Effects of Mechanical and Electrical Anharmonicities on Local Mode Spectrum

The first and second order dipole moment parameters along the CH bond in benzene, chloroform, and cyclohexane were obtained by deducing the local mode intensities on the basis of the one-dimensional Morse oscillator model. The validity of the one-dimensional model is discussed according to a newly introduced formula which helps us to understand the nature of local mode absorptions. The relative intensities of the first overtones to the other bands were found to be crucially important in determining M2/M1 together with its sign, because the contributions of the mechanical and electrical anharmonicities to M1/M2 are often cancelled out each other accidentally. The M2/M1 values for benzene, cyclohexane, and chloroform were 2.3, 1.45, and −0.93 Å−1 respectively. These values for benzene and cyclohexane were excellently reproduced by the ab initio calculations for benzene and methane respectively.