High-frequency hydrogen-bond stretching vibrations as indicators of the character of molecular aggregation in solutions

A complementary analysis of the theoretically predicted structure, stability, and vibrational spectra of the H-bonded clusters with an emphasis on the high-frequency IR range typical of proton vibrations makes it possible to judge the probable character of molecular aggregation in solutions if it is driven by H-bonding. This is illustrated by an example of pentamethylene diaziridine (PMDA) solutions in CDCl3, the spectra of which involve two characteristic bands with peaks at 3213 and 3265 cm−1, the relative intensity of which depends on the PMDA concentration. The analysis showed that the basic building blocks of the crystal structure are scarcely present in the solutions. Actual aggregates are probably stabilized by bi- and trimolecular joints. At the PMDA–chloroform interface, anti single H-bridges between molecules should prevail. At the low concentrations, syn (twisted) double H-bond bridges between the neighboring molecules screened from the solvent can be formed. Anti (nearly planar) double H-bridges should appear at a concentration no lower than 0.5 M, while adjacent syn and anti joints (arranged successively) can probably be found in the inner parts of the aggregates above a content of 0.8 M. The larger H-bonded rings seem to be formed at still higher concentrations. The ring-like joints are expected to be arranged successively with no fused H-bonded rings, which form the basis of the crystal structure. The increase in the experimental IR absorption within the lower-frequency band with an increase in the concentration of PMDA solutions reflects the better structuring inside PMDA aggregates predetermined by the alternation of anti, equatorial, and syn arrangement of the neighboring hydrophobic pentamethylene fragments. This variant becomes possible in large clusters if they have a helical staircase organization. •High-frequency IR range of N–H stretching vibrations is a fingerprint range for H-bonding patterns in the case of aggregation.•The population of H-bonding motives depending on the solution concentration can be estimated by analyzing experimental and theoretical spectra.•Spectral analysis can be used for characterizing the H-aggregation of solute particles even under the effect of solute-solvent interactions.•Spectral analysis reveals the changes in H-bonding architecture upon dissolution of crystalline solids.