Spectroscopic Investigation of Subtle CH-Bond Changes between Solid and Vapor Phase Naphthalene

We have recorded CH-stretching overtone spectra of naphthalene- d 0 (N- d 0 ) and selectively deuterated naphthalene- 2, 3, 6, 7 - d 4 (N-β d 4 ) both in the vapor and in the solid phase. We have used intracavity and extracavity laser photoacoustic as well as conventional absorption spectroscopy to measure these overtone transitions. The N- d 0 spectra can be unequivocally assigned on the basis of comparison of the N- d 0 and N-β d 4 spectra. Subtle changes of the two nonequivalent CH-stretching oscillators are observed from the vapor to the solid phase. The peaks assigned to CH β experience a red shift whereas peaks assigned to CH α are blue shifted. These changes are explained on the basis of a condensed phase red shift and a selective blue shift due to repulsive intermolecular steric crowding of CH α in the naphthalene crystal structure.