Since Donald J. Cram's first synthesis of a carcerand, which permanently entrapped a single guest molecule, many other carcerands and hemicarcerands have been synthesized and studied. Slowly, we begin to understand the role of the template in the formation of hemicarceplexes and carceplexes, why some hemicarceplexes are more stable than others, and how guests enter and exit the inner phases of molecular containers. In this Account we discuss our new insight in the chemistry of molecular containers, as well as recent highlights in through-shell chemistry and in the inner-phase stabilization of reactive intermediates.

Recent highlights in hemicarcerand chemistry.

The intra- and intermolecular chemistry of phenylnitrene (PhN), its singlet−triplet energy separation, and its electronic spectra are interpreted with the aid of ab initio molecular orbital theory. The key to understanding singlet PhN is the recognition that this species has an open-shell electronic structure, in contrast to the related species, phenylcarbene, which has a closed-shell electronic structure. The thermodynamics of nitrenes, benzazirines, dehydroazepines, aminyl radicals, and their hydrocarbon analogues are also discussed.

The interplay of theory and experiment in the study of phenylnitrene.

Reaction coordinates are computed for the Bergman cyclizations of hex-3-en-1,5-diyne and neutral and protonated 3-azahex-3-en-1,5-diyne at various levels of correlated electronic structure theory, as are singlet−triplet splittings for intermediate arynes. To be effective in low-symmetry situations showing high degrees of biradical character, CCSD(T) calculations benefit from use of Brueckner orbitals. Replacement of a CH fragment by N is predicted to increase the stability of the aryne relative to the iminediyne, and to increase drastically the stability of the isomeric enynenitrile. The barrier for retro-aza-Bergman cyclization of 2,5-pyridyne to pent-3-en-1-ynenitrile is predicted to be only 0.9 kcal/mol, which, combined with a predicted singlet−triplet splitting of −11.6 kcal/mol, suggests that 2,5-pyridynes are poor hydrogen atom abstracting agents. Protonation of nitrogen decreases the singlet−triplet splitting and raises the barrier to retro-aza-Bergman cyclization such that protonated 2,5-pyridynes...

Bergman, aza-bergman, and protonated aza-bergman cyclizations and intermediate 2,5-arynes : chemistry and challenges to computation

The Bergman-type cyclizations of parent, 2,3-dimethyl, and monocyclic (ring sizes = 7−12) enediynes were studied in detail at the Becke−Lee−Yang−Parr (BLYP) density functional (DFT) level with 6-31G* as well as 6-311+G** basis sets for geometry optimizations and relative energy evaluations, respectively. Pure DFT methods work reasonably well for these reactions; the errors are somewhat larger (ca. 3−7 kcal mol-1) than for the much more time-consuming complete active space (CASPT2) and coupled-cluster [CCSD(T)] (both in error by ca. 2 kcal mol-1) methods with high-quality basis sets. The hybrid method B3LYP is unsuitable for this type of chemistry (errors of 14−20 kcal mol-1). The singlet−triplet energy separations (ΔEST) for p-benzynes are underestimated systematically by about 2 kcal mol-1 at BLYP; the ΔEST of 2-methyl-p-benzyne (−3.1 kcal mol-1) is close to that of p-benzyne (−3.8 kcal mol-1, i.e., singlet ground state) but the 2,3-dimethyl-p-benzyne ΔEST is only −0.6 kcal mol-1 due to singlet destabili...

Monocyclic Enediynes: Relationships between Ring Sizes, Alkyne Carbon Distances, Cyclization Barriers, and Hydrogen Abstraction Reactions. Singlet−Triplet Separations of Methyl-Substituted p-Benzynes

Cycloheptatrienyl, alkyl and aryl PCP-pincer complexes: Ligand backbone effects and metal reactivity

The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-ζ quality were estimated by (R)MP2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)6 surface which was examined in the present study, followed by planar 3PC. The strained BCT is found to be nearly degenerate in energy with 1PC, but the high activation energy for its formation from 1PC together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental...

and Eric V. Patterson

Papers

The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment