Abstract
The geometry of thirty alkyl-substituted 1,2-dioxetanes derivatives was optimized using theoretical methods. It was found that AM1 and PM3 semiempirical methods do not adequately predict dihedral angles of the peroxidic ring of highly stabilized 1,2-dioxetanes. Geometric parameters calculated by ab initio and hybrid DFT methods are in better agreement with experimental activation parameter data than the one obtained by semiempirical calculations. Among those, the B3LYP method with the 6-31G(d) basis set is the most adequate one. Very good correlation between theoretical carbon-carbon bond distances and experimental activation parameters was found for all ab initio and hybrid DFT methods, whereas, oxygen-oxygen bond distances and dihedral angles do not correlate well with the activation parameters. Results obtained by different methods are compared and a qualitative explanation for the stabilization effect of alkyl groups on the 1,2-dioxetane ring is proposed.
Highlights
The geometry of thirty alkyl-substituted 1,2-dioxetanes derivatives was optimized using theoretical methods
In order to investigate the effect of alkyl substituents on thermal stability of 1,2-dioxetane, thirty derivates were chosen, submitted to a full conformational analysis and their geometry optimized using the AM1, PM3 and B3LYP/6-31G(d) methods (Table 1)
Both the semiempirical AM1 and density functional theory (DFT)-B3LYP/6-31G(d) methods predict the same behavior for all derivatives, d(O-O) is nearly constant whereas d(C-C) is longer for more stable derivatives, i.e., the smallest d(O-O)/d(C-C) values are calculated for the tetrasubstituted derivates 7, 21, 28, 29, and 30 (Tables S1 and S3)
Summary
The geometry of thirty alkyl-substituted 1,2-dioxetanes derivatives was optimized using theoretical methods. Up to now, no definitive explanation for the chemiexcitation mechanism in 1,2dioxetane thermolysis was obtained, neither by experimental nor theoretical means.[2,7,13,15] Most attempts on mechanistic analysis of this process focused on the question whether the O-O and CC bond cleavage occurs concertedly or in a stepwise manner.[1,12,13,14,16,17,18] Theoretical evidence reported so far indicates that the C-C bond stretches along the reaction coordinate, which is consistent with the intuitively assessed merged dioxetane cleavage mechanism, which predicts the concerted, not simultaneous, cleavage of the O-O and C-C bonds, with the elongation of the O-O bond being more advanced than that of the C-C-bond (concerted biradicallike mechanism).[19,20] This mechanism was first proposed by Adam and used to rationalize both thermal stability and singlet/triplet quantum yields in the series of methyl-substituted 1,2-dioxetanes, including the parent 1,2-dioxetane.[18,19,20] In this study, stability trends in the series of methyl-substituted 1,2dioxetanes could be rationalized by the effect of nonbonding repulsive interactions of the methyl substituents on the O-O and C-C bonds of the four-membered peroxide ring. Trends in the chemiexcitation yields of this series could be understood on the basis of the merged mechanism and relative C-C and O-O bond strength, leading to the conclusion that a concerted, almost simultaneous, decomposition pathway would be responsible for highly efficient chemiexcitation (as observed in the case of tetramethyl-1,2-dioxetane), whereas, a biradical-like decomposition pathway would result in the formation of mainly ground state carbonyl products (as observed in the case of the unsubstituted 1,2-dioxetane).[19]
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