Scaled quantum chemical calculations and FTIR, FT-Raman spectral analysis of 3,4-diamino benzophenone

The vibrational spectra of 3,4-diamino benzophenone (DABP) have been computed using B3LYP methodology and 6-31G* and 6-31G** basis sets. The solid phase FTIR and FT-Raman spectra were recorded in the region 4000-400 cm-1 and 3500-100 cm-1, respectively. A close agreement was achieved between the observed and calculated frequencies by employing normal coordinate calculations. The observed and simulated spectra were found to be well comparable.     

FT-IR study of self-association of some hydroperoxides

Self-association of cumyl, tertiary butyl and 3-phenylmethyl hydroperoxides in solutions of n-decane, carbon tetrachloride and chlorobenzene were studied by IR spectroscopy (3100–3700 cm⁻¹, 293–353 K). The experimental data were interpreted by factor analysis and band contour resolution. The di- and trimerization constants and thermodynamic parameters of self-associates were determined. Intramolecular hydrogen bond of cumyl hydroperoxide was investigated. The conformations of tertiary butyl and cumyl hydroperoxides were studied. The solvent influence on the thermodynamic parameters of hydrogen bond was found.     

Conformational analysis of N-methylglycine and N,N-dimethylglycine by ab initio calculations

Eight of the most stable conformers of N-methylglycine (NMG) and five of N,N-dimethylglycine (DMG) were analyzed by high level ab initio calculations. Since NMG has only one amino hydrogen and a carboxylic acid hydrogen, it is capable of the formation of various types of hydrogen-bonded conformers and as a result is ideally suited to studying the importance of hydrogen-bonding on the relative stabilities of the various types of conformers of glycine and N-alkylated glycines. Comparisons of the relative energies of the various NMG and DMG conformers that have different types and number of hydrogen bonds (H-bonds) reveal the importance of hydrogen bonds to the stability of the different types of conformers. For NMG, conformer Ib which has two types of H-bonds and a dipole moment of 1.2 debyes is the most stable. Conformer Ib is similar to that of the most stable conformer of glycine. For DMG, on the other hand, IIc is the most stable conformer. IIc has a dipole moment of 5.6 debyes (compared to a value of 1.1 debyes for another of its conformers, Ic) and only one H-bond which involves the carboxylic acid and amino functionalities. The stability of IIc is attributed to the relative strength of the type H-bond formed — a similar type H-bond of glycine and NMG is predicted to be weaker. Thus, for a particular conformer, the relative strength and number of possible H-bonds that can be formed, and not necessarily the magnitude of the dipole moment, play key roles in the relative stability of amino acid conformers in the gas phase.     

2(1H)-pyridinone (2-pyridone): self-association and association with water. Spectral and structural characteristics: infrared study and ab initio calculations

DFT calculations of 2(1H)-pyridinone (2-pyridone NHP), the centrosymmetric dimer (NHP)2 and the closed complexes (NHP, H2O) and (NHP, 2H2O), with their deuterated homologues NDP, (NDP)2, (NDP, D2O) and (NDP, 2D2O), are compared with vibrational spectra of NHP and NDP in ternary mixtures CH3CN, NHP, H2O. Experimental data are also obtained for NHP or NDP in various solvents. The protic solvent effects demonstrate that mechanical couplings are different in the 1500-1700 cm(-1) range for the nuC=O and nu8b (valence of the ring) modes in NHP and NDP (or (NHP, H2O) and (NDP, D2O)). For the first time, data are obtained for NDP in the dimer (NHP, NDP). Comparison of data for pyridone, monomer, dimer or complexed with water, shows that in the complexes, water is a weaker proton acceptor and a stronger proton donor than a second pyridone molecule in the centrosymmetric dimer.     

