Carol A. Deakyne

Proton affinities and gas-phase basicities: theoretical methods and structural effects

Abstract The inherent effects of molecular structure on acid–base behavior have been a topic of debate for over 30 years. Three of the models developed to probe these effects are reviewed in this article. Each model is described briefly and the results from its application to representative systems are summarized. The first model was proposed by Taft and coworkers and subdivides intrinsic substituent effects on proton-transfer equilibria into field effects, polarizability effects and resonance effects. Various techniques used to factor out the individual contributions of these effects are described. The importance of intramolecular hydrogen bonding in determining proton affinity is also discussed. The second model was proposed by Maksic and Vianello and separates the protonation process into three sequential steps in which an electron is removed from the base, the electron is captured by the proton, and a homolytic chemical bond is formed between the radical cation and hydrogen atom. The third model was proposed by Perez and coworkers and analyzes substituent effects on proton affinity with the aid of global and local descriptors of reactivity used in conjunction with the hard and soft acids and bases principle. Several of the composite methods designed to calculate accurate thermochemical data are also discussed briefly.

Hydrogen bonding between the nitrate anion (conventional and peroxy forms) and the water molecule

Ab initio quantum mechanical methods have been applied to the complexes of the nitrate anion (conventional D3h NO−3 and Cs ONOO− forms) with the water molecule, namely, NO−3⋅H2O and ONOO−⋅H2O. Equilibrium geometries and vibrational frequencies for a number of energy minima on the ground state potential energy surface have been determined using analytic energy first and second derivative techniques. The highest level of theory employed is the self‐consistent field method with a double‐zeta plus polarization basis set. The minimum energy structure (lying ∼15 kcal/mol below separated NO−3 +H2O) incorporates two hydrogen bonds in C2v symmetry. This theoretical dissociation energy agrees well with experiment. A second, more conventional single‐hydrogen‐bond structure is predicted to lie 2.6 kcal/mol higher in energy. At the most reliable level of theory, no distinct minimum was found for the hydrogen bonded structure between HNO3 (nitric acid) and OH−. Several minima (high lying energetically) between ONOO− an...

Noble gas compounds and chemistry: a brief review of interrelations and interactions with fluorine-containing species

Abstract This review is composed of six vignettes. They deal with respectively: the reaction of Xe and PtF 6 ; the reaction of O 2 and O 3 with PtF 6 ; salts of O + , the covalent OF, and noble gas-containing cations; synthesis, reactions and structure of [XeO 2 ] + en route to [XeF] + salts; [Xe 2 ] + , green and related species; neutral xenon oxides, nonmetal oxyanions, and a nonmetal fluoride “mid-valence” crisis. Interrelations and interactions are emphasized.

Experimental and theoretical study of the reaction of HO− with NO

Hydroxide ion (HO−) reacts with nitric oxide by slow reactive electron detachment with a rate coefficient ∼4×10−12 cm3 s−1 at 298 K. The detachment process is presumably associative detachment forming nitrous acid and an electron. Observations, data analysis, and alternative explanations for these observations are discussed. The associative detachment reaction was also investigated theoretically through calculations of the geometries, relative energies, and normal‐mode vibrational frequencies of the relevant species HO−, HO, NO, cis‐ and trans‐HONO, and cis‐ and trans‐HONO−. These calculations indicate that in the ion HONO−, the cis conformer is more stable, while in the neutral HONO, the trans conformer is more stable. The HO–NO bond in HONO, which is formed in this reaction, is much stronger than the HO−–NO bond in HONO− with an energy of 198.7±1.8 kJ mol−1 for cis‐HONO [J. Phys. Chem. Ref. Data 14, 1 (1985)] and 52.2±5 kJ mol−1 for cis‐HONO− at 0 K. HONO− is bound with respect to HONO. The adiabatic el...

Filling of solvent shells about ions: Part 3. Isomeric clusters of (HCN)n(NH3)mH+

Abstract Clustering enthalpies of (HCN) n (NH 3 ) m H + ions have been studied via high pressure mass spectrometry and ab initio calculations. Fully optimized geometries have been obtained for n + m ⩽ 5 at the HF/6–31G ∗ level of calculation. Five isomeric forms of the (HCN) n (NH 3 ) m H + ions have been considered. All of the complexes were found to contain an NH + 4 core ion. One isomer has all of the ligands bound to the NH + 4 ion whereas the other four have one of the ligands bound to another ligand. The former isomer is the most stable for all clusters; however, the latter isomers containing an NCH ⋯ NH 3 or an NCH ⋯ NCH hydrogen bond become competitive in stability as n + m increases. MP2/6–31 + G(2d,2p)//HF/6-31G ∗ stabilization enthalpies adjusted for BSSE have been compared to the experimental values. With two exceptions the values agree to within ± kJmol −1 . The agreement between the calculated and experimental Δ S ° and Δ G ° values is less good at ± 27 J K −1 mol −1 and ± 12 kJ mol −1 , respectively. The smooth decrease with n in the plot of -Δ H n −1 °, n vs. n for (HCN) n (NH 3 )H + and the ab initio results for (HCN) n (NH 3 )H + and (HCN)(NH 3 ) m H + suggest that mixtures of isomeric clusters may be present at equilibrium when n and m are ⩾ 4.

