Detection and structure of HOON: microwave spectroscopy reveals an O-O bond exceeding 1.9 Å

Photochemistry of Microsolvated Nitrous Acid: Observation of the Water-Separated Complex of Nitric Oxide and Hydroxyl Radical

The photochemistry of nitrous acid (HONO) plays a crucial role in atmospheric chemistry as it serves as a key source of hydroxyl radicals (OH) in the atmosphere; however, our comprehension of the underlying mechanism for the photochemistry of HONO especially in the presence of water is far from being complete as the transient intermediates in the photoreactions have not been observed. Herein, we report the photochemistry of microsolvated HONO by water in a cryogenic N2 matrix. Specifically, the 1:1 hydrogen-bonded water complex of HONO was facially prepared in the matrix through stepwise photolytic O2 oxidation of the water complex of imidogen (NH-H2O) via the intermediacy of the elusive water complex of peroxyl isomer HNOO. Upon photolysis at 193 nm, the matrix-isolated HONO-H2O complex decomposes by yielding the ternary water complex of OH and NO due to the matrix cage effect. The identification of this rare water-separated radical pair (OH-H2O-NO) with matrix-isolation infrared and ultraviolet-visible spectroscopy is aided by D, 15N, and 18O isotope labeling and quantum chemical calculations at the (U)CCSD/AVTZ level of theory, and its most stable structure exhibits separate hydrogen bonding interactions of the OH and NO radicals with H2O via OH···OH2 and ON···HOH contacts, respectively. This ternary complex is extremely unstable, as it undergoes spontaneous radical recombination to reform the HONO-H2O complex in the temperature range of 4-12 K through quantum-mechanical tunneling with 16/18O, H/D, 14/15N kinetic isotopic effects of 1.43, 2.33, and 0.91, respectively. At increased temperatures from 15 to 21 K, the recombination proceeds predominantly by overcoming the activation barrier with an estimated height of 0.12(1) kcal/mol.

Direct Observation of HOON Intermediate in the Photochemistry of HONO

The photochemistry of nitrous acid (HONO), encompassing dissociation into OH and NO as well as the reverse association reaction, plays a pivotal role in atmospheric chemistry. Here, we report the direct observation of nitrosyl-O-hydroxide (HOON) in the photochemistry of HONO, employing matrix-isolation IR and UV-vis spectroscopy. Despite a barrier of approximately 30 kJ/mol, HOON undergoes spontaneous rearrangement to the more stable HONO isomer through quantum mechanical tunneling, with a half-life of 28 min at 4 K. Kinetic isotope effects and instanton theory calculations reveal that the tunneling process involves the concerted motion of the NO moiety (65.2%) and the hydrogen atom (32.3%). Our findings underscore the significance of HOON as a key intermediate in the photolytic dissociation-association cycle of HONO at low temperatures.

The location of the chemical bond. Application of long covalent bond theory to the structure of silica

Oxygen is the most abundant terrestrial element and is found in a variety of materials, but still wanting is a universal theory for the stability and structural organization it confers. Herein, a computational molecular orbital analysis elucidates the structure, stability, and cooperative bonding of α-quartz silica (SiO₂). Despite geminal oxygen-oxygen distances of 2.61-2.64 Å, silica model complexes exhibit anomalously large O-O bond orders (Mulliken, Wiberg, Mayer) that increase with increasing cluster size-as the silicon-oxygen bond orders decrease. The average O-O bond order in bulk silica computes to 0.47 while that for Si-O computes to 0.64. Thereby, for each silicate tetrahedron, the six O-O bonds employ 52% (5.61 electrons) of the valence electrons, while the four Si-O bonds employ 48% (5.12 electrons), rendering the O-O bond the most abundant bond in the Earth's crust. The isodesmic deconstruction of silica clusters reveals cooperative O-O bonding with an O-O bond dissociation energy of 4.4 kcal/mol. These unorthodox, long covalent bonds are rationalized by an excess of O 2p-O 2p bonding versus anti-bonding interactions within the valence molecular orbitals of the SiO₄ unit (48 vs. 24) and the Si₆O₆ ring (90 vs. 18). Within quartz silica, oxygen 2p orbitals contort and organize to avoid molecular orbital nodes, inducing the chirality of silica and resulting in Möbius aromatic Si₆O₆ rings, the most prevalent form of aromaticity on Earth. This long covalent bond theory (LCBT) relocates one-third of Earth's valence electrons and indicates that non-canonical O-O bonds play a subtle, but crucial role in the structure and stability of Earth's most abundant material.

