Density functional theory and ab initio studies of vibrational spectra of 2-bis (2-chloroethyl) aminoperhydro-1,3,2-oxazaphosphorinane-2-oxide

The Fourier transform Raman (FTR) and Fourier transform infrared (FTIR) spectra of 2-bis (2-chloroethyl) aminoperhydro-1,3,2-oxazaphosphorinane-2-oxide were recorded in the regions 4000–100 cm^? 1 and 4000–400 cm¹, respectively, in the solid phase. Molecular electronic energy, geometrical structure, harmonic vibrational spectra, infrared intensities and Raman scattering activities, highest occupied molecular orbital, lowest unoccupied molecular orbital energy, energy gaps and thermodynamical properties such as zero-point vibrational energies, rotational constants, entropies and dipole moment were computed at the Hartree–Fock/6-31G(d,p) and three parameter hybrid functional Lee–Yang–Parr/6-31G(d,p) levels of theory. The vibrational studies were interpreted in terms of potential energy distribution. The results were compared with experimental values with the help of scaling procedures. The observed wave number in FTIR and FTR spectra was analysed and assigned to different normal modes of the molecule. Most of the modes have wave numbers in the expected range and are in good agreement with computed values.     

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.     

Structure and vibrational frequencies of 6-chloro-8-thia-1,4-epoxybicyclo[4.3.0]non-2-ene: density functional theory study

The Fourier transform infrared (FT-IR) spectrum of 6-chloro-8-thia-1,4-epoxybicyclo[4.3.0]non-2-ene has been recorded in the region 4000–525 cm^? 1. The optimised geometry, frequency and intensity of the vibrational bands of the title compound have been calculated using the ab initio Hartree–Fock and the density functional theory method with 6-31G(d,p) and 6-311G(d,p) basis set levels. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR spectrum. The observed and the calculated frequencies are found to be in good agreement. The theoretical vibrational spectrum of the title compound were interpreted by means of potential energy distributions using VEDA 4 program.     

Experimental and DFT computational studies on 5-benzyl-4-(3,4-dimethoxyphenethyl)-2H-1,2,4-triazol-3(4H)-one

The triazole compound, 5-benzyl-4-(3,4-dimethoxyphenethyl)-2H-1,2,4-triazol-3(4H)-one, has been synthesized and characterized by ¹H-NMR, ¹³C-NMR, IR, and X-ray single-crystal determination. The compound crystallizes in the monoclinic space group P2₁ with a?=?11.8844(3) Å, b?=?17.5087(4) Å, c?=?17.3648(6) Å, β?=?99.990(2)? and Z?=?8. In addition to the molecular geometry from X-ray experiment, the molecular geometry, vibrational frequencies and gauge including atomic orbital (GIAO) ¹H- and ¹³C-NMR chemical shift values of the title compound in the ground state have been calculated using the density functional method (B3LYP) with 6-31G(d,p) basis set. The calculated results show that the optimized geometries can well reproduce the crystal structure and the theoretical vibrational frequencies and chemical shift values show good agreement with experimental ones. Besides, molecular electrostatic potential (MEP), natural bond orbital (NBO), and frontier molecular orbitals (FMO) analysis of the title compound were performed by the B3LYP/6-31G(d,p) method.     

Theoretical investigation on the structure and thermodynamic properties of the 2,4-dinitroimidazole complex with methanol

The structure and thermodynamic properties of the 2, 4-dinitroimidazole complex with methanol were investigated using the B3LYP and MP2(full) methods with the 6-31++G(2d,p) and 6-311++G(3df,2p) basis sets. Four types of hydrogen bonds [N–H?O, C–H?O, O–H?O (nitro oxygen) and O–H?π] were found. The hydrogen-bonded complex having the highest binding energy had a N–H?O hydrogen bond. Analyses of natural bond orbital (NBO) and atoms-in-molecules (AIM) revealed the nature of the intermolecular hydrogen-binding interaction. The changes in thermodynamic properties from monomers to complexes with temperatures ranging from 200.0 to 800.0 K were investigated using the statistical thermodynamic method. Hydrogen-bonded complexes of 2,4-dinitroimidazole with methanol are fostered by low temperatures.

