Physical organic chemistry plays a unifying role in chemistry by combining a large variety of experimental and theoretical techniques to create simplified, intuitively understandable concepts of chemical processes. The concept of “antiaromaticity” is used to demonstrate how complex experiments may result in general models that allow chemists to predict the chemical and physical properties of difficult to access molecules and even of transition states. Cyclooctatetraene and the fluorenyl cation are used as examples to illustrate some working methods of physical organic chemistry.

The photochemistry of diphenylcarbene 5, matrix-isolated in argon at 3–5 K was investigated by UV–Vis, IR, and EPR spectroscopy. Although carbene 5 proved to be stable toward broadband UV irradiation, it slowly rearranges to 1-phenyl-1,2,4,6-cycloheptatetraene 8 during irradiation with the intense light of a 445-nm diode laser. Other intermediates were not observed. Allene 8 is the key intermediate that was missing in the proposed mechanism of the rearrangement of 5 to fluorene 7. Copyright © 2014 John Wiley & Sons, Ltd.

Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H₂ through a two-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H₂ at cryogenic temperatures. Here we show that 1-azulenylcarbene with a singlet ground state readily inserts into H₂, and that QMT governs the insertion into both H₂ and D₂. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.

Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H₂ through a two-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H₂ at cryogenic temperatures. Here we show that 1-azulenylcarbene with a singlet ground state readily inserts into H₂, and that QMT governs the insertion into both H₂ and D₂. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.

The role of N-heterocyclic carbenes in the chemistry of ionic liquids based on imidazolium salts has long been discussed. Here, we present experimental evidence that 1-ethyl-3-methylimidazolium-2-ylidene (EMIm) can coexist with its protonated imidazolium cation (EMImH⁺) at low temperatures. If the vapor of the ionic liquid [EMImH⁺][AcO⁻] is trapped in solid argon or nitrogen at 9 K, only acetic acid (AcOH) and the carbene, but no ionic species, are found by IR spectroscopy. This indicates that during the evaporation of [EMImH⁺][AcO⁻] proton transfer occurs to form the neutral species. If the vapor of [EMImH⁺][AcO⁻] is trapped at 9 K as film in the absence of a host matrix, a solid consisting of EMImH⁺, EMIm, AcO⁻, and AcOH is formed. During warming to room temperature the proton transfer in the solid to form back the IL [EMImH⁺][AcO⁻] can be monitored by IR spectroscopy. This clearly demonstrates that evaporation and condensation of the IL [EMImH⁺][AcO⁻] results in a double proton transfer, and the carbene EMIm is only metastable even at low temperatures.

The benzyl radical (1) is a key intermediate in the combustion and tropospheric oxidation of toluene. Because of its relevance, the reaction of 1 with molecular oxygen was investigated by matrix-isolation IR and EPR spectroscopy as well as computational methods. The primary reaction product of 1 and O₂ is the benzylperoxyl radical (2), which exists in several conformers that can easily interconvert even at cryogenic temperatures. Photolysis of radical 2 at 365 nm results in a formal [1,3]-H migration and subsequent cleavage of the OO bond to produce a hydrogen-bonded complex between the hydroxyl radical and benzaldehyde (4). Prolonged photolysis produces the benzoyl radical (5) and water, which finally yield the phenyl radical (7), CO, and H₂O. Thus, via a sequence of exothermic reactions 1 is transformed into radicals of even higher reactivity, such as OH and 7. Our results have implications for the development of models for the highly complicated process of combustion of aromatic compounds.

4-Oxocyclohexa-2,5-dienylidene is a highly reactive triplet ground state carbene that is hydrogenated in solid H₂, HD, and D₂ at temperatures as low as 3 K. The mechanism of the insertion of the carbene into dihydrogen was investigated by IR and EPR spectroscopy and by kinetic studies. H or D atoms were observed as products of the reaction with H₂ and D₂, respectively, whereas HD produces exclusively D atoms. The hydrogenation shows a very large kinetic isotope effect and remarkable isotope selectivity, as was expected for a tunneling reaction. The experiments, therefore, provide clear evidence for both hydrogen tunneling and the rare deuterium tunneling in an intermolecular reaction.

