Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

N‐Heterocyclic Carbenes as Organocatalysts

Organophosphorus π-Conjugated Materials

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

Aromaticity as a cornerstone of heterocyclic chemistry.

Energetic Aspects of Cyclic Pi-Electron Delocalization: Evaluation of the Methods of Estimating Aromatic Stabilization Energies

A photoelectron spectroscopic study of keto-enol tautomerism in acetylacetones - a new application of photoelectron spectroscopy

The CNDO/S method is extended to second-row molecules and parametrized for phosphorus and sulphur. Both sp and spd basis sets are considered. The method is applied to the aromatic molecules phosphorin and thiophen. The uv transitions, ionization potentials and dipole moments of these molecules are satisfactorily explained.

CNDO CI calculations on second-row molecules

Measurement of relative conformational stabilities by variable temperature photoelectron spectroscopy. A study of rotational isomerism in thioanisole

MNDOC Study of Excited States

A theoretical method involving the plane‐wave approximation and the use of semiempirical LCAO‐SCF‐MOs of the CNDO and MINDO type is developed for the calculation of photoionization cross sections of arbitrary molecules consisting of atoms from the first three rows of the Periodic Table. Three qualitative concepts for the interpretation of the formalism are discussed, and the dependence of the cross sections on the photon energy, on the electron density distribution in the initial orbital, and on the different behaviour of s, p, and d electrons is treated explicitly. Examples for the application to HeI and HeII photoelectron spectroscopy are given, too.

Armin Schweig

Papers

A photoelectron spectroscopic study of keto-enol tautomerism in acetylacetones - a new application of photoelectron spectroscopy

CNDO CI calculations on second-row molecules

Measurement of relative conformational stabilities by variable temperature photoelectron spectroscopy. A study of rotational isomerism in thioanisole

MNDOC Study of Excited States

Photoionization cross sections in the valence electron approximation. III. Nonlinear molecules (theory)