Study on the molecular geometry of phosphorescent pyrazine by optical detection of zero-field magnetic resonance and T ← S excitation spectroscopy

Abstract

The phosphorescent state of pyrazine has been studied with the T → S emission and T ← S excitation methods in combination with microwave techniques in the systems of pyrazine-h ₄ in pyrazine-d ₄, pyrazine-h ₄ and -d ₄ in p-dioxane, p-xylene, and benzene, and pyrazine-h ₄ x-trap. The three out-of-plane hydrogen bending vibrations of v _10a , v ₅ and v _17a are found to be active in the T_x sublevel emission of the isotopic mixed crystal. By modifying the force constants used by Scully, the frequencies of the out-of-plane vibrations were reexamined and the result was compared with the observed frequencies.

The microwave-induced changes in the phosphorescence intensity were used as a measure of the relative populations in the triplet sublevels, when the system was excited with light having the wavelength corresponding to a particular vibronic band of the T ← S absorption. In this way, the vibrational structure of the absorption spectra was successfully analysed. The overtone bands of v _10a and v ₅ were observed strongly in the excitation spectra of pyrazine crystals. This observation and others lead to the conclusion that the potential function of phosphorescent pyrazine has two double minima along the v _10a and v ₅ normal coordinates in the crystal. The analysis of the potentials showed that the C-H bonds of pyrazine lean by about 4° (=4(π/180) rad) out of the molecular plane in the ³ B _3u state in the crystal.