MOLECULAR-STRUCTURE AND CONFORMATIONAL COMPOSITION OF 1-CHLOROBUTANE, 1-BROMOBUTANE, AND 1-IODOBUTANE AS DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

Gas-phase electron diffraction (ED), together with ah initio molecular orbital calculations, have been used to determine the structure and conformational composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane. These molecules may in principle exist as mixtures of five different conformers, but only three or four of these were observed in gas phase at temperatures of the ED experiments, 18 degrees C, 18 degrees C, and 23 degrees C, respectively. The observed conformational compositions (1-chlorobutane, 1-bromobutane, and 1-iodobutane) were AA (13 +/- 12%, 21 +/- 14%, 19 +/- 17%), GA (60 +/- 13%, 33 +/- 32%, 17 +/- 31%), AG (12 +/- 16%, 8 +/- 12%, <1%), and GG (12 +/- 16%, 38 +/- 34%, 64 +/- 31%). A and G denotes anti and gauche positions for the X-C-1-C-2-C-3 (X = Cl, Br, I), and the C-1-C-2-C-3-C-4 torsion angles. The results for the most important distances (r(g)) and angles (angle(alpha)) from the combined ED/ab initio study for the GA conformer of 1-chlorobutane, with estimated 2 sigma uncertainties, are r(C-1-C-2) = 1.519(3) Angstrom, r(C-2-C-3) = 1.530(3) Angstrom, r(C-3-C-4) = 1.543(3) Angstrom, r(C-1-Cl) = 1.800(4) Angstrom, angle C1C2C3 = 114.3(6)degrees, angle C2C3C4 = 112.0(6)degrees, angle CCCl = 112.3(5)degrees. The results for the GA conformer of 1-bromobutane are r(C-1-C-2) = 1.513(4) Angstrom, r(C-2-C-3) = 1.526(4) Angstrom, r(C-3-C-4) = 1.540(4) Angstrom, r(C-1-Br) = 1.959(8) Angstrom, angle C1C2C3 = 115.3(11)degrees, angle C2C3C4 = 112.8(11)degrees, angle CCBr = 112.1(14)degrees. The results for 1-chlorobutane and 1-bromobutane are compared with those from earlier electron diffraction investigations. The results for the GA conformer of 1-iodobutane are r(C-1-C-2) = 1.506(5) Angstrom, r(C-2-C-3) = 1.518(5) Angstrom, r(C-3-C-4) = 1.535(5) Angstrom, r(C-1-I) = 2.133(11) Angstrom, angle C1C2C3 = 116.8(15)degrees, angle C2C3C4 = 115.3(15)degrees, angle CCI = 110.2(14)degrees. Differences in length between the different C-H bonds in each molecule, between the different C-C bonds, between the different CCH angles, and between the different CCC angles were kept constant at the values obtained from the ab initio calculations.