Designing a σ0π2 singlet ground state carbene from dicationic carbones.

Carbenes with a σ0π2 singlet ground state are rare and little is known about their chemistry. Here, we study the potential formation of such carbenes by removal of two electrons from carbones/donor-substituted allenes. The desired electron configuration becomes favorable in the case of bis-diiminium substitution (CAAC motif).

From Criegee to Breslow: How π-Donors Steer the Route of Olefin Ozonolysis.

While π-bonds typically undergo cycloaddition with ozone, resulting in the release of much-noticed carbonyl O-oxide Criegee intermediates, lone-pairs of electrons tend to selectively accept a single oxygen atom from O3, producing singlet dioxygen. We questioned whether the introduction of potent electron-donating groups, akin to N-heterocyclic olefins, could influence the reactivity of double bonds - shifting from cycloaddition to oxygen atom transfer or generating lesser-known, yet stabilized, donor-substituted Criegee intermediates. Consequently, we conducted a comparative computational study using density functional theory on a series of model olefins with increasing polarity due to (asymmetric) π-donor substitution. Reaction path computations indicate that highly polarized double bonds, instead of forming primary ozonides in their reaction with O3, exhibit a preference for accepting a single oxygen atom, resulting in a zwitterionic species formally identified as a carbene-carbonyl adduct. This previously unexplored reactivity potentially introduces aldehyde umpolung chemistry (Breslow intermediate) through olefin ozonolysis. Considering solvent effects implicitly reveals that increased solvent polarity further directs the trajectories toward a single oxygen atom transfer reactivity by stabilizing the zwitterionic character of the transition state. The competing modes of chemical reactivity can be explained by a bifurcation of the reaction valley in the post-transition state region.

Conformations and Rearrangements of Collinolactone - Experiments and Theory on a Dynamic Cyclodecatriene.

Collinolactone A is a microbial specialized metabolite with a unique 6-10-7 tricyclic bislactone skeleton which was isolated from Streptomyces bacteria. The unusual cyclodecatriene motif features dynamic interconversions of two rotamers. Given the biological profiling of collinolactone A as neuroprotective agent, semisynthetic modifications represent an invaluable strategy to enhance its efficacy. Since understanding conformations and reactions of bioactive substances is crucial for rational structure-based design and synthesis of derivatives, we conducted computational studies on conformational behavior as well as experiments on thermal and acid induced rearrangements of the cyclodecatriene. Experimental conformer ratios of collinolactone A and its biosynthetic ketolactone precursor are well reproduced by computations at the PW6B95-D3/def2-QZVPP//r2 SCAN-3c level. Upon heating collinolactone A in anhydrous dioxane at 100 °C, three collinolactone B stereoisomers exhibiting enollactone structures form via Cope rearrangements. Our computations predict the energetic preference for a boat-like transition state in agreement with the stereochemical outcome of the main reaction pathway. Constriction of the ten-membered ring forms collinolactone C with four annulated rings and an exocyclic double bond. Computations and semisynthetic experiments demonstrate strong preference for an acid-catalyzed reaction pathway over an alternative Alder-ene route to collinolactone C with a prohibitive reaction barrier, again in line with stereochemical observations.

A Metastable Ketenyl Radical-Water Complex from UV Photolysis of the Carboxymethyl Radical.

The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical-molecule (and especially radical-water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C-H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C-H···O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C-O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.

Synthesis of Sterically Encumbered Thiourea S-Oxides through Direct Thiourea Oxidation.

