H-complexes in the “4-n-alkoxybenzoic acid: 4-pyridyl 4′-n-alkoxybenzoate” system. IR spectroscopy and quantum chemical calculations

Abstract

IR spectra of individual 4-n-dodecyloxybenzoic acid (A) and 4-pyridyl 4′-n-dodecyloxybenzoate (B) compounds as well as IR spectra of 2A:1B and 1A:1B systems were recorded. For the assignment of the experimental vibrational spectra a series of quantum chemical calculations of DFT(B97-D)/6-311++G** level was carried out. Hydrogen-bonded complexes of types A⋯A, A⋯B, as well as various trimers that can be formed at different component ratios in system A-B were simulated. The geometric structure of these complexes was optimized and the vibrational frequencies were calculated. The conclusions on the molecular organization of system A-B for different ratios of the components A and B were based on the interpretation of IR spectra and the analysis of calculated thermodynamic characteristics of self-assembly processes. Thus, it is determined that the system A consists of cyclic A⋯Acycl dimers; in 1A:1B system the H-complexes of A⋯B type are formed. In the 2A:1B system in the process of self-assembly, instead of the complexes A⋯A⋯B and A⋯B⋯A of stoichiometric composition, the complexes A⋯B and A⋯Acycl are formed in the ratio 2:1. These results are confirmed by the dilatometric method data.