Lowest Electronic Transition Research Articles

Structural and spectroscopic characterization of ClC(O)SNSO. A theoretical and experimental study

N-(Sulfinylimine)chlorocarbonylsulfane, ClC(O)SNSO, was prepared by the reaction of ClC(O)SCl with Hg(NSO)2. The crystal structure of ClC(O)SNSO was determined by X-ray diffraction analysis from crystals obtained at low temperature using a miniature zone melting procedure. The molecule exhibits only one form with Cs symmetry: the CO double bond syn with respect to the S–N single bond, the C–S single bond anti to the NS double bond and the S–N single bond syn with respect to the SO double bond. The following skeletal parameters were determined (distances in A, angles in degrees, errors between parentheses expressed as sigma): Cl–C=1.771(2), CO=1.184(3), C–S=1.750(2), S–N=1.666(2), NS=1.534(2), SO=1.455(2), ClCO=122.9(2), ClCS=107.1(1), OCS=130.0(2), CSN=97.4(1), SNS=122.3(1). NSO=118.0(1), OCSN=-1.9, ClCSN=178.0, CSNS=-178.0, SNSO=-1.2. These experimental parameters compare satisfactorily with those obtained by abinitio and DFT calculations. The best agreement was found with the B3PW91/6–31+G* calculation. These quantum chemical methods were also employed to predict the vibrational spectra providing a good agreement with the experimental Raman spectra measured from the liquid sample. It is shown that the analysis of the vibrational spectra provides a sound basis for the determination of the conformation and configuration of R–NSO compounds. Raman excitation profiles were determined in the range from 514 to 413 nm. Most of the modes are predominantly enhanced via the π→π* transition at 296 nm but additional intensity is derived from low-electronic transitions at ca. 450 and 430 nm which are too weak to be detectable in the UV/VIS absorption spectra.

Theoretical Study of the Structural and Spectroscopic Properties of Stellacyanin

The electronic spectrum of the azurin Met121Gln mutant, a model of the blue copper protein stellacyanin, has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method), including the effect of the protein and solvent by point charges. The six lowest electronic transitions have been calculated and assigned with an error of less than 2400 cm-1. The ground-state singly occupied orbital is found to be a predominantly π antibonding orbital involving Cu3d and Scys3pπ. However, it also contains a significant amount (18%) of Cu−Scys σ antibonding character. This σ interaction is responsible for the appearance in the absorption spectrum of a band at 460 nm, with a significantly higher intensity than observed for other, axial, type 1 copper proteins (i.e., plastocyanin or azurin). The π−σ mixing is caused by the axial glutamine ligand binding at a much shorter distance to copper than the corresponding methionine ligand in the normal blue copper proteins. This explains why, b...

Color Prevision of Activated Forms of Photochromic Spirooxazines and Chromenes

Different kinds of semi-empirical quantic calculations have been used to model the lower electronic transition of photomerocyanines issued from photochromic spirooxazines and chromenes. PPP-MO leads to satisfying correlations between experimental and calculated λmax of an extended set of compounds. This interesting result allows to predict with a good assumption the effect of a given substitution on the electronic absorption. In contrast, all valence-electrons methods gave deceptive results as well concerning the value of the λmax as concerning the substituent effect on the shift of this λmax. It is suggested that a precise study of the fundamental and excited reaction pathways of the photochromic process could bring decisive informations about the modelling of the electronic absorption of the colored forms of spirooxazines and chromenes.

Theoretical Study of the Electronic Spectrum of Plastocyanin

The electronic spectrum of the blue copper protein plastocyanin has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method). The six lowest electronic transitions have been calculated and assigned with an error of less than 2000 cm-1. The singly occupied orbital in the ground state is Cu 3d-S(Cys) 3pπ antibonding with some N(His) 2pσ character. The bright blue color originates from an electron transfer to this orbital from the corresponding Cu 3d-S(Cys)3pπ bonding orbital. The influence of different ligand models on the spectrum has been thoroughly studied; Cu(imidazole)2(SCH3)(S(CH3)2)+ as a model of CuHis2CysMet is the smallest system that gives converged results.The spectrum is surprisingly sensitive to changes in the geometry, especially in the Cu-S bond distances; a 5 pm change in the Cu-S(Cys) bond length may change the excitation energies by as much as 2000 cm-1. The effect of the surrounding protein and solvent on the transition energies has been modeled by point charges and is found to be significant for some of the transitions (up to 2000 cm-1). (Less)

