Photochemical reaction mechanism of cyclobutanone: CASSCF study

Abstract

ABSTRACT:The photochemical reaction channels of cyclobutanone have been studied at the CASSCF level with a 6-31G* basis set. Starting from the n-* excited-state (S 1 ) cyclobutanone, the three reactions can take place: decarbonylation (produce CO and cyclopropane or propylene), cycloelimination (produce ketene and ethylene), and ring expansion (produce oxacarbene). Our computation indicates that decarbonylation products CO and triplet trimethylene are formed on the triplet n-* excited state (T 1 ) in a stepwise way via a biradical intermediate after intersystem crossing (ISC) to T 1 from S 1 . And, then, the triplet trimethylene undergoes a second ISC to the ground state (S 0 ) to produce the singlet trimethylene from which cyclopropane can be produced rapidly only overcoming a 1 to 2-kcal/mol barrier while propylene can be formed as a secondary product. The cycloelimination products ketene and ethylene are formed on the S 0 in a concerted mechanism after internal conversion (IC) to S 0 from S 1 via a biradical conical intersection. The reaction channels corresponding to ring expansion on the S 0 , T 1 , and S 1 states have also been discussed, and the likeliest reaction path is that oxacarbene is formed on the ground state following S 1 /S 0 internal conversion. The surface topology of cyclobutanone on the S 1 surface is characterized by a transition state separating the minimum from the S 1 /S 0 conical intersection, which is consistent with the previous computations and can explain the wavelength dependence of the fluorescence emission yield.

Excited-State Ring-Opening Mechanism of Cyclic Ketones: A MS-CASPT2//CASSCF Study

“…For cyclobutanone, the S 1 barrier increases to 6.7 kcal/mol and the T 1 one remains zero. Our prediction is close to 4.9 and 0.8 kcal/mol of Zewail et al but much lower than 15.7 and 10.6 kcal/mol of Chen et al For cyclopentanone and cyclohexanone, the S 1 barriers increase significantly, to more than 14 kcal/mol at the MS-CASPT2 level; even, the T 1 barriers are also close to 10 kcal/mol (see Figure ).…”

X‐ray photoelectron spectroscopy study on the photodegradation of copolyester model compounds

“…These results are in agreement with our XPS studies as well as previous studies which stated that excited cyclobutanone undergoes an efficient cleavage to produce the hypothetical 1,4-acyl-alkyl biradical. 31,[46][47][48][49] TMCD-T-TMCD and TMCD-T-TMCD-ketone theoretical models were also optimized at the singlet excited states by using TDDFT and reactive sites for optimized structures in the excited states and these are given in Figure 15(b). It was determined that one of the alcohol end groups underwent ring opening when both ends were TMCD alcohol end groups.…”

“…30 The photochemical reaction channels of cyclobutanone have been studied at the CASSCF level theoretically and carbene intermediate followed by cyclic acetal formation in the singlet excited state was validated. 31 The most crucial step for this reaction is biradical formation in the first excited singlet state. It was observed experimentally that increasing biradical stability not only increases the efficiency of cyclic acetal formation, but lowers the efficiencies of competing decarbonylation and cycloelimination side reactions.…”

Transformation of a Phosphorus-Bound Cp* Moiety in the Coordination Sphere of Manganese Carbonyl Complexes

“…However, the all-carbon analogues, the cyclobutanones, represent a broadly studied class of compounds. Their photochemical decarbonylation leading to the formation of 1,3-biradical was shown to be the predominant pathway for triplet-sensitized reactions. − The further photodecomposition of 5 can be facilitated by coordination of the P atom to two metal centers, as well as by the release of the stable Me 4 Fv in the reaction course. Imine elimination (Pr i NCMe 2 ) was reported for a μ-PN i Pr 2 ligand coordinated to two metal centers upon α-C–H activation, which is somewhat comparable to the process found in 5 .…”