CH and CD Stretching Anharmonicities and Radiationless Decay of Triplet Benzene

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, ν20(e1u) + nν2(a1g), 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.