Theoretical Characterization of Photoisomerization Channels of Dimethylpyridines on the Singlet and Triplet Potential Energy Surfaces

Abstract

Photoexcitations and photoisomerizations due to low-lying n pi* and pi pi* excited states of dimethylpyridines are investigated by density functional theory, CASSCF, CASPT2 and MRCI methodologies. Mechanistic details for the formation of Dewar dimethylpyridines and the interconversions of dimethylpyridines are rationalized through the characterization of minima and transition states on the singlet and triplet potential energy surfaces of relevant intermediates. Our present theoretical schemes suggest that Mobius dimethylpyridine intermediate 14 and azabenzvalene intermediate 10 can serve as possible precursors to Dewar dimethylpyridines and singlet phototransposition products, respectively. The calculations suggest that an S1(pi pi*)/S0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S2(pi pi*)/S1(n pi*) interaction followed by an S1/S0 decay in dimethylpyridine 6. Calculated barriers of isomerizations from 14 to Dewar dimethylpyridine 7 and from 10 to 4 are 8.4 and 28.5 kcal mol(-1) at the B3LYP/6-311 G** level, respectively. In the suggested triplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T1(3B1) excited state of 3,5-dimethylpyridine 6 is a precursor of the interconversion of 6 to 2.4-dimethylpyridine 4. The formation of a triplet azaprefulvene 21 with a barrier of 20.7 kcal mol(-1) is a key step during the triplet migration process leading to another out-of-plane distorted structure 27. Subsequent rearomatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.

Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations

“…As can be seen from Fig. A conical intersection between S 0 and S 2 was found to connect the D2(S 0 ) structure in the previous studies, 15,21 which is about 145 kcal/mol above the S 0 minimum. One electron excitation from π to π * orbital weakens to a large extent the π bond in the ring.…”

“…One electron excitation from π to π * orbital weakens to a large extent the π bond in the ring. 11,12,15 However, the prefulvene isomer was confirmed to have a closed-shell ground state by the B3LYP/6-311G** calculations. The vertical and adiabatic S 0 → S 2 excitation energies were calculated to be 118.…”

“…13 The observed infrared bands were assigned to an isomer that has a C-C bridging bond (Dewar isomers) with the aid of density functional theory (DFT) calculation. 20 Cao and co-workers 15 have performed a systematic theoretical study on photoisomerization processes of 2,3dimethylpyridine and 3,5-dimethylpyridine. Zewail and co-workers reported the direct determination of the molecular structure during the nonradiative transition in aromatic pyridine excited at ∼266 nm (∼37 594 cm −1 ).…”

“…But no other isomer bands were observed in the FT-IR spectroscopy. 15 Formation of Dewar isomer by photoexcitation of pyridine at 254 nm (39 370 cm −1 ) was examined by the CASSCF and CASSCF-MP2 calculations with basis sets of 6-311G(d,p) and 6-311++G(3df,3pd). 14, 18 A ringopened diradical structure resulting from C-N bond scission was observed as the primary product with the time scale of ∼17 ps.…”

Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

“…It is expected that methyl substitution in the 2-and 6positions of pyridine will have significant effects on the topology of the relevant conical intersections, on the basis of previous theory and experience with the methyl substituted. Peyerimhoff and coworkers have predicted this molecule is the most stable isomer [13], using density functional theory, CASSCF, CASPT2, and MRCI. The presence of two methyl groups in the 2-and 6positions of pyridine further stabilizes the ring through increased charge density [14].…”