Very Slow Intramolecular Vibrational Energy Redistribution (IVR) for Molecules in Planar Conformations

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2001-07, Vol.105 (28), p.6800-6807
• Main Authors: Engelhardt, Caitlin, Keske, John C, Rees, Frances S, Self-Medlin, Yehudi B, Yoo, Hyun S, Pate, Brooks H
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp010300i

Evidence is presented that shows very slow intramolecular energy redistribution (IVR) in the acetylenic C−H stretch fundamental of molecules in planar conformations. Each molecule,4-chlorobut-1-yne, 4-bromobut-1-yne, 4-fluorobut-1-yne, methyl propargyl ether, 1-pentyne, and pent-1-en-4-yne have a planar trans conformer (except for pent-1-en-4-yne where the planar conformer is cis) that is connected to a nonplanar gauche (skew) conformer by rotation about a C−C bond (C−O for methyl propargyl ether). The planar forms (observed for each molecule except methyl propargyl ether) of these molecules exhibit some of the slowest hydride stretch IVR rates measured, with τIVR for the acetylenic C−H stretch ranging from 1 to 3 ns. These results are compared to the IVR lifetimes for the nonplanar conformers of 1-pentyne and methyl propargyl ether (τIVR ≈ 300 ps). Also presented are a series of molecules ((Z)-pent-3-en-1-yne, (E)-pent-3-en-1-yne, 1-butyne, 3-fluorobut-1-yne, and 2-methyl-1-buten-3-yne) that have a single methyl group substituent and do not form different conformations upon rotation about the C−C bond. The barrier to internal rotation of the methyl tops range from 389 to 2016 cm-1; however, the IVR lifetimes of the acetylenic C−H stretch for each is near 100 ps.