Computational study on the electronic g-tensors of hydrophilic and hydrophobic nanodiamonds interacting with water.

Hydrogenated and hydroxylated nanodiamonds (NDs) are modeled by putting emphasis on the most common paramagnetic impurities-dangling bonds as well as single substitutional nitrogen atoms-and their interaction with water. It is shown that, despite its overall hydrophobicity, hydrogenated ND can become locally hydrophilic due to the introduced defects; therefore, water molecules may be attracted to the particular sites at its surface. To assess the direct influence of water on the magnetic behavior of NDs, the solvent-induced shift of the g-tensor was employed, indicating that for the same types of impurities, the impact the water has strongly depends on their positions in ND. In addition, water molecules at the locally hydrophilic sites of hydrogenated ND may influence the magnetic behavior of defects to the same extent as it may be influenced in the case of hydroxylated ND. Moreover, the overall hydrophilic nature of the latter does not necessarily guarantee that water, although being strongly attracted to the vicinity of impurity, will form a hydrogen bond network with a substantial impact on the local environment of the unpaired electron. The obtained data imply that in the context of the Overhauser effect, for which the solvent-induced shift of the g-tensor is proposed as a tool to reveal whether some NDs are more favorable for it to occur compared to the others, hydrogenated NDs should perform no worse than hydroxylated ones, despite only the local hydrophilicity of the former.

Electronic g-tensors of nanodiamonds: Dependence on the size, shape, and surface functionalization.

The electronic g-tensor dependence on the size, shape, and surface functionalization of nanodiamonds (NDs) is theoretically investigated by selecting dangling bonds and single substitutional nitrogen atoms as a main source of the unpaired electrons. The performed g-tensor calculations reveal that aforementioned paramagnetic impurities introduced into octahedrally shaped ND of C84H64 size behave in a very similar manner as those embedded into a smaller octahedral model of C35H36 size. Since cubic and tetrahedral NDs-C54H48 and C51H52-demonstrate a wider range of g-shift values than octahedral systems, the g-tensor dependence on different shapes can be considered as more pronounced. However, a different surface functionalization scheme, namely, fluorination, results in a much larger variation of the g-shifts, pointing to a significant impact the F atoms have on the local environment of the unpaired electrons in C35F36. A partial surface functionalization of C35H36 with benzoic acid and aniline groups indicates that, in some special cases, these linkers might induce a noticeable spin density redistribution which in turn substantially modifies the g-shift values of the system. Additional infrared (IR) spectra calculations show that some of paramagnetic defects in C35H36 and C35F36 possess clearly expressed signatures which could be useful while analyzing the experimental IR spectra of NDs.

On the magnetic properties of nanodiamonds: Electronic g-tensor calculations.

The electronic g-tensor calculations are carried out for various paramagnetic defects introduced into hydrogenated diamond nanocrystal C35H36, showing that such a system can be successfully used to model magnetic properties of nanodiamonds (NDs) with paramagnetic centers containing no vacancies. In addition, it is revealed that, depending on the geometric positions in ND, paramagnetic centers of the same type produce noticeable variations of the g-tensor values. A side-by-side comparison of the performance of effective nuclear charge and spin-orbit mean field (SOMF) approaches indicates that the latter is more sensitive to the quality of basis sets, especially concerning diffuse functions, the inclusion of which is found to be nonbeneficial. What is more, the SOMF method also exhibits a much more pronounced gauge-origin dependence. Compared to electronic charge centroid, spin centers (SCs) demonstrate a superior suitability as gauge origins, providing a better agreement with diamagnetic and paramagnetic contributions of g-tensor obtained employing gauge-including atomic orbitals (GIAOs). Therefore, SCs can be recommended for the g-tensor calculations of NDs whenever GIAOs are not available.

Quantum mechanics capacitance molecular mechanics modeling of core-electron binding energies of methanol and methyl nitrite on Ag(111) surface.

We study a newly devised quantum mechanics capacitance molecular mechanics (QMCMM) method for the calculation of core-electron binding energies in the case of molecules adsorbed on metal surfaces. This yet untested methodology is applied to systems with monolayer of methanol/methyl nitrite on an Ag(111) surface at 100 K temperature. It was found out that the studied C, N, and O 1s core-hole energies converge very slowly as a function of the radius of the metallic cluster, which was ascribed to build up of positive charge on the edge of the Ag slab. Further analysis revealed that an extrapolation process can be used to obtain binding energies that deviated less than 0.5 eV against experiments, except in the case of methanol O 1s where the difference was as large as 1.8 eV. Additional QM-cluster calculations suggest that the latter error can be connected to the lack of charge transfer over the QM-CMM boundary. Thus, the results indicate that the QMCMM and QM-cluster methods can complement each other in a holistic picture of molecule-adsorbate core-ionization studies, where all types of intermolecular interactions are considered.

Quantum mechanics/molecular mechanics modeling of photoelectron spectra: the carbon 1s core-electron binding energies of ethanol-water solutions.

Using ethanol-water solutions as illustration, we demonstrate the capability of the hybrid quantum mechanics/molecular mechanics (QM/MM) paradigm to simulate core photoelectron spectroscopy: the binding energies and the chemical shifts. An integrated approach with QM/MM binding energy calculations coupled to preceding molecular dynamics sampling is adopted to generate binding energies averaged over the solute-solvent configurations available at a particular temperature and pressure and thus allowing for a statistical assessment with confidence levels for the final binding energies. The results are analyzed in terms of the contributions in the molecular mechanics model-electrostatic, polarization, and van der Waals-with atom or bond granulation of the corresponding MM charge and polarizability force-fields. The role of extramolecular charge transfer screening of the core-hole and explicit hydrogen bonding is studied by extending the QM core to cover the first solvation shell. The results are compared to those obtained from pure electrostatic and polarizable continuum models. Particularly, the dependence of the carbon 1s binding energies with respect to the ethanol concentration is studied. Our results indicate that QM/MM can be used as an all-encompassing model to study photoelectron binding energies and chemical shifts in solvent environments.

