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1

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Application of artificial neural networks and genetic algorithms to modeling molecular electronic spectra in solution (2001)

Lilichenko, Mark ; Kelley, Anne Myers

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 114 (2001), S. 7094-7102

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: A novel approach is presented for finding the vibrational frequencies, Franck–Condon factors, and vibronic linewidths that best reproduce typical, poorly resolved electronic absorption (or fluorescence) spectra of molecules in condensed phases. While calculation of the theoretical spectrum from the molecular parameters is straightforward within the harmonic oscillator approximation for the vibrations, "inversion" of an experimental spectrum to deduce these parameters is not. Standard nonlinear least-squares fitting methods such as Levenberg–Marquardt are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here we employ a genetic algorithm to force a broad search through parameter space and couple it with the Levenberg–Marquardt method to speed convergence to each local minimum. In addition, a neural network trained on a large set of synthetic spectra is used to provide an initial guess for the fitting parameters and to narrow the range searched by the genetic algorithm. The combined algorithm provides excellent fits to a variety of single-mode absorption spectra with experimentally negligible errors in the parameters. It converges more rapidly than the genetic algorithm alone and more reliably than the Levenberg–Marquardt method alone, and is robust in the presence of spectral noise. Extensions to multimode systems, and/or to include other spectroscopic data such as resonance Raman intensities, are straightforward. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1358835

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Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes (2001)

Waterland, Mark R. ; Stockwell, David ; Kelley, Anne Myers

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 114 (2001), S. 6249-6258

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1355657

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3

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Temperature-dependent total emission spectra of azulene in polymers: Modeling using spectral densities (1999)

Gupta, Vinita ; Kelley, Anne Myers

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 111 (1999), S. 3599-3611

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Monochromatically excited total emission spectra have been measured for the S1→S0 transition of azulene in polyethylene, polystyrene, and poly(methylmethacrylate) matrices over a temperature range from 1.4 to 100 K. The spectra in all three polymers exhibit strong zero-phonon lines (excitation of azulene vibrations only) accompanied by well-defined Stokes-shifted phonon sidebands at the lowest temperatures. As the temperature is raised the phonon bands broaden and gain relative intensity at the expense of the zero-phonon lines, and the spectra become qualitatively similar to the room-temperature liquid-phase spectra with sharp Raman lines on a broad fluorescence background. The near-origin-excited data are simulated by calculating the complete emission spectrum as a χ(3) process that assumes no artificial partitioning between "Raman" and "fluorescence." The internal vibrations of azulene are modeled as simple undamped displaced harmonic oscillators while the intermolecular or matrix phonons are either modeled as a Brownian oscillator or treated as effective spectral densities extracted from published neutron scattering and/or low-frequency nonresonant Raman data in the same polymers. While the qualitative features of the spectra and their temperature dependence are reproduced, none of the spectral densities employed give a fully satisfactory fit to the experimental spectra. The results demonstrate the sensitivity of total emission spectra to the chromophore–matrix interactions, and suggest that the spectral densities describing these interactions are functions not only of the matrix but also of the chromophore involved. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.479676

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4

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Far-ultraviolet resonance Raman spectroscopy of nitrate ion in solution (2000)

Waterland, Mark R. ; Myers Kelley, Anne

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 113 (2000), S. 6760-6773

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the (approximate)60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ν1 band. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1310615

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Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile (2002)

Moran, Andrew M. ; Egolf, Debra S. ; Blanchard-Desce, Mireille ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 116 (2002), S. 2542-2555

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The linear absorption spectra and absolute resonance Raman excitation profiles of the "push-pull" chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band. Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies. The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure. These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N′-diethylthiobarbituric acid. The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state. We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance. This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1433966

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Solvent effects on ground and excited electronic state structures of p-nitroaniline (2001)

Moran, Andrew M. ; Kelley, Anne Myers

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 115 (2001), S. 912-924

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Resonance Raman intensities of p-nitroaniline, a prototypical "push–pull" chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the Cphenyl–Nnitro, N–O, and phenyl C2–C3 bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1378319

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