Moustafa M. Habeeb

Vibrational spectroscopic studies of hydrogen‒bonded complexes between 2,5-dihydroxy-P-benzoquinone and amines

A series of 1 : 1 hydrogen-bonded complexes between 2,5-dihydroxy-P-benzoquinone (DHBQ) and various nitrogen bases of different strengths were prepared. The FT-IR spectra of the prepared complexes were examined in different regions. It has been found that the protonic and carbonyl vibrations are strongly affected by protonation. The intensity of protonic vibrations was estimated, the base line was corrected by using Perkin-Elmer Paragon 1000 program. A maximum was found at pKa = 5.6 in the correlation between the intensity and pKa (amines) suggesting a critical behavior. The same value was located as a deep minimum in the correlation between the center of gravity of the protonic vibrations, v cg cm 1 ,a nd the pK a of the amines confirming the critical behavior in some of the studied complexes. The effect of the contribution of the second OH group to the IR absorption profile was discussed. Finally, the FT-IR spectra of some deuterated complexes were presented and analysed.

Spectroscopic studies and molecular orbital calculations on the charge transfer reaction between DDQ and 2-aminopyridine

Charge transfer complex formation between 2-aminopyridine (donor, 2AP) with 2,3-dichloro-5,6-dicyano-p-benzoquinone (acceptor, DDQ) has been studied spectrophotometrically in acetonitrile (CH3CN). The newly formed CT-complex has reddish brown color and is characterized by the appearance of new absorption bands in the 375-650 nm regions where acceptor and donor do not have any absorption. Maximum and constant absorbance of the complex was obtained after 10 min at 20 °C with 1 mL 5×10(-3) M DDQ in CH3CN. Based on photometric titration method, the stoichiometry of the formed CT-complex was found to be 1:1 [(2AP)(DDQ)]. Minimum-maximum absorbances method has been applied to estimate the formation constant of the complex where it recorded large value confirming its high stability. Molecular orbital calculations utilizing GAMESS computations were carried out in order to record changes in the electronic structure and molecular geometry of the formed CT-complex. In addition, the infrared vibrational frequencies of the complex were computed and compared with experimental results.

SPECTROSCOPIC STUDIES OF PROTON TRANSFER EQUILIBRIA IN HYDROGEN BONDED COMPLEXES

Abstract Recent studies on proton transfer equilibria for many hydrogen bonded complexes are discussed. These studies employ various spectroscopic techniques as infrared, ultraviolet, 1H NMR, 13C NMR, 15N NMR and nuclear quadrupole resonance (NQR) spectroscopy. Special attention has been paid to Fourier transform infrared (FTIR), and it forms the main focus of this review, in particular for the study of proton transfer equilibria in proton sponges hydrogen bonded complexes. The influence of proton transfer equilibria on the physical, chemical and biological properties of hydrogen bonded complexes is shown. Some applications of proton transfer equilibria are also discussed. *Dedicated to Prof. Samir A. Abou-Ali, in appreciation for his efforts in modernizing our department.

Spectroscopic characterization of hydrogen-bonded proton transfer complex between 4-aminopyridine with 2,6-dichloro-4-nitrophenol in different solvents and solid state

Proton transfer reaction between the proton donor 2,6-dichloro-4-nitrophenol (DCNP) with the proton acceptor 4-aminopyridine (4APy) has been investigated spectrophotometrically in different solvents included the aprotic solvent acetonitrile (MeCN), the protic one methanol (MeOH) and a mixture consists of 50% acetonitrile+50% dichloroethane (ANDC). The proton transfer complex is produced instantaneously with deep yellow color and absorption maxima in the range 395-425nm. The composition of the complex was characterized spectrophotometrically to be 1:1 in all solvent proving that the solvent has no effect on the complex stoichiometry. The proton transfer formation constant has been estimated by using Benesi-Hildebrand equation where the highest value was recorded in the mixture ANDC. This proofs the high stability of the complex in less polar solvent as a result of the high stability of the complex ground state. The solid complex has been synthesized and characterized by elemental analysis to be 1:2 [(proton donor) (proton acceptor)2]. The obtained solid complex was analyzed by infrared spectroscopy where two broad bands at 3436 and 2500cm(-1) characterized for asymmetric NHN(+) hydrogen bond were identified. Molecular modeling utilizing GAMESS computations as a package of ChemBio3D Ultra12 program was carried out where asymmetric NHN(+) was explored with NN bond distance 2.77Å. The computations showed a difference in molecular geometry of the complex compared with reactants especially bond lengths, bond angles and distances of close contact.

Intermolecular hydrogen bonds and proton transfer equilibrium in some nitro cresols-aliphatic amines-acetonitrile or methanol systems

Abstract Intermolecular hydrogen bonds between two nitro cresols, 2-nitro-4-methylphenol (NMP) and 3-methyl-4-nitrophenol (MNP) with three aliphatic amines morpholine (MO), Benzylamine (BA) and triethylamine (TEA) have been investigated in acetonitrile and methanol applying UV-Vis spectroscopy. It has been found that the proton transfer complex formation is retarded in acetonitrile leading to a sharp decrease in the proton transfer equilibrium constants (KPT). On the other hand, the proton transfer complex formation is strongly favored in methanol leading to a sharp increase in KPT. The presence of a single ortho substituent and strong intramolecular hydrogen bond does not inhibit the proton transfer process. It has been recorded from this work that the intramolecular hydrogen bond is not broken in acetonitrile but broken in methanol. It has been found alsa that TEA is sterically hindered in forming hydrogen bonds when compared to primary and secondary amines. Moreover, the present contribution proved that acetonitrile binds the phenol OH as hydrogen bonding proton acceptor while methanol binds as a proton donor with the phenol OH.