Unusual In-plane Aromaticity Facilitates Intramolecular Hydrogen Transfer in Long-Bonded cis-Isonitrosyl Methoxide

The hydrogen-atom transfer from methoxy radical to nitric oxide, leading to the formation of formaldehyde and nitroxyl, represents a secondary reaction of photodissociation of methyl nitrite, which is used as rocket fuel. In this study, we explored the potential energy profile of the hydrogen-atom transfer using the electronic structure calculations at the DLPNO-CCSD(T)/aug-cc-pVTZ level of theory for two isomeric forms (cis and trans) of the pre-reaction complex. The cis-oriented pre-reaction complex has a weak elongated O─O bond, which gets further elongated in the hydrogen transfer transition state. This O─O bond stabilizes the pre-reaction complex by 32.9 kJ/mol. The O─O-induced stabilization is even greater for the transition state (48.2 kJ/mol), which was unexpected because of the larger O─O distance in the transition state structure. To address this paradox, we performed the electronic structure analysis of the reaction participants using the valence bond (VB) theory, natural resonance theory, topological analysis of the electron density and its derivatives, and analysis of the electron localization function distribution. This combined analysis led to the conclusion that the cis-transition state for hydrogen transfer, instead of being directly stabilized by the O─O interaction, gained substantial stabilization from the in-plane five-center six-electron aromaticity.

Exceptionally Long Covalent CC Bonds-A Local Vibrational Mode Study

For decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D3d form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C−C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C−C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants ka(CC) and related bond strength orders BSO n(CC), computed at the ωB97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C−C bond, electronic effects can lead to even longer and weaker C−C bonds. Within our set of molecules the electron deficient ethane radical cation, in D3d symmetry, acquires the longest C−C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C−C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO n value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C−C bonds.

The Triplet Hydroxyl Radical Complex of Phosphorus Monoxide

Phosphorus monoxide (^. PO) is a key intermediate in phosphorus chemistry, and its association with the hydroxyl radical (^. OH) to yield metaphosphorous acid (cis-HOPO) contributes to the chemiluminescence in the combustion of phosphines. When photolyzing cis-HOPO in an Ar-matrix at 2.8 K, the simplest dioxophosphorane HPO₂ and an elusive hydroxyl radical complex (HRC) of ^. PO form. This prototypical radical-radical complex reforms into cis-HOPO at above 12.0 K by overcoming a barrier of 0.28±0.02 kcal mol^-1 . The vibrational spectra of this HRC and its D- and ¹⁸ O-isotopologues suggest a structure of ^. OH⋅⋅⋅OP^. , for which a triplet spin multiplicity with a binding energy of -3.20 kcal mol^-1 has been computed at the UCCSD(T)-F12a/aug-cc-pVTZ level.

Hydrogen-Atom Tunneling in Metaphosphorous Acid

Metaphosphorous acid (HOPO), a key intermediate in phosphorus chemistry, has been generated in syn- and anti-conformations in the gas phase by high-vacuum flash pyrolysis (HVFP) of a molecular precursor ethoxyphosphinidene oxide (EtOPO→C₂ H₄ +HOPO) at ca. 1000 K and subsequently trapped in an N₂ -matrix at 2.8 K. Unlike the two conformers of the nitrogen analogue HONO, the anti-conformer of HOPO undergoes spontaneous rotamerization at 2.8 K via hydrogen-atom tunneling (HAT) with noticeable kinetic isotope effects for H/D (>10⁴ for DOPO) and ¹⁶ O/¹⁸ O (1.19 for H¹⁸ OPO and 1.06 for HOP¹⁸ O) in N₂ -matrices.

Caged Nitric Oxide-Thiyl Radical Pairs

S-Nitrosothiols (RSNO) are exogenous and endogenous sources of nitric oxide in biological systems due to facile homolytic cleavage of the S-N bonds. By following the photolytic decomposition of prototypical RSNO (R = Me and Et) in Ne, Ar, and N₂ matrixes (<10 K), elusive caged radical pairs consisting of nitric oxide (NO•) and thiyl radicals (RS•), bridged by O···S and H···N connections, were identified with IR and UV/vis spectroscopy. Upon red-light irradiation, both caged radical pairs (RS•···•ON) vanish and reform RSNO. According to the calculation at the CASPT2(10,8)/cc-pVDZ level (298.15 K), the dissociation energy of MeS•···•ON amounts to 4.7 kcal mol^-1.

NOO Peroxy Isomer Exposed with Velocity-Map Imaging

The chemistry of NO₂, a key atmospheric trace gas, has historically been interpreted in terms of the C_2v isomer ONO, with the peroxy isomer NOO only postulated to be stable. In this work, a velocity-map-imaged photoelectron spectrum of the nitrite anion, NO₂^-, reveals energetic-electron structure that may only occur by photodetachment from the NOO^-(X̃¹A') isomer. This measurement defines NOO(X̃²A') bond frequencies and an electron affinity of only 335(30) cm^-1, which, supported by ab initio calculations, confirm the first observation of this important reactive species.

Microwave spectral taxonomy: A semi-automated combination of chirped-pulse and cavity Fourier-transform microwave spectroscopy

Because of its structural specificity, rotational spectroscopy has great potential as an analytical tool for characterizing the chemical composition of complex gas mixtures. However, disentangling the individual molecular constituents of a rotational spectrum, especially if many of the lines are entirely new or unknown, remains challenging. In this paper, we describe an empirical approach that combines the complementary strengths of two techniques, broadband chirped-pulse Fourier transform microwave spectroscopy and narrowband cavity Fourier transform microwave spectroscopy, to characterize and assign lines. This procedure, called microwave spectral taxonomy, involves acquiring a broadband rotational spectrum of a rich mixture, categorizing individual lines based on their relative intensities under series of assays, and finally, linking rotational transitions of individual chemical compounds within each category using double resonance techniques. The power of this procedure is demonstrated for two test cases: a stable molecule with a rich spectrum, 3,4-difluorobenzaldehyde, and products formed in an electrical discharge through a dilute mixture of C2H2 and CS2, in which spectral taxonomy has enabled the identification of propynethial, HC(S)CCH.

