Theoretical study on the photodissociation reaction of α-cyclohexanedione in ground state

Abstract

The α-cyclohexanedione (α-CHD) molecule is an important structural unit in the six-membered ring systems with a large number ofbiologically meaningfulmoleculeswhich have been found. It has important applications in synthetic science also. It is found that some fragments can be obtained through vacuum ultraviolet absorption spectrum and induction photolysis experiments for α-CHD molecules. In order to understand the dissociation reaction mechanism of α-CHD and reveal the resource of those fragments, the potential energy surface of the dissociation reaction for α-CHD molecules in ground state is studied by B3LYP and CCSD(T) methods. The reaction paths of the products are obtained, such as P1(c-C5H8O+ CO), P2(2 C2H4+ 2 CO), P3 (CH2CHCH2CH2CHO+ CO), P4(2 C2H2O+ C2H4), P5(CH3CHCO+ CH2CHCHO). And the structure parameters of the reactant, products, intermediates and transition states in the reaction processes are also obtained. Their reaction mechanisms can be summarized as the isomerization and dissociation processes, and these processes mainly involve the hydrogen atom transfer, ring-opening and C–C bond cleavages. A reactionchannel in which α-CHD dissociates into cyclopentanone and CO needs lower energy, so it is more advantage our to make dissociation study than other studies. In addition, we think that α-dissociationreaction cannotoccur directly in ground state from our calculations. Based on the UV photolysis experiment of α-CHD with a wavelength of 253.7 nm (112.7 kcal/mol) and the theoretical calculation of potential energy surface in ground state, we obtain that Path 1 (α-CHD→ c-C5H8O+ CO) is the most possible channel, Path 3 (α-CHD→ CH2CHCH2CH2CHO+ CO) is the next, and Path 5(α-CHD→ CH3CHCO+ CH2CHCHO) is the third, while Path 2 (α-CHD→ 2 C2H4+ 2 CO) and Path 4 (α-CHD→ 2 CH2CO+ C2H4) are difficult to be achieved. So c-C5H8O and CO are the major fragment products, CH2CHCH2CH2CHO is the subsidiary one, maybe a minor distribution of CH3CHCO and CH2CHCHO, but the fragments C2H4 and CH2CO are difficult to obtain. This agrees well with the analysis using mass spectrometry in experiment. Results can clarify the microcosmic reaction mechanism of the photodissociation reaction for α-CHD molecule in ground state. It may provide an important reference for realizing its spectrum in-depth. The obtained results are informative for future studies on α-CHD relative.