Vibrational spectra and structures of urazole and 4-methylurazole: DFT calculations of the normal modes in aqueous solution and in the solid state,and the influence of hydrogen bonding

Solid-state IR and Raman as well as aqueous solution state Raman spectra are reported for urazole, 4-methylurazole and their deuterated derivatives. DFT calculations, at the B3-LYP/cc-pVTZ level, established that the structures and vibrational spectra of the molecules can be interpreted using models with hydrogen-bonded water molecules, in conjunction with the polarizable continuum solvation method. The vibrational spectra were computed at the optimised molecular geometry in each case, enabling normal coordinate analysis, which yielded satisfactory agreement with the experimental IR and Raman data. Computed potential energy distributions of the normal modes provided detailed vibrational assignments. Solid-state pseudopotential-plane-wave DFT calculations, using the PW91 functional were also carried out, reflecting the importance of intermolecular hydrogen bonding in the solid state.     

Spectroscopic and theoretical studies on nickel(II) complex of maleonitriledithiolate and 2,2'-bipyridine

The complete IR spectra of the title complex Ni(mnt)(bpy) (mnt=maleonitriledithiolate, bpy=2,2'-bipyridine) and a new method to analyze vibrational spectra for such a complicated metal complex are reported in this paper. The molecular geometry, binding, electronic structure and spectroscopic property of it have been studied in detail by theoretical calculations. The geometry optimization from PM3 calculations give that this molecule is of a planar structure with the symmetry point group C(2v) and its ground state is the spin triplet state. The vibrational and electronic spectra were calculated by PM3 and ZINDO/S methods, respectively. The scientific method of analyzing vibrational spectra is established herein by giving main fixed points and pivotal vibrational units. Besides the regular symbols, the new defined symbols eta and M play an important role in describing the vibration modes accurately and vividly.     

Crystal structure and density functional theory study on structural properties and energies of a isonicotinohydrazide compound

An X-ray and a theoretical study of the structure of the isoniazid derivative N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate (1) are reported. In this work, we will report a combined experimental and theoretical study on the molecular structure, vibrational spectra and energies of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate. The calculated parameters are in good agreement with the corresponding X-ray diffraction values. The FTIR spectrum in the range of 400-4000 cm-1 of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate has been recorded. The molecular geometry and vibrational frequencies and energies in the ground state are calculated by using the DFT (B3LYP, PBE1PBE) methods with 6-311G** basis sets. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The geometries and normal modes of vibrations obtained from B3LYP/PBE1PBE/6-311G** calculations are in good agreement with the experimentally observed data.     

Fourier transform infrared and Raman spectra, vibrational assignment and density functional theory calculations of naphthazarin

FT Raman and FTIR spectra of Naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) and its deuterated analogue are recorded. Comparison between the spectra obtained by two techniques, a series of density functional theory (DFT) calculations and the spectral behavior upon deuteration were used for the assignment of the vibrational spectra of this compound. The calculated vibrational frequencies by the B3LYP, B3PW91, G96LYP, G96P86, and MPWLYP density functionals are generally consistent with the observed spectra. Infrared and Raman vibrational transitions predicted by B3LYP/6-311++G** are reported for the titled compound and its deuterated analogous and the assignments are discussed. All experimental and theoretical results support a relatively weak hydrogen bond in naphthazarin (NZ), compared with that in the enol form of normal beta-diketones. The observed nuOH/nuOD and gammaOH/gammaOD appear at about 3060/2220 and 790/560 cm(-1), respectively, which are consistent with the calculated hydrogen bond geometry and proton chemical shift results. Two bands at about 350 and 290 cm(-1) are assigned to the O...O stretching modes belong to A1 and B2 species, respectively.     

DFT studies and vibrational spectra of isoquinoline and 8-hydroxyquinoline

The geometry, frequency and intensity of the vibrational bands of isoquinoline (IQ) and 8-hydroxyquinoline (8-HQ) were obtained by the density functional theory (DFT) calculations with the B3LYP functional and 6-31 G* basis set. The vibrational spectral data obtained from the solid phase mid and far FT-IR and FT-Raman spectra of IQ and 8-HQ are assigned based on the results of the normal coordinate calculations. The observed and the calculated spectra are found to be in good agreement.     