Proton transfer equilibria, temperature and substituent effects on hydrogen bonded complexes between chloranilic acid and anilines

The proton transfer equilibrium constants (KPT) for 1 : 1 complex formation between Chloranilic Acid (CA) and a series of p -a ndm-substituted anilines have been measured in 1,4-dioxane spectrophotometrically. The results supported the concept of amine-solvent hydrogen bond formation (short range solvation effect). Beside, this effect, the KPT values were apparently affected by the electron donation power of the aniline ring substituent, which was transmitted to the interaction center via resonance and inductive effects. Linear relationships between KPT and -Hammett substituent constants, or pKa values for m -a ndp-substituted anilines,were obtained verifying the above conclusions. The solute-solvent hydrogen bond formation might increase the reactivity of the aniline nitrogen than would the inductive effect of the alkyl group, in case of CA-N -alkyl aniline complexes. The thermodynamic parameters for the proton transfer complex formation were estimated and it was indicated that the solvent-aniline hydrogen bond formation was preferred in the case of p-substituted aniline complexes more than in the case of the corresponding m-isomer. It has been found that the proton transfer process was enthalpy and entropy controlled.

Spectrophotometric Determination of Proton Transfer Equilibria and Thermodynamic Parameters of 1:1 Hydrogen Bonded Complexes in Chloranilic Acid-Amine Systems in 1,4-Dioxan

Abstract The equilibrium constants for 1:1 proton transfer complex formation between chloranilic acid (CA) and quinoline, α-picoline, benzyl amine (BA), triethyl amine (TEA) and diethyl amine (DEA) have been measured in 1,4-dioxan by using UV spectroscopy. A linear correlation between the logarithm of the proton-transfer equilibrium constant and the pK of the amine has been found, except for primary and secondary amines. The results support the concept of amine-solvent hydrogen bond formation, which increases the reactivity of tha amine as a base. Such hydrogen bonds increase amine basicity more than would the inductive effect of an added alkyl group. The thermodynamic parameters were found in agreement with these results. Moreover, the thermodynamic data showed that the strength of the hydrogen bonds involved are among the strongest hydrogen bonds known.

Spectral studies on some phenol derivatives

Abstract The electronic absorption spectra of 2-nitrophenol, salicylic acid, 2-aEinophenol, catechol, pyrogallol and gallic acid, have been measured in different solvents. The solvent effects on the spectra have been discussed and the solvent Induced spectral shifts have been analysed as a function of different solvent polarity parameters. Moleculer orbital calculations of the different singlet-singlet and triplet-triplet transitions in these systems have been done using the PPP method.

Synthesis and infrared spectroscopic investigation of hydrogen-bonded complexes between 1,8-bis(dimethylamino)naphthalene and 2,6-dihydroxynaphthalene in 2:1 and 1:2 ratios in the crystalline forms and in acetonitrile

Two hydrogen bonded complexes between 1,8- bis(dimethylamino) naphthalene (DMAN) and 2,6-dihydroxynaphthalene (DHN) in 2:1 and 1:2 ratios were synthesised and studied using FTIR the spectra of the solid 2:1 complex revealed incomplete protonation of DMAN with the appearance of two broad absorptions representing ν(NHN)+ and ν(OHN) in the ranges 700–400 and 1600–700 cm−1 respectively, while the 1:2 complex indicated the complete protonation of DMAN with the appearance of two broad absorptions representing ν(NHN)+ and ν(OHO-) in the ranges 700–400 and 1600–700 cm−1 respectively. On the other hand the IR spectra in acetonitrile indicated that the 1:2 complex is more stable than the 2:1 complex.

HYDROGEN BONDING INTERACTION OF THE MANNICH BASE, 4-NITRO-2-(DIETHYLAMINO)METHYL PHENOL WITH AMINES

Abstract Spectral studies of the title compound showed that its intramolecular hydrogen bond is relatively strong. Hydrogen bond formation between some amines and the phenolic -OH group is not inhibited in presence of strong intramolecular hydrogen bonding between the -OH group and the ortho-diethylamino group in the title compound. Proton transfer equilibrium constants (KPT) between the studied compound and different amines in methylene chloride were measured. A linear correlation was found between log KPT and the proton affinities, PA, of amines except for α-picoline. The KPT values due to the proton transfer equilibrium between the Mannich base and α-picoline in different solvents were calculated and they gave a good correlation with the Onsager parameter. The estimated thermodynamic parameters showed that the proton transfer equilibrium in our system is entropy- and enthalpy controlled, and proved more polarizability of the intermolecular OHN bridge with increasing the solvent polarity.