A scheme that reduces the calculations of ground-state properties of systems of interacting particles exactly to the solution of single-particle Hartree-type equations has obvious advantages. It is not surprising, then, that the density functional formalism, which provides a way of doing this, has received much attention in the past two decades. The quality of the energy surfaces calculated using a simple local-density approximation for exchange and correlation exceeds by far the original expectations. In this work, the authors survey the formalism and some of its applications (in particular to atoms and small molecules) and discuss the reasons for the successes and failures of the local-density approximation and some of its modifications.

The density functional formalism, its applications and prospects

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

Photolysis of azides 1 – 4 in methanol, which have a built-in intramolecular triplet sensitizer, yields mainly carbamates 5 – 8 . Laser flash photolysis of 1 – 4 shows formation of their triplet-excited ketone, which decays by intramolecular energy transfer to form triplet nitrenes 1n – 4n . Irradiating 1 – 3 in matrices yields isocyanic acid, whereas photolysis of 4 forms isocyanate 4i . The depletion rate of the azide bands between 2100 and 2200 cm −1 is different than the rate of formation for the isocyanic acid bands at ∼2265 cm −1 ; thus, the formation of isocyanic acid is a stepwise process. Irradiating 1 in matrices produces an absorption band due to nitrene 1n ( λ max  ∼ 343 nm), which is depleted upon further irradiation, whereas the absorption due to 4-acetyl benzaldehyde ( λ max  ∼ 280 nm) increases with prolonged irradiation. We propose that formation of isocyanic acid in matrices must come from secondary photolysis of nitrenes 1n – 3n . This mechanism is further supported by calculation, which show that the estimated transition state for 1n – 4n to fall apart to yield alkoxy and cyanato radicals is only ∼34 kcal/mol above the ground state of the triplet nitrenes and thus the cleavage can take place photochemically. Thus, nitrenes 1n – 4n can be formed selectively, but these intermediates are highly photosensitive and undergo secondary photolysis in matrices.

Triplet-sensitized photolysis of alkoxycarbonyl azides in solution and matrices

Infrared spectra of matrix-isolated calcium-44 substituted oxides

The matrix isolation technique, combined with infrared spectroscopy and theoretical calculations, has been employed to investigate the thermal and photochemical reactions of dimethylsulfoxide (DMSO) with CrCl 2 O 2 and OVCl 3 . Twin jet codeposition leads to formation and isolation of a photochemically unstable 1:1 complex. The photoproduct in the twin jet DMSO + CrCl 2 O 2 experiments is identified as dimethyl sulfone, (CH 3 ) 2 SO 2 , interacting with the Cl 2 CrO fragment, while in the analogous OVCl 3 reaction, the photoproduct bands were too weak to allow product identification. Merged jet codeposition led to rapid gas phase reaction, and in the case of DMSO + CrCl 2 O 2 , dimethyl sulfone is formed in high yield. This marks the first demonstration of a gas phase thermal oxygen atom transfer from CrCl 2 O 2 to an organic substrate. For the reaction of DMSO with OVCl 3 , no volatile products are deposited in the matrix and dimethyl sulfone is not a product. These results support differing pathways for the reactions of CrCl 2 O 2 and OVCl 3 , a conclusion that is supported by density functional calculations.

Bruce S. Ault

Papers

Triplet-sensitized photolysis of alkoxycarbonyl azides in solution and matrices

Infrared spectra of matrix-isolated calcium-44 substituted oxides

Matrix isolation study of the reaction of dimethyl sulfoxide with CrCl2O2 and OVCl3

Matrix Isolation and Cryogenic Thin Film Studies of Molecular Complexes of Dimethylzinc with Group V and VI Hydrides

Excimer-laser-induced oxidation of diborane: Formation and isolation of HBO, HB18O AND H3B3O3 in argon matrices