Internal Hydrogen Bonding In Gaseous 3-Aminoacrolein: An Electron-Diffraction Investigation Augmented By Ab Initio Calculations Of Its Molecular Structure And Conformational Composition

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Journal Of Molecular Structure


The molecular structure of 3-aminoacrolein has been investigated in the gas phase by electron diffraction, aided by ab initio molecular orbital calculations at the HF/6-31G* level and normal coordinate analyses. Of the four possible planar or near planar conformers cis/s-cis (CSC), trans/s-cis (TSC), trans/s-trans (TST), and cis/s-trans (CST) where the symbols refer respectively to orientations about the C=C and C-C bonds, CSC is found to be the overwhelmingly dominant conformer in agreement with the MO results. Assuming planarity of the CSC form, which is consistent with the theoretical results to within 0.1 degrees in all torsion angles, the values of its structural parameters (r(g)/Angstrom and angle(alpha)) and mole fraction (chi) with rough estimates of 2 sigma uncertainties are [r(NH)] = 1.002 (26), [r(C-H)] = 1.086 (20), r(C=O) = 1.232 (7), r(C-N) = 1.358 (26), r(C=C) = 1.363 (31), r(C-C) = 1.424 (18), angle OCC = 127.0 (32), angle NCC = 124.2 (57), angle CCC = 120.7 (28), angle H6NC = 119.5 (33), angle H7NC = 122.1 (32), angle H18CC = 119.3 (33), angle H10CC = 116.1 (33), angle CCNH7 = 180.0, angle CCNH6 = 0.0, angle CCCN = 0.0, angle CCCO = 0.0, chi = 0.98 (41). These values are reasonable, but should be accepted with care because of data contamination caused by sample decomposition during the experiments. However, although the measured composition is subject to considerable experimental uncertainty, the combined theoretical and experimental evidence leaves no doubt that only very small amounts of conformers other than CSC can be present. The relative stability of the CSC form is surely due in large part to the formation of a N-H ... O internal hydrogen bond. (C) 1998 Elsevier Science B.V.

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