Maria Rospenk

Proton transfer and self-association of sterically modified Schiff bases

Abstract The intramolecular proton transfer process has been studied by measuring of the dipole moment, an average molecular weight, and UV/VIS spectra as a function of temperature and concentration in a few solvents of low polarity. Ten Schiff bases were selected for such studies, where steric repulsion of alkyl- and aryl-substituents in azomethine moiety enhances the strength of hydrogen bond and the proton transfer process. The self-association of species with intramolecular proton transfer has been investigated as a function of solvent polarity. Behaviour of these systems has been compared with those of Mannich bases, where intramolecular electron coupling between acid and base centres is considerably prevented by –CH 2 – bridge. A substantially different behaviour of these two types of compounds has been found out.

Low temperature studies on ultraviolet and infrared spectra of ortho Mannich bases

Abstract Low temperature studies of UV and IR spectra have been performed for 2-(N,N-dimethylaminomethyl)-4,6-dichlorophenol (Cl2MB), 2-(N,N-diethylaminomethyl)-3,4,6-trichlorophenol (Cl3MB) and 2-(N,N-diethylaminomethyl)-tetrachlorophenol (Cl4MB) in n-butylchloride, which undergoes transition to the glassy state at 150 K. The proton transfer (PT) equilibrium in the liquid state is clearly seen only for Cl4MB. In the case of Cl3MB, negligible amounts of the PT species are observed in liquid phase above 150 K, and complete PT takes place below the glassy state transition. No PT was detected for Cl2MB down to 90 K. The temperature effects on the high frequency region and background IR absorption extended down to 650 cm−1 are analyzed. The broad absorption below 1600 cm−1 vanishes for Cl4MB and Cl3MB on cooling when they transform to the PT ion pairs. The potential energy curve for ion-pairs is characterized by a single minimum.

Dipole moments and conformation of Schiff bases with intramolecular hydrogen bonds

Abstract The dipole moments of series of Schiff bases, which are derivatives of salicylic aldehydes in CCl4 solution were measured. The influence of the substituents on the conformation of the molecules, the intramolecular hydrogen bond energy and tautomerism were studied by means of semi-empirical PM3, AM1 and ab initio methods.

Proton transfer and secondary deuterium isotope effect in the 13C NMR spectra of ortho-aminomethyl phenols

Abstract The temperature dependence of the deuterium isotope effect on the 13 C NMR signals for various positions in the phenyl ring of 2-(N,N-diethylaminomethyl) tetrachlorophenol was studied in the moderately polar solvent CH 2 Cl 2 . The critical temperature, T c = 245 K, corresponds to the compensation of opposite effects for non-proton-transfer and proton-transfer (PT) forms existing in equilibrium. The UV spectra show that the contribution of the PT form at 245 K amounts to 20 ± 5%. Semi-empirical calculations taking into account the Onsager reaction field allowed us to discuss the role of permittivity and dimerization of polar PT forms in the charge separation.

A strong effect of hydrogen bonding upon δ(NC3) bands in the IR spectra of trimethylamine

Abstract Remarkable effects of hydrogen-bond formation and its strengthening on the NC 3 bending modes of trimethylamine (TMA) are reported. The relative value of the blue shift of the δ NC 3 (A 1 ) frequency reaches about 30% for strong hydrogen bonds. The integrated intensity of corresponding IR band increases about 9 times. The ab initio calculations performed for some model TMA complexes confirm the tendency observed in the experiments, the maximum effects being predicted for the homoconjugated (CH 3 ) 3 NH + N(CH 3 ) 3 cation. The optimized structure of the cation, resulting from the calculations, implies the double-well profile of the potential surface and splitting of internal TMA frequencies. The doublet structure of δ (A 1 ) and other CNC bands with frequencies close to those predicted for the homoconjugated cation have been detected for a TMA–pentachlorophenol complex of 2:1 composition in chloroform solution.

