Coupled-states statistical investigation of vibrational and rotational relaxation of OH(2pi) by collisions with atomic hydrogen

We report state-to-state cross sections and thermal rate constants for vibrational and rotational relaxation of OH(2pi) by collision with H atoms. The cross sections are calculated by the coupled-states (CS) statistical method including the full open-shell character of the OH + H system. Four potential energy surfaces (PESs) ((1,3)A' and (1,3)A') describe the interaction of OH(X2pi) with H atoms. Of these, three are repulsive, and one (1A') correlates with the deep H2O well. Consequently, rotationally and ro-vibrationally inelastic scattering of OH in collisions with H can occur by scattering on the repulsive PESs, in a manner similar to the inelastic scattering of OH by noble gas atoms, or by collisions which enter the H2O well and then reemerge. At 300 K, we predict large (approximately 1 x 10(-10) cm3 molecule(-1) s(-1)) vibrational relaxation rates out of both v = 2 and v = 1, comparable to earlier experimental observations. This anomalously fast relaxation results from capture into the H2O complex. There exists a significant propensity toward formation of OH in the pi(A') lambda-doublet level. We also report state-resolved cross sections and rate constants for rotational excitation within the OH v = 0 manifold. Collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted lambda-doublet population.     

Reorientation,polarization and scaling laws in rotational transfer experiments

Reorientation of angular momentum in molecules excited by polarized light, following rotationally inelastic collisions results in depolarization of the light emitted by these molecules. We show here how this effect can be quantitatively predicted from degeneracy, i.e. orientation-averaged rotational transfer rates, as usually measured in cell experiments. For this we first develop a classical phenomenology of rotational relaxation of diatomic molecules in the ¹Σ state, based on a vector model for angular momenta. Different simple scaling relationships are then given and applied to analyze the Li₂

+He system for which both orientation-averaged rate constants and polarization measurements for rotationally inelastic collisions already exist. It is found indeed that the latter data set agrees quite well with what can be predicted from the former data set. Finally we show that a propensity for conserving 0, the orientation of the angular momentum with respect to a laboratory-fixed axis results generally from geometrical reasons and is fairly independent of the collision dynamics.     

Mechanism of vibrational relaxation of NO upon collisions with Ar

The vibrational relaxation of NO diluted in Ar is considered. The conventional adiabatic channel and the nonadiabatic vibronic channel of vibrational d     

A theoretical study of rate coefficients for the O + NO vibrational relaxation

Quasi-classical trajectories have been integrated to study the vibrational relaxation of the O + NO(v) process as a function of the initial vibrational quantum number for T = 298 K, 1500 K, and 3000 K. Two reliable potential energy surfaces have been employed for the A' and A' doublet states of NO2. The calculated vibrational relaxation rate constants show a nearly v-independent behavior at room temperature and a moderate increase with v for higher temperatures. Although deviating significantly from the recommended values, good agreement with recent experimental results has been obtained. The importance of multi-quantum transitions is also analyzed.     

Cross sections for rotational/vibrational transitions in HF-HF collisions: effect of initial state

Recent semiclassical calculations of cross sections for vibrational and rotational transitions in HF + HF collisions have been extended by considering the effect of changing the initial state, resonance rotational excitation and by increasing the basis set expansion to a 157- and 249-state “dynamical” basis.     

Speed dependence of collisional relaxation in ground vibrational state of OCS: rotational behaviour

Accurate experimental data on pressure broadened profiles of (16)O(12)C(32)S pure rotational lines in a broad range of quantum number J have been analyzed taking into account the speed dependence of collisional relaxation. Refined values of collisional self-broadening coefficients are determined and compared to previously known data. New quantitative information on departures of observed line shapes from the traditional Voigt profile is obtained. It is shown that these departures result mainly from the speed dependence of collisional relaxation. Theoretical calculations of self-broadening parameters are performed in the framework of the semiclassical impact Robert-Bonamy formalism where the mean relative molecular speed as well as the Maxwell-Boltzmann distribution of relative speeds is considered. The necessity of allowance for the speed dependence in line shape models is confirmed and satisfactory results have been obtained by arbitrarily limiting the integration of the differential cross section to a finite value of the impact parameter. It is shown for the first time for the whole rotational spectrum that speed dependent models not only improve accuracy of modeling the observed line profiles but also give physically grounded values of collisional relaxation parameters.     

