Theoretical and experimental studies of vibrational spectra and thermodynamical analysis of 3�?bromopropiophenone and 4�?bromo-3-chloropropiophenone

Quantum chemical calculations of molecular geometries, vibrational wavenumbers and thermodynamical properties of 3'-bromopropiophenone and 4'-bromo-3-chloropropiophenone were carried out using Hartree-Fock (HF) and density functional theory (DFT) using hybrid functional B3LYP with 6-31?G (d,p) as basis set. The optimized geometrical parameters obtained by HF and DFT calculations are in good agreement with the experimental FTIR and FT Raman spectral datas. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed simulated spectrograms.     

Density functional theory,restricted Hartree–Fock simulations and vibrational spectroscopic studies of nicorandil

Fourier transform infrared and Raman spectra of nicorandil have been recorded. The structure, conformational stability, geometry optimisation and vibrational frequencies have been investigated. Complete vibrational assignments were made for the stable conformer of the molecule using restricted Hartree–Fock (RHF) and density functional theory (DFT) calculations (B3LYP) with the 6-31G(d,p) basis set. Comparison of the observed fundamental vibrational frequencies of the molecule and calculated results by RHF and DFT methods indicates that B3LYP is superior for molecular vibrational problems. The thermodynamic functions of the title molecule were also performed using the RHF and DFT methods. Natural bond order analysis of the title molecule was also carried out. Comparison of the simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibration modes.     

Density functional and Møller–Plesset studies of cyclobutanone⋯HF and ⋯HCl complexes

The molecular structure (hydrogen bonding, bond distances and angles), dipole moment and vibrational spectroscopic data [vibrational frequencies, IR and vibrational circular dichroism (VCD)] of cyclobutanone?HX (X?=?F, Cl) complexes were calculated using density functional theory (DFT) and second order Møller–Plesset perturbation theory (MP2) with basis sets ranging from 6–311G, 6–311G^**, 6–311 + + G^**. The theoretical results are discussed mainly in terms of comparisons with available experimental data. For geometric data, good agreement between theory and experiment is obtained for the MP2 and B3LYP levels with basis sets including diffuse functions. Surface potential energy calculations were carried out with scanning HCl and HF near the oxygen atom. The nonlinear hydrogen bonds of 1.81 Å and 175° for HCl and 1.71 Å and 161° for HF were calculated. In these complexes the C=O and H–X bonds participating in the hydrogen bond are elongated, while others bonds are compressed. The calculated vibrational spectra were interpreted and the band assignments reported are in excellent agreement with experimental IR spectra. The C=O stretching vibrational frequencies of the complexes show red shifts with respect to cyclobutanone.     

FT-IR,FT-Raman spectra and DFT vibrational analysis of 2-aminobiphenyl

In this work, the Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectra of 2-aminobiphenyl (2ABP) were recorded in the solid phase. The optimised geometry, frequency and intensity of the vibrational bands of 2ABP were obtained by the density functional theory (BLYP and B3LYP) methods with complete relaxation in the potential energy surface using 6-31G(d) basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed spectrograms.     

FTIR,FT-Raman spectra and quantum chemical studies of nebivolol

Fourier transform infrared and Raman spectra of nebivolol have been recorded. The structure, conformational stability, geometry optimisation, and vibrational wave numbers have been investigated. Satisfactory vibrational assignments were made for the stable conformer of the molecule using Restricted Hartree–Fock (RHF) and density functional theory (DFT) calculation (B3LYP) with the 6-31G(d,p) basis set. Comparison of the observed fundamental vibrational wave numbers of the molecule and calculated results by RHF and DFT methods indicates that B3LYP is superior for molecular vibrational problems. Comparison of the simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. The RHF and DFT-based NMR calculation procedure was also done. It was used to assign the ¹³C NMR chemical shift of nebivolol.     

