Alkylation Effects on Strong Collisions of Highly Vibrationally Excited Alkylated Pyridines with CO2.

The role of alkylation on the energy partitioning in strong collisions with CO2was investigated for highly vibrationally excited 2-ethylpyridine (2EP) and 2-propylpyridine (2PP) prepared with Evib38 570 and 38 870 cm-1, respectively, using 266 nm light. Nascent energy gain in CO2(0000) rotation and translation was measured with high-resolution transient absorption spectroscopy at 4.3 m and the results are compared to earlier relaxation studies of pyridine (Evib37 950 cm-1) and 2-methylpyridine (2MP, Evib38 330 cm-1). Overall, the alkylated donors impart less rotational and translational energy to CO2than 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 CO2via a less repulsive part of the intermolecular potential than does pyridine. [ABSTRACT FROM AUTHOR]