Theoretical investigations on the hydrogen bonding of nitrile isomers with H2O,HF, NH3 and H2S

The hydrogen bonds formed by the interaction of nitriles with water, hydrogen fluoride, ammonia and hydrogen sulphide have been studied using B3LYP and second-order Møller–Plesset perturbation (MP2) methods and 6-311+ + G(d,p) basis set. The energies and structures of 80 hydrogen-bonded complexes between nitriles and small molecules were examined systematically using B3LYP and MP2 procedure. Categorisation of the hydrogen bonds involved in the various complexes led to an ordering of hydrogen bond donor and acceptor abilities for some functional groups. The interaction energies have been corrected for the basis set superposition error using Boy's counterpoise correction method. The Morokuma energy decomposition analysis reveals that the strong interactions are due to the attractive contributions from the electrostatic (ES), polarisation (PL) and charge transfer (CT) components. The topological parameters, electron density and Laplacian of electron density show excellent correlation with the hydrogen bond length. Natural bond orbital (NBO) analysis has also been performed to study the CT from proton acceptor to the antibonding orbital of the H–Y bond in the proton donor part of complexes. The frequency analysis of C–H…Y bond in the complexes indicates the blue-shifting nature largely in case of sp² hybridised carbon atom.     

Spectroscopic (FTIR,FT-Raman,NMR and UV) and molecular structure investigations of 1,5-diphenylpenta-2,4-dien-1-one: a combined experimental and theoretical approach

The title molecule 1,5-diphenylpenta-2,4-dien-1-one (cinnamylideneacetophenone, CA) has been synthesised and characterised by FTIR, FT-Raman, NMR and UV–vis spectral analyses. The possible stable conformers of the CA molecule were searched by potential energy surface scan at B3LYP level of theory. The molecular geometry from X-ray determination of the CA molecule in the ground state has been compared using the density functional theory (DFT) with 6-31G(d,p) basis set. The harmonic vibrational modes, the corresponding wavenumbers and IR and Raman intensities of most stable conformer were calculated by the DFT method. The assignments of the fundamentals were proposed on the basis of total energy distribution calculations. The calculated ¹³C and ¹H NMR chemical shifts using gauge including atomic orbitals approach are in good agreement with the observed chemical shifts. The molecular stability and bond strength have been investigated by applying natural bond orbital analysis. Using the time-dependent DFT method, the electronic absorption spectrum of the title compound has been predicted and the electronic transitions within the molecule have been interpreted. The molecular electrostatic potential map was used for predicting possible hydrogen and oxygen bonding sites in the CA molecule.     

Structural and vibrational spectroscopic elucidation of sulpiride in solid state

The study on the conformational and vibrational behaviors of sulpiride molecule which is known as a neuroleptic or antipsychotic drug that is widely used clinically in the treatment of schizophrenic or depressive disorders is an important scientific and practical task. In here, a careful enough study of monomer and dimeric forms of sulpiridine {5-(aminosulfonyl)-N-[(1-ethyl-2-pyrrolidinyl) ethyl]-2-methoxy-benzamide (C₁₅H₂₃N₃O₄S)} is undertaken by density functional theory (DFTB3LYP) method with the B3LYP/6-31G(d,p) basis set. The conformations of free molecule were searched by means of torsion potential energy surfaces scan studies through dihedral angles D₁ (8?N, 18C, 20C, 23?N), D₂ (18C, 20C, 23?N, 25C) and D₃ (28C, 30C, 41S, 44?N) in electronically ground state, employing 6-31G basic set. The final geometrical parameters for the obtained stable conformers were determined by means of geometry optimization, carried out at DFT/B3LYP/6-31G(d,p) theory level. Afterwards, the possible dimer forms of the molecule were formed and their energetically preferred conformations were investigated. Moreover, the effect of basis set superposition error on the structure and energy of the three energetically favourable sulpiride dimers has been determined. The optimized structural parameters of the most stable monomer and three low energy dimer forms were used in the vibrational wavenumber calculations. Raman and IR (4000–400?cm^?1) spectra of sulpiride have been recorded in the solid state. The assignment of the bands was performed based on the potential energy distribution data. The natural bond orbital analysis has been performed on both monomer and dimer geometries in order to elucidate delocalization of electron density within the molecule. The predicted frontier molecular orbital energies at DFT/B3LYP/6-31G(d,p) theory level show that charge transfer occurs within the molecule. The first-order hyperpolarizability (β₀) and related properties (μ and α) of the title molecule were also calculated.     

