Hans Peter Reisenauer, Jarosław Romański, Grzegorz Mlostoń, Peter R. Schreiner

Chem. Commun. 2015, 51, 10022–10025. DOI: 10.1039/c5cc02168e

Highlight: Holm Petzold, Nachr. Chem. 2015, 63, 760

The atmospherically highly relevant methylsulfinyl radical (CH3(O)S•) readily reacts with molecular triplet oxygen in cryogenic argon matrices containing small amounts of 3O2. Comparison of experimental and computed IR- and UV/Vis spectra, including isotope exchange, show that the initially formed 3O2 adduct has the structure of a peroxyl radical (CH3(O)SOO•), which upon irradiation with UV light isom-erizes to the methylsulfonoxyl radical (CH3SO3•). The latter transforms into the methansulfonic acid radical (•CH2SO3H) by irradiation with visible light. During the matrix photolysis small amounts of SO3 and methyl radical were detected indi-cating competitive direct photodissociation.

Hans-Peter Reisenauer, Jaroslaw Romanski, Peter R. Schreiner, and Grzegorz Mloston
J. Phys. Chem. A 2015, Article ASAP. DOI: 10.1021/jp5036647

The atmospherically highly relevant methylsulfonyl radical (CH₃SO₂^•) was generated by high-vacuum flash pyrolysis (HVFP) of allylmethylsulfone and isolated in an argon matrix at 10 K; the allyl radical formed as the cofragment. Upon thermolysis, the methylsulfonyl radical undergoes partial decomposition, leading to substantial amounts of sulfur dioxide in the matrix. The title compound was characterized through the assignment of eight fundamental IR bands of its CD₃ and ¹³CH₃ isotopologues and the excellent agreement with the B3LYP/6-311+G(3df,3pd) computed harmonic vibrational frequencies. The two most intense absorptions were found at 1267.1 and 1067.6 cm^–1. In extension of this study S-methyl methanethiosulfonate was found to be another suitable, although less efficient, precursor for the gas-phase generation of the methylsulfonyl radical.

Hans Peter Reisenauer, Jan Philipp Wagner, Peter R. Schreiner 

Angew. Chem. Int. Ed. 2014, 53, 11766–11771 (“hot paper”), DOI: 10.1002/anie.201406969.

Highlights:

a) Front cover of this issue;

b) Perspective: Götz Bucher and Wolfram Sander Science 2014, 346, 544–545; c) Feature: Carbonic Acid Crystal Forms Identified. Jyllian N. Kemsley C & EN News 2014, 92 (41), 28–29;

d) ChemistryViews: Carbonic Acid – And Yet It Exists!;

e) Innovations Report: Carbonic Acid—And Yet It Exists!;

f) Physorg.com: Preparation and characterization of gas-phase carbonic acid and its monomethyl ester.

Carbonic acid (H₂CO₃), an essential molecule of life (e.g., as bicarbonate buffer), has been well characterized in solution and in the solid state, but for a long time, it has eluded its spectral characterization in the gas phase owing to a lack of convenient preparation methods; microwave spectra were recorded only recently. Here we present a novel and general method for the preparation of H₂CO₃ and its monomethyl ester (CH₃OCO₂H) through the gas-phase pyrolysis of di-tert-butyl and tert-butyl methyl carbonate, respectively. H₂CO₃ and CH₃OCO₂H were trapped in noble-gas matrices at 8 K, and their infrared spectra match those computed at high levels of theory [focal point analysis beyond CCSD(T)/cc-pVQZ] very well. Whereas the spectra also perfectly agree with those of the vapor phase above the β-polymorph of H₂CO₃, this is not true for the previously reported α-polymorph. Instead, the vapor phase above α-H₂CO₃ corresponds to CH₃OCO₂H, which sheds new light on the research that has been conducted on molecular H₂CO₃ over the last decades.

Michael L. McKee, Hans-Peter Reisenauer, and Peter R. Schreiner,

J. Phys. Chem. A. 2014, 118, 2801–2809. DOI: 10.1021/jp501107b.

Car–Parrinello molecular dynamics was used to explore the reactions between triplet and singlet carbon atoms with benzene. The computations reveal that, in the singlet C atom reaction, products are very exothermic where nearly every collision yields a product that is determined by the initial encounter geometry. The singlet C atom reaction does not follow the minimum energy path because the bimolecular reaction is controlled by dynamics (i.e., initial orientation of encounter). On the other hand, in a 10 K solid Ar matrix, ground state C(³P) atoms do tend to follow RRKM kinetics. Thus, ab initio molecular dynamics (AIMD) results indicate that a significant fraction of C–H insertion occurs to form phenylcarbene whereas, in marked contrast to previous theoretical and experimental conclusions, the Ar matrix isolation studies indicate a large fraction of direct cycloheptatetraene formation, without the intermediacy of phenylcarbene. The AIMD calculations are more consistent with vaporized carbon atom experiments where labeling studies indicate the initial formation of phenylcarbene. This underlines that the availability of thermodynamic sinks can completely alter the observed reaction dynamics. 

