Some electronic spectra of dibenzofuran

The low-energy electronic states of dibenzofuran have been investigated by absorption and emission spectroscopy. The absolute intensity of absorption parallel to the a and b axes of a single crystal at room temperature has been measured from 32 000 to 42 000 cm^?1, and qualitative information about the strength of the c-axis absorption was obtained from a study of finely-ground dibenzofuran compressed in KBr discs. The symmetries of three transitions were determined and tentative assignments of two others were made. The absorption bands in the spectrum of the crystal held at 4.2 K remained broad, an effect attributed to exciton-phonon interaction. Spectra of dibenzofuran in a polycrystalline n-heptane matrix at 4.2 K were sharp and vibrational analyses of fluorescence and phosphorescence are given in terms of the known ground state fundamentals.     

Theoretical prediction of vibrational spectra: The out-of-plane force field and vibrational spectra of pyridine

The harmonic force field for the out-of-plane vibrations of pyridine has been calculated from ab initio Hartree-Fock wavefunctions obtained with a 4–21 basis set of contracted Gaussians. To account for systematic errors, the calculated force constants were scaled, using only two independent sclae factors which were transferred unchanged from benzene. The resulting scaled quantum mechanical force field, which is strictly a priori in that it is not based on any experimental data on pyridine, predicts the 64 out-of-plane fundamental frequencies of pyridine and its deuterated isotopomers of C_2v symmetry with a mean deviation from experiment of only 8.5 cm^?1. Addition of polarization functions to the basis set for the nitrogen atom and refinement of the two scale factors by fitting them to the observed pyridine spectra produce no significant improvement in the fit. Assignments of the vibrational spectra are discussed.     

The vibrational spectra of magnesium pyrophosphate polymorphs

The Raman spectra were measured and analyzed for both α- and β-Mg₂P₂O₇. The IR and Raman spectra were interpreted for both phases using factor group analyses. The spectral features predicted with factor groups arising from the X-ray crystallographic space groups $\text{P2}{}_1\text{c}$ ? c⁵_2h and $\text{C2}\text{m}$ ? C³_2h for α-Mg₂P₂O₇ and β-Mg₂P₂O₇, respectively, fit the observed results. Bands observed in the Raman spectrum for β-Mg₂P₂O₇ are consistent with a linear bond angle while those for α-Mg₂P₂O₇ are consistent with a bent P-O-P bond angle. No soft modes were observed in the Raman spectra indicating that the phase transition between the two phases is not a second order process.     

The vibrational spectra of some isotopic species of propionitrile

The infrared and Raman spectra of CH₃CH₂CN, CH₃CD₂CN, and CD₃CH₂CN, and the infrared spectrum of CH₃CH₂¹³CN were investigated in detail between 6000 and 100 cm⁻¹. Some infrared measurements of other isotopic species are also reported and partial assignments given. All fundamentals of propionitrile-d₀, -d₂, -d₃, and -¹³CN were assigned, together with a large number of mainly binary combination bands for which a general method of assignment is given. Several Fermi resonances were detected and the unperturbed positions of some of the levels involved were calculated. Special attention was paid to the CH stretching vibrations for which persisting wrong assignments exist in the literature, and to the methyl torsion frequencies which were determined for the four isotopic species above. A valence force field was calculated, and the potential energy distribution of the normal vibrations is tabulated.     

Diethynylbenzenes; The vibrational spectra of some deuterated isomers

The infrared and Raman spectra of para, meta- and ortho-diethynylbenzene and six deuterated isomers of these molecules have been measured. Comparative analyses of the spectra have enabled the 42 normal frequencies of vibration of each molecule to be assigned.     

Dicyanobenzens; The vibrational spectra of some deuterated isomers

The infrared and Raman spectra of the para, meta, and ortho dicyanobenzenes (phthalonitriles) and their ring-deuterated isomers have been measured. Comparative analyses of the spectra have enabled the 36 normal frequencies of vibration of each molecule to be assigned.     

The computed force constants and vibrational spectra of cubane

The geometry, complete harmonic force field, and dipole moment derivatives of cubane, C₈H₈, have been calculated at the Hartree-Fock level using a 4–21 Gaussian basis set. The infrared and Raman spectra of cubane and four deuterated derivatives were calculated and compared with previously observed spectra. A set of five scale factors for the calculated force constants was then derived by least-squares fitting of the fundamental vibrational frequencies calculated from the scaled force field to the frequencies obtained by direct experimental measurement. The resulting scaled quantum-mechanical (SQM) force field, containing 73 unique elements, is believed to give an accurate representation of the harmonic vibrational potential of cubane. In most cases, the spectral assignments previously made from purely empirical considerations were confirmed, but a few corrections are proposed. The only major alteration is for an A_2u mode revised to appear at 1030 cm^?1 in the undeuterated molecule. Coriolis constants and approximate infrared intensities are also calculated.     

