Search

Joint studies by IR spectroscopy, dipole moments, average molecular weight measurements and DFT calculations on the self-aggregation of N , N ′-diallylureas and N , N ′-diallylthioureas in solvents of different polarities were performed. Simultaneous uses of all these methods are required for better understanding the mechanism of aggregation and the effects of different polarity of solvents. In this study also the measurements of IR spectra in polarized light were additionally performed, which gives information on arrangement of aggregates in liquid crystal matrix—built of 4-CN biphenyl derivative. It was demonstrated that in such conditions two forms of dimers the linear and cyclic ones are in equilibrium with different arrangements according the axis of CN group.

The paper consists of the joint studies of average molecular weight, dipole moments and the IR spectra of symmetric dialkyl, from dimethyl to dihexyl, thioureas, performed in function of concentration in low polar solvents, with the aim to determine those substances’ relative ability of self-aggregation and the form of the subsequent aggregates. The studies were accompanied by the DFT calculations at B3PW91/6-31+G(d,p) level. It was demonstrated that association in CCl 4 and benzene is much stronger than in chloroform and 1,2-dichloroethane, which results from the competition of interaction of acidic C H groups of solvent molecules with sulfur atom of thioureas. By way of comparing the association constant with the related values for urea derivatives it was shown that the aggregation ability of thioureas is clearly lower than of ureas. This results from the fact that the basicity of sulfur atom is lower than of the oxygen one. Interesting difference between urea and thiourea derivatives is the dependence of dipole moments on concentration, when in urea derivatives dipole moment systematically grow with concentration, showing predominance of near-linear aggregation, in thioureas with shorter chains dipole moments decrease with concentration. Increase the size of chains leads to some preference of linear aggregation – dipole moments increase in non-active solvents. It can be explained by change the conformation in direction of trans–trans forms. Such conclusion was supported by results of DFT calculations.

The mechanisms of self-aggregation of N-alkyl and N,N'-dialkyl derivatives of urea and thiourea in weakly polar solvents (chloroform and 1,2-dichloroethane) were examined. The C-H···O or C-H···S hydrogen bonds formed with these two acidic solvents compete with the N-H···O or N-H···S hydrogen bonds formed between solute molecules, influencing the self-aggregation of urea derivatives in a particular solvent. The peculiarities of the solvent interactions were discussed and the stronger interaction of chloroform was noted. Aggregation of the N-alkyl derivatives was followed using IR spectroscopy, with two gradual aggregation constants (K1 and K2) determined. The average molecular weight and dipole moments were shown to depend on the concentration, and the form of aggregation was analyzed through the study of the dipole moments. All of the urea derivatives demonstrated an increase in dipole moment with increased concentration, resulting in stronger NH2···O hydrogen bond interactions and leading to linear-type aggregation. Contrastingly, the dipole moments of the mono-N-alkyl-substituted thioureas decreased with concentration. Density-functional theory calculation of these processes showed that reliable results could only be obtained if solvent interactions were considered, with a specific combination of local and bulk effects. It was also shown that going from N,N'-disubstituted to N-monoalkyl derivatives the ability to aggregate increases, which is related to a diminished steric hindrance to hydrogen bonding. Finally, it was demonstrated that the mechanisms of self-aggregation depend on the acid-base properties of the solute, hydrogen bonding to the solvent molecules, and steric interactions of the aliphatic chains.

Combined IR spectroscopy, dipole moment, and average molecular weight measurements and DFT calculations on the self-aggregation of N,N'- and N,N-dialkylureas in solvents of different polarities were performed. It was found that, to acquire a better understanding of the mechanisms of associations, the simultaneous use of all of these methods is required. It was found that symmetric dialkyl derivatives of urea associate much more strongly, giving in CCl 4 even a 12-fold mass of monomers, in contrast to asymmetric ones, where the average molecular weight reaches only a 2-fold mass of monomers. The very strong influence of solvents was discovered. The aggregation is much weaker in more polar chloroform and 1,2-dichloroethane leading to only a 2-fold increase in the average molecular weight. A strong influence of the symmetry of dialkylurea molecules was also found. Dipole moments of symmetric N,N'-dialkylurea increase with concentration, suggesting a rather linear arrangement of vectors in an aggregate. For asymmetric N,N-derivatives dipole moments decrease with concentration. DFT calculations were used to predict the form of aggregation.