Photo-initiated ground state chemistry: How important is it in the atmosphere?

Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a dataset of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2-loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implication to global H2 production and tautomerisaton has implication to the atmospheric production of organic acids.

Evaluation of Dissociation Energies of the Diatomic (NdO, FeH and BaF) Molecules

The ground and excited state dissociation energies are determined by curve fitting techniques using the five parameters Hulburt-Hirschfelder (H-H) function. The estimated dissociation energies are 7.33± 0.15eV, 2.90 ± 0.13 eV and 6.04± 0.12eV for NdO, FeH and BaF respectively. The computed values are in good agreement with the literature values. The nature of binding is discussed in the light of the percentage ionic\characters of these molecules.

Multiswitchable photoacid–hydroxyflavylium–polyelectrolyte nano-assemblies

Light- and pH-responsive nano-assemblies with switchable size and structure are formed by the association of a photoacid, anthocyanidin, and a linear polyelectrolyte in aqueous solution. Specifically, anionic disulfonated naphthol derivatives, neutral hydroxyflavylium, and cationic poly(allylamine) are used as building blocks for the ternary electrostatic self-assembly, forming well-defined supramolecular assemblies with tunable sizes of 50 to 500 nm. Due to the network of possible chemical reactions for the anthocyanidin and the excited-state dissociation of the photoacid upon irradiation, different ways to alter the ternary system through external triggering are accessible. The structure and trigger effects can be controlled through the component ratios of the samples. Dynamic and static light scattering (DLS, SLS) and ζ-potential measurements were applied to study the size and the stability of the particles, and information on the molecular structure was gained by UV–vis spectroscopy. Isothermal titration calorimetry (ITC) provided information on the thermodynamics and interaction forces in the supramolecular assembly formation.

Kinetic Stability of Si2C5H2 Isomer with a Planar Tetracoordinate Carbon Atom

Dissociation pathways of the global minimum geometry of Si2C5H2 with a planar tetracoordinate carbon (ptC) atom, 2,7-disilatricyclo[4.1.0.01,3]hept-2,4,6-trien-2,7-diyl (1), have been theoretically investigated using density functional theory and coupled-cluster (CC) methods. Dissociation of Si-C bond connected to the ptC atom leads to the formation of 4,7-disilabicyclo[4.1.0]hept-1(6),4(5)-dien-2-yn-7-ylidene (4) through a single transition state. Dissociation of C-C bond connected to the ptC atom leads to an intermediate with two identical transition states and leads back to 1 itself. Simultaneous breaking of both Si-C and C-C bonds leads to an acyclic transition state, which forms an acyclic product, cis-1,7-disilahept-1,2,3,5,6-pentaen-1,7-diylidene (19). Overall, two different products, four transition states, and an intermediate have been identified at the B3LYP/6-311++G(2d,2p) level of theory. Intrinsic reaction coordinate calculations have also been done at the latter level to confirm the isomerization pathways. CC calculations have been done at the CCSD(T)/cc-pVTZ level of theory for all minima. Importantly, all reaction profiles for 1 are found be endothermic in Si2C5H2. These results are in stark contrast compared to the structurally similar and isovalent lowest-energy isomer of C7H2 with a ptC atom as the overall reaction profiles there have been found to be exothermic. The activation energies for Si-C, C-C, and Si-C/C-C breaking are found to be 30.51, 64.05, and 61.85 kcal mol−1, respectively. Thus, it is emphasized here that 1 is a kinetically stable molecule. However, it remains elusive in the laboratory to date. Therefore, energetic and spectroscopic parameters have been documented here, which may be of relevance to molecular spectroscopists in identifying this key anti-van’t-Hoff-Le Bel molecule.

Capturing roaming molecular fragments in real time

Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (H2CO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel–resolved measurements, but not in real-time observations of the roaming fragment itself. Using time-resolved Coulomb explosion imaging (CEI), we directly imaged individual “roamers” on ultrafast time scales in the prototypical formaldehyde dissociation reaction. Using high-level first-principles simulations of all critical experimental steps, distinctive roaming signatures were identified. These were rendered observable by extracting rare stochastic events out of an overwhelming background using the highly sensitive CEI method.

Photoresponsive Photoacid-Macroion Nano-Assemblies

In this study, light-responsive nano-assemblies with light-switchable size based on photoacids are presented. Anionic disulfonated napthol derivates and cationic dendrimer macroions are used as building blocks for electrostatic self-assembly. Nanoparticles are already formed under the exclusion of light as a result of electrostatic interactions. Upon photoexcitation, an excited-state dissociation of the photoacidic hydroxyl group takes place, which leads to a more highly charged linker molecule and, subsequently, to a change in size and structure of the nano-assemblies. The effects of the charge ratio and the concentration on the stability have been examined with absorption spectroscopy and ζ-potential measurements. The influence of the chemical structure of three isomeric photoacids on the size and shape of the nanoscale aggregates has been studied by dynamic light scattering and atomic force microscopy, revealing a direct correlation of the strength of the photoacid with the changes of the assemblies upon irradiation.