Thiourea S-oxides can be viewed as formal analogs of the currently unknown diamino-substituted Criegee intermediates (urea O-oxides). However, the preparation of such S-oxides is rather challenging, and the direct oxidation of thioureas typically only leads to formation of desulfurized products. Employing the accurate revDSD-PBEP86-D4 double hybrid density functional, it was found that the peracid mediated oxidation of thiourea S-oxides exhibits a lower reaction barrier than the oxidation of the corresponding thiourea itself in contrast to most other ordinary thioketones. The undesired overoxidation reactivity, which is associated with strong π-donation from the thiourea's nitrogen atoms, can be partially suppressed by introduction of bulky substituents and the utilization of protic solvents. In this regard, we managed to prepare two sterically encumbered thiourea S-oxides in isolated yields of 35-40 %. The S-oxides are stable in the solid state and in alcoholic solutions at room temperature for extended periods of time, but swiftly decompose in aprotic solvents by disproportionation. A dimesityl-substituted thiourea S-oxide complexed with residual mCBA could be characterized by means of X-ray crystallography, confirming the importance of hydrogen bonding in the stabilization of the amino-substituted C=S+ -O- moiety.

The activated reaction of dichlorocarbene with triplet molecular oxygen.

The well-known dichlorocarbene (CCl2, 1) is deemed to undergo an extremely facile addition reaction with triplet molecular oxygen (3O2) under formation of the corresponding singlet Criegee intermediate, phosgene O-oxide. This is unexpected, because the carbene possesses a singlet ground state with a large singlet-triplet gap and, typically, only triplet carbenes react swiftly with triplet dioxygen. Hence, we deployed a careful theoretical study of this reaction and computed the oxygen addition barrier at levels of electron correlation as high as CCSD(T) and BD(TQ) and basis sets as large as cc-pV5Z. Our results firmly establish the existence of a reaction barrier, and we estimate its height to amount to 8.8 kcal mol-1. Furthermore, the initially formed triplet dioxygen adduct is prone to facile O-O bond breaking rendering phosgene and triplet oxygen atoms likely products of the overall reaction. As a general conclusion, we find that carbenes are ambiphiles in oxygen additions and more electrophilic as well as that more nucleophilic carbenes show greater reactivity.

2H-Imidazol-2-one O-Oxide: A Criegee Intermediate from a σ0π2 Singlet Ground-State Carbene.

Triplet carbenes and triplet molecular oxygen can combine to form singlet carbonyl O-oxide Criegee intermediates in an overall spin-allowed transformation. While this reaction runs at the diffusion limit in the case of triplet carbenes, singlet carbenes are commonly more reluctant to bind 3O2. In contradiction to this customarily encountered spin selectivity, the σ0π2 singlet ground-state carbene 2H-imidazol-2-ylidene 1 reacts extremely rapidly with 3O2 at temperatures as low as 30 K. The product of this cryogenic reaction is singlet 2H-imidazol-2-one O-oxide 7, an N-heterocyclic Criegee intermediate. The addition reaction becomes possible through the electrophilic activation of dioxygen in the triplet state, in which O2 binding can initially proceed without a barrier. Criegee intermediate 7 exhibits an unprecedented high O-O stretching vibration at 1105 cm-1, which can be explained by a resonance structure with an O2 double bond. Furthermore, 2H-imidazol-2-one 5 and spiro-dioxirane 6 could be identified as the photodecomposition products of the herein-reported carbonyl oxide.

σ0π2 Singlet Ground State Carbenes Undergo Least-Motion Reactions with H2 and Alkenes.

Ground state singlet carbenes commonly feature σ2π0 orbital occupations and are known for their concerted σ-bond insertion and cycloaddition reactions. Despite the facility of these transformations, orbital symmetry conservation forces them into non-least-motion π-approach reaction pathways. This situation completely changes when the singlet σ0π2 electron configuration becomes the ground state, which we show here by means of high-level CCSD(T) geometry optimizations. Carbenes like the experimentally known 2H-imidazol-2-ylidene react with H2 and ethylene with negligible or no barrier in a σ-fashion, which effectively corresponds to a least-motion reaction trajectory.

Infrared spectroscopy of the protonated HCl dimer and trimer.