Molecular structure and effects of intermolecular hydrogen bonding on the vibrational spectrum of trifluorothymine, an antitumor and antiviral agent

In the present work, the experimental and the theoretical vibrational spectra of trifluorothymine were investigated. The FT-IR (400-4000?cm(-1)) and μ-Raman spectra (100-4000?cm(-1)) of trifluorothymine in the solid phase were recorded. The geometric parameters (bond lengths and bond angles) and vibrational frequencies of the title molecule in the ground state were calculated using ab initio Hartree-Fock (HF) method and density functional theory (B3LYP) method with the 6-31++G(d,p) and 6-311++G(d,p) basis sets for the first time. The optimized geometric parameters and the theoretical vibrational frequencies were found to be in good agreement with the corresponding experimental data and with results found in the literature. Vibrational frequencies were assigned based on the potential energy distribution using the VEDA 4 program. The dimeric form of trifluorothymine was also simulated to evaluate the effect of intermolecular hydrogen bonding on the vibrational frequencies. It was observed that the stretching modes shifted to lower frequencies, while the in-plane and out-of-plane bending modes shifted to higher frequencies due to the intermolecular N-H?O hydrogen bonds.     

Molecular structure,spectroscopic (FT-IR,FT-Raman,UV–vis), NBO,thermochemistry analysis of bis(thiourea)zinc(II) chloride crystals

The experimental and theoretical studies on the molecular structure and vibrational spectra of bis(thiourea)zinc(II) chloride (BTZC) crystals were investigated. The Fourier transform infrared, Fourier transform Raman and UV–vis spectra of BTZC were recorded. The molecular geometry and vibrational frequencies of BTZC in the ground state were calculated by using B3LYP with LANL2DZ as basis set. Comparison of the observed structural parameters of BTZC with single-crystal X-ray studies yields a good agreement. Vibrational analysis of the simultaneous IR and Raman activation of the Zn–Cl stretching mode in the molecule provides the evidence for the charge transfer interaction taking place within the molecule. The energy and oscillator strength are calculated by time-dependent density functional theory. The simulated spectra satisfactorily coincide with the experimental spectra.     

DFT studies on the intrinsic conformational properties of non-ionic pyrrolysine in gas phase

B3LYP/6-31G(d,p) level of theory is used to carry out a detailed gas phase conformational analysis of non-ionized (neutral) pyrrolysine molecule about its nine internal back-bone torsional angles. A total of 13 minima are detected from potential energy surface exploration corresponding to the nine internal back-bone torsional angles. These minima are then subjected to full geometry optimization and vibrational frequency calculations at B3LYP/6-31++G(d,p) level. Characteristic intramolecular hydrogen bonds present in each conformer, their relative energies, theoretically predicted vibrational spectra, rotational constants and dipole moments are systematically reported. Single point calculations are carried out at B3LYP/6-311++G(d,p) and MP2/6-31++G(d,p) levels. Six types of intramolecular H-bonds, viz. O…H–O, N…H-O, O…H–N, N…H–N, O…H–C and N…H–C, are found to exist in the pyrrolysine conformers; all of which contribute to the stability of the conformers. The vibrational frequencies are found to shift invariably toward the lower side of frequency scale corresponding to the presence of intramolecular H-bond interactions in the conformers.     

Vibrational and electronic circular dichroism monitoring of copper(II) coordination with a chiral ligand

Novel copper(II) coordination compounds with chiral macrocyclic imine ligands derived from R-/S-camphor were asymmetrically synthesized and characterized with the aid of chiroptical spectroscopies. Crystal structures of both enantiomers were determined by single crystal X-ray diffraction analysis. Circular dichroism (CD) spectra were analyzed using a simplified exciton model as well as quantum chemical computations. The absolute configuration of the copper(II) coordination compounds determined from CD was found consistent with the crystal data. The copper(II) complexes were further investigated by vibrational CD (VCD) measurement combined with density functional theory calculation. The complex formation was evidenced by spectral shifts of the characteristic bands in the CD and VCD spectra.     

Vibrational spectra of deuterated methane and water molecules in structure I clathrate hydrate from ab initio MD simulation

The deuteration of the lattice molecules in clathrate hydrates is a widely used experimental technique to clearly separate the vibrational modes. However, the effect of the deuteration on the vibrational spectra and molecular motions is not fully understood. Since the guest–host coupling may change the vibrational spectra, a detailed analysis of the vibrational spectra of deuterated clathrate hydrate is significant in the understanding of the mechanism of the vibrational shift. In this study, the vibrational spectra of the deuterated methane hydrates were calculated by ab initio molecular dynamics simulation. The intramolecular vibrational frequency of the methane in D₂O lattice and deuterated methane in H₂O lattice was calculated and compared with the pure methane hydrate. The bending, rocking and overtone of the bending mode was also reported. The effect of coupling of the rattling motions of guest and host molecules on the vibrational spectra was revealed.     