The highly strained 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 1 is metastable, and rearranges to 4-oxacyclohexa-2,5-dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K. The kinetics for this rearrangement show pronounced matrix effects, but in a given matrix, the reaction rate is independent of temperature between 3 and 20 K. This temperature independence means that the activation energy is zero in this temperature range, indicating that the reaction proceeds through quantum mechanical tunneling from the lowest vibrational level of the reactant. At temperatures above 20 K, the rate increases, resulting in curved Arrhenius plots that are also indicative of thermally activated tunneling. These experimental findings are supported by calculations performed at the CASSCF and CASPT2 levels by using the small-curvature tunneling (SCT) approximation.

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: OH insertions are assumed to be characteristic of singlet states, whereas CH insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen-bonded singlet ground state complex that was synthesized in low-temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of OH insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: OH insertions are assumed to be characteristic of singlet states, whereas CH insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen-bonded singlet ground state complex that was synthesized in low-temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of OH insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.

The photochemistry of N-methylformamide (MF) is elucidated by investigating its photodissociation products generated by UV irradiation (248 nm) in an argon matrix (10 K). We find that, starting from trans-MF, prolonged irradiation produces cis-MF, CH₃NH₂ and CO fragments as major products. Another photoproduct is identified as methylformimidic acid (FIA). Nonadiabatic dynamics simulations starting from both MF conformers revealed that the internal conversion occurs within 1 ps through a CN dissociation channel. The major product is a weakly bound complex between CH₃NH and HCO radicals. This complex owes its existence to the cage effect of the matrix which allows for H-transfer reactions and recombination. By identifying the primary photoisomerization and photodissociation pathways of MF, we gain new insights into the photochemistry of peptide bonds in general, which is a prerequisite for a better understanding of the effect of UV irradiation on living systems.

The photochemistry of 3-iodo-2,5,6-trifluoropyridyl azide (6c), matrix-isolated in argon, was investigated by IR and EPR spectroscopy. The primary photoproduct is 3-iodo-2,5,6-trifluoropyridylnitrene (7c) in its triplet ground state. Further irradiation produced very low yields of nitrene radical 3c, which shows a characteristic EPR spectrum. The yield of 3c, however, was too low for IR detection. Instead, azirinyl radical 11, formed from 3c by ring closure, could clearly be identified in the IR spectra. Other products observed were azirine 8c′ and the ketenimine 9c′, which are formed in a photostationary equilibrium together with nitrine 7c.

The photochemistry of 2,4,6-triazido-3,5-difluoropyridine 21 was investigated by matrix infrared and electron paramagnetic resonance spectroscopies. Ultraviolet irradiation (>260 nm) of 21 results in the formation of 3,5-difluoropyridyl-2,4,6-trinitrene 26 in yields high enough for characterization by infrared spectroscopy. The experimental infrared spectrum is in good agreement with density functional theory calculations. Under similar conditions, a very strong electron paramagnetic resonance spectrum of septet trinitrene 26 was obtained. Shorter irradiation times resulted in more complex product mixtures containing, in addition, mononitrenes and dinitrenes. Surprisingly, azirines and keteneimines, the typical photoproducts of arylnitrenes, were not observed. Copyright © 2011 John Wiley & Sons, Ltd.

The UV (λ>305 nm) photolysis of triazide 3 in 2-methyl-tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm⁻¹, E_T=0 cm⁻¹), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm⁻¹, E_Q=0.035 cm⁻¹), and septet trinitrene 8 (g=2.003, D_S=−0.0904 cm⁻¹, E_S=−0.0102 cm⁻¹). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ∼1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line-shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line-broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8. According to these line-broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6-311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one-center spin–spin interactions in combination with weak positive two-center spin–spin interactions, as predicted by theoretical considerations.

The isomeric 2-dehydro- and 4-dehydro-1,3-benzoquinodimethanes, 18 and 19, were generated by irradiation of the corresponding triiodo compounds in cryogenic argon matrices and characterized by electron paramagnetic resonance spectroscopy. In agreement with multireference computations, both systems possess quartet ground states, whereas in the isomeric 5-dehydro-m-xylylene 20 the ²B₂ doublet state lies energetically below the ⁴B₂ state, so that no characteristic quartet signals could be observed for this triradical under similar experimental conditions. The best estimates for the adiabatic doublet–quartet energy splittings (CAS(7,7)-AQCC/cc-pVTZ//CAS(9,9)-RS2c/cc-pVTZ) of 18–20 are 10.4, 7.7, and −1.3 kcal/mol, respectively. The measured zero-field splitting parameters of 18 and 19 are discussed in terms of the contributions of carbenoid resonance structures (spin polarization of the π-system) to the resonance hybrid of the title triradicals. Copyright © 2011 John Wiley & Sons, Ltd.

