ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.448881
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