Mass spectrometric study of some protonated and lithiated 2,5-disubstituted-1,3,4-oxadiazoles

The mass spectrometric behavior of lithiated derivatives of 2,5-disubstituted-1,3,4-oxadiazoles has confirmed the skeletal rearrangement presented earlier for protonated derivatives. In the case of [M + H] + ions the loss of isocyanic acid was observed and for [M + Li] + ions the loss of lithium isocyanate occurred. On the other hand, benzoyl ions [RCO] + were formed from [M + H] + ions, but not from [M + Li] + ions. Formation of benzoyl ions was in agreement with the differences between bond orders calculated for [M + H] + ions and neutral molecules. From [M + Li] + ions the [RCNLi] + fragment ions were formed, but the formation of [RCNH] + fragment ions from [M + H] + ions was not observed. This result can be explained on the basis of theoretically calculated stabilities of these fragment ions, since the calculated heats of formation of [RCNLi] + ions were found to be substantially lower than those of the respective [RCNH] + ions.