STRUCTURE AND CONFORMATION OF 1-BROMOPROPANE - A GAS-PHASE ELECTRON-DIFFRACTION INVESTIGATION USING MICROWAVE-SPECTROSCOPY DATA AND RESULTSFROM AB-INITIO CALCULATIONS AS CONSTRAINTS

1-Bromopropane has been studied by gas-phase electron diffraction (ED) at 24 degrees C. Earlier published values for rotational constants fr om microwave spectroscopy (MW), together with results from ab initio m olecular-orbital calculations, have been included in the ED analysis. Two conformers with C-C-C-Br torsion angles of 180 degrees (anti) or 6 6.0(17)degrees (gauche) have been observed. The results obtained for t he bond distances (r(g)) and valence angles (angle(alpha)) from this c ombined ED/MW analysis, with the ab initio results used as constraints are [r(C-H)] = 1.114(9) Angstrom, r(C-1-C-2) = 1.521(5) Angstrom, r(C -2-C-3) = 1.535(5) Angstrom, r(C-1-Br) = 1.962(6) Angstrom, angle(C-C- C)(anti) = 110.0(11)degrees, angle(C-C-C)(gauche) = 113.3(11)degrees, angle(C-C-Br)(anti) = 111.1(6)degrees, angle(C-C-Br)(gauche) = 112.1(6 )degrees angle C-2-C-1-H = 112.1 degrees (ab initio value), angle C-2- C-3-H = 111.4 degrees (ab initio value), angle H-C-2-H = 107.0 degrees (ab initio value). Error limits are given as 2 sigma, where sigma (st andard deviation) includes estimates of uncertainties in voltage/heigh t measurements and correlation in the experimental data. The observed amount of gauche conformer was 64(14)%. Using the entropy difference b etween conformers obtained in the ab initio calculations, this composi tion corresponds to an energy difference of Delta E = E(anti) - E(gauc he) = 0.03(36) kcal/mol. The results are compared with those earlier o btained for other 1-halopropanes.