A matter of quantum voltages.

Voltages inside matter are relevant to crystallization, materials science, biology, catalysis, and aqueous chemistry. The variation of voltages in matter can be measured by experiment, however, modern supercomputers allow the calculation of accurate quantum voltages with spatial resolutions of bulk systems well beyond what can currently be measured provided a sufficient level of theory is employed. Of particular interest is the Mean Inner Potential (V(o))--the spatial average of these quantum voltages referenced to the vacuum. Here we establish a protocol to reliably evaluate V(o) from quantum calculations. Voltages are very sensitive to the distribution of electrons and provide metrics to understand interactions in condensed phases. In the present study, we find excellent agreement with measurements of V(o) for vitrified water and salt crystals and demonstrate the impact of covalent and ionic bonding as well as intermolecular/atomic interactions. Certain aspects in this regard are highlighted making use of simple model systems/approximations. Furthermore, we predict V(o) as well as the fluctuations of these voltages in aqueous NaCl electrolytes and characterize the changes in their behavior as the resolution increases below the size of atoms.

Charge and electric field fluctuations in aqueous NaCl electrolytes.

Crystalloluminescence, the long-lived emission of visible light during the crystallization of certain salts, was first observed over 200 years ago; however, the origin of this luminescence is still not well understood. The observations suggest that the process of crystallization may not be purely classical but also involves an essential electronic structure component. Strong electric field fluctuations may play an important role in this process by providing the necessary driving force for the observed electronic structure changes. The main objective of this work is to provide a basic understanding of the fluctuations in charge, electric potentials, and electric fields for concentrated aqueous NaCl electrolytes. Our charge analysis reveals that the water molecules in the first solvation shell of the ions serve as a sink for electron density originating on Cl(-). We find that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our results show that the potential and field distributions are largely independent of concentration. We also find the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged nonzero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials, and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence.

Laser pulse trains for controlling excited state dynamics of adenine in water.

We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical-molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing.

Singlet and triplet potential surfaces for the O2+C2H4 reaction.

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.

Azomethane: nonadiabatic photodynamical simulations in solution.

The nonadiabatic deactivation of trans-azomethane starting from the nπ* state has been investigated in gas phase, water, and n-hexane using an on-the-fly surface-hopping method. A quantum mechanical/molecular mechanics (QM/MM) approach was used employing a flexible quantum chemical level for the description of electronically excited states and bond dissociation (generalized valence bond perfect-pairing complete active space). The solvent effect on the lifetime and structural parameters of azomethane was investigated in detail. The calculations show that the nonadiabatic deactivation is characterized by a CNNC torsion, mainly impeded by mechanic interaction with the solvent molecules. The similar characteristics of the dynamics in polar and nonpolar solvent indicate that solvent effects based on electrostatic interactions do not play a major role. Lifetimes increase by about 20 fs for both solvents with respect to the 113 fs found for the gas phase. The present subpicosecond dynamics also nicely show an example of the suppression of C-N dissociation by the solvent cage.

Photodynamics of azomethane: a nonadiabatic surface-hopping study.

The internal conversion and hot ground-state dynamics of trans- and cis-azomethane starting in the S(1) state have been investigated by nonadiabatic ab initio surface hopping dynamics using MCSCF-GVB-CAS and MRCISD methods and by determining energy minima and saddle points, minima on the crossing seam, and minimum energy pathways on the ground and first excited-state surfaces. The lifetimes and photoproducts from the dynamics simulations, geometric properties, excitation energies of selected stationary points and minimum energy pathways between them are reported. Our results favor a statistical model with trans-AZM moving to the ground-state minima before the first CN dissociation takes place. A detailed discussion in comparison to recent experimental and theoretical data is presented.

O((3)P) + C(2)H(4) potential energy surface: study at the multireference level.

The O((3)P) + C(2)H(4) reaction provides a crucial, initial understanding of hydrocarbon combustion. In this work, the lowest-lying triplet potential energy surface is extensively explored at the multiconfiguration self-consistent field (MCSCF) and MRMP2 levels with a preliminary surface crossing investigation; and in cases that additional dynamical correlation is necessary, MR-AQCC stationary points are also determined. In particular, a careful determination of the active space along the intrinsic reaction pathway is necessary; and in some cases, more than one active space must be explored for computational feasibility. The resulting triplet potential energy surface geometries mostly agree with geometries from methods using single determinant references. However, although the selected multireference methods lead to energetics that agree well, only qualitative agreement was found with the energetics from the single determinant reference methods. Challenges and areas of further exploration are discussed.

Dynamics starting at a conical intersection: application to the photochemistry of pyrrole.

The photochemical ring opening process in pyrrole has been investigated by performing classical on-the-fly dynamics using the multiconfiguration self-consistent field method for the computation of energies and energy gradients. As starting point for the dynamics the conical intersection corresponding to the ring-puckered ring-opened structure, determined previously [Barbatti et al., J. Chem. Phys. 125, 164323 (2006)], has been chosen. Two sets of initial conditions for the nuclear velocities were constructed: (i) nuclear velocities in the branching (g,h) plane of the conical intersection and (ii) statistical distribution for all atoms. Both sets of initial conditions show very similar results. Reactive trajectories are only found in a very limited sector in the (g,h) plane and reaction products are very similar. Within the simulation time of 1 ps, ring opening of pyrrole to the biradical NH=CH-CH(*)-CH=CH(*) chain followed by ring closure to a substituted cyclopropene structure (NH=CH-C(3)H(3)) is observed. The computed structural data correlate well with the experimentally observed dissociation products.

Molecular dynamics simulations of beta-cyclodextrin-aziadamantane complexes in water.

Force-field-based atomistic simulations of host-guest supramolecular complexes between beta-cyclodextrin and several aziadamantane derivatives have been analyzed with respect to relative orientation and interaction energies, explicitly considering solvent (water) molecules. For each case, the calculations revealed two stable orientations of the guest within the host that are different in interaction energy. Fluctuation of and correlation between characteristic properties were analyzed. Among other things, it turned out that orientation angle and inclusion depth are clearly correlated. In addition, for the unsubstituted aziadamantane, the enthalpy of complex formation was calculated and compared to experimental results.