Triazine-based molecular glasses frustrate the crystallization of barbiturates

Abstract

Hydrogen bonding is a key element in crystal engineering to direct crystal packing. Here, hydrogen bonding with molecular glasses is rather exploited to thwart crystallization.

Comparing C2=O and C2=S Barbiturates: Different Hydrogen-Bonding Patterns of Thiobarbiturates in Solution and the Solid State

“…As for TB, the IR spectra remain almost identical to those at room temperature, similar to the spectra of B in the melt state, indicative for an already dynamic character with more broken H-bonds in thiobarbiturates TB within the whole temperature range. Owing to the strong and ordered H-bonds in barbiturates, barbiturate B presents as a white powder that can form crystals, which is consistent with previous reports to use the H-bonds to engineer its ordering via crystallization [15,50,51]. Thiobarbiturate TB, in contrast, presents itself as a highly viscous liquid at room temperature.…”

“…The surface pressure-mean molecular area (π-mmA) isotherms of the model compounds B and TB are shown in Figure 4a. Owing to the strong and ordered H-bonds in barbiturates, barbiturate B presents as a white powder that can form crystals, which is consistent with previous reports to use the Hbonds to engineer its ordering via crystallization [15,50,51]. Thiobarbiturate TB, in contrast, presents itself as a highly viscous liquid at room temperature.…”

“…For TB , the increase takes place at ~97 Å 2 , explained by the formation of molecular stacks of the film when compressed. After an increase with an almost identical slope, the transition occurs at 71 Å 2 and 73 Å 2 for B and TB , respectively, which could either be explained by crystallization or multilayer formation, since it was reported that the nature of the H-bonds can change during crystallization [ 15 , 50 , 51 ]. As shown in Figure 4 b, several crystalline domains can clearly be observed for B , but most parts of the film remain amorphous.…”

Glass engineering of aminotriazine-based materials with sub-ambient Tg and high kinetic stability

“…Correlations were observed previously between the Tg of aminotriazine molecular glasses and the association constants (Ka) of their NH groups, which are indicative of hydrogen bond strength. 30,50,51 As the constants are measured by variableconcentration NMR spectroscopy in CDCl3 solutions, we can assume that the impact of the weaker NH -π interactions is negligible. The Ka values of compounds 1NHMe-3NHMe (Table 3 and Figure S8) range from 0.2 to 0.4 M -1 .…”

“…These small Ka values are substantially lower than that of their analogues with NH linkers, which typically ranged from 0.7 to 2.1 M -1 . 30,50 This can be explained by the presence of only one NH group located on the headgroup. In our previous work, the headgroup NH Please do not adjust margins Please do not adjust margins typically displayed a weaker Ka than the linker NH groups.…”