Ab initio vibrational calculations on ara-T molecule: application to analysis of IR and Raman spectra

The FTIR and FT-Raman spectra are reported for the arabinonucleoside ara-T (1-beta-D-arabinofuranosylthymine), which shows antiviral activity. The accurate knowledge of the vibrational modes is a prerequisite for the elucidation of drug-nucleotide and drug-enzyme interactions. The FTIR and FT-Raman spectra of ara-T were recorded from 4000 to 30 cm(-1). A tetradeuterated derivative (deuteration at N3, and hydroxyl groups O'2, O'3, and O'5) was synthesized and the observed isotopic shifts in its spectra were used for the vibrational analysis of ara-T. The theoretical frequencies and the potential energy distribution (PED) of the vibrational modes of ara-T were calculated using the ab initio Hartree-Fock/3-21G method. An assignment of the vibrational spectra of ara-T is proposed considering the scaled PED and the observed band shifts under deuteration. The scaled ab initio frequencies were in reasonable agreement with the experimental data.     

Experimental and theoretical studies of the structure of tellurate-borate glasses network

The structural properties of the xTeO₂·(1-x)B₂O₃ glasses (x = 0.6; 0.7) were investigated by FT-IR spectroscopy. From the analysis of the FTIR spectra, it is reasonable to assume that by the increasing of boron ions content, the tetrahedral [BO₄] units are gradually replaced by the trigonal [BO₃] units. The increase in the number of non-bridging oxygen atoms would decrease the connectivity of the glass network and will yield the depolymerization of the borate chains. The molecular structure and vibrational frequencies of the proposed structural models have been studied by exploring the density functional theory (DFT) calculations. The FTIR spectra of the xTeO₂·(1-x)B₂O₃ vitreous systems were compared with the calculated spectrum. This procedure allowed us to assign most of the observed IR bands.     

Vibrational analysis and spectra of orotic acid

The IR and Raman spectra of polycrystalline anhydrous orotic acid and its N1, N3, and O12 trideuterated isotopomer are recorded in the 4000-40 cm(-1) spectral interval as part of a series of vibrational analyses of nucleosides, nucleotides, and related compounds carried out in our laboratory. The frequencies of the fundamental transitions and the potential energy distributions of the 39 normal modes of orotic acid and its isotopomer are calculated by an ab initio density functional theory Becke3P86/6-311G** treatment. Assignments of the vibrational modes are proposed that consider the results of these calculations and the observed spectra. The results of the ab initio treatment are related to crystallographic and spectral data, and they are compared with previous assignments for similar molecules.     

Electronic structure simulations of 2,6-dimethyl-2,5-heptadien-4-one by FTIR,FT-Raman,NMR, UV–vis,NBO and density functional theory

Density functional theory (DFT) (B3LYP and B3PW91) calculations have been carried out for 2,6-dimethyl-2,5-heptadien-4-one (DMHD4O) using 6–311++ G** basis set. Complete vibrational assignment and analysis of the fundamental modes of the compound were carried out from the FTIR and FT-Raman spectral data. The theoretical electronic absorption has been calculated by using time-dependent DFT (TD-DFT) methods and compared with the experimental spectra. The theoretically computed Frontier energy gaps and TD-DFT calculations are in good agreement with the experimental UV–vis spectral absorption. The chemical hardness measured from the Frontier molecular orbital energies of DMHD4O is 0.0693 eV. Electronic stability of the compound arising from hyperconjugative interactions and charge delocalisation were also investigated based on the natural bond orbital (NBO) analysis. Effective stabilisation energy E ⁽²⁾ associated with the interactions of the π and the lone pair of electrons was determined by the NBO analysis. ¹³C and ¹H NMR chemical shifts of the compound have been calculated by means of Gauge-Invariant Atomic Orbital using B3LYP/6–311++ G** method. The partial ionic character of the carbonyl group due to resonance render a partially positive charge to the carbonyl carbon, and thus C4 chemical shift lie in the very downfield 191.6 ppm. Comparison between the experimental and the theoretical results indicates that B3LYP method is able to provide satisfactory results for predicting vibrational, electronic and NMR properties.     

