Chemistry of the diazeniumdiolates - 1. Structural and spectral characteristics of the [N(O)NO](-) functional group

Ions of structure X[N(O)NO](-), examples of which have seen increasing use as probes for studying the biology of nitric oxide (NO) over the past decad e, have a varied chemical history spanning nearly two centuries. Neverthele ss, they have not been widely appreciated for their physicochemical similar ities. Here we begin a series of systematic inquiries into the fundamental chemistry of such compounds aimed at identifying both the characteristics t hat justify considering them as a group and the factors that contribute to observed differences in their physicochemical properties. In the present pa per, X-ray structures in which X is SO3- (1), O- (2), Ph (3), and Et2N (5), as well as that of the gem-disubstituted carbon derivative CH2[N(O)NO](2)( 2-) (4), are compared. All their O-N-N-O systems are essentially planar, wi th cis oxygens and an N-N linkage exhibiting considerable double-bond chara cter. The ultraviolet spectrum of the isolated chromophore consists of a re latively intense (similar to6-10 mM(-1) cm(-1) per [N(O)NO](-) group) absor ption at 248-250 nm (for 2 and 5) that is red shifted by through-space Star k interactions (e.g., by similar to 10 nm in 1 and 4) as well as by conjuga tive interaction with X (lambda (max) = 284 run for 3). Infrared and Raman spectra for the widely used pharmacological probe 5 were determined, with a nalysis of vibrational modes being aided by comparison with the spectra of the [N-15(O)(NO)-N-15](-) isotopomer and density functional theory calculat ions at the B3LYP/6-311++G** level. To address confusion that has arisen in the literature resulting from rather widespread use of differing trivial d esignations for this class of compounds, a unifying nomenclature system is recommended in which compounds containing the [N(O)NO](-) moiety are named as diazeniumdiolates. It is hoped that these and other efforts to understan d and predict the physicochemical similarities and differences among differ ent members of the diazeniumdiolate class will aid in reaping their full po tential in the area of rational drug design.