The investigation on the relative stability of different clusters of thiourea dioxide in water using gas phase quantum chemical calculations

The relative stability of different clusters of thiourea dioxide (TDO) in water is examined using gas phase quantum chemical calculations at the MP2 and B3LYP level with 6‐311++G(d,p) basis set. The possible equilibrium structures and other energetic and geometrical data of the thiourea dioxide clusters, TDO‐(H₂O)_n (n is the number of water molecules), are obtained. The calculation results show that a strong interaction exists between thiourea dioxide and water molecules, as indicated by the binding energies of the TDO clusters progressively increased by adding water molecules. PCM model is used to investigate solvent effect of TDO. We obtained a negative hydration energy of ?20.6 kcal mol^?1 and free‐energy change of ?21.0 kcal mol^?1 in hydration process. On the basis of increasing binding energies with adding water molecules and a negative hydration energy by PCM calculation, we conclude thiourea dioxide can dissolve in water molecules. Furthermore, the increases of the C? S bond distance by the addition of water molecules show that the strength of the C? S bonds is attenuated. We find that when the number of water molecules was up to 5, the C? S bonds of the clusters, TDO‐(H₂O)₅ and TDO‐(H₂O)₆ were ruptured. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Electronic and steric effects of substituents on the conformational diversity and hydrogen bonding of N-(4-X-phenyl)-N′,N″-bis(piperidinyl) phosphoric triamides (X = F, Cl, Br, H, CH3): A combined experimental and DFT study

Phosphoric triamides of the general formula (4-X-C₆H₄NH)P(O)(NC₅H₁₀)₂, X = F (1), Cl (2), Br (3), H (4) and CH₃ (5), have been synthesized and characterized. X-ray crystallography at 120 K reveals that the compounds 1, 3, 4·H₂O and 5 are composed of one, four, two and four conformers, respectively. DFT calculations were performed to investigate the electronic structures of the compounds. The X-ray data and DFT calculations revealed that the conformational diversity in these compounds is mainly governed by the steric effects of the substituent X rather than by electronic effects. Although substituent X does not participate directly in hydrogen bonding, the crystal packing of the compounds is influenced by the size of X. Atoms in molecules (AIM) and natural bond orbital (NBO) analyses confirm that the para substituent X has no significant effect on the electronic features of the amidic proton and the phosphoryl oxygen atom (O_P). Using X-ray crystallography, AIM and NBO analyses, the structural and electronic aspects of inter- and intramolecular hydrogen bonds of the compounds have been studied. The charge density (ρ) at the bond critical point (bcp) of the N-H bond decreases from the fully optimized monomers to their corresponding hydrogen bonded clusters. The N-H stretching frequency decreases from the calculated values to the experimental results.     

Density functional theory calculations and vibrational spectral analysis of 3,5-(dinitrobenzoic acid)

The FT-IR and Raman spectra of 3,5-dinitrobenzoic acid (DNBA) have been recorded and analyzed. The equilibrium geometry, various bonding and harmonic vibrational wavenumbers have been calculated with the help of density functional theory (DFT) method. Most of the vibrational modes are observed in the expected range. Mulliken population analysis shows the interactions C-N-O?H-C and C-O?H-C. The most possible interaction is explained using natural bond orbital (NBO) analysis. The strengthening and polarization of the CO bond increases due to the degree of conjugation. HOMO-LUMO energy and the thermodynamic properties are also evaluated.     

Spectroscopic studies of the 1:1 complex of piperidine-4-carboxylic acid (isonipecotic acid) with 2,6-dichloro-4-nitrophenol

The 1:1 complex of piperidine-4-carboxylic acid (isonipecotic acid, P4C) with 2,6-dichloro-4-nitrophenol (DCNP), has been investigated by single-crystal X-ray analysis, Raman and FTIR spectroscopy and theoretical calculations. The hydrogen-bonded-ion-pair complex is observed in the crystalline state with the O⋯H⋯OOC hydrogen bond of 2.453(16) Å. FTIR spectrum shows a broad absorption in the 1600–400 cm⁻¹ region characteristic of very short OHO hydrogen bond, broken by the Evans holes. The complexes are joined through NH⋯O into a H-bonding network. The NH⋯O mode appears as a broad band in the range of 3100–2000 cm⁻¹. In the structure optimized at the B3LYP/6–311 + +G(d,p) level of theory the proton is transferred from DCNP to P4C, and molecules are joined through the O⋯HOOC hydrogen bond of 2.640 Å. The experimental and theoretical infrared spectra are discussed. Detail interpretation of the vibrational spectra has been carried out with the use of computed Potential Energy Distribution (PED).     

