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Collision rates and energy transfer distributions are reported for HOD with highly vibrationally excited 2-methylpyridine (2-picoline, E = 38 310 cm(-1)) and 2,6-dimethylpyridine (2,6-lutidine, E = 38 700 cm(-1)). High resolution transient IR absorption is used measured to complete product state distributions of scattered HOD(000) molecules with E(rot) = 109 to 1331 cm(-1). Doppler-broadened line profiles characterize the depletion and appearance for HOD molecules due to collisions with hot donors and show that the product translational and rotational energy distributions are similar for both donors with DeltaE(rel) = 370 cm(-1) and DeltaE(rot) approximately 75 cm(-1). The energy transfer rate for picoline (E)/HOD is 2.5 times larger than the Lennard-Jones collision rate. The energy transfer rate for lutidine(E)/HOD is 3.2 times larger than the Lennard-Jones collision rate. Previous work ( Havey, Liu, Li, Elioff, and Mullin, J. Phys. Chem. A 2007, 111, 13321-9 ) reported similar energy transfer values for pyrazine/HOD collisions and an energy transfer rate that is 1.7 times the Lennard-Jones collision rate. The observed collision rates are discussed in terms of hydrogen bonding interactions between HOD and the aromatic donor molecules. Energy gain profiles for HOD are compared with those for H(2)O.

This work presents state-resolved measurements of weak and strong collisions between HOD and highly vibrationally excited pyrazine (Evib = 37,900 cm(-1)). Transient IR absorption line profiles of HOD(000) rotational states that are populated in single collisions with pyrazine (Evib) are fit using double-Gaussian functions to extract Doppler-broadened line widths and energy transfer rates for appearance and depletion populations. We recently reported the use of this new approach to determine collision rates for energy transfer (Havey, D. K.; Liu, Q.; Li, Z. M.; Elioff, M.; Fang, M.; Neudel, J.; Mullin, A. S. J. Phys. Chem. A 2007, 111, 2458-2460). Here we present a full description of transient measurements of weak collisions, along with rate constants and the full energy transfer distribution function for the vibrational-to-rotation/translation (V-RT) pathway. The low- and high-J populations of scattered HOD(000) are characterized by a single rotational distribution with Trot = 430 +/- 50 K. The average translational energy of the scattered HOD(000) increases modestly with rotational energy suggesting an impulsive energy transfer mechanism. The energy gain distribution P(Delta E) for HOD(000) shows that approximately 99% of collisions have DeltaE values less than 3000 cm(-1). These data yield a lower limit to the collision rate that is approximately 70% larger than the calculated Lennard-Jones collision rate. These findings show that water is an efficient quencher due to a large collisional energy transfer cross section and not as a result of a large supercollision tail.

The role of alkylation on the energy partitioning in strong collisions with CO2 was investigated for highly vibrationally excited 2-ethylpyridine (2EP) and 2-propylpyridine (2PP) prepared with E(vib) approximately 38,570 and 38,870 cm(-1), respectively, using lambda = 266 nm light. Nascent energy gain in CO2 (00(0)0) rotation and translation was measured with high-resolution transient absorption spectroscopy at lambda approximately 4.3 microm and the results are compared to earlier relaxation studies of pyridine (E(vib) = 37,950 cm(-1)) and 2-methylpyridine (2MP, Evib = 38,330 cm(-1)). Overall, the alkylated donors impart less rotational and translational energy to CO2 than does pyridine. 2PP consistently imparts more translational energy in collisions than does 2EP and has larger energy transfer rates. Of the alkylated donors, 2MP and 2PP have larger probabilities for strong collisional energy transfer than does 2EP. Two competing processes are discussed: donors with longer alkyl chains have lower average energy per mode and fewer strong collisions but longer alkyl chains increase donor flexibility, leading to higher state densities that enhance energy loss via strong collisions. A comparison of state density effects based on Fermi's Golden Rule shows that 2PP has more strong collisions than predicted while 2EP has fewer. The role of torsional motion in the hot donors is considered. Comparison of effective impact parameters shows that the alkylated donors undergo strong collisions with CO2 via a less repulsive part of the intermolecular potential than does pyridine.