Matrix Isolation Spectroscopy Research Articles

Regularities and Anomalies in Neon Matrix Shifts of Hydrogen-Bonded O-H Stretching Fundamentals.

O-H bond stretching vibrations in hydrogen-bonded complexes embedded into cryogenic neon matrices are subtly downshifted from cold gas phase reference wavenumbers. To the extent that this shift is systematic, it enables neon matrices as more universally applicable spectroscopic benchmark environments for quantum chemical predictions. Outliers are indicative of either an assignment problem in one of the two cryogenic experiments or they reveal interesting dynamics or structural effects on the complexes as a function of the environment. We compile 6 literature-known pairs of experimental data in jet and neon matrix expansions and realize a 6-fold expansion of that number through targeted matrix isolation and/or slit jet expansion spectroscopy presented in this work. In many cases, the neon matrix shift is less than the uncertainty of the currently best-performing blind quantum chemical predictions for the gas phase, but in specific cases, it may exceed the currently achievable theoretical accuracy. Some evidence for a positive correlation of the matrix shift with the hydrogen bond shift is found, similar to observations for helium nanodroplets. Outliers in particular for water acting as a donor are discussed, and in a few cases they call for a future reinvestigation. Substantial improvement in the correlation of the matrix shift with the hydrogen bond shift is achieved for ketone monohydrates by removing a vibrational resonance. New insights into nitrile hydration isomerism are obtained, and the linear OH stretching spectrum of the jet-cooled ammonia-water complex is presented for the first time. Vibrational spectroscopy in weakly perturbing solid rare gas quantum matrices for the benchmarking of gas phase theory and future explicit theoretical treatments of the quantum matrix environment to better understand the outliers are both encouraged.

Experimentally Delineating the Catalytic Effect of a Single Water Molecule in the Photochemical Rearrangement of the Phenylperoxy Radical to the Oxepin-2(5H)-one-5-yl Radical.

Catalysis plays a pivotal role in both chemistry and biology, primarily attributed to its ability to stabilize transition states and lower activation free energies, thereby accelerating reaction rates. While computational studies have contributed valuable mechanistic insights, there remains a scarcity of experimental investigations into transition states. In this work, we embark on an experimental exploration of the catalytic energy lowering associated with transition states in the photorearrangement of the phenylperoxy radical-water complex to the oxepin-2(5H)-one-5-yl radical. Employing matrix isolation spectroscopy, density functional theory, and post-HF computations, we scrutinize the (photo)catalytic impact of a single water molecule on the rearrangement. Our computations indicate that the barrier heights for the water-assisted unimolecular isomerization steps are approximately 2-3 kcal mol-1 lower compared to the uncatalyzed steps. This decrease directly coincides with the energy difference in the required wavelength during the transformation (Δλ = λ546nm - λ579nm ≡ 52.4-49.4 = 3.0 kcal mol-1), allowing us to elucidate the differential transition state energy in the photochemical rearrangement of the phenylperoxy radical catalyzed by a single water molecule. Our work highlights the important role of water catalysis and has, among others, implications for understanding the mechanism of organic reactions under atmospheric conditions.

Investigation of Isolated IrF5 -, IrF6 - Anions and M[IrF6] (M=Na, K, Rb, Cs) Ion Pairs by Matrix-Isolation Spectroscopy and Relativistic Quantum-Chemical Calculations.

The molecular IrF5 -, IrF6 - anions and M[IrF6] (M=Na, K, Rb, Cs) ion pairs were prepared by co-deposition of laser-ablated alkali metal fluorides MF with IrF6 and isolated in solid neon or argon matrices under cryogenic conditions. The free anions were obtained as well by co-deposition of IrF6 with laser-ablated metals (Ir or Pt) as electron sources. The products were characterized in a combined analysis of matrix IR spectroscopy and electronic structure calculations using two-component quasi-relativistic DFT methods accounting for spin-orbit coupling (SOC) effects as well as multi-reference configuration-interaction (MRCI) approaches with SOC. Inclusion of SOC is crucial in the prediction of spectra and properties of IrF6 - and its alkali-metal ion pairs. The observed IR bands and the computations show that the IrF6 - anion adopts an Oh structure in a nondegenerate ground state stabilized by SOC effects, and not a distorted D4h structure in a triplet ground state as suggested by scalar-relativistic calculations. The corresponding "closed-shell" M[IrF6] ion pairs with C3v symmetry are stabilized by coordination of an alkali metal ion to three F atoms, and their structural change in the series from M=Na to Cs was proven spectroscopically. There is no evidence for the formation of IrF7, IrF7 - or M[IrF7] (M=Na, K, Rb, Cs) ion pairs in our experiments.