The millimeter-wave spectra of germyl fluoride in excited vibrational states

Millimeter-wave spectra of GeH₃F in the v₃ = 1, v₆ = 1, v₂ = 1, and v₅ = 1 excited states have been recorded. Strong Coriolis resonance between the v₂ = 1 and v₅ = 1 states results in a highly perturbed spectrum which shows no obvious resemblance to the normal symmetric top excited state pattern. A similar, but not so strong coupling, exists between the v₃ = 1 and v₆ = 1 states. These spectra are analyzed by setting up the Coriolis coupled Hamiltonian matrices for these states. It has been shown that the apparently anomalous distortion constants of these states are due to the Coriolis coupling and the distortion parameters obtained with the model presented are, as expected, close to the ground state values.     

Davydov splitting of vibrational spectra of adsorbates

A monolayer of adsorbed molecules that form a two-dimensional lattice whose unit cell contains several orientationally nonequivalent molecules is considered. General expressions for the polarizability tensor of this system and the frequencies and integral intensities of s-and p-polarized spectral lines with regard for the electronic polarizability components of the molecules are found. As a result, the discrepancy between the calculated and experimentally observed values of the Davydov splitting for the system CO/NaCl(100) is reduced from 25 to 5%.     

First-principles calculation of nuclear resonance vibrational spectra

We present a ‘first-principles’ methodology for the calculation of the parameters that are required for the simulation of nuclear resonance vibrational spectra (NRVS) of molecular systems. Formulae are given for the intensities of vibrational transitions corresponding to the so-called single- and double-phonon contributions to the NRVS signal. The method is also valid for those vibrations that are not in the high-frequency/low-temperature limit. We have rigorously treated the issue of orientational averaging of the Lamb–Mössbauer factor and the effect of the neglect of its anisotropy on the calculated NRVS pattern. Normal mode composition factors are determined in a compact form as appropriate components of an orthogonal matrix that diagonalizes the Hessian matrix. The method is illustrated by simulating the NRVS spectra and the partial vibrational density of states of [FeO(H₂O)5]²⁺ on the basis of vibrational frequencies and normal mode composition factors obtained from density functional theory (DFT) calculations.     

Microwave and low-frequency vibrational spectra of bullvalene

The microwave spectrum of bullvalene has been investigated in the region 18–40 GHz. In addition to transitions in the ground vibrational state, transitions arising from five excited vibrational states below 600 cm⁻¹ have also been observed. A combination of microwave intensity measurements and infrared and Raman data has been utilized to assign these vibrations. Three of the vibrations are E-type modes at 241, 355, and 588 cm⁻¹. One is an A₁-type mode at 445 cm⁻¹, and another is an A₂-type at 266 cm⁻¹. The microwave spectrum indicates the presence of a first-order Coriolis interaction for the E modes at 241 and 588 cm⁻¹. The first-order Coriolis coupling constant q = 0.557 MHz for the 241 cm⁻¹ vibration. The spectral results are consistent with C_3v symmetry for bullvalene.     

Modeling vibrational spectra of indole in water

Vibrational spectra of indole in the isolated state and in aqueous solution have been calculated using a B3LYP/6-311++G(d, p) method. The influence of water as a solvent on vibrational spectra of indole was examined with due regard for intermolecular interactions (SCRF method) and explicit consideration of the effect of hydrogen bonds (1:1 indole:water complex).     

The vibrational levels of methane obtained from analyses of high-resolution spectra

Methane and its tetrahedral isotopologues are spherical-top molecules whose high-resolution rovibrational spectra can only be analyzed in detail, thanks to sophisticated symmetry-adapted tensorial models. However, the effective Hamiltonian parameters of such models do not give direct access to the positions of the vibrational sublevels. In this paper, we present a calculation of the vibrational level positions for ¹²CH₄, ¹³CH₄, ¹²CD₄ and ¹³CD₄ performed using the effective Hamiltonian parameters obtained through recent analyses. We also include the results of a re-analysis of the octad system of ¹²CH₄ performed with a higher order of the development which slightly improves the previous work on this polyad.     

The effect of disorder-induced mode coupling on vibrational spectra of glasses

A theoretical approach based on the Green's function method is proposed for investigation of disorder-induced mode coupling in glasses and of its effect on the intensity of the first order Raman and Infrared spectra. It is shown that while the spectral line widths depend on the Green's function of the glass, dG¢, the mode coupling coefficients depend on dG²¢. The numerical results, obtained in self-consistent approximation for dG¢ and in ladder approximation for dG²¢, demonstrate that the structural disorder leads to strong coupling between modes only if they are of the same type and with close frequencies.