title: Revisiting the concept of the (a)synchronicity of Diels-Alder reactions based on the dynamics of quasiclassical trajectories.
abstract: A number of model Diels-Alder (D-A) cycloaddition reactions (H2C=CH2 + cyclopentadiene and H2C=CHX + 1,3-butadiene, with X = H, F, CH3, OH, CN, NH2, and NO) were studied by static (transition state - TS and IRC) and dynamics (quasiclassical trajectories) approaches to establish the (a)synchronous character of the concerted mechanism. The use of static criteria, such as the asymmetry of the TS geometry, for classifying and quantifying the (a)synchronicity of the concerted D-A reaction mechanism is shown to be severely limited and to provide contradictory results and conclusions when compared to the dynamics approach. The time elapsed between the events is shown to be a more reliable and unbiased criterion and all the studied D-A reactions, except for the case of H2C=CHNO, are classified as synchronous, despite the gradual and quite distinct degrees of (a)symmetry of the TS structures.
authors.affiliation: Instituto De Química, Universidade Federal Do Rio Grande Do Norte, Natal, Rio Grande do Norte, 59072-970, Brazil.|Departamento De Química, CCEN, Universidade Federal Da Paraíba, João Pessoa, Paraíba, 58.059-900, Brazil.|Departamento De Química Fundamental, Universidade Federal De Pernambuco, Recife, Pernambuco, 50.740-560, Brazil.
authors.forename: Miguel A F
authors.initials: MA
authors.lastname: de Souza
authors.affiliation: Departamento De Química, CCEN, Universidade Federal Da Paraíba, João Pessoa, Paraíba, 58.059-900, Brazil.
authors.forename: Elizete
authors.initials: E
authors.lastname: Ventura
authors.affiliation: Departamento De Química, CCEN, Universidade Federal Da Paraíba, João Pessoa, Paraíba, 58.059-900, Brazil.
authors.forename: Silmar A
authors.initials: SA
authors.lastname: do Monte
authors.affiliation: Instituto De Química, Universidade De São Paulo, Caixa Postal 26077, São Paulo, 05599-970, Brazil.|Centro De Ciências Naturais E Humanas, Universidade Federal Do ABC, Rua Santa Adelia 166, Santo André, São Paulo, 09210-170, Brazil.
authors.forename: José M
authors.initials: JM
authors.lastname: Riveros
authors.affiliation: Departamento De Química Fundamental, Universidade Federal De Pernambuco, Recife, Pernambuco, 50.740-560, Brazil.
authors.forename: Ricardo L
authors.initials: RL
authors.lastname: Longo
mesh_terms: D002070:Butadienes / Q000138:chemical synthesis / Q000737:chemistry*; D003198:Computer Simulation; D061565:Cycloaddition Reaction; D003517:Cyclopentanes / Q000138:chemical synthesis / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D013816:Thermodynamics
chemical_list: D002070:Butadienes; D003517:Cyclopentanes; C031763:1,3-butadiene
keywords: cycloaddition; dynamics; mechanism; synchronicity; temporal dependence