19F NMR transverse and longitudinal relaxation filter experiments for screening: a theoretical and experimental analysis

Ligand‐based ¹⁹F NMR screening represents an efficient approach for performing binding assays. The high sensitivity of the methodology to receptor binding allows the detection of weak affinity ligands. The observable NMR parameters that are typically used are the ¹⁹F transverse relaxation rate and isotropic chemical shift. However, there are few cases where the ¹⁹F longitudinal relaxation rate should also be used. A theoretical and experimental analysis of the ¹⁹F NMR transverse and longitudinal relaxation rates at different magnetic fields is presented along with proposed methods for improving the sensitivity and dynamic range of these experiments applied to fragment‐based screening. Copyright © 2016 John Wiley & Sons, Ltd.     

Vibrational dephasing and hydrodynamic effects on vibrational relaxation rates in acetophenone: Raman bandshape analysis

The isotropic component of Raman band for C=O stretching mode of acetophenone in solution was analyzed by estimating the correlation coefficient with reference to Lorentzian lineshape. In the intermediate region of solute/solvent concentration there is a sharp decrease in the correlation coefficient and there appears to be a transition from non-Lorentzian to Lorentzian lineshape. The vibrational relaxation rates have been estimated from the isotropic component of Raman band in different solvents. The rate is shown to be dependent on several macroscopic as well as microscopic properties of the solute-solvent system and intermolecular interactions. The hydrodynamic and dispersion forces appear to play a major role in determining the vibrational relaxation rate and the broadening of the bands.     

Low-temperature rotational relaxation of CO in self-collisions and in collisions with Ne and He

The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1' scheme has been employed through the A(1)Pi state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of approximately 20 and 7 A(2) at 100 K to values of approximately 65-70 and approximately 20 A(2) in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 A(2) at 100 K to a maximum of 60 A(2) at 20 K and then decreases again to 40 A(2) at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N(2)-N(2), N(2)-Ne, and N(2)-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N(2). Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments.     

Validity of the rotational sudden approximation for vibrational relaxation in He + CO

Calculations of cross sections and rate constants are reported for the vibrational relaxation of CO in collision with ³He and ⁴He. Results computed using a semiclassical technique and a vibrational close-coupling, rotational infinite-order sudden method are compared.     

Competition between vibrational relaxation and photochemistry: relevance of vibronic quantum effects

Two molecules showing photochemistry but no fluorescence have been investigated at 80 K in a rigid matrix regarding the behavior of the quantum yield for bond fragmentation as a function of the vibrational/vibronic level and electronic excited state. A new equation was developed to determine the photochemical quantum yield under ambient conditions (80 K). The levels/bands involved were those within a given vibrational progression, in different progressions as well as in combination. The yield was low (phi = 0.1) with excitation into the n = 0 level of S1 but very rapidly increased with excitation into higher levels whether they were harmonics or combination levels. A parallel result was observed upon excitation into S2. Vibrational relaxation/deactivation occurs only between levels of the same vibrational progression. Deactivation from the 0 level of S2 does not occur via levels of S1. The photochemically active modes correspond to the vibrational modes present in the region of the molecule where bond breakage occurs. These results add further proof of the complex nature and number of processes that can occur within excited states of photochemically active molecules.     

Incorporation of rotational effects into vibrational distributions in chemical reactions

Many models of chemical reactions yield only vib-translational distributions. A method by which such distributions are modified to account for a final assumed or calculated rotational distribution is presented. Two examples demonstrate the reliability of the method.     

Coriolis interaction and intramolecular vibrational redistribution: A high-resolution spectroscopic probe of the rotational contribution to vibrational

We present here high-resolution fluorescence excitation spectra of the 12₀² band of pyrimidine in a molecular beam, which provide compelling ev     

Vibrational energy relaxation of isotopically labeled amide I modes in cytochrome c: theoretical investigation of vibrational energy relaxation rates and pathways

With use of a time-dependent perturbation theory, vibrational energy relaxation (VER) of isotopically labeled amide I modes in cytochrome c solvated with water is investigated. Contributions to the VER are decomposed into two contributions from the protein and water. The VER pathways are visualized by using radial and angular excitation functions for resonant normal modes. Key differences of VER among different amide I modes are demonstrated, leading to a detailed picture of the spatial anisotropy of the VER. The results support the experimental observation that amide I modes in proteins relax with subpicosecond time scales, while the relaxation mechanism turns out to be sensitive to the environment of the amide I mode.     

Restricted intramolecular vibrational relaxation in polyatomics and laser selective effects

A theory for unimolecular dissociation applicable to selective laser excitation is presented. Expressions for lifetimes, yields, and product fragment translational energy distributions are given as a function of IVR rate. We analyze an experiment purporting to “ demonstrate” rapid IVR.     

