Conformational studies of aminomethylene-malonic acid dimethylester and its N-methyl derivatives using vibrational spectroscopy, X-ray analysis and ab initio calculations

The IR and Raman spectra of aminomethylene-malonic acid dimethylester (AMDME) [NH2CHC(COOCH3)2] and its N-methyl derivatives (MAMDME and DMAMDME) were measured in solid phase and in different solvents at various temperatures. X-ray analysis revealed that AMDME exists in solid phase as EZ conformer, MAMDME as ZZa conformer and DMAMDME as ZE conformer (the first and second E or Z letters express the orientation of the carbonyl oxygen to the CC double bond for trans and cis methylester group, respectively, and the third letter a denotes anti position of methylamino group with respect to the CC double bond). In less polar solutions dominantly two ZZ and EZ conformational forms of AMDME and ZZa and EZa of MAMDME are observed, whereas in more polar environments a third conformational form (ZE and ZEa, respectively) also appeared.The behaviour of DMAMDME is different because there is no intramolecular hydrogen bond and in less polar solutions exists in two ZZ and ZE conformational forms. Very weak indications of a third conformational form (probably EZ conformer) were observed only in more polar surroundings. From the solution IR temperature dependent spectra the energy difference between ZE and ZZ conformers of ΔH = 1.8 ± 0.5 kJ mol–1 in chloroform and ΔH = 4.2 ± 0.5 kJ mol–1 in acetonitrile was estimated with the ZZ one being more stable.The geometries and relative energies of the possible conformers of all three compounds were evaluated using ab initio MP2 and DFT B3LYP methods in 6-31G∗∗ basis set with PCM solvent effect inclusion. The influence of environment polarity on the conformational equilibrium is discussed.