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 i n the DELTAv(CH)=2-6 regions by conventional near infrared-visible spe ctroscopy, and in the DELTAv(CH)=4-7 regions by intracavity dye/titani um:sapphire, laser photoacoustic spectroscopy (ICL-PAS). Gas phase vib rational overtone spectra of 1,3-butadiene-d, are recorded in the DELT Av(CD)=2-5 regions with conventional spectroscopy and in the DELTAv(CD )=5-8 regions by ICL-PAS. Oscillator strengths are calculated from wav e functions that are obtained from a harmonically coupled anharmonic o scillator (HCAO) local mode model and from a dipole moment function th at is obtained from ab initio calculations. The experimental oscillato r strengths are compared to the values that are calculated for both th e CH- and CD-stretching components of the spectrum. Our simple calcula tions, which contain no adjustable parameters, are in very good agreem ent with the relative intensities of the peaks corresponding to the th ree different CH oscillators in 1,3-butadiene. As expected, the local mode description is not as good for the CD oscillators in 1,3-butadien e-d6. Nonetheless, the calculations can provide a reasonable explanati on of the CD-stretching intensity distribution in the higher overtone spectra of 1,3-butadiene-d6. Small hydrogen impurities in the fully de uterated sample give rise to isolated CH-stretching overtones. The rel ative intensities of the CD peaks and the CH impurity peaks in the 1,3 -butadiene-d6 sample spectra are predicted by the calculations. A comp arison of the 1,3-butadiene-d6 sample spectra in the CH-stretching reg ion with the CH-stretching overtone spectra in 1,3-butadiene dramatica lly illustrates the effects of vibrational coupling between CH oscilla tors.