Intensity of CH‐ and NH‐stretching transitions in the overtone spectra of cyclopropylamine

Vapor phase room temperature overtone spectra of cyclopropylamine are measured both with conventional absorption spectroscopy for the lower overtones and with intracavity laser photoacoustic spectroscopy for the higher overtones. Spectra are recorded in both the CH‐stretching (Δv_CH=2–7) and NH‐stretching (Δv_NH=2–6) regions. The relative intensities of XH‐stretching peaks are explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the XH‐stretching modes are neglected. The dipole moment function is expanded in the XH‐stretching coordinates, with the expansion coefficients determined from ab initio molecular orbital calculations. The high number of peaks throughout the spectra makes assignment difficult, but allows the overlap of spectra from different experimental conditions. Thus it is possible to approximately determine the absolute intensities for the higher overtone spectra that are obtained from the laser experiments. The simple intensity calculation, which contains no adjustable parameters, can account very well for the magnitude of the intensities and for the relative intensities of the pure local mode peaks for all XH oscillators in the overtone spectral region from Δv_CH=3 to 7.