Conformational stability of ethylenediamine from temperature dependent infrared spectra of liquid xenon solutions, r0 structural parameters, ab initio calculations, and vibrational assignments

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000–400 cm−1) of ethylenediamine, NH2CH2CH2NH2, dissolved in liquid xenon and the infrared spectra in the gaseous phase have been carried out. From these data, three of the possible ten conformers have been identified, and the enthalpy differences have been determined among the most stable g′G′g′ conformer and the second stable conformer, g′G′t, to be 64 ± 6 cm−1 (0.77 ± 0.07 kJ mol−1) and the third conformer, gG′g′, to be 210 ± 19 cm−1 (2.51 ± 0.23 kJ mol−1). The first indicator is the lone pair of electrons on the nitrogen-1 atom with respect to the NCC angle (g = gauche or t = trans) and the second indicator is the NCCN dihedral angle (G = gauche or T = trans) and the third one similar to the first one but with respect to nitrogen-2. The percentage of each conformer at ambient temperature is estimated to be 47 ± 1% of g′G′g′, 36 ± 2% of g′G′t and 17 ± 2% of gG′g′. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-PVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the two most stable conformers. The determined heavy atom structural parameters for the g′G′g′ [g′G′t] conformer are: the distances (Å) N1–C2 = 1.472(3) [1.470(3)], C2–C3 = 1.526(3) [1.532(3)], N4–C3 = 1.464(3) [1.463(3)] and the angles (°)∠N1C2C3 = 109.7(5) [109.7(5)], ∠N4C3C2 = 109.5(5) [115.3(5)] and τN1C2C3N4 = 63.5(5) [59.7(5)]. Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies and infrared intensities for all three conformers. The results are discussed and compared to the corresponding properties of some similar molecules.