronic decoherence. Finally, we show that NSD alone cannot explain the >3× shorter experimentally observed coherence times and suggest that methyl groups at key protein sites may explain this discrepancy. [ABSTRACT FROM AUTHOR]

Nuclear long-lived spin states represent spin density operator configurations that are exceptionally well protected against spin relaxation phenomena. Their long-lived character is exploited in a variety of Nuclear Magnetic Resonance (NMR) techniques. Despite the growing importance of long-lived spin states in modern NMR, strategies for their identification have changed little over the last decade. The standard approach heavily relies on a chain of group theoretical arguments. In this paper, we present a more streamlined method for the calculation of such configurations. Instead of focusing on the symmetry properties of the relaxation superoperator, we focus on its corresponding relaxation algebra. This enables us to analyze long-lived spin states with Lie algebraic methods rather than group theoretical arguments. We show that the centralizer of the relaxation algebra forms a basis for the set of long-lived spin states. The characterization of the centralizer, on the other hand, does not rely on any special symmetry arguments, and its calculation is straightforward. We outline a basic algorithm and illustrate advantages by considering long-lived spin states for some spin-1/2 pairs and rapidly rotating methyl groups. [ABSTRACT FROM AUTHOR]

Carbon–carbon coupling is an important step in many catalytic reactions, and performing sp3–sp3 carbon–carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C–I cleavage and selective sp3–sp3 carbon–carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp3–sp3 carbon–carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants. [ABSTRACT FROM AUTHOR]

The microwave spectrum of 2,5-dimethylpyrrole was recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. Only one stable conformer was observed as expected and confirmed by quantum chemical calculations carried out to complement the experimental analysis. The two equivalent methyl groups cause each rotational transition to split into four torsional species, which is combined with the quadrupole hyperfine splittings in the same order of magnitude arising from the 14N nucleus. This results in a complicated spectrum feature. The spectral assignment was done separately for each torsional species. Two global fits were carried out using the XIAM code and the BELGI-C2v-2Tops-hyperfine code, a modified version of the BELGI-C2v-2Tops code, giving satisfactory root-mean-square deviations. The potential barriers to internal rotation of the two methyl groups were determined to be V3 = 317.208(16) cm−1. The molecular parameters were obtained with high accuracy, providing all necessary ground state information for further investigations in higher frequency ranges and on excited torsional-vibrational states. [ABSTRACT FROM AUTHOR]

We present a new technique for detecting chirality in the gas phase: Chiral molecules are spatially aligned in three dimensions by a moderately strong elliptically polarized laser field. The momentum distributions of the charged fragments, produced by laser-induced Coulomb explosion, show distinct three-dimensional orientation of the enantiomers when the laser polarization ellipse is rotated by a non-right angle with respect to the norm vector of the detector plane. The resulting velocity-map-image asymmetry is directly connected to the enantiomeric excess and to the absolute handedness of molecules. We demonstrated our scheme computationally for camphor (C10H16O), with its methyl groups as marker fragments, using quantum-mechanical simulations geared toward experimentally feasible conditions. Computed sensitivity to enantiomeric excess is comparable to other modern chiroptical approaches. The present method can be readily optimized for any chiral molecule with an anisotropic polarizability tensor by adjusting the polarization state and intensity profile of the laser field. [ABSTRACT FROM AUTHOR]

