Gas-phase structure and conformation of diethyl ketone studied by molecular orbital constrained electron diffraction

Abstract

The molecular structure and conformation of diethyl ketone (3-pentanone), C(3)H 3C(2)H 2C(1)OC(4)H 2C(5)H 3, at 27°C were studied by gas electron diffraction (GED). Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Structural constraints were obtained from RHF/6-31G∗ ab initio calculations. Diffraction data were reproduced best by a mixture of two conformers, trans-trans (TT, dihedral angles F 1(C 4C 1C 2C 3) = F 2(C 2C 1C 4C 5) = 180°) and trans-gauche (TG). The structural parameters of the TT form determined by GED, with error estimates (3σ) in parentheses, are: r g( CO) = 1.217(2) A ̊ , r g( C 1C ) = 1.519(1) A ̊ , r g( C 2C 3 ) = 1.524(1) A ̊ , 〈r g( CH)〉 = 1.103(3) A ̊ , ∠ α CC(O)C = 116.0(6)°, ∠ α C(O)CC = 114.2(6)°, ∠ α C(O)CH = 108.2(7)°, and 〈∠ α C 2C 3H 〉 = 111.4(7)° , where 〈 〉 denotes average value. The difference between r (C 1C) and r(C 2 3) was assumed. Similarly ∠C(O)CH and ∠C 2C 3H were refined in groups. The structural differences between the TT and TG conformers were taken from the 6–31G∗ calculations except for the dihedral angles. The dihedral angles of the TG form were determined to be F 1 = 61(7)° and F 2 = 151(14)°. The relative abundances of the TT and TG conformers are 50(10) and 50%, respectively.