The energetics of nitric oxide generation upon protonation of diazeniumdiolates

The enthalpies for formation of nitric oxide (NO) upon protonation of gas-phase diazeniumdiolates were examined by combining the enthalpy for loss of two NO molecules from the ion, obtained from collision-induced dissociation (CID) threshold energy measurements, with the gas-phase acidity, DeltaH(acid)(R2NH), of the corresponding amine. CID of the N-methylaniline-substituted diazeniumdiolate ion resulted in the formation of N-methylanilide anion by loss of two NO and the nitrosamine radical anion that results from single NO loss. Simultaneous modeling of the cross-sections for the two channels gave a DeltaH(298) = 1.23 +/- 0.17 eV for loss of two NO molecules. From the dissociation energy of the ion and the gas-phase acidity of N-methylaniline, the enthalpy for formation of the amine and two NO molecules upon protonation of the diazeniumdiolate is determined to be -335.7 +/- 4.4 kcal/mol. CID of the diethyl and piperidyl-substituted diazeniumdiolates gave NO- and N2O2.- ions in addition to the corresponding amide. Attempts to model the data to obtain dissociation energies were unsuccessful. Density functional calculations predict a small substituent effect on the enthalpy on formation of NO upon protonation of diazeniumdiolates in the gas phase, but little difference for the solvated ions.