MOLECULAR-STRUCTURE AND CONFORMATIONAL COMPOSITION OF 1-IODOPROPANE AS DETERMINED FROM GAS-PHASE ELECTRON-DIFFRACTION, MICROWAVE SPECTROSCOPY DATA AND AB-INITIO CALCULATIONS

1-Iodopropane has been studied by gas-phase electron diffraction (ED). Earlier published values for rotational constants from microwave spec troscopy, together with results from ab initio molecular orbital calcu lations (HF/LANL1DZ) have been included in the ED analysis. Two confor mers with C-C-C-I torsion angles of 180 degrees (anti) or 66.1(5) (gau che) have been observed. The results obtained for the bond distances ( r(g)) and valence angles (angle(alpha)) from this combined ED/microwav e spectroscopy analysis, with the ab initio calculations used as const raints, are: (r(C-H)) = 1.110(4)Angstrom, r(C-1-C-2) = 1.521(5)Angstro m, r(C-2-C-3) = 1.537(5)Angstrom, r(C-1-I) = 2.164(8)Angstrom, angle(C -C-C)(anti), = 110.2(9)degrees, angle(C-C-C)(gauche) = 113.9(9)degrees , angle(C-C-I)(anti) = 111.8(4)degrees, angle(C-C-I)(gauche) = 113.0(4 )degrees, angle C-2-C-1-H = 112.4 degrees (ab initio value), angle C-2 -C-3-H = 111.0 degrees (ab initio value), angle H-C-2-H = 109.5 degree s (ab initio value). Error limits are given as 2 sigma where sigma inc ludes estimates of uncertainties in voltage/height measurements and co rrelation in the experimental data. The observed amount of gauche conf ormer is 72(13)%, corresponding to an energy difference of Delta E =E( anti) - E(gauche) = 0.2(4) kcal mol(-1). The results are compared with those obtained earlier for other halopropanes.