Crystal and molecular structure, hydrogen bond and electrostatic interactions of bis(1-methylisonicotinate)hydrogen perchlorate studied by X-ray diffraction, DFT calculations, FT-IR, Raman and NMR spectroscopes

The crystal structure of bis(1-methylisonicotinate)hydrogen perchlorate, (MIN)₂H·ClO₄, has been studied by X-ray diffraction, DFT calculations, FT-IR, Raman, ¹H and ¹³C NMR spectra. The crystals are monoclinic, space group P2₁/n, with a pair of MIN molecules bridged by a short asymmetrical O·H·O hydrogen bond of 2.461(5) Å. The COO groups are twisted by 80.55° with respect to the plane of the pyridine ring. The anion interacts electrostatically with the positively charged nitrogen atoms of the neighbouring MIN molecules. The most stable conformer of isolated (MIN)₂H·ClO₄ and two homoconjugated cations, (MIN)₂H, have been analyzed by the B3LYP/6-31G(d,p) calculations in order to determine the influence of the anion on the hydrogen bonds in MIN·H·MIN unit. The FT-IR spectrum of the (MIN)₂H·ClO₄ shows a broad and intense absorption in the 1500–400 cm⁻¹ region, typical of short hydrogen bonds. The isotopic ratio, νOHO/νODO, is close to unity, indicating that the hydrogen bond is acentric (pseudo-type A).     

A quantum chemical study of red-shift and blue-shift hydrogen bonds in bimolecular and trimolecular methylhydrazine-hydrate complexes

This study presents a theoretical discussion of the geometry and molecular parameters of the methylhydrazine-hydrate complex. Using B3LYP/6-311++G(d,p) calculations, two geometries for the methylhydrazine-hydrate complex were analyzed by considering the interaction between water in: (i) a lone nitrogen pair assisted by a methyl (I); and (ii) the nitrogen of the NH₂ group (II). These geometries were examined by examining the formation of (N···H) hydrogen bonds, which were characterized using ChelpG charge transfer amounts, as well as by means of topological parameters derived from Quantum Theory of Atoms in Molecules (QTAIM) calculations. In a qualitative evaluation, both complexes (a) and (b) were compared with the corresponding trimolecular system (c) formed by methylhydrazine and two water molecules. A conclusion was then obtained by means of vibrational analysis, in which, in addition to δυ(H–O) red-shifts in water molecules, the stretch frequency of the H–C bond of methyl group shifted upwards, indicating the formation of a blue-shifting hydrogen bond in the methylhydrazine-bihydrate complex.     

NBO analysis and vibrational spectra of 2,6-bis(p-methyl benzylidene cyclohexanone) using density functional theory

Vibrational analysis of the 2,6-bis(p-methyl benzylidene cyclohexanone) [PMBC] compound was carried out by using NIR FT-Raman and FT-IR spectroscopic techniques. The equilibrium geometry, various bonding features and harmonic vibrational frequencies of PMBC have been investigated with the help of B3LYP/6-31G(d) density functional theory method. The optimized geometry clearly demonstrates cyclohexanone ring chair conformation is changed into half-chair conformation. The shortening of C–H bond length and blue shifting of the CH stretching wavenumber suggest the existence of improper weak C–HO hydrogen bonding, which is confirmed by the natural bond orbital analysis. The Mulliken population analysis on atomic charges and the HOMO–LUMO energy are also calculated.     

Theoretical calculations of hypersurfaces of the C chemical shift anisotropy in the COHN hydrogen bond and the benefit for the ab initio structure determination

