Infrared and Raman spectra, r0 structural parameters, conformational stability, and vibrational assignment of 2-cyanoethylamine

► The heavy atom and NH2 structural parameters have been determined for the two most abundant forms. ► Vibrational data indicate the presence of four of the five possible conformers. ► Complete vibrational assignments have been obtained for two lowest energy conformers. ► Energy differences have been predicted for the five possible conformers. ► Experimental enthalpy difference has been determined between the two most abundant conformers. The infrared spectra (4000–200cm−1) of the gas and solid and the Raman spectrum (4000–40cm−1) of the liquid have been recorded. Vibrational fundamentals have been identified for four of the possible five stable conformers and complete vibrational assignments have been made for the Gg and Gt forms where the Capital G is for Gauche for the orientation around the CC bond and the lower cases g and t for gauche and trans orientation for the amine. These forms are the two lowest energy conformers. Vibrational assignments have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, and depolarization ratios. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. The enthalpy differences between the Gg and Gt conformers was determined to be 75cm−1 and for the Gg to Tg form 333cm−1. The r0 structural parameters have been obtained for the two most stable conformers from predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations adjusted to fit the previously reported microwave rotational constants. The determined heavy atom structural values for the Gg [Gt] conformer are: the distances (Å) N1C2=1.461(3) [1.453(3)], C2C3=1.535(3) [1.545(3)], C3C4=1.466(3) [1.463(3)], C4N5=1.161(3) [1.161(3)] and angles (°) ∠N1C2C3=109.5(5) [116.0(5)], ∠C2C3C4=111.1(5) [111.1(5)], ∠C3C4N5=177.4(5) [177.0(5)]. These parameters are compared to those estimated from the microwave study. The results are discussed and compared to the corresponding properties of some similar molecules.