Generation and Identification of the Trifluorosilylarsinidene F3SiAs and Isomeric Perfluorinated Arsasilene FAsSiF2.

The trifluorosilylarsinidene F3SiAs in the triplet ground state has been generated through the reaction of laser-ablated silicon atoms with AsF3 in cryogenic Ne- and Ar-matrices. The reactions proceed with the initial formation of perfluorinated arsasilene FAsSiF2 in the singlet ground state by two As-F bonds insertion reaction on annealing. The trifluorosilylarsinidene F3SiAs was formed via F-migration reactions of FAsSiF2 under irradiation at UV light (λ = 275 nm). The characterization of FAsSiF2 and F3SiAs by IR matrix-isolation spectroscopy is supported by computations at CCSD(T)-F12/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory.

Isolated Dimers Versus Solid-State Dimers of N-Heteropolycycles: Matrix-Isolation Spectroscopy in Concert with Quantum Chemistry.

In this work, matrix-isolation spectroscopy and quantum-chemical calculations are used together to analyse the structure and properties of weakly bound dimers of the two isomers benzo[a]acridine and benzo[c]acridine. Our measured experimental electronic absorbance spectra agree with simulated spectra calculated for the equilibrium structures of the dimers in gas-phase, but in contrast, disagree with the simulated spectra calculated for the structures obtained by optimising the experimental solid-state structures. This highlights the sensitivity of the electronic excitations with respect to the dimer structures. The comparison between the solid-state and gas-phase dimers shows how far the intermolecular interactions could change the geometric and electronic structure in a disordered bulk material or at device interfaces, imposing consequences for exciton and charge mobility and other material properties.

A DFT and Matrix–Isolation IR/UV-Visible Study of High-Coordinated Lanthanide-CO Complexes

Recent joint mass spectrometric and IR photodissociation studies have provided proof on the existence of octa-coordinated ionic lanthanide-carbonyl complexes under those extreme gaseous conditions. In contrast, in older literature concerning cryogenic studies of neutral Ln(CO)x species, the highest coordination was assigned to hexa-coordinated Ln(CO)6 molecules. The present study aims to clarify the above controversy using matrix isolation spectroscopy and DFT calculations. In order to ensure the maximum possible coordination, the Ln(CO)x complexes were synthesized in neat CO cryogenic matrices at 10 K and were investigated by infrared and UV-visible spectroscopy. The formed complexes were identified on the basis of the characteristic CO stretching frequencies of the ground-state molecules predicted by DFT calculations. Our joint experimental–theoretical analysis confirmed the preference of octa-coordinated Ln(CO)8 complexes in cryogenic neat CO matrices.

Isolation, Structural and Physical Characterization as well as Reactivity of Persistent Acenium Radical Cation Salts.

The unsubstituted acenium radical cations (ARCs) are extremely sensitive and were hitherto only studied in situ, i. e. in the gas phase, as dilute solutions in strong acids or by matrix isolation spectroscopy at about 10 K. In this study, room temperature stable ARC salts with the weakly coordinating anion [F{Al(ORF )3 }2 ]- (ORF =-OC(CF3 )3 ) supported by the weakly coordinating solvent 1,2,3,4-tetrafluorobenzene (TFB) were prepared and structurally, electrochemically and spectroscopically characterized. Reaction of the neutral acenes with Ag+ [F{Al(ORF )3 }2 ]- led, non-innocent,[54] to intermediate [Ag2 (acene)2 ]2+ complexes, which decompose over time to Ag0 and the corresponding (impure) ARC salts. By contrast, direct deelectronation with the recently developed innocent[54] deelectronator radical cation salt [anthraceneHal ]+⋅ [F{Al(ORF )3 }2 ]- led to phase-pure products [acene]+⋅ [F{Al(ORF )3 }2 ]- (anthraceneHal =9,10-dichlorooctafluoroanthracene; acene=anthra-, tetra-, pentacene). For the first time, a homogenous set of spectroscopic data on analytically pure ARC salts was obtained. In addition, cyclovoltammetric measurements of the acenes connected the potentials in solution with those in the gas-phase. Hence, the data complement the existing isolated gas-phase, strong acid or matrix isolation studies. A first entry to follow-up chemistry of the acenium radical cations as ligand forming oxidizers was demonstrated by reaction with Co2 (CO)8 giving [Co(anthracene)(CO)2 ]+ .

