Through a retrospective examination of the author’s half-century immersion in physical organic chemistry, an attempt is made to identify some broader currents in the field. Among these are the cyclical nature of physical organic chemistry, the persistence of research themes, the driving force of new instrumentation, paradigm shifts in theory, greater computational power, enhanced time resolution of experiments, and the export of mechanism-based analysis to other foci of contemporary science.

Reviewed herein are equilibria between halocarbenes and halocarbanions, carbenes and carbene complexes and carbenes and O-ylides. The transient species were visualized by laser flash photolysis coupled with UV–Visible spectroscopy. This methodology enabled the determination of equilibrium constants and the extraction of associated thermodynamic parameters. Parallel computational studies provided anticipated structures and energies for the transient species, as well as electronic absorption wavelengths and oscillator strengths.

Decades of research on the structure of the 2-norbornyl cation afford a new understanding of bonding in carbon compounds and of the electron-donor capacity of carbon–carbon σ bonds. The bridged or “nonclassical” structure of the 2-norbornyl cation, which features 3-center, 2-electron bonding, is supported by (inter alia) X-ray crystallography, NMR spectroscopy, calculations, thermochemical studies, and copious mechanistic evidence. Copyright © 2014 John Wiley & Sons, Ltd.

Complexes of several carbenes with aryl ethers are documented by laser flash photolysis with UV-visible spectroscopic detection. The carbenes include methylchlorocarbene, benzylchlorocarbene, phenylchlorocarbene, p-nitrophenylchlorocarbene, and dichlorocarbene, whereas the complexing ligands are anisole, various methylanisoles, dibenzo-18-crown-6, dibenzofuran, 1,3-dimethoxybenzene, and 1,3,5-trimethoxybenzene. The carbene complexes are characterized both by spectroscopy and computation. Complexation retards the additions of the carbenes to olefins. The complexation of phenylchlorocarbene by 1,3,5-trimethoxybenzene is shown to be an equilibrium process, and the pertinent thermodynamic parameters are obtained by experiment and computed from theory. Copyright © 2011 John Wiley & Sons, Ltd.

The Graham oxidation of amidines to halodiazirines, coupled with the diazirine exchange reaction, makes available many new diazirine precursors of carbenes. Nitration of the phenoxy moieties of phenoxychloro- or phenoxyfluorodiazirine converts them to leaving groups, ultimately permitting the preparations of CCl₂, CClF, and CF₂. Absolute rate constants and activation parameters are measured for the additions of these carbenes to several alkenes. The comparative contributions of ΔH^‡ and ΔS^‡ to ΔG^‡ are assessed for these reactions, as are the dependences of ΔH^‡ and ΔS^‡ on the structures and reactivities of the carbenes and alkenes. Copyright © 2009 John Wiley & Sons, Ltd.

Fragmentations of secondary or tertiary alkoxyhalocarbenes in polar solvents generate carbocations as components of ion pairs. A variety of carbocations can be produced including acyclic, alicyclic, benzyl, bridgehead, cyclopropyl, cyclopropylcarbinyl, and norbornyl examples. Laser flash photolysis (LFP) studies provide kinetics and activation parameters for the carbene fragmentations, which are orders of magnitude faster, and require considerably reduced activation energies, compared to analogous solvolytic carbocation-generative processes. In some cases, the time required for solvent and anion equilibration of the ion pairs can be estimated. In nonpolar solvents, the gas phase, or cryogenic matrices, homolytic carbene fragmentation may, in certain cases, supplant heterolytic fragmentation. Copyright © 2008 John Wiley & Sons, Ltd.