Theoretical study of dynamic electron-spin-polarization via the doublet-quartet quantum-mixed state (II). Population transfer and magnetic field dependence of the spin polarization

The population transfer to the spin-sublevels of the unique quartet (S = 3/2) high-spin state of the strongly exchange-coupled (SC) radical-triplet pair (for example, an Acceptor-Donor-Radical triad (A-D-R)) via a doublet-quartet quantum-mixed (QM) state is theoretically investigated by a stochastic Liouville equation. In this work, we have treated the loss of the quantum coherence (de-coherence) due to the de-phasing during the population transfer and neglected the effect of other de-coherence mechanisms. The dependences on the magnitude of the exchange coupling or the fine-structure parameter of the QM state are investigated. The dependence on the velocity of the population transfer (by the electron transfer or the energy-transfer) from the QM state to the SC quartet state is also clarified. It is revealed that the de-coherence during the population transfer mainly originates from the fine-structure term of the QM state in the doublet-triplet exchange coupled systems. This de-coherence leads to the unique dynamic electron polarization (DEP) on the high-field spin sublevels of the SC state, which is similar to the unique DEP pattern of the photo-excited triplet states of the reaction centers of photosystems I and II. The magnetic field dependence of the population transfer leading to the populations of the spin-sublevels of the SC states is also calculated. The possibility of the control of energy transport, spin transport and information technology by using the QM state is discussed based on these results. The knowledge obtained in this work is useful in the spin dynamics of any doublet-triplet exchange coupled systems.     

The numerical calculation of ESR spectra of randomly oriented quartet radicals

The ESR spectra of randomly oriented quartet radicals were calculated with the aid of an analytical formula. A line broadening through Lorentz lines was explicity taken into consideration. The experimental spectrum of an alternant hydrocarbon radical was simulated with sufficiently good agreement.     

Design,synthesis and electronic states of an iminonitroxide radical with excited quartet high-spin state and photo-induced electron transfer in its CT complex

In this paper, we report the design, synthesis and electronic state of a π-conjugated stable iminonitroxide radical, 1, the electron donor property of which is improved by an attachment of dimethylamino group. The photo-excited quartet high-spin state was observed by a time-resolved ESR experiment. CV measurement has clarified that the π-HOMO of the ground state is located higher ca. 0.35 eV in energy than 2 reported previously. These results show that the electron donor property is improved and the nature of the photo-excited spin alignment is conserved. Two CT complexes (1-TCNQ and 1-BQF₄) were synthesized using 1 as an electron donor. Their complexes have shown strong CT bands. The time-resolved ESR signal without the fine-structure splitting was observed for 1-TCNQ CT complex. The signal may arise from the photo-induced electron transfer from the quartet excited state of 1 to the TCNQ acceptor.     

Theoretical and gas-phase studies of specific cationized purine base quartet

Guanine tetraplexes are biological non-covalent systems stabilized by alkali cations. Thus, self-clustering of guanine, xanthine and hypoxanthine with alkali cations (Na(+), K(+) and Li(+)) is investigated by electrospray ionization mass spectrometry (ESI-MS) in order to provide new insights into G-quartets, hydrogen-bonded complexes. ESI assays displayed magic numbers of tetramer adducts with Na(+), Li(+) and K(+), not only for guanine, but also for xanthine bases. The optimized structures of guanine and xanthine quartets have been determined by B3LYP hybrid density functional theory calculations. Complexes of metal ions with quartets are classified into different structure types. The optimized structures obtained for each quartet explain the gas-phase results. The gas-phase binding sequence between the monovalent cations and the xanthine quartet follows the order Li(+) > Na(+) > K(+), which is consistent with that obtained for the guanine quartet in the literature. The smallest stabilization energy of K(+) and its position versus the other alkali metal ions in guanine and xanthine quartets is consistent with the fact that the potassium cation can be located between two guanine or xanthine quartets, for providing a [gua(or (xan))(8)+K](+) octamer adduct. Even if an abundant octamer adduct with K(+) for xanthine was detected by ESI-MS, it was not the case for guanine.     

Theoretical study of IR spectra of paraphenylenediamine

Normal coordinate analysis of the paraphenylenediamine (1,4-diaminobenzene, PPD) molecule has been carried out and complete interpretation of the vibrational spectrum is given for both trans and cis isomers. The reliable force field and electro-optical parameters of PPD have been determined by refinement in order to fit the experimental wavenumbers and intensities of PPD molecule. The initial force field parameters of PPD were refined from the corresponding parameters of aniline molecule. The initial values of bond dipole moments of the molecule were calculated by MINDO/3 method. The combination of the calculated IR spectra of trans and cis isomers of PPD is found to reproduce the experimental IR spectrum of solid PPD, satisfactorily, indicating that PPD exists as a mixture of both conformations.     

