TOWARD THE GENERATION OF NO IN BIOLOGICAL-SYSTEMS - THEORETICAL-STUDIES OF THE N2O2 GROUPING

Substituent effects for R[N2O2]H (R = NH2, O-, CH3, H, CF3, and CN) have been studied by ab initio methods and were found to have considerable effect in determining whether or not these derivatives exist as the N-nitroso,N-hydroxylamine 3 or the tautomeric hydroxydiazenium N-oxide 4. When the substituents (R) were NH2, O-, CH3, and H, a clear preference for 4 over 3 at the MP2 level was found, attributable to intramolecular hydrogen bonding and favorable substituent effects. The hydroxydiazenium N-oxide structures 4 were all planar while the N-nitroso,N-hydroxylamine structures 3 were tetrahedral, with the exception of R = O-. For R = H and NH2 it is estimated that hydrogen bonding stabilizes 4 by approximately 4.5 kcal mol(-1). Although hydrogen bonding was also present to the same extent when the substituents were CF3 and CN, little preference for either 3 or 4 was found and for these two substituents it is expected that both tautomers are likely to coexist in protic media. Similar charges for both the interior oxygen (O1) and exterior oxygen (O2) in the corresponding anions, suggest that protonation will occur without preference at either O1 or O2 for all substituents, however, when R = NH2, O-, CH3, and H rapid tautomerism to the thermodynamically more stable 4 is expected. The difference in stabilities between 3 and 4 when the substituent is a nucleophilic residue (for example, NH2 or O-) allows a clear mechanism for the release of NO to be postulated.