A number of systems in spectroscopy, astrochemistry and combustion chemistry are influenced by spin-forbidden processes between electronic states of differing spin, coupled through the spin-orbit interaction. Though typically modeled using time independent Landau-Zener theory, time dependent molecular dynamic trajectory surface hopping methods can be employed. We revisit the spin-forbidden 3B1 to 1A1 transition for SiH2 through direct on-the-fly molecular dynamics simulations incorporating the Tully’s Fewest Switches trajectory surface hopping method for trajectories spanning 2 ps. For an improved description of the hopping, the time-uncertainty method is utilized as well as the gradV (∇V) method for improved momentum adjustment upon hopping. The resulting dynamics illustrate a large distribution of associated hopping geometries and spin-orbit couplings, but their average lies near the values used in Landau-Zener analysis. A challenge in applying Landau-Zener theory to molecular systems, lies in selecting an appropriate model to describe the system velocity at the MECP, particularly for complex systems with nonequilibrium energy distribution between the vibrational degrees of freedom. We believe that both, computationally inexpensive Landau-Zener theory and direct nonadiabatic molecular dynamics, will be useful in describing transitions between spin-orbit coupled electronic states in complex molecular systems.
Molecular hydrogen reduction and oxidation currently require expensive platinum group catalysts. Promising alternatives to these catalysts are structural models of the active sites of hydrogenases, which contain abundant and inexpensive first-row transition metals. To create catalytically effective structural models, it is essential to know the mechanisms of hydrogen reduction and oxidation on the active site. We investigate H2 binding to the active site of [NiFe]-hydrogenase through a triplet/singlet spin-forbidden pathway using DFT and MCQDPT2. Firstly, the H2 molecule prefers to bind to the Fe atom of the active site, both in singlet and triplet states. However, the H2 binding to the triplet state of the active site is more energetically favorable. We then demonstrate that the rotation of the terminal thiolate ligands around the Ni center induces a crossing between the lowest energy singlet and triplet electronic states. At the crossing, nonadiabatic spin-forbidden transitions, mediated by spin-orbit coupling between the singlet and triplet states, can occur. We found the probability of these transitions by utilizing Landau-Zener theory in the 270-370 K temperature range. These nonadiabatic transitions could play an important role in hydrogen catalysis on the active site of [NiFe]-hydrogenase, and could explain the current inability of structural models with small ligands to bind molecular hydrogen.
The diatomic alkali molecules have been proposed as possible candidates for applications in ultracold chemistry, quantum computing, and for high-precision measurements of fundamental constants. These applications require very low temperatures (μK-nK range), which reduces the probability of transitions of the molecule to other quantum states and increases its average lifetime in a specific quantum state. To estimate the vibrational state lifetime of the ground and excited states of heteronuclear alkali dimers XY (X, Y = Li, Na, K, Rb, Cs) we solve the vibrational Schrödinger equation using the CCSDT potential energy and dipole moment curves. The dissociation energies are overestimated by only 14 cm-1 for LiNa and by no more than 114 cm-1 for the other molecules. The discrepancies between the experimental and calculated harmonic vibrational frequencies are less than 1.7 cm-1, and the discrepancies for the anharmonic correction are less than 0.1 cm-1. The transition dipole moments between all vibrational states, the Einstein coefficients, and the lifetimes of the vibrational states are calculated. For all studied alkali dimers the ground vibrational state has the largest lifetime. Therefore, for applications where lifetime is important, such as quantum computing, molecules should be in the ground state.
The recent discoveries of complex organic molecules such as cyclopropenone and glycoaldehyde in interstellar space have renewed the interest in astrochemical reaction mechanisms. We investigate three previously proposed reaction mechanisms for cyclopropenone formation in interstellar medium using ab initio quantum chemical methods. The nonadiabatic spin-forbidden reaction between atomic oxygen and cyclopropenylidene characterized by very small activation barrier and significant spin-orbit coupling between the lowest energies singlet and triplet states. We calculate the Landau-Zener probability of transition between the triplet and singlet states, and use nonadiabatic transition state theory to estimate the reaction rate constant of this spin-forbidden reaction. The reaction between acetylene and carbon monoxide, and between molecular oxygen and cyclopropenylidene, are two spin-allowed cyclopropenone formation pathways, also investigated in this work. Of the three studied reactions, the most probable mechanism of cyclopropenone formation in cold regions of interstellar space is between molecular oxygen and cyclopropenylidene since it is found to be a barrier free reaction.