Inelastic Neutron Scattering (INS) Study of Low Frequency Vibrations and Hydrogen Bonding of (E)-2-Hydroxyimino-2-Cyanoacetic Acid Ethyl Ester

Abstract

The title unsubstituted (normal) (E)-2-hydroxyimino-2-cyanoacetic acid ethyl ester (the abbreviated name: (E)-2-cyanoethyl ester-2-oxime or 2-oxime) and its 2-deuterated analogue have been prepared and characterized using infrared, Raman and inelastic, incoherent neutron scattering (IINS) spectroscopy. Molecular structures (molecular conformations) of the compounds are proposed for the vapour state. Four molecular conformers, namely two pairs of the s-cis, s-trans rotational isomers (rotamers) from six lower energetic conformations of E-2-hydroxyimino-2-cyanoacetic ethyl ester, are predominating for the single molecule of the compound. They amount together to almost 100% of the conformational populations of both rotational isomers as it was theoretically calculated using DFT (B3LYP) method with 6-311G++(d,p) basis set. Additionally dimers of 2-oxime were considered using B3LYP/6-31G^* method in order to improve the results obtained for single molecule. In both parent (=N–OH) and deuterated (=N–O–D) molecules of the (E)-2-ethylcyano ester-2-oxime respectively, extra bands corresponding primarily to hydrogen (deuterium) bond vibrations (γ–bending out of plane of hydrogen bond, σ–stretching of hydrogen bond bridge, δ–bending in plane of hydrogen bond, λ–bending of hydrogen bond bridge have been identified, namely σ (N–O–H···O) at 132.8, 212.5, 279, 358.7, 424, 448, 523.6, 872, γ (N–O–H···O) at 334, 564, 592, 745, 820, δ (N–OH···O) at 1461, 1600, 1862cm⁻¹ in the low temperature IINS spectrum, whereas the frequency at 365cm⁻¹ has been assigned as the out of plane (o.o.p.) (γ_O–D···O) vibrational mode. Formation of the O–H···O hydrogen bridge is reflected in shifting of the bands of the stretching and bending vibrations of OH group as well as arising of the bridge vibrations in the low frequency range. Moreover, equilibrium geometries and all harmonic vibrational frequencies of the (E)-2-cyano ethyl ester-2-oxime molecules with potential energy distribution (PED) were also calculated by means of the above mentioned theoretical programs.