Line shapes in electronic absorption spectra. Phenanthrene

The 3000 Å, (¹B₂ ← ¹A₁), absorption system of phenanthrene in durene crystals at 4°K illustrates an electronic transition, which is subject to near-resonance vibronic perturbations whose effect is intermediate to both the small (sparse intermediate) and large molecule (statistical) limits. Both broad (300 cm^?1) and narrow (10 cm^?1) lines are evident. A model is proposed which incorporates both these features by fast allowing for a consideration of the interaction between a small number of discrete levels, those associated with the largest coupling, followed by a treatment of the broadening of these levels through interaction with the remaining near continuum of states of the lower electronic state. Thus, one and the same electronic state provides both a sparse and dense manifold of levels. An important result of the model is that in terms of absorption intensities all the lines emerge with the same heights but differ in widths. When the intensities are summed with respect to energies this aspect is obscured. This approach has been shown to satisfactorily reproduce many of the features of the ¹B₂ absorption spectra of phenanthrene and phenanthrene-d₁₀. The ¹B₂ absorption systems have also been measured in the vapour phase and fine structure attributable to vibronic coupling and sequence band development are discussed.     

Computer simulation of band shapes in electronic absorption spectra of dimers of organic dyes

The band shapes in the absorption spectra of dimers of cyanine dyes were simulated using a combination of an empirical molecular force field for the ground state with quantum-chemical calculations of the electron excitation energy as a function of normal nuclear coordinates. The shape and the width of an absorption band strongly depend on the mutual arrangement of the monomers. If the monomers are located one directly above the other, the sublevels arising from intramolecular vibrations disappear in the spectrum, and a large hypsochromic shift of the 0-0-transition band is observed, which results mainly from through-space interaction of monomer orbitals. If the monomers are strongly shifted relative to each other, the sublevels mentioned are also absent in the spectrum, but the bathochromic shift of the 0-0-transition band is small and results from interaction of dipole moments of electron transitions. A rather broad region of intermediate structures is found between these dimer forms, where the interaction of dipole moments of electron transitions in monomers is low, and the shapes of absorption bands are similar to those of the monomers.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1705–1710, September, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08539) and the International Science Foundation (Grant M1Z 000).     

The nature of the electronic states and photoelectron spectra of oxyanion crystals

The densities of states, atomic charges, and partial components were calculated by the B3LYP method for lithium, sodium, and potassium nitrites, nitrates, chlorates, perchlorates, sulfites, and sulfates using a localized basis of atomic orbitals and CRYSTAL06 software. The calculated densities of states N(E) are in good agreement with the experimental photoelectron spectrum (UPS). The crystallographically nonequivalent metal and oxygen atoms are in different charged states, which leads to a splitting of the N(E) bands.     

Raman spectra of shock compressed pentaerythritol tetranitrate single crystals: anisotropic response

To gain insight into the anisotropic sensitivity of shocked pentaerythritol tetranitrate (PETN) single crystals, single-pulse Raman spectroscopy was used to examine the response of crystals shocked along the [100] (insensitive) and [110] (sensitive) orientations. High-resolution Raman spectra revealed several orientation-dependent features under shock compression: (i) substantially different stress dependence of the Raman shift for the CH(2) and NO(2) stretching modes for the two orientations, (ii) discontinuity in the stress dependence of the Raman shift for the CH(2) stretching modes above 4 GPa for the [110] orientation, and (iii) large broadening for the CH(2) and NO(2) asymmetric stretching modes for stresses above 4 GPa for the [110] orientation. The present data in combination with previous static pressure results provide support for conformational changes in PETN molecules for shock compression along the [110] (sensitive) orientation. Implications of the present results for the anisotropic sensitivity of shocked PETN are discussed.     

The equilibrium shapes of crystals and of cavities in crystals

Surface free energies are assumed to be the sum of the excess free energies of bonding of molecules in or near the surface, and the stable form of a crystal or cavity is assumed to be the form that makes the sum of these excess free energies a minimum. When only plane surfaces are allowed, this model predicts the same shapes for crystals as an equation of Wulff (2. Kristallogr. 34, 449 (1901)), which is based on the macroscopic thermodynamic relation of Gibbs (“The Collected Works, Vol. 1.: Thermodynamics,” Longmans, Green, New York (1931)). The model predicts rounding of edges and corners of kinds which are not allowed by the Wulff relation and predicts that spherical forms of particles and cavities can be stable despite anisotropic surface free energies. The model provides a useful framework for analysis of whether unstable crystal or cavity shapes will evolve into stable or metastable forms. Some crystals and cavities that have been assumed to have equilibrium shapes instead have metastable shapes.     

Investigation of molecule properties from electronic absorption spectra of solid and liquid crystals

Among the achievements of 20th century, there is the origin and violent development of the low-temperature technique and low-temperature spectroscopy of molecular crystals in the polarized light. Many obtained results became possible due to the close cooperation between experiment investigators and theorists. This short review traces the evolution of only one trend in the physics of molecular crystals, namely, the investigation of energetic and spatial structure of molecules from the analysis of electronic spectra of molecular crystals. First, for this purpose the possibilities of using the electronic spectra of molecular crystals at low temperatures benzene derivatives and the electronic spectra of liquid ionic crystals are considered. The results of investigations of the electronic absorption spectra for the new class of liquid crystals, namely, ionic metal-organic smectics are presented. Changes in the structure of doping molecules in the ionic liquid crystals under the influence of the dc electric field are analyzed.     