Molecular structure, vibrational spectra and NBO analysis of phenylisothiocyanate by density functional method

The molecular geometry, vibrational frequencies and NBO analysis of phenylisothiocyanate (PITC) in the ground state have been calculated by using density functional theory calculation (B3LYP) with 6-311++G(d,p) basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with experimental values. Comparison of the observed fundamental vibrational frequencies of the PITC and calculated result by density functional theory (B3LYP) indicates B3LYP is superior for molecular vibrational problems. The entropy of the title compound was also performed at HF/B3LYP/6-311++G(d,p) levels of theory. Natural bond orbital (NBO) analysis of title molecule is also carried out. A detailed interpretation of the IR and Raman spectra of PITC is reported on the basis of the calculated potential energy distribution (PED). The theoretical spectrogram for IR spectrum of the title molecule has been constructed.     

High-resolution electronic spectra of ethylenedioxythiophene oligomers

The photophysical properties of a series of 3,4-ethylenedioxythiophene oligomers (OEDOT) with up to five repeat units are studied as function of conjugation length using absorption, fluorescence, phosphorescence, and triplet-triplet absorption spectroscopy at low temperature in a rigid matrix. At 80 K, a remarkably highly resolved vibrational fine structure can be observed in the all electronic spectra which reveals that the electronic structure of the oligomers strongly couples to two different vibrational modes (approximately 180 and approximately 50 meV). The energies of the 0-0 transitions in absorption, and fluorescence, phosphorescence, and triplet-triplet absorption all show a reciprocal dependence on the inverse number of repeat units. The triplet energies inferred from the phosphorescence spectra are accurately reproduced by quantum chemical DFT calculations using optimized geometries for the singlet ground state (S0) and first excited triplet state (T1). Using vibrational IR and Raman spectroscopy and quantum chemical DFT calculations for the normal modes in the ground state, we have been able to assign the vibrations that couple to the electronic structure to fully symmetric normal modes. The high-energy mode is associated with the well-known carbon-carbon bond stretch vibration, and the low-energy mode involves a deformation of the bond angles within the thiophene rings and a change of C-S bond lengths. Experimentally obtained Huang-Rhys parameters and theoretical normal mode deformations are used to analyze the geometry changes between T1 and S0 and to semiexperimentally predict the geometry in the S1 state for 2EDOT.     

Density functional theory studies on vibrational and electronic spectra of 2-chloro-6-methoxypyridine

The Fourier transform infrared (FTIR) and FT-Raman spectra of 2-chloro-6-methoxypyridine have been recorded in the range 3700-400 and 3700-100 cm(-1), respectively. The complete vibrational assignment and analysis of the fundamental modes of the compound was carried out using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with the theoretical frequencies computed by DFT gradient calculations (B3LYP method) employing the 6-31G(d,p), cc-pVTZ and/6-311++G(d,p) basis sets for the optimised geometry of the compound. The geometry and normal modes of vibration obtained from the DFT methods are in good agreement with the experimental data. The normal co-ordinate analysis was also carried out using DFT force fields utilising Wilson's FG matrix method. The influence of the substituents bulky chlorine atom and the methoxy group on the spectral characteristics of the compound has been discussed. The electronic spectrum determined by TD-DFT method is compared with the observed electronic spectrum.     

Theoretical and experimental vibrational spectrum study of 4-hydroxybenzoic acid as monomer and dimer

Theoretical calculations on the molecular geometry and the vibrational spectrum of 4-hydroxybenzoic acid were carried out by the Density Functional Theory (DFT/B3LYP) method. In addition, IR and Raman spectra of the 4-hydroxybenzoic acid in solid phase were newly recorded using them in conjunction the experimental and theoretical data (including SQM calculations), a vibrational analysis of this molecular specie was accomplished and a reassignment of the normal modes corresponding to some spectral bands was proposed. The geometries of monomers and dimers in gas phase were optimized using the DFT B3LYP method with the 6-31G*, D95** and 6-311++G** basis sets. Also, both the vibrational spectra recorded and the results of the theoretical calculations show the presence of one stable conformer for the 4-hydroxybenzoic acid cyclic dimer. The B3LYP/6-31G* method was used to study the structure for cyclic dimer of 4-hydroxybenzoic acid and for a complete assignment our results were compared with results of the cyclic dimer of benzoic acid. A scaled quantum mechanical analysis was carried out to yield the best set of harmonic force constants. The formation of the hydrogen bond was investigated in terms of the charge density by the AIM program and by the NBO calculations.     