The H2O2-NO2− and H2NO4− isomers of the nitrate anion-water complex

Abstract Ab initio quantum mechanical methods have been applied to two isomers of the complex of the conventional nitrate anion (D 3h NO 3 − ) with the water molecule, namely, H 2 NO 4 − (two conformers) and H 2 O 2 -NO 2 − . The nitrogen atom in H 2 NO 4 − is tetrahedrally coordinated to the four oxygen atoms, while H 2 O 2 -NO 2 − is a doubly hydrogen-bonded system. Equilibrium geometries and vibrational frequencies for these three energy minima on the ground state (singlet) potential energy surface have been determined using analytic energy first and second derivative techniques. The standard level of theory employed is the self-consistent-field (SCF) method with a double-zeta plus polarization (DZP) basis set. However, the effects of diffuse basis set and electron correlation effects were also carefully considered. At this level of theory, all three structures studied lie energetically well above the complex between the conventional nitrate anion and the water molecule. The system H 2 O 2 -NO 2 - , with two strong, equivalent, and nearly linear hydrogen bonds, lies energetically closest (∼46 kcal/mol) to the nitrate anion-water molecule complex.

A comparative study of isoelectronic and isogyric reactions.: Part 2. Molecular orbital calculations of diatomic sulfides, oxides and halides

Abstract Enthalpies of reaction for reactions involving diatomic sulfides and oxides have been calculated at the MP n /6–31G ∗∗ ( n = 2–4) and MP4SDTQ/6–311G(2df,2pd) levels. Enthalpies of formation and singlet-triplet and doublet-quartet energy splittings have also been obtained. The species investigated include ArS + , ArO + , NeS + , NeO + , ArCl + , NeCl + , HS and HS + . The reactions investigated include proton transfer, halogen atom transfer and charge transfer. In order to facilitate the study of reaction enthalpies, the reactions have been divided into four categories: (1) isogyric and the reactant and product pairs are isoelectronic; (2) isogyric and the reactant and product pairs are valence isoelectronic; (3) isogyric; (4) not even isogyric. The results from this study are compared to those from our earlier study. It is found that the MP4SDTQ/6–311G(2df,2pd) enthalpies of reaction reproduce the experimental enthalpies within 3.5 kcal mol −1 for all but nonisogyric reactions. MP n /6–31G ∗* results reproduce the higher level computational results and the experimental results consistently well only for reactions in category (1). There is also disagreement in the energy splittings obtained with the two basis sets. Finally, some suggestions are made to experimentalists about ion-molecule reactions that may produce the above ions.

The temperature dependence of absolute gas phase acidities

Abstract Using both ab initio and semiempirical calculations, the temperature dependence of the thermochemical cycle relating gas phase enthalpies of acidity to electron affinities and bond dissociation energies is examined. In almost all cases, the effect of temperature is less than the uncertainties in the thermochemical quantities themselves.

The structure and energetics of [B, C, F, H3]+: quantum chemistry shows multiple minima

There is a marked paucity of reliable energetics data for ions containing boron, carbon, fluorine and hydrogen. Even one of the conceptually most simple ions, [B, C, F, H3]+, is poorly understood because its sole measurement is part of a 40-year-old electron impact study on CH3BF2. Intuition suggests this ion has the structure [CH3BF]+. What else could it be—it is isoelectronically related to the well-known CH3CN, [CH3CO]+ and [CH3N2]+? What about [BH3CF]+, isoelectronic to the well-known BH3CO and [BH3CN]−? We use high level quantum chemical calculations in the current study to disclose 10 minima: [CH3BF]+ is the most stable and [BH3CF]+ is not even a minimum. Derived quantitative energy differences and qualitative reasoning are used for the understanding of the various isomeric forms of the [B, C, F, H3]+ ion, and by inference and extension in future studies, other ions containing boron, carbon and fluorine.

The relative Lewis acidities of BF3 and SO3: the enthalpies of complexation with the anionic bases F−, OH−, NH2−, CH3− and CF3−

Abstract Ab initio quantum chemical calculations, literature thermochemical studies in both gas and condensed phases, and simple chemical rules and regularities are used to show that SO3 is a stronger Lewis acid than BF3. We show that this is due to the composite effect of relative bond strengths to sulfur and to boron, and the relative resonance energies of the two Lewis acids and the derived tetracoordinate complexes.