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Ab initio NMR and NICS Study of Deprotonation of Cycloproparenes

Aromaticity is usually characterized by geometric (lack of the bond length alternation, uniform distribution of bond orders and ring current indices), energetic (stabilization energies) as well as magnetic (NMR chemical shifts, diamagnetic susceptibility anisotropy and exaltation and more recently by the nucleus independent chemical shift, NICS) criteria. As a part of our ongoing studies of cycloproparene chemistry we present here results of ab initio calculations of NMR chemical shifts and NICS values for a series of cycloproparenes and their deprotonated forms. The NICS values are calculated 1 angstrem above the unweighted geometric centre of each of the aromatic rings. All calculations were performed by using GIAO method at the HF/6-31+G*//BVWN5/6-31+G* level of theory. It apears that deprotonation leads to significant change in both 1-H and 13-C NMR chemical shifts and NICS(1) values. The aromatic protons in anions exhibit upfield shifts, whilst 13-C chemical shifts show a more pronounced alternation than in parent molecules. The largest changes in the NICS(1) values are observed in the aromatic ring fused to the three-membered moiety. In anions 6-8, this ring is practically nonaromatic, whereas in 9 its aromaticity is reduced to a considerable extent. In contrast, a slight increase in aromaticity is observed in 10.

NMR; NICS; calculations; ab initio; naphthocyclopropenes; cyclopropaantracenes

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