Role of zero-point vibrational corrections to carbon hyperfine coupling constants in organic π radicals.

By analyzing a set of organic π radicals, we demonstrate that zero-point vibrational corrections give significant contributions to carbon hyperfine coupling constants, in one case even inducing a sign reversal for the coupling constant. We discuss the implications of these findings for the computational analysis of electron paramagnetic spectra based on hyperfine coupling constants evaluated at the equilibrium geometry of radicals. In particular, we note that a dynamical description that involves the nuclear motion is in many cases necessary in order to achieve a semi-quantitatively predictive theory for carbon hyperfine coupling constants. In addition, we discuss the implications of the strong dependence of the carbon hyperfine coupling constants on the zero-point vibrational corrections for the selection of exchange-correlation functionals in density functional theory studies of these constants.

Spatial quantum beats in vibrational resonant inelastic soft x-ray scattering at dissociating states in oxygen.

Resonant inelastic soft x-ray scattering (RIXS) spectra excited at the 1σ(g) → 3σ(u) resonance in gas-phase O2 show excitations due to the nuclear degrees of freedom with up to 35 well-resolved discrete vibronic states and a continuum due to the kinetic energy distribution of the separated atoms. The RIXS profile demonstrates spatial quantum beats caused by two interfering wave packets with different momenta as the atoms separate. Thomson scattering strongly affects both the spectral profile and the scattering anisotropy.

Theoretical study of specific solvent effects on the optical and magnetic properties of copper(II) acetylacetonate.

Specific and basicity solvent effects on the visible near-infrared electronic transitions and the electron paramagnetic resonance (EPR) parameters of the copper(II) acetylacetonate complex, Cu(acac)2, have been investigated at the density functional theory level. The computed absorption transitions as well as the EPR parameters show a strong dependence on the direct coordination environment around the Cu(II) complex. High solvatocromic shifts are observed for 3d-3d transitions, with the highest effect observed for the dz(2)→dxy transition, which is red-shifted by 6000 cm(-1) and 9000 cm(-1) in water and pyridine solvent models, respectively. Compared to the electronic g-tensors, the hyperfine coupling constants of the Cu(acac)2 complex show a more pronounced dependence on the effect of base strength of solvent. Overall, the present methodology satisfactorily models the solvent effect on the optical and magnetic properties of the Cu(acac)2 complex, and theory and experiment agree sufficiently well to warrant the use of the computed optical and EPR parameters to elucidate the coordination environment of the Cu(II) systems in basic solutions.

Solvatochromic shift of phenol blue in water from a combined Car-Parrinello molecular dynamics hybrid quantum mechanics-molecular mechanics and ZINDO approach.

The present work addresses the solvatochromic shift of phenol blue (PB) dye. For this purpose the results of Car-Parrinello molecular dynamics (CPMD) simulations for PB in gas phase are compared with results obtained for PB in water from CPMD hybrid quantum mechanics-molecular mechanics (CPMD-QM/MM) calculations. The absorption spectra were obtained using the intermediate neglect of differential overlap/spectroscopic-configuration interaction (INDO/CIS) method and were calculated for a multitude of configurations of the trajectory. The calculated lambda(max) for PB in gas phase was found to be about 535 nm, which is considerably lower than the lambda(max) reported for PB in nonpolar solvents. Different solvation shells for PB in water have been defined based on the solute-all-atoms and solvent center of mass radial distribution function (g(r(X-O))). The electronic excitation energies for PB computed in the presence of solvent molecules in an increasing number of solvation shells were calculated in a systematic way to evaluate their contributions to the solvatochrmic shift. The inclusion of solvent molecules in the hydration shell yields a lambda(max) of 640 nm, which contributes to almost 78% of the solvatochromic shift. The inclusion of solvent molecules up to 10 A in the g(r(X-O)) rdf yields a lambda(max) of 670 nm which is in good agreement with the experimentally reported value of 654-684 nm. Overall, the present study suggests that the combined CPMD-QM/MM and INDO-CIS approach can be used successfully to model solvatochromic shifts of organic dye molecules.

Many-photon dynamics of photobleaching.

A detailed dynamical theory of photobleaching by periodical sequences of laser pulses is presented. The theory is used for interpretation of recent experiments with pyrylium salts. Our simulations are based on first-principles simulations of photoabsorption cross-sections and on empirical rate constants. Two competitive channels of photobleaching, namely, photobleaching from the lowest excited singlet and triplet states and from higher excited states, are found to explain different intensity dependences of the photobleaching rates in different samples. The process includes two-photon excitation from the ground state to the first or second excited singlet states and one-photon excitation from the first singlet or triplet states to higher excited states. The fluorescence follows double-exponential dynamics with two characteristic times. The first and the shorter one is the equilibrium settling time between the ground and the lowest triplet states. The second characteristic time, the time of photobleaching, is responsible for the long-term dynamics. The effective rate of photobleaching from the first excited singlet and lowest triplet states depends differently on the irradiance in comparison with the photobleaching in higher states. The first channel is characterized by a quadratic intensity dependence in contrast to the second channel that shows a cubic dependence. The competition between these photobleaching channels is very sensitive to the rate constants as well as to the repetition rate, the pulse duration, and the peak intensity. The double-exponential decay of the fluorescence is explained by the spatial inhomogeneity of the light beam. The findings in this work are discussed in terms of the possibility of using many-photon-induced photobleaching for new three-dimensional read-write devices.