Photodissociation of Iso-Propoxy (I-C3H7O) Radical at 248 Nm

<p>Photodissociation of the <i>i</i>-C₃H₇O radical is investigated using fast beam photofragment translational spectroscopy. Neutral <i>i</i>-C₃H₇O radicals are produced through the photodetachment of a fast beam of <i>i</i>-C₃H₇O^- anions and are subsequently dissociated using 248 nm (5.0 eV). The dominant product channels are CH₃ + CH₃CHO and OH + C₃H₆ with some contribution from H + C₃H₆O. CH₃ and H loss are attributed to dissociation on the ground electronic state of <i>i</i>-C₃H₇O, but in a nonstatistical manner because RRKM dissociation rates exceed the rate of energy randomization. Translational energy and angular distributions for OH loss are consistent with ground state dissociation, but the branching ratio for this channel is considerably higher than predicted from RRKM rate calculations. These results corroborate what has been observed previously in C₂H₅O dissociation at 5.2 eV that yields CH₃, H, and OH loss. Additionally, <i>i</i>-C₃H₇O undergoes three-body fragmentation to CH₃ + CH₃ + HCO and CH₃ + CH₄ + CO. These three-body channels are attributed to dissociation of <i>i</i>-C₃H₇O to CH₃ + CH₃CHO, followed by secondary dissociation of CH₃CHO on its ground electronic state.</p>

Photodissociation of Iso-Propoxy (I-C3H7O) Radical at 248 Nm

<p>Photodissociation of the <i>i</i>-C₃H₇O radical is investigated using fast beam photofragment translational spectroscopy. Neutral <i>i</i>-C₃H₇O radicals are produced through the photodetachment of a fast beam of <i>i</i>-C₃H₇O^- anions and are subsequently dissociated using 248 nm (5.0 eV). The dominant product channels are CH₃ + CH₃CHO and OH + C₃H₆ with some contribution from H + C₃H₆O. CH₃ and H loss are attributed to dissociation on the ground electronic state of <i>i</i>-C₃H₇O, but in a nonstatistical manner because RRKM dissociation rates exceed the rate of energy randomization. Translational energy and angular distributions for OH loss are consistent with ground state dissociation, but the branching ratio for this channel is considerably higher than predicted from RRKM rate calculations. These results corroborate what has been observed previously in C₂H₅O dissociation at 5.2 eV that yields CH₃, H, and OH loss. Additionally, <i>i</i>-C₃H₇O undergoes three-body fragmentation to CH₃ + CH₃ + HCO and CH₃ + CH₄ + CO. These three-body channels are attributed to dissociation of <i>i</i>-C₃H₇O to CH₃ + CH₃CHO, followed by secondary dissociation of CH₃CHO on its ground electronic state.</p>

Photodissociation Dynamics of the Tert-Butyl Perthiyl Radical

<p>The photodissociation dynamics of the <i>tert</i>-butyl perthiyl (<i>t</i>-BuSS) radical are investigated by fast-beam coincidence translational spectroscopy. A fast (6-8 keV) beam of neutral <i>t</i>-BuSS radicals is produced via photodetachment of the corresponding anion, followed by photodissociation at 248 nm (5.00 eV) or 193 nm (6.42 eV) and coincident detection of the neutral products. Photofragment mass and translational energy distributions are obtained at both wavelengths. At 248 nm, the dominant product channel (90%) is found to be S loss, with a product translational energy distribution that peaks close to the maximum available energy and an anisotropic photofragment angular distribution, indicating dissociation along a repulsive excited state. A minor channel (10%) leading to the formation of S₂ + <i>t</i>-Bu is also observed. At 193 nm, both two- and three-body dissociation are observed. Formation of S₂ + <i>t</i>-Bu is the dominant two-body product channel, with multiple electronic states of the S₂ molecule produced via excited state dissociation processes. Formation of S + <i>t</i>-BuS is a minor two-body channel at this dissociation energy. The three-body channels are S₂ + H + isobutene, S₂ + CH₃ + propene, and S + SH + isobutene. The first two of these channels result from a sequential dissociation process in which loss of S₂ from <i>t</i>-BuSS results in ground state <i>t</i>-Bu with sufficient internal energy to undergo secondary fragmentation. The third three-body channel, S + SH + isobutene, is attributed to loss of internally excited HS₂ from <i>t</i>-BuSS, which then rapidly dissociates to form S + SH in an asynchronous concerted dissociation process. </p>

Photodissociation Dynamics of the Tert-Butyl Perthiyl Radical

<p>The photodissociation dynamics of the <i>tert</i>-butyl perthiyl (<i>t</i>-BuSS) radical are investigated by fast-beam coincidence translational spectroscopy. A fast (6-8 keV) beam of neutral <i>t</i>-BuSS radicals is produced via photodetachment of the corresponding anion, followed by photodissociation at 248 nm (5.00 eV) or 193 nm (6.42 eV) and coincident detection of the neutral products. Photofragment mass and translational energy distributions are obtained at both wavelengths. At 248 nm, the dominant product channel (90%) is found to be S loss, with a product translational energy distribution that peaks close to the maximum available energy and an anisotropic photofragment angular distribution, indicating dissociation along a repulsive excited state. A minor channel (10%) leading to the formation of S₂ + <i>t</i>-Bu is also observed. At 193 nm, both two- and three-body dissociation are observed. Formation of S₂ + <i>t</i>-Bu is the dominant two-body product channel, with multiple electronic states of the S₂ molecule produced via excited state dissociation processes. Formation of S + <i>t</i>-BuS is a minor two-body channel at this dissociation energy. The three-body channels are S₂ + H + isobutene, S₂ + CH₃ + propene, and S + SH + isobutene. The first two of these channels result from a sequential dissociation process in which loss of S₂ from <i>t</i>-BuSS results in ground state <i>t</i>-Bu with sufficient internal energy to undergo secondary fragmentation. The third three-body channel, S + SH + isobutene, is attributed to loss of internally excited HS₂ from <i>t</i>-BuSS, which then rapidly dissociates to form S + SH in an asynchronous concerted dissociation process. </p>