Surface-enhanced Raman scattering study of the adsorption of croconate violet on colloidal silver particles

Very stable silver particle suspension has been synthesized for use in surface-enhanced Raman scattering (SERS) spectroscopy with near-infrared exciting radiation. Such citrate-stabilized silver particles were obtained through a suitable control of the nucleation and growth process during the synthesis. The SERS spectra of the bis(dicyanomethylene) croconate dianion or croconate violet (CrocV) were obtained, with excitation in the near-infrared and in the visible region. The differences in the spectral patterns were correlated with a pre-resonance Raman effect of the adsorbate. The vibrational frequencies of CrocV isolated and interacting with silver surface were obtained through theoretical calculations using DFT method that together the surface selection rules allowed to perform the vibrational assignment of the SERS spectra and to infer the adsorption geometry.     

Theoretical investigation of the structure and vibrational spectra of carbamoyl azide

Molecular structure and vibrational frequencies of carbamoyl azide NH2CO-NNN have been investigated with ab initio and density functional theory (DFT) methods. The molecular geometries for all the possible conformers of the molecule were optimized using DFT-B3LYP, DFT-BLYP and MP2 applying the standard 6-311++G** basis set. From the calculations, the molecule was predicted to exist predominantly in cis conformation with the cis-trans rotational barrier of about 7.91-9.10 kcal/mol depending on the level of theory applied. The vibrational frequencies and the corresponding vibrational assignments of carbamoyl azide in Cs symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule in the cis conformation were plotted. Observed frequencies for normal modes were compare with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G** basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

Vibrational assignment and structure of dibenzoylmethane. A density functional theoretical study

Molecular structure and vibrational frequencies of 1,3-diphenyl-1,3-propanedione, known as dibenzoylmethane (DBM), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA) and acetylacetone (AA), the parent molecule. IR and Raman spectra of DBM and its deuterated analogue were clearly assigned.The calculated hydrogen bond energy of DBM is 16.15 kcal/mol, calculated at B3LYP/6-311++G** level of theory, which is 0.28 kcal/mol more than that of AA. This result is in agreement with the vibrational and NMR spectroscopy results. The molecular stability and the hydrogen bond strength were investigated by applying the Natural Bond Orbital analysis (NBO) and geometry calculations. The theoretical calculations indicate that the hydrogen bond in DBM is relatively stronger than that in BA and AA.     

Molecular structure, spectroscopic studies and first-order molecular hyperpolarizabilities of ferulic acid by density functional study

Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of ferulic acid (FA) (4-hydroxy-3-methoxycinnamic acid) were carried out by using density functional (DFT/B3LYP/BLYP) method with 6-31G(d,p) as basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with single crystal XRD data. The vibrational spectral data obtained from solid phase FT-IR and FT-Raman spectra are assigned based on the results of the theoretical calculations. The observed spectra are found to be in good agreement with calculated values. The electric dipole moment (μ) and the first hyperpolarizability (β) values of the investigated molecule have been computed using ab initio quantum mechanical calculations. The calculation results also show that the FA molecule might have microscopic nonlinear optical (NLO) behavior with non-zero values. A detailed interpretation of the infrared and Raman spectra of FA was also reported. The energy and oscillator strength calculated by time-dependent density functional theory (TD-DFT) results complements with the experimental findings. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The theoretical FT-IR and FT-Raman spectra for the title molecule have been constructed.     

FT-IR,FT-Raman,ab initio and DFT structural and vibrational frequency analysis of 6-aminopenicillanic acid

Quantum mechanical calculations of energies, geometries and vibrational wavenumbers of 6-aminopenicillanic acid were carried out by using ab initio HF and density functional theory (DFT/B3LYP) methods with 6-311G(d,p) basis set. The optimized geometrical parameters obtained by HF and DFT calculations are in good agreement with experimental X-ray data. A detailed interpretation of the infrared spectra has also been reported. The theoretical IR and Raman spectrograms have been constructed and compared with the experimental FT-IR and FT-Raman spectra. The differences between the observed and scaled wavenumber values of most of the fundamentals are very small. The thermodynamic parameters have also been computed.     

