The effect of the valent state of the heteroatom on the spectrochemical evidence of conjugation in allyl compounds of sulfur and phosphorus

The effect of valency of the heteroatom on the spectrochemical evidence of the conjugation in allyl derivatives of S^II,VI and P^III,IV was studied by quantum chemical methods. The geometric and electronic structures of thetrans- andgauche-conformers, including those for allyl compounds of P^V with equatorial or axial P-C bonds, were calculated. According to the calculations, inversion of the higher occupied molecular orbitals (MO) and the convergence of their energies resulting from the replacement of weak n,-conjugation by strong ,-conjugation should occur on going from S^II and P^III to S^VI and P^V. Increasing the interaction between the higher occupied MO of the fragments in the non-planar conformation of the molecule causes strengthening of the spectral evidence of conjugation: a decrease in the first ionization potential and a bathochromic shift of the long-wavelength absorption band in the electronic spectrum. The increased relative stability of thegauche-conformers of allyl compounds of hypervalent elements was explained by the through-space interaction between the vacant d-orbitals of the S^VI and P^V atoms and the -orbital of the C=C bond. Charge transfer between the molecular fragments makes the major contribution to the energy of conjugation.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 43–47, January, 1995.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-5566).