Relaxed redundant coordinate scans along the C−Br−Br angle in the isomer species suggest that iso-CHBr3 is connected to the reactant (CHBr3) through a transition state (TS).
Relaxed redundant coordinate scans along the C−Br−Br and C−Br geometrical parameters in iso-CHBr3 (towards the reactant and the molecular products) and single-point energy calculations at the radical pair asymptote performed to explore different possible decay channels of this isomer species in acetonitrile and methylcyclohexane.
Intrinsic reaction coordinate calculations of the ground-state isomerization path connecting CHBr3 and iso-CHBr3 at the B3LYP/aug-cc-pvTZ level of theory (solid line), and the results of single point CASSCF (16,12)/ANO-RCC-VTZP calculations on the ground and lowest-lying excited singlet states (S0 and S1, solid and open symbols) along the B3LYP path are shown.
(Left) The B3LYP/aug-cc-pVTZ IRC scans connecting the isomerization TS and the iso-CHBr3 minimum in methylcyclohexane (a) and acetonitrile (b). Vertical arrows mark the selected points along the scans at which the VETs of iso-CHBr3 were calculated. (CASPT2//CASSF/ANO-RCC-VTZP, transition energies given in nm with oscillator strengths in parentheses.) The insets show the full IRC scans connecting the CHBr3 and iso-CHBr3 ground-state minima. Solvent was simulated using the PCM model with the acetonitrile or methylcyclohexane parameters. (Right) B3LYP/aug-cc-pVTZ/PCM relaxed redundant coordinate two-dimensional mapping of the ground-state surface of iso-CHBr3 in methylcyclohexane along the C−Br−Br angle and Br−Br bond length. The red circles mark the angle and bond length coordinates at which the CASPT2//CASSF/ANO-RCC-VTZP VETs were computed with the PCM solvent description (the bold font values expressed in nm shown inside the red circles).
(Left) Energetics of the gas-phase CHBr3 system. 17,20 The absorption spectrum 76,77 is shown adjacent to the assignment of the excited electronic states computed at the CCSD/aug-cc-pVDZ level of theory. 13 (Right) Steady-state absorption spectra of CHBr3 in methylcyclohexane and acetonitrile (maximum extinction coefficient ɛ M = 2452 and 2050 M−1 cm−1, respectively) as well as in the gas-phase shown for comparison. The vertical bars correspond to the CCSD/aug-cc-pVDZ VET energies, where the bar height is proportional to the computed oscillator strength (f). 13 Note, that all f values for the singlet-triplet excitation are multiplied by a factor of 5. The inset shows the sample absorbance measured at 280 nm in a set of cells with different pathlengths. The linear plot of the absorbance divided by the thickness of the cell used vs. the CHBr3 concentration demonstrates that the sample obeys Lambert-Beer law in both solvents up to 200 mM.
255-nm ΔA spectra of CHBr3 in methylcyclohexane. Panels a, b, and c correspond to short, intermediate, and long time delays, which are expressed in picoseconds and given in legends. Note, the 100-fs ΔA spectrum for λ > 330 nm was obtained by subtracting appropriately scaled solvent contribution. Panel c shows the product absorption spectrum (□) previously assigned to a Br·CHBr3 complex 60 and the product absorption spectrum produced following irradiation of CHBr3 in a ∼5 K Ne matrix 61 assigned to iso-CHBr3 product (▲). Top insets: the B3LYP/PCM VETs of the iso-CHBr3 and CHBr2· species carrying oscillator strengths larger than 0.05.
Early- and intermediate-time ΔA kinetic traces (symbols) for CHBr3 in methylcyclohexane (left panels) and acetonitrile (right panels). The inset shows kinetic traces up to 1150 ps, the longest investigated time delay. The solid red lines are multiexponential fits to the data points. The ΔA signals measured upon excitation of neat solvent under the same experimental conditions are shown as blue lines.
Decay-associated spectra (ɛ i) of the global fit analysis for CHBr3 in methylcyclohexane (left) and acetonitrile (right) under the assumption of consecutive exponential decay. The analysis of the 340–765 nm range of measured ΔA spectra yields five components (A, B, C, D, E) and a permanent spectrum (F). The resulting time constants are given besides the corresponding ɛ i spectra.
255-nm ΔA spectra of CHBr3 in acetonitrile. Panels (a), (b), and (c) show the ΔA spectra at short, intermediate, and long-time time delays, which are expressed in picoseconds and given in panel legends. (Top insets) The B3LYP VETs of the iso-CHBr3, CHBr2, and CHBr2OH product species carrying oscillator strengths larger than 0.05.
The ΔA kinetic traces (symbols) for CHBr3 (20 mM) in acetonitrile with 0.2% of water, and acetonitrile with 2% and 5% of water measured in a 0.2-mm thick flow cell at 500 nm, 218 nm, and 250-nm following 255-nm excitation with an incident energy of 0.85 μJ pulse−1.
The inset displays transient absorption spectra of Br2 (10 mM) in CHBr3 measured in a 2-mm spinning cell after 420-nm excitation with an energy of 4.8 μJ pulse−1. The main window shows the 1-ps ΔA spectrum from the inset (line plus symbols) superimposed on the transient absorption measured 1 ps after 267-nm excitation of a 90%/10% (v/v) mixture of CHBr3 in cyclohexane (line) previously reported by Crim and co-workers. 59
Proposed photochemical reaction pathways following UV excitation of bromoform. The lifetimes and absorption maxima are shown in blue and red colors for acetonitrile and methylcyclohexane, respectively.
TD DFT/PCM vertical transition energies and corresponding oscillator strengths (in parentheses) calculated for the fully optimized iso-CHBr3 structures using the aug-cc-pvTZ basis set in the gas-phase and two solvents considered in this work, simulated using the corresponding PCM parameters.
TD DFT vertical transition energies and corresponding oscillator strengths (in parenthesis) computed for possible photoproducts of CHBr3 upon UV excitation (CHBr2·, CHBr2 +, and CHBr2OH) using the aug-cc-pvTZ basis set in the three different media considered in this work.
Energies of the transition state (TS) and the iso-CHBr3 species relative to the energy of the CHBr3 parent structure calculated at the B3LYP, CAM-B3LYP, M06-2X, and MP2 levels all using the aug-cc-pvTZ basis set.
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