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.     

The molecular, electronic structures and vibrational spectra of metal-free, N,N'-dideuterio and magnesium tetra-2,3-pyridino-porphyrazines: Density functional calculations

A theoretical investigation of the fully optimized geometries and electronic structures of the metal-free (TPdPzH(2)), N,N'-dideuterio (TPdPzD(2)), and magnesium (TPdPzMg) tetra-2,3-pyridino-porphyrazine has been conducted based on density functional theory. The optimized geometries at density functional theory level for these compounds are reported here for the first time. A comparison between the different molecules for the geometry, molecular orbital, and atomic charge is made. The substituent effect of the N atoms on the molecular structures of these compounds is discussed. The IR and Raman spectra for these three compounds have also been calculated at density functional B3LYP level using the 6-31G(d) basis set. Detailed assignments of the NH, NM, and pyridine ring vibrational bands in the IR and Raman spectra have been made based on assistance of animated pictures. The simulated IR spectra of TPdPzH(2) are compared with the experimental absorption spectra, and very good consistency has been found. The isotope effect on the IR and Raman spectra is also discussed.     

Probing intrazeolite space

Molecular sieves are inorganic framework structures generally composed of crystalline aluminosilicate tetrahedra which are arranged to form channels of 2–10 Å, diameters and cages with dimensions from 6–15 Å. Absorption of probe molecules of varying geometries and sizes is used to characterize the framework dimensions and topography in concert with X-ray diffraction identification of the specific structure. From unit cell dimensions and assumptions about the size of the framework forming species, a pore volume can be calculated. The volumes of the absorbed probe molecules, using their liquid densities, are then compared to the calculated pore volume. The constraint on the packing of the absorbed molecules is quantified by comparing their packing density to their density in the liquid state. Further, the packing of different probe molecules into the same pore volume is compared via a ratio technique called the packing ratio. The effect of the lattice geometry and framework dimensions on the packing ratios is to provide a set of characteristic values for a given molecular sieve. The packing ratios for zeolites rho and ZSM-5 are presented as expectation values for other scientists to use as bases of comparison.     

Electronic structure and electronic absorption spectra of molybdenum and tungsten oxotetrachlorides

Electronic absorption spectra of the molecules MoOCl₄ and WOCl₄ have been measured and their electronic structure has been calculated on the basis of the SCF-X-SW theory in the overlapping atomic sphere model. Ionisation potentials and allowed optical transition energies have been found in the transition state approximation. The interpretation of the electronic absorption spectra of gaseous MoOCl₄ and WOCl₄ is given.     

Electronic structure and optical properties of chelating heteroatomic conjugated molecules: a SAC-CI study

The electronic structure and optical properties of 13 chelating heteroatomic conjugated molecules such as pyridine, benzoxazole, and benzothiazole derivatives, which are used as C–N ligands in organometallic compounds, have been investigated. The geometries of the ground and first excited states were obtained by the DFT and CIS methods, respectively, followed by the SAC-CI calculations of the transition energies for absorption and emission. For six compounds whose experimental data are available, the SAC-CI calculations reproduced the experimental values satisfactorily with deviations of less than 0.3 eV for absorption and 0.1 eV for emission except for benzoxazoles. For other molecules, the theoretical absorption and emission spectra were predicted. The lowest ππ* excited-state geometries was calculated to be planar for most of the molecules with two or three conjugated rings connected by single bond. The geometry change due to the ππ* excitation was qualitatively interpreted by electrostatic force theory based on SAC/SAC-CI electron density difference. The excitations are relatively localized in the central region and in the lowest ππ* excited state, the inter-ring single bond shows large change, with a contraction of 0.05–0.09 Å. The present calculations provide reliable information regarding the energy levels of these chelating heteroatomic conjugated compounds.     

