A Local Mode Study of Ring Puckering Effects in the Infrared Spectra of Cyclopentane

We report and interpret recently recorded high-resolution infrared spectra for the fundamentals of the CH2 scissors and CH stretches of gas phase cyclopentane at −26.1 and −50 ○C, respectively. We extend previous theoretical studies of this molecule, which is known to undergo barrierless pseudorotation due to ring puckering, by constructing local mode Hamiltonians of the stretching and scissor vibrations for which the frequencies, couplings, and linear dipoles are calculated as functions of the pseudorotation angle using B3LYP/6-311++(d,p) and MP2/cc-pVTZ levels of theory. Symmetrization (D5h) of the vibrational basis sets leads to simple vibration/pseudorotation Hamiltonians whose solutions lead to good agreement with the experiment at medium resolution, but which miss interesting line fractionation when compared to the high-resolution spectra. In contrast to the scissor motion, pseudorotation leads to significant state mixing of the CH stretches, which themselves are Fermi coupled to the scissor overtones

High energy vibrational excitations of nitromethane in liquid water

The pathways and timescales of vibrational energy flow in nitromethane are investigated in both gas and condensed phases using classical molecular mechanics, with a particular focus on relaxation in liquid water. We monitor the flow of excess energy deposited in vibrational modes of nitromethane into the surrounding solvent. A marked energy flux anisotropy is found when nitromethane is immersed in liquid water, with a preferential flow to those water molecules in contact to the nitro group. The factors that permit such anisotropic energy relaxation are discussed, along with the potential implications on the molecule's non-equilibrium dynamics. In addition, the energy flux analysis allows us to identify the solvent motions responsible for the uptake of solute energy, confirming the crucial role of water librations. Finally, we also show that no anisotropic vibrational energy relaxation occurs when nitromethane is surrounded by argon gas.A.J.R. acknowledges the financial support from the Departament de Recerca i Universitats de la Generalitat de Catalunya. C.C. acknowledges support from Spanish Grant No. PID2021-124297NB-C31, funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe.” E.L.S. gratefully acknowledges support from the NSF via Grant No. CHE-1900095. A.J.R. and R.R. acknowledge support from Grant No. PID2021-124297NB-C32, funded by MCIN/AEI/10.13039/501100011033 and FEDER, and from the Generalitat de Catalunya (Grant No. 2021 SGR 01411).Peer ReviewedPostprint (published version

VIBRATIONAL ENERGY RELAXATION OF CHOLOROIODOMETHANE IN COLD ARGON

Author Institution: Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison, WI 53706Electronically exciting the C-I stretch in the molecule chloroiodomethane CH$_{2}$ClI embedded in a matrix of argon at 12K can lead to an isomer, iso-chloroiodomethane CH$_{2}$Cl-I, that features a chlorine iodine bond., 114503 (2011)} By temporally probing the isomer at two different frequencies of 435 nm and 485 nm, multiple timescales for isomerization are inferred. To gain further mechanistic insights into this process we have studied the isomerization theoretically using molecular dynamics. Two and three low frequency modes (C-Cl-I bend, Cl-I stretch and C-Cl stretch) are included in the model. The experiment is simulated by equilibrating the molecule in the parent configuration and providing an energy of 37,500 cm$^{-1}$, corresponding to the electronic excitation of the C-I stretch. Subsequent time evolution of the classical trajectories is monitored, and the decay rates of energy are compared to the experimental spectroscopy results. Varying different parameters related to the potential energy surface can lead to different results and their implications to the energy flow are discussed. The decay rates in the isomer well are also compared to the classical Landau Teller theory

HOW DO HYDROGEN BONDS BREAK IN SMALL ALCOHOL OLIGOMERS ?

Author Institution: Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison, WI 53706Hydrogen bonding liquids play vital roles in condensed phase chemical processes. It is well-known that in the gas phase alcohol clusters undergo vibrational predissociation after hydroxyl (OH) stretch excitation. Recent infrared pump probe studies on alcohol oligomers in CCl$_4$ solution suggest that similiar processes might occur in the condensed phase too. Following OH stretch excitation, ultrafast hydrogen bond breaking events have been observed on the timescale of 1 ps, the mechanism of which is still unclear. To answer the titular question, we consider vibrational predissociation of the hydrogen bonded methanol dimer. We construct a 5-dimensional model for the dimer including the hydrogen bond stretch, the donor OH stretch, the donor OH bend, and both the OH and methyl rotation about the CO axis of the donor. The potential energy surface is computed ab initio. A vibrational self-consistent field representation is adopted to calculate predissociation rates with the Fermi's Golden Rule approach. In addition, close-coupling calculations are performed yielding results in good agreement with the Fermi's Golden Rule approach. The predissociation rates strongly depend on the hydrogen bond strength. For our model methanol dimer, the timescale is on the order of 100ps. When the potential energy surface is scaled to correspond to the hydrogen bond strength of the methanol oligomers, a timescale on the order of 1ps is obtained. It has been suggested in the literature that predissociation occurs as a result of energy deposited in the OH stretch flowing directly into the hydrogen bond stretch. However, our results clearly demostrate that the predissociation occurs via highly excited OH rotational states. From our results, a simple nonadiabatic curve crossing picture for the predissociation process emerges, which provides a usefull framework for future study of solvent assisted vibrational predissociation