Barrier potentials,molecular structure,force filed calculations and quantum chemical studies of some bipyridine di-carboxylic acids using the experimental and theoretical using (DFT,IVP) approach

ABSTRACT

FT-IR and FT-Raman spectra of 2,2′-bipyridine-3,3′-dicarboxylic acid (B3DA), 2,2′-bipyridine-4,4′-dicarboxylic acid (B4DA) and 2,2′-bipyridine-5,5′-dicarboxylic acid (B5DA) were recorded and analysed. The quantum chemical calculations of the title compounds begin with barrier potentials at different rotation angles around the C–C′ and C–Cα bonds in order to arrive conformation of lowest energy using DFT employing B3LYP functional with 6-311++G(d,p) basis set. This confirmation was further optimised to get the global minimum geometry. The vibrational frequencies along with IR, Raman intensities were computed, the rms error between observed and calculated frequencies were 11.2 cm^?1, 10.2 cm^?1 and 12.2 cm^?1 for B3DA, B4DA, and B5DA. An 87-element modified valence force field is derived by solving the inverse vibrational problem using Wilson’s GF matrix method. This force field is refined using 163 observed fundamentals employing in overlay least-squares technique. The average error between computed and experimental frequencies was found as 12.85 cm^?1 using potential energy distribution (PED) and eigenvectors. By using the gauge-independent atomic orbital (GIAO) method calculate the ¹H and ¹³C NMR chemical shifts of the molecules and compared with experimental results. The first-order hyperpolarisability, HOMO and LUMO energies, molecular electrostatic potential (MESP) and natural orbital analysis (NBO) of titled compounds were evaluated using DFT.     

Quantum chemical investigation of intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives

The one step intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives has been studied through the use of electronic structure methods. Due to the absence of experimental data for the parent molecule of 1,2,4-triazole-3-thione the structure and energetics of aforementioned tautomers were derived using various basis sets and levels including HF, B3LYP, and MP2 methods. The gas phase results show that in all different levels of theory the most stable tautomer is the thione form. It has also been revealed that B3LYP/6-31G(d,p) level is quite well suited and reliable to investigate these kinds of tautomerism. To account the influence of substituents on the mentioned tautomerization, the tautomerism and conformational properties as well as vibrational analysis of 20 halophenyl and isopyridyl derivatives were investigated using B3LYP/6-31G(d,p) calculations. In all cases the calculations indicate that substituents have no considerable effects on relative stabilities and energy barriers for the thione-thiol proton transfer and the thione forms are the predominant species in the gas phase. In order to figure out the relative stabilities of the species involved in the tautomerism, geometrical and natural bond orbital (NBO) analyses have been employed. It has also been shown that the computed vibrational frequencies of tautomers with different scaling factors could be used to interpret the vibrational frequencies in IR spectrum of similar species.     

