A Monte Carlo revisiting of N-methylformamide and acetone

Abstract

Os líquidos N-metilformamida e acetona foram revisitados através de simulações Monte Carlo feitas no ensemble NPT, a 1 atm e 25 °C. As moléculas são rígidas com todos os átomos explicitados e o potencial intermolecular utilizado é o clássico 6-12 Lennard-Jones mais Coulomb. Os g(r)'s teóricos dos dois líquidos foram comparados com aqueles obtidos através de difratometria de nêutrons e simulação de refinamento de estrutura. Os resultados indicam a existência de ligações de hidrogênio determinando a estrutura da N-metilformamida, enquanto que na acetona as correlações são orientadas pelo momento dipolar. A estrutura do líquido N-metilformamida é guiada por um dímero preponderante, no qual as moléculas estão posicionadas de maneira que o ângulo entre os vetores de dipolo é 73°, enquanto que no líquido acetona, que é muito menos organizado, a orientação dos dipolos das moléculas muda de antiparalela a curtas distâncias para mais paralela à medida que a distância aumenta.The pure liquids N-methylformamide and acetone have been revisited via Monte Carlo simulations in the NTP ensemble at 1 atm and 25 °C. The molecules are all-atom rigid structures, and the intermolecular potential used is the classical 6-12 Lennard-Jones plus Coulomb. The theoretical g(r)s of both liquids were compared with those obtained from neutron diffraction and empirical potential structure refinement simulations. The results point to the existence of H-bonds driving the N-methylformamide structure, while in acetone the correlations are dipole moment oriented. The structure of the liquid N-methylformamide is mainly guided by a dimer whose molecules are arranged in such a way that the angle between their dipole moments is 73°, while liquid acetone is much less organized and the orientation of the molecules changes from an antiparallel dipolar correlation at short distances to more parallel alignments of the molecular dipole moments for larger distances.Keywords: Monte Carlo, acetone, N-methylformamide, hydrogen bonds, liquid structure Introduction N-methylformamide (NMF) and acetone (ACT) belong to a group of dipolar organic solvents, which are common media for a variety of important chemical reactions. Table 1 shows some of the most common physical-chemistry properties of the liquids to give an idea of the differences.NMF is one of the simplest molecules among those that include a peptide bond in their structure. The molecule has long been of great interest because of the presence of that structure as a repeat unit in proteins and peptides. The liquid, either pure or as a component of mixtures, has been the subject of studies, theoretically [2][3][4][5][6][7][8][9][10] as well as experimentally. [11][12][13][14][15][16][17][18] The NMF molecules act as proton donors and acceptors via their C=O and H-N groups and consequently form C=O•••H-N hydrogen bonds (H-bond) with each other, the same type of H-bond that is known to play an important role in stabilizing the ordered intramolecular structure of peptides and proteins.19 Moreover, these molecule...

Absolute cross sections for elastic electron scattering from methylformamide

“…Although this restriction is not significant in general for the relatively high energies (as used in the present experimental work), in the case of molecules with a relatively high permanent dipole moment, rotational excitation may become more important, especially at lower incident energies [11,28]. Note that the dipole moment for NMF has been reported to be about 3.86D [32]. Still, it is important to point out that the ICSs for these molecular targets can be rather accurately reproduced by the present low-cost calculations, even down to about 10 eV.…”

A hybrid neutron diffraction and computer simulation study on the solvation of N-methylformamide in dimethylsulfoxide

“…With a "bond length" of 1.6 Å the OD-HN correlation is even shorter than it is in other H-bonded liquids, 24 including water, 42,43,59 ethanol, 58 and NMF itself. 27,34 In the NMF-water mixture the O(H 2 O)-HN peak is located around 1.9 Å. 60 These results point to a NMF-DMSO H-bond stronger than others found in classical H-bonded liquids and imply an extensive solvation of NMF molecules by DMSO.…”

“…25,26 Pure liquid NMF and its aqueous solutions have been the subject of study in recent years, both theoretically and experimentally. [27][28][29][30][31][32][33][34] Figure 1 shows a sketch of both molecules with the dipole moment vector indicated on them.…”

Detailing the Structure of Liquid THF Based on an EPSR Study

“…Differently of other liquids like acetone and N-methylformamide (NMF) [37] or dimethylsulfoxide (DMSO) and water [38], which present an structured pair energy distribution, consequence of existence of pairs with different stability in the liquids, for THF the pair energy distribution curve show no structure. There is a unique band of energy, indicating that the energy relative to the different pairs corresponding to the OCM is encompassed for that band.…”