We investigated the influence of structural changes on the anisotropic part of the carbonyl ¹³C chemical shift tensor in a model complex containing hydrogen bonded cyanuric acid and pyrrole. The model was chosen for its chemical resemblance to cyameluric acid. In the solid state this compound comprises three different hydrogen bonds which are well distinguishable based on the anisotropy parameters δ_aniso and η of the carbonyl ¹³C atoms. The variation of six relevant structural variables in the model system produced hypersurfaces for the isotropic shift, δ_aniso and η. Our goal was to investigate whether such surfaces can be used for the ab initio structure determination of hydrogen bonds. With a medium size basis set it could be shown that although the absolute values differ DFT describes the relative change in δ_aniso and η close to the quality of MP2 calculations. Due to the high dimensionality of the hypersurface we had to reduce the number of variables in our study. We systematically created subsurfaces each described by three of the six variables and investigated their isolated influence on the NMR observables. We identified the most important structure parameters and on this base built a minimal model. For a fixed NO distance the hydrogen bond arrangement was altered by two angular variations and one dihedral distortion. In this model evidence was found that the η surfaces for different NO distances exhibit a uniform shape and can be transformed into one another by a simple shift and multiplication by a mean factor. Furthermore, the experimental parameters δ_aniso and η of cyameluric acid were taken as a base for the extraction of structures from the hypersurfaces. δ_aniso and η unequivocally selected ensembles of similar structure and the COHN arrangement in two of the three cyameluric hydrogen bonds could be predicted with good quality from the theoretical model. Our results show that it is possible to predict the distance and at least qualitatively the orientation in a hydrogen bond environment from an analysis of the anisotropic part of the ¹³C chemical shift tensor.     

FT-Raman and FTIR spectra, normal coordinate analysis and ab initio computations of (2-methylphenoxy)acetic acid dimer

The Fourier Transform Raman and infrared spectra of the crystallized herbicide (2-methylphenoxy)acetic acid (MPA) have been recorded in the region 4000–400 cm⁻¹. The geometry, intermolecular hydrogen bond, and harmonic vibrational frequencies of MPA have been investigated with the help of B3LYP density functional theory (DFT) methods. The calculated molecular geometry has been compared with the experimental data obtained from XRD data. The assignments of the vibrational spectra have been carried out with the aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology (SQMFF). The strong doubly hydrogen bonded interface of the dimerized system is well demonstrated by the red shift in OH stretching frequency concomitant with the elongation of bond length. The most stable structure of the dimer possesses center of symmetry and interaction energy of −83.642 kJ mol⁻¹ after the basis set superposition error (BSSE) correction by the counterpoise (CP) method. The natural bond orbital analysis (NBO) ascertains that the delocalization of unpaired electron of oxygen atom onto the CO bond causes double bond character.     

Vibrational assignment of the spectral data and thermodynamic properties of 2-chloro-4-fluorobenzophenone using DFT quantum chemical calculations

The FT-Raman (3500-100 cm⁻¹) and FT-IR (4000-450 cm⁻¹) spectra of 2-chloro-4-fluorobenzophenone were recorded in the solid phase. Density functional theory calculations with B3LYP/6-31G (d, p) basis set was used to determine the ground state molecular geometries (bond lengths and bond angles), harmonic vibrational frequencies, infrared intensities and Raman activities of this compound. Potential energy distributions (PEDs) and normal modes, for the spectral data computed at B3LYP/6-31G (d, p) level, have also been obtained from force-field calculations. The wavenumbers found after scaling of the force field showed very good agreement with the experimentally determined values. A comparison of the theoretical spectra and experimental FT-IR and FT-Raman spectra of the title molecule has been made and full vibrational assignments of the observed spectra have been proposed. On the basis of vibrational analyses, the thermodynamic properties of title compound at different temperatures have been calculated.     

Association of model peptides and dehydropeptides: N-acetyl-l-alanine- and dehydroalanine N′,N′-dimethylamides

The comparative studies on the association of Ac-ΔAla-NMe₂ and Ac-l-Ala-NMe₂ in carbon tetrachloride were performed by the analysis of their average molecular weight, dipole moments and FTIR spectra. To aid spectroscopic interpretation and gain some deeper insight into the nature of associates, the geometries of the minimum energy of the dimers of Ac-ΔAla-NMe₂ and Ac-l-Ala-NMe₂ were calculated by the B3LYP/6-31+G** method. The average molecular weight in the studied concentration range, for the ΔAla and l-Ala peptide, as determined by the osmometric method, did not exceed 1.5 and 1.2 of the monomeric mass, respectively. Accordingly, the percentage of the monomeric form (α) decreased as concentration was increased more significantly for the ΔAla analogue than for its saturated counterpart. In the studied concentrations, the dipole moment of the unsaturated compound decreases and that of its counterpart is almost constant. We identified a wider range of dimeric forms of Ac-ΔAla-NMe₂ than those of Ac-l-Ala-NMe₂. While Ac-ΔAla-NMe₂ mainly forms cyclic dimers, built of open conformers H/F, specific for α,β-dehydroamino acids, Ac-l-Ala-NMe₂ forms cyclic and linear dimers, characteristic for the usual amino acids. Ac-ΔAla-NMe₂ has a greater tendency to associate than its saturated variant, which is the result of stronger hydrogen bonds.     

