Principal Energy Transfer Pathways in the Collision of N₂O(00⁰1) with Toluene-d₈. A (WKB) Semiclassical Study

Abstract

Vibrational energy transfer from N2O(0001) to deuterated toluene has been studied by use of the Wentzel−Kramers−Brillouin (WKB) semiclassical procedure in the distorted-wave approximation in the temperature range of 100−500 K. Energy transfer to the CD stretch modes (νa‘ ‘ and νa‘) on the methyl group of C6D5CD3 is shown to be the principal pathway occurring through long-range interactions. The energy transfer to the CD stretch modes (ν7, ν20, and ν2) on the benzene ring are found to be of minor importance in removing vibrational energy from N2O(0001). Energy transfer to these ring modes is shown to occur through short-range interactions where the energy mismatch is supplied by the translational motion. In all cases, energy transfer probabilities decrease with increasing temperature.

Vibrational Energy Transfer from DF(1) to Toluene. Competition between the Benzene Ring CH and Methyl Group CH Stretches

“…We note that the DF−DF hydrogen-bond energy is as large as 6 kcal/mol or 4.17 × 10 -13 erg, but use of such a large value is not appropriate for the gas-phase interaction between DF and a saturated hydrocarbon. We expect the range parameter a for C 6 H 5 CH 3 −DF to be significantly larger than 0.20−0.25 Å, the values that have often been used for simple molecular systems. , In fact, we have used a = 0.34 Å for C 6 H 5 CH 3 −N 2 O . Since the values of a for both DF and N 2 O are close to 0.20 Å, ,, we choose the same magnitude of 0.34 Å for the present system.…”

“…We expect the range parameter a for C 6 H 5 CH 3 −DF to be significantly larger than 0.20−0.25 Å, the values that have often been used for simple molecular systems. , In fact, we have used a = 0.34 Å for C 6 H 5 CH 3 −N 2 O . Since the values of a for both DF and N 2 O are close to 0.20 Å, ,, we choose the same magnitude of 0.34 Å for the present system. When we use the polarizability of 13 Å 3 estimated for toluene 28 and the dipole moment of 1.82D for DF, the dipole−induced dipole energy is U DID = −2.15 × 10 -11 (3 cos θ‘ + 1)/ x 6 in erg, where x is in Å.…”

“…The integrals in the exponent are table integrals, which can be readily evaluated. Such WKB wave functions have been used in the evaluation of the perturbation integral F if that appears in the quantum mechanical treatment of molecular interactions. ,,− Equation 8 describes the collisions at short-range ( x < x t ), where repulsive interactions are of major importance, whereas eq 9 is appropriate for the collision taking place at long range ( x > x t ), where the attractive part of the intermolecular potential causes vibrational energy to be transferred. Thus, throughout in this paper, we shall refer to the use of these two equations in the evaluation of the perturbation integrals as the short-range and long-range interaction models, respectively.…”

“…In recent years, collisions involving large organic molecules have been studied actively, revealing valuable information on the rates and the mechanisms of vibrational energy transfer. Large organic molecules can provide a number of near-resonant energy transfer pathways, − so they are important systems for studying the problem of vibration-to-vibration (VV) energy transfer. Except for the case of exact resonance, intermolecular VV energy transfer involves the energy mismatch Δ E , which has to be transferred to or from other motions such as translation (VT) or rotation (VR).…”

“…When the magnitude of Δ E is small, the translational motion of the colliding molecules can transfer it efficiently even at long range; i.e., the overall energy transfer process is VVT. Among such collision systems which have been studied are energy transfer between aromatic hydrocarbons and N 2 O(00°1) or CO 2 (00°1). − ,, Normal or deuterated hydrocarbons have their CH stretching frequencies in near resonance with the asymmetric stretch of these triatomic molecules and vibrations of many diatomic species such as DF, CO + , N 2 + and OH + . − Of these hydrocarbons, toluene has its CH stretching frequencies within 200 cm -1 of DF(1). In particular, toluene has two groups of CH stretches (the benzene ring and methyl CH modes) so the collision of this molecule with a vibrationally excited molecule such as DF(1) is a particularly attractive system for studying the competition among the two groups of modes for energy transfer and the mechanism for energy transfer, especially whether it is the short-range repulsive or long-range attractive part of the interaction energy which causes energy transfer.…”