Molecular dynamics of cis/transN-methylformamide liquid mixture using a new optimized all atom rigid force field

Concentration fluctuations and microheterogeneity in aqueous amide mixtures

The relationship between concentration fluctuations and the microheterogeneous status of aqueous amide mixtures is addressed through the molecular dynamics study of three different amides, namely, formamide, N-methylformamide, and dimethylformamide. The computer simulations provide structural evidence that these mixtures exhibit considerable microheterogeneity, in apparent contrast to the experimentally obtained Kirkwood-Buff integrals which indicate that these mixtures should be near ideal. This contradiction is addressed by distinguishing microheterogeneity from concentration fluctuations. The former is the result of mixing H-bonding species under specific constraints due to various bonding possibilities between the molecules, while the second is related to the average relative distribution of the molecules. The relationship between these two different quantities is analyzed and illustrated in terms of the partial site-site structure factors. Small wave-number prepeaks relate to the microheterogeneity while zero wave-number value relates to the concentration fluctuations. A simple analytical statistical model for the microheterogeneity is formulated, which allows to discuss the small wave-number behavior of these structure factors in terms of the kinetics of the transient cluster formation, as observed in the computer simulations.

Density functional theory study of self-association of N-methylformamide and its effect on intramolecular and intermolecular geometrical parameters and thecis/transpopulation

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 B3LYP6-31G* 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 counterpoise 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 (1)H- 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, DeltaE(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 DeltaE(CP) values of the cyclic cis- and trans-homooligomers of NMF is positive for n=2 and 3 but negative for n > or =4.

Liquid Structure and Preferential Solvation of Metal Ions in Solvent Mixtures of N,N-Dimethylformamide and N-Methylformamide

Raman spectra of aprotic N,N-dimethylformamide (DMF) and protic N-methylformamide (NMF) mixtures containing manganese(II), nickel(II), and zinc(II) perchlorate were obtained, and the individual solvation numbers around the metal ions were determined over the whole range of solvent compositions. Variation profiles of the individual solvation numbers with solvent composition showed no significant difference among the metal systems examined. In all of these metal systems, no preferential solvation occurs in mixtures with DMF mole fraction of x(DMF) < 0.5, whereas DMF preferentially solvates the metal ions at x(DMF) > 0.5. The liquid structure of the mixtures was also studied by means of small-angle neutron scattering (SANS) and low-frequency Raman spectroscopy. SANS experiments demonstrate that DMF molecules do not appreciably self-aggregate in the mixtures over the whole range of solvent composition. Low-frequency Raman spectroscopy suggests that DMF molecules are extensively hydrogen-bonded with NMF in NMF-rich mixtures, whereas NMF molecules extensively self-aggregate in DMF-rich mixtures, although the liquid structure in neat NMF is partly ruptured. The bulk solvent structure in the mixtures thus varies with solvent composition, which plays a decisive role in developing the varying profiles of the individual solvation numbers of metal ions in the solvent mixtures.