SPECTROSCOPIC PROPERTIES OF [Ru(bpy)2(bpz)]2+

We report on the temperature and magnetic field dependence of the low-temperature emission and excitation spectra of [Ru(bpy)2(bpz)]2+ doped into [Zn(bpy)3](ClO4)2. We could identify, for the first time, the origins corresponding to the three lowest electronic states. They are assigned to be triplet sublevels of a Ru4d – (bpz)π* metal to ligand charge transfer (MLCT) excitation. The energies of these zero-phonon lines are 15804 cm −1 (line I), 15822 cm −1 (line II), and 15899 cm −1 (line III). These sharp electronic transitions (halfwidths ≈ 2 cm −1) are associated with broad emission bands, which result from superpositions of vibrational satellites and multi-phonon structures. In particular, it is shown that magnetic fields have a drastic influence on the lowest electronic transition (including the vibronic structure). This transition is strongly forbidden. Thus, at zero field the origin line I is only detected in emission (τ = 130 μ s ). However, application of magnetic fields of B ≥ 8 T leads to a growing in of the corresponding excitation line, being in resonance with the luminescence line. For B = 12 T, this lowest origin line exhibits a Zeeman red-shift of ≈ 4 cm −1. However, the associated broad emission maximum is blue-shifted by ≈ 400 cm −1, where the shift attains a saturation near B = 4 T. These results are explained by a field-induced mixing of the wavefunctions of the lowest excited states. The described effects result from the large differences in the vibronic radiative deactivation paths and the radiative rates of the involved states.

ELECTRONIC ABSORPTION PROPERTIES OF SYMMETRICAL DIALKOXYANTHRACENES. LINEAR DICHROISM AND MAGNETIC CIRCULAR DICHROISM*

Abstract— Linear dichroism (LD) and magnetic circular dichroism (MCD) of symmetrical dialkoxy‐anthracenes and some related compounds were recorded in an attempt to elucidate electronic absorption properties and assign the nature of the lowest singlet excited state, recognized to control both the fluorescence and the photodimerization of anthracenes. The spectroscopic measurements proved that the lowest electronic transition is of the 1La type in all the compounds. The peculiar photoreactivity pattern is therefore governed by factors other than an inversion of the order of the 1La and 1Lb states and may be related to electronic density repartition on the different positions of the anthracene nucleus dictated by the position of the electron‐donating substituents.

Nonadiabatic electronic spin transition in ligand–heme protein binding kinetics and the influence of the heme Fe molecular environment

Nonadiabatic spin transitions can significantly reduce rates in reactions involving transition metals. Zero field spin splittings can further modulate the reaction rate at temperatures with thermal energies of the same size as the zero field splittings, and the low temperature reaction rate can be completely shut off in ligand fields of relatively low symmetry, C2v or D2, for example, due to a vanishing of the electronic matrix element between the initial and final ground spin states. We show in this paper that if the site symmetry is lowered even further, the electronic matrix element coupling the ground spin states can become nonzero, allowing the reaction to occur as the temperature approaches zero. We show what factors in the metal’s ligand environment are important for the electronic coupling between the initial and final electronic states. This is especially relevant for transition metals in sites of low symmetry, such as are found in biomolecules. We focus specifically on heme proteins and show how factors in the immediate environment of the Fe, such as the proximal histidine, may lower the site symmetry sufficiently to allow a nonzero low temperature nonadiabatic electronic spin transition that may be necessary for ligand binding. We also use these ideas to make predictions that could resolve the question of whether the reversible binding of CO to heme proteins is a nonadiabatic electronic process.

Low-energy electron-energy-loss spectroscopy of amorphous ice: Electronic excitations.

We report high-resolution electron-energy-loss spectra at 14.3 and 19 eV incident electron energy for electronic excitations in multilayer films of amorphous ice. The lowest electronic transition has its threshold at 7.3 eV and its maximum at 8.5 eV. It is followed by a broad shoulder around 10.4 eV and a rise above 11.6 eV.

New molecular metals composed of peri-condensed heterocycles

We reported design and synthesis of new peri-condensed heterocycles which do not contain TTF type conjugated systems, that is, three isomers of dithiaperylenes (DTPR), one isomer of dithiapyrene (DTPY) and their methylthio or methylseleno derivatives. These donors showed two-stage redox behavior whose potentials are comparable to that of TTF. Good correlation was obtained between the oxidation potentials and the HOMO energies calculated by the PM3 method. A number of CT complexes prepared were classified into three groups, neutral, ionic, and non-V-shape groups. Among them, the non-V-shape group complexes contained highly conducting complexes including molecular metals. Most of the CT salts with inorganic anions having high conductivities showed lower electronic transition energies than 0.5 eV.

