4-Nitropyrazole: a nitrogen or an oxygen base in the gas phase?

Protonated 3- and 4-nitropyrazoles were subject to collisional activation a nd neutralization-reionization mass spectrometric studies using a large sca le tandem mass spectrometer. The gas phase basicities of 2- and 4-nitroimid azoles were determined by means of Fourier transform ion cyclotron resonanc e spectroscopy. These and other neutral and protonated molecules were studi ed by ab initio methods up to and including the CCSD(T)/6-31 + G* level. Th is information was used to assess the site of protonation of 3-nitro- and 4 -nitropyrazole in the gas phase: at the equilibrium these compounds protona te on the heterocyclic nitrogen rather than on the oxygen of the nitro grou p. Copyright (C) 1999 John Wiley & Sons, Ltd.