Absorption band shape in five-coordinated copper(ii) complexes: influence of the Jahn-Teller effect

A Monte Carlo method has been used to analyze the absorption band shape for A → E electronic transitions, with Jahn-Teller and pseudo-Jahn-Teller interactions included in the Hamiltonian. The results obtained suggest a reconsideration of the assignment of d-d transitions in trigonal bipyramidal copper(II) complexes may be in order.     

X‐ray photoelectron spectroscopy and the pattern recognition method—their application to surface studies in CoPd alloys

In the present work, polycrystalline CoPd alloys in varying range of bulk atomic percent composition (Co30Pd70, Co50Pd50 and Co70Pd30) are investigated by means of X‐ray photoelectron spectroscopy (XPS). The results of conventional XPS quantitative multiline (ML) approach are compared to the results obtained on the basis of XPS lines shape analysis, where the selected XPS or X‐ray induced Auger electron (XAES) transitions, are processed using the pattern recognition method known as the fuzzy k‐nearest neighbour (fkNN) rule. The fkNN rule is applied to the following spectra line shapes: Pd MNV, Co 2p, Co LMM, Pd 3d and valence band, analysing electrons in a varying range of selected kinetic energies. Both methods showed the surface segregation of Pd in Co30Pd70 and Co50Pd50 alloys. The results of the ML, the binding energy shift (ΔBE) analysis and the fkNN rule remained in agreement. Discrepancies in quantitative results obtained using different approaches are discussed within the accuracy of the applied methods, differences due to mean escape depth (MED) of electrons in considered transitions, their depth distribution function, the sensitivity of electron transition line shape on the environmental change (weaker effect for the inner shell transitions, and stronger effect for the outer shell transitions and Auger electron spectroscopy (AES) electrons transitions) and the non‐uniform depth profile concentrations. Copyright © 2005 John Wiley & Sons, Ltd.     

Line shape evolution in pressure-broadened parallel and perpendicular bands of N2O

High pressure band profile has been investigated for two transitions of N₂O in the 2 μm region. The primary focus of these experiments is on the line shape evolution in moderately dense media. Experimental results suggest that the analysis of the band shape can give a test for the two major assumptions of the collision theory. It is shown that the impact approximation can be tested on a parallel band (40°0-00°0) for pressures up to 15 atm ; while interference effects can be pointed out in a perpendicular band (01′2-00°0) for much lower pressures.     

Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (g(lum)) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and g(lum) are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. g(lum) for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions[g(lum)] decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, [g(lum)] is almost constant throughout the series of vibronic transitions but for weak coupling [g(lum)] becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.     

UV absorption spectra of 1,1'-binaphtyle and 2,2'-binaphtyle molecules and their self-associations from the atom monopole-dipole interaction (AMDI) model

The Atom Monopole-Dipole interaction (AMDI) model is used for the first time in calculation of UV absorption spectra of 1,1'-binaphtyl and 2,2'-binaphtyl and their self-associations for the case of frequency-dependant atom polarisability with Lorentzian band shape. UV absorption spectra are obtained from a set of electronic normal modes. The variation of spectra with the conformation of these molecules concord with the results of the medium effect on the corresponding electronic transitions. The calculated auto-association spectra are qualitatively in good agreement with polycyclic aromatic hydrocarbon aggregations behaviour.     

Conformational energetics and excited state level ordering in 11-cis retinal.

Semiempirical molecular orbital theory and semiclassical solvent effect theory are used to analyze the conformational and electronic properties of the 12-s-cis and 12-s-trans conformers of 11-cis retinal. The goal is to examine the influence of solvent environment on the equilibrium geometries of these conformers as well as to provide a perspective on the electronic transitions that contribute to the four band systems that are observed in the 200-500 nm region of the optical spectrum. We conclude that the 12-s-cis isomer is more stable in vacuum, but that the 12-s-trans conformer is preferentially stabilized in both polar and nonpolar solvent environment due to dispersive as well as electrostatic interactions. This observation is in substantial agreement with previous literature results. In contrast, our analysis of the excited state manifold indicates that the spectral features observed in the absorption spectrum are associated with a complex set of overlapping transitions. A total of 18 pi*<--pi transitions contribute to the four bands, and in some cases, conformation changes the relative contribution of the individual transitions that define the overall band shape. This study provides the first definitive assignments for all four band systems.     

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).     

