Theoretical insights into photo-induced isomerization mechanisms of phenylsulfinyl radical PhSO˙

Identification of the Photoreactive Species of Protonated N-Nitrosopiperidine in Acid Medium: A CASPT2 and DFT Study

In this work, we have studied the initial reaction step after photoexcitation of protonated N-nitrosopiperidine both in the gas and condensed phases. To achieve this end, we have applied the CASPT2 and MP2 wave function methods and the density functional theory approach. It is found that the site of protonation of N-nitrosopiperidine in acid medium depends on the solvent: protonation occurs at the oxygen atom in protic solvents, while in aprotic solvents, the proton is bonded at the N-atom of the amine moiety. Furthermore, protonation at such an N-atom is the unique protonated species that absorbs in the visible range and directly dissociates into aminium radical cation and nitric oxide.

A theoretical study on the photochemical generation of phenylborylene from phenyldiazidoborane

Organic borylenes are a kind of highly reactive species, which play important roles in a lot of reactions as vigorous intermediates. In this work, we investigated the photochemical generation mechanisms of phenylborylene (PhB) together with the side product N-phenylnitrenoiminoborane (PhNBN) from phenyldiazidoborane (PhBN₆) by extrusion of dinitrogen in the two lowest electronic singlet states (S₀ and S₁) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the reaction PhBN₆ → PhB + 3N₂ involves stepwise N₂ extrusion three times and the azido region rearrangement. Moreover, we found that the studied photo-induced processes are kinetically feasible because the highest energy barrier is only 0.36 eV and excitation with light of wavelength 254 nm can provide enough excess energy to overcome these energy barriers. Importantly, we revealed that several conical intersections between S₁ and S₀ states participate and facilitate the studied photochemical processes. Our results not only clarify the experimental observations, (H. F. Bettinger, J. Am. Chem. Soc. 2006, 128, 2534), but also provide valuable insights into borylene chemistry.

Photochemistry of 1-Phenyl-1-diazopropane and Its Diazirine Isomer: A CASSCF and MS-CASPT2 Study

In this work, we studied the wavelength (520 or 350 nm) dependence of the photochemical decomposition of 1-phenyl-1-diazopropane (PDP) and 1-phenyl-1-propyl diazirine (PED) by means of high-level ab initio quantum chemical calculations (CASSCF and MS-CASPT2) to obtain qualitative and quantitative results. It is found that the photochemistry of PDP is governed by nonradiative deactivation processes that can involve one or two S1/S0 conical intersections (CI1 and CI2) depending on the wavelength of the radiation; CI2 is only accessible at the shortest wavelength. It is demonstrated that the main intermediate of the photochemistry of the titled compounds is 1-ethyl-1-phenyl carbene (EPC). Upon irradiation of PDP with the 520 nm light, the carbene is always generated in its ground state as closed-shell singlet carbene. In contrast, the 350 nm radiation can directly decompose PDP into S1 carbene (open shell) and N2 when the conical intersection CI2 is avoided. Once the carbene is formed in the S1 state, it can experience excited state intramolecular proton transfer along a seam of crossing (ESIPT-SC) of the S1 and S0 states to yield the alkene derivative; that is, the proton transfer reaction takes places on a degenerate potential energy surface where the two electronic states have equal energy. In addition, it is found that EPC absorbs at 350 nm (double excitations); therefore, there is another possible route that can induce as well a slightly different photochemistry in changing the wavelength of the radiation because the shortest wavelength (when it is intense enough) decreases the amount of available EPC or generates a highly vibrationally excited state of the carbene; that is, after 350 nm excitation, the carbene intermediate can deactivate via radiation emission or can decay through a cascade of conical intersections to its first excited state (S1), where ESIPT-SC is operative again.