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.     

Fourier transforms infrared spectra and structure of triformylmethane. A density functional theoretical study

Molecular structure and vibrational frequencies of triformylmethane have been investigated by means of density functional theory (DFT) calculations. The geometrical parameters and vibrational frequencies obtained in the B3LYP, B3PW91, BLYP, BPW91, G96LYP and G96PW91 levels of DFT and compared with the corresponding parameters of malonaldehyde (MA). Fourier transform infrared spectra of triformylmethane and its deuterated analogue were clearly assigned. Theoretical calculations show that the hydrogen bond strength of triformylmethane is stronger than that of MA, which is in agreement with spectroscopic results.     

Vibrational structure of titanium silicate catalysts. A spectroscopic and theoretical study.

A thorough analysis of the vibrational features of the titanium silicalite-1 (TS-1) catalyst is presented, based on quantitative IR measurements, Raman and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm(-1) associated with titanium insertion into the zeolite framework. The enhancement of the intensity of the 1125 cm(-1) feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of resonant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theory provide the basis for the assignment of the main vibrational contributions and for the understanding of Raman enhancement. The resonance-enhanced 1125 cm(-1) mode is unambiguously associated with a totally symmetric vibration of the TiO(4) tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as a totally symmetric stretching of the four Si-O bonds pointing toward Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO(4) units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxygen atom belongs to an OH group, such as in terminal tetrahedra of cluster models and in real defective zeolites. Asymmetric SiO(4) and TiO(4) stretching modes appear above and below 1000 cm(-1), respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm(-1) (in both SiO(4) and TiO(4)) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features associated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO(4) modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO(4) vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti caused by water adsorption quenches the UV-Raman enhancement of the 1125 cm(-1) band.     

Intramolecular hydrogen bond, molecular structure and vibrational assignment of tetra-acetylethane. A density functional study

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of tetra-acetylethane have been investigated by means of high-level density functional theory (DFT) methods with most popular basis sets. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were recorded in the regions 400-4000 cm(-1) and 40-4000 cm(-1), respectively. The calculated geometrical parameters of tetra-acetylethane were compared to the experimental results of this compound and its parent molecule (acetylacetone), obtained from X-ray diffraction. The O...O distance in tetra-acetylethane, about 2.424A, suggests that the hydrogen bond in this compound is stronger than acetylacetone. This conclusion is well supported by the NMR proton chemical shifts and O-H stretching mode at 2626 cm(-1). Furthermore, the calculated hydrogen bond energy in the title compound is 17.22 kcal/mol, which is greater than the acetylacetone value. On the other hand, the results of theoretical calculations show that the bulky substitution in alpha-position of acetylacetone results in an increase of the conjugation of pi electrons in the chelate ring. Finally, we applied the atoms in molecules (AIM) theory and natural bond orbital method (NBO) for detail analyzing the hydrogen bond in tetra-acetylethane and acetylacetone. These results are in agreement with the vibrational spectra interpretation and quantum chemical calculation results. Also, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.     

Vibrational spectra and density functional study of propylgermane

Raman and infrared spectra of propylgermane, CH₃CH₂CH₂GeH₃, and its Ge-deuterated analog, CH₃CH₂CH₂GeD₃, were investigated in their gaseous, liquid and solid states. The normal coordinate treatment was carried out by density functional theory (DFT) calculation, using B3LYP/6-31G^* and 6-311++G^** basis sets, and the corresponding fundamental vibrations were assigned. The trans (T) and gauche (G) forms around the central C–C bond coexisted in the gaseous and liquid states and only the T form existed in the solid state. From the temperature dependent measurements of the Raman spectra in the liquid state, the enthalpy difference was found to be ΔH(T−G)=−0.36±0.02 kcalmol⁻¹ with the T form being more stable. The energy differences between the isomers obtained by DFT calculations were ΔE(T−G)=−0.46 kcalmol⁻¹ and ΔE(T−G)=−0.87 kcalmol⁻¹ by the 6-31G^* basis set and 6-311++G^** basis set, respectively.     

Structure and vibrational spectra of the enol form of hexafluoro-acetylacetone. A density functional theoretical study

Molecular and vibrational structure of 1,1,1,6,6,6-hexafluoropentane-2,4-dione (hexafluoro-acetylacetone) have been investigated by means of density functional theory (DFT) calculations and have been compared with those of acetylacetone, the parent molecule. According to the theoretical calculations HFAA has an asymmetric structure with hydrogen bond strength of about 12 kcal mol(-1), about 6 kcal mol(-1) less than that of acetylacetone. This weakening of hydrogen bond is consistent with frequency shifts for OH/OD stretching, OH/OD out of plane bending and O...O stretching modes upon substitution of methyl hydrogen atoms with fluorine atoms. The symmetric structure based on electron diffraction data is interpreted as superposition of two asymmetric structures.     

