Phase shift cavity ring down and Fourier transform infrared measurements of C–H vibrational transitions, energy levels, and intensities of (CH3)3Si–C≡C–H

Abstract

Phase shift cavity ring down and Fourier transform IR techniques have been used to observe the C-H stretch fundamental and overtone absorptions of the acetylenic (Δυ = 1-5) and methyl (Δυ = 1-6) C-H bonds of trimethyl-silyl-acetylene [(CH3)3CSi≡CH] at 295 K. Harmonic frequencies ω(ν1), ω(a), and ω(s) and anharmonicities x(ν1), ω(a)x(a), ω(s)x(s) were calculated for the acetylenic, methyl out-of-plane, and methyl in-plane C-H bonds, respectively. The harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. A hot band, assigned as υν1 + ν24 - ν24 is observed for transitions with Δυ = 1-5 in a region near the acetylenic stretch. The intensity of the hot band is reduced considerably at 240 K. The strength of a Fermi resonance between C-Ha transition (υν(a)) and the combination band ((υ-1)ν(a) + 2ν(bend)) with (υ = 3-6) was calculated using the experimental perturbed energies and relative intensities. The main bands are separated by computer deconvolution and are integrated at each level to get the experimental band strengths. For methyl absorptions, the dipole moment function is expanded as a function of two C-H stretching coordinates and the intensities are calculated in terms of the HCAO model where only the C-H modes are considered. Acetylenic intensities are derived with a one dimensional dipole moment function. The expansion coefficients are obtained from molecular orbital calculations. The intensities are calculated without using adjustable parameters and they are of the same order of magnitude of the experimental intensities for all C-H transitions.