Theoretical study of nonlinear optical properties of cobalt bis (dicarbollide) derivatives: the effect of substituents

In the present work, molecular first-order hyperpolarizability (\(\beta _{\mathrm{tot}}\)) and dipole moment (d) are obtained at B3LYP/6–31G(d,p) level of theory by coupled perturbed Hartree–Fock method within the static approach. The investigated molecules are a series of substituted cobalt bis (dicarbollide) derivatives: Hydrogens bonded to the two carbon atoms are replaced by acceptor and donor electron substituents. Correlations between the Hammett electronic parameters of the substituents and the molecular properties are tested. Among them, the named push–pull compounds produced the largest calculated values of \(\beta _{\mathrm{tot}}\) and d. The UV–Vis spectra are reported for all studied compounds.     

Effect of molecular vibrations on the MD/QC‐simulated absorption spectra

Effect of molecular vibrations on the absorption spectra simulated via a sequential approach combining molecular dynamics (MD) with quantum‐chemical calculations has been investigated. Simulated spectra have been obtained from the time‐dependent density functional theory results averaged over series of molecular geometries retrieved from Born–Oppenheimer MD trajectories. Distributions of bond lengths have been analyzed and related to the features of calculated spectra. For NVE simulations of small systems, absorption spectra exhibit bimodal bandshape as a result of classical treatment of vibrations. For NVE trajectories of larger systems or simulations in the NVT ensemble calculated absorption bands are symmetric, however, they may not agree with the results of Franck–Condon analysis. These results are practical manifestations of effects predicted theoretically from general principles. Consequences for the modeling of absorption spectra have been discussed. © 2013 Wiley Periodicals, Inc.     

Structural and spectral properties of 3-substitutedphenyl-1,5-diphenylformazans: A quantum chemical study

The structural and optical properties of 3-substitutedphenyl-1,5-diphenylformazans are studied by quantum chemical methods. The density functional theory (DFT) is employed to optimize the ground state geometries of formazans substituted with different electron donating and withdrawing groups in both gas and solvent phases. The absorption spectra of formazan derivatives are calculated using time dependent density functional theory (TD-DFT). The polarizable continuum model (PCM) calculations of 3-substitutedphenyl-1,5-diphenylformazans are performed for bulk solvent effects. The geometrical parameters, vibrational frequencies, and relative stabilities of isomers of 3-substitutedphenyl-1,5-diphenylformazans are studied. The results obtained by TD-DFT calculations reveal that the substitution of electron withdrawing and donating substituents affects the absorption spectra of 3-substitutedphenyl-1,5-diphenylformazans. The calculated maximum absorption wavelengths (λ_max) are highly consistent with the experimental values as found from UV-vis spectra.     

Electronic Structure and Circular Dichroism of Natural Alboatisins Isolated from Aerial Parts of Isodon Albopilosus: DFT and TDDFT Study

The stable conformations of a series of bioactive molecules, (?)-alboatisins A?C, are identified via Monte Carlo searching with the MMFF94 molecular mechanics force field. Then, the optical rotation (OR) values, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD) spectra were calculated using the gradient-corrected density functional theory method. The vibrational and transition modes of molecular chirality were explored in terms of their microscopic origin. The calculated specific rotations are in agreement with the experimental values. From the OR analysis, it was concluded that optical rotation values areregulated by hydroxyl substitution. Vibrations occurring on the chiral skeleton may cause strong absorption in VCD spectra; VCD spectra are thus the spectral response to deformation vibrations on the chiral carbon skeleton. The lowest-energy negative Cotton effect is caused by σ→π^* transition. Frontier molecular orbital analysis showed that strong ECD absorptions are produced when the dominant transition on the chiral skeleton is asymmetric; ECD spectra show the result of transitions lacking asymmetry on the chiral skeleton.     

Ultraviolet bands of Na2 and K2

The ultraviolet spectra of Na₂ and K₂ molecules have been investigated. These studies were made in absorption in the second order of a 3.4 m Ebert Spectrograph with a reciprocal dispersion of 2.6 Å/mm. A number of new bands in the spectra of both the molecules not previously reported have been observed. Computer methods have been used to calculate the term values and to evaluate molecular constants.     

