Dennis Gerbig

Methylhydroxycarbene: tunneling control of a chemical reaction.

Quantum tunneling induces the opposite outcome expected from traditional kinetic factors in a chemical rearrangement. Chemical reactivity is conventionally understood in broad terms of kinetic versus thermodynamic control, wherein the decisive factor is the lowest activation barrier among the various reaction paths or the lowest free energy of the final products, respectively. We demonstrate that quantum-mechanical tunneling can supersede traditional kinetic control and direct a reaction exclusively to a product whose reaction path has a higher barrier. Specifically, we prepared methylhydroxycarbene (H3C–C–OH) via vacuum pyrolysis of pyruvic acid at about 1200 kelvin (K), followed by argon matrix trapping at 11 K. The previously elusive carbene, characterized by ultraviolet and infrared spectroscopy as well as exacting quantum-mechanical computations, undergoes a facile [1,2]hydrogen shift to acetaldehyde via tunneling under a barrier of 28.0 kilocalories per mole (kcal mol–1), with a half-life of around 1 hour. The analogous isomerization to vinyl alcohol has a substantially lower barrier of 22.6 kcal mol–1 but is precluded at low temperature by the greater width of the potential energy profile for tunneling.

σ/σ- And π/π-interactions are equally important: multilayered graphanes.

The properties of single-sheet [n]graphanes, their double-layered forms (diamondoids), and their van der Waals (vdW) complexes (multilayered [n]graphanes) were studied for n = 10-97 at the dispersion-corrected density functional theory (DFT) level utilizing B97D with a 6-31G(d,p) basis set; for comparison, we also computed a series of structures at M06-2X/6-31G(d,p) as well as B3LYP-D3/6-31G(d,p) and evaluated SCS-MP2/cc-pVDZ single-point energies. The association energies for the vdW complexes reach 120 kcal mol(-1) already at 2 nm particle size ([97]graphane dimer), and graphanes adopt layered structures similar to that of graphenes. The association energies of multilayered graphanes per carbon atom are rather similar and independent of the number of layers (ca. 1.2 kcal mol(-1)). Graphanes show quantum confinement effects as the HOMO-LUMO gaps decrease from 8.2 eV for [10]graphane to 5.7 eV for [97]graphane, asymptotically approaching 5.4 eV previously obtained for bulk graphane. Similar trends were found for layered graphanes, where the differences in the electronic properties of double-sheet CH/σ vdW and double-layered CC/σ diamondoids vanish at particles sizes of 1 nm. For comparison, we studied the parent CC/π systems, i.e., the single- and double-sheet [n]graphenes (n = 10-130) for which the association energies demonstrate the same trends as in the case of [n]graphanes; in both cases the band gaps decrease with an increase in system size. The [112]graphene dimer (HOMO-LUMO gap = 0.5 eV) already approaches the 2D metallic properties of graphite.

Light- and Heavy-Atom Tunneling in Rearrangement Reactions of Cyclopropylcarbenes

We investigated both light- and heavy-atom tunneling in the rearrangements of a series of cyclopropylcarbenes using canonical variational transition state theory with multidimensional tunneling corrections (CVT/MT) and the Wentzel-Kramers-Brillouin (WKB) formalism. Halogeno- and hydroxy-substituted cyclopropylcarbenes were found not to undergo carbon tunneling owing to wide reaction barriers. However, while carbon tunneling plays a major role in the ring expansion of parent cyclopropylcarbene yielding cyclobutene, cyclopropylmethylcarbene is prone to undergo hydrogen tunneling to give cyclopropylmethylene.

Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene

Elusive tert-butylhydroxycarbene was generated in the gas phase via high-vacuum flash pyrolysis of tert-butylglyoxylic acid at 960 °C. The pyrolysis products were subsequently matrix isolated in solid Ar at 11 K and characterized by means of IR spectroscopy. While still being exposed to the harsh pyrolysis conditions, the hydroxycarbene undergoes CH-insertion to dimethylcyclopropanol, as well as a CC-insertion to novel methylbutenol, with activation barriers of 23.8 and 31.0 kcal mol−1, respectively. Once embedded in the cold Ar matrix, the carbene transforms to its isomer pivaldehyde not only by photolysis, but it also cuts through the barrier of 27.3 kcal mol−1 by quantum mechanical tunneling. The temperature independent half-life is measured as 1.7 h; the tunneling pathway was entirely blocked upon O-deuteration. The experimental half-life of tert-butylhydroxycarbene was verified by tunneling computations applying the Wentzel–Kramers–Brillouin formalism on the minimum energy path evaluated at the computationally feasible M06-2X/6-311++G(d,p) level of theory. Our experimental findings are supported by relative energy computations at the CCSD(T)/cc-pVDZ level of theory.

Intramolecular hydroxycarbene C-H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene

Summary The first C–H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable (o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C–H bond. The insertion product, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/cc-pVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy)benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.