Vibrationally resolved electronic spectroscopy of rhodamine B cation at less than 5 K: combining gas phase absorption and matrix isolation fluorescence spectroscopy

Helium tagging photodissociation action spectroscopy in the gas phase together with laser-induced fluorescence (LIF) spectroscopy in neon matrix has been used to probe the photophysics of electrosprayed rhodamine B cation cooled to < 5 K and studied in two different minimally perturbing environments ([RhB•He]+ as well as [RhB]+ in solid neon). Helium tagging experiments were performed using a newly developed apparatus He-TAG. LIF measurements were carried out using an improved matrix-isolation spectroscopy machine Depo-II. The resulting, partially vibrationally resolved electronic spectra of the S0→S1, resp. S1→S0 transition provide benchmarks for gauging the theoretical description of this prototypical organic dye. As a first approach, we have compared our spectra to TD-DFT-based predictions of vibronic structure at the Franck-Condon (FC) and Franck-Condon Herzberg-Teller (FCHT) levels. Depletion action and fluorescence spectroscopy of rhodamine B cation at < 5 K

Ultraviolet Photodissociation of Proteinogenic Amino Acids.

The ultraviolet photochemistry of the amino acids glycine, leucine, proline, and serine in their neutral forms was investigated using parahydrogen matrix-isolation spectroscopy. Irradiation by 213 nm light destroys the chirality of all three chiral amino acids as a result of the α-carbonyl C-C bond cleavage and hydrocarboxyl (HOCO) radical production. The temporal behavior of the Fourier-transform infrared spectra revealed that HOCO radicals rapidly reach a steady state, which occurs predominantly due to photodissociation of HOCO into CO + OH or CO2 + H. In glycine and leucine, the amine radicals generated by the α-carbonyl C-C bond cleavage rapidly undergo hydrogen elimination to yield methanimine and 3-methylbutane-1-imine, respectively. Breaking of the α-carbonyl C-C bond in proline appeared to yield 1-pyrroline, although due to its weak absorption it remains unconfirmed. In serine, additional products were formaldehyde and E/Z ethanimine. The present study shows that the direct production of HOCO previously observed in α-alanine generalizes to other amino acids of varying structure. It also revealed a tendency for amino acid photolysis to form imines rather than amine radicals. HOCO should be useful in the search for amino acids in interstellar space, particularly in combination with simple imine molecules.

Electronic spectroscopy of heptacene ions in the search for carriers of diffuse interstellar bands

Context. The absorption bands of the interstellar medium (ISM) in the optical and near-infrared regions called the diffuse interstellar bands (DIBs) have been known for almost a century, yet their origins remain largely unknown. Knowledge of molecular carriers of DIBs would allow for a much better understanding of the chemistry and physics of the ISM. Polycyclic aromatic hydrocarbons (PAHs) and, among them, polyacenes have been suggested as promising candidates for carriers of DIBs. Aims. In this paper, we report on the spectroscopy of heptacene (Hep), the polyacene molecule consisting of seven aromatic rings in a linear arrangement, in its cationic and anionic forms (Hep+/−). The performed spectroscopic studies made it possible to accurately determine the Hep+/− absorption band positions and to conduct a direct comparison of laboratory and observational spectra. Methods. We utilized helium-tagging action spectroscopy to measure the spectra of Hep+/− in a wide spectral range of 3000–13 000 Å. In most cases, the spectra obtained by this method can be directly compared with the observational spectra. By analyzing the spectral shift as a function of the number of attached helium atoms, we obtained precise estimates of the gas-phase band positions. Quantum-chemical computations were used to support and interpret the findings. Matrix isolation spectroscopy provided information on the spectral properties of neutral Hep and extended the spectral range for Hep+. Results. We found several absorption bands characterized by a rather large full width at half maximum in the spectra of Hep+/−. The two most intense bands were found at 4714 ± 5 Å and 12 250 ± 12 Å for Hep+ and at 4673 ± 14 Å and 11326 ± 4 Å for Hep−. We did not find any good match between laboratory and observational spectra. In particular, the intrinsic width of the absorption bands of Hep+/− is much higher than that of most observed DIBs. Conclusions. The non-detection of Hep+/− in the observational spectra excludes the bottom-up formation route for polyacenes in the ISM. Larger polyacene molecules could still be considered as potential carriers of DIBs in the case of an efficient top-down formation route. All currently measured polyacene ions exhibit relatively broad absorption bands. Therefore, additional spectroscopy studies of neutral polyacenes and larger polyacene ions as well as the study of possible top-down formation routes are suggested.

