Carbenes : Synthesis, properties, and organometallic chemistry

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.

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Single-site heterogeneous catalysts.

Prepared within biocompatible poly(amidoamine) dendrimer hosts, blue emitting Au8 nanodots were prepared with fluorescence quantum yields more than 100 times larger than any previously prepared. The strong size-dependent blue emission from these from monodisperse Au8 nanodots was measured to be 41% in aqueous solution and 52% in methanol. These easily prepared, water-soluble, and high quantum yield gold nanodots lend insight into the molecular nature of small metal nanoclusters and may find application as novel biocompatible fluorophores.

High quantum yield blue emission from water-soluble Au8 nanodots

A review of the theoretical and computational modeling of bimolecular reactions is given. The review is divided into several sections which are as follows: gas-phase thermal reactions; gas-phase state-selected reactions and product state distributions; and condensed-phase bimolecular reactions. The section on gas-phase thermal reactions covers the enthalpies and free energies of reaction, kinetics, saddle points and potential energy surfaces, rate theory for simple barrier reactions and bimolecular reactions over potential wells. The section on gas-phase state-selected reactions focuses on electronically adiabatic reactions and electronically nonadiabatic reactions. Finally, the section on condensed-phase bimolecular reactions covers reactions in liquids, reactions on surfaces and in solids and tunneling at low temperature.

Modeling the Kinetics of Bimolecular Reactions

This work presents a detailed analysis of enhanced Raman scattering of the pyridine−Ag20 model system using time-dependent density functional theory. A consistent treatment of both the chemical and electromagnetic enhancements (EM) is achieved by employing a recently developed approach based on a short-time approximation for the Raman cross section. A strong dependence of the absolute and relative intensities on the binding site and excitation wavelength is found. The analysis of the Raman scattering cross sections shows the importance of different contributions to the enhancements, including static chemical enhancements (factor of 10), charge-transfer enhancements (103), and EM enhancements (105). The largest enhancement found (105−106) is due to the EM mechanism, with a small contribution from the chemical interaction. This suggests that the enhanced Raman scattering due to atomic clusters is comparable to findings on single nanoparticles. A combination of information about the vibrational motion and th...

Pyridine-Ag20 cluster: a model system for studying surface-enhanced Raman scattering.

We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 × 10 −6 per second in nitrogen and 4 × 10 −5 per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.

https://science.sciencemag.org/content/sci/299/5608/867.full.pdf

Carbon Tunneling from a Single Quantum State

Irradiation of 3-(3-noradamantyl)-3-chlorodiazirine produced the corresponding noradamantylchlorocarbene, which could be detected in solution with laser flash photolysis via its pyridinium ylide, and in N2 or Ar matrixes at 9 K with IR and UV/vis spectroscopy. At ambient temperatures, or on irradiation in the cryogenic matrixes, the carbene rearranged to 2-chloro-1-adamantene, which could be trapped in solution and which was characterized by IR and UV/vis at low temperatures. In the dark at 9 K, the carbene also slowly ring-expanded to the chloroadamantene, at a rate ca. 10111 times faster than predicted by its B3LYP-calculated activation barrier. It is proposed that the low-temperature rearrangement occurs through carbon quantum mechanical tunneling.

Peter S. Zuev

Papers

Carbon Tunneling from a Single Quantum State

Carbon tunneling in the ring expansion of noradamantylchlorocarbene.

Tunneling in the C-H insertion of a singlet carbene: tert-butylchlorocarbene

Low-temperature hydrogenation of triplet carbenes and diradicaloid biscarbenes--electronic state selectivity.

p-Phenylenebis(chloromethylene): resonance interaction of two singlet carbenes