Structure and conformation of 1-Bromopropane: A gas-phase electron-diffraction investigation using microwave-spectroscopy data and results from ab initio calculations as constraints |
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Authors: | Kolbjørn Hagen Reidar Stølevik |
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Institution: | (1) Department of Chemistry, University of Trondheim, AVH, N-7055 Trondheim, Norway |
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Abstract: | 1-Bromopropane has been studied by gas-phase electron diffraction (ED) at 24C. Earlier published values for rotational constants from microwave spectroscopy (MW), together with results from ab initio molecular-orbital calculations, have been included in the ED analysis. Two conformers with C-C-C-Br torsion angles of 180 (anti) or 66.0(17) (gauche) have been observed. The results obtained for the bond distances (r
g) and valence angles () from this combined ED/MW analysis, with the ab initio results used as constraints are r (C-H)=1.114(9) å,r(C1-C2)=1.521(5) å,r(C2-C3)=1.535(5) å,r (C1-Br)=1.962(6) å, <(C-C-C)anti,=110.0(11), <(C-C-C)gauche=113.3(11), < (C-C-Br)anti=111.1(6), < (C-C-Br)gauche=112.1(6), <C2-C1-H=112.1 (ab initio value), <C2-C3-H=111.4 (ab initio value), <H-C2-H=107.0 (ab initio value). Error limits are given as 2, where (standard deviation) includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The observed amountof gauche conformer was 64(14)%. Using the entropy difference between conformers obtained in the ab initio calculations, this composition corresponds to an energy difference of E=E
anti —E
gauche=0.03(36) kcal/mol. The results are compared with those earlier obtained for other 1-halopropanes. |
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Keywords: | Electron diffraction structure conformation 1-bromopropane |
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