Validation of reducing-difference procedure for the interpretation of non-polarized infrared spectra of n-component solid mixtures

Validation of reducing-difference procedure based on subtracting of non-polarized infrared (IR) spectra of n-component solid mixtures is presented. The accuracy and precision are established. The limits of detection are 3.0, 2.5, 1.5 and 1.0 wt.% for 5-, 4-, 3-and 2-component mixtures, respectively. Smoothing procedures for IR spectral analysis, based on Savitzky-Golay or Fourier methods are applied as well. The mean values and relative standard deviations for peak position (ν_i) and integral absorbance (A_i) obtained by this data processing approach have been examined using Student's t-test.     

H-bonds of imidazole crystals. I. Measurements of polarized light IR spectra

This article is the first of a series on the v_s bands of imidazole crystals. It deals with experimental results on fully protonated and N—D deuterated species. We first describe the experimental conditions which enable us to obtain polarized spectra of monocrystals of imidazole which are sufficiently thin (≈1 μm) to give non-saturated v_s bands, so that we can measure with good precision the first moments of the v_s bands of these two species at various temperatures ranging from 10 to 300 K.     

Vibrational spectra of the ethylenediammonium ion in crystals

The IR and Raman spectra of three isotopic varieties of the ethylenediammonium ion in the tetrachlorocadmate(II) have been measured. It is shown that the ion assumes the centrosymmetric trans conformation in this salt. A complete vibrational assignment has been proposed for the observed fundamentals. Certain assignments for related compounds in the literature are modified. The spectra of the ion in the hexachloroplatinate(IV) and hexachlorostannate(IV) have also been obtained; compared to these two compounds hydrogen bonding in the tetrachlorocadmate is appreciable. Combination bands observed in the 2100-1800 cm^?1 region of the IR and Raman spectra of compounds containing the C-NH₃⁺ grouping are discussed and assignments are suggested for those observed in ethylenediammonium salts.     

A refined model of dielectric relaxation in uniaxial liquid crystals

A refined approach to dielectric relaxation in uniaxial liquid crystals based on the linear response concept is presented. The model is based on the expansion of the conditional probability function in terms of the probability function evolution operator eigenfunctions (instead of the Wigner functions). As a result, the time dependence of the expansion coefficients a_lm^j(t) is not, in general, a single exponential Wigner matrix element autocorrelation function.     

Influence of molecular electronic properties on the IR spectra of dimeric hydrogen bond systems: polarized spectra of 2-hydroxybenzothiazole and 2-mercaptobenzothiazole crystals

We have studied the polarized IR spectra of the hydrogen-bonded molecular crystals of 2-hydroxybenzothiazole (HBT) and 2-mercaptobenzothiazole (MBT). The crystal structure of 2-hydroxybenzothiazole was determined by X-ray diffraction. The polarized spectra of the crystals were measured, in the frequency ranges of the ν_N-H and ν_N-D bands, at room temperature, and at 77 K. In both systems an extremely strong H/D isotopic effect in the spectra was observed, involving reduction of the well-developed ν_N-H band fine structure to a single prominent ν_N-D line only. The two ν_N-H bands were also shown to exhibit almost identical properties, band shapes, temperature and dichroic properties included. The spectra were quantitatively reconstituted, along with the strong isotopic effect, when calculated using the ‘strong-coupling’ theory, assuming the centrosymmetric dimers of HBT or MBT to be the structural units responsible for the crystalline spectral properties. The similarity of the spectra of the two crystalline systems was considered to be a result of longer-distance couplings between the proton vibrations in the dimers, via the aromatic ring electrons. When investigating the ‘residual’ ν_N-H band shapes for crystals isotopically diluted by deuterium, we observed some ‘self-organization’ effects in the spectra, indicating the energetically favored presence of two identical hydrogen isotopes in each hydrogen bond dimer.     

On the interpretation of dielectric relaxation times for uniaxial liquid crystals

The two extensions of the Debye theory of rotational diffusion to include the effect of the orientational ordering potential in uniaxial liquid crystals are compared. The more accurate of these is then employed to analyse the dielectric relaxation times observed for 4-methoxybenzylidene-4′-n-butylaniline. The relationship between the diffusion constant, determined by the analysis, and the twist viscosity coefficient is also discussed.     

Band shapes and moment analysis of the far-infrared absorptions of dipolar molecules in solutions

The moment analyses were performed on the far-infrared absorption bands of CH₃CN, CHCl₃, CH₂Cl₂, and their deuterated compounds dissolved in nonpolar solvents. The values of torque acting on the solute molecules were estimated from the fourth moments. The band profiles were simulated by means of the continued-fraction representation of the time correlation function by Mori, using the moment values obtained. The third order long-time approximation of the correlation function yields essentially good simulations. The effective rotation angles of molecules between collisions were estimated from the first positive-to-negative turning points of the memory functions. The solvent effects on the torque and the rotation angle were consistent with each other.     

Recalling the appropriate representation of electronic spectra

This paper presents a critical discussion on the representation of UV-vis-NIR light absorption and fluorescence emission spectra of polyatomic molecules in liquid solution. A general overview how to handle the experimental spectra in order to extract physical data from them is presented together with some simulations to show the errors that can arise from unfortunate manipulation.