Structure of Aggregates of Dialkyl Urea Derivatives in Solutions

Abstract

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.

Solution Conformation and Self‐Assembly of Ferrocenyl(thio)ureas

“…These data support dimerization of 4 and 5 in predominantly cis‐trans conformation through NH a ··· O hydrogen bonds (Figure c), which has been also proposed for an N , N′ ‐diarylurea in CCl 4 . Furthermore, the formation of dimers of 4 and 5 fits with the low reported relative molecular masses (average molecular mass of aggregates/molecular mass of monomer) of aggregates of N , N′ ‐dialkylurea monomers in CHCl 3 . In this monomer/dimer model, the proton chemical shift ( δ ) of the NH a signal is the fraction‐weighted average of the chemical shifts for the dimer ( δ D ) and for the monomer ( δ M ).…”

“…The dimerization constants for 4 [ K = (4.9 ± 0.8) m –1 , δ M = (5.46 ± 0.01) ppm, δ D = (8.18 ± 0.27) ppm, R 2 = 0.9991] and 5 [ K = (4.1 ± 0.3) m –1 , δ M = (5.42 ± 0.01) ppm, δ D = (8.45 ± 0.12) ppm, R 2 = 0.9996] are identical within uncertainty. Furthermore, the dimerization constants ( K ) are similar to those of N , N′ ‐dialkylureas, as determined IR spectroscopically in chloroform from a two‐equilibrium‐constant approach (dimerization/oligomerization) , . The dependence of the chemical shift ( δ ) of the NH b signal on the concentration is small and shows a linear behavior with an extrapolated axis intercept ( δ 0 ) that corresponds to the chemical shift at infinite dilution ( δ 0 = 4.90 ppm for 4 , 5.01 ppm for 5 ).…”

“…[8][9][10][11][12][13] In solution, trans-trans, cistrans, and cis-cis isomers of N,N′-disubstituted (thio)ureas equilibrate and direct the self-assembly process (Scheme 1). [14][15][16][17][18][19] In noncoordinating solvents, ureas prefer the trans-trans conformation, leading to supramolecular polymeric tape structures with bifurcated intermolecular hydrogen bonds (Scheme 1). [17,18] In contrast, thioureas show a significantly reduced aggregation tendency, forming dimers of cis-trans isomers (Scheme 1).…”

Electrical conductivity studies for hydrogen-bonded supramolecular polymer formed by dialkylurea in non-polar solvent

“…[7] A specificity of the ureide unit is related to its two acidic protons which can form with two lone pairs of the oxygen atom of other molecule the bifurcated hydrogen bonds. [8] That circumstance leads to an unusually efficient development of the self-association of N,N -disubstituted ureas into the polymeric chains according to the scheme: [8][9][10][11][12] It is obvious that such type of bifurcated linkages, which effectively are quadruple linkages, make the supramolecular structures much more rigid and energetically more stable in comparison to the structures where the monomers are linked with two, nonbifurcated hydrogen bonds, as it is in the case of the secondary amides, for example.…”

Crystal Structure of a Head-to-Head Bis(cholic)-Urea Dimer

“…The crystal of N,N'-diphenylurea is a nice example. 21 Reasons why the interactions of urea derivatives through hydrogen bonding are important have been commented on elsewhere, 20,22 the formation of inclusion compounds being among them. 23 This property is also characteristic of bile acids 24 and it has been used for practical purposes as the enantioresolution of chiral compounds, [25][26] the formation of polymers, [27][28] and delayed release of drugs.…”