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(C H3)=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 solve nts in the region 4000-50 cm(-1). AE and DMAE can exist only as single conf ormers. From the vibrational and NMR spectra of MAE in solutions, the exist ence 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 betwe en the conformers was measured to be 1.9 +/- 1.3 kH mol(-1) from the NMR sp ectra in DMSO solution. Semi-empirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations usin g a DZP basis set were carried out for all the three compounds. The calcula tions support the existence of two conformers anti and syn for MAE with ant i 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, respecti vely. Finally, complete assignments of the vibrational spectra for all the three compounds were made with the aid of normal coordinate calculations employin g scaled ab initio force constants. The scale factors from the similar amin omethylene propanedinitrile and its N-methyl derivatives were used and a ve ry good agreement between calculated and experimental frequencies was achie ved. (C) 1999 Elsevier Science B.V. All rights reserved.