Electron correlation effects on geometries and F-19 shieldings of fluorobenzenes

In order to include the effects of electron correlation in ab initio molecu lar orbital calculations it is necessary to go beyond the single determinan t Hartree-Fock (BF) level of theory. In the present investigation the influ ences of both dynamic and nondynamic correlation effects on the optimised g eometries and F-19 nuclear shielding calculations of the twelve fluorobenze nes are reported. The non-dynamic electron correlation effects are represen ted by complete-active space self-consistent field (CASSCF) calculations. S econd- and fourth-order Moller-Plesset (MP2 and MP4) calculations are used to describe the dynamic electron correlation effects, Some density-function al (DFT) results are also reported which do not distinguish between dynamic and non-dynamic electron correlation. Following the correlated geometry op timisations 19F nuclear shielding calculations were performed using the gau ge-included atomic orbitals (GIAO) procedure, these were undertaken with wa vefunctions which include various levels of electron correlation including HF, CASSCF and MP2. For the calculations of the optimised geometries, and s ome of the nuclear shieldings the 6-31G** basis set is used whereas the loc ally-dense [6-31G** on C and Il and G-311++G(2d,2p) on F] set is used for s ome of the shielding calculations. A comparison of the results of HF shield ing calculations using other basis sets is included. Comparison of the calc ulated geometry and shielding results with relevant, reported, experimental data is made.