Two potential paths for OH radical formation on surfaces of pure water microdroplets.

Experimental findings by others suggest that OH radicals are formed in unexpected abundance on or near surfaces of 1-50 µm microdroplets comprised of pure water, but the mechanism by which these radicals are generated is not yet fully resolved. In this work, we examine two possibilities using ab initio electronic structure methods: (1) electron transfer (ET) from a microdroplet surface-bound OH- anion to a nearby H3O+ cation and (2) proton transfer (PT) from such a H3O+ cation to a nearby OH- anion. Our findings suggest that both processes are possible but only if the droplet's underlying water molecules comprising the microdroplet provide little screening of the Coulomb interaction between the anion and cation once they reach ∼10 Å of one another. In the ET event, an OH radical is formed directly; for PT, the OH formation occurs because the new O-H bond formed by the transferred proton is created at a bond length sufficiently elongated to permit homolytic cleavage. Both the ET and PT pathways predict that H atoms will also be formed. Finally, we discuss the roles played by strong local electric fields in mechanisms that have previously been proposed and that occur in our two mechanisms.

Strongly Bound Polynuclear Anions Comprising Scandium Fluoride Building Blocks.

The stability of polynuclear anions composed of ScF3 building blocks was studied by using ab initio and density functional theory electronic structure methods and flexible basis sets. Thorough exploration of ground state potential energy surfaces of (Sc2F7)-, (Sc3F10)-, and (Sc4F13)- anions which may be viewed as comprising ScF3 fragments and the additional fluorine atom led to determining the isomeric structures thereof. It was found that the most stable isomers which are predicted to dominate at room temperature correspond to the compact structures enabling the formation of a large number of Sc-F-Sc bridging linkages rather than to the chain-like structures. The vertical electron detachment energies of the (ScnF3n+1)- anions were found to be very large (spanning the 10.85-12.29 eV range) and increasing with the increasing number of scandium atoms (n) and thus the ScF3 building blocks involved in the structure. Thermodynamic stability of (ScnF3n+1)- anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

An ab initio study on the stability of isolated phosphaalkene synthons.

Using ab initio methods and flexible basis sets, we examined the electronic, geometric, and thermodynamic stabilities of selected phosphaalkene synthons matching the (PCR2)- formula (R = H, CH3, C6H5, C6F5, and Mes). All isolated synthons considered were found to be electronically stable and susceptible to neither fragmentation nor isomerization processes. The structures corresponding to the most stable isomers of the studied phosphaalkene synthons contain a PC double-bond (whose presence was confirmed by natural bond orbital occupancies of σ(P-C) and π(P-C) approaching 2 electrons) and two R substituents connected to the carbon atom in either (PCR2)- (for R = H, CH3, C6H5, and Mes) or (PCF-R-R)- (for R = C6F5) manner. Vertical electron detachment energies (spanning the 0.924-3.118 eV range) characterizing the phosphaalkene synthons were predicted and discussed.

Degradation of selected perfluoroalkyl substances (PFASs) using AlF3 in water.

The conversion of representative perfluorinated carboxylic acids and perfluorinated sulfonic acids in aqueous solutions into the corresponding perfluoroalkenes is investigated by using electronic structure methods. It is demonstrated that the use of aluminum trifluoride enables such conversions even at room temperature (with the reaction completion time not exceeding 1 minute). The mechanism of the reactions studied involves the withdrawal of F- from either the carboxylic or the sulfonic anion by AlF3 which leads to the formation of a stable AlF4- anion and a perfluoroalkene compound (which can be further decomposed to a series of nonfluorinated products) accompanied by CO2 or SO3 loss.

Finding Valence Antibonding Levels while Avoiding Rydberg, Pseudo-continuum, and Dipole-Bound Orbitals.

