INTENSITIES OF CH-STRETCHING AND CD-STRETCHING OVERTONES IN 1,3-BUTADIENE AND 1,3-BUTADIENE-D6

Gas phase vibrational overtone spectra of 1,3-butadiene are recorded in the DELTAv(CH)=2-6 regions by conventional near infrared-visible spectroscopy, and in the DELTAv(CH)=4-7 regions by intracavity dye/titanium:sapphire, laser photoacoustic spectroscopy (ICL-PAS). Gas phase vibrational overtone spectra of 1,3-butadiene-d, are recorded in the DELTAv(CD)=2-5 regions with conventional spectroscopy and in the DELTAv(CD)=5-8 regions by ICL-PAS. Oscillator strengths are calculated from wave functions that are obtained from a harmonically coupled anharmonic oscillator (HCAO) local mode model and from a dipole moment function that is obtained from ab initio calculations. The experimental oscillator strengths are compared to the values that are calculated for both the CH- and CD-stretching components of the spectrum. Our simple calculations, which contain no adjustable parameters, are in very good agreement with the relative intensities of the peaks corresponding to the three different CH oscillators in 1,3-butadiene. As expected, the local mode description is not as good for the CD oscillators in 1,3-butadiene-d6. Nonetheless, the calculations can provide a reasonable explanation of the CD-stretching intensity distribution in the higher overtone spectra of 1,3-butadiene-d6. Small hydrogen impurities in the fully deuterated sample give rise to isolated CH-stretching overtones. The relative intensities of the CD peaks and the CH impurity peaks in the 1,3-butadiene-d6 sample spectra are predicted by the calculations. A comparison of the 1,3-butadiene-d6 sample spectra in the CH-stretching region with the CH-stretching overtone spectra in 1,3-butadiene dramatically illustrates the effects of vibrational coupling between CH oscillators.