Fluorescence Excitation and Dispersed Fluorescence Spectra of the 1-Hydronaphthyl Radical (1-C10H9) in Solid para-Hydrogen.

Matrix isolation spectroscopy with para-hydrogen (p-H2) has previously been employed to record IR absorption spectra of hydrogenated and protonated polycyclic aromatic hydrocarbons (PAHs), prospective carriers of unidentified infrared and diffuse interstellar bands. Despite the promising prospects of p-H2 as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H2 matrix isolation spectroscopy has rarely been applied to study electronic transitions of guest molecules. Here, we present the dispersed fluorescence and fluorescence excitation spectrum of the 1-hydronaphthyl radical (1-C10H9) isolated in solid p-H2. We observed a strong 000 band associated with the electronic transition to the first excited electronic state at 18881 cm-1, red-shifted by ∼68 cm-1 relative to a value reported for jet-cooled 1-C10H9. From a comparison of our experimental results to simulated vibrationally resolved electronic absorption and emission spectra computed on the basis of (TD-)DFT geometry optimizations and scaled harmonic vibration calculations using the FCclasses code, we derived assignments for observed vibronic transitions. The dispersed fluorescence spectrum of 1-C10H9 is new; it complements the infrared spectrum and identified many vibrational modes unidentifiable with infrared. The excitation spectrum covers a much wider spectral range than previous reports. We compare the excitation spectrum in solid p-H2 to the reported electronic absorption spectrum of jet-cooled gaseous 1-C10H9 and that of 1-C10H9 isolated in solid Ne to assess the influence of p-H2 as a matrix host on the electronic transition of 1-C10H9 and discuss a potential contribution of 1-C10H9 to the diffuse interstellar bands.

Infrared and Laser-Induced Fluorescence Spectra of Sumanene Isolated in Solid para-Hydrogen.

The para-hydrogen (p-H2) matrix-isolation technique has been scarcely used to record electronic absorption and emission spectra. It is expected that its small matrix shifts due to diminished molecular interactions and the softness of the lattice might be advantageous to help identify the carriers of the diffuse interstellar bands. In this article, we present infrared, fluorescence excitation, and dispersed fluorescence spectra of sumanene (C21H12), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C60, isolated in solid p-H2. The recorded vibrational wavenumbers from infrared and dispersed fluorescence agree with the scaled harmonic vibrational wavenumbers calculated with the B3PW91/6-311++G(2d,2p) and B3LYP/6-311++G(2d,2p) methods. The recorded fluorescence excitation spectra are consistent with the spectra of jet-cooled gas-phase C21H12 reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm-1 for the S1-S0 electronic transition origin located at 27 888 cm-1. Vibrational wavenumbers associated with the S1 state of C21H12 inferred from the experimental spectrum can be assigned mostly to fundamental normal modes; they are in satisfactory agreement with scaled harmonic vibrational wavenumbers calculated at the TD-B3PW91/6-311++G(2d,2p) level of theory. Significantly more vibrational modes of the S1 state were identified as compared with those in the reported gas-phase work. The potential of p-H2 matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C21H12 isolated in solid p-H2 and in solid Ne, a matrix host commonly employed in astrochemistry.

Photo-Induced Reactions between Glyoxal and Hydroxylamine in Cryogenic Matrices.

