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

TY - JOUR

T1 - Electron correlation effects on geometries and F-19 shieldings of fluorobenzenes

AU - Webb, G A

AU - Karadakov, P B

AU - England, J A

PY - 2000

Y1 - 2000

N2 - In order to include the effects of electron correlation in ab initio molecular orbital calculations it is necessary to go beyond the single determinant Hartree-Fock (BF) level of theory. In the present investigation the influences of both dynamic and nondynamic correlation effects on the optimised geometries and F-19 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 19F 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 Il and G-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.

AB - In order to include the effects of electron correlation in ab initio molecular orbital calculations it is necessary to go beyond the single determinant Hartree-Fock (BF) level of theory. In the present investigation the influences of both dynamic and nondynamic correlation effects on the optimised geometries and F-19 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 19F 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 Il and G-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.

KW - electron correlation effects

KW - geometry optimisation

KW - F-19 nuclear shieldings

KW - fluorobenzenes

KW - ab initio molecular orbital calculations

KW - locally-dense basis set

KW - RESONANCE CHEMICAL-SHIFTS

KW - PERTURBATION-THEORY

KW - ATOMIC ORBITALS

KW - NUCLEAR

KW - CONSTANTS

KW - TENSORS

KW - MOLECULES

M3 - Article

SN - 0239-7285

VL - 48

SP - 101

EP - 112

JO - BULLETIN OF THE POLISH ACADEMY OF SCIENCES-CHEMISTRY

JF - BULLETIN OF THE POLISH ACADEMY OF SCIENCES-CHEMISTRY

IS - 1

ER -