Electronic structure methods are now widely used to assist in the interpretation of many varieties of experimental data. The energies and physical characteristics (e.g., sizes, shapes, and spatial localization) of valence antibonding π* and σ* orbitals play key roles in a variety of chemical processes including photochemical reactions and electron attachment reductions and are used in Woodward-Hoffmann-type analyses to probe reaction energy barriers and energy surface intersections leading to internal conversion or intersystem crossings. One's ability to properly populate such valence antibonding orbitals within electronic structure calculations is often hindered by the presence of other molecular orbitals having similar energies. These intruding orbitals can be of Rydberg, pseudo-continuum, or dipole-bound characteristic. This article shows how, within the most widely available electronic structure codes, one can avoid the pitfalls presented by these intruding orbitals to properly populate a valence π* or σ* orbital and how to subsequently use that orbital in a calculation that includes electron correlation effects and thereby offers the possibility of chemically useful precision. Special emphasis is given to cases in which the electronic state is metastable.

Superhalogen Anions Supported by the Systems Comprising Alternately Aligned Boron and Nitrogen Central Atoms.

Using DFT/(B3LYP/wB97XD/B2PLYPD) and OVGF electronic structure methods with flexible atomic orbital basis sets, we examined the series of polynuclear superhalogen anions matching the (BF3(BN) n F4n+1)- formula (for n = 1-10,13,18-20) containing alternately aligned boron and nitrogen central atoms decorated with fluorine ligands. It was found that the equilibrium structures of these anions correspond to fully extended chains (with each B and N central atom surrounded by four substituents arranged in a tetrahedral manner) and thus mimic the globally stable fully extended (all-trans) conformations of higher n-alkanes. The vertical electron detachment energies of the (BF3(BN) n F4n+1)- anions were found to exceed 8 eV in all cases and gradually increase with the increasing number of n. The approximate limiting value of vertical electron binding energy that could be achieved for such polynuclear superhalogen anions was estimated as equal to ca. 10.7 eV.

The Mechanism of a Retro-Diels-Alder Fragmentation of Luteolin: Theoretical Studies Supported by Electrospray Ionization Tandem Mass Spectrometry Results.

The mechanisms of retro-Diels-Alder fragmentation of luteolin are studied theoretically using the Density Functional Theory method (B3LYP hybrid functional) together with the 6-311++G(d,p) basis set and supported by electrospray ionization tandem mass spectrometry (ESI-MS) results. The reaction paths leading to the formation of 1,3A- and 1,3B- fragment ions observed as the main spectral features in the ESI-MS spectrum are described and discussed, including the structures of the transition states and intermediate products. The heights of the activation energy barriers which have to be overcome along the reaction paths corresponding to 1,3-retrocyclization cleavage of the ionized luteolin are predicted to span the 69-94 kcal/mol range (depending on the initial isomeric structure) for the concerted retrocyclization mechanism and the 60-89 kcal/mol (first barrier) and 24-52 kcal/mol (second barrier) barriers for the stepwise mechanism (also depending on the initial isomeric structure). It is also demonstrated that the final fragmentation products (1,3A- and 1,3B-) are in fact represented by various isomeric systems which are not experimentally distinguishable. In addition, the absence of the spectral feature corresponding to the [M-B]- fragment ion formed by the rupture of the C-C bond connecting luteolin's B and C rings (which does not occur during the ESI-MS experiment) is explained by much larger energy barriers predicted for such a process.

Photoresponsive Amide-Based Derivatives of Azobenzene-4,4'-Dicarboxylic Acid-Experimental and Theoretical Studies.

Azobenzene derivatives are one of the most important molecular switches for biological and material science applications. Although these systems represent a well-known group of compounds, there remains a need to identify the factors influencing their photochemical properties in order to design azobenzene-based technologies in a rational way. In this contribution, we describe the synthesis and characterization of two novel amides (L1 and L2) containing photoresponsive azobenzene units. The photochemical properties of the obtained compounds were investigated in DMSO by UV-Vis spectrophotometry, as well as 1H NMR spectroscopy, and the obtained results were rationalized via Density Functional Theory (DFT) methods. After irradiation with UV light, both amides underwent trans to cis isomerization, yielding 40% and 22% of the cis isomer of L1 and L2 amides, respectively. Quantum yields of this process were determined as 6.19% and 2.79% for L1 and L2, respectively. The reverse reaction (i.e., cis to trans isomerization) could be achieved after thermal or visible light activation. The analysis of the theoretically determined equilibrium structure of the transition-state connecting cis and trans isomers on the reaction path indicated that the trans-cis interconversion is pursued via the flipping of the substituent, rather than its rotation around the N=N bond. The kinetics of thermal back-reaction and the effect of the presence of the selected ions on the half-life of the cis form were also investigated and discussed. In the case of L1, the presence of fluoride ions sped the thermal relaxation up, whereas the half-life time of cis-L2 was extended in the presence of tested ions.