The vacuum-ultraviolet threshold photoelectron spectrum of methyl isocyanate CH3NCO has been recorded from 10.4 eV to 12 eV using synchrotron radiation and a coincidence technique allowing for a mass-discrimination of the photoelectron signal. A significant improvement is achieved over previous investigations as this experimental setup leads to a much more resolved spectrum. Ten sharp peaks and a broad feature spanning 1.2 eV were recorded. This spectrum consists of X̃+ 2A″←X̃¹A′ and Ã+ 2A′←X̃ ¹A′ ionizing transitions. For the former, the adiabatic ionization energy was determined experimentally to be 10.596(6) eV; for the latter, its value was estimated to be 10.759(50) eV. Seven sharp peaks could be assigned to vibrational modes of the cation X̃+ 2A″ and neutral X̃¹A′ ground electronic states involving only the NCO group atoms. Theoretical modeling of the threshold photoelectron spectrum has proven difficult as methyl isocyanate is a non-rigid molecule displaying large amplitude internal rotation of the methyl group and ∠CNC bending mode, leading to the quasi-symmetry. With the help of ab initio calculations, a theoretical model in which these two large amplitude motions are included in addition to the five small amplitude vibrational modes involving NCO group atoms is proposed. Comparison with the experimental spectrum shows that the broad feature and the strongest peak line positions are well accounted for; their intensities are also fairly well reproduced after adjusting a few parameters. [ABSTRACT FROM AUTHOR]

The rotational spectrum of 2-methylthiazole was recorded using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range of 2–40 GHz. Due to the internal rotation of the methyl group, all rotational transitions were split into A and E symmetry species lines, which were analyzed using the programs XIAM and BELGI-Cs-hyperfine, yielding a methyl torsional barrier of 34.796 75(18) cm−1. This value was compared with that found in other monomethyl substituted aromatic five-membered rings. The 14N quadrupole coupling constants were accurately determined to be χaa = 0.5166(20) MHz, χbb − χcc = −5.2968(50) MHz, and χab = −2.297(10) MHz by fitting 531 hyperfine components. The experimental results were supplemented by quantum chemical calculations. [ABSTRACT FROM AUTHOR]

The acetonyl radical (•CH2COCH3) is relevant to atmospheric and combustion chemistry due to its prevalence in many important reaction mechanisms. One such reaction mechanism is the decomposition of Criegee intermediates in the atmosphere that can produce acetonyl radical and OH. In order to understand the fate of the acetonyl radical in these environments and to create more accurate kinetics models, we have examined the reaction system of the acetonyl radical with O2 using highly reliable theoretical methods. Structures were optimized using coupled cluster theory with singles, doubles, and perturbative triples [CCSD(T)] with an atomic natural orbital (ANO0) basis set. Energetics were computed to chemical accuracy using the focal point approach involving perturbative treatment of quadruple excitations [CCSDT(Q)] and basis sets as large as cc-pV5Z. The addition of O2 to the acetonyl radical produces the acetonylperoxy radical, and multireference computations on this reaction suggest it to be barrierless. No submerged pathways were found for the unimolecular isomerization of the acetonylperoxy radical. Besides dissociation to reactants, the lowest energy pathway available for the acetonylperoxy radical is a 1-5 H shift from the methyl group to the peroxy group through a transition state that is 3.3 kcal mol−1 higher in energy than acetonyl radical + O2. The ultimate products from this pathway are the enol tautomer of the acetonyl radical along with O2. Multiple pathways that lead to OH formation are considered; however, all of these pathways are predicted to be energetically inaccessible, except at high temperatures. [ABSTRACT FROM AUTHOR]

The photochemical dynamics of double-bond-containing hydrocarbons is exemplified by the smallest alkenes, ethylene and butadiene. Chemical substituents can alter both decay timescales and photoproducts through a combination of inertial effects due to substituent mass, steric effects due to substituent size, and electronic (or potential) effects due to perturbative changes to the electronic potential energy surface. Here, we demonstrate the interplay of different substituent effects on 1,3-butadiene and its methylated derivatives using a combination of ab initio simulation of nonadiabatic dynamics and time-resolved photoelectron spectroscopy. The purely inertial effects of methyl substitution are simulated through the use of mass 15 "heavy-hydrogen" atoms. As expected from both inertial and electronic influences, the excited-state dynamics is dominated by pyramidalization at the unsubstituted carbon sites. Although the electronic effects of methyl group substitution are weak, they alter both decay timescales and branching ratios by influencing the initial path taken by the excited wavepacket following photoexcitation. [ABSTRACT FROM AUTHOR]