Density functional theory study of the rotational barriers,conformational preference,and vibrational spectra of 2-formylfuran and 3-formylfuran

The torsional potentials, molecular structures, conformational stability, and vibrational wavenumbers for the rotational isomers of 2-formylfuran and 3-formylfuran are computed using the density functional theory (B3LYP) method with the 6-31+G* basis set. All structures are fully optimized and the optimized geometries, rotational constants, dipole moments, and energies are presented. From the computations, both 2-formylfuran and 3-formylfuran are predicted to exist predominantly in trans conformation with a cis–trans rotational barrier of 11.19 kcal/mol and 8.10 kcal/mol, respectively. The vibrational wavenumbers and the corresponding vibrational assignments of the molecules in the C _s symmetry are examined and the infrared spectra of the molecules are simulated using the wavenumbers and the corresponding intensities obtained from the computations. The effect of solvents on the conformational stability of all the molecules in nine different solvents (heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethylsulfoxide, and water) is investigated. The integral equation formalism in the polarizable continuum model (IEF-PCM) is used for all solution phase computations.     

Theoretical and Experimental Investigation of the 2-Hydroxyquinoxaline Structure: Study of the Tautomerization Equilibrium System and Analysis of the Electronic Properties

All reasonable tautomers of 2-hydroxyquinoxaline (2HQ) are investigated by the DFT B3LYP/6-311G(d) method. The optimized geometries corresponding to the minimum energy show that the keto form QX2 is the most stable form. The geometry optimization parameters (bond lengths, bond angles) are compared to the X-ray values. Calculated FTIR, UV, and NMR spectra of QX2 are compared to the experimental data to achieve a synergetic computational and spectroscopic approach for the structure analysis of 2HQ. The electronic properties, frontier molecular orbitals, and Mulliken atomic charges are calculated.     

Star-shaped Organic Molecules That Comprise a 1,3,5-Trisubs-tituted Benzene Core and Three Oligoaryleneethynylene Arms as Light-emitting Materials

Star‐shaped rigid molecules that comprise a 1,3,5‐trisubstitued benzene core and three oligoaryleneethynylene arms have great potential application in organic light‐emitting devices (OLEDs). Their optical and electronic properties are tuned by the star‐shaped molecular size. To reveal the relationship between the properties and structures, we perform a systemic investigation for these organic molecules. The ground and excited state molecules are studied using density functional theory (DFT), the ab initio HF, and the single excitation configuration interaction (CIS), respectively. And the electronic absorption and emission spectra are investigated with time‐dependent density functional theory (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IP), electron affinities (EA), absorption and emission spectra are controlled by the star‐shaped molecular size, which favor the hole and electron injection into OLEDs. With increasing the molecular conjugated length, the absorption and emission spectra exhibit red shifts to some extent and are in good agreement with the experimental ones. Also, the calculated emission spectra range from 330 to 440 nm. All the calculated show that the star‐shaped molecules are promising as blue light emitting materials     

Optical absorption and emission properties of fluoranthene, benzo[k]fluoranthene, and their derivatives. A DFT study

Fluoranthene and benzo[k]fluoranthene-based oligoarenes are good candidates for organic light-emitting diodes (OLEDs). In this work, the electronic structure and optical properties of fluoranthene, benzo[k]fluoranthene, and their derivatives have been studied using quantum chemical methods. The ground-state structures were optimized using the density functional theory (DFT) methods. The lowest singlet excited state was optimized using time-dependent density functional theory (TD-B3LYP) and configuration interaction singles (CIS) methods. On the basis of ground- and excited-state geometries, the absorption and emission spectra have been calculated using the TD-DFT method with a variety of exchange-correlation functionals. All the calculations were carried out in chloroform medium. The results show that the absorption and emission spectra calculated using the B3LYP functional is in good agreement with the available experimental results. Unlikely, the meta hybrid functionals such as M06HF and M062X underestimate the absorption and emission spectra of all the studied molecules. The calculated absorption and emission wavelength are more or less basis set independent. It has been observed that the substitution of an aromatic ring significantly alters the absorption and emission spectra.     

