Chemical ionization mass spectrometry protonation sites

The basicity of simple 4,5-dihydropyrazoles (A -pyrazolines) has been discussed on the basis of protonation at N-1 in the case of 1-unsubstituted, 1-methyl, and 1-phenyl derivatives <2000JP0372>. The pA values of 15 4,5-dihydropyrazoles substituted at N-1 by /i-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl groups were determined. After examining some linear free energy relationships, to discuss these values further, DFT calculations, including temperature effects, were carried out on the parent compounds (no C-substituents) for the 1-unsubstituted, 1-methyl, 1-phenyl, l-/)-nitrophenyl, and 1-(2,4,6-trinitrophenyl) series. These calculations predicted an inversion of N-1 and N-2 basicities between 1-phenyl- and l-(/>-nitrophenyl)-4,5-dihydropyrazoles. Since there were no experimental data for the protonation of 4,5-dihydropyrazoles in the gas phase, chemical ionization mass spectrometry was used to try to determine the structure of protonated 1-methyl- and l,3-dimethyl-5-phenyl-4,5-dihydropyrazoles. The substituent effects and protonation sites of 1-phenyl-3-methyl-5-A-benzylideneaminopyrazoles were studied by NMR and ab initio (6-31G ) MO calculations <1996J(P2)2383>. 

The sites of protonation of aromatic compounds, including the possible three mono fluoronitrobenzenes, have been studied by neutralization-reionization mass spectrometry (NRMS)22. The NRMS experiments on the MD+ species generated by D2 chemical ionization clearly indicated that the D+ attachment takes place to the nitro group rather than to the aromatic ring, as evidenced by the abundant losses of OD and DNO2 (NO + OD )22. 

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