Comparison of simulated and actual DSC measurements for first-order transitions

A system of differential equations modeling a heat flux DSC is solved and the results are compared with those obtained using a TA Instruments Q1000™ DSC.¹ It incorporates a new heat flow rate measurement technique that determines the heat flow rate between the sample and its pan. Two types of first-order transitions are investigated: melting of a pure substance and solidification of a pure substance including super-cooling. In both transitions, the peak shape obtained using the new heat flow rate measurement and predicted by the model is quite different from that measured using conventional DSC. It is shown that the differences are the result of simplifications implicit in the conventional heat flow rate measurement that is based solely on the difference between sample and reference calorimeter temperatures. Heat flow rates measured using the improved measurement agree very well with the model predictions for heat exchange between the sample and its pan.     

Adaptive umbrella sampling of the potential energy: modified updating procedure of the umbrella potential and application to peptide folding

Adaptive umbrella sampling of the potential energy is used as a search method to determine the structures and thermodynamics of peptides in solution. It leads to uniform sampling of the potential energy, so as to combine sampling of low-energy conformations that dominate the properties of the system at room temperature with sampling of high-energy conformations that are important for transitions between different minima. A modification of the procedure for updating the umbrella potential is introduced to increase the number of transitions between folded and unfolded conformations. The method does not depend on assumptions about the geometry of the native state. Two peptides with 12 and 13 residues, respectively, are studied using the CHARMM polar-hydrogen energy function and the analytical continuum solvent potential for treatment of solvation. In the original adaptive umbrella sampling simulations of the two peptides, two and six transitions occur between folded and unfolded conformations, respectively, over a simulation time of 10 ns. The modification increases the number of transitions to 6 and 12, respectively, in the same simulation time. The precision of estimates of the average effective energy of the system as a function of temperature and of the contributions to the average effective energy of folded conformations obtained with the adaptive methods is discussed. Received: 11 July 1998 / Accepted: 22 September 1998 / Published online: 17 December 1998     

Frontier orbitals and ligand-to-metal charge transfer electronic transitions in <Emphasis Type="Italic">d</Emphasis><Superscript>0</Superscript>-metal complexes

Properties of frontier orbitals and low-energy electronic transitions in a d ⁰-organometallic complex have been studied by TDDFT and DFT methods using B3LYP hybrid functional and 3-21G*, 6-31G**, SDD, CEP-121G, and DGDZVP basis sets. It has been shown that the electronic transition between frontier orbitals in the excitation and absorption spectra is associated with charge transfer mainly from π-type ligands to a central metal d ⁰-ion. The good agreement of the data (the shape and band position of the spectra of electronic absorption and excitation, energy of electronic transitions, and strength of the harmonic oscillator) of quantum-chemical and photophysical studies is demonstrated.     

Infrared laser spectroscopy of jet-cooled carbon clusters: the nu 5 band of linear C9

The nu 5 antisymmetric stretching vibration of 1 sigma+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079-2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported nu 6 transitions yielded the following molecular constants for the nu 5 band: nu 0 = 2 079.673 58(17) cm-1, B"= 0.014 321 4(10) cm-1, and B'=0.014 288 9(10) cm-1. The IR intensity of the nu 5 band relative to nu 6 was found to be 0.108 +/- 0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).     

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, ³(n→π^*), in formamide (HCONH₂) and N,N-dimethyl formamide (HCONMe₂) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the ³(π→π^*) and ³(n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me₂CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/dE) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.     

Experimental and theoretical study of the polarized infrared spectra of the hydrogen bond in 3‐thiophenic acid crystal

This article presents the results of experimental and theoretical studies of the v_{O H} and v_{O D} band shapes in the polarized infrared spectra of 3‐thiophenic acid crystals measured at room temperature and at 77 K. The line shapes are studied theoretically within the framework of the anharmonic coupling theory, Davydov coupling, Fermi resonance, direct and indirect damping, as well as the selection rule breaking mechanism for forbidden transitions. The adiabatic approximation allowing to separate the high‐frequency motion from the slow one of the H‐bond bridge is performed for each separate H‐bond bridge of the dimer and a strong nonadiabatic correction is introduced via the resonant exchange between the fast‐mode excited states of the two moieties. The spectral density is obtained within the linear response theory by Fourier transform of the damped autocorrelation functions. The approach correctly fits the experimental line shape of the hydrogenated compound and predicts satisfactorily the evolution in the line shapes with temperature and the change in the line shape with isotopic substitution. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Vibration—rotation spectra of CO in liquid alkanes

Vibration—rotation infrared spectra have been obtained for CO dissolved in a variety of branched and normal alkanes. The normalized band profiles yield rotational correlation functions, correlation times and the torques acting on the CO molecule. These quantities and the band shape depend only on the alkane free volume, contrasting with the situation for DC1 solute where they showed a strong dependence on solvent molecular shape. The Gordon m-diffusion model for rotational diffusion gives good predictions for the correlation functions and a complete fit is obtained using the generalized m-diffusion model incorporating a small frequency-dependence of the effectiveness of a collision in changing the rotor angular momentum. This success of simple stochastic theory contrasts with its failure for DCl in these alkanes.     

