MOLECULAR-STRUCTURES OF METHYLDIFLUOROARSINE, CH3ASF2, AND DIMETHYLFLUOROARSINE, (CH3)(2)ASF, IN THE GAS-PHASE AS DETERMINED BY ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

The structures of gaseous CH3AsF2 and (CH3)(2)AsF have been determined by electron diffraction incorporating vibrational amplitudes derived from ab initio force fields scaled by experimental frequencies and, fo r the difluoride, restrained by microwave constants. The following par ameters (r(alpha)(o) structure, distances in pm, angles in degrees) ha ve been determined for CH3AsF2: r(As-C) = 194.6(4), r(As-F) = 173.1(1) , angle CAsF = 95.2(1), angle FAsF = 97.0(1). For (CH3)(2)AsF structur al refinement gives r(As-C) = 195.1(1), r(As-F) = 175.4(1), angle CAsF = 95.3(5), and angle CAsC = 96.9(8). For the series (CH3)(3)As, (CH3) (2)AsF, CH3AsF2, and AsF3, both As-C and As-F bond lengths are shorten ed with increasing numbers off atoms, but the angles CAsF and FAsF are almost invariant.