On the Role of Methyl Torsional Modes in the Intersystem Crossing Dynamics of Isolated Molecules

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 111; no. 49; pp. 12802 - 12809
• Main Authors: Alvarez-Valtierra, Leonardo, Tan, Xue-Qing, Pratt, David W
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13.12.2007
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp076568c

Rotationally resolved fluorescence excitation spectra of the bands of the S1 ← S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the σ = 0 torsional level and the other associated with the σ = ±1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion−rotation Hamiltonian yields the methyl group torsional barriers of V 6‘ ‘ = 1.56 and V 6‘ = 8.28 cm-1 in 2MP and V 6‘ ‘ = 4.11 and V 6‘ = 58.88 cm-1 in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the σ = ±1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the σ = 0 torsional levels.