VIBRATIONAL PREDISSOCIATION OF THE METHANOL DIMER

Author Institution: Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison, WI 53706Examples of vibrational predissociation of hydrogen bonded (H-bonded) complexes in solution are rare while the gas phase counterpart is relatively well-understood today. Recent infrared pump probe studies of H-bonded alcohol oligomers in CCl$_4$ solution revealed that excitation of the OH (D) stretch of the H-bonded alcohol molecules leads to fast vibrational energy relaxation (VER) and concomitant H-bond breaking. The timescales of the relaxation and H-bond breaking events are on the order of 1 ps, which is faster than the VER timescale of a non-H-bonded monomer. To account for above experimental findings, it was suggested that the excited oligomers predissociate via a direct energy transfer from the OH (D) stretch to the H-bond degrees of freedom, which immediately breaks the H-bond. However, subsequent theoretical studies have failed to corroborate the proposed mechanism. Here we consider the vibrational predissociation of the H-bonded methanol dimer. Our reduced dimensional model for the methanol dimer includes three degrees of freedom, the H-bond donor OH stretch, the H-bond donor OH torsion and the H-bond stretch. The potential energy surface is computed using electronic structure program. Gas phase predissociation rates are obtained using both simple Fermi's Golden Rule approach and more rigorous close-coupling calculations. Our results demonstrate that a direct energy transfer from the OH stretch to the H-bond stretch is extremely slow. However, predissociation pathways involving highly excited torsional states are much more efficient, with timecales on the order of 100 ps. In solution, the solvent molecules could potentially facilitate the predissociation process of the methanol dimer either by modifying the potential energy surface or by taking up excess energy. Current work attempts to include the methanol-solvent interaction in a system-bath coupling sense. This will hopefully provide some insights into the previous experimental work

VIBRATIONAL DYNAMICS AROUND THE CONICAL INTERSECTION RESULTING FROM THE A~→X~\tilde{A} \rightarrow \tilde{X} LASER INDUCED FLUORESCENCE OF THE METHOXY (CH3_3O) RADICAL

Author Institution: Department of Chemistry and Theoretical Chemistry Institute, University of Wisconsin-Madison, WI 53706The results of a theoretical calculation of the spectra associated with the laser induced fluorescence $\tilde{A}^2A_1\rightarrow \tilde{X}^2E$ of both the methoxy molecule and CH$_2$DO are presented and discussed. The form of the vibronic dipole moment is determined by symmetry and the corresponding dipole expansion coefficients are calculated using {\it ab initio} methods. The calculated spectra include states up to 3000 cm$^{-1}$ above the zero point energy. We describe how the various features of the spectrum are related to coordinate dependent terms in the dipole expansion as well as the spin-orbit couplings, Jahn-Teller couplings, and vibrational anharmonicities

A DIABATIC BASIS APPROACH TO THE DETERMINATION OF TUNNELING SPLITTINGS IN FORMIC ACID DIMER

Author Institution: University of Wisconsin-Madison, Madison, Wisconsin 53706\maketitle Formic Acid Dimer (FAD) is the smallest molecule which isheld together through a double hydrogen bond. This type of hydrogen bonding structure is found in many large biological systems, however, high level theoretical treatment of such systems is currently intractable. By studying the FAD system in detail insights into other larger systems may be possible. Tunneling splitting of FAD for the ground state and with one quantum of CO stretch have recently been measured by Madeja and Havenith (J. Chem. Phys. {\bf{117}}, 7162 (2002)). We have calculated a three dimensional reaction surface potential for FAD while treating all other degrees of freedom as a set of coupled harmonic oscillators. This problem provides a challenging case of coupling between the degrees of freedom. Great care must be taken between adiabatic and diabatic representations. By making use of a diabatic representation for the reaction surface potential a small and efficient basis set is defined. Our goal is to investigate the effects of vibrational excitation on the calculated tunneling splittings