Density functional theory study of self-association of N -methylformamide and its effect on intramolecular and intermolecular geometrical parameters and the cis / trans population

The single-point total energy ( E ) of several acyclic and cyclic oligomers of N -methylformamide (NMF) was computed by the first time without any geometrical restriction, using the B 3 LYP ∕ 6 - 31 G * method of the density functional theory in order to determine the effect of self-association on intramolecular geometrical parameters of cis - and trans -NMF, the intermolecular distances of the hydrogen-bonding chains formed by NMF as well as intermolecular association energies including couterpoise corrections. It is concluded that liquid NMF exists mainly as polymers formed by self-association of trans -NMF units, whereas the cis -NMF isomer occurs as isolated units inserted along the chains. These computational results are in accordance with the experimentally determined predominance (ca. 90%) of trans -NMF population by means of H 1 - NMR and other spectroscopic techniques, but in severe contradiction with a recent interpretation of x-ray diffraction data on liquid NMF, postulating a cyclic trimer of cis -NMF ( c - C 3 ) as the predominating species. The counterpoise-corrected values of the association energy, Δ E ( CP ) , calculated for cyclic oligomers, increase with the polymerization degree ( n ) revealing a high grade of cooperative effect for amidic hydrogen-bonded chains. Noteworthy, the difference between the Δ E ( CP ) values of the cyclic cis - and trans -homooligomers of NMF is positive for n = 2 and 3 but negative for n ⩾ 4 .