Teresa Dziembowska

Photochromism and thermochromism of solid trans-N,N′-bis-(salicylidene)-1,2-cyclohexanediamines and trans-N,N′-bis-(2-hydroxy-naphylidene)-1,2-cyclohexanediamine

Abstract The photochromic and thermochromic properties of trans - N , N ′-bis(salicylidene)-1,2-cyclohexanediamine ( 1 ), trans - N , N ′-bis(3,5-dichloro-salicylidene)-1,2-cyclohexanediamine ( 2 ), trans - N , N ′-bis(3,5-di- t -butyl-salicylidene)-1,2-cyclohexanediamine ( 3 ) and trans - N , N ′-bis(2-hydroxy-naphylidene)-1,2-cyclohexanediamine ( 4 ) were investigated by UV and fluorescence spectroscopies in the crystalline state at various temperatures and the molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction. The existence of the two Schiff base groups on a single molecule does not seem to differentiate the chromobehavior of the present compounds (except possibly for 3 ) with respect to the usual Schiff bases of salicylaldehyde. It is suggested that for this class of compounds also, what determines the thermochromic behavior is the enhanced basicity of the nitrogen atom, due the absence of π,π- and n ,π-conjugation with an aryl ring bound to it. The role of the crystal structure in this case is important only in so far as it affects the electron density on the nitrogen atom. Photochromism, however, is structure dependent and requires space for the generation of the photoproduct that involves cis to trans isomerization in the excited state.

Synthesis and spectroscopic study of Schiff bases derived from trans-1,2-diaminocyclohexane. Deuterium isotope effect on 13C chemical shift

Abstract The proton transfer in the intramolecular hydrogen bond in a series of Schiff bases derivatives of aromatic ortho-hydroxyaldehydes and trans-1,2-diaminocyclohexane has been studied by means of IR, UV–Vis, 1H and 13C NMR spectroscopies. The measurement of deuterium isotope effect on 13C chemical shift suggests that the proton transfer equilibria in both salicylidene moieties are not independent. Substitution of H by D in one hydrogen bond shifts the proton transfer equilibrium in one direction in this moiety, while in the opposite direction in the other.

Intramolecular hydrogen bonding in N-salicylideneanilines. X-ray diffraction and solid-state NMR studies

The crystal structures of four Schiff bases: N-(3,5-dichlorosalicylidene)-4-dimethylaminoaniline, N-(3,5-dichlorosalicylidene)-4-diethylaminoaniline, N-(3-nitrosalicylidene)-4-diethylaminoaniline and N-(2- hydroxynaphthylidene)-4-dimethylaminoaniline have been determined by single-crystal X-ray diffraction and solid-state NMR. We discuss the relationship between the variation of the hydrogen bonding on the one hand, and the geometry and electron density distribution in the N-salicylideneaniline fragment on the other. Mesomerism involving ionic structures is the dominant effect in changing the geometric parameters of the fragment. The concentration of ionic structures correlates very well with the C—O bond length and other structural parameters.

The 15N and 13C solid state NMR study of intramolecular hydrogen bond in some Schiff's bases

A series of 11 Schiffs bases derived from substituted salicylaldehyde and aliphatic amines has been studied in the solid state by 15N and 13C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR). 15N CPMAS is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers. In the solid state, three of the compounds examined were shown by 15N NMR to exist as OH tautomeric forms, and the remaining eight as the corresponding NH forms. This was confirmed by 13C CPMAS. The results reported were compared with those obtained in CDCI3 solutions.

Intramolecular hydrogen bond investigations in Schiff bases derivatives of 2-hydroxy-1-naphthaldehyde and 2-hydroxy-1-acetonaphthone in CDCl3 solution and in the solid state by NMR methods

Abstract Three Schiff bases: 1-( N -methyl-iminomethyl)-2-naphthol ( I ), 1-( N -isopropyl-iminomethyl)-2-naphthol ( II ) and 1-( N -isopropyl-α-iminoethyl)-2-naphthol ( III ) have been investigated by 15 N and 13 C NMR spectroscopy in order to study the tautomeric equilibrium in both the solid and liquid phases. For two derivatives of 2-hydroxy-1-naphthaldehyde ( I and II ), the NH tautomers have been found to be dominant in both the phases. The Schiff base derived from 2-hydroxy-1-acetonaphthone ( III ) exists in the CDCl 3 solution mainly in the NH form but in the solid state only the OH tautomer has been detected. The steric effect of the methyl group at the imine carbon atom seems to be responsible for such exceptional behaviour of the Schiff base investigated. Additionally, the principal components of 15 N chemical shift (CS) tensor of the compound III and two model compounds (5-methoxy-salicylidenmethylamine ( IV ) and 4-methoxy-salicylidenemethylamine ( V )) have been measured to confirm the electronic structure of III .

Solvent effect on intramolecular proton transfer equilibrium in some N-(R-salicylidene)-alkylamines

Abstract The solvent effect on tautomerism of Schiff bases has been studied by measuring the deuterium isotope effect on their 15 N NMR and UV–Vis spectra in different solvents. The interactions of the Schiff bases with solvent molecules are of local character and they vary for the series of solvents under study. The experimental results and PM3 calculations show that the solvent effect on the position of the proton transfer equilibrium depends on the electronic structure of particular Schiff bases.

Proton transfer equilibrium in Schiff bases derived from 5-nitrosalicylaldehyde. a study of deuterium isotope effects on 13C NMR chemical shifts

The proton transfer equilibrium in series of Schiff bases derived from 5‐nitrosalicylaldehyde and aliphatic amines was investigated by means of variable‐temperature NMR spectra and the deuterium isotope effect on 13C NMR chemical shifts. Copyright

15N NMR study of the intramolecular hydrogen bond in N-salicylidene-alkylamines

Abstract 15 N NMR spectra of a series of N -(R-salicylidene)-alkylamines in CDCl 3 solution were measured. Proton transfer equilibrium has been established on the basis of the temperature dependence of the 15 N chemical shift and 1 J NH coupling constants.

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.

A multinuclear NMR study of proton transfer processes in Schiff bases

Abstract 1 H, 13 C and 15 N NMR data are reported for seven Schiff bases in order to study the possibility of proton transfer from oxygen to nitrogen. The compounds contain various substituents which influence the acidity of the OH group and the basicity of the nitrogen atom. All the NMR results provide a consistent view of the extent of proton transfer. The reported value of 1 J( 15 N − 1 H ) is used to give a quantitative estimate of this extent. Low temperature 15 N NMR results for compound 7 show an increase in proton transfer with respect to that found for this molecule at room temperature.