Tracing the acid-base catalytic properties of MON2O mixed oxides (M = Be, Mg, Ca; N = Li, Na, K) by theoretical calculations.

The stability and acid-base properties of MON2O mixed oxides (where M = Be, Mg, Ca; N = Li, Na, K) are studied by using ab initio methods. It is demonstrated that (i) the basicity of such designed systems evaluated by estimation of electronic proton affinity and gas-phase basicity (defined as the electronic and Gibbs free energies of deprotonation processes for [MON2O]H+) were found significant (in the ranges of 272-333 and 260-322 kcal/mol, respectively); (ii) in each series of MOLi2O/MONa2O/MOK2O, the basicity increases with an increase of the atomic number of alkali metal involved; (ii) the Lewis acidity of the corresponding [MON2O]H+ determined with respect to hydride anion (assessed as the electronic and Gibbs free energies of H- detachment processes for [MON2O]H2) decreases as the basicity of the corresponding oxide increases. The thermodynamic stability of all [MON2O]H2 systems is confirmed by estimating the Gibbs free energies for the fragmentation processes yielding either H2 or H2O.

An Excess Electron Bound to Magnesium Halides and Basic Grignard Compounds (RMgX and RMgR, R = Me, Et, Ph; X = F, Cl, Br).

Grignard reagents are commonly used in organic synthesis, yet their ability to form stable anionic states has not been recognized thus far. In this work, representative examples of RMgF, RMgCl, and RMgBr molecules involving methyl, ethyl, and phenyl functional groups serving as R substituents are investigated regarding their equilibrium structures, adiabatic electron affinities, and vertical electron detachment energies of their daughter anions. The electronic stabilities determined for the negatively charged Grignard compounds are then compared to those predicted for their corresponding magnesium halides. The anions formed by RMgX (R = Me, Et, Ph; X = F, Cl, Br) molecules are found to be adiabatically electronically stable valence-bound systems characterized by relatively large vertical electron detachment energies spanning the 0.79-1.62 eV range. In addition, significant structural relaxation upon attachment of an excess electron is predicted for all Grignard compounds considered. Furthermore, the re-examination of the anions formed by magnesium halides resulted in recognizing them as valence-bound rather than dipole-bound anions, in contrast to the earlier interpretations.

Electron attachment to representative cations composing ionic liquids.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we investigated the electronic structure and stability of reduction products of selected representative cations (C+) constituting ionic liquids. We found that an electron attachment to such cations leads to the neutral radicals, whereas a subsequent attachment of another (i.e., excess) electron leads to adiabatically stable anions only in two cases {[P(CH3)4]- and [MeMePyr]-}. The possibility of the formation of various dimers (such as CC+, CC, and CC-) was also considered, and the resulting systems were characterized by predicting their lowest energy structures, ionization potentials, electron affinities, and susceptibilities to the fragmentation process. Among the cations studied, only the [MeMePyr]+ was found to form a typical Rydberg radical (MeMePyr) and double-Rydberg anion ([MeMePyr]-), whereas the remaining cations were predicted to form neutral radicals of a primarily valence (MeMeIm and MePy) or mixed Rydberg-valence [P(CH3)4] character. Our calculations confirmed the stability of all CC+ and CC dimers against fragmentation yielding the corresponding monomers (the binding energies of 12.2-20.5 kcal/mol and 11.3-72.3 kcal/mol were estimated for CC+ and CC dimers, respectively). [(MeMePyr)2]- was identified as the only adiabatically stable CC- dimeric anion having its vertical electron detachment energy of 0.417 eV. We also found that in the [(MeMePyr)2]- anionic state, three outermost electrons are described by Rydberg orbitals, which results in the (σ)2(σ*)1 configuration.

