Conformational Aspects of the p-Nitrosophenolate Anion and Related Compounds: NMR Study and ab initio 6-31G Optimizations of Molecular Structures

The p-nitrosophenolate anion, PNPA (3), represents an excellent example of variable π-electron transfer demanded by a network of different hydrogen bonds. The limiting structures of mono-protonation in the solid state and in solution are either the p-nitrosophenol (1) or the p-quinonoxime (2) molecules. For the systems 1 to 3 and the related compounds 4 to 7 are results of ab initio 6-31G HF MO optimizations of molecular structures presented which are calculated additionally in variously rotated conformations. Solid state C-CP/MAS NMR spectra of two PNPA salts 8 and 9 have been recorded at 7.0 Tesla. The protonation of 8 and 9 was studied in solution using a 400 MHz NMR spectrometer recording the variations in H and C NMR chemical shifts on changing the solvent from DMSO-d to D O in steps of 25%. The 2D H- C correlated spectrum leads to unambigous assignment of NMR signals. From the coalescence temperature of H signals of 9 the experimental barrier of rotation in D O was determined as 16.9 ± 0.1 kcal mol . The calculated gas phase rotational barrier for free PNPA is with 26.9 kcal mol much higher, but is lowered by calculations for the Li-salt 6 c to 14.5 kcal mol and to 11.8 kcal mol for the protonated PNP If. The in-plane inversion of PNPA through 3c is calculated to be extremely high with 52.4 kcal mol . The effects of rotation of substituents to orthogonal conformations on calculated energies and on intramolecular distances were studied thoroughly.