Generalized Valence Bond Description of Chalcogen-Nitrogen Compounds. III. Why the NO-OH and NS-OH Bonds Are So Different

Crabtree et al. recently reported the microwave spectrum of nitrosyl-O-hydroxide (trans-NOOH), an isomer of nitrous acid, and found that this molecule has the longest O-O bond ever observed: 1.9149 Å ± 0.0005 Å. This is in marked contrast to the structure of the valence isoelectronic trans-NSOH molecule, which has a normal NS-OH bond length and strength. Generalized valence bond calculations show that the long bond in trans-NOOH is the result of a weak through-pair interaction that singlet couples the spins of the electrons in singly occupied orbitals on the hydroxyl radical and nitrogen atom, an interaction that is enhanced by the intervening lone pair of the oxygen atom in NO. The NS-OH bond is the result of the formation of a stable recoupled pair bond dyad, which accounts for both its length and strength.

Expanding the applicability of electrostatic potentials to the realm of transition states

Central to any reaction mechanism study, and sometimes a challenging job, is tracing a transition state in a reaction path. For the first time, electrostatic potentials (ESP) of the reactants were used as guiding tactics to predict whether there is a possibility of any transition state in a reaction surface. The main motive behind this strategy is to see whether the directionality nature of the transition state has something to do with the anisotropic natures of the ESP with their embedded directionalities. Strategically, some atmospherically important, but simple, reactions have been chosen for this study, which heretofore were believed to be barrierless. By carefully analysing the ESP maps of the reactants, regions of possible interactions were located. Using the bilinear interpolation of the 2D grids of the ESP surfaces, search co-ordinates were fine-tuned for a local gradient based approach for the search of a transition state. Out of the three reactions studied in this work, we were able to successfully locate transition states, for the first time, in two cases and the third one still proved to be barrierless. This gives a clear indication that though ESP maps can qualitatively predict the possibility of a transition state; it is not always true that there should definitely be a transition state, as some of the reaction surfaces may genuinely be barrierless. But, nevertheless this strategy definitely has credential to be tested for many more reactions, either new or already established, and may be applied to create the initial search co-ordinates for any well-established transition state search method. Moreover, we have observed that the analysis of the ESP maps of the reactants were very much useful in explaining the nature of interactions existing in those observed transition states and we hope the same can also be extended to any transition state in an electrostatically driven reaction potential energy surface.

A Database of Formation Enthalpies of Nitrogen Species by Compound Methods (CBS-QB3, CBS-APNO, G3, G4)

Accurate thermochemical data for compounds containing C/H/N/O are required to underpin kinetics simulation and modeling of the reactions of these species in different environments. There is a dearth of experimental data so computational quantum chemistry has stepped in to fill this breach and to verify whether particular experiments are in need of revision. A number of composite model chemistries (CBS-QB3, CBS-APNO, G3, and G4) are used to compute theoretical atomization energies and hence enthalpies of formation at 0 and 298.15 K, and these are benchmarked against the best available compendium of values, the Active Thermochemical Tables or ATcT. In general the agreement is very good for some 28 species with the only discrepancy being for hydrazine. It is shown that, although individually the methods do not perform that well, collectively the mean unsigned error is <1.7 kJ mol(-1); hence, this approach provides a useful tool to screen published values and validate new experimental results. Using multiple model chemistries does have some drawbacks but can produce good results even for challenging molecules like HOON and CN2O2. The results for these smaller validated molecules are then used as anchors for determining the formation enthalpies of larger species such as methylated hydrazines and diazenes, five- and six-membered heterocyclics via carefully chosen isodesmic working reactions with the aim of resolving some discrepancies in the literature and establishing a properly validated database. This expanded database could be useful in testing the performance of computationally less-demanding density function methods with newer functionals that have the capacity to treat much larger systems than those tested here.

New coordination features; a bridging pyridine and the forced shortest non-covalent distance between two CO32- species

The aerobic reaction of the multidentate ligand 2,6-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-pyridine, H4L, with Co(ii) salts in strong basic conditions produces the clusters [Co4(L)2(OH)(py)7]NO3 (1) and [Co8Na4(L)4(OH)2(CO3)2(py)10](BF4)2 (2). Analysis of their structure unveils unusual coordination features including a very rare bridging pyridine ligand or two trapped carbonate anions within one coordination cage, forced to stay at an extremely close distance (dO···O = 1.946 Å). This unprecedented non-bonding proximity represents a meeting point between long covalent interactions and "intermolecular" contacts. These original motifs have been analysed here through DFT calculations, which have yielded interaction energies and the reduced repulsion energy experimented by both CO32- anions when located in close proximity inside the coordination cage.