HF, MP2 and DFT calculations and spectroscopic study of the vibrational and conformational properties of N-diethylendiamine

The conformational stability and vibrational modes of the N-diethylendiamine organic cation (N-DD2+) were studied by experimental (Raman) spectroscopy combined with theoretical calculations. Various ab initio theories were used: Hartree–Fock (HF) theory, Möller–Plesset second-order perturbation (MP2) theory and density functional theory (DFT). Three stable conformers of N-DD2+, trans–trans, gauche–gauche and gauche–trans were calculated. A comparison between the computed structural parameters of the conformers at both levels of theory and the X-ray data was made. It is demonstrated that the N-DD2+ cation adopts more probably the gauche–gauche conformation at room temperature. In order to make a more detailed interpretation of the low temperature phase transition of N-DDHP, the Raman spectra of N-DDHP were recorded at room and low temperature in the 200–3400 cm−1 region. The vibrational frequencies of the different conformers of N-DD2+ were also calculated using the DFT/B3LYP (6-31G(d)) level of theory. By comparison between the experimental and theoretical results, the conformational dynamic of the N-DD2+ organic cation was confirmed. It is shown that the N-DD2+ cation configuration changes from gauche–gauche conformer to gauche–trans conformer when decreasing the temperature.