Quantum chemical study of the structure and properties of citrinin

Detailed structures and electronic properties of three tautomeric forms of the toxin citrinin were investigated using several quantum calculation methods. Energetic preference of the predominant p- and o-quinone methide tautomeric forms is dependent on the method of calculation. A previously unstudied carboxylic acid enol tautomer was calculated to be surprisingly stable in vacuo, being within 2.5 kcal mol^? 1 at the B3LYP/6-311++G(2d,2p) level of theory. Despite differences in bond nature and connectivity of tautomers, the natural bond orbital analysis revealed that tautomeric forms share similar natural charges and natural electron configurations. Calculated bond lengths corresponded with experimentally observed values and assignments for the calculated infrared vibrational frequencies are reported.     

Comparative theoretical study of the UV/Vis absorption spectra of styrylpyridine compounds using TD-DFT calculations

This study examined absorption properties of 2-styrylpyridine, trans-2-(m-cyanostyryl)pyridine, trans-2-[3-methyl-(m-cyanostyryl)]pyridine, and trans-4-(m-cyanostyryl)pyridine compounds based on theoretical UV/Vis spectra, with comparisons between time-dependent density functional theory (TD-DFT) using B3LYP, PBE0, and LC-ωPBE functionals. Basis sets 6–31G(d), 6–31G(d,p), 6–31+G(d,p), and 6–311+G(d,p) were tested to compare molecular orbital energy values, gap energies, and maxima absorption wavelengths. UV/Vis spectra were calculated from fully optimized geometry in B3LYP/6–311+G(d,p) in gas phase and using the IEFPCM model. B3LYP/6–311+G(d,p) provided the most stable form, a planar structure with parameters close to 2-styrylpyridine X-ray data. Isomeric structures were evaluated by full geometry optimization using the same theory level. Similar energetic values were found: ～4.5 kJ?mol^?1 for 2-styrylpyridine and ～1 kJ?mol^?1 for derivative compound isomers. The 2-styrylpyridine isomeric structure differed at the pyridine group N-atom position; structures considered for the other compounds had the cyano group attached to the phenyl ring m-position equivalent. The energy difference was almost negligible between m-cyano-substituted molecules, but high energy barriers existed for cyano-substituted phenyl ring torsion. TD-DFT appeared to be robust and accurate approach. The B3LYP functional with the 6–31G(d) basis set produced the most reliable λ_max values, with mean errors of 0.5 and 12 nm respect to experimental values, in gas and solution, respectively. The present data describes effects on the λ_max changes in the UV/Vis absorption spectra of the electron acceptor cyano substituent on the phenyl ring, the electron donor methyl substituent, and the N-atom position on the electron acceptor pyridine ring, causing slight changes respect to the 2-styrylpyridine title compound.     

Vibrational spectral assignments of paraldehyde by ab initio and density functional methods

The FTIR and Laser-Raman spectra of paraldehyde have been recorded in the regions 4000–400 cm⁻¹ and 3500–250 cm⁻¹ respectively. Molecular electronic energy, geometrical structure, harmonic vibrational spectra, infrared intensities and Raman scattering activities have been computed at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. The results were compared with experimental values with the help of scaling procedures. The observed wave numbers in FTIR and Laser-Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wave numbers in the expected range and are in good agreement with computed values.     

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.     

Anion–π interactions: CP-corrected vs. standard optimisation,AIM and NBO analyses

The anion–π interactions between Br^? , Cl^? , F^?  and H^?  anions and hexafluorobenzene (HFB), 1,2,4,5-tetracyanobenzene (TCB) and tetracyanopyrazine (TCP) have been studied by standard and counterpoise (CP) corrected methods at HF, B3LYP and MP2/6-31+ + G (d,p) levels of theory. The complexation energies were corrected for basis set superposition error (ΔE _BSSE) and zero point energy (ΔE _BSSE + ZPE). Also, the B3LYP results were corrected by single-point calculation at B3LYP/aug-cc-PVTZ level of theory. Although the CP-corrected method results in higher distances between anions and rings, the standard method gives lower complexation energies. TCP…X^?  series gives higher complexation energies in both CP-corrected and standard methods. Topological analysis of the charge density ρ(r) has been performed by the means of atoms in molecules method on the wave functions obtained at MP2/6-31+ + G (d,p) level of theory. The number and the nature of critical points depend on aromatic ring and anion. Natural bond orbital analysis indicates that n_X → π*_CC and n_X → π*_CN interactions are the most important interactions for TCB (and HFB)…X^?  and TCP…X^?  complexes, respectively.     