Vibrational frequencies and structure of 2-thiouracil by Hartree-Fock,post-Hartree-Fock and density functional methods

Vibrational study of the biomolecule 2-thiouracil was carried out. Ab initio and density functional calculations were performed to assign the experimental spectra. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted frequencies of 2-thiouracil. Several scaling procedures were used. The geometry structure of the molecule was determined. The effect of sulfur substitution at C2 position in the uracil molecule, on the N1-H and N3-H frequencies and intensities reflects changes in proton donor abilities of these groups. Calculations with the 6-31 G** basis set with HF and DFT methods appear in general to be useful for interpretation of the general features of the IR and Raman spectra of the molecule. Using specific scale factors a very small error was obtained. The use of these specific scale factors resolve and correct some of the controversial assignments in the literature.     

Vibrational frequencies of hydrazoic acid and methyl azide: density functional theory study

Harmonic vibrational frequencies of HN3 and CH3N3 molecules and their several isotopomers are calculated using HF, MP2 and five popular density functional theory (DFT) methods. On the basis of the comparison between calculated and experimental results, assignments of fundamental vibrational modes arc examined. HF and MP2 results are in bad agreement with experimental values. Of the five DFT methods, BLYP reproduces the observed fundamental frequencies the most satisfactorily. Two hybrid DFT methods are found to yield frequencies generally higher than the observed fundamental frequencies. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.     

Molecular structure and vibrational assignment of α‐chloro acetylacetone: A density functional theory study

The intramolecular hydrogen bond, molecular structure, and vibrational frequencies of α‐chloro acetylacetone have been investigated. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were recorded in the regions 400–4,000 cm^?1 and 50–4,000 cm^?1, respectively. Rigorous normal coordinate analysis has been performed at the B3LYP/6‐311++G** level of theory for purposes of comparison. The complete vibrational assignment for TFAA has been made on the basis of the calculated potential energy distribution. We also applied the atoms in molecules theory and natural bond orbital method for the analysis of the hydrogen bond in α‐Chloro acetylacetone and acetylacetone. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A combined density functional and x-ray diffraction study of Pt nanoparticle structure

The structure of 1.7 nm Pt nanoparticles is investigated using x-ray diffraction (XRD) measurements and density functional theory (DFT) calculations. Two types of particles are compared, those made by solution chemistry which are capped either by thiol or amine ligands, and dendrimer encapsulated particles (DENs) which do not have capping ligands. All particles were dried before analyzing their structure. Pair distribution function (PDF) data from XRD measurements show that the ligand-capped particles are more disordered than the DENs. To determine the structure of the particles and the nature of the ligand-induced disorder, we use a hybrid reverse Monte Carlo approach. A weighted average of the calculated binding energy of the particles and a goodness-of-fit parameter to the PDF data is taken as the object function, which is minimized to determine the optimal structure. A scan over different weights gives the set of pareto optimal structures, which show how well simultaneous agreement can be reached to both experiment and theory. Using an embedded atom potential to sample configuration space and DFT to refine the optimal structures, we show that the DEN structure is most consistent with a face centered cubic lattice of truncated octahedral shape. The disorder induced by the capping ligands is consistent with surface relaxation of the particle rather than disorder of the crystal structure.     

Molecular structure and vibrational spectra of 2,6-bis(benzylidene)cyclohexanone: a density functional theoretical study

The near-infrared Fourier transform (NIR-FT) Raman and Fourier transform infrared (FT-IR) spectral analyses of 2,6-bis(benzylidene)cyclohexanone (BBC) molecule, a potential drugs for the treatment of P388 leukemia cells, were carried out along with density functional computations. The optimized geometry of BBC using density functional theory shows that the energetically favored chair conformation is not observed for central cyclohexanone ring and is found to possess a nearly ‘half chair’ conformation and shows less expansion of the angles and more rotation about the bonds. The existence of intramolecular C–H?O improper, blue-shifted hydrogen bond was investigated by means of the NBO analysis. The lowering of carbonyl stretching vibration can be attributed to the mesomeric effect and the π-orbital conjugation induced by the unsaturation in the α-carbon atoms and co-planarity of the (–CHC–(CO)–CCH–) group.     

Reaction of CH3CHO with Y+: A density functional theoretical study

The reaction mechanism of the Y⁺ cation with CH₃CHO has been investigated with a DFT approach. All the stationary points are determined at the UB3LYP/ECP/6-311++G** level of the theory. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of a CH₃CHO-metal complex followed by C-C, aldehyde C-H, methyl C-H and C-O activation. These reactions can lead to four different products (Y⁺CH₄ + CO, Y⁺CO + CH₄, Y⁺COCH₂ + H₂ and Y⁺O + C₂H₄). The minimum energy reaction path is found to involve the spin inversion in the different reaction steps, this potential energy curve-crossing dramatically affects reaction exothermic. The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.     

Vibrational assignment of acetylacetone

The infrared and Raman spectra of acetylacetone and its deuterated analogues have been analyzed by the aid of ab initio calculations at post Hartree-Fock level and considering the spectral behavior upon deuteration. By deconvolution of the infrared spectra of acetylacetone and d6-acetylacetone at 1600 cm(-1) region a broad and strong band is found and correlated with the strong Raman lines observed for these compounds in this region. The broadness of this infrared band at room temperature and it's splitting at low temperature is attributed to free rotation of methyl group attached to carbonyl group at room temperature. Furthermore it is found that all ring modes in 1200-1600 cm(-1) region more or less are mixed with the OH in plane bending motion.     

