The vibrational and NMR spectra, conformations and ab initio calculations of 1-aminoethylidene propanedinitrile and its N-methyl derivatives

The IR and Raman spectra of 1-aminoethylidene propanedinitrile (AE) [H2N-C(CH3)=C(CN)(2)], 1-(methylamino)ethylidene propanedinitrile (MAE) [CH3NH-C(CH3)=C(CN)(2)] and 1-(dimethylamino)ethylidene propanedinitrile (DMAE) [(CH3)(2)N-C(CH3)=C(CN)(2)] were recorded as solids and solutes in various solvents in the region 4000-50 cm(-1). AE and DMAE can exist only as single conformers. From the vibrational and NMR spectra of MAE in solutions, the existence of two conformers with the methyl group oriented anti and syn towards the double C=C bond were confirmed. The enthalpy difference Delta H-0 between the conformers was measured to be 1.9 +/- 1.3 kH mol(-1) from the NMR spectra in DMSO solution. Semi-empirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all the three compounds. The calculations support the existence of two conformers anti and syn for MAE with anti being 9.4 kJ mol(-1) more stable than syn from ab initio and 7.4, 12.0, 7.8 and 9.2 kJ mol(-1) from AM1, PM3, MNDO and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all the three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The scale factors from the similar aminomethylene propanedinitrile and its N-methyl derivatives were used and a very good agreement between calculated and experimental frequencies was achieved. (C) 1999 Elsevier Science B.V. All rights reserved.