Electronic structure and Raman spectroscopy study of dibarium magnesium orthoborate,Ba2Mg(BO3)2

The samples of dibarium magnesium orthoborate Ba₂Mg(BO₃)₂ were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba₂Mg(BO₃)₂ were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba₂Mg(BO₃)₂ polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5E_u + 4A_2u + 5E_g + 4A_1g + 1A_2g + 1A_1u, of which 5E_g + 4A_1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba₂Mg(BO₃)₂ were assigned as E_g (42 cm⁻¹), A_1g (85 cm⁻¹), E_g (156 cm⁻¹), E_g (237 cm⁻¹), A_1g (286 cm⁻¹), E_g (564 cm⁻¹), A_1g (761 cm⁻¹), A_1g (909 cm⁻¹), E_g (1165 cm⁻¹). The strongest band at 928 cm⁻¹ in the experimental spectrum is assigned to totally symmetric stretching mode of the BO₃ units.     

Structure of the complex of dimethylphenyl betaine with dichloroacetic acid studied by X-ray diffraction,DFT calculations,infrared and Raman spectra

The structure of the complex of dimethylphenyl betaine (DMPB) with dichloroacetic acid (DCA) (1) has been investigated by X-ray diffraction, FTIR and Raman spectroscopy, and B3LYP/6-311 + + G(d,p) calculations. The crystal is monoclinic, space group P2₁. The acid is connected with betaine through the OH⋯O hydrogen bond of 2.480(2) Å. In the optimized structure the short, asymmetric O⋯O distance is 2.491 Å. FTIR spectrum shows a broad absorption in the 1500–400 cm⁻¹ region characteristic of very short OH⋯O hydrogen bond caused by Fermi resonance between νOH and overtones of δOH and γOH. In the Raman spectrum this broad absorption is not observed. The potential energy distributions (PED) were used for the assignments of IR and Raman frequencies in the experimental and calculated spectra. The FTIR and Raman spectra of the crystal complex are consistent with the X-ray results.     

Complexes of aluminium(III) with isoquercitrin: spectroscopic characterization and quantum chemical calculations

The composition and conformation of complexes of aluminium(III) with isoquercitrin (Iso) have been studied in methanol solution. This molecule presents two potential chelating sites in competition. UV–Vis spectroscopy provides evidence of three different species in neutral solution: Al(Iso)²⁺ Al(Iso)₂ ⁺ and Al₂(Iso)³⁺. This last one is formed only if a great quantity of Al(III) is presented in the medium. The first site involved in complexation is the 5-hydroxy-4-keto group. FT-Raman spectroscopy has allowed to confirm this mechanism. The stability constants of this complexes have been determined using the program. In acidic condition, only the first complex is obtained. In alkaline medium, Al(Iso)⁺ and Al(Iso)₂ ⁻ complexes are formed, the catechol group is then the chelating site involved in these species To propose a molecular conformation for the free isoquercitrin and its complexes in neutral medium, both semiempirical molecular orbital and DFT calculations have been performed. It has been showed that the participation of cynnamoyl and benzoyle mesomeric forms stabilise the structure of Al(Iso)²⁺. The structural models have been validated by the good agreement between theoretical and experimental electronic spectra.     

Experimental and quantum chemical studies of structure and vibrational spectra of platinum(II) and palladium(II) thiourea chlorides

Equilibrium geometries of platinum(II) and palladium(II) tetrathiourea dichlorides have been determined by the X-Ray diffraction studies as well as calculated by the Density Functional Theory using the MPW1PW/LanL2DZ functional/basis set. Infrared spectra of the species have also been studied in the 4000–400 cm⁻¹ frequency range both experimentally and theoretically. The experimental and theoretical data remain in satisfactory agreement.