The protonated HCl dimer and trimer complexes were prepared by pulsed discharges in supersonic expansions of helium or argon doped with HCl and hydrogen. The ions were mass selected in a reflectron time-of-flight spectrometer and investigated with photodissociation spectroscopy in the IR and near-IR regions. Anharmonic vibrational frequencies were computed with VPT2 at the MP2/cc-pVTZ level of theory. The Cl-H stretching fundamentals and overtones were measured in addition to stretch-torsion combinations. VPT2 theory at this level confirms the proton-bound structure of the dimer complex and provides a reasonably good description of the anharmonic vibrations in this system. The trimer has a HCl-HClH+-ClH structure in which a central chloronium ion is solvated by two HCl molecules via hydrogen bonding. VPT2 reproduces anharmonic frequencies for this system, including several combinations involving core ion Cl-H stretches, but fails to describe the relative band intensities.

Criegee Intermediates in Autoxidation Reactions: Mechanistic Considerations.

Products of Criegee intermediate (CI) chemistry were recently detected in radical chain autoxidation reactions involving ß-hydroxyperoxy radicals. Here, we demonstrate by means of accurate G4 computations that direct scission of the latter to CIs and radical byproducts is thermodynamically highly unfavorable. Instead, the reaction becomes possible through a hydrogen abstraction reaction that could proceed by reversible formation of a dimeric tetroxide and a subsequent [1,6] hydrogen shift of the hydroxy hydrogen.

Intramolecular London Dispersion Interactions Do Not Cancel in Solution.

We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two t-butyl groups are in close proximity (H···H distance ≈ 2.5 Å), but they are far apart in the unfolded 1,4-isomer (H···H distance ≈ 7 Å). We determined the relative strengths of these noncovalent intramolecular σ-σ interactions via temperature-dependent nuclear magnetic resonance measurements. The origins of the interactions were elucidated with energy decomposition analysis at the density functional and ab initio levels of theory, pinpointing the predominance of London dispersion interactions enthalpically favoring the folded state in any solvent measured.

Infrared spectroscopy of H+(CO)2 in the gas phase and in para-hydrogen matrices.

The H+(CO)2 and D+(CO)2 molecular ions were investigated by infrared spectroscopy in the gas phase and in para-hydrogen matrices. In the gas phase, ions were generated in a supersonic molecular beam by a pulsed electrical discharge. After extraction into a time-of-flight mass spectrometer, the ions were mass selected and probed by infrared laser photodissociation spectroscopy in the 700 cm-1-3500 cm-1 region. Spectra were measured using either argon or neon tagging, as well as tagging with an excess CO molecule. In solid para-hydrogen, ions were generated by electron bombardment of a mixture of CO and hydrogen, and absorption spectra were recorded in the 400 cm-1-4000 cm-1 region with a Fourier-transform infrared spectrometer. A comparison of the measured spectra with the predictions of anharmonic theory at the CCSD(T)/ANO1 level suggests that the predominant isomers formed by either argon tagging or para-hydrogen isolation are higher lying (+7.8 kcal mol-1), less symmetric isomers, and not the global minimum proton-bound dimer. Changing the formation environment or tagging strategy produces other non-centrosymmetric structures, but there is no spectroscopic evidence for the centrosymmetric proton-bound dimer. The formation of higher energy isomers may be caused by a kinetic effect, such as the binding of X (=Ar, Ne, or H2) to H+(CO) prior to the formation of X H+(CO)2. Regardless, there is a strong tendency to produce non-centrosymmetric structures in which HCO+ remains an intact core ion.

Difficulties of Popular Density Functionals to Describe the Conformational Isomerism in Iodoacetic Acid.

