The structure of the dihydroxyphenylselenonium ion (C_{6}H_{7}O_{2}Se^{+}) has been determined in its benzenesulfonate (C_{6}H_{5}O_{3}Se^{-}) and p-toluenesulfonate (C_{7}H_{7}O_{3}S ^{-}) salts. Whereas the former salt is disordered, the latter less dense salt is well defined. This difference in crystallization behaviour is attributed to a C—H...O hydrogen bond involving the methyl group of the p-toluenesulfonate ion. The two salts display very similar hydrogen-bond arrangements and differ only with respect to the stacking of the phenyl groups. The dihydroxyselenonium ion is a strong acid with a pK value of −0.9 determined from the variation of the 77Se chemical shift. A comparison with the two deprotonated species reveals a systematic increase in the Se—O bond lengths and the pyramidal configuration around Se with the number of protons attached.
Some aspects of the NMR chemical shift/structure correlation in the structural characterization of polymers and biopolymers