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm−1 and 584–588 cm−1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations. [ABSTRACT FROM AUTHOR]

In order to study the effect of the chemical structure of the equatorial ligand on the spin state of the Fe(iii) ion in complexes with invariable photoisomerizable 4-styrylpyridine axial ligands and different tetradentate Schiff bases, several new Fe(iii) complexes have been first synthesized, characterized, and studied by UV-vis, NMR, and EPR spectroscopies. The general chemical formula of the complexes is [Fe(SB)Sp2]BPh4·MeOH, where Sp is trans-4-styrylpyridine and SB are dianions of Schiff bases: salen, bzacen, and acen [salen = N,N′-ethylenebis(salicylaldimine) 1, acen = N,N′-ethylenebis(acetylacetonylideneimine) 2, and H2bzacen = N,N′-ethylenebis(benzoylacetonylideneimine) 3]. The results of the EPR and NMR measurements of the complexes both in the solid state and in solutions showed that the more methyl groups and less aromatic rings in the equatorial ligand, the more abrupt spin-crossover was observed in the complex. The dependence of the magnetic properties of the complexes on the state of matter and the presence of a solvent (powder, liquid solutions, and vitrified solutions) are noted. [ABSTRACT FROM AUTHOR]

In this contribution, the relative hyperfine couplings are determined for the 1H nuclei of the flavin mononucleotide (FMN) radical in an aqueous environment. In addition, three structural analogs with different methylation patterns are characterized and the influence of the substituents at the isoalloxazine moiety on the electronic structure of the radicals is explored. By exploiting nuclear hyperpolarization generated via the photo-CIDNP (chemically induced dynamic nuclear polarization) effect, it is possible to study the short-lived radical species generated by in situ light excitation. Experimental data are extracted by least-squares fitting and supported by quantum chemical calculations and published values from electron paramagnetic resonance and electron-nuclear double resonance. Furthermore, mechanistic details of the photoreaction of the investigated flavin analogs with l-tryptophan are derived from the photo-CIDNP spectra recorded at different pH values. Thereby, the neutral and anionic radicals of FMN and three structural analogs are, for the first time, characterized in terms of their electronic structure in an aqueous environment. [ABSTRACT FROM AUTHOR]

We present a generalized nuclear spin bath model for embedded electron spin decoherence in organic solids at low temperatures, which takes the crucial influence from hindered methyl group rotation tunneling into account. This new, quantum many body model, after resolved using the cluster correlation expansion method, predicts the decoherence profiles directly from the proton relative position and methyl group tunneling splitting inputs. Decoherence profiles from this model explain adequately the influence from both strongly and weakly hindered methyl groups to embedded electron spin decoherence: The former accelerates decoherence by increasing the nearest neighbor nuclear spin coupling, while the latter enhances coherence through a novel confinement like' mechanism, in which the very strong nuclear spin coupling from the tunneling splitting term suppresses those protons on the methyl rotors from participating in the bath dynamics. Both types of influences are successfully proven experimentally in representative organic polycrystalline matrices: methyl malonic acid for strongly hindered and acetamide for weakly hindered methyl groups, respectively. [ABSTRACT FROM AUTHOR]

Vinoxy radicals are involved in numerous atmospheric and combustion mechanisms. High-level theoretical methods have recently shed new light on the reaction of the unsubstituted vinoxy radical with O2. The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k∞, 5–7 times higher than that of the vinoxy plus O2 reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O2 system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O2 addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O2 addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O2 addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O2. This is attributed to the favorable interaction between the incoming O2 molecule and the methyl group of the 2-methylvinoxy radical. Via the focal point approach, energetics for this reaction were determined, in most cases, to chemical accuracy. The coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] correlation energy and Hartree–Fock energies were independently extrapolated to the complete basis set limit. A correction for the effect of higher excitations was computed at the CCSDT(Q)/6-31G level. Corrections for the frozen-core approximation, the Born–Oppenheimer approximation, the nonrelativistic approximation, and the zero-point vibrational energy were included. From the 1-oxo-2-propylperoxy radical, dissociation to reactants is competitive with the lowest energy isomerization pathway. The lowest energy isomerization pathway ultimately forms acetaldehyde, CO, and ·OH as the final products. [ABSTRACT FROM AUTHOR]

