STRUCTURE AND CONFORMATION OF 1,4-DIFFUOROBUTANE AS DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION, AND BY MOLECULAR MECHANICS AND AB-INITIO CALCULATIONS

Gas-phase electron diffraction data at 23 degrees C together with mole cular mechanics (MM3) and ab initio (HF/6-31G, GAUSSIAN 86) calculati ons have been used to determine the structure and conformations of 1,4 -difluorabutane. The object was to ascertain whether effects similar t o the gauche effect in 1,2-difluoroethane, which serves to stabilize t he gauche form with the fluorine atoms in close proximity, could also operate in 1,4-difluorobutane. It was found both theoretically and exp erimentally that the proportion of those conformers having close fluor ine atoms was small, implying the absence of effects similar to the ga uche effect. The conformational composition estimated from the theoret ical calculations is in good agreement with the experimental data. The experimental electron diffraction results constrained by assumptions drawn from the theoretical calculations, ED/MM3 [ED/ab initio], for th e principal distances (r(g)/Angstrom) and angles (angle(alpha)/deg) wi th estimated 2 sigma uncertainties are as follows: r(C-H) = 1.105(3) [ 1.106(3)], r(C-F) = 1.398(2) [1.398(2)], r(C-1-C-2) = 1.513(2) [1.516( 2)], r(C-2-C-3) = 1.537(2) [1.532(2)], angle F-C-C=110.9(3) [111.1(3)] , angle C-C-C = 112.9(4) [112.9(4)], and angle H-C-H = 100(3) [100(3)] .