Prediction of Aqueous Reduction Potentials of X, ChH, and XO Radicals with X = Halogen and Ch = Chalcogen.

The aqueous electron affinity and aqueous reduction potentials for F, Cl, Br, I, OH, SH, SeH, TeH, ClO, BrO, and IO were calculated using electronic structure methods for explicit cluster models coupled with a self-consistent reaction field (SMD) to treat the aqueous solvent. Calculations were conducted using MP2 and correlated molecular orbital theory up to the CCSD(T)-F12b level for water tetramer clusters and MP2 for octamer cluster. Inclusion of explicit waters was found to be important for accurately predicting the redox potentials in a number of cases.

Absolute Hydration Free Energy of Small Anions and the Aqueous p of Simple Acids.

Heats of formation and gas phase acidities for the simple acids and their deprotonated anions (A = F, Cl, Br, I, OH, SH, SeH, TeH, OCl, OBr, and OI) were calculated using the Feller-Peterson-Dixon (FPD) method with large basis sets including Douglass-Kroll scalar relativistic corrections. Hydration of the neutral and anionic species was predicted using the supermolecule-continuum approach, resulting in absolute hydration free energies that, when combined with calculated gas phase acidities, produce aqueous acidities and p values for these simple acids that are, in general, in excellent agreement with experimental literature values. Absolute hydration free energy values converged quickly in terms of the experimental values for neutral species, requiring only four explicit HO molecules.

Synergistic Coupling of CO and HO during Expansion of Clays in Supercritical CO-CH Fluid Mixtures.

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na-, NH-, and Cs-montmorillonites (SWy-2) in supercritical fluid mixtures of HO, CO, and CH. Building on our prior work with Na-clay that demonstrated that HO facilitated CO intercalation at relatively low RH, here we show that increasing CO/CH ratios promote HO intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO intercalated and expanded the Cs-clay even in the absence of HO, while increasing fluid CO/CH ratios inhibited HO intercalation.

Polarizabilities of neutral atoms and atomic ions with a noble gas electron configuration.

Atomic polarizabilities play an important role in the development of force fields for molecular simulations, as well as for the development of qualitative concepts of atomic and molecular behavior. Coupled cluster theory at the coupled cluster singles doubles triples level with very large correlation-consistent basis sets with extended diffuse functions has been used to predict the polarizabilities of the atomic neutrals, mono-cations and mono-anions with a noble gas configuration. Additional corrections for scalar relativistic and spin-orbit effects were also included for the electron configurations of Kr, Xe, and Rn.

Crystallographic evidence of Watson-Crick connectivity in the base pair of anionic adenine with thymine.

Utilizing an ionic liquid strategy, we report crystal structures of salts of free anionic nucleobases and base pairs previously studied only computationally and in the gas phase. Reaction of tetrabutylammonium ([N]) or tetrabutylphosphonium ([P]) hydroxide with adenine (HAd) and thymine (HThy) led to hydrated salts of deprotonated adenine, [N][Ad]·2HO, and thymine, [P][Thy]·2HO, as well as the double salt cocrystal, [P][Ad][Thy]·3HO·2HThy. The cocrystal includes the anionic [Ad(HThy)] base pair which is a stable formation in the solid state that has previously not even been suggested.

The H•/H Redox Couple and Absolute Hydration Energy of H.

A supermolecule-continuum approach with water clusters up to = 16 HO molecules has been used to predict the absolute hydration free energies at 298 K (Δ) of both hydrogen (H•) and hydride (H) to be 4.6 ± 1 and -78 ± 3 kcal/mol, respectively. These values are combined with a high accuracy prediction of the gas-phase electron affinity (Δ = -16.

Elucidation of Bottom-Up Growth of CaCO Involving Prenucleation Clusters from Structure Predictions and Decomposition of Globally Optimized (CaCO) Nanoclusters.

Low-energy minima structures for (CaCO), ≤ 28, are predicted using bottom-up genetic algorithms in conjunction with density functional theory electronic structure calculations, in comparison with the frozen and relaxed top-down clusters generated by cuts from the calcite, vaterite, and aragonite crystal structures. Similarities in structural motifs for the bottom-up and relaxed top-down are revealed using a fragment recognition technique. Fragment energy decomposition analysis shows that the bottom-up and relaxed top-down clusters belong to two classes of amorphous clusters with distinct intracluster energy distributions, despite their structural similarity.

Reaction Energetics and C Fractionation of Alanine Transamination in the Aqueous and Gas Phases.

The alanine transaminase enzyme catalyzes the transfer of an amino group from alanine to α-ketoglutarate to produce pyruvate and glutamate. Isotope fractionation factors (IFFs) for the reaction HNCH(CH)COO + OOCCHCHC(O)COO ↔ CHC(O)COO + HNCH(CHCHCOO)COO (zwitterionic neutral alanine + doubly deprotonated α-ketoglutarate ↔ pyruvate + zwitterionic glutamate anion) were calculated from the partition functions of explicitly and implicitly solvated molecules at 298 K. Calculations were done for alanine (noncharge separated, zwitterion, deprotonated), pyruvic acid (neutral, deprotonated), glutamic acid (noