Competition between Photochemistry and Energy Transfer in UV-Excited Diazabenzenes. 4. UV Photodissociation of 2,3-, 2,5-, and 2,6-Dimethylpyrazine

Abstract

The quantum yield for HCN formation via 248 nm photodissociation of 2,3-, 2,5-, and 2,6-dimethylpyrazine (DMP, C6N2H8) was measured using diode laser probing of the HCN photoproduct. The total quantum yield is phi = 0.039 +/- 0.07, 0.14 +/- 0.02, and 0.30 +/- 0.06 for 248 nm excitation of 2,3-, 2,5- and 2,6-DMP, respectively. Analysis of the quenching data within the context of a gas kinetic, strong collision model allows an estimate of the rate constant for HCN production via DMP photodissociation, ks = 4.1 x 10(3), 1.0 x 10(3), and 1.3 x 10(4) s(-1) for 2,3-, 2,5- and 2,6-DMP, respectively. Unlike HCN produced from the photodissociation of pyrazine and methylpyrazine, the amount of HCN produced via a prompt, unquenched dissociation channel was essentially zero, suggesting little multiphoton UV absorption. The rate constants for HCN formation together with previously measured rate constants for HCN production from photodissociation of pyrazine and methylpyrazine have been used to investigate possible reaction mechanisms. The position of the methyl group affects the HCN rate constant, suggesting that the mechanism for pyrazine dissociation involves an initial step that is hindered by the addition of the methyl groups. The proposed initial molecular motion of the mechanism, an out-of-plane H atom migration across a N atom, is consistent with (1) the position of the methyl groups, (2) the dissociation lifetime of the various pyrazine molecules studied, and (3) the observed large energy transfer magnitudes from pyrazine near dissociation. These so-called "supercollisions" have been linked to low-frequency, out-of-plane motion, suggesting that the molecular motions leading to efficient energy transfer are the same motions involved in dissociation. In addition, the pyrazine (C4N2H4) 248 nm photoproduct (C3H3N) was identified as acrylonitrile using IR spectroscopy, an observation that aids in understanding the dissociation mechanism.

Quenching of highly vibrationally excited pyrimidine by collisions with CO2

“…"Superexcited" pyrimidine molecules on the verge of breaking apart undergo large amplitude anharmonic motion that is likely to affect the magnitude of the supercollisions. Photodissociation studies performed in our laboratory 45 indicate that the motions leading to dissociation are the same motions responsible for large energy transfer processes. Thus as a molecule approaches threshold, the energy transfer modes ͑the motions that also lead to dissocia-tion͒ become more active.…”

Full State-Resolved Energy Gain Profiles of CO2 (J = 2−80) from Collisions of Highly Vibrationally Excited Molecules. 1. Relaxation of Pyrazine (E = 37900 cm−1)

“…33 Until recently, experiments that measure energy transfer dynamics of highly excited molecules have focused on large ∆E "supercollisions" where energy-accepting bath molecules are propelled to energy states that are well above the initial thermal distribution. A number of experimental [34][35][36][37][38][39][40][41][42][43][44][45][46][47] and theoretical [48][49][50][51][52] studies have addressed the dynamics of large ∆E transitions. Oref, Steel, and co-workers first saw evidence for supercollisions in the isomerization of cyclobutene caused by collisions with excited hexafluorobenzene.…”

“…40,41,44,59,60 Sevy and co-workers have studied the interplay of photodissociation dynamics with collisional deactivation for several donor molecules with CO 2 . 38,39,47 While strong collisions are effective at deactivating highenergy molecules by removing large amounts of energy in single collisions, they occur with very low probabilities. It is far more likely that vibrational relaxation in molecules occurs through sequential weak collisions.…”

“…Until recently, experiments that measure energy transfer dynamics of highly excited molecules have focused on large Δ E “supercollisions” where energy-accepting bath molecules are propelled to energy states that are well above the initial thermal distribution. A number of experimental − and theoretical − studies have addressed the dynamics of large Δ E transitions. Oref, Steel, and co-workers first saw evidence for supercollisions in the isomerization of cyclobutene caused by collisions with excited hexafluorobenzene. , Supercollision dynamics were first characterized by Flynn and co-workers for the pyrazine(E):CO 2 system using transient IR absorption. , The Mullin group has investigated supercollision dynamics for a number of highly excited donor molecules (pyrazine, pyridine, alkylated pyridines, and azulene) with a range of bath molecules (CO 2 , H 2 O, HOD, and DCl) ,,,,− and investigated the energy dependence of supercollisions for pyrazine(E), pyridine(E), and azulene(E) with CO 2 . ,,,, Sevy and co-workers have studied the interplay of photodissociation dynamics with collisional deactivation for several donor molecules with CO 2 . ,, While strong collisions are effective at deactivating high-energy molecules by removing large amounts of energy in single collisions, they occur with very low probabilities.…”

Rotationally Resolved IR-Diode Laser Studies of Ground-State CO2 Excited by Collisions with Vibrationally Excited Pyridine

“…Key to understanding the overall rate of product formation in unimolecular processes is the energy-transfer probability distribution function, P(E,E′), which describes the probability that a molecule initially at energy E′, will possess, following a collision, energy E. 2 Despite the importance of this function, and 85 years of interest in energy transfer and its relationship to unimolecular processes, it has only been in the past 10 years that experimental techniques have measured this function. [3][4][5] The transient IR laser probe technique, developed by Flynn, 6-10 and currently used by and our lab, [21][22][23] is capable of studying the relaxation of relatively complex molecules with chemically significant amounts of energy by collisions with spectroscopically tractable bath molecules. The power of this technique lies in the use of high-resolution spectroscopy to study energy gain by simple bath molecules.…”