Electronic spectra of phenyl-, 3-pyridyl-, furfuryl-, and 2-theinyl-imino derivatives of thiazole: Molecular orbital treatment

The electronic spectra of phenylmethylene, 3-pyridylmethylene, furfurylmethylene, and theinylmethylene derivatives of 2-aminothiazole have been investigated. Gaussian analysis for the spectra indicates the existence of four electronic transitions in the 350–220 nm region. Geometry optimization using AM1 method followed by INDO/S-CI calculations was carried out. The results confirmed the absence of n–π* transitions. The coefficients of configuration interaction (CI) wave functions defined the type of electronic transitions, which are of major charge transfer (CT) character. The anticonformer is the predominant one. The gap energies of the studied compounds are of the same order, which indicates that their reactivities are expected to be the same. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 415–423, 1998     

Vibrational spectra of partially oriented molecules having two conformers in nematic and isotropic solutions: furfural and 2-chlorobenzaldehyde

Vibrational analysis of the two conformers of furfural and 2-chlorobenzaldehyde has been carried out on the basis of their IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystalline) solvent. The average orientation of the individual conformers in the nematic solvent has been determined by means of a recently developed approach for low symmetry planar molecules using DFT calculations of the vibrational transitions moments. The complex shape of the carbonyl band additionally split into several components is interpreted as an effect of Fermi resonance.     

Band moments and rotational autocorrelation function in infrared for perpendicular transitions of linear molecules

By use of Gordon's theory the second and fourth band moments are computed. Some of these results are also obtained by relating rotational autocorrelation functions of dipolar moment for parallel and perpendicular transitions. Theoretical and experimental results are compared.     

Size-dependent extinction coefficients of PbS quantum dots

We report here on a detailed study on PbS colloidal quantum dots. A characterization via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) allowed us to reliably determine the diameter and the shape of the nanocrystals. These data, together with second-derivative analysis of the absorption spectra, allowed us to determine the size dependence of seven transitions in the absorption spectrum; some of these transitions were identified on the basis of their normalized confinement energy. The size dependence of the first excitonic transition was best modeled by a four-band envelope approach which considers the anisotropy of the band edges (Andreev, A. D.; Lipovskii, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15402-15404). The extinction coefficients were calculated using concentrations obtained from inductively coupled plasma atomic emission spectrometry (ICP-AES), and their size dependence was found to follow a power law with exponent equal to approximately 2.5. In contrast with what was expected from the effective mass approximation, the per particle absorption cross section of the lowest transition was found to be strongly dependent on the particle size.     

Absorption spectrum and solvatochromism of the [Ru(4,4'-COOH-2,2'-bpy)2(NCS)2] molecular dye by time dependent density functional theory

We present a combined Density Functional/Time Dependent Density Functional study of the molecular structure, electronic states, and optical absorption spectrum of [Ru(4,4'-COOH-2,2'-bpy)(2)(NCS)(2)], a widely used charge-transfer sensitizer in nanocrystalline TiO(2) solar cells. Calculations have been performed both for the complex in vacuo and in ethanol and water solvents, using a continuum model to account for solute-solvent interactions. Inclusion of the solvent leads to important changes of the energies and composition of the molecular orbitals of the complex; as a consequence, whereas the computed spectrum for the Ru-complex in vacuo deviates from the experimental one in both energy and shape, the spectra calculated in the presence of the solvent are in good agreement with the experiment. The first two absorption bands are found to originate from mixed ruthenium-NCS to bipyridine-pi* transitions rather than to pure metal-to-ligand-charge-transfer (MLCT) transitions, whereas the third band arises from intraligand pi --> pi* transitions. The experimentally observed blue-shift of the spectrum in water with respect to ethanol is well reproduced by our calculations and appears to be related to a decreased dipole moment in the excited state.