In this paper, the photochemistry of glyoxal–hydroxylamine (Gly–HA) complexes is studied using FTIR matrix isolation spectroscopy and ab initio calculations. The irradiation of the Gly–HA complexes with the filtered output of a mercury lamp (λ > 370 nm) leads to their photoconversion to hydroxyketene–hydroxylamine complexes and the formation of hydroxy(hydroxyamino)acetaldehyde with a hemiaminal structure. The first product is the result of a double hydrogen exchange reaction between the aldehyde group of Gly and the amino or hydroxyl group of HA. The second product is formed as a result of the addition of the nitrogen atom of HA to the carbon atom of one aldehyde group of Gly, followed by the migration of the hydrogen atom from the amino group of hydroxylamine to the oxygen atom of the carbonyl group of glyoxal. The identification of the products is confirmed by deuterium substitution and by MP2 calculations of the structures and vibrational spectra of the identified species.

Vibrational-Excitation-Induced and Spontaneous Conformational Changes in Solid Para-H2—Diminished Matrix Effects

Both vibrational-excitation-induced (by (N)IR laser) and spontaneous (by H atom tunneling) conformational changes are often investigated by matrix-isolation spectroscopy. It is well known that rigid hosts, such as solid noble gases, N2, or normal-H2, can largely affect both the quantum efficiency of the (N)IR photon-induced process and the tunneling rate. In the present study, the conformational changes of formic and acetic acids, as well as glycine, were investigated in a soft quantum host, solid para-H2. It is shown that the tunneling rates in para-H2 are orders of magnitude larger than those in rigid hosts. Furthermore, our results also suggest that the quantum efficiencies of some (N)IR-light-induced conformational changes are larger than in rigid matrices. These results can open a door for the applications of para-H2 host in conformational and tunneling studies and can help understand the details of these complex processes.

Spectroscopy of Alkali Atoms in Solid Matrices of Rare Gases: Experimental Results and Theoretical Analysis

We present an experimental and theoretical investigation of the spectroscopy of dilute alkali atoms in a solid matrix of inert gases at cryogenic temperatures, specifically Rubidium atoms in a solid Argon or Neon matrix, and related aspects of the interaction energies between the alkali atoms and the atoms of the solid matrix. The system considered is relevant for matrix isolation spectroscopy, and it is at the basis of a recently proposed detector of cosmological axions, exploiting magnetic-type transitions between Zeeman sublevels of alkali atoms in a magnetic field, tuned to the axion mass, assumed in the meV range. Axions are one of the supposed constituents of the dark matter (DM) of the Universe. This kind of spectroscopy could be also relevant for the experimental search of new physics beyond the Standard Model, in particular the search of violations of time-reversal or parity-charge-conjugation (CP) symmetry. In order to efficiently resolve the axion-induced transition in alkali-doped solid matrices, it is necessary to reduce as much as possible the spectral linewidth of the electronic transitions involved. The theoretical investigation presented in this paper aims to estimate the order of magnitude of the inhomogeneous contribution to the linewidth due to the alkali–matrix interactions (Coulomb/exchange and dispersion), and to compare the theoretical results with our experimental measurements of spectra of dilute Rubidium atoms in Argon and Neon solid matrix. The comparison of the expected or measured spectral linewidths will be important for selecting the most appropriate combination of alkali atoms and matrix inert elements to be used in the proposed axion detection scheme. It is finally suggested that dilute Lithium atoms diffused in a cold parahydrogen solid matrix could be, overall, a good system upon which the proposed detector could be based.

Rotational and Vibrational Spectra of the Pyridyl Radicals: A Coupled-Cluster Study.

Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm-1 were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron-spin, spin-spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.

Spectroscopic characterization of two peroxyl radicals during the O2-oxidation of the methylthio radical