Low barriers to internal rotations are especially challenging for both the experimental and theoretical determinations because they result in large tunneling splittings which are hard to assign and in potential functions that can be difficult to model. In the present work, the internal rotations of two methyl groups of 2,4-dimethylanisole were analyzed and modeled using a newly developed computer code, called ntop, adapted for fitting the high-resolution torsion-rotation spectra of molecules with two or more methyl rotors. The spectrum was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range of 2.0–26.5 GHz, revealing internal rotation tunneling quintets with splittings of up to several gigahertz. The V3 potential barriers are 441.139(23) cm−1 and 47.649(30) cm−1 for the o- and p-methyl groups, respectively. Quantum chemical calculations predicted only one conformer with the methoxy group in the anti position related to the neighboring o-methyl group. While the results from geometry optimizations were reliable, ab initio calculations at the MP2 level did not reproduce the low torsional barriers, calling for further experiments on related systems and additional theoretical models. [ABSTRACT FROM AUTHOR]

Zero-kinetic-energy (ZEKE) spectra are presented for m-fluorotoluene, employing different low-lying (<350 cm−1) intermediate torsional and vibration-torsional ("vibtor") levels of the S1 state. The adiabatic ionization energy (AIE) is found to be 71 997 ± 5 cm−1 (8.9265 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)]. For cation torsional levels, the most intense band corresponds to changes in the torsional quantum number, in line with the known change in the phase of the torsional potential upon ionization. This leads to the observation of an unprecedented number of torsions and vibtor levels, with the pronounced vibtor activity involving out-of-plane vibrations. Interactions between levels involving torsions are discussed, with evidence presented, for the first time it is believed, for modification of a torsional potential induced by a vibration. Also, we discuss the possibility of distortion of the methyl group leading to a change from G6 molecular symmetry to Cs point group symmetry. [ABSTRACT FROM AUTHOR]

The use of relaxation interference in the methyl Transverse Relaxation-Optimized SpectroscopY (TROSY) experiment has opened new avenues for the study of large proteins and protein assemblies in nuclear magnetic resonance. So far, the theoretical description of the methyl-TROSY experiment has been limited to the slow-tumbling approximation, which is correct for large proteins on high-field spectrometers. In a recent paper, favorable relaxation interference was observed in the methyl groups of a small protein at a magnetic field as low as 0.33 T, well outside the slow-tumbling regime. Here, we present a model to describe relaxation interference in methyl groups over a broad range of magnetic fields, not limited to the slow-tumbling regime. We predict that the type of multiple-quantum transition that shows favorable relaxation properties change with the magnetic field. Under the condition of fast methyl-group rotation, methyl-TROSY experiments can be recorded over the entire range of magnetic fields from a fraction of 1 T up to 100 T. [ABSTRACT FROM AUTHOR]

