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Singlet States (singlet + states)
Selected AbstractsSinglet states open the way to longer time-scales in the measurement of diffusion by NMR spectroscopyCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2008Simone Cavadini Abstract Nuclear magnetic resonance is a powerful nonintrusive technique for measuring diffusion coefficients through the use of pulsed field gradients. The main limitation to the application range of this method is imposed by the relaxation time constants of the magnetization. The recently introduced singlet-state spectroscopy affords obtaining relaxation time constants for pairs of coupled spins which can be longer by more than an order of magnitude than the spin-lattice relaxation time constants. We review in this paper the advantages that are offered by these long relaxation time constants for diffusion measurements. Using experiments that combine singlet-state and diffusion spectroscopy, slower diffusion constants can be determined. The coupling of the two methods constitutes an alternative to the use of special probes equipped with strong gradients for the study of large molecules that diffuse slowly in solution. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 68,78, 2008. [source] Optical spectra and covalent chemistry of fulleropyrrolidinesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2007B. S. Razbirin Abstract Low-temperature vibronic spectra of two fulleropyrrolidines (1-methyl-3,4-FP and 1-methyl-2(4-pyridine)-3,4-FP) embedded in crystalline toluene matrix have been studied. Two-component composition of the spectra has been established and charge-transfer-excitation origin of the structureless component has been suggested. Fine-structured Shpol'skii spectra were observed for 1-methyl-3,4-FP, which made possible to perform the vibrational analysis of its vibronic spectra. General similarities of the absorption spectra of fulleropyrrolidines and C60 molecules along with significant difference in their details have been discussed. A detailed interpretation of the C60 spectra serves as a basis for analyzing the spectra of the derivatives. Quantum-chemical study is based on the effectively-unpaired-electron concept for the fullerene molecule. Computations have been performed for the singlet states of the molecules in unrestricted Hartree,Fock approximation implemented in AM1 semiempirical quantum chemical codes of the CLUSTER-Z1 software. The population of the HOMO and LUMO of the molecules under study alongside with the lowering of the molecules symmetry have been proposed to explain the spectral features observed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Theoretical study on the gas-phase reaction mechanism between nickel monoxide and methane for syngas productionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2009Hua-Qing Yang Abstract The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH3 is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH3NiOH and HNiOCH3, locate on the deep well, while CH3NiOH is more energetically favorable than HNiOCH3 on both the triplet and singlet states. The main products shall be syngas once HNiOCH3 is created on the singlet state, whereas the main products shall be methyl radical if CH3NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH3 formation involving an oxidative addition of NiO molecule into the CH bond of methane, with an energy barrier of 120.3 kJ mol,1. The syngas formation would be more effective under higher temperature and photolysis reaction condition. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Computational study of the cooperative effects of nitrogen and silicon atoms on the singlet,triplet energy spacing in 1,3-diradicals and the reactivity of their singlet statesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2010Takeshi Nakamura Abstract Quantum chemical calculations were performed to investigate the cooperative effect of the nitrogen and silicon atoms on the singlet,triplet energy spacing and the reactivity of the singlet state in 1,2-diazacyclopentane-3,5-diyls and 1,2-diaza-4-silacyclopentane-3,5-diyls. The largest singlet,triplet energy gap (,=,,36.1,kcal/mol) found so far in localized 1,3-diradicals was in the C2v symmetry of 4,4-difluoro-1,2-diaza-4-silacyclopentane-3,5-diyl at the UB3LYP/6-31G(d) level of theory. The cooperative effect was also found in the energy differences of singlet diradicals with the corresponding ring-closing compounds, bicyclo[2.1.0]pentane derivatives. The singlet state of the 1,2-diaza-4-silacyclopentane-3,5-diyls was calculated to be energetically more stable than the ring-closing compound. The notable finding on the stability of the singlet diradicals may be attributed to the resonance structures that specifically stabilize the singlet state of diradicals. The computational studies predict that the singlet 1,2-diaza-4-silacyclopentane-3,5-diyl is a persistent molecule under conditions