Mechanistic Photodissociation of Glycolaldehyde: Insights from Ab Initio and RRKM Calculations

Abstract

Herein we report a theoretical study on mechanistic photodissociation of glycolaldehyde, HOCH(2) CHO. Equilibrium structures, transition states, and intersection structures for the α-C-C and -C-H bond fissions and the β-C-O bond fission in the excited states are determined by the complete active space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations are refined by performing single-point calculations using the multi-state multi-reference CASSCF second order perturbation (MS-MR-CASPT2) method. With a low excitation energy of 280-340 nm, the T(1) α-C-C and β-C-O bond fissions following intersystem crossing from the S(1) state are the predominant and comparable channels, whereas the α-C-H bond fissions both in the S(1) and in the T(1) states are nearly prohibited due to the relevant high barriers. The rate constants for the T(1) α-C-C and β-C-O bond fissions are also calculated by RRKM theory. Furthermore, the S(0) reactions can occur as a consequence of intersystem crossing via T(1)/S(0) intersection points resulting from the T(1) C-C and C-O bond cleavages. This photodissociation mechanism is consistent with recent experimental studies.

Photochemistry of glycolaldehyde in cryogenic matrices

“…1). These values are in excellent agreement with those obtained with the CASSCF/cc-pVDZ method, 5 with the MP4(SDTQ)/cc-pVQZ method, 6 and with the CCSD(T)/cc-pVTZ//B3LYP/6-31G(d,p) hybrid quartic force field. 7 Energies and dihedrals of the four conformers are listed in Table VI. Our method, therefore, succeeds in capturing the structural and energetic properties of GA conformers at a limited computational cost, which is required in order to perform extensive calculations of PES.…”

“…9 and Table VI). Note that the structures of minCC are similar to the minima obtained by Cui et al in the S 1 and T 1 excited states 5 (Table S1 in the supplementary material 28 ).…”

“…17 This is also the case of GA as estimated by the spin-orbit coupling (23.4 cm −1 ) calculated in Ref. 5 for the structure at the minimum energy on S 1 . As a consequence, calculations were performed on both S 1 and T 1 states.…”

“…The radical pathways are supported by the recent theoretical study of Cui et al who calculated the potential energy profiles in the S 0 ground state, the excited S 1 singlet, and T 1 triplet states of GA considering the three C-C, C-H, and C-O bond cleavage channels upon excitation of the most stable form. 5 They propose that after excitation to S 1 , GA relaxes to T 1 through intersystem crossing due to a high S 1 /T 1 spin-orbit coupling (23.4 cm −1 ). Because the T 1 state is adiabatically correlated with the radicals' ground-state along these three cleavage channels, GA dissociates to radicals.…”

“…[2][3][4] GA molecule mainly appears as a pseudo-cyclic form named CC (Cis-Cis) with an intramolecular H-bond. According to theory, this CC form is the most stable conformer but three "open" conformers, i.e., with no H-bond, are also stable, lying at higher energies: TT, TG, and CT forms [5][6][7][8] ( Fig. 1).…”