Conformational stability of allyl amine from temperature dependent infrared spectra of rare gas solutions, ab initio calculations, ro structural parameters, and vibrational assignment

Variable temperature (-55 to -150degreesC) studies of the infrared spectra (3500-60 cm(-1)) of allyl amine (3-aminopropene), CH2=CHCH2NH2, dissolved in liquid krypton and/or liquid xenon have been carried out. From these data it has been possible to identify four of the possible five stable conformers as well as to determine their relative stabilities. The order of the conformers stabilities has been determined to be where the first indicator is for the relative position of the amino group (C = cis or G = gauche) to the double bond (rotation around the C-C bond) and the second one (t = trans, g = gauche, g' = gauche) is the relative position of the amino rotor, i.e. rotation around the C-N bond: Ct, Gt approximate to Gg, Cg, and Gg. The enthalpy differences have been determined between the most stable Ct conformer and the second most stable rotamer, Gt, to be 92 +/- 8 cm(-1) (1.10 +/- 0.10 kJ/mol), and the third most stable rotamer, Gg, to be 122 +/- 12 cm(-1) (1.46 +/- 0.14 kJ/mol) with the Cg having a significant larger value of 173 +/- 12 cm(-1) (2.07 +/- 0.14 kJ/mol) than either of these forms. No evidence was obtained for the fifth conformer Gg' which is predicted by most of the ab initio calculations to be less stable by more than 600 cm(-1) than the Ct form. The percentage of each conformer at ambient temperature is estimated to be: Ct (23%), Gt (29%), Gg (25%) and Cg (20%). Utilization of diffuse functions for the ab initio calculations gives a distinctive preference for the gauche conformer for both the amino torsion and the relative position of the NH2 group to the double bond. Therefore it is not possible to predict the conformer stability order from the ab initio calculations. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/6-311 + G(2df,2pd) and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for six isotopomers of the Ct conformer and one isotopomer for the Gt, Gg and Cg.forms along with ab initio predicted structural values, r(0) parameters have been obtained for all four observed-conformers. The results are discussed and compared to the corresponding properties of some similar molecules. (C) 2002 Elsevier Science B.V. All rights reserved.