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1 Bulletin of the Polish Academy of Sciences. Chemistry

1 2000

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Electron correlation effects on geometries and 19F shieldings of fluorobenzenes

Webb G. A., Karadakov P. B., England J. A.

Bulletin of the Polish Academy of Sciences. Chemistry

2000

Vol. 48, No. 1

101-112

In order to include the effects of electron correlation in ab initio molecu- lar orbital calculations it is necessary to go beyond the single determinant Hartree-Fock (HF) level of theory. In the present investigation the influences of both dynamic and nondynamic correlation effects on the optimised geometries and F nuclear shielding calculations of the twelve fluorobenzenes are reported. The non-dynamic electron correlation effects are represented by complete-active space self-consistent field (CASSCF) calculations. Second- and fourth-order Moller-Plesset (MP2 and MP4) calculations are used to describe the dynamic electron correlation effects. Some density-functional (DFT) results are also reported which do not distinguish between dynamic and non-dynamic electron correlation. Following the correlated geometry optimisations F nuclear shielding calculations were performed using the gauge-included atomic orbitals (GIAO) procedure, these were undertaken with wavefunctions which include various levels of electron correlation including HF, CASSCF and MP2. For the calculations of the optimised geometries, and some of the nuclear shieldings the 6-31G** basis set is used whereas the locally-dense [6-31G** on C and H and 6-311++G(2d,2p) on F] set is used for some of the shielding calculations. A comparison of the results of HF shielding calculations using other basis sets is included. Comparison of the calculated geometry and shielding results with relevant, reported, experimental data is made