Matrix isolation studies in solid argon and neon at 4.2 K reveal that iodoacetic acid initially only exists as its ground state (c,x) conformer with an almost perpendicular I-C-C═O dihedral angle, but UV irradiation in the 240-255 nm range leads to population of the 0.8 kcal mol-1 less stable (c,c) isomer. The latter structure exhibits a close 3.23 Å contact of the iodine and carbonyl oxygen atoms decidedly below the sum of their van der Waals radii (3.50 Å). Increasing the matrix temperature by only a few Kelvin triggers the thermal back reaction of (c,c) to (c,x) and leads to an estimated upper limit of 0.38 kcal mol-1 for the associated torsional barrier. While wave function methods including completely uncorrelated Hartree-Fock theory have no problem to identify (c,c) as a proper minimum, many popular density functionals fail to describe the C-C torsional potential in cis-iodoacetic acid qualitatively correct. We assessed the performance of 12 density functionals of different levels of sophistication, namely, the BLYP, PBE, TPSS, B3LYP, BHandHLYP, PBE0, M06-2X, CAM-B3LYP, ωB97X-D3, B2-PLYP, B2GP-PLYP, and DSD-PBEP86 methods, against accurate extrapolated CCSD(T)/CBS(T-Q)//MP2/def2-TZVPP energies and found that almost all of them yield acceptable relative energies. Still, even some of the best performers fail to find a reasonably deep minimum in the region of the (c,c) conformer, and addition of the empirical D3-dispersion correction does not remedy the qualitative shortcoming. Instead, inclusion of a sufficient amount of (long-range) exact exchange and likely a proper treatment of medium-range correlation effects all along the torsional coordinate play an important role in the proper description of the sub-van der Waals iodine-oxygen contact. More modern, recommended functionals do not suffer from the described shortcoming.

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2.

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O-O bond scission is hampered by a barrier of 11.9 kcal mol-1 at the ROCCSD(T)/cc-pVTZ/UB3LYP/6-311G(d,p) level of theory.

The Role of Tunneling in the Spectra of H5+ and D5+ up to 7300 cm-1.

The spectra for H5+ and D5+ are extended to cover the region between 4830 and 7300 cm-1. These spectra are obtained using mass-selected photodissociation spectroscopy. To understand the nature of the states that are accessed by the transitions in this and prior studies, we develop a four-dimensional model Hamiltonian. This Hamiltonian is expressed in terms of the two outer H2 stretches, the displacement of the shared proton from the center of mass of these two H2 groups, and the distance between the H2 groups. This choice is motivated by the large oscillator strength associated with the shared proton stretch and the fact that the spectral regions that have been probed correspond to zero, one, and two quanta of excitation in the H2 stretches. This model is analyzed using an adiabatic separation of the H2 stretches from the other two vibrations and includes the non-adiabatic couplings between H2 stretch states with the same total number of quanta of excitation in the H2 stretches. Based on the analysis of the energies and wave functions obtained from this model, we find that when there are one or more quanta of excitation in the H2 stretches the states come in pairs that reflect tunneling doublets. The states accessed by the transitions in the spectrum with the largest intensity are assigned to the members of the doublets with requisite symmetry that are localized on the lowest-energy adiabat for a given level of H2 excitation.

Gauging stability and reactivity of carbonyl O-oxide Criegee intermediates.

In this study, we evaluated the effect of substitution on the stability and reactivity of carbonyl O-oxide Criegee intermediates (CIs). In this regard, we computed a set of more than 50 carbonyl oxides at the CBS-QB3 level of theory and assessed their stability by means of an isodesmic reaction equation defining a carbonyl oxide stabilization energy (COSE). Almost all substituents are stabilizing and amino groups in particular leading to COSE values of almost 60 kcal mol-1. As opposed to π-donors, substituents with a strong σ-electron pull destabilize the C[double bond, length as m-dash]O-O group. Furthermore, we studied how the intrinsic stabilization of the Criegee intermediate is reflected in its C[double bond, length as m-dash]O and O-O bond lengths as well as the partial charges on the individual atoms of the carbonyl oxide moiety. As a potential measure for reactivity, we determined the adiabatic singlet-triplet energy gap of all carbonyl oxides. Amino substituted CIs exhibit high-lying triplet states and have relatively large barriers towards addition of water or the OH radical. However, the ΔES-T cannot serve as a rigorous measure for carbonyl oxide reactivity.

