density functional calculations
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3H3NS and C3H3NS.+ isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol-1, it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol-1. The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCH=CHNC (4), which is 34.5 kcalmol-1 higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol-1). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol-1) relative to their singlet states. Interestingly, in addition to 2.+ and 4.+, the cyclic radical cation 3.+ is predicted to be stable although it is substantially higher in energy than ionised thiazole 1.+ (by 53.9 kcalmol-1), whereas 2.+ and 4.+ are much closer in energy (only 10.2 and 27.0 kcalmol-1 higher, respectively). Dissuading 2.+ and 3.+ from isomerising to 1.+ are energy barriers of 52.6 and 15.3 kcalmol-1, respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2.+, which could be differentiated from 1.+ by collisional activation mass spectrometry. Reduction of the ylide ion 2.+ in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.
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