ESR study of the photoexcited triplet state of substituted 2-acetonaphthones

Electron spin resonance of phosphorescent triplet states of a number of 6-substituted-2-acetonaphthones is studied at 77 K in toluene glass. The life-times of the triplet states and zero-field parameter D* have been measured from Δm_s = ± 2 resonance. The life-times and the zero-field splitting parameters indicate that the lowest triplet states have mainly π, π* character.     

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: an ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.     

Steady state and time-resolved fluorescence study of isoquinoline: reinvestigation of excited state proton transfer

In the present work we report some hitherto unnoticed features in the steady state and time-resolved measurements of isoquinoline in water and trifluoroethanol (TFE). Absorption spectra reveal that in water, neutrals as well cationic species are present. Emission spectrum shows structured features at shorter wavelengths accompanied with a broad band around 375 nm, which correspond to neutrals and cations respectively. However, time-resolved data indicate that protonation does not take place in the excited state in water. On the contrary, in stronger hydrogen bonding solvent TFE, distribution of decay components is observed and at longer wavelengths a small rise time is present. This is ascribed to neutral and cation-like species present in the ground as well as in the excited state. The difference in the results is explained in terms of different excited state potential energy surfaces for water and TFE; particularly, the presence of a rather small barrier for protonation in case of TFE.     

Theoretical study of the X-ray photoionization spectra of polycycloalkanes

The dependence of X-ray photoionization spectra on structural characteristics is assessed by one-particle Green's function calculations on the nearly isomeric adamantane, 1,4-dimethyl bicyclo [2,2,2] octane and 1,1,4,4-tetramethyl cyclohexane compounds. These calculations are carried out using the quasi-particle approximation and the diagonal 2ph-TDA renormalization scheme for the self-energy. The simulated spectra fingerprint remarkably the atomic connectivity from their inner valence part, through the interplay of energy degeneracies and the influence of torsional strains on electron binding energies. The boundary region between the inner and outer valence bands provides also specific signatures for molecular characteristics pertaining to conformational questions, namely the enforced conversion from the chair to the boat form of the cyclohexane units generating these compounds. © 1996 John Wiley & Sons, Inc.     

Theoretical study of electronic spectra of the DNA bases

Excited π-electronic states of cytosine and uracil are calculated by the CI method. The effects of a transition from the single-excited configuration set to the set involving all double-excited configurations are considered. The set expansion is shown to affect essentially the transition energies and oscillator strengths, in particular, an additional electron transition related to the first absorption band occurs in the singlet uracil spectrum. When doubly excitations are taken into account the triplet transition energies considerably increase and become practically insensitive to repulsion integral parametrization.     

An ESR study of the hyperconjugation in the phosphorescent state of acenaphthene

ESR absorption has been observed in a single-crystal solid solution of acenaphthene in durene under irradiation at 120K with light from a high-pressure mercury lamp. The observed hyperfine structure gives evidence for hyperconjugation of the ethylene group protons. The guest and host molecular planes are nearly parallel to each other, and the guest in-plane axes deviate from the host ones by ca 40°. For comparison, 1,1,2,2-tetradeuteroacenaphthene was also prepared and studied.     

Theoretical study with vibration–rotation and dipole moment calculations of quartet states of the CrCl molecule

The potential energy curves have been investigated for the 10 lowest quartet electronic states in the ^2s+1Λ^± representation below 30,000 cm^?1 of the molecule CrCl via CASSCF and MRCI (singly and doubly excitation with Davidson correction) calculations. The harmonic frequency ω_e, the internuclear distance r_e, the rotational constant B_e, the electronic energy with respect to the ground state T_e, and the permanent dipole moment μ have been calculated. By using the canonical functions approach, the eigenvalues E_v, the rotational constant B_v, and the abscissas of the turning points r_min and r_max have been calculated for the considered electronic states up to the vibrational level v = 19. Seven electronic states have been studied here theoretically for the first time. The comparison of these values to the theoretical results available in the literature shows a good agreement. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

秋水仙碱立体异构体电子结构及圆二色谱性质的理论研究

采用量子化学密度泛函理论(DFT)在B3LYP/6-311++G二水平上对秋水仙碱四个立体异构体分子几何构型进行了优化,在优化的基础上进行了振动圆二色谱(VCD),紫外-可见光谱(UV-Vis)和电子圆二色谱(ECD)研究.为模拟真实条件,以水为溶剂,计算其对分子电子结构和光谱性质的影响.研究结果表明:秋水仙碱四个立体...     

ESR spectra and the behavior of substituted indophenoxyl radicals

ESR spectra and structure are compared for radicals formed from substituted indophenols by oxidation (substituents CH₃, i-C₃H₇, t-C₄H₉, t-C₅H₁₁, cyclohexyl). The unpaired electron interacts with the nitrogen and with the hydrogen in the ortho and meta positions of both benzene rings. A kinetic equation of first order with respect to the indophenol applies to the reaction of the latter with benzoyl peroxide. The loss of indophenoxyl radicals in benzene obeys an equation of second order in the radical concentration. The rate constants for radicals with various substituents indicate the radical stability, which falls greatly in going from ones with ortho-t-alkyl substituents to ones with less highly branched groups.