An ab initio and DFT study of structure and vibrational spectra of γ form of Oleic acid: Comparison to experimental data

Oleic acid (cis-9-octadecenoic acid) is the most abundant cis-unsaturated fatty acid in nature; it is distributed in almost all organisms. In this work, we present a detailed vibrational spectroscopy investigation of Oleic acid by using infrared and Raman spectroscopies. These data are supported by quantum mechanical calculations, which allow us to characterize completely the vibrational spectra of this compound. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP with complete relaxation in the potential energy surface using 6-311G(d, p) basis set. After a proper scaling the calculated wavenumbers show a very good agreement with the observed values. A complete vibrational assignment is provided for the observed Raman and infrared spectra of Oleic acid. In this work, we also investigate the deviation of vibrational wavenumbers computed with two quantum chemical methods (HF and B3LYP).     

Isotopic difference spectra as an aide in determining absolute configuration using vibrational optical activity: vibrational circular dichroism of 13C- and 2H-labelled nonamethoxy cyclotriveratrylene

The use of isotopic difference spectra in vibrational optical activity is demonstrated as a supplemental aide in determining the absolute configuration of chiral molecules. It is shown that IR and VCD difference spectra associated with isotopic substitution observed in experimental spectra can be accurately reproduced by density functional theory calculations when the IR and VCD spectra of the original isotopomer are calculated to reasonable accuracy. Results for isotopically substituted nonamethoxy cyclotriveratrylene are presented to illustrate the degree of agreement between measured and calculated IR and VCD difference spectra for several isotopomers of this molecule. These findings highlight the utility of isotoptic substitution as an aide to verifying the determination of absolute configuration using vibrational optical activity.     

Molecular structure,vibrational spectra and first-order hyperpolarisability analysis of 2-amino-6-nitrobenzothiazole by DFT method

Vibrational analysis of 2-amino-6-nitrobenzothiazole (2A6NBT) molecule has been carried out at room temperature using FT-IR and FT-Raman spectroscopic techniques. The equilibrium geometry, bonding features and harmonic vibrational frequencies have been investigated with the help of the density functional theory DFT method. The non-linear optical (NLO) behaviour of the examined molecule has been studied followed by the determination of the electric dipole moment μ, the polarisability α and hyperpolarisability β using HF/6-31G(d,p) method. Stability of the molecule arising from hyperconjugative interactions and charge delocalisation have been analysed using the natural bond orbital analysis. The results show that charge in electron density in the σ^* and π^* antibonding orbitals and second-order delocalisation energies (E²) confirms the occurrence of intramolecular charge transfer within the molecule. The assignments of the vibrational spectra have been carried out with the help of normal coordinate analysis following the scaled quantum mechanical force field methodology. The energy and oscillator strength calculated by time-dependent density functional theory complements with the experimental findings. The simulated spectra satisfactorily coincide with the experimental spectra.     

Time-resolved step-scan FTIR difference spectroscopy for the study of photosystem I with different benzoquinones incorporated into the A1 binding site

Time-resolved step-scan FTIR difference spectroscopy has been used to study photosystem I (PSI) with plastoquinone-9 (PQ) and two other benzoquinones (2,6-dimethyl-1,4-benzoquinone and 2,3,5,6-tetrachloro-1,4-benzoquinone) incorporated into the A₁ binding site. By subtracting a (P700⁺A₁^????P700A₁) FTIR difference spectrum for PSI with the native phylloquinone (PhQ) incorporated from corresponding spectra for PSI with different benzoquinones (BQs) incorporated, FTIR double difference spectra are produced, that display bands associated with vibrational modes of the quinones, without interference from features associated with protein vibrational modes.Molecular models for BQs involved in asymmetric hydrogen bonding were constructed and used in vibrational mode frequency calculations. The calculated data were used to aid in the interpretation and assignment of bands in the experimental spectra. We show that the calculations capture the general trends found in the experimental spectra.By comparing four different FTIR double difference spectra we are able to verify unambiguously bands associated with phyllosemiquinone in PSI at 1495 and 1415?cm^?1. We also resolve a previously unrecognized band of phyllosemiquinone at 1476?cm^?1 that calculations suggest is due in part to a C₄$\stackrel{?}{?}$O stretching mode.For PSI with PQ incorporated, calculations and experiment taken together indicate that the C₁$\stackrel{?}{?}$O and C₄$\stackrel{?}{?}$O vibrational modes of the semiquinone give rise to bands at 1487 and 1444?cm^?1, respectively. This is very distinct compared to PSI with PhQ incorporated.From the calculated and experimental spectra, we show that it is possible to distinguish between two possible orientations of PQ in the A₁ protein binding site.