A. Koll

STRENGTHENING OF THE INTRAMOLECULAR O...H...N HYDROGEN BONDS IN SCHIFF BASES AS A RESULT OF STERIC REPULSION

Abstract Three ortho hydroxy Schiff bases 2-( N- methyl- α -iminoethyl)-phenol ( I ), 2-( N- benzyl- α -iminoethyl)-phenol ( II ) and 2-( N- benzyl- α -iminopropyl)-phenol ( III ) were synthesised in which the hydrogen atoms in C–C(H)N group were replaced by an alkyl substituent. The crystal structures were determined and ab initio calculations at the B3LYP/6-31G** basis set were performed. One of the shortest known O–H⋯N hydrogen bonds were found with O⋯N distance equal to 2.459(3), 2.497(3) and 2.494(3) A, respectively in I , II and III . Steric repulsion of substituted alkyl results in this unusual strengthening of the hydrogen bonds. FT-IR spectra in solution and in the solid state in function of the temperature were determined. Strong influence of the temperature on the character of the hydrogen bond was observed.

Unusual spectroscopic properties of CF3H dissolved in liquified Ar, N2, CO, and CO2

Abstract The FTIR spectra of CF 3 H have been studied in liquefied Ar, N 2 , CO, and CO 2 . The effect of a blue shift of frequency of the C – H stretching vibration accompanied by a decrease in the integral intensity of the corresponding ν 1 band was found in the studied series of solvents. The combination bands ascribed to simultaneous excitation of vibrations of interacted molecular partners have been revealed in CO and CO 2 solutions. DFT/B3LYP and ab initio MP2 calculations utilizing the 6-311++G(3df,3pd) basis set suggest weak linear F 3 CH⋯ B complex formation, reproduce the experimentally observed unusual trends for B= Ar , N 2 , CO, CO 2 , and predict the conventional red frequency shift of the ν 1 band for B =NH 3 .

Car-Parrinello simulation of an O–H stretching envelope and potential of mean force of an intramolecular hydrogen bonded system: Application to a Mannich base in solid state and in vacuum

Car-Parrinello molecular dynamics (CPMD) study was performed for an anharmonic system-an intramolecularly hydrogen bonded Mannich-base-type compound, 4,5-dimethyl-2(N,N-dimethylaminemethyl)phenol, to investigate the vibrational spectrum associated with the O-H stretching. Calculations were carried out for the solid state and for an isolated molecule. The classical CPMD simulation was performed and then the proton potential snapshots were extracted from the trajectory. The vibrational Schrodinger equation for the snapshots was solved numerically, and the (O-H) envelope was calculated as a superposition of the 0-->1 transitions. The potential of mean force for the proton stretching mode was calculated from the proton vibrational eigenfunctions and eigenvalues incorporating statistical sampling, nuclear quantum effects, and effects of the environment. Perspectives for application of the presented methodology in the computational support of biocatalysis are given in the study.

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.

Specificity of the intramolecular hydrogen bond. The differences in spectroscopic characteristics of the intermolecular and intramolecular H-bonds

Abstract IR spectroscopic properties—positions of ν (OH) bands and their integrated intensities for series of nine Schiff bases, 16 complexes of phenols with N , N -dimethylbenzylamine and 11 complexes of phenols with N -benzylidenemethylamine—were determined in CCl 4 solutions and compared with their Δp K a values. Characteristics of particular groups of compounds with the hydrogen bond were determined. It was shown that the strong decrease of the intensity of the ν (OH) bands is a specific feature of the intramolecular hydrogen bonding in comparison to related intermolecular complexes. It results from bending of the intramolecular hydrogen bond and is not limited to pseudoaromatic chelate rings only. Such conclusion results also from semiempirical and ab initio calculations.

MANNICH BASES AS MODEL COMPOUNDS FOR INTRAMOLECULAR HYDROGEN BONDING. I. SOLID STATE STRUCTURES AND MOLECULAR CALCULATIONS

SummaryA survey of the structures and the properties of the hydrogen bonds inortho-aminomethylphenols and -naphthols in the solid state is presented. The results are compared with molecular structure calculations. The solid state complexes can exist in the molecular or in zwitterionic form. In the last case, only intermolecular hydrogen bonds forming cyclic dimers or linear chains were detected.ZusammenfassungEine Übersicht über Strukturen und Eigenschaften von Wasserstoffbrücken inortho-Aminomethylphenolen und -naphtholen wird präsentiert. Die Ergebnisse werden mit Molekülrechnungen verglichen. Im Festkörper existieren diese Verbindungen in neutraler oder in zwitterionischer Form. Für den zweiten Fall können nur intermolekulare Wasserstoffbrücken nachgewiesen werden, und zwar in Dimeren oder in linearen Ketten.

Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature.

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecond spectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (approximately 11000 cm(-1)) at a selected excited energy in alcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in most of the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates. At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethylene glycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitation energy.

Spectroscopic and structural consequences of intramolecular hydrogen bond formation in ortho-dimethylaminomethylphenol

Abstract Ab initio and density functional calculations are applied to analyse the structure of the Mannich base ortho -dimethylaminomethylphenol with emphasis on a detailed description of the properties of the intramolecular hydrogen bond. The calculated structures of the components, phenol and dimethylbenzylamine, are compared to the geometries of the hydrogen-bonded and open forms of the Mannich base. Additionally, the gas-phase infrared spectra of the Mannich base and of dimethylbenzylamine are presented. The experimental spectra are confronted with theoretical infrared spectra and interpreted with the aid of a detailed normal coordinate analysis. Trends in the modifications of computed harmonic force constants upon closure of the intramolecular hydrogen bond are monitored and discussed.

Interactions of Dihydrochloride Fluphenazine with DPPC Liposomes: ATR-IR and 31P NMR Studies

The influence of dihydrochloride fluphenazine (FPh) on the dipalmitoylphosphatidylcholine (DPPC) bilayer structure was investigated using ATR-IR and (31)P NMR methods. The ATR-IR results indicate an increase in conformational disorder in the hydrophobic part compared with pure DPPC liposomes and a decrease in temperature of the chain-melting phase transition in FPh/DPPC liposomes. These effects depended on the concentration of the drug in the DPPC bilayer. The dihydrochloride fluphenazine molecules form H-bonds with the proton-acceptor carbonyl groups of DPPC molecules. At a higher concentration of the drug, the lipid bilayer structure is destroyed, and an isotropic phase is observed using (31)P NMR spectroscopy. The interactions between FPh and the lipid bilayer have a crucial role in MDR (multidrug-resistant) activity of this drug. These results improve one possible strategy of cancer chemoprevention with FPh accompanied by fluidization and destabilization of the model lipid bilayer structure.

Gas phase IR spectra of systems with intramolecular hydrogen bonds

Abstract The IR spectra of intramolecular hydrogen bonds in ortho Mannich bases have been measured in the gas phase and in diluted CCl 4 solutions. It is demonstrated that the intramolecular hydrogen bonds are stable both in the gas phase and in CCl 4 solutions. The hydrogen bonds in the gas phase are weaker but the ν s (OH) bands are still broad with a halfwidth of about 400–500 cm −1 . It demonstrates that the coupling with internal modes of a molecule is a sufficient condition for such band broadening. Spectral moments of ν s (OH) bands were determined and correlated with Δp K a values both in the gas phase and in CCl 4 solutions.