Semi-empirical ?-electron calculations

Bailey, Margaret L.

doi:10.1007/bf00527319

Cited by 27 publications

(13 citation statements)

References 35 publications

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“…The parameterizations and functional forms for the PPP matrix elements are, of course, not uniquely defined. The particular functional form and parameterization that are given below are those that have been successful in reproducing transition energies and intensities in aromatic heterocycles given by where e is the magnitude of the electron charge, R μν is the distance between atoms μ and ν, and γ νν is the one-center repulsion parameter.…”

Section: Methodsmentioning

confidence: 99%

Solvent and Intramolecular Effects on the Absorption Spectrum of Betaine-30

2000

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The effect of solvent electrostatics and solute torsional modes on the absorption spectrum of betaine-30 in acetonitrile is examined. Combined quantum/classical molecular dynamics ground state simulations are used to calculate the electronic absorption spectrum in acetonitrile. The model for betaine-30 includes the electronic degrees of freedom of the π system of the molecule and their interactions with the electric field of the solvent, treating the electronic wave function at the level of Pariser−Parr−Pople semiempirical electronic structure theory. The absorption intensity, width, and maximum of the S0 to S1 band are well reproduced by the model. In solution, the S0 molecular dipole moment is found to be strongly enhanced due to solvent-induced electronic reorganization. The width of the absorption band in acetonitrile is found to be a function of solvent orientational fluctuations and is not correlated with conformational changes caused by torsional motion in the molecule. This fact, combined with the good agreement between the classical reorganization energies inferred from the simulated and experimental spectra indicates that, at least in acetonitrile, the classical component of the reorganization energy is fully determined by solvent orientational polarization. The spectral band maximum of the lowest energy transition is found to be blue shifted over 7000 cm-1, compared to a calculation in which the coupling of the betaine-30 electronic structure to the solvent molecules is eliminated, in agreement with the shift found experimentally for betaine-30 in acetonitrile compared to alkanes. However, in contrast to the result found in acetonitrile, the transition energy in the absence of solvent interactions is found to be strongly correlated with the central phenolate−pyridinium dihedral ring angle. This contrasting behavior implies that in nonpolar solvents, the classical reorganization energy does have a contribution from that torsional mode. Correspondingly, this difference in behavior with solvent indicates that the assumption of a solvent independent intramolecular contribution to the reorganization energy is questionable.

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Section: Methodsmentioning

confidence: 99%

Solvent and Intramolecular Effects on the Absorption Spectrum of Betaine-30

2000

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“…The electrostatic potential produced by the solvent molecules couples to the betaine-30 electronic degrees of freedom via a one electron contribution to the diagonal elements of the Fock matrix. The transition energies are calculated using single CI, which has proven to be very accurate in the PPP method in the calculation of transition energies of heteronuclear aromatic molecules . The model has proven to be accurate in calculating the wavelength, intensity, and bandwidth of the lowest energy transition of betaine-30 in acetonitrile, as well as the shift in these compared to a nonpolar medium .…”

Section: Methodsmentioning

confidence: 99%

Computer Simulation of the Excited State Dynamics of Betaine-30 in Acetonitrile

Lobaugh

Rossky

1999

J. Phys. Chem. A

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Time-dependent studies of the excited state dynamics of betaine-30 in acetonitrile at room temperature have been carried out using a mixed classical/quantum molecular dynamics simulation methodology. The π-electron system of the solute molecule is treated quantum mechanically using the semiempirical Pariser−Parr−Pople Hamiltonian, including the solvent influence on electronic structure. The remaining interactions are treated via empirical potentials. Transition probabilities between adiabatic electronic states are evaluated using surface hopping methods, including all nuclear degrees of freedom in the coupling. The dynamics treats the (rigid) solvent and the dihedral angles for relative rotation of rings of an otherwise rigid solute classically. The contribution of all remaining solute intramolecular vibrations is included in the nonadiabatic coupling via an approximate, but purely quantum mechanical, treatment. Analysis of the dynamics reveals that, after excitation to the first excited state, the energy gap between ground and first excited states of the molecule exhibits an ultrafast (∼100 fs) decrease due to the inertial response of the solvent that accounts for about 70% of the solvent response, followed immediately by a further subpicosecond solvent component. The times and amplitudes of these solvation components are in accord with the results inferred from resonance Raman spectra, and the solvent contribution to the Stokes shift observed is in accord with values inferred from ground state absorption spectral line shape analysis. However, we also find that the energy gap exhibits a slower picosecond time scale response of comparable magnitude due to relative rotation of the central phenolate and pyridinium rings. This relaxation has not been previously noted or incorporated in corresponding electron transfer models. Analysis of contributions to the electronic nonadiabatic coupling shows that this is dominated by a small set of high-frequency intramolecular modes of the betaine-30 molecule, with the solvent making a relatively very small contribution, also in agreement with previous experimental inference.

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“…1) according to a crystal structure determination by Stemplc and Watson (1972). Standard parameters according to Bailey (1969) have been used.…”

Section: Methodsmentioning

confidence: 99%

Interactions Between Dna and Psoralenamines Studied With Dichroism Techniques

Nordén

Wirth

Ygge

et al. 1986

Photochem & Photobiology

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The interaction of a series of water‐soluble mono‐ and bis‐psoralenamines with calf thymus DNA has been studied with flow UV linear dichroism (LD), circular dichroism (CD) and equilibrium techniques. The positive charge of a protonated amino group strongly enhances the DNA affinity compared to that of the parent compound, 8‐methoxypsoralen. The orientation of the psoralen when bound to DNA, depends on the position of the amino substituent. With amino substituents in the 5‐position (on the‘hydrophobic edge’of psoralen) psoralenamines tend to bind with a considerable tilt relative to the average orientation of the DNA base‐pairs. The tilt generally increases with an increased psoralen: base‐pair ratio, indicating a more random, nonintercalated binding. With the amino substituents in the 8‐position the psoralen binds with its plane parallel to that of the DNA bases as expected for intercalation. The DNA CD supports that these psoralenamines induce a considerable perturbation of the DNA structure, and the CD induced in the psoralen chromophore is in qualitative agreement with intercalation. The study also includes a theoretical and an experimental determination of the UV transition moments of the psoralen chromophore.

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Semi-empirical ?-electron calculations

Cited by 27 publications

References 35 publications

Solvent and Intramolecular Effects on the Absorption Spectrum of Betaine-30

Solvent and Intramolecular Effects on the Absorption Spectrum of Betaine-30

Computer Simulation of the Excited State Dynamics of Betaine-30 in Acetonitrile

Interactions Between Dna and Psoralenamines Studied With Dichroism Techniques

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