Temperature-dependent luminescence spectra and lifetime measurements of octahedral Se(IV) and Te(IV) hexahalogeno coordination compounds

Emission spectra and lifetime measurements of microcrystalline A 2SeCl 6, A=Cs, Rb and Cs 2TeX 6, X=Cl, Br excited in the lowest electronic transition are reported for the temperature interval from T=1.9 to 240 K. The results are explained by a model which considers spin-orbit coupling, electron-electron interaction and the electron-phonon coupling (Jahn-Teller effect) of the complex molecule MX 6 2−. Evaluation of the measured data indicates that the lowest excited states Γ 1 and Γ 4 resulting from the sp electron configuration differ by only about 110 and 95 cm −1 for the Se(IV) and Te(IV) compounds, respectively. For the SeCl 6 2− complex, emission only from Γ 1 is observed at temperatures below 2.1 K. The emission properties of octahedrally coordinated s 2 ions can be explained by simple models including kinetic aspects. The relative orders of magnitude calculated for the decay rates correspond to predictions obtained from selection rules.

Quantum chemical calculations of spectroscopic parameters of thiones and selenones of the heteroaromatic series and corresponding heterofulvalenes based on them

1. Using quantum chemical calculations by the CNDO/CI method with a modified spectroscopic set of parameters, we have shown that asymmetric isomeric thiones and selenones of the heteroaromatic series and also the heterofulvalenes corresponding to them have smaller ionization potentials and energies of the lowest electronic transitions than do their symmetric isomers and heterofulvalenes based on them. 2. The increase in the dimensions of classical donors by their annealation by heterocyclic fragments insignificantly changes the ionization potentials and electronic transition energies, which is explained by the insignificant contributions from the AO′s of the atoms of these fragments to the composition of the highest occupied MO; in this case the major contributions to the ionization potentials comes from the 4pπ orbitals of the heteroatoms found in positions 2 and 3, and also from the 2pπ orbitals of the C atoms forming the double bond.

Resonance ion dissociation spectroscopy of naphthalene ions prepared in a supersonic expansion

Resonance ion dissociation (RID) spectra are reported for cations of naphthalene and 2-methylnaphthalene cooled in a supersonic beam. Discrete vibronic resonances were observed in the ultraviolet region of both ions. A discrete red system was also observed for 2-methylnaphthalene that is subject to strong degenerate vibronic interaction with an underlying quasicontinuum associated with a lower energy electronic transition. This leads to effective power broadening of the RID spectra at moderately low laser intensities. The red two-photon dissociation process of 2-methylnaphthalene was successfully modeled by classical rate equations applied to a four level system, consisting of a ground state, the directly excited state, a lower energy excited state, and a final dissociative state.

More light on the low electronic transition energy bands of dithizone

The visible electronic spectral behaviour of different concentrations of dithizone in pure and mixed various organic solvents has been investigated. It is identified that in dilute basic solvents solutions, dithizone (H 2DZ) exists mainly in monovalent anionic form (HDZ −), where its extent of ionization is largely dependent on the solvent basicity effect. The visible absorption band belonging to absorption of HDZ − form and the shorter visible one belonging to absorption of H 2DZ form are assigned to a transition involving the whole solute associated with intramolecular CT interaction. On the other hand, the longer wavelength visible band observed in the spectrum of the H 2DZ form is assigned to absorption of hydrogen bonding solvated molecular complex. This involves an electron transfer from the lone pair of electrons belonging to solvent molecule to the σ*-antibonding orbital of the acidic NH bond belongs to H 2DZ form.

