Enhanced red shifts ( -AAvmux) of the [ +R2N==Cr-*-C4=NC>2-] band in the ultraviolet spectrum for 4-nitroaniline relative to A,V-diethyl-4-nitroaniline on going from cyclohexane to oxygen-base solvents are attributable primarily to the bathochromic influence of hydrogen bonding by 4-nitroaniline to these solvents and are considered to be measures of relative solvent-proton affinity. Such AA¡/max values show good correlation with * values of R in a series of ROH solvents which includes water. Good correlations between -AAvmax and * values are also obtained from the following pairs: Ar-ethyl-4-nitroaniline/iV,Ar-diethyl-4-nitroaniline and 4-nitrophenol/4nitroanisole. These probes of relative solvent-proton affinity do not, however, show ethers to be better hydrogen-bond acceptors than alcohols, probably because of steric factors.

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