Measurement of time dependent product branching ratios indicates two-state reactivity in metal mediated chemical reactions

For several decades, the influence of Two State Reactivity (TSR) has been implicated in a host of reactions, but has lacked a stand-alone, definitive experimental kinetic signature identifying its occurrence. Here, we demonstrate that the measurement of a temporally dependent product branching ratio is indicative of spin inversion and is a kinetic signature of TSR. This is caused by products exiting different hypersurfaces with different rates and relative exothermicities. The composite measurement of product intensities with the same mass but with different multiplicities yield biexponential temporal dependences with the sampled product ratio changing in time. These measurements are made using the single photon initiated dissociative rearrangement reaction (SPIDRR) technique which identifies TSR but further determines the kinetic parameters for reaction along the original ground electronic surface in competition with spin inversion and its consequent TSR.

Singlet-Triplet States Interaction Regions in DNA/RNA Nucleobase Hypersurfaces

The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet crossing region directly connecting the lowest singlet and triplet ππ* excited states. Adenine, thymine, and uracil display additional higher-energy crossing regions related to the presence of low-lying singlet and a triplet nπ* state. These funnels are absent in guanine and cytosine, which have the bright (1)ππ* state lower in energy and less accessible nπ* states. Knowledge of the location and accessibility of these regions, in which the singlet-triplet interaction is related to large spin-orbit coupling elements, may help to understand experimental evidence such as the wavelength dependence measured for the triplet formation quantum yield in nucleobases and the prevalence of adenine and thymine components in the phosphorescence spectra of DNA.

Potential nonadiabatic reactions: ring-opening 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives to aromatic reactive intermediates

Potential singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives were explored using density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods. Since these ring openings involve relatively high energy species that lead to relatively stable aromatic species, a good scenario for potential nonadiabatic events, we posited that the reaction paths of these ring openings might come close to or cross excited state surfaces. At the DFT level of theory, all reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths. 6-Methylidenebicyclo[3.1.0]hex-3-en-2-one and a closely related derivative (4-methylidenebicyclo[3.1.0]hex-2-en-6-one) were explored at the CASSCF level of theory; CASSCF results were qualitatively similar to DFT results and yielded spin-orbit couplings of 1.1-1.4 cm(-1) at the singlet-triplet crossing points.