Vibronic analyses of the Rydberg and lower intravalence electronic transitions in thioacetone

The absorption spectrum of thioacetone, (CH3)2CS, has been surveyed over the region 700-190 nm, and five distinct absorptions have been identified. These are the spin-allowed and spin-forbidden n → π∗, Ã(1A2) ← X̃(1A1), and ã(3A2) ← X̃(1A1) transitions; the orbitally allowed π → π∗, B̃(1A1) ← X̃(1A1) transition; and the Rydberg transitions n → 4s, C̃(1B2) ← X̃(1A1), n → 4pz, D̃(1B2) ← X̃(1A1), and n → 4py, Ẽ(1A1) ← X̃(1A1). In addition, the laser phosphorescence excitation spectrum has been obtained for the ã ← X̃ transition. Partial vibrational assignments have been obtained for all of the excited electronic states. The analyses showed that the ã, Ã, and C̃ states are all planar or pseudo-planar. Of interest is the vibrational activity of the methyl torsional modes. The presence of torsional progressions in the visible absorption spectrum led to the conclusion that the CH3 groups are rotated in the ã state by 60° from the X̃ state. Observation of torsional sequence transitions rather than progressions in the C̃ ← X̃ spectrum indicates that the methyl groups do not change their configuration on excitation into the C̃ state.

Thioketone spectroscopy: An analysis of the lower electronic transitions in thioacetone and thioacetaldehyde

Visible and ultraviolet spectra of the unstable species, thioacetaldehyde, CH 3CHS, thioacetone, (CH 3) 2CS, and thioacetone- d 6 have been studied in the gas phase. The valence n → π* excitations, Ã ← X̃ and ã ← X̃, have been identified. Rydberg n → 4s, 4p y and 4p z and π → π* valence excitations have been found in thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the n,4s and π,π* states which leads to a broad absorption of mixed character between 200 and 220 nm.

Theoretical studies of reactions in a laser field: F( 2P 3/2, 2P 1/2) + H 2+ ħω (0.469 eV)

The collinear F( 2P 3/2, 2P 1/2) + H 2 system in a strong laser field is studied employing the trajectory-surface-hopping method. The results are compared with a recent quantal study by Zimmerman, Baer and George. The two treatments yield low electronic non-adiabatic transition probabilities (⩽0.1). Good agreement is found in the high-energy region.

ChemInform Abstract: THE METHYLBENZENES VIS‐A‐VIS BENZENE. COMPARISON OF THEIR SPECTRA IN THE VALENCE‐SHELL TRANSITIONS REGION

Abstract Absolute extinction coefficient and oscillator strength values of nine gaseous benzenes (toluene, o-, m-, p-xylene, 1,2,4-, 1,3,5-trimethylbenzene, 2,3,5,6-tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) were recorded with moderate resolution in the region of the lower valence-shell electronic transitions (4 to 7 eV). The spectra are very similar to benzene except for a new band that appears in the heavier substituted compounds.

The methylbenzenes vis-à-vis benzene: Comparison of their spectra in the valence-shell transitions region

Absolute extinction coefficient and oscillator strength values of nine gaseous benzenes (toluene, o-, m-, p-xylene, 1,2,4-, 1,3,5-trimethylbenzene, 2,3,5,6-tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) were recorded with moderate resolution in the region of the lower valence-shell electronic transitions (4 to 7 eV). The spectra are very similar to benzene except for a new band that appears in the heavier substituted compounds.

Small computer-based system for determination of second- and third-order optical nonlinearities

An apparatus is described which has been designed for the purpose of measuring second- and third-order nonlinear optical susceptibilities of electronic character. The optical power derives from a Q-switched Nd3+/YAG laser which through a variety of frequency converters allows thorough characterization in the spectral window between the high vibrational states in the near infrared and the low electronic transitions in the visible or near UV. Data collection, treatment, and analysis are performed by a small computer/controller. As example of the potentiality of the apparatus the observation of multiple cascading in summation third-order wave mixing by crystalline quartz is reported.

Electronic structure of porphyrins. II. Mutual influence between ligands of Co(III)porphyrins

CNDO/2 calculations of hexacoordinate Co(III)porphyrins, [Co(III)(Po)XY] and hexacoordinate tetraammine Co(III) complexes, [Co(III)(NH 3) 4XY], where X = CO and CN − and Y = CO, Im, Py and NH 3 have been carried out. As the energy differences between HOMO and LUMO are almost constant for all hexacoordinate Co(III)porphyrins, their electronic spectra associated with the lowest electronic transitions are similar to each other. In hexacoordinate metal porphyrins, there seems to occur charge breathing between the axial ligand and the H's in porphyrin. The strengths of the metalporphyrin bond is larger than that of the metalNH 3 bond. This brings the bond weakening of MX bond in metal porphyrins in comparison with tetraammine complexes, which seems to be responsible for the interesting catalytic ability of metal porphyrins. The effect of the fifth ligand is almost absorbed by the 4pπ–2pπ interaction between the metal and porphyrin.