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Register a new userVibrational effects in the quantum dynamics of the H + D_2^+ charge transfer reaction
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Added by
Octavio Roncero Dr.
Publication date
2021
fields
Physics
and research's language is
English
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The H + D_2^+(v=0,1 and 2) charge transfer reaction is studied using an accurate wave packet method, using recently proposed coupled diabatic potential energy surfaces. The state-to-state cross section is obtained for three different channels: non-reactive charge transfer, reactive charge transfer, and exchange reaction. The three processes proceed via the electronic transition from the first excited to the ground electronic state. The cross section for the three processes increases with the initial vibrational excitation. The non-reactive charge transfer process is the dominant channel, whose branching ratio increases with collision energy, and it compares well with experimental measurements at collision energies around 0.5 eV. For lower energies the experimental cross section is considerably higher, suggesting that it corresponds to higher vibrational excitation of D_2^+(v) reactants. Further experimental studies of this reaction and isotopic variants are needed, where conditions are controlled to obtain a better analysis of the vibrational effects of the D_2^+ reagents.
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The non-adiabatic quantum dynamics of the H+H$_2^+$ $rightarrow$ H$_2$+ H$^+$ charge transfer reactions, and some isotopic variants, is studied with an accurate wave packet method. A recently developed $3times$3 diabatic potential model is used, which is based on very accurate {it ab initio} calculations and includes the long-range interactions for ground and excited states. It is found that for initial H$_2^+$(v=0), the quasi-degenerate H$_2$(v=4) non-reactive charge transfer product is enhanced, producing an increase of the reaction probability and cross section. It becomes the dominant channel from collision energies above 0.2 eV, producing a ratio, between v=4 and the rest of vs, that increases up to 1 eV. H+H$_2^+$ $rightarrow$ H$_2^+$+ H exchange reaction channel is nearly negligible, while the reactive and non-reactive charge transfer reaction channels are of the same order, except that corresponding to H$_2$(v=4), and the two charge transfer processes compete below 0.2 eV. This enhancement is expected to play an important vibrational and isotopic effect that need to be evaluated. For the three proton case, the problem of the permutation symmetry is discussed when using reactant Jacobi coordinates.
We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling (CVT/$mu$OMT) were applied using a fitted potential energy surface [J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval $ 4 cdot 10^{-20}$ to $4 cdot 10^{-17}$ cm$^3$ s$^{-1}$ , demonstrating that even deuterat
We perform the fixed-node diffuse Monte Carlo (FN DMC) calculations to determine the barrier height and reaction energy of a critical reaction, the H-transfer reaction from syn-CH3CHOO to vinyl hydroperoxide. The FN DMC barrier height is found to be 16.60+/-0.35 kcal/mol which agrees well with the experimental measurement within a few tenths of kcal, justifying the reliability of the FN DMC method for predicting barrier height of the rapid unimolecular reaction of Criegee intermediates. By comparing the predictions from the CCSD(t), G3 (MCG3), DFT and MP2 methods with respect to the FN DMC results and available experiment measurement, we found that the CCSD(t) barrier heights agree with the FN DMC counterpart within statistical errors, and is within a closer agreement with experiment and FN DMC prediction than the G3(MCG3) models. Barrier heights predicted from the relatively more economic DFT methods are within a few tenths kcal of the FN DMC prediction. MP2 method severely underestimates the barrier height. FN DMC prediction for the reaction energy is -17.25+/-0.31 kcal/mol, setting an upper limit for the reaction energies predicted by the post Hartree-Fock methods and a lower limit for the DFT reaction energies. We provide FN DMC input for clarifying the energetic uncertainties in the critical H-transfer reaction of syn-CH3CHOO. The quantitatively close agreements between the FN DMC barrier height and experimental measurement, and between the predictions from the FN DMC and G3 model for the reaction energy provide a theoretical basis for resolving the energy uncertainty in this reaction.
For a small fraction of hot CO2 molecules immersed in a liquid-phase CO2 thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO2 molecules and a cavity mode accelerates hot-molecule relaxation. The physical mechanism underlying this acceleration is the fact that polaritons, especially the lower polariton, can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates (i) resonantly depend on the cavity mode detuning, (ii) cooperatively depend on molecular concentration or Rabi splitting, and (iii) collectively scale with the number of hot molecules, which is similar to Dickes superradiance. For larger cavity volumes, due to a balance between this superradiant-like behavior and a smaller light-matter coupling, the total VSC effect on relaxation rates can scale slower than $1/N$, and the average VSC effect per molecule can remain meaningful for up to $N sim10^4$ molecules forming VSC. Moreover, we find that the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than equally distributing it to all thermal molecules. Finally, we highlight the similarities of parameter dependence between the current finding with VSC catalysis observed in Fabry-Perot microcavities.
We introduce a heterodimer model in which multiple mechanisms of vibronic coupling and their impact on energy transfer can be explicitly studied. We consider vibronic coupling that arises through either Franck-Condon activity in which each site in the heterodimer has a local electron-phonon coupling and as Herzberg-Teller activity in which the transition dipole moment coupling the sites has an explicit vibrational mode-dependence. We have computed two-dimensional electronic-vibrational (2DEV) spectra for this model while varying the magnitude of these two effects and find that 2DEV spectra contain static and dynamic signatures of both types of vibronic coupling. Franck-Condon activity emerges through a change in the observed excitonic structure while Herzberg-Teller activity is evident in the appearance of significant side-band transitions that mimic the lower-energy excitonic structure. A comparison of quantum beating patterns obtained from analysis of the simulated 2DEV spectra shows that this technique can report on the mechanism of energy transfer, elucidating a means of experimentally determining the role of specific vibronic coupling mechanisms in such processes.
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