Excited-State Tautomerization of 7-Azaindole in Nonpolar Solution: A Theoretical Study Based on Liquid-Phase Potential Surfaces of Mean Force

Thermally-Activated Tunneling in the Two-Water Bridge Catalyzed Tautomerization of Phosphinylidene Compounds

Phosphinylidenes are an important class of organophosphorus compounds that can exhibit tautomerization between tricoordinated P(III) hydroxide (R¹ R² POH) and a pentacoordinated P(V) oxide (R¹ R² P(O)H) form. Herein we show, using the canonical variational transition state theory combined with multidimensional small-curvature tunneling approximation, the dominance of proton tunneling in the two-water-bridged tautomerizations of phosphinous acid and model phosphinylidenes comprising phosphosphinates, H-phosphonates, H-phosphinates and secondary phosphine oxides. Based on the studied system, the contribution of thermally-activated tunneling is predicted to speed up the semiclassical reaction rate by ca. threefold to as large as two orders of magnitude at 298.15 K in the gas phase. The large KIE and the concavity in the Arrhenius plots are further fingerprints of tunneling. The simulations also predicted that the rapid tunneling rate and short half-life span for the forward reaction, as opposed to the reverse reaction in fluorinated secondary phosphine oxides, would result in P(V) being elusive and only P(III) being isolable, which agrees with previous experiments where only P(III) was detected by IR and NMR spectroscopy. We also explored the role of solvent and predicted tunneling to be substantial.

Excited-state intramolecular proton transfer to carbon atoms: nonadiabatic surface-hopping dynamics simulations

Excited-state intramolecular proton transfer (ESIPT) between two highly electronegative atoms, for example, oxygen and nitrogen, has been intensely studied experimentally and computationally, whereas there has been much less theoretical work on ESIPT to other atoms such as carbon. We have employed CASSCF, MS-CASPT2, RI-ADC(2), OM2/MRCI, DFT, and TDDFT methods to study the mechanistic photochemistry of 2-phenylphenol, for which such an ESIPT has been observed experimentally. According to static electronic structure calculations, irradiation of 2-phenylphenol populates the bright S1 state, which has a rather flat potential in the Franck-Condon region (with a shallow enol minimum at the CASSCF level) and may undergo an essentially barrierless ESIPT to the more stable S1 keto species. There are two S1/S0 conical intersections that mediate relaxation to the ground state, one in the enol region and one in the keto region, with the latter one substantially lower in energy. After S1 → S0 internal conversion, the transient keto species can return back to the S0 enol structure via reverse ground-state hydrogen transfer in a facile tautomerization. This mechanistic scenario is verified by OM2/MRCI-based fewest-switches surface-hopping simulations that provide detailed dynamic information. In these trajectories, ESIPT is complete within 118 fs; the corresponding S1 excited-state lifetime is computed to be 373 fs in vacuum. Most of the trajectories decay to the ground state via the S1/S0 conical intersection in the keto region (67%), and the remaining ones via the enol region (33%). The combination of static electronic structure computations and nonadiabatic dynamics simulations is expected to be generally useful for understanding the mechanistic photophysics and photochemistry of molecules with intramolecular hydrogen bonds.

Excited-state proton transfer in 4-2′-hydroxyphneylpyridine: full-dimensional surface-hopping dynamics simulations

We have employed combined electronic structure calculations and “on-the-fly” fewest switches surface-hopping dynamics simulations to study the S₁ excited-state intramolecular proton transfer (ESIPT) and decay dynamics of 4-(2′-hydroxyphenyl)pyridine.

Investigation of ESIPT in a panel of chromophores presenting N-H∙∙∙N intramolecular hydrogen bonds

Thermodynamic and kinetic aspects of excited state intramolecular proton transfer (ESIPT) are investigated in 11 chromophores harboring an intramolecular N-H∙∙∙N hydrogen bond [pyridyl pyrazole, pyridyl pyrrole, azaindole, pyridyl indole, pyrroloquinoline, and an analogue of the Blue Fluorescent Protein (BFP) chromophore] with the help of quantum mechanical calculations. For pyridyl pyrazoles, simulated spectra are used to help the interpretation of experimental ones and the effects of several substituents are investigated. Then it is shown that Time-Dependent Density Functional Theory fails to satisfactorily describe the energetic aspects of ESIPT for the BFP chromophore analogue. Equation-of-Motion Coupled Cluster theory is thus used to reach accurate insights for this challenging case.