Thermodynamic parameters for the proton-transfer reaction in mannich bases

The results of u.v. spectroscopic studies on thermodynamic parameters of the intramolecular proton-transfer (PT) reaction in Mannich bases in polar solvents are presented. It is shown that for these hydrogen-bonded systems additional phenomena such as association, self-association of ionic pairs and dissociation into ions are excluded. Determined ΔHPT values are within the region –7 to –16 kJ mol–1; ΔSPT values are negative and relatively high; from –30 to –60 J K–1. The results obtained indicate that a distinct reorganization of solvent molecules proceeds in the course of the intramolecular proton-transfer reaction.

DFT theoretical, X-ray diffraction and IR matrix-isolated studies on 4-chloro-2′-hydroxy-4′-ethoxyazobenzene

Abstract Results of X-ray diffraction, IR spectra of the solute either in an Ar matrix or in benzene and carbon tetrachloride solutions as well as DFT theoretical studies are used to discuss the structure and dynamics of 4-chloro-2′-hydroxy-4′-ethoxyazobenzene with particular attention to the OH⋯N intramolecular hydrogen bonding. The packing in the crystalline lattice suggests preferences for the formation of sandwich type dimers in stacks with OH⋯N bridges in two opposite directions. The analysis of the IR spectra supported by DFT calculations was particularly devoted to the manifestations of hydrogen bonding in the ν(OH) and γ(OH) vibrations. It is demonstrated that there is a substantial contribution of the γ(OH) overtone to the intense complex absorption in the spectral region around 1600 cm −1 . The mechanical and electrical anharmonicities of the OH group are discussed based on the H/D frequency and intensity isotope effects. The extremely small intensity of the ν(OH) band (41 km mol −1 ) is also particular in the present case.

The influence of steric effects of proton-transfer equilibrium in intramolecular hydrogen bonds

Abstract The results of UV spectroscopic studies on thermodynamic properties of the proton transfer reaction in two series of ortho Mannich bases which are the condensation products of 2,4,5-trichlorophenol and p -NO 2 -phenol with dialkylamines of varying chain length (from methyl- to n-decyl), are presented. It was found that in methanol all thermodynamic functions correlate satisfactorily with 1/ V , where V is the molar volume of the aliphatic chain. The results are discussed in terms of the Onsager reaction field theory.

Solvent effect on the infrared spectra of a trichlorosubstituted Mannich base and its deuterated OD analogue

Abstract The i.r. spectra of 2,4,5-trichloro-6-[(diethylamino)methyl] phenol and its deuterated OD analogue have been studied in the 3000-700 cm −1 range in 10 different solvents. Two bands are observed in the ν OH range, and these have been ascribed to a Fermi resonance between the ν OH and 2γ OH levels. The real values of the isotopic ratio ν OH /ν OD decrease with the polarity of the solvent, going from 1.285 in n -hexane to 1.252 in acetonitrile. These values correspond to a hydrogen bond characterized by an asymmetric double minimum potential with a relatively low barrier. The δ OH vibrations are coupled with the aromatic ring and ν CO vibrations and decoupled in the OD compound. The γ OH vibrations are solvent sensitive and are observed between 950 and 1050 cm −1 .

Temperature effect on proton-transfer equilibrium and IR spectra of chlorophenol–tributylamine systems

Binary solutions of chlorophenols in tributylamine (TBA) have been studied by UV and IR spectroscopy at temperatures down to the glassy transition state of TBA (123 K). The pentachlorophenol–TBA system has also been studied at 13–123 K in TBA matrices prepared by co-deposition of the gaseous components on a cooled window. 2,4-Dichlorophenol, 2,4,5-trichlorophenol, 2,6-dichlorophenol and 2,4,6-trichlorophenol in solution show proton-transfer (PT) equilibria which shift sharply toward the PT species on cooling. No PT is detected in the case of 2-chlorophenol. Pentachlorophenol appears entirely as a PT ion pair across the whole temperature range both in solution and in a matrix. Significant influence of ion-pair self-association on the IR spectra of solutions has been found by comparison with spectra obtained in rigid matrices. All systems studied reveal broad, intense, temperature-dependent bands corresponding to protonic vibrations. An extremely intense and broad absorption, with a maximum shifted to 700–500 cm–1, is observed when the PT equilibrium constant is close to unity. The contribution of different processes, e.g. hydrogen-bond strengthening, proton transfer and association of ion pairs, to the temperature transformations of IR spectra is discussed.