Quantification of the (anti)aromaticity of fulvenes subject to ring size

• Published in: Tetrahedron letters Vol. 49; no. 17; pp. 2776 - 2781
• Main Authors: Kleinpeter, Erich, Fettke, Anja
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 21.04.2008
• Subjects: Antiaromaticity; Aromaticity; Fulvenes; GIAO, NICS; MP2 ab initio calculations; Through space NMR shieldings; GIAO, NICS; Through space NMR shieldings; MP2 ab initio calculations; Antiaromaticity; Aromaticity; Fulvenes
• ISSN: 0040-4039
• DOI: 10.1016/j.tetlet.2008.02.137

Spatial magnetic properties (TSNMRS) prove tria- and pentafulvenes to attain negligibly small partial aromaticity via conjugation with the exocyclic CC double bond but heptafulvene to be slightly antiaromatic. Tria-, penta-, hepta- and nonafulvenes (1–4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding πCC orbital of the exocyclic CC double bond, obtained by NBO analysis, quantitatively proves π-electron delocalization which can reveal partial 2-, 6- and 10-π-electron aromaticity, and 4-, 8- and 12-π-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic CC double bond whilst the (anti)aromaticity of the ring moieties of 1–4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).