Chlorine NQR and Phase Transitions in NOCl

Nitrosyl chloride NOC1 has previously mainly been studied in the gaseous state by different methods, e. g. the microwave technique [1, 2]. It is suitable as a model compound for studying the N 0 and N-Cl chemical bonding. The gas phase investigations indicate a considerable degree of ionic character of the N-Cl bond [3]. Vibrational spectra in the solid [4, 5] show an even increased ionicity. and below 85 K the appearance of new low frequency Raman lines suggests a solid phase with completely ionized Cl atoms and NO groups [5]. Because of the reactivity of NOC1. samples are usually kept in sealed glass tubes at lower temperatures. At room temperature the vapor pressure rises to about 2 bars and the compound slowly decomposes. The red liquid crystallizes at 212 K to ruby-red crystals of space group Cmcm. Upon cooling the color gradually lightens, and at liquid nitrogen temperature the material is said to change into bright-yellow microcrystalline "snow"' [5], The facts mentioned above, and recent X-ray diffraction measurements [6| show that this compound deserves more attention. We have undertaken Cl NQR investigations in order to learn more about the N-Cl and N 0 chemical bonding in solid NOC1. where there are also indications of interesting structural transformations.