Environmental effects on vibrational properties of carotenoids: experiments and calculations on peridinin

Carotenoids are employed in light-harvesting complexes of dinoflagellates with the two-fold aim to extend the spectral range of the antenna and to protect it from radiation damage. We have studied the effect of the environment on the vibrational properties of the carotenoid peridinin in different solvents by means of vibrational spectroscopies and QM/MM molecular dynamics simulations. Three prototypical solvents were considered: cyclohexane (an apolar/aprotic solvent), deuterated acetonitrile (a polar/aprotic solvent) and methanol (a polar/protic solvent). Thanks to effective normal mode analysis, we were able to assign the experimental Raman and IR bands and to clarify the effect of the solvent on band shifts. In the 1500-1650 cm(-1) region, seven vibrational modes of the polyene chain were identified and assigned to specific molecular vibrations. In the 1700-1800 cm(-1) region a strong progressive down-shift of the lactonic carbonyl frequency is observed passing from cyclohexane to methanol solutions. This has been rationalized here in terms of solvent polarity and solute-solvent hydrogen bond interactions. On the basis of our data we propose a classification of non-equivalent peridinins in the Peridinin-Chlorophyll-Proteins, light-harvesting complexes of dinoflagellates.     

Theoretical study of the low-temperature vibrational relaxation of o-H2 in collisions with 4He

Cross sections and rate constants for the vibrational relaxation of H₂(υ = 1, j = 1) in collisions with ⁴He were determined using the coupled states method with a fully-converged channel basis. The interaction potential was taken to be that of Gordon and Secrest with the elastic matrix elements modified to include the spherically-symmetric component of the semi-empirical Shafer-Gordon potential. First-order forbidden transitions play a significant role in the overall relaxation process. The cross sections for the de-excitation of the υ = 1, j = 1 level are slightly larger than those for the υ = 1, j = 0 level. The ratio of the corresponding rate constants for vibrational relaxation varies smoothly from a value of 1.25 at 500 K to 1.63 at 60 K.     

A theoretical study of P4O10: vibrational analysis, infrared and Raman spectra

The normal mode frequencies and the corresponding vibrational assignments of tetraphosphorus decaoxide (P4O10) in tetrahedral (Td) symmetry are examined theoretically and experimentally. The Gaussian 98 set of quantum chemistry codes at the HF/6-311G*, MP2/6-311G*, and DFT/B3LYP/6-311G* levels of theory are used. By comparison to experimental normal mode frequencies deduced by Gilliam et al. [J. Phys. Chem. B 107 (2003) 2892], Chapman [Spectrochim. Acta A, 24 (1968) 1687], Beattie et al. [J. Chem. Soc. A (1970) 449], Konings et al. [J. Mol. Spectrosc. 152 (1992) 29] and the present work, correction factors for predominant vibrational motions are determined and compared. Normal modes were decomposed into five non-redundant motions (P-O stretch, P=O stretch, P-O-P bend, P-O-P wag, and P=O wag). Standard deviations found for the HF, MP2, and DFT corrected frequencies compared to experiment are 9, 5, and 4 cm(-1), respectively. Electron distribution for selected molecular orbitals is considered.     

"Proton holes" in long-range proton transfer reactions in solution and enzymes: A theoretical analysis

Proton transfers are fundamental to chemical processes in solution and biological systems. Often, the well-known Grotthuss mechanism is assumed where a series of sequential "proton hops" initiates from the donor and combines to produce the net transfer of a positive charge over a long distance. Although direct experimental evidence for the sequential proton hopping has been obtained recently, alternative mechanisms may be possible in complex molecular systems. To understand these events, all accessible protonation states of the mediating groups should be considered. This is exemplified by transfers through water where the individual water molecules can exist in three protonation states (water, hydronium, and hydroxide); as a result, an alternative to the Grotthuss mechanism for a proton transfer through water is to generate a hydroxide by first protonating the acceptor and then transfer the hydroxide toward the donor through water. The latter mechanism can be most generally described as the transfer of a "proton hole" from the acceptor to the donor where the "hole" characterizes the deprotonated state of any mediating molecule. This pathway is distinct and is rarely considered in the discussion of proton-transfer processes. Using a calibrated quantum mechanical/molecular mechanical (QM/MM) model and an effective sampling technique, we study proton transfers in two solution systems and in Carbonic Anhydrase II. Although the relative weight of the "proton hole" and Grotthuss mechanisms in a specific system is difficult to determine precisely using any computational approach, the current study establishes an energetics motivated framework that hinges on the donor/acceptor pKa values and electrostatics due to the environment to argue that the "proton hole" transfer is likely as important as the classical Grotthuss mechanism for proton transport in many complex molecular systems.