Caralumane Superacids of Lewis and Brønsted Character.

Carborane Brønsted superacids have proven to be useful reagents in a variety of organic and inorganic synthetic processes. In this work, analogs in which the icosahedral CB11 carborane core is replaced by a CAl11 core are studied using ab initio electronic structure tools. Each so-called caralumane Brønsted acid is formed by adding HF, HCl, or HH to a corresponding caralumane Lewis acid possessing a vacant Al-centered orbital that acts to accept an electron pair from the HF, HCl, or HH. The Lewis acid strengths of the species involved, as measured by their F- ion affinities, are all found to exceed the threshold for labeling them Lewis superacids. Also, the deprotonation Gibbs free energies of the Brønsted acids are found to be small enough for them to be Brønsted superacids. When HF or HCl is bound to a caralumane Lewis acid to form the Brønsted acid, the HF or HCl is bound datively to a single Al atom, and hydrogen bonds can be formed between this molecule's H atom and nearby F or Cl atoms attached to other Al atoms. In contrast, when HH is bound to the Lewis acid to form the Brønsted acid, two novel low-energy structures arise, both of which are Brønsted superacids. One has an essentially intact HH molecule attached to a single Al atom in a η2 fashion. In the other, the HH molecule is heterolytically cleaved to generate a hydride ion that attaches to a single Al atom and a proton that binds in a multicenter manner to other Al atoms. The structures and relative energies of a multitude of such caralumane Lewis and Brønsted superacids are provided and discussed.

Unusual and Conventional Dative Bond Formation by s2 Lone Pair Donation from Alkaline Earth Metal Atoms to BH3, AlH3, and GaH3.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns2 electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH3, AlH3, or GaH3. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH3, MgAlH3, MgGaH3, and CaBH3 are found to be much as one would expect for datively bound molecules, CaAlH3 and CaGaH3 were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH3 species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH3, AlH3, and GaH3 groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Fate of Dipole-Bound Anion States when Hydrated.

Many strongly polar molecules can form an anion by attaching an electron to either an empty or half-filled valence-bound (VB) orbital or a so-called dipole-bound (DB) orbital. These two families of orbitals can be very different in their radial extent (the former are usually more compact, while the latter are quite diffuse) and in the degree to which they are affected by surrounding solvent molecules. In this study, the effects of hydration (representative of strong solvation) on the DB state of a model polar species are investigated with an eye toward determining whether this state is stabilized or even persists when a few to 100 water molecules surround the polar molecule. It is found that in the presence of up to ca. 10-12 water molecules, the excess electron can remain in a DB orbital. However, once there are enough water molecules to form a complete first hydration shell (or more), the excess electron migrates into an orbital localized on the outer surface of the water solvent cage. These findings have implications on the possible role of DB states as doorways to facilitating electron attachment and subsequent electron transfer to VB states. It is shown that even when the electron is bound to the surface of the surrounding solvent, the dipole potential of the solute molecule can influence where on the surface the electron binds. It is also illustrated that using continuum dielectric methods to describe the hydration of DB states is fraught with danger because much of the outermost electron density in such states penetrates outside the boundary of the cavity used in these methods.

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages.

Theoretical investigation of the H(CHB11 X11 ) (X=H, F, Cl, CN), H(CHB11 Xn Y11-n ) (X,Y=F, Cl; n=1,5), and dimeric (H(CHB11 X11 ))2 (X=F, Cl) carborane superacids performed at the B3LYP/6-311++G(d,p) theory level revealed the similarity of their equilibrium structures and the possibility of nearly barrierless hydrogen atom migration among the substituents attached to one side of the icosahedral CB11 cage. The vertical electron detachment energies predicted at the OVGF/6-311++G(3df,2pd) theory level for the conjugate bases (CHB11 X11 )- were found to span the 5.82-9.00 ev range. The acid strengths (manifested by the Gibbs free deprotonation energies spanning the 213-266 kcal/mol range) predicted for the icosahedral H(CHB11 X11 ) carborane systems confirm their superacidic properties which might be increased even further by the attachment of the second carborane H(CHB11 X11 ) unit that leads to a dimeric structure mimicking a part of an experimentally observed H-bridged polymeric chain. The Gibbs free deprotonation energy of the dimeric (H(CHB11 Cl11 ))2 acid was predicted to be smaller by 17 kcal/mol than that of the corresponding monomeric H(CHB11 Cl11 ) acid.

