Benzene; high level quantum chemical calculations, gas electron diffraction pattern recorded on Fuji imaging plates and a method to explore systematic discrepancies which was used to determine an improved sector correction

The C-C bond distance in molecular benzene is used by several electron diffraction groups to calibrate the electron wavelength in a gas electron diffraction experiment. It is therefore important to compare the applied r(a) value against the currently best available ab initio r(e) result. A high level CCSD(T = Full)/cc-pVTZ calculation, which has proven to give r(e) distances close to the experimental values, gave r(e)(C-C) = 1.392 angstrom. When our wavelength calibration distance r(a)(C-C) = 1.3975 angstrom is corrected for atomic displacements in the curvilinear approach and for anharmonic vibrations, it matches exactly the calculated r(e)(C-C) value. From a B3LYP/cc-pVTZ molecular force field the distance correction terms (d(hn)) and the root-mean-square vibration amplitudes were computed both in the linear and the curvilinear approximations. Gas electron diffraction intensities for benzene were registered on Fuji imaging plates. The data are very reproducible and revealed that systematic discrepancies might be present. A method to investigate systematic errors in electron diffraction is proposed. A multiplicative correction likely due to very small errors in the applied sector correction could be estimated. Applying this new correction the agreement is improved due to the high reproducibility of the imaging plates, however, hardly all systematic errors are removed. The agreement is very good and the errors left are unlikely to be caused by the Fuji imaging plate system. (c) 2006 Elsevier B.V. All rights reserved.