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Quantitative Raman spectroscopic measurements of the intensity of the v ( N 0 ) symmetric stretching band of NO2+ in aqueous solutions of nitric and sulfuric acid mixtures have been carried out over the temperature range 29S313 K. From the spectroscopic data it was found that the NO,+ species concentration decreased with an increase in temperature. Equilibrium constants for the reaction HNO, + 2H2S04+ NO2+ + H,O+ + 2HS04were determined at 293 and 313 K and the average enthalpy of NO,+ formation was found to be AH = -25 & 5 kJ mol-'.

The Raman spectra of the cyclohexyl halide-thiourea clathrates have been investigated. It is found that whereas cyclohexyl chloride and bromide exist only in the axial conformation in the clathrate, cyclohexyl iodide exists in both the axial and equatorial conformations in the clathrate. Journal of Raman Spectroscopy 4 ( I 976) 285-294. All Rights Reserved Copyright 0 1976 by D. Reidet Publishing Company, Dordrecht-Holland

Resonance Raman spectra (RRS) of azobenzene, 4-aminoazobenzene, 4-methylaminobenzene and 4-dimethylaminoazobenzene in the solid state and in a range of solvents have been recorded. The RRS of the 4-aminoazobenzenes are much more solvent dependent than those of azobenzene. Their electronic absorption spectra suggest that their azo-nitrogens are strongly hydrogen bonded in aqueous media. The RRS were interpreted in terms of intra-and inter-molecular interactions. These molecules probably have a near-planar geometry in both the solid state and in aqueous media owing to intermolecular interactions. The intramolecular interactions of the nNZN orbitals with the n MOs tend to cause ring twisting about the axis of the N=N bond. The RRS of the aminoazobenzenes in non-polar solvents show differences from that of the powder, and these could only be attributed to the formation of non-planar structures. INTRO D U CTI 0 NMany theoretical and spectroscopic studies of azobenzene and its derivatives have been made in recent years, but most studies have been concerned with the changes associated with cis-trans isomerization and acid-base equilibria. Resonance Raman spectroscopy (RRS) has often been used to study these molecules, but it appears that the solvent sensitivity of their spectra has not been fully appreciated. The main purpose of this paper is to present the results of a study of the solvent dependence of the RRS of azobenzene (AB) and its derivatives 4-aminoazobenzene (AAB), 4-methylaminoazobenzene (MAB) and 4-dimethylaminoazobenzene (DAB). The 4-aminoazobenzenes are well known as dyes, indicators and probes. EXPERIMENTAL MaterialsAB and AAB were purchased from BDH Chemicals Ltd and used without further purification. DAB was purchased from Aldrich Chemical Co. and recrystallized, first from ethanol-water (1 : 1) and then from hexanebenzene (10: 1). MAB was prepared from benzene diazonium chloride and N-methylaniline by a method devised by Miller and Baumann.' Equipment and data processingThe RRS were recorded using a Spectra-Physics argon ion laser ( ho = 488.0 nm and power output at sample