Determination of Heats of Tautomerization of Nitrile—Ketenimine by Mass Spectrometry

Abstract

Tautomerism of some nitriles has been studied by mass spectrometry. The analysis of the corresponding mass spectra has allowed to assign some fragmentations to specific tautomers and to determine heats of tautomerization through temperature effects and electron energy studies. Experimental determinations are supported by theoretical calculations. The joint analysis of mass spectrometry and DFT-B3LYP data indicate that this tautomeric equilibrium can be studied by the experimental spectrometric strategy employed.

Experimental Study of Tautomeric Equilibria of 2-Cyanobenzoic Acids in Gas, Solution and Solid Phase

“…Besides, for GC/MS (Gas Chromatography/Mass Spectrometry) experiments, once the solvent is separated after injection in the injection port of the gas chromatograph, tautomerism mechanisms (intermolecular or unimolecular) would not seem to take place even without chromatographic separation of the tautomers (under the selected experimental conditions). These conclusions are supported by temperature studies at the ion source (negligible effect) and at the injection port of the gas chromatograph (shifts of the relative abundances of tautomer-specific ions are in agreement with the corresponding heats of tautomerization) [4,[8][9][10]. In fact, tautomerism would take place very fast in the injection port of the GC under the working conditions.…”

“…As shown in several papers [4][5][6][7][8][9][10][11] the usefulness of mass spectrometry (and GC/MS) to predict tautomeric behavior is demonstrated here along with additional support provided by the IR and NMR spectroscopy.…”

“…It has been demonstrated in the case of keto-enol tautomerism of a variety of carbonyl and thiocarbonyl compounds [4][5][6][7][8][9][10] that there is no significant interconversion of the tautomeric forms in the gas phase following electron impact ionization in the ion source of the mass spectrometer prior to fragmentation (molecular ions, M +. , do not seem to undergo *Address corresponding to this author at the Laboratorio de Estudio de Compuestos Orgánicos (Laboratorio LADECOR), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calle 47 y 115, B1900 La Plata, Pcia.…”

“…Separation of tautomers in analytical columns is usually very difficult, even when it has been carried out successfully for some compounds (previous work has reported chromatographic separation of the tautomeric forms for -ketoesters [11]). Consequently, the different pathways of fragmentation of the tautomeric forms have to be used for identification of individual tautomers [4,[8][9][10]. For this reason and because of the high similarity between MS (commercial databases) and GC/MS spectra (the GC separation would not contribute to the complex distribution of internal energies of the ions formed in the ion source), the analytical separation has not been considered critical for the present work.…”

Tautomerism of uracil and related compounds: A mass spectrometry study

“…Figure 1 shows the mass spectrum of uracil, in which the spectra of all tautomers in equilibrium (that arrive to the ion source) are overlapped. Previous works carried out on a wide variety of carbonyl and thiocarbonyl compounds [25][26][27][28][29][30][31] support the following two assumptions and they all lead to the conclusion that the observed spectrum is the superposition of all tautomers:…”