ABINITIO CALCULATIONS OF THE MILLS-NIXON EFFECT IN INDAN, TETRALIN AND IN RELATED SYSTEMS

Structural properties of indan, tetralin, indene and the indenyl radic al are examined by the 3-21G ab initio procedure. It was found that di stal bonds of the benzene ring in tetralin exhibit a tiny distortion w hich resembles an anti-Mills-Nixon (MN) effect. However, it is so smal l that it is more of conceptual than practical significance. The other three compounds on the other hand show the MN type of deformation but to a different extent. The effect is extremely small in indan and qui te substantial in the indenyl radical. Hence the difference in electro philic substitution reactivity at beta- and alpha-positions in the for mer molecule should be ascribed to features of the transition structur es. Geometric variations are interpreted in terms of rehybridization a t the carbon junction atoms and changes in pi-bond orders. Experimenta l evidence on the structural features is very limited, but available d ata are in accordance with theoretical findings.