Mid-Infrared Spectroscopy of C7H7+ Isomers in the Gas Phase: Benzylium and Tropylium.

Both prominent C7H7+ isomers, the benzylium and the tropylium cations, were generated in an electrical discharge/supersonic expansion from toluene and cycloheptatriene precursors. Their infrared spectra were measured in the region of 1000-3500 cm-1 using photodissociation of the respective argon- and nitrogen-tagged complexes with a broadly tunable OPO/OPA laser system. Spectral signatures of both isomers were observed independent of the precursor, albeit in different relative intensities. The spectra were assigned based on scaled harmonic B3LYP-D3/cc-pVTZ frequency computations and comparisons to previous experimental studies. Consistent with its high symmetry, only two bands were observed for the (nitrogen-tagged) tropylium ion at 3036 and 1477 cm-1, corresponding to C-H stretching and C-C-H deformation/C═C stretching vibrations, respectively. Furthermore, the C-H stretching region of the benzylium ion is reported for the first time.

Communication: Infrared photodissociation spectroscopy of the H6+ cation in the gas phase.

The H6+ cation was generated in a pulsed-discharge supersonic expansion of hydrogen and mass-selected in a time-of-flight spectrometer. Its vibrational spectrum was measured in the region of 2050-4550 cm-1 using infrared photodissociation with a tunable OPO/OPA laser system. The H6+ photodissociates, producing H5+, H4+, and H3+ fragments; each of these fragment channels has a different spectrum. Computational studies identify two low-lying isomers described in previous work, whose energies were evaluated at the CCSD(T)/cc-pVTZ//MP2/cc-pVTZ level of theory. A D2d species having an H2+ cation bridging between two perpendicular H2 molecules is the global minimum structure. A Cs structure with an H3+ core ion bound to both H2 and an H atom lies 4.0 kcal mol-1 higher in energy. Anharmonic vibrational spectra were computed for each of these isomers with second-order vibrational perturbation theory (VPT2) in combination with density functional theory at the B2PLYP/cc-pVTZ level. The comparison between experimental and predicted spectra confirms the presence of both the D2d and Cs structures and explains the spectra in different fragmentation channels. Although we find reasonable agreement between the experiment and the spectra predicted by VPT2 computations, a more sophisticated computational approach is needed to better understand this complex system.

Spectroscopy of Proton Coordination with Ethylenediamine.

Protonated ethylenediamine monomer, dimer, and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C2H8N2, en) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm-1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a gauche conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm-1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol-1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer's NH3+ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm-1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within 2 kcal mol-1 of the experimentally observed structure. The spectrum of the trimer is rather diffuse, and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum.

Intricate Conformational Tunneling in Carbonic Acid Monomethyl Ester.

Disentangling internal and external effects is a key requirement for understanding conformational tunneling processes. Here we report the s- trans/ s- cis tunneling rotamerization of carbonic acid monomethyl ester (1) under matrix isolation conditions and make comparisons to its parent carbonic acid (3). The observed tunneling rate of 1 is temperature-independent in the 3-20 K range and accelerates when using argon instead of neon as the matrix material. The methyl group increases the effective half life (τeff) of the energetically disfavored s- trans-conformer from 3-5 h for 3 to 11-13 h for 1. Methyl group deuteration slows the rotamerization further (τeff ≈ 35 h). CCSD(T)/cc-pVQZ//MP2/aug-cc-pVTZ computations of the tunneling probability suggest that the rate should be almost unaffected by methyl substitution or its deuteration. Thus the observed relative rates are puzzling, and they disagree with previous explanations involving fast vibrational relaxation after the tunneling event facilitated by the alkyl rotor.