The effects of fluorine and chlorine substitution on bond lengths in ethanes and disilanes: comparisons of ab initio and experimental information

Ab initio and some density functional theory calculations of bond lengths i n fluoro- and chloro-ethanes and disilanes are reported with a precision of +/-0.0001 Angstrom under strictly comparable conditions. The resulting cha nges in MH and MX (M = C, Si; X = F, Cl) bond length are analysed for the e ffects of halogens substituted in geminal (alpha), or vicinal (gauche or tr ans) positions. The shortening effect of a halogen on an MH bond is markedl y reduced or even reversed by the introduction of electron correlation at t he MP2 or B3LYP level, MX bonds are little affected. gauche halogen consist ently shortens both MH and MX bonds, while trans halogen has no effect on a n MH bond but a small and variable effect on the MX bond. The reality of these calculated changes in bond length is tested in two way s. Mil bond lengths are plotted against experimental values of the isolated stretching frequencies nu(is)MH, which themselves correlate well with expe rimental r(0) bond lengths. Agreement on the resulting substituent effects is generally good for the gauche and trails effects of halogen but variable for a effects. Unobserved nu(is)MH values are predicted from computed bond lengths in fluoroethanes, chloroethanes and chlorodisilanes. Calculated MX and MM bond lengths are compared with experimental values, no tably those from electron diffraction studies amongst the ethanes. Most cal culations underestimate the changes found experimentally in CF and CCl bond lengths. CC bond length changes are underestimated in fluoroethanes and ov erestimated in the chloro-compounds. The 'offset' value (r(e)(calc) - r(e)(true)) for a CH or SiH bond calculate d with a given basis set and level of theory in most cases varies markedly throughout the series of compounds. The same is true for CF, CCl, CC and Si Si bonds if the corresponding offset values for the r(a) lengths are consta nt. The need is stressed for extended experimental work on many of the compound s, especially the disilanes. It is recommended that structures should be re fined with ab initio derived constraints on the bond lengths involved and d ifferences between spectroscopic and diffraction-based geometries reconcile d through the calculation of ri structures. (C) 2000 Elsevier Science B.V. All rights reserved.