We report results of a two dimensional laser induced fluorescence study of torsional states, low frequency vibrations, and combinations of torsion with low frequency vibration in m-fluorotoluene up to 560 cm−1 in S0 and 350 cm−1 in S1. Evidence is presented for interactions between torsion and low frequency vibrations in both S0 and S1, demonstrating that the coupling of torsion and vibration observed previously in toluene and p-fluorotoluene extends to a molecule with a threefold torsional barrier. This barrier is low in S0 (20 cm−1) and modest in S1 (116 cm−1). The methyl torsion-vibration interaction is much larger for the mode involving out-of-plane wagging of the methyl group with respect to the planar frame compared with the analogous out-of-plane fluorine atom motion. Methyl group out-of-plane modes were found to be most important for torsion-vibration interactions in toluene and p-fluorotoluene, and the evidence is accumulating that this motion is fundamental in torsion-vibration interactions. Fits of the experimental band positions yield torsion-vibration coupling constants, torsional potential terms (V3 and V6), and rotational constants (F) for the methyl torsion in S0 and S1. The inclusion of torsion-vibration coupling primarily affects V6 and F: |V6| is reduced and F increased, as was seen previously for the G12 molecules, toluene and p-fluorotoluene. The torsional barrier height does not appear to influence the magnitude of the torsion-vibration interaction: the coupling constants for the out-of-plane CH3 wag mode are almost the same in S0 and S1 (15.5 cm−1 and 14.0 cm−1, respectively). [ABSTRACT FROM AUTHOR]

Rotational spectra of thioacetic acid (CH3COSH) have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for both the syn and anti conformers of the parent species, as well as the 34S and 13C carbonyl isotopologues. Transitions arising from the lowest A and E internal rotor states of the methyl group have been observed and analyzed. Experimental values of the three-fold internal rotation barrier, V3, for the syn and anti conformers of the parent isotopologue are 76.300(12) and 358.056(51) cm−1, respectively, indicating a large effect of the S−H orientation on the CH3 internal rotation potential. M06-2X/6-311+G(d,p) calculations are in good agreement with these results. The block localized energy decomposition method has been applied to understand the origins of this strong dependence of V3 on conformation. The results indicate that π conjugation from the SH to the carbonyl group and steric repulsion between the SH and the methyl group in the anti form are main contributors to the difference. [ABSTRACT FROM AUTHOR]

The vibrational spectra of simple C H 3 + —Rg (Rg = He, Ne, Ar, Kr) complexes have been studied by vibrational configuration interaction theory relying on multidimensional potential energy surfaces (PESs) obtained from explicitly correlated coupled cluster calculations, CCSD(T)-F12a. In agreement with experimental results, the series of rare gas atoms leads to rather unsystematic results and indicates huge zero point vibrational energy effects for the helium complex. In order to study these sensitive complexes more consistently, we also introduce configuration averaged vibrational self-consistent field theory, which is a generalization of standard vibrational self-consistent field theory to several configurations. The vibrational spectra of the complexes are compared to that of the methyl cation, for which corrections due to scalar-relativistic effects, high-order coupled-cluster terms, e.g., quadruple excitations, and core-valence correlation have explicitly been accounted for. The occurrence of tunneling splittings for the vibrational ground-state of C H 3 + —He has been investigated on the basis of semiclassical instanton theory. These calculations and a direct comparison of the energy profiles along the intrinsic reaction coordinates with that of the hydronium cation, H3O+, suggest that tunneling effects for vibrationally excited states should be very small. [ABSTRACT FROM AUTHOR]

While the electrostatic potential and the counterion distribution produced by interfaces with idealized geometries can be well-described by analytical models, the same does not hold true for the interaction between surfaces with different and arbitrary geometries. Besides, the geometry of a charged interface may also affect the counterion adsorption, potentially modulating the electrostatic potential and the solvent organization close to the interfaces, demanding molecular details to be taken into account. The complex electrostatics of a sodium dodecyl sulfate micelle in the presence of monolayers of the same surfactant at the water-vapor interface was assessed by a set of molecular dynamics simulations. The electrostatic potential was evaluated numerically, and its total magnitude was decomposed into contributions arising from each species comprising the system. The counterion adsorption was stronger at the flat interfaces due to the more favorable formation of sodium bridges, where the same counterion is bounded to two or more anionic heads, while water reorientation was more pronounced near the micelle. These opposing effects counteracted each other so that the overall electrostatic potential changes were similar for both interfaces. The increase in the counterion concentration between the micelle and the interface originates a double layer mediated repulsion amounting to a free energy barrier of at least 14 kJ/mol, preventing the micelle to get closer to the monolayers. It is noteworthy that the hydrophobic regions had electrostatic potential contributions as large as those arising from the hydrophilic regions, mostly due to the orderly orientation of the terminal methyl groups. [ABSTRACT FROM AUTHOR]

