Internal hydrogen bonding in gaseous 3-aminoacrolein: an electron-diffraction investigation augmented by ab initio calculations of its molecular structure and conformational composition

Abstract

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 6–31 G∗ 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° in all torsion angles, the values of its structural parameters ( r g A ̊ and ∠ α) and mole fraction ( x) with rough estimates of 2σ 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), ∠OCC = 127.0 (32), ∠NCC = 124.2 (57), ∠CCC = 120.7 (28), ∠H 6NC = 119.5 (33), ∠H 7NC = 122.1 (32), ∠H 1.8CC = 119.3 (33), ∠H 10CC = 116.1 (33), ∠CCNH 7 = 180.0, ∠CCNH 6 = 0.0, ∠CCCN = 0.0, ∠CCCO = 0.0, x = 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.