Preparation of a Silanone through Oxygen Atom Transfer to a Stable Cyclic Silylene

We report the evaporation of a stable cyclic silylene and its oxidation (with ozone or N2 O) through oxygen atom transfer to form the corresponding silanone under matrix isolation conditions. As uncomplexed silanones are rare owing to their very high reactivity, this method provides an alternative route to these sought-after molecules. The silanone, as well as a novel bicyclic silane with a bridgehead silicon atom derived from an intramolecular silylene CH bond insertion, were characterized by comparison of high-resolution infrared spectra with density functional theory (DFT) computations at the M06-2X/cc-pVDZ level of theory.

Two‐Dimensional Infrared Spectroscopy Reveals the Structure of an Evans Auxiliary Derivative and Its SnCl4 Lewis Acid Complex

Determining the structure of reactive intermediates is the key to understanding reaction mechanisms. To access these structures, a method combining structural sensitivity and high time resolution is required. Here ultrafast polarization-dependent two-dimensional infrared (P2D-IR) spectroscopy is shown to be an excellent complement to commonly used methods such as one-dimensional IR and multidimensional NMR spectroscopy for investigating intermediates. P2D-IR spectroscopy allows structure determination by measuring the angles between vibrational transition dipole moments. The high time resolution makes P2D-IR spectroscopy an attractive method for structure determination in the presence of fast exchange and for short-lived intermediates. The ubiquity of vibrations in molecules ensures broad applicability of the method, particularly in cases in which NMR spectroscopy is challenging due to a low density of active nuclei. Here we illustrate the strengths of P2D-IR by determining the conformation of a Diels-Alder dienophile that carries the Evans auxiliary and its conformational change induced by the complexation with the Lewis acid SnCl(4), which is a catalyst for stereoselective Diels-Alder reactions. We show that P2D-IR in combination with DFT computations can discriminate between the various conformers of the free dienophile N-crotonyloxazolidinone that have been debated before, proving antiperiplanar orientation of the carbonyl groups and s-cis conformation of the crotonyl moiety. P2D-IR unequivocally identifies the coordination and conformation in the catalyst-substrate complex with SnCl(4), even in the presence of exchange that is fast on the NMR time scale. It resolves a chelate with the carbonyl orientation flipped to synperiplanar and s-cis crotonyl configuration as the main species. This work sets the stage for future studies of other catalyst-substrate complexes and intermediates using a combination of P2D-IR spectroscopy and DFT computations.

Synthesis and Stereochemical Assignment of Crypto-Optically Active (2) H6 -Neopentane.

The determination of the absolute configuration of chiral molecules is at the heart of asymmetric synthesis. Here we probe the spectroscopic limits for chiral discrimination with NMR spectroscopy in chiral aligned media and with vibrational circular dichroism spectroscopy of the sixfold-deuterated chiral neopentane. The study of this compound presents formidable challenges since its stereogenicity is only due to small mass differences. For this purpose, we selectively prepared both enantiomers of (2) H6 -1 through a concise synthesis utilizing multifunctional intermediates. While NMR spectroscopy in chiral aligned media could be used to characterize the precursors to (2) H6 -1, the final assignment could only be accomplished with VCD spectroscopy, despite the fleetingly small dichroic properties of 1. Both enantiomers were assigned by matching the VCD spectra with those computed with density functional theory.

Computational methods for contemporary carbene chemistry

The use of computational methods in carbene chemistry has a long‐standing tradition. Indeed, the field has come a long way since the first ab initio calculations on methylene. Computations now routinely accompany most experimental studies, either to validate the obtained results or to help design appropriate experiments. Advances in computational carbene chemistry within the last decade are covered in this text, encompassing a plethora of studies on alkyl‐, aryl‐, halo‐, and heterocarbenes (N, P, O, S) as well as on persistent triplet carbenes. Moreover, the conceptual advancements in the fields of theoretical chemistry and computing technology have enabled researchers to conduct intricate ab initio studies. The application of leading‐edge theory to multireference problems, high‐accuracy thermochemical evaluations, atom tunneling, and the description of bonding is thoroughly reviewed. In addition, general recommendations for the choice of an appropriate method for a specific computational problem are given. Practitioners of the art are likely to discover new computational approaches in carbene chemistry applied to various examples from the current literature.

Formation of a Tunneling Product in the Photorearrangement of o-Nitrobenzaldehyde

The photochemical rearrangement of o-nitrobenzaldehyde to o-nitrosobenzoic acid, first reported in 1901, has been shown to proceed via a distinct ketene intermediate. In the course of matrix isolation experiments in various host materials at temperatures as low as 3 K, the ketene was re-investigated in its electronic and vibrational ground states. It was shown that hitherto unreported H-tunneling dominates its reactivity, with half-lives of a few minutes. Unexpectedly, the tunneling product is different from o-nitrosobenzoic acid formed in the photoprocess: Once prepared by irradiation, the ketene spontaneously rearranges to an isoxazolone via an intriguing mechanism initiated by H-tunneling. CCSD(T)/cc-pVTZ computations reveal that this isoxazolone is neither thermodynamically nor kinetically favored under the experimental conditions, and that formation of this unique tunneling product constitutes a remarkable and new example of tunneling control.