3,5-Dimethylpyrazole Complexes with Strong Carboxylic Acids

Abstract

3,5-Dimethylpyrazole and halogen-containing carboxylic acids in solution give rise to equilibrium between the free molecules and 1 : 1 and 1 : 2 H-complexes. These complexes either have a ion pair-like structure with two equivalent hydrogen bonds O ! ...HN + (1 : 1) or include homoconjugate anion with strong hydrogen bond C3O3H...O3C (1 : 2). At low temperatures, 2 : 1 complexes were detected. The structure and harmonic vibration frequencies of the complexes were calculated in terms of the density functional theory.An important role of strong hydrogen bonds in enzymatic catalysis [133] and molecular self-organization [436] stimulates studies in this line. The problem is complicated by the fact that H-bond systems involved in both cases strongly interact with each other and are dependent on the molecular environment. Pyrazoles are promising model compounds and convenient components for building up supramolecular systems. Due to the presence of an =NN(H) fragment, pyrazole derivatives possess a unique ability to form various self-associates (linear chains, cyclic dimers, trimers, and tetramers, and complexes with proton donor and proton acceptors) in the gaseous phase, in crystal, and in solution [7310]. Structural diversity of such associates and considerable differences in their spectral parameters make it possible to examine the effect of substituents on the geometry and thermodynamic properties of the complexes, interactions between several hydrogen bonds, and specific behavior of IR absorption bands typical of N3H stretching vibrations, chemical shifts and coupling constants in NMR spectra, and H/D-isotope effects.

Density Functional Study of the Proton Transfer Effect on Vibrations of Strong (Short) Intermolecular O−H···N/O−···H−N+Hydrogen Bonds in Aprotic Solvents

“…The present study deals with strong (short) intermolecular O−H···N bonds in the 1:1 hydrogen bonded complexes formed by benzoic acids and phenols with pyridines. In the gas phase the bridging (shared) proton is always located near oxygen and the only state without proton transfer (HB state) exists. , In the condensed phase the bridging proton shifts along the hydrogen bond (H-bond) toward the base. − In the extreme case the proton transfer state (PT state) is formed. − This state is characterized by a large value of the dipole moment and can exist only by being stabilized by an external electric field or a system of coupled H-bonds. Thus, the PT state can appear as a local or the global minimum on the potential (free) energy surface only as a result of the environmental effect.…”

“…The frequency and intensity of IR transitions attributed to the OH stretching vibration are used in numerical correlation to estimate geometry, energy and other features of the HB state. , The situation is different for the PT state. Only a limited number of compounds with the O − ···H−N + fragment were described in detail. ,,− The reason is that this fragment does not exist in the gas phase , and often cannot be identified in solution. , Thus, it is of significant interest to inspect the spectroscopic properties of the PT state for different 1:1 complexes with a strong (short) intermolecular H-bond.…”

“…Only a limited number of compounds with the O -• • • H-N + fragment were described in detail. 7,10,[19][20][21][22] The reason is that this fragment does not exist in the gas phase 1,2 and often cannot be identified in solution. 8,23 Thus, it is of significant interest to inspect the spectroscopic properties of the PT state for different 1:1 complexes with a strong (short) intermolecular H-bond.…”

Columnar Liquid Crystalline Self‐Assembly of Hydrogen‐Bonded Rod–Coil Diblock Complexes Created from Pyrazole/Benzoic acid Derivatives

“…As a result, it forms a two‐fold helical chain‐like structure, known as a catemer. In addition to self‐assembly of itself, it has been observed that the 4‐dimethyl‐1 H ‐pyrazole unit can form hydrogen bonds with various carboxylic acids . This shows that the pyrazoles can adopt cyclic structures or open‐chain dimers via hydrogen‐bonded pair complexes.…”

Structure of adducts of the intermolecular interaction of dimethylpyrazole and diphenylformamidine with hydrogen halides in the solution

“…Due to the bifunctionality of such azo compounds much attention has previously been paid to their formation of closed cyclic complexes with two (or more) ɇ-bonds [4][5][6][7][8][9]. In the studies of the interaction of selected molecules with carboxylic acids (ɇɋɈɈɇ, ɋɇ 3 ɋɈɈɇ, and ɋɇ 2 ɋlɋɈɈɇ) the formation of cyclic hydrogen-bonded complexes with Ɉɇ…N and C=O…HN ɇ-bonds was proved, while in the formation of complexes with stronger acids (CHCl 2 COOH, CCl 3 COOH, and CF 3 COOH) in the solution, the possibility of the proton transfer in the complex with the formation of hydrogen-bonded ionic pairs was shown [4,6,8,10].…”