Hans Peter Reisenauer,* Jaroslav Romanski, Grzegorz Mloston,* and Peter R. Schreiner,

Chem. Commun. 2013, 9467–9469. DOI: 10.1039/c3cc45379k

The atmospherically highly relevant methylsulfinyl radical CH3(O)S• was generated thermally under flash pyrolysis conditions and isolated in Ar matrices at 10 K; the allyl radical is a byproduct. CH3(O)S• and its D₃- and ¹³C-isotopologues were characterized through the excellent agreement between experimental and computed IR and UV/Vis spectra.

Hans-Peter Reisenauer,* Grzegorz Mloston,* Jaroslaw Romanski, and Peter R. Schreiner,

Eur. J. Org. Chem. 2012, 3408–3415. DOI: 10.1002/ejoc.201200146

Flash vacuum pyrolysis of 3,3,5,5-tetramethyl-1,2,4-trithiolane 1-oxide performed at 700 °C yields the 1-oxatrisulfan-3-yl radical (HOSS·) along with disulfur monoxide (S₂O) and diisopropyl sulfide, which were isolated in argon matrices at 10 K. Upon irradiation with UV light, the 1-oxatrisulfan-3-yl radical undergoes isomerization to the 1-oxatrisulfan-1-yl radical (HSSO·). Both radicals were identified by comparison of their computed and experimental IR and UV/Vis spectra. In addition, density functional theory (DFT) computations offer a plausible explanation of the most likely reaction mechanism, suggesting that the initial step is a 1,3-H shift with simultaneous ring opening. A 1-oxatrisulfane derivative formed thereby undergoes fragmentations via a radical and a competitive concerted pathway leading to the observed final products. The same mechanism also governs the thermal fragmentation of di-tert-butyl disulfide S-oxide. Its pyrolysis at 700 °C affords an analogous set of products, including the 1-oxatrisulfan-3-yl radical (HOSS·) as the key intermediate.

Udo H. Brinker, Alexander A. Bespokoev, Hans Peter Reisenauer, and Peter R. Schreiner

J. Org. Chem. 2012, 77, 3800–3807. DOI: 10.1021/jo3001035

Bicyclo[3.2.1]oct-6-en-8-ylidene (1) can assume either the conformation of “classical” carbene 1a or that of foiled carbene 1b in which the divalent carbon bends toward the double bond. Oxadiazoline precursors for the generation of 1 were prepared, followed by photochemical and thermal decomposition as well as flash vacuum pyrolysis (FVP) of a tosyl hydrazone sodium salt precursor, to give a number of rearrangement products. Matrix isolation experiments demonstrate the presence of a diazo intermediate and methyl acetate in all photochemical and thermal precursor reactions. The major product from rearrangements of “classical” bridged carbene 1a is bicyclo[3.3.0]octa-1,3-diene as a result of an alkyl shift, while dihydrosemibullvalene formed from a 1,3-C–H insertion. In contrast, thus far unknown strained bicyclo[4.2.0]octa-1,7-diene formed by a vinyl shift in foiled carbene 1b. The experimental results are corroborated by density functional theory (DFT), MP2, and G4 computations.

Peter R. Schreiner, Hans Peter Reisenauer, Jaroslaw Romanski, and Grzegorz Mloston

J. Am. Chem. Soc. 2010, 132, 7240–7241. Download

We describe the first preparation of the long sought after parent oxathiirane from sulfine through photochemical rearrangement with light at 313 ± 10 nm in an Ar-matrix at 11 K. Oxathiirane was characterized by the extraordinarily good agreement  of experimentally measured and CCSD(T)/cc-pVTZ  (unscaled) computed vibrational frequencies both for the perhydrogenated and perdeuterated species.  The title molecule is about 10 kcal mol–1 less stable than sulfine, in marked contrast to the isomer energy difference of dioxirane vs. carbonyl oxide (ca. –25 kcal mol–1).  This is due to the strong positive polarization (blue potential) vs. the highly electronegative oxygen atom (red).  The stability ordering and the relative energy differences of carbonyl vs. thiocarbonyl groups underlines the likely role oxathiiranes play in sulfur transfer reactions.

