Application of 4-hydroxy-, 4-oxo-, and 4-hydroxyimino derivatives of stable 2,2,6,6-tetramethylpiperidine N-oxyl radical for the measurements of Lewis acidity of solvents

UV-VIS spectra of 2,2,6,6-tetramethylpiperidine N-oxyl (TMPNO) stable radic al measured in 18 solvents were compared with the spectra of its derivative s functionalized in the 4 position (4-hydroxy- (h-TMPNO), 4-oxo- (o-TMPNO) and 4-hydroxyimino-(i-TMPNO)). The results obtained proved, that the lambda (max) values corresponding to the n --> pi* transition in the N-O-. chromop hore are for all TMPNO derivatives intercorrelated, and they also correlate very well with other acidity parameters (Z, E-T and AN). Thus, all these c ompounds could be used as standard bases for determination of the experimen tal acidity of solvents. Detailed analysis of the spectral characteristics of all compounds leads to the conclusion, that the molar absorptivities (ep silon) of TMPNO and h-TMPNO corresponding to the n --> pi* transition are o f the order of 10 dm(3) mol(-1) cm(-1), whereas those of the o-TMPNO and i- TMPNO are two times lower. In the case of the n --> pi* transition we obser ved an opposite behaviour - the molar absorptivities (epsilon) of TMPNO and h-TMPNO are close to 2x10(3) dm(3) mol(-1) cm(-1), whereas those of the ox o and hydroxyimino derivatives are equal to similar to 4.4x 10(3) dm(3) mol (-1) cm(-1). The integral intensities of both bands behave accordingly. The se differences were attributed to the formation of the head-to-tail dimers, in which the N-O-. chromophore interacts with the exocyclic double bond in the 4-position of the piperidine.