Pressure-Dependent NMR Spectroscopy Indicates That Internal Rotation of Gas-Phase Formamide Follows Statistical Kinetics

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 38; pp. 8609 - 8616
• Main Authors: Taha, Angela N, True, Nancy S
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.09.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp001666i

Pressure-dependent internal rotation rate constants for [15N]formamide in gas mixtures containing 1.3 Torr of [15N]formamide and SF6 pressures ranging from 150 to 5225 Torr were determined from analysis of exchange-broadened 1H NMR spectra obtained at 333 K. They agree well with RRKM calculations using experimental vibrational frequencies and rotational constants for the ground state and appropriately scaled 6-311++G** parameters for the transition states. The strong collision model, with a hard sphere diameter of 4.8 Å, yields satisfactory agreement. Activated formamide molecules have an average calculated energy specific rate constant, 〈k(E)〉 , of ca. 5.3 × 109 s-1 and a state density of ca. 7.2 cm-1. Statistical internal rotation kinetics requires activated formamide molecules to have vibrational lifetimes of <200 ps and average intervibrational coupling matrix elements of ≥0.1 cm-1. These results are compared to kinetic studies of similar low-energy unimolecular processes.