Theoretical Study of the Enthalpies of Formation for C₄₀H₅₆ Carotenes

Abstract

Theoretical study of the enthalpies of formation (DeltaHf) for polyenes up to nine ethylene units and for several C40H56 carotenes including beta-carotene, alpha-carotene, lycopene, and prolycopene is presented. For polyenes and small branched alkenes, we used G2, G3, and G3MP2B3 theories, and the DeltaHf values were evaluated with the atomization, isodesmic bond separation, and homodesmic schemes. The applicability of six DFT functionals were evaluated by comparing their predictions with those obtained using G3 theory within the atomization scheme. Additivity approaches, including atom equivalents and group equivalents using DFT and semiempirical theories, were explored. We found that group equivalents associated with isodesmic reactions are able to provide the most accurate predictions within the test set. The predictions from the six functionals are in good agreement with the G3 results. Among them, B3LYP performs the best, with an average absolute deviation of only 0.30 kcal/mol. The application of DFT in the prediction for the DeltaHf value of C40H56 carotenes is promising.

A Theoretical Approach for Accurate Predictions of the Enthalpies of Formation of Carotenes

“…In a previous study, ∆H f values of the four C 40 H 56 isomers;lycopene, prolycopene, R-carotene, and β-carotene;were predicted using B3LYP with the GEQ scheme in a previous study. 21 However, the predictions for the ∆H f in this previous study now seem significantly overestimated when compared with the CE and CCAZ values obtained in this study. As shown in the figure, MM4 predictions are noticeably smaller than those from CCAZ-B3LYP and CCAZ-MPWB1K.…”

“…The AEQ results for MPWB1K are slightly underestimated, while GEQ results for MPWB1K are in close agreement with CCAZ. In a previous study, Δ H f values of the four C 40 H 56 isomers―lycopene, prolycopene, α-carotene, and β-carotene―were predicted using B3LYP with the GEQ scheme in a previous study . However, the predictions for the Δ H f in this previous study now seem significantly overestimated when compared with the CE and CCAZ values obtained in this study.…”

“…18 In a study for the predictions of the ∆H f of C 40 H 56 carotenes, we have demonstrated that predictions using the isodesmic and homodesmic reactions are more accurate than those obtained using atomization reactions. 21 However, noticeable accumulations of error were observed.…”

“…Thus, in these studies, isodesmic or homodesmic reactions were applied, and improved results were obtained for extended alkanes . In a study for the predictions of the Δ H f of C 40 H 56 carotenes, we have demonstrated that predictions using the isodesmic and homodesmic reactions are more accurate than those obtained using atomization reactions . However, noticeable accumulations of error were observed.…”

Comparação entre métodos compostos no cálculo de afinidades por próton e elétron em sistemas moleculares

“…[32][33][34][35][36][37][38][39][40][41][42][43][44] Outra aplicação interessante é no estudo do mecanismo em processos envolvidos em espectroscopia de massas. 45 A variedade de aplicações utilizando métodos compostos vem aumentando tanto em trabalhos teóricos como em trabalhos experimentais, devido à sua praticidade, confiabilidade e precisão em prever parâmetros físico-químicos.…”