The benzoyl radical 1 was synthesized in argon matrices by the thermal reaction of the phenyl radical 2 with CO. The IR spectrum with the C=O str. vibration at 1824.4 cm^–1 is in good agreement with DFT calculations. The formation of 1 is reversible and UV irradiation results in the cleavage back to 2 and CO. The benzoyl radical 1 can react with molecular oxygen in the matrix to produce the benzoylperoxy radical 3. Radical 3 was also characterized by IR spectroscopy in combination with DFT calculations.

Septet 3,5-dicyanopyridyl-2,4,6-trinitrene was synthesized together with quintet 2-azido-3,5-dicyanopyridyl-4,6-dinitrene, quintet 4-azido-3,5-dicyanopyridyl-2,6-dinitrene, triplet 2,6-diazido-3,5-dicyanopyridyl-4-nitrene, and triplet 2,4-diazido-3,5-dicyanopyridyl-6-nitrene by photolysis of 2,4,6-triazido-3,5-dicyanopyridine in solid argon at 4 K. The electronic and magnetic properties of the matrix-isolated nitrenes were studied using electron paramagnetic resonance (EPR) spectroscopy in combination with density functional theory (DFT) calculations. The fine-structure parameters of the nitrenes were determined with high accuracy from spectral simulations. All signals in the EPR spectra of the nitrenes, randomly oriented in the solid phase, were unambiguously assigned based on eigenfield calculations of the Zeeman energy levels and angular dependences of resonance fields. Copyright © 2009 John Wiley & Sons, Ltd.

The reaction of the phenyl radical 1 with water has been investigated by using matrix isolation spectroscopy and quantum chemical calculations. The primary thermal product of the reaction between 1 and water is a weakly bound complex stabilized by an OH⋅⋅⋅π interaction. This complex is photolabile, and visible-light irradiation (λ>420 nm) results in hydrogen atom transfer from water to radical 1 and the formation of a highly labile complex between benzene and the OH radical. This complex is stable under the conditions of matrix isolation, however, continuous irradiation with λ>420 nm light results in the complete destruction of the aromatic system and formation of an acylic unsaturated ketene. The mechanisms of all reaction steps are discussed in the light of ab initio and DFT calculations.

The photochemistry of 2-iodo-3,4,5,6-tetrafluorophenyl azide (7 d) has been investigated in argon and neon matrices at 4 K, and the products characterized by IR and EPR spectroscopy. The primary photochemical step is loss of a nitrogen molecule and formation of phenyl nitrene 1 d. Further irradiation with UV or visible light results in mixtures of 1 d with azirine 5 d′, ketenimine 6 d′, nitreno radical 2 d, and azirinyl radical 9. The relative amounts of these products strongly depend on the matrix and on the irradiation conditions. Nitreno radical 2 d with a quartet ground state was characterized by EPR spectroscopy. Electronic structure calculations in combination with the experimental results allow for a detailed understanding of the properties of this unusual new type of organic high-spin molecules.

The phenylperoxy radical 1 has been synthesized by the reaction of the phenyl radical 2 with ³O₂. Radical 1 could be either generated in the gas phase and subsequently trapped in solid argon at 10 K, or directly synthesized in argon matrices. By reacting 2 as well as its perdeuterated isotopomer [D₅]-2 with ¹⁶O₂ and with ¹⁸O₂, respectively, the four isotopomers [H₅]-¹⁶O₂-1, [D₅]-¹⁶O₂-1, [H₅]-¹⁸O₂-1, and [D₅]-¹⁸O₂-1 were matrix-isolated and characterized by IR spectroscopy. The experimental IR spectra are in excellent agreement with results from DFT calculations. Irradiation of 1 with visible light produces the 2-oxepinoxy radical 5 in a clean reaction. Subsequent irradiation results in ring-opening and formation of several conformers of ketoketene 6. The radicals 1, 5, and 6 play an important role in the combustion of aromatic hydrocarbons and could now be isolated and spectroscopically characterized for the first time.