Mass Spectral Studies on Pyrazaboles

Abstract

The mass spectra of several pyrazaboles of type 1 (R = H) have been studied. The data illustrate that electron impact causes three major fragmentation patterns for the species. Symmetrical cleavage of the pyrazabole followed by further breakdown is common to all compounds. In addition, those pyrazaboles containing H or CH3 at the C atoms of the pyrazole rings undergo an electron impact-induced rearrangement which appears to result in the formation of a species containing a B2N3 ring as a structural entity; subsequent breakdown leads to a B2N2 ring system. The mass spectra of halo-or pseudohalo-substituted pyrazaboles evidence the ready loss of hydrogen halide as a predominant feature; no rearrangement ions are observed.

Spectroscopic Studies on Pyrazaboles

“…This is in contrast to the mass spectra of such pyrazaboles of type 1 with R = R' = H, where three major fragmentation patterns have been observed [3]. Hence, substitution of the boron atoms of pyrazaboles with ethyl groups seems to enhance the stability of the B2N4 ring system toward electron impact.…”

“…In analogy to the previously studied pyrazaboles [3] rearrangement to an ion containing a B2N3 ring system (M/z centered at 215) is also observed; however, this process is very minor although the respective peaks increase in intensity in the lowvoltage spectra.…”

“…The most striking feature of this mass spectrum as compared to those of other pyrazaboles reported ealier [3] (where R = R'= H) is the lack of a molecular ion at M/z 270. Rather, the base peak in the spectrum of la is observed at M/z 243, corresponding to the extremely ready loss of C2H4 from the molecular ion.…”

“…Summarizing the mass spectral data it is apparent that boron substitution of a pyrazabole (1, where R = R' = C2H5) considerably influences the mass spectral fragmentation as compared to compounds of type 1 where R = R' = H. In the latter case, the Unauthenticated Download Date | 6/6/19 10:42 PM mass spectra were characterized by an intense molecular ion and the observation of various rearrangement ions [3], whereas the data on the former type suggest that the loss of boron-bonded hydrocarbon moieties is the predominant feature of the spectra. Not surprisingly, the spectrum of li is intermediate between the two series.…”

“…A wide variety of these chemically extremely stable compounds has been prepared [2] but relatively few studies on the spectroscopic properties of such species are known. In an extension of previous mass spectroscopic studies [3] on some pyrazaboles of type 1 where R = R' = H, the mass spectra of a series of compounds with R and/or R' = C2H5 have been recorded and some nuclear magnetic resonance and ultraviolet data have been collected.…”