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Nuclear Motion (nuclear + motion)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Time-dependent density functional theory for nonadiabatic processes

ISRAEL JOURNAL OF CHEMISTRY, Issue 1-2 2005
Roi Baer
Time-dependent density functional theory (TDDFT) is a general and robust method allowing the study of electron dynamics whether induced by nuclear motion or by external fields. We give a brief overview of the theory and some numerical methods together with recent applications stressing the generality and wide applicability of the method. We also discuss recent attempts to extend the present TDDFT by incorporating memory terms into the exchange correlation potentials. [source]

QM/MM calculation of solvent effects on absorption spectra of guanine

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010
Maja Parac
Abstract Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited-state calculations were extracted from ground-state molecular dynamics (MD) simulations using the self-consistent-charge density functional tight binding (SCC-DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited-state calculations used time-dependent density functional theory (TDDFT) and the DFT-based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto-N7H and keto-N9H guanine, with particular focus on solvent effects in the low-energy spectrum of the keto-N9H tautomer. When compared with the vertical excitation energies of gas-phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC-DFTB-based rather than B3LYP-based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas-phase and solution spectra, typically ca. 0.1,0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent-induced structural changes and from electrostatic solute,solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source]

On the effect of neglecting anharmonic nuclear motion in charge density studies

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2010
Kathrin Meindl
The effect of neglecting anharmonic nuclear motion when it is definitely present is studied. To ensure the presence of anharmonic nuclear motion a model was used that was previously refined against experimental data including anharmonic nuclear motion, and these calculated structure factors were used as observed data for a multipole refinement. It was then studied how the neglect of anharmonic nuclear motion and noise in the data affects the usual crystallographic quality measure R, the density parameters and the residual density distribution. It is demonstrated that the neglect of anharmonic nuclear motion leads to a characteristic imprint onto the residual density distribution in terms of residual density peaks and holes, in terms of the whole residual density distribution and in terms of the number, location and strength of valence shell charge concentrations (VSCCs). These VSCCs differ from that of the input model in a way which heavily influences and misleads the chemical interpretation of the charge density. This imprint vanishes after taking anharmonic nuclear motion into account. Also the input model VSCCs are restored. The importance of modeling anharmonic nuclear motion is furthermore shown by the characteristic imprint on the residual density distribution, even in the case of a numerically almost unaffected R value. [source]

Synthesis, Structure, and Physico-optical Properties of Manganate(II)-Based Ionic Liquids

CHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2010
Slawomir Pitula Dr.
Abstract Several ionic liquids (ILs) based on complex manganate(II) anions with chloro, bromo, and bis(trifluoromethanesulfonyl)amido (Tf2N) ligands have been synthesized. As counterions, n- alkyl-methylimidazolium (Cnmim) cations of different chain length (alkyl=ethyl (C2), propyl (C3), butyl (C4), hexyl (C6)) were chosen. Except for the 1-hexyl-3-methylimidazolium ILs, all of the prepared compounds could be obtained in a crystalline state at room temperature. However, each of the compounds displayed a strong tendency to form a supercooled liquid. Generally, solidification via a glass transition took place below ,40,°C. Consequently, all of these compounds can be regarded as ionic liquids. Depending on the local coordination environment of Mn2+, green (tetrahedrally coordinated Mn2+) or red (octahedrally coordinated Mn2+) luminescence emission from the 4T(G) level is observed.1 The local coordination of the luminescent Mn2+ centre has been unequivocally established by UV/Vis as well as Raman and IR vibrational spectroscopies. Emission decay times measured at room temperature in the solid state (crystalline or powder) were generally a few ms, although, depending on the ligand, values of up to 25,ms were obtained. For the bromo compounds, the luminescence decay times proved to be almost independent of the physical state and the temperature. However, for the chloro- and bis(trifluoromethanesulfonyl)amido ILs, the emission decay times were found to be dependent on the temperature even in the solid state, indicating that the measured values are strongly influenced by nuclear motion and the vibration of the atoms. In the liquid state, the luminescence of tetrahedrally coordinated Mn2+ could only be observed when the tetrachloromanganate ILs were diluted with the respective halide ILs. However, for [C3mim][Mn(Tf2N)3], in which Mn2+ is in an octahedral coordination environment, a weak red emission from the pure compound was found even in the liquid state at elevated temperatures. [source]