title: Theoretical and structural analysis of long C-C bonds in the adducts of polycyanoethylene and anthracene derivatives and their connection to the reversibility of Diels-Alder reactions.
abstract: X-ray structure determinations on four Diels-Alder adducts derived from the reactions of cyano- and ester-substituted alkenes with anthracene and 9,10-dimethylanthracene have shown the bonds formed in the adduction to be particularly long. Their lengths range from 1.58 to 1.62 Å, some of the longest known for Diels-Alder adducts. Formation of the four adducts is detectably reversible at ambient temperature and is associated with free energies of reaction ranging from -2.5 to -40.6 kJ  mol(-1). The solution equilibria have been experimentally characterised by NMR spectroscopy. Density-functional-theory calculations at the MPW1K/6-31+G(d,p) level with PCM solvation agree with experiment with average errors of 6 kJ  mol(-1) in free energies of reaction and structural agreement in adduct bond lengths of 0.013 Å. To understand more fully the cause of the reversibility and its relationship to the long adduct bond lengths, natural-bond-orbital (NBO) analysis was applied to quantify donor-acceptor interactions within the molecules. Both electron donation into the σ*-anti-bonding orbital of the adduct bond and electron withdrawal from the σ-bonding orbital are found to be responsible for this bond elongation.
authors: Institut de Sciences et d'Ingénierie Supramoléculaires (ISIS), Université de Strasbourg, 8 allée Gaspard Monge, 67000 Strasbourg (France); Current address: Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 7, 9747 AG (The Netherlands).
authors: Anna K H
authors: AK
authors: Hirsch
authors: Philippe
authors: P
authors: Reutenauer
authors: Marc
authors: M
authors: Le Moignan
authors: Sébastien
authors: S
authors: Ulrich
authors: Peter J
authors: PJ
authors: Boul
authors: Jack M
authors: JM
authors: Harrowfield
authors: Peter D
authors: PD
authors: Jarowski
authors: Jean-Marie
authors: JM
authors: Lehn
keywords: X-ray diffraction; bond lengths; cycloaddition; density functional calculations; natural bond orbitals