without intermolecular-trapping reagents. Copyright © 2010 John Wiley & Sons, Ltd. [source] Magneto-electric effect on frequency mixing in atomsLASER PHYSICS LETTERS, Issue 7 2004V. D. Ovsiannikov Abstract Steady electric and magnetic fields can stimulate frequency mixing of two laser waves in ensemble of free atoms. In addition to coherence conditions, the steady fields may induce additional resonance singularities essentially enhancing the cross section for scattering the sum-frequency wave. Interference between different components of the electric- and magnetic-field induced frequency summation amplitudes may cause significant effects on the efficiency of conversion. The dependence on the incident wave polarization and the atomic resonance structure is calculated analytically for the frequency mixing in atoms with a singlet structure of the ground and resonance states. Numerical estimates for the quantitative characteristics of the effect are presented for helium, alkali-earth and mercury atoms in their ground n1S0 -state in the case of the two-photon resonance on excited singlet states with angular momentum 0, 1, and 2. (© 2004 by ASTRO, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Influence of electron correlations on strong field ionization of calciumLASER PHYSICS LETTERS, Issue 1 2004E. Charron Abstract Non-perturbative time-dependent calculations of single and double ionization of a one-dimensional model of atomic calcium by short and intense laser pulses were performed at various wavelengths. The comparison of the probabilities calculated within a two-active electron (TAE) approach with those obtained using a single-active electron (SAE) approximation clearly demonstrates the crucial role played by the electronic correlation and by doubly excited states within this model, even for the formation of Ca+. Experimental and calculated energy spectra of the singlet states of Ca. The experimental values (b) are taken from [20], and the calculated values (a) on the left and (c) on the right correspond to the present SAE and TAE models respectively. (© 2004 by HMS Consultants. Inc. Published exclusively by WILEY-VCH Verlag GmbH & Co.KGaA) [source] Upper electron-excited states in bioluminescence: experimental indicationLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 3 2001N. S. Kudryasheva Abstract The involvement of upper electron-excited states in bacterial bioluminescence process was studied with excitation energy-accepting molecules. The fluorescent aromatic compounds, anthracene and 1.4-bis(5-phenyloxazol-2-yl)benzene, were chosen. Energies of their lowest excited singlet states are higher than the energy of the analogous state of the bioluminescence emitter; their absorption spectra and bioluminescence do not overlap. Hence, the excitation of these molecules by singlet,singlet energy transfer or by light absorption is excluded. Sensitized fluorescence of these compounds in the bioluminescence systems has been recorded, indicating the activity of upper electron-excited states in the bioluminescent process. Copyright © 2001 John Wiley & Sons, Ltd. [source] Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2009Fernando Lorenzo The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (, = 36800 and 17000 dm3 mol,1 cm,1, respectively). Both the absorption and emission properties of SFX are pH-dependent; pKa values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet,triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm3 mol,1 cm,1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (3A*) was found to react with phosphate buffer to form triplet 3B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens. [source] Primary Photophysical Properties of Moxifloxacin, A Fluoroquinolone AntibioticPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2008Fernando Lorenzo The photophysical properties of the fluoroquinolone antibiotic moxifloxacin (MOX) were investigated in aqueous media. MOX in water, at pH 7.4, shows two intense absorption bands at 287 and 338 nm (, = 44 000 and 17 000 dm3 mol,1 cm,1, respectively). The absorption and emission properties of MOX are pH-dependent, pKa values for the protonation equilibria of both the ground (6.1 and 9.6) and excited singlet states (6.8 and 9.1) of MOX were determined spectroscopically. MOX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Phosphorescence from the excited triplet state in frozen ethanol solution has a quantum yield of 0.046. Laser flash photolysis and pulse radiolysis studies have been carried out to characterize the transient species of MOX in aqueous solution. On laser excitation, MOX undergoes monophotonic photoionization with a quantum yield of 0.14. This leads to the formation of a long-lived cation radical whose absorption is maximum at 470 nm (,470 = 3400 dm3 mol,1 cm,1). The photoionization process releases hydrated electron which rapidly reacts (k = 2.8 × 1010 dm3 mol,1 s,1) with ground state MOX, yielding a