Soft Coulomb hole method applied to molecules

The soft Coulomb hole method introduces a perturbation operator, defined by ?e/r₁₂ to take into account electron correlation effects, where ω represents the width of the Coulomb hole. A new parametrization for the soft Coulomb hole operator is presented with the purpose of obtaining better molecular geometries than those resulting from Hartree–Fock calculations, as well as correlation energies. The 12 parameters included in ω were determined for a reference set of 12 molecules and applied to a large set of molecules (38 homo‐ and heteronuclear diatomic molecules, and 37 small and medium‐size molecules). For these systems, the optimized geometries were compared with experimental values; correlation energies were compared with results of the MP2, B3LYP, and Gaussian 3 approach. On average, molecular geometries are better than the Hartree–Fock values, and correlation energies yield results halfway between MP2 and B3LYP. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Syntheses,Electronic Structure,and Second-Order Optical Nonlinearity of Two New Bidentate Monoazo Schiff-Base Compounds

Two new bidentate salicylaldiminato Schiff-base ligands have been synthesized by condensation of various precursor ligands and n-propylamine. The results of elemental analysis are in conformity with the suggested formula. The main bands in IR spectra, signals in ¹H NMR and electronic absorption spectra are assigned and discussed in terms of molecular structure. Semiempirical calculations of the structures and second-order nonlinear optical polarizabilities () of molecules were performed. The value for one of these Schiff-base compounds is 134 times that of urea.     

IR spectroscopic study of the conformational lability of 4′-ethyl-4-cyanobiphenyl

The conformational lability of 4-ethyl-4-cyanobiphenyl molecules in solid crystal (SC) and isotropic liquid (IL) states was investigated by IR spectroscopic techniques (experiment and theory). IR absorption spectra were measured at 28°C–95°C in the frequency range 400 cm^–1–4000 cm^–1. Spectrum simulation was performed using the fragment method with allowance for the conformational fluctuations of molecules. The experimental and calculated spectra were compared and analyzed, and it was shown that in the IL, the samples are mixtures of conformers. The temperature changes in the spectra in the stated range are caused by the conformational lability of molecules.Original Russian Text Copyright © 2004 by L. M. Babkov, I. I. Gnatyuk, G. A. Puchkovskaya, and S. V. TrukhachevTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 398–405, May–June, 2004.     

Theoretical investigation on structures, electronic spectra and nonlinear optical properties of gold compounds [X-“Au(PMe3)”2]

The structures, electronic spectra, polarizability and third-order nonlinear optical properties of six gold compounds [X-“Au(PMe₃)”₂] were investigated by density functional theory (DFT) B3LYP and BhandHLYP methods. It was found that the calculation methods and basis set are rational for the object of study, and molecular structures change slightly when PPh₃ is replaced by PMe₃. The spatial effects of the bridging section have a significant influence on the polarizability, but indistinctive to the third-order nonlinear optical (NLO) coefficient. As a result of the conjugated effect in different compounds, the third-order polarizability of molecule 1a is the smallest, while that of molecule 2a is the largest. Au has donor ability in molecule 1a but acceptor ability in molecules 2a–6a by analyzing the electronic spectra and frontier molecular orbitals constitute maximal absorption, which indicates the contribution of Au to NLO properties in the six molecules is different.     

Molecular analysis of water clusters: Calculation of the cluster structures and vibrational spectrum using density functional theory

Density functional theory (DFT) is applied to obtain absorption spectra at THz frequencies for molecular clusters of H₂O. The vibrational modes of the clusters are calculated. Coupling among molecular vibrational modes explains their spectral features associated with THz excitation. THz excitation is associated with vibrational frequencies which are here calculated within the DFT approximation of electronic states. This is done for both isolated molecules and collections of molecules in a cluster. The principal result of the paper is that a crystal-like cluster of 38 water molecules together with a continuum solvent background is sufficient to replicate well the experimental vibrational frequencies.