Trioxatriangulenium (TOTA) platform molecules were functionalized with methyl, ethyl, ethynyl, propynyl, and hydrogen and sublimated onto Au(111) surfaces. Low-temperature scanning tunneling microscopy data reveal that >99% of ethyl-TOTA and methyl-TOTA remain intact, whereas 60% of H-TOTA and >99% of propynyl-TOTA and ethynyl-TOTA decompose. The observed tendency toward fragmentation on Au(111) is opposite to the sequence of gas-phase stabilities of the molecules. Although Au(111) is the noblest of all metal surfaces, the binding energies of the decomposition products to Au(111) destabilize the functionalized platforms by 2 to 3.9 eV (190–370 kJ/mol) and even render some of them unstable as revealed by density functional theory calculations. Van der Waals forces are important, as they drive the adsorption of the platform molecules. [ABSTRACT FROM AUTHOR]

A large bandgap is critical for the applications of quantum spin Hall (QSH) insulators at room temperature. Based on the first-principles calculations, we predict that the methyl and ethynyl functionalized TlSb monolayers, namely, TlSb(CH3)2 and TlSb(C2H)2 films, own QSH states with large bandgaps of 0.13 and 0.272 eV, which possess potential applications at room temperature. For TlSb(CH3)2, the QSH phase arises from the spin-orbit coupling (SOC) induced s-p band inversion, while for TlSb(C2H)2, the QSH phase results from the SOC induced p-p bandgap opening. The QSH effect is further characterized by the Z2 topological invariant and topologically protected edge states. Significantly, the QSH states in TlSb(CH3)2 and TlSb(C2H)2 films are robust against external strain and various methyl/ethynyl coverages, making them especially flexible in the substrate selection. Besides, we find that h-BN is an ideal substrate for TlSb(CH3)2 and TlSb(C2H)2 films to keep QSH states with large bandgaps. Thus, the methyl and ethynyl functionalized TlSb films may be good QSH effect platforms for the design and fabrication of topological electronic devices. [ABSTRACT FROM AUTHOR]

In the present contribution, we study dissociative electron attachment to 1-methyl-2-thiouracil that has been synthesized and purified prior to the measurements. We compare the results with those previously obtained from 2-thiouracil. The comparison of the yield of the dehydrogenated parent anion from both the compounds allows us to assign the site from which the H atom is expulsed and to predict the mechanism that is involved in the formation of the peaks within the ion yield curve. It appears that the dehydrogenation observed for 2-thiouracil arising from the vibrational Feshbach resonances (at 0.7 and 1.0 eV) and a π*/σ* transition (at 0.1 eV) involves the bond cleavage at the N1 site, while that at the N3 site operates via the π*/σ* transition and occurs in the energy range of 1.1–3.3 eV. Besides the loss of the H atom from 1-methyl-2-thiouracil, we observe a relatively strong signal due to the loss of an entire methyl group (not observed from methyl-substituted thymine and uracil) that is formed from the N1–CH3 bond cleavage and can mimic the N-glycosidic bond cleavage within the DNA macromolecule. [ABSTRACT FROM AUTHOR]

We investigated the influence of the emitter (amorphous-Si, a-Si, or polythiophene derivatives: poly(3-hexylthiophene), P3HT, and poly(3-[3,6-dioxaheptyl]-thiophene), P3DOT) and the interface passivation (intrinsic a-Si or SiOX and methyl groups or SiOX) on the c-Si based 1×1 cm2 planar hybrid heterojunction solar cell parameters. We observed higher short circuit currents for the P3HT or P3DOT/c-Si solar cells than those obtained for a-Si/c-Si devices, independent of the interface passivation. The obtained VOC of 659mV for the P3DOT/SiOX/c-Si heterojunction solar cell with hydrophilic 3,6-dioxaheptyl side chains is among the highest reported for c-Si/polythiophene devices. The maximum power conversion efficiency, PCE, was 11% for the P3DOT/SiOX/c-Si heterojunction solar cell. Additionally, our wafer lifetime measurements reveal a field effect passivation in the wafer induced by the polythiophenes when deposited on c-Si. [ABSTRACT FROM AUTHOR]

