CH and CD Stretching Anharmonicities and Radiationless Decay of Triplet Benzene

Abstract

The near‐ir spectra of liquid chloroform and benzene have been analyzed to evaluate the CH and CD diagonal and cross anharmonicity constants. For HCCl₃ and DCCl₃, the diagonal anharmonicities (−124 and −66 cm⁻, respectively) and the rate of intensity decrease of successive overtones show an isotope effect. The more complex C₆H₆, C₆H₅D, C₆HD₅, and C₆D₆ spectra yield both diagonal and cross terms which are −116 and −150 for C₆H₆ and −63 and −87 cm⁻¹ for C₆D₆, respectively) and show the same isotope effects. The analysis indicates that the only CH stretching progression arises from a combination‐overtone, ${\nu }_{20}{\mathrm{(e}}_{\text{1u}}{\mathrm{)\hspace{0.17em}+\hspace{0.17em}n\nu }}_2{\mathrm{(a}}_{\text{1g}})$ , and is due to the considerable anharmonicity associated with an asymmetric potential function. The pattern of anharmonicity in C₆D₆ is greatly changed by replacing one D by an H. The relationship of the CH stretching anharmonicity to the radiationless rate constants derived from phosphorescence lifetimes is discussed.