long-lived anion radical with maximum absorption at 390 nm (,390 = 2400 dm3 mol,1 cm,1). The cation radical of MOX is able to oxidize protein components tryptophan and tyrosine. The bimolecular rate constants for these reactions are 2.3 × 108 dm3 mol,1 s,1 and 1.3 × 108 dm3 mol,1 s,1, respectively. Singlet oxygen sensitized by the MOX triplet state was also detected only in oxygen-saturated D2O solutions, with a quantum yield of 0.075. [source] The formation of neutral CCCO2H and HCCCO2 molecules from anionic precursors in the gas phase: a joint experimental and theoretical studyRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 24 2005Mark Fitzgerald Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that the anions ,CCCO2H and HCCCO are stable species in their singlet states. Upon collision-induced, vertical one-electron oxidation under neutralisation-reionisation (,NR+) conditions, they produce the neutral molecules CCCO2H and HCCCO2, respectively. Some of the CCCO2H neutrals should be stable for the duration of the neutralisation-reionisation experiment (10,6,s), while others will dissociate to CCCO and OH (requires 125,kJ,mol,1). In contrast, neutral HCCCO2 is expected to be much less stable, and dissociate to HCC and CO2 (37,kJ,mol,1). Neither CCCO2H nor HCCCO2 is expected to interconvert, or to rearrange to other isomers. The anions ,CCCO2H and HCCCO have been formed in the ion source of the mass spectrometer by the reactions between (CH3)3SiCCCO2H and F, and HCCCO2Si(CH3)3 and F,, respectively. The ,NR+ spectrum of ,CCCO2H shows a recovery signal and also indicates that the lowest energy dissociation pathway of neutral CCCO2H corresponds to the loss of OH. The ,NR+ spectrum of HCCCO2 displays little or no recovery signal, and the spectrum is dominated by the [CO2]+ ion. The experimental observations are in agreement with the predictions of the extensive theoretical studies. Copyright © 2005 John Wiley & Sons, Ltd. [source] Application of the Perimeter Model to the Assignment of the Electronic Absorption Spectra of Gold(III) Hexaphyrins with [4n+2] and [4n] ,-Electron SystemsCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2009Atsuya Muranaka Dr. Abstract Expanded porphyrins: The electronic excited states of two forms of meso -hexakis(pentafluorophenyl)-substituted gold(III) hexaphyrin(1.1.1.1.1.1), such as that depicted, have been investigated by density functional calculations and magnetic circular dichroism spectroscopy to assign their low-energy excited singlet states. The electronic excited states of two forms of meso -hexakis(pentafluorophenyl)-substituted gold(III) hexaphyrin(1.1.1.1.1.1) have been investigated by density functional calculations and magnetic circular dichroism (MCD) spectroscopy, in order to assign their low-energy excited singlet states. We found that the perimeter model can be successfully applied to the interpretation of the electronic states. In the case of the neutral forms (Au2 -N, Au-N), the absorption bands observed in the NIR and visible region can be assigned to ,,,* transitions referred to as the L and B bands, respectively, analogous to the Q and Soret bands of regular porphyrins. In marked contrast with the neutral forms, the absorption bands of the reduced forms (Au2 -R and Au-R) are attributed to ,,,* transitions involving six frontier molecular , orbitals. By applying the 4N -electron perimeter model, the six orbitals are labeled as h,, h+, s,, s+, l,, and l+, while the observed absorption bands can be assigned to the S, N1, N2, P1, and P2 transitions, in order of increasing energy. [source] Electronic Structure of the Ground and Excited States of ,-CarbolineCHEMPHYSCHEM, Issue 6 2008Vicente Pérez Mondéjar Abstract Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of ,-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects. [source] Spin-Orbit Ab Initio Investigation of the Ultraviolet Photolysis of DiiodomethaneCHEMPHYSCHEM, Issue 6 2007Ya-Jun Liu Dr. Abstract The UV photodissociation (<5 eV) of diiodomethane (CH2I2) is investigated by spin-orbit ab initio calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH2I2 and CH2II lead to geminate-radical products, CH2I,.+I(2P3/2) or CH2I,.+ I*(2P1/2). The recombination process from CH2II to CH2I2 is explained by an isomerization process and a secondary photodissociation reaction of CH2II. Finally, the study reveals that spin-orbits effects are significant in the quantitative analysis of the electronic spectrum of the CH2II species. [source] The Low-Lying Excited States of 2,2,-Bithiophene: A Theoretical AnalysisCHEMPHYSCHEM, Issue 12 2003Mercedes Rubio Dr. Abstract The low-energy regions of the singlet,singlet, singlet,triplet, and triplet,triplet electronic spectra of 2,2,-bithiophene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 11Bu and 21Bu, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 23Ag and 23Bu, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states. [source] | ||||||||||