DFT and ab initio quantum chemical studies on p-cyanobenzoic acid

The Fourier transform infrared (FTIR) and FT-Raman spectra of p-cyanobenzoic acid (CBA) have been recorded in the range 4000-400 and 4000-100 cm(-1), respectively. The complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with theoretical wavenumbers obtained from ab initio HF and DFT-B3LYP gradient calculations employing 6-31G**, 6-311++G** and cc-pVTZ basis sets for the optimised geometry of the compound. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method. The interactions of cyano and carboxylic acid groups with the skeletal vibrational modes were investigated.     

Vibrational spectra and structures of the anions of urazole and 4-methylurazole: DFT calculations of the normal modes and the influence of hydrogen bonding

Solid state IR and Raman as well as aqueous solution state Raman spectra are reported for the anions of urazole and 4-methylurazole, and their N-deuterated derivatives. DFT calculations, at the B3-LYP/cc-pVTZ level, established that the structures and vibrational spectra of both anions can be interpreted using a model that incorporates hydrogen-bonded water molecules, in conjunction with the polarizable continuum solvation method. In the case of the urazole anion it is shown that deprotonation occurs primarily at N1 rather than N4, but there is also evidence for the second tautomer both in the solid state and in aqueous solution. The vibrational spectra were computed at the optimised molecular geometry in each case, enabling normal coordinate analysis, which yielded satisfactory agreement with the experimental IR and Raman data. Computed potential energy distributions of the normal modes provided detailed vibrational assignments.     

Spectroscopic investigations and ab initio computations of the dye Chromotrope 2R

Detailed analysis of the NIR FT-Raman, FT-IR and UV–visible spectra of the dye Chromotrope 2R (C2R) has been performed. The optimized geometry of the dye is theoretically computed with the HF and DFT levels using the standard 6-31G(d) and LANL2DZ basis sets. Optimized geometry and vibrational spectra indicate that the major species in the solid state are the trans form of hydrogen bonded hydrazone tautomer. The effect of H-bonding in stabilizing a particular type of structure is also discussed. The most preferred trans-configuration for its photochemical activity has been demonstrated on the basis of torsional potential energy surface (PES) scan studies. The optimized geometries and calculated vibrational wavenumbers are evaluated via comparison with experimental values. Electronic spectra are in accordance with the nature of the electronic transitions predicted by time-dependent B3LYP/DZ calculations.     

The inelastic neutron scattering spectrum of H3B:NH3 and the reproduction of its solid-state features by periodic DFT

The inelastic neutron scattering (INS) and periodic density functional theory (DFT) vibrational spectra of H3B:NH3 are reported to 1600 cm-1. The H3B:NH3 structural and INS features, specifically the reduced solid-state B:N dative bond length and the altered B:N stretching frequency, are reproduced by the periodic DFT calculations, placing the B:N stretching mode at 800 cm-1, in excellent agreement with experiment relative to previous nonperiodic theoretical treatments of this molecule.     

Density functional theory calculations of the internal rotations and vibrational spectra of 2-, 3- and 4-formyl pyridine

The torsional potentials, molecular conformations and vibrational spectra, of 2-, 3- and 4-formyl pyridine have been investigated using density functional theory (DFT) method with 6-31+G* basis set. From the calculations, 2-formyl pyridine and 3-formyl pyridine were predicted to exist predominantly in cis conformation with the cis-trans rotational barrier of 9.38 kcal/mol and 8.55 kcal/mol, respectively. The two equivalent planar structures of 4-formyl pyridine are separated by an energy barrier of 7.18 kcal/mol. The vibrational wavenumbers and the corresponding vibrational assignments of molecules in C(s) symmetry were examined theoretically and the calculated Infrared of the molecules in the cis conformation was plotted. Observed wavenumbers for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of DFT force fields using the standard 6-31+G* basis set of the theoretical optimized geometry.     

FT-IR, FT-Raman spectra and ab initio DFT vibrational analysis of p-bromophenoxyacetic acid

The Fourier transform Raman and Fourier transform infrared spectra of p-bromophenoxyacetic acid were recorded in the solid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities were calculated by HF and DFT (B3LYP) method with the 6-31G(d,p) basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental ones. A detailed interpretation of the infrared and Raman spectra of p-bromophenoxyacetic acid is reported on the basis of the calculated potential energy distribution. The theoretical spectrograms for the IR spectrum of the title molecule have been constructed.