Grzegorz Mloston, Jaroslaw Romanski, Michael L. McKee, Hans-Peter Reisenauer,  and Peter R. Schreiner

Eur. J. Org. Chem. 2010, 2132–2137. Download

The products of the gas phase pyrolysis of two regioisomeric 1,2,4-trithiolane S-oxides were collected in an argon matrix at 10 K and studied by means of spectroscopic as well as computational methods. Whereas the main products of the pyrolysis of the ‘symmetrical’ S-oxide were identified as thioformaldehyde S-oxide and thioformaldehyde S-sulfide, the ‘non-symmetrical’ S-oxide gave predominantly dithioformic acid, which exists as a mixture of s-cis and s-trans conformers. We present a rationalization of the reaction pathways including density functional theory computations.

Peter R. Schreiner, Hans Peter Reisenauer, Jaroslaw Romanski, and Grzegorz Mloston 

Angew. Chem. Int. Ed. 2009, 48, 8133–8136. Download; Highlight: Neil Withers Nature Chem. 2009. Link; Robert Berger Nachr. Chem. 2010, 58, 7. Blog

Extremely rare: a CS triple bond can be assigned to HCSOH, a new molecule prepared through a photochemical [1,3]H-shift of sulfine H2C=S=O. But does this formal description agree with analyses on the basis of IR vibrations, bond lengths, bond orders, molecular orbitals, and compliance constants? Such types of molecules challenge and refine our current understanding of chemical bonding.

Adelina Nemirowski, Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston, and Peter R. Schreiner,

J. Phys. Chem. A, 2008, 50, 13244. Download

Abstract.  The current case study focuses on the generation, identification, and characterization of two representative mono- and disubstituted alkyl phosphonatocarbenes by means of matrix isolation techniques in conjunction with density functional theory [B3LYP/6-311++G(d,p)] and coupled cluster [CCSD(T)/cc-pVXZ, X = D, T] computations. The EPR measurements identify both carbenes as triplet ground-state species with D values of 0.660 and 0.623 cm–1 respectively, exhibiting persistency toward intramolecular reactions (the EPR signal observable in perfluoromethylcyclohexane up to around 70 K for the disubstituted molecule). While the reaction of the carbene center of the conformationally rich tetramethyl bisphosphonatocarbene with the CH bonds of the methyl groups leads to phosphaoxetane at room temperature, its fragmentation via a Wittig-type reaction during high vacuum flash pyrolysis (HVFP) results in dimethyl vinylphosphonate and methyl metaphosphate. The latter has been observed for the first time as an isolated entity.

Adelina Nemirowski and Peter R. Schreiner, J. Org. Chem. 2007, 72, 9533. Download

Abstract.  Based on systematic ab initio (CCSD(T)/cc-pVDZ) studies of substituent effects, we present a concept for the construction of electronically stabilized triplet ground state carbenes with singlet−triplet energy separations (∆E_ST) exceeding that of methylene. Sterically demanding and conjugating substituents were excluded from the selection of model compounds under investigation, as these either destabilize both the singlet and the triplet states or delocalize unpaired spins away from the carbene carbon. Negative partial charges on the carbene center allow for the prediction of the electronic stabilization of substituted carbenes. To decrease carbene reactivity, we chose b-substituents with strong polar bonds. Among them, highly electronegative elements such as fluorine and oxygen enlarge the ∆E_ST value with respect to hydrogen, while chlorine does not due to p-orbital participation.

Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston, and Peter R. Schreiner,

Eur. J. Org. Chem. 2006, 4813. Download

    Abstract.  Dimethoxycarbene was prepared from an oxadiazoline precursor under high-vacuum flash pyrolysis (HVFP) conditions and was trapped at low temperatures by matrix isolation techniques (Ar, 10 K). The excellent agreement between the computed [CCSD(T)/cc-pVDZ] IR spectrum of the mixture of conformers of dimethoxycarbene and the experimentally measured IR absorptions allows a detailed analysis ofthe conformational preference of dimethoxycarbene. ItsUV spectrum is in agreement with earlier studies and our TD-B3LYP/6-311+G(d,p) computations. The computed [CCSD(T)/cc-pVDZ] rotational profile is rather steep and separates the s-trans,s-trans and s-cis,s-trans conformers by a 16 kcal mol^-1 barrier, whilst the s-cis,s-cis conformer is too high-lying to be observable (+17 kcal mol^-1). In marked contrast with the gauche,gauche minimum of dimethoxymethane, the s-trans,s-trans conformer of dimethoxycarbene is slightly preferred (0.5 kcal mol^-1). The s-cis,s-trans conformer equilibrates at the high temperatures required during HFVP generation and both conformers can be identified in the IR spectrum of the argon matrix at 10 K. The conformational preference is partly due to the minimization of the overall dipole moment in the s-trans,s-trans conformer.

Peter R. Schreiner, Hans Peter Reisenauer, Kurt W. Sattelmeyer, and Wesley D. Allen,

J. Am. Chem. Soc. 2005, 127, 12156. Download