MOLECULAR-STRUCTURE AND CONFORMATIONAL COMPOSITION OF 1,4-DICHLOROBUTANE AND 1,4-DIBROMOBUTANE - A GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO INVESTIGATION

Gas-phase electron diffraction (ED) data for 1,4-dichlorobutane and 1,4-dibromobutane obtained at 60 and 80 degrees C, respectively, together with results from ab initio calculations have been used to determine the structures and conformational compositions of these compounds. For both molecules (1,4-dichlorobutane/1,4-dibromobutane)three conformers: G-AA (25 +/- 12%/22 +/- 36%, respectively), G-G-G-(33 +/- 12%/21 +/- 36%, respectively) and G+AG- (21 +/- 12%/31 +/- 18%, respectively) were found to dominate the conformational composition. These results:are in fairly good agreement with the ab initio results calculated at the HF/6-31G* level for 1,4-dichrorobutane and HF/6-31G*/Binningen or HF/STO-3G** level for 1,4-dibromobutane. The results for the principal distances (r(g)) and angles angle alpha from the combined ED/ab initio study for the G-G-G-conformer of 1,4-dichrorobutane; with estimated 2 sigma uncertainties, are r(C-1-H) = 1.108(7) Angstrom, r(C-2-H) = 1.104(7) Angstrom, r(C-1-C-2) = 1.523(3) Angstrom, r(C2C3) = 1.530(3) Angstrom, r(C-1-Cl) = 1.802(3) Angstrom, angle CCC = 116.3(6)degrees, angle C2C1Cl = 112.4(4)degrees, angle C2C1H = 112.2(34)degrees, angle C1C2H = 109.1(24)degrees. The results for the G+AG-conformer of 1,4-dibromobutane are r(C-1-H) 1.099(15) Angstrom, r(C-2-H) = 1.103(15) Angstrom, r(C-1-C-2) = 1.524(6) Angstrom, r(C-2-C-3) = 1.532(6) Angstrom, r(C-1-Br) = 1.966(8) Angstrom, angle CCC = 113.3(19)degrees, angle C2C1Br = 112.8(7)degrees, angle C2C1H = 110.2(47)degrees,angle C1C2H = 109.0(5).