The mechanisms of isobutene hydration yielding tert-butanol catalyzed by a strong mineral acid (H2SO4) and Lewis-Brønsted superacid (HF/SbF5).

The mechanism of the (CH3)2CCH2+H2O→(CH3)3COH reaction catalyzed by two strong acids (H2SO4 and HSbF6) was investigated theoretically using the ab initio MP2 and CCSD(T) methods and the aug-cc-pVDZ/LANL2DZ and aug-cc-pVTZ/LANL2DZ basis sets. The effects of surrounding solvent molecules were approximated by employing the polarized continuum solvation model. The most important findings include the observation that both acids are capable of catalyzing isobutene hydration but the reaction is predicted to proceed faster when the HSbF6 superacid plays the catalyst role.

The correlation-bound anion of p-chloroaniline.

The p-chloroaniline anion was generated by Rydberg electron transfer and studied via velocity-map imaging anion photoelectron spectroscopy. The vertical detachment energy (VDE) of the p-chloroaniline anion was measured to be 6.6 meV. This value is in accord with the VDE of 10 meV calculated by Skurski and co-workers. They found the binding of the excess electron in the p-chloroaniline anion to be due almost entirely to electron correlation effects, with only a small contribution from the long-range dipole potential. As such, the p-chloroaniline anion is the first essentially correlation-bound anion to be observed experimentally.

The Acid Strength of the Lewis-Brønsted Superacids - A QSPR Study.

The acidity of Lewis-Brønsted superacids can be derived from the theoretical calculations as the Gibbs free energy of the deprotonation reaction (ΔGacid ), which describes the tendency of a studied compound to donate a proton. This paper presents the first Quantitative Structure - Property Relationship (QSPR) model that correlates the ΔGacid of superacid (HF/MeX3 formula (X=F, Cl, Br)) with their structure. Developed model is well fitted, roubustness, has good predictive abilities, fulfills all OECD recommendation for good model. Obtained results provide the insight into the relation of structural features of superacids, which are responsible for their acid strength - the structures characterized by strong F-Me dative bond (with relatively large vibrational frequency), small positive partial atomic charge on Me central atom, possibly large polarity exhibit large acid strength. Such assumption can be used in the future as valuable information in the process of the designing new, stronger, more effective superacids.

Functionalization of the transition metal oxides FeO, CoO, and NiO with alkali metal atoms decreases their ionization potentials by 3-5 eV.

The existence and stabilities of various neutral metal oxides of formula MON and MON2 (M = Fe, Co, Ni; N = Li, Na) and their corresponding cations MON+ and MON2+ are predicted using density functional theory (B3LYP) with the 6-311 + G(d) basis set. Ab initio calculations carried out at the CCSD(T)/6-311 + G(3df) level of theory reveal that the ionization potentials (IPs) of the oxides MO decrease by ca. 3-5 eV upon functionalization with N to give either MON or MON2. The influences of the chemical constitution and local spin magnetic moment (on the transition metal atom) of the oxide or cation on its IP are presented and discussed.

Formation of Enormously Strongly Bound Anionic Clusters Predicted in Binary Superacids.

The possible formation of the (AlF4(HF) n)- ( n = 1-8 and 12), (AsF6(HF) n)-, and (SbF6(HF) n)- ( n = 1-6 and 12) anionic clusters of a superhalogen nature is predicted in the solutions of binary HF/AlF3, HF/AsF5, and HF/SbF5 Lewis-Brønsted superacids on the basis of ab initio calculations. Our results show that all systems investigated represent extremely strongly bound anions characterized by vertical electron detachment energies (VDEs) that significantly exceed 10 eV. The VDE values estimated for the (AlF4(HF)12)-, (AsF6(HF)12)-, and (SbF6(HF)12)- systems are predicted to be 13.96, 14.03, and 14.03 eV, respectively, and are the largest vertical electron detachment energies reported in the literature thus far.