Vibrational spectral investigation of four second order nonlinear optical azobenzene-containing materials: a combination of experimental and density functional theoretical (DFT) study

In this work, four-second order nonlinear optical (NLO) azobenzene-containing materials are studied in-depth by using vibrational spectra and density functional theory (DFT). The Fourier transform infrared (FT-IR) spectra and FT-Raman spectra are recorded in the range of 50-4000 and 100-3600cm(-1), respectively. Meanwhile, the DFT computations are performed at B3LYP/6-31G (d, p) level to derive equilibrium geometry, vibrational wavenumbers and intensities, and first hyperpolarizability, and the scaled theoretical wavenumbers are also shown to be in good agreement with experimental data. The calculated results show that these four azobenzene-containing compounds are good materials and the compound with nitro substituent groups possesses a larger first molecular hyperpolarizability (β) value. Moreover, the simultaneous infrared and Raman activation of R1 group and CC stretching suggest that the charge transfer interaction might occur between the R1 group and phenyl ring, and the HOMO-LUMO gap analysis also supports this viewpoint.     

Vibrational spectroscopic and density functional theory studies of chloranil-imidazole interaction

The present work reports vibrational spectra and density functional theory calculations for chloranil, imidazole and their complexes. The experimentally observed infrared and Raman bands have been assigned with the help of calculated vibrational frequencies and potential energy distribution analysis. Some bands of chloranil and imidazole have been found to shift on the complex formation due to partial electronic charge transfer from imidazole to chloranil. The charge transfer between these molecules is also corroborated by the electronic absorption spectroscopy and calculations. The theoretical values of the interaction energy of various possible chloranil-imidazole interactions suggest that the two molecules interact preferably via N and H atoms of imidazole and CO group of chloranil with their molecular planes almost perpendicular to each other.     

Vibrational spectra and assignments using ab initio and density functional theory analysis on the structure of biotin

Density functional theory (DFT) and Hartree-Fock calculations were performed using the following models: HF/6-311G(d), B3LYP/6-311G(d), B3LYP/6-311+G(d) and B3PW91/6-311G(d) calculations were performed for biotin. It has been characterized by IR and X-ray. The calculated results show that the predicted geometry can well reproduce the structural parameters. Predicted vibrational frequencies have been assigned and compared with experimental IR spectra and they supported each other. On the basis of vibrational analyses, the thermodynamic properties of the title compound at different temperatures have been calculated, revealing the correlations between Cp,m degrees, Sm degrees, Hm degrees and temperatures.     

Quantum parameters based study of some heterocycles using density functional theory method: A comparative theoretical study

Density functional theory can envisage a vast assortment of molecular possessions such as molecular structures, vibrational frequencies, molecular energies, ionization energies, polar, electric and magnetic properties etc. The efficacy of this method relies on the study of electronic parameters to categorize the reactive sites to comprehend the plausible action of these scaffolds. Further it also facilitates the correlation between the structural characteristics of drug and their inhibition efficiency against infectious microorganisms. In light of the above facts, we have studied the structural parameters such as energy (total), variation of electron density over highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), charge distribution, absolute electronegativity (χ), softness/hardness (σ/ɳ) and fraction of electron transfer (ΔN) of some previously synthesized heterocycles.     

Signature of nanodiamond in Raman spectra: a density functional theoretical study

Nanocrystalline diamond (NCD) has attracted great attention both experimentally and theoretically in the past few years. The identification of the presence and the amount of NCD in amorphous carbon film has been a challenging issue. Although Raman spectroscopy has become a standard tool for the characterization of various carbon phases, a simple criterion for identifying NCD has not yet been well-established. In the present work, a theoretical study of the Raman spectra of the model compounds of NCD is presented on the basis of the density functional calculations. The reliability of the computational approach has been tested by comparing the predicated Raman spectra of several reference molecules to those obtained experimentally. To show the unique Raman spectrum of a NCD phase, a series of the model compounds of various carbon materials including tetrahedral and hexagonal clusters, and trans-polyacetylene fragments, were considered, and subsequently their Raman spectra below 2000 cm(-1) were calculated and compared with each other. The calculated results indicate that the relatively stronger broad peak at about 480 cm(-1) could be used as the signature of a NCD phase in the sample.     

Molecular dynamics and density functional theory simulations of matrix deposition. II. Absolute site structure assignment for porphyrin in xenon

Molecular dynamics calculations reveal that the main trapping site for porphyrin embedded in a xenon matrix corresponds to a hexagonal cavity formed after removal of seven host atoms. Tautomerization involving two inner hydrogen atoms leads to two trans forms that interact differently with the matrix cage. Therefore, both electronic and infrared spectra are split into doublets. Comparison of the experimentally observed splitting patterns with the results of density functional theory calculations that explicitly include the nearest xenon atoms allows assigning each spectral feature to one of two different configurations of the chromophore inside the xenon cavity. The main factor responsible for the splittings is a distortion of the molecular skeleton from a squarelike towards rectangular geometry.