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 in fluoro- and chloro-ethanes and disilanes are reported with a precision of ±0.0001 Å under strictly comparable conditions. The resulting changes in MH and MX (M=C, Si; X=F, Cl) bond length are analysed for the effects of halogens substituted in geminal (), or vicinal (gauche or trans) positions. The shortening effect of halogen on an MH bond is markedly reduced or even reversed by the introduction of electron correlation at the MP2 or B3LYP level. MX bonds are little affected. gauche halogen consistently shortens both MH and MX bonds, while trans halogen has no effect on an 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 ways. MH bond lengths are plotted against experimental values of the isolated stretching frequencies ν^isMH, which themselves correlate well with experimental r₀ bond lengths. Agreement on the resulting substituent effects is generally good for the gauche and trans effects of halogen but variable for effects. Unobserved ν^isMH values are predicted from computed bond lengths in fluoroethanes, chloroethanes and chlorodisilanes.

Calculated MX and MM bond lengths are compared with experimental values, notably those from electron diffraction studies amongst the ethanes. Most calculations underestimate the changes found experimentally in CF and CCl bond lengths. CC bond length changes are underestimated in fluoroethanes and overestimated in the chloro-compounds.

The ‘offset’ value (r_e(calc)−r_e(true)) for a CH or SiH bond calculated 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 SiSi bonds if the corresponding offset values for the r_a lengths are constant.

The need is stressed for extended experimental work on many of the compounds, especially the disilanes. It is recommended that structures should be refined with ab initio derived constraints on the bond lengths involved and differences between spectroscopic and diffraction-based geometries reconciled through the calculation of r_z structures.