Theoretical study of the geometrical and electronic structures and thermochemistry of spherophanes

A set of supramolecular cage-structures—spherophanes—was studied at the density functional B3LYP level. Full geometrical structure optimisations were made with 6–31G and 6–31G(d) basis sets followed by frequency calculations, and electronic energies were evaluated at B3LYP/6–31++G(d,p). Three different symmetries were considered: C1, Ci, and Oh. It was found that the bonds between the benzene rings are very long to allow π-electron delocalisation between them. These spherophanes show portal openings of 2.596 Å in Spher1, 4.000 Å in Meth2, 3.659 Å in Oxa3, and 4.412 Å in Thia4. From the point of view of potential host–guest interaction studies, it should also be noted that the atoms nearest to the centre of the cavities are carbons bonded to X groups. These supramolecules seem to exhibit relatively large gap HOMO?LUMO: 2.89 eV(Spher1), 5.26 eV(Meth2), 5.73 eV(Oxa3), and 4.82 eV(Thia4). The calculated ΔH°_f (298.15 K) values at B3LYP/6–31G(d) are (in kcal mol^?1) 750.98, 229.78, ?10.97, and 482.49 for Spher1, Meth2, Oxa3, and Thia4, respectively. Using homodesmotic reactions, relative to Spher1, the spherophanes Meth2, Oxa3, and Thia4 are less strained by ?399.13 kcal mol^?1, ?390.40 kcal mol^?1, and ?411.38 kcal mol^?1, respectively. Their infrared and ¹³C NMR calculated spectra are reported.     

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.     

Molecular structure and vibrational and chemical shift assignments of 5-(2-Hydroxyphenyl)-4-(p-tolyl)-2,4-dihydro-1,2,4-triazole-3-thione by DFT and <Emphasis Type="Italic">ab initio</Emphasis> HF calculations

The molecular geometry, vibrational frequencies, gauge including atomic orbital (GIAO) ¹H and ¹³C chemical shift values and several thermodynamic parameters of 5-(2-Hydroxyphenyl)-4-(p-tolyl)-2,4-dihydro-1,2,4-triazole-3-thione in the ground state have been calculated by using the Hartree-Fock (HF) and density functional method (DFT/B3LYP) with 6–31G(d), 6–31 + G(d,p) and LANL2DZ basis sets. The results of the optimized molecular structure are presented and compared with the experimental X-ray diffraction. The computed vibrational frequencies are used to determine the types of molecular motions associated with each of the experimental bands observed. Also, calculated ¹H chemical shift values compared with the experimental ones. The data of the title compound display significant molecular structure and IR, NMR analysis provide the basis for future design of efficient materials having the of 1,2,4-triazole core.     

Density functional computational studies on (E)-2-[(2-Hydroxy-5-nitrophenyl)-iminiomethyl]-4-nitrophenolate

Density functional calculations of the structure, atomic charges, molecular electrostatic potential and thermodynamic functions have been performed at B3LYP/6-31G(d,p) level of theory for the title compound (E)-2-[(2-hydroxy-5-nitrophenyl)-iminiomethyl]-4-nitrophenolate. The results show that the phenolate oxygen atom and all of the nitro group oxygen atoms have bigger negative charges, and the coordination ability of these atoms differs in different solvents. The energetic behavior of the title compound in solvent media has been examined using B3LYP method with the 6-31G(d,p) basis set by applying the Onsager method and the isodensity polarized continuum model (IPCM). The results obtained with these methods reveal that the IPCM method yielded a more stable structure than Onsager’s method. In addition, natural bond orbital and frontier molecular orbital analysis of the title compound were performed using the B3LYP/6-31G(d,p) method.