Kinetically constrained ring-polymer molecular dynamics for non-adiabatic chemical reactions.

We extend ring-polymer molecular dynamics (RPMD) to allow for the direct simulation of general, electronically non-adiabatic chemical processes. The kinetically constrained (KC) RPMD method uses the imaginary-time path-integral representation in the set of nuclear coordinates and electronic states to provide continuous equations of motion that describe the quantized, electronically non-adiabatic dynamics of the system. KC-RPMD preserves the favorable properties of the usual RPMD formulation in the position representation, including rigorous detailed balance, time-reversal symmetry, and invariance of reaction rate calculations to the choice of dividing surface. However, the new method overcomes significant shortcomings of position-representation RPMD by enabling the description of non-adiabatic transitions between states associated with general, many-electron wavefunctions and by accurately describing deep-tunneling processes across asymmetric barriers. We demonstrate that KC-RPMD yields excellent numerical results for a range of model systems, including a simple avoided-crossing reaction and condensed-phase electron-transfer reactions across multiple regimes for the electronic coupling and thermodynamic driving force.

Dissociative photoionization of glycerol and its dimer occurs predominantly via a ternary hydrogen-bridged ion-molecule complex.

The photoionization and dissociative photoionization of glycerol are studied experimentally and theoretically. Time-of-flight mass spectrometry combined with vacuum ultraviolet synchrotron radiation ranging from 8 to 15 eV is used to investigate the nature of the major fragments and their corresponding appearance energies. Deuterium (1,1,2,3,3-D5) and (13)C (2-(13)C) labeling is employed to narrow down the possible dissociation mechanisms leading to the major fragment ions (C3H(x)O2(+), C2H(x)O2(+), C2H(x)O(+), CH(x)O(+)). We find that the primary fragmentation of the glycerol radical cation (m/z 92) occurs only via two routes. The first channel proceeds via a six-membered hydrogen-transfer transition state, leading to a common stable ternary intermediate, comprised of neutral water, neutral formaldehyde, and a vinyl alcohol radical cation, which exhibits a binding energy of ≈42 kcal/mol and a very short (1.4 Å) hydrogen bond. Fragmentation of this intermediate gives rise to experimentally observed m/z 74, 62, 44, and 45. Fragments m/z 74 and 62 both consist of hydrogen-bridged ion-molecule complexes with binding energy >25 kcal/mol, whereas the m/z 44 species lacks such stabilization. This explains why water- or formaldehyde-loss products are observed first. The second primary fragmentation route arises from cleaving the elongated C-C bond. Also for this channel, intermediates comprised of hydrogen-bridged ion-molecule complexes exhibiting binding energies >24 kcal/mol are observed. Energy decomposition analysis reveals that electrostatic and charge-transfer interactions are equally important in hydrogen-bridged ion-molecule complexes. Furthermore, the dissociative photoionization of the glycerol dimer is investigated and compared to the main pathways for the monomeric species. To a first approximation, the glycerol dimer radical cation can be described as a monomeric glycerol radical cation in the presence of a spectator glycerol, thus giving rise to a dissociation pattern similar to that of the monomer.

Restricted active space spin-flip (RAS-SF) with arbitrary number of spin-flips.

The restricted active space spin-flip (RAS-SF) approach is a multistate, spin-complete, variational and size consistent method applicable to systems featuring electronic (near-)degeneracies. In contrast to CASSCF it does not involve orbital optimizations and so avoids issues such as root-flipping and state averaging. This also makes RAS-SF calculations roughly 100-1000 times faster. In this paper RAS-SF method is extended to include variable orbital active spaces and three or more spin-flips, which allows the study of polynuclear metal systems, triple bond dissociations and organic polyradicals featuring more than four unpaired electrons. Benchmark calculations on such systems are carried out and comparison to other wave-function based, multi-reference methods, such as CASSCF and DMRG yield very good agreement, provided that the same active space is employed. Where experimental values are available, RAS-SF is found to substantially underestimate the exchange coupling constants, if the minimal active space is chosen. However, the correct ground state is always obtained. Not surprisingly, inclusion of bridge orbitals into the active space can cause the magnitude of the coupling constants to increase substantially. Importantly, the ratio of exchange couplings in related systems is in much better agreement with experiment than the magnitude of the coupling. Nevertheless, the results indicate the need for the inclusion of dynamic correlation to obtain better accuracy in minimal active spaces.

Restricted active space spin-flip configuration interaction: theory and examples for multiple spin flips with odd numbers of electrons.

The restricted active space spin flip (RAS-SF) method is extended to allow ground and excited states of molecular radicals to be described at low cost (for small numbers of spin flips). RAS-SF allows for any number of spin flips and a flexible active space while maintaining pure spin eigenfunctions for all states by maintaining a spin complete set of determinants and using spin-restricted orbitals. The implementation supports both even and odd numbers of electrons, while use of resolution of the identity integrals and a shared memory parallel implementation allow for fast computation. Examples of multiple-bond dissociation, excited states in triradicals, spin conversions in organic multi-radicals, and mixed-valence metal coordination complexes demonstrate the broad usefulness of RAS-SF.

Mechanism for singlet fission in pentacene and tetracene: from single exciton to two triplets.

Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a dynamic interplay between SF and triplet-triplet annihilation.

Theoretical study of substituted PBPB dimers: structural analysis, tetraradical character, and excited states.

The radicaloid nature of para and meta 1,3-diborata-2,4-diphosphoniocyclobutane-1,3-diyl doubly substituted benzene is assessed from several electronic structure perspectives. Orbital occupation numbers computed by perfect pairing (PP), complete active space SCF (CASSCF), and restricted active space double spin-flip (RAS-2SF) reveal the presence of less than one unpaired electron in the planar molecules. Thus, the surprising stability of the "para tetraradical" can be rationalized by its moderate extent of radical character. Estimation of the delocalization energy, low-lying excited states, and short and long-range magnetic coupling constants all indicate a rather weak interaction to occur between two singlet PBPB units. Communication between two triplet units was found to be negligible. Comparison between para and meta isomers confirms a distinctly larger communication via the pi framework for the former. However, this communication, which was recently proposed to be the main factor for the different behavior of meta and para isomers regarding their preferred geometries, was found to account for only one-third of their energy difference. The study shows the important contribution of steric and/or electronic effects of the bulky (i)Pr and (t)Bu substituents on P and B.

Computational Studies on the Pt(II)-Catalyzed Cycloisomerization of 1,6-dienes into Bicyclopropanes: A Mechanistic Quandary Evaluated by DFT.

The mechanism of the (bis(phosphanylethyl)phosphane)Pt(2+) catalyzed cyclo-isomerization reaction of 7-methyl-octa-1,6-diene to form 1-isopropylbicyclo[3.1.0]hexane was studied using computational methods. The cyclopropanation step was found to be the turnover-limiting step. The overall reaction proceeds via both a 5-exo and a 6-endo route. W conformations were shown to facilitate cyclopropanation, but do not have any influence on the rate of the 1,2-hydride shifts.