title: Energetics of the allyl group.
abstract: Aiming to improve our understanding of the stability of radicals containing the allylic moiety, carbon-hydrogen bond dissociation enthalpies (BDEs) in propene, isobutene, 1-butene, (E)-2-butene, 3-metylbut-1-ene, (E)-2-pentene, (E)-1,3-pentadiene, 1,4-pentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have been determined by quantum chemistry calculations. The BDEs in cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have also been obtained by time-resolved photoacoustic calorimetry. The theoretical study involved a DFT method as well as ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)). By taking the C(sp3)-H BDE in propene as a reference, we have concluded that one methyl group bonded to C3 in propene (i.e., 1-butene) leads to a decrease of 12 kJ mol(-1) and that a second methyl group bonded to C3 (3-methylbut-1-ene) further decreases the BDE by 8 kJ mol(-1). When the methyl group is bonded to C2 in propene (isobutene), an increase of 7 kJ mol(-1) is observed. Finally, a methyl group bonded to C1 in propene (2-butene) has essentially no effect (-1 kJ mol(-1)). While this trend can be rationalized in terms of stabilization of the corresponding radical (through hyperconjugation and pi-delocalization), the BDE values observed for the dienes can only be understood by considering the thermodynamic stabilities of the parent compounds.
authors.affiliation: Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016, Lisboa, Portugal.
authors.forename: Filipe
authors.initials: F
authors.lastname: Agapito
authors.forename: Paulo M
authors.initials: PM
authors.lastname: Nunes
authors.forename: Benedito J Costa
authors.initials: BJ
authors.lastname: Cabral
authors.forename: Rui M Borges
authors.initials: RM
authors.lastname: dos  Santos
authors.forename: José A Martinho
authors.initials: JA
authors.lastname: Simões
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry*; D002151:Calorimetry; D008956:Models, Chemical; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D000498:Allyl Compounds