Spectroscopy of high vibrational levels in pyramidal molecules: C-H stretching overtones of N(CH₃)₃, P(CH₃)₃, and as(CH₃)₃

Photoacoustic spectra of the overtones of the C-H stretches of N(CH₃)₃, P(CH₃)₃, and As(CH₃)₃ in the gas phase are reported. The observed spectral features are assigned on the basis of the local-mode model. Two bands are observed for each overtone which are assigned to nonequivalent methyl C-H bonds. The higher energy absorption band in each overtone region (Δν = 5, 6, and 7) is assigned to the out-of-plane methyl C-H bonds while the lower energy absorption band is assigned to the in-plane C-H bond. Deconvolution of the overtone bands for Δν(C-H) = 5, 6, and 7 provides additional information with respect to peak position, profile, and line width of the overtone absorptions. Local-mode harmonic frequencies (ω_e) and anharmonicities (ω_ex_e) are obtained from an analysis of the spectra. Data suggest that the major factor contributing to the width of P(CH₃)₃ and As(CH₃)₃ overtone absorptions is the rotational envelope of the molecule while line widths obtained for N(CH₃)₃ indicate there is also a major contribution to the width from homogeneous broadening.