The c i s influence of bromine in SF5Br. A combined electron diffraction–microwave study

Abstract

The gas phase molecular structure of SF5Br has been determined by electron diffraction and by joint analyses, including rotational constants of only one isotopic species (32S79Br or32S81Br, respectively). The following parameters (rz values with 2σ uncertainties) were derived in the joint analyses: S–Fe=1.671(2), S–Fa=1.560(6), S–Br=2.232(2) Å and FeSFa=89.3(1)°. These results demonstrate a small cis influence of Br on the SF5 group, i.e., the equatorial bonds (cis to Br) are affected more strongly by the substituent than is the axial bond. Ab initio calculations for SF6 and HSF5 with various basis sets (3-21G, 3-21G*, and 6-31G*) confirm the cis influence of a σ-donating substituent and allow for a qualitative interpretation of the substituent effects. The microwave spectrum has been recorded in the range of the J=28–29 transitions and the ground state transition of the 34S isotopic species has been reassigned (B0=1169.54 MHz for 34S79Br and 1157.10 MHz for 34S81Br). Test calculations demonstrate that geometric parameters derived from spectroscopic data alone (rotational constants and centrifugal distortion constant R6) are highly sensitive to isotopic differences and to the force field used in the calculation of R6.