Semiempirical Model of Vibrational Relaxation for Estimating Absolute Rate Coefficients

Collisional Intermolecular Energy Transfer from a N2 Bath at Room Temperature to a Vibrationlly "Cold" C6F6 Molecule Using Chemical Dynamics Simulations

Chemical dynamics simulations were performed to study collisional intermolecular energy transfer from a thermalized N₂ bath at 300 K to vibrationally "cold" C₆F₆. The vibrational temperature of C₆F₆ is taken as 50 K, which corresponds to a classical vibrational energy of 2.98 kcal/mol. The temperature ratio between C₆F₆ and the bath is 1/6, the reciprocal of the same ratio for previous "hot" C₆F₆ simulations (J. Chem. Phys. 2014, 140, 194103). Simulations were also done for a C₆F₆ vibrational temperature of 0 K. The average energy of C₆F₆ versus time is well fit by a biexponential function which gives a slightly larger short time rate component, k₁, but a four times smaller long time rate component, k₂, compared to those obtained from the "hot" C₆F₆ simulations. The average energy transferred per collision depends on the difference between the average energy of C₆F₆ and the final C₆F₆ energy after equilibration with the bath, but not on the temperature ratio of C₆F₆ and the bath. The translational and rotational degrees of freedom of the N₂ bath transfer their energies to the vibrational degrees of freedom of C₆F₆. The energies of the N₂ vibrational mode and translational and rotational modes of C₆F₆ remain unchanged during the energy transfer. It is also found that the energy distribution of C₆F₆ broadens as energy is transferred from the bath, with an almost linear increase in the deviation of the C₆F₆ energies from the average C₆F₆ energy as the average energy of C₆F₆ increases.

Chemical Dynamics Simulations of Intermolecular Energy Transfer: Azulene + N2 Collisions

Chemical dynamics simulations were performed to investigate collisional energy transfer from highly vibrationally excited azulene (Az*) in a N2 bath. The intermolecular potential between Az and N2, used for the simulations, was determined from MP2/6-31+G* ab initio calculations. Az* is prepared with an 87.5 kcal/mol excitation energy by using quantum microcanonical sampling, including its 95.7 kcal/mol zero-point energy. The average energy of Az* versus time, obtained from the simulations, shows different rates of Az* deactivation depending on the N2 bath density. Using the N2 bath density and Lennard-Jones collision number, the average energy transfer per collision ⟨ΔEc⟩ was obtained for Az* as it is collisionally relaxed. By comparing ⟨ΔEc⟩ versus the bath density, the single collision limiting density was found for energy transfer. The resulting ⟨ΔEc⟩, for an 87.5 kcal/mol excitation energy, is 0.30 ± 0.01 and 0.32 ± 0.01 kcal/mol for harmonic and anharmonic Az potentials, respectively. For comparison, the experimental value is 0.57 ± 0.11 kcal/mol. During Az* relaxation there is no appreciable energy transfer to Az translation and rotation, and the energy transfer is to the N2 bath.

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene Viewed by IR−UV Double Resonance Spectroscopy. 2. Perturbed States with J = 17 and 18

Collision-induced state-to-state molecular energy transfer between rovibrational states in the 12,700 cm(-1) 4nu(CH) manifold of the electronic ground state X of acetylene (C(2)H(2)) is monitored by time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. Rotational J-states associated with the (nu(1) + 3nu(3)) or (1 0 3 0 0)(0) vibrational combination level, initially prepared by an IR pulse, are probed at approximately 299, approximately 296, or approximately 323 nm with UV laser-induced fluorescence via the Alpha electronic state. The rovibrational J-states of interest belong to a congested manifold that is affected by anharmonic, l-resonance, and Coriolis couplings, yielding complex intramolecular dynamics. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-DeltaJ features but also supposedly forbidden odd-DeltaJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4nu(CH) manifold (particularly those with J = 0 and J = 1) whereas this paper examines locally perturbed states at higher values of J (particularly J = 17 and 18, which display anomalous doublet structure in IR-absorption spectra). Three complementary forms of IR-UV DR experiments (IR-scanned, UV-scanned, and kinetic) are used to address the extent to which intramolecular perturbations influence the efficiency of J-resolved collision-induced energy transfer with both even and odd DeltaJ.