Methyl 2-furoate (FAME2) is a model for the potential renewable biofuel of dimethyl furan-2,5-dicarboxylate, with the development of its new synthesis method. The potential energy surfaces of H-abstractions and OH-additions between FAME2 and hydroxyl radical (OH) were studied using CCSD(T)/CBS//M062X/cc-pVTZ. The subsequent isomerization and decomposition reactions were also determined for the primary radicals produced. The results showed that H-abstraction on the branched methyl group was the dominant channel and that the OH-addition reactions on the furan ring had a significant pressure dependency. The rate coefficients presented here provide important kinetic data to support future improvement of the combustion mechanism of FAME2, and present a sound basis for further research into practical fuels. [ABSTRACT FROM AUTHOR]

We study the influence of rotational coupling between a pair of methyl rotators on the tunneling spectrum in condensed phase. Two interacting adjacent methyl groups are simulated within a coupled-pair model composed of static rotational potential created by the chemical environment and the interaction potential between two methyl groups. We solve the two-dimensional time-independent Schrödinger equation analytically by expanding the wave functions on the basis set of two independent free-rotor functions. We investigate three scenarios which differ with respect to the relative strength of single-rotor and coupling potential. For each scenario, we illustrate the dependence of the energy level scheme on the coupling strength. It is found that the main determinant of splitting energy levels tends to be a function of the ratio of strengths of coupling and single-rotor potential. The tunnel splitting caused by coupling is maximized for the coupled rotors in which their total hindering potential is relatively shallow. Such a weakly hindered methyl rotational potential is predicted for 4-methylpyridine at low temperature. The experimental observation of multiple tunneling peaks arising from a single type of methyl group in 4-methylpyridine in the inelastic neutron scattering spectrum is widely attributed to the rotor-rotor coupling. In this regard, using a set of first-principles calculations combined with the nudged elastic band method, we investigate the rotational potential energy surface (PES) of the coaxial pairs of rotors in 4-methylpyridine. A Numerov-type method is used to numerically solve the two-dimensional time-independent Schrödinger equation for the calculated 2D-density functional theory profile. Our computed energy levels reproduce the observed tunneling transitions well. Moreover, the calculated density distribution of the three methyl protons resembles the experimental nuclear densities obtained from the Fourier difference method. By mapping the calculated first-principles PES on the model, it is confirmed that the hindering potential in 4-methylpyridine consists of proportionally shallow single-rotor potential to coupling interaction. [ABSTRACT FROM AUTHOR]

Heat capacity measurements were made down to 0.35 K for the isotopic modifications of methanol, CH3-nDnOH, and methyl iodide, CH3-nDnI, (n = 0, 1, 2, 3) to determine the orientation of the partially deuterated methyl group in the solid phase. The mono-deuterated modifications favor the symmetric conformation, whereas the di-deuterated ones favor the asymmetric conformation. Infrared spectroscopy demonstrates that some vibrational modes change in intensity depending on temperature, which supports the energy scheme obtained by calorimetry. Zero-point kinetic energies were obtained by single molecule density functional theory calculations. Although the favorable conformations of CH2DOH and CHD2OH were confirmed, the energy difference between symmetric and asymmetric conformations was twice as large as that determined experimentally, which indicates that intermolecular forces significantly decrease the energy difference. For CHD2OH, the conversion between the two asymmetric conformations becomes very slowat lowtemperature and results in a residual entropy of R ln 2. [ABSTRACT FROM AUTHOR]