The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL-PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH-stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6-31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall-off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.

Gas phase overtone spectra of dimethyl ether (6.V CH = 2-5) and acetone (6.V CH = 3) are measured using conventional near infrared (NIR) spectroscopy. Intracavity dye laser photoacoustic spectroscopy (lDL-PAS) has been used to measure the 6.v CH = 5-7 gas phase spectra of both dimethyl ether and acetone. Oscillator strengths are calculated using a harmonically coupled anharmonic oscillator local mode description to obtain the vibrational wave functions and ab initio molecular orbital (MO) calculations to obtain the dipole moment function. The calculations, which use no adjustable parameters, can account for the magnitude of the intensities and for the falloff in intensity with increasing v, for both molecules. It can also account reasonably well for the relative intensity of various peaks within a given vibrational manifold and for the relative intensity in the spectra of these two molecules and of propane. Rop = 1.0863 A, R,p = 1.0811 A,LH"pCH"p = 107.21°, and LH,pCH"p = 109.54° for acetone.

The liquid phase overtone spectrum of CD2Cl2 is measured in the CD-stretching regions corresponding to ΔvCD=2 to 4, and the peaks are assigned in terms of the local mode model. Oscillator strengths are determined from these spectra and from the previously measured overtone spectra of CH2Cl2. A general theoretical description is developed for the overtone intensities in these molecules. The theory considers the functional dependence of the dipole moment on local coordinates, as determined from CNDO calculations and numerical differentiation techniques. Morse oscillator wave functions are used to describe the vibrational state within a symmetrized local mode description, and matrix elements of these wave functions are evaluated over powers of the local CH/CD coordinate. It is evident that any description of overtone intensities in terms of harmonic oscillator wave functions would be totally inadequate. Vibrational mixing of the symmetrized Morse oscillator product states is detemined from a harmonic coupling model. The theory predicts that vibrational mixing is much more important as a source of intensity for combinations than off-diagonal terms in the local coordinate expansion of the electric dipole moment. Significant contributions to overtone intensity arise from terms involving the first, second, and third derivatives of the dipole moment. The linear and quadratic terms always appear with opposite sign. Comparison of the calculated and experimental oscillator strengths reveals that, although the calculated oscillator strengths are too small, they account reasonably well for the observed exponential fall off in overtone intensity with increasing v, and the greater intrinsic overtone intensity of CH2Cl2 as compared to CD2Cl2. The calculations also account for several detailed features of the individual spectra.

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Oscillator strengths for the CH-stretching components of the overtone spectrum of dichloromethane are calculated for ΔvCH ≤4. The calculations use local mode theory to obtain the vibrational wave functions and SCF theory, with a number of different basis sets, to obtain the dipole moment functions. A comparison with experimental results shows the calculations can reproduce the relative ordering of the oscillator strengths as a function of the particular vibrational state, and give a reasonably quantitative account of the magnitude of the intensities, even with simple split-valence basis sets.