Knowledge Bank

The pure chlorine nuclear quadrupole resonance (NQR) absorption frequencies of several crystalline chlorobenzenes and their deuterium containing analogues have been measured at various temperatures between $77\circ K$ and their respective melting points. Raman spectra of single crystals of some of these compounds have been obtained near room temperature. Thermal averaging of chlorine quadrupolar parameters is dominated by librational lattice modes and low frequency intramolecular chlorine bending motions. The greater mass of the deuterium containing compounds produces smaller vibrational amplitudes and less effective averaging of quadrupolar parameters, leading to prediction of higher NQR frequency for any deuterated compound than for the corresponding hydrogen compound at any temperature. In earlier work, the opposite effect was observed. When deuterium is substituted for hydrogen, both increases and decreases of NQR frequency are observed in the present work; in the case of p-dichlorobenzene, the NQR frequency decreases in the monoclinic phase but increases in the $\gamma$ phase. The theoretical discussion given attempts to correlate the Raman and NQR data obtained without invoking the hypothesis of deuterium substitution induced electronic structure changes.