Oxygenation of the phenylhalocarbenes. Are they spin-allowed or spin-forbidden reactions?

Abstract

Oxygenation mechanisms of phenylhalocarbenes in vacuum are investigated through the use of density functional theory and CASSCF-PT2 approaches. Reactions with both substituted and unsubstituted phenylhalocarbenes are strongly exothermic. The reactions with nitrosubstituted halocarbenes are predicted to be spin allowed due to the practical degeneracy of the singlet and triplet carbene states. Conversely, for the unsubstituted compounds, the equilibrium between the triplet carbene and (3)O(2) requires a previous singlet-triplet state crossing. The large spin-orbit coupling at this crossing suggests a rather efficient tunneling rate. This would help to explain why the rate constant in solution in these cases is only one order of magnitude smaller than those involving a spin-allowed process. Moreover, the action of the solvent as a third body appears to play a fundamental role in prompting relatively smooth rates.

Modulation of Reactive Oxygen Species Photogeneration of Bacteriopheophorbide a Derivatives by Exocyclic E-Ring Opening and Charge Modifications

“…Time-dependent DFT (TDDFT) methods were employed to calculate the vertical excitation energies of 2a – 5a and 2b – 5b to estimate whether the compounds were photodynamically active. According to literature procedures, the use of TDDFT with the B3LYP functional and the 6-31G* basis set provides a useful estimate of the energy difference between the singlet ground-state (S 0 ) and the vertical triplet excited state. ,, We optimized the geometry using symmetry-unrestricted DFT methods at B3LYP/6-31G* and then performed single-point energy calculations using TDDFT methods at the same level of theory to determine the vertical excitation energies of 2a – 5a and 2b – 5b . − The calculations were performed in the gas phase because it has been shown that for excitation calculations at this level the effects of the solvent are negligible. , These calculations were used to investigate the potential of these bacteriochlorins to generate type 2 PDT ROS. Type 2 PDT require the vertical singlet–triplet excitation energy gap (Δ E ST ) of PSs to be greater than 0.98 eV to transfer energy from their triplet excited state to ground-state molecular oxygen ( 3 O 2 ) to produce singlet oxygen ( 1 O 2 ). − These Δ E ST calculations showed that 2a – 5a and 2b – 5b were each capable of undergoing type 2 PDT (Table S3) and that the Δ E ST values of the compounds without the exocyclic E ring ( 2b – 5b : Δ E ST = 1.5445–1.6218 eV) are consistently higher than that of compounds with the exocyclic E ring ( 2a – 5a ; Δ E ST = 1.4926–1.5172 eV) (Figure ).…”

“…Following this, symmetry-unrestricted, gas-phase TDDFT single-point energy calculations were performed for the triplet excited state using the ground-state-optimized geometry. The vertical excitation energies for the compounds were obtained from this calculation. − ,− …”

“…Although the spectroscopic properties of 2a – 5a and 2b – 5b were explored, the primary goal of this study was to investigate if taurine-driven exocyclic E-ring opening of bacteriochlorophyll a derivatives and net-charge variations would modulate ROS photogeneration. Using the time-dependent extension of DFT (TDDFT), − the vertical excited-state energy was calculated to determine whether 2a – 5a and 2b – 5b showed the potential for type 2 PDT ROS photogeneration. − ROS-activated fluorescence probes were then used to elucidate experimentally the relative photogeneration of ROS in organic-based solution, aqueous-based solution, and cell culture media. These experimental results were then analyzed to delineate the effects of taurine-driven exocyclic E-ring opening and net-charge variations on ROS photogeneration.…”