The atmospheric oxidation of dimethyl sulfide (DMS) yields sulfuric acid and methane sulfonic acid (MSA), which are key precursors to new particles formed via homogeneous nucleation and further cluster growth in air masses. Comprehensive experimental and theoretical studies have suggested that the oxidation of DMS involves the formation of the methylthio radical (CH3S•), followed by its O2-oxidation reaction via the intermediacy of free radicals CH3SOx• (x = 1–4). Therefore, capturing these transient radicals and disclosing their reactivity are of vital importance in understanding the complex mechanism. Here, we report an optimized method for efficient gas-phase generation of CH3S• through flash pyrolysis of S-nitrosothiol CH3SNO, enabling us to study the O2-oxidation of CH3S• by combining matrix-isolation spectroscopy (IR and UV–vis) with quantum chemical computations at the CCSD(T)/aug-cc-pV(X + d)Z (X = D and T) level of theory. As the key intermediate for the initial oxidation of CH3S•, the peroxyl radical CH3SOO• forms by reacting with O2. Upon irradiation at 830 nm, CH3SOO• undergoes isomerization to the sulfonyl radical CH3SO2• in cryogenic matrixes (Ar, Ne, and N2), and the latter can further combine with O2 to yield another peroxyl radical CH3S(O)2OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CH3S(O)2OO• to CH3SO2•, CH2O, SO2, and SO3. The IR spectroscopic identification of the two peroxyl radicals CH3SOO• and CH3S(O)2OO• is also supported by 18O- and 13C-isotope labeling experiments.

UV Photolysis Study of Para-Aminobenzoic Acid Using Parahydrogen Matrix Isolated Spectroscopy

Many sunscreen chemical agents are designed to absorb UVB radiation (and in some cases UVA) to protect the skin from sunlight, but UV absorption is often accompanied by photodissociation of the chemical agent, which may reduce its UV absorption capacity. Therefore, it is important to understand the photochemical processes of sunscreen agents. In this study, the photolysis of para-aminobenzoic acid (PABA), one of the original sunscreen chemical agents, at three different UV ranges (UVA: 355 nm, UVB: &gt;280 nm, and UVC: 266 nm and 213 nm) was investigated using parahydrogen (pH2) matrix isolation Fourier-Transform Infrared (FTIR) Spectroscopy. PABA was found to be stable under UVA (355 nm) irradiation, while it dissociated into 4-aminylbenzoic acid (the PABA radical) through the loss of an amino hydrogen atom under UVB (&gt;280 nm) and UVC (266 nm and 213 nm) irradiation. The radical production supports a proposed mechanism of carcinogenic PABA-thymine adduct formation. The infrared spectrum of the PABA radical was analyzed by referring to quantum chemical calculations, and two conformers were found in solid pH2. The PABA radicals were stable in solid pH2 for hours after irradiation. The trans-hydrocarboxyl (HOCO) radical was also observed as a minor secondary photoproduct of PABA following 213 nm irradiation. This work shows that pH2 matrix isolation spectroscopy is effective for photochemical studies of sunscreen agents.

Photochemistry of phosphenic chloride (ClPO2): isomerization with chlorine metaphosphite (ClOPO) and reduction by carbon monoxide

Phosphenic chloride (ClPO2) is an elusive congener of nitryl chloride (ClNO2). By high-vacuum flash pyrolysis of 2-chloro-1,3,2-dioxaphospholane in the gas phase, ClPO2 has been efficiently generated and subsequently isolated in cryogenic N2, Ar, and CO matrices (10 K) for a first time study on its photochemistry. Upon 193 nm laser irradiation, ClPO2 isomerizes to the novel chlorine metaphosphite (ClOPO) by initial cleavage of the Cl-P bond (→ ˙Cl + ˙PO2) with subsequent Cl-O bond formation inside the N2 and Ar matrix cages. The reverse transformation becomes feasible under further irradiation at 266 nm. This photochemistry is consistent with the observed absorptions of ClPO2 and ClOPO at 207 and 250 nm, respectively. When the photolysis was performed in solid CO ice, no isomerization occurs due to CO-trapping of the initially generated ˙Cl atoms by forming caged radical pair ClCO˙⋯˙PO2. Concomitantly, photolytic reduction of ClPO2 to ClPO by CO has been observed, yielding a weakly bonded molecular complex consisting of ClPO and CO2 bonded through short intermolecular C⋯O contact (2.910 Å). The characterization of ClPO, ClPO2, ClOPO, and the molecular complexes of ClPO2-CO and ClPO-CO2 using matrix-isolation IR and UV-vis spectroscopy is supported by the theoretical calculations at the B3LYP/6-311 + G(3df) level, and the photochemistry of ClPO2 is also compared with the revisited photochemistry of ClNO2 in the N2-matrix.

Characterization of a Triplet Vinylidene

Singlet vinylidenes (R2C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm–1 and |E|/D = 0.028) were determined, and the 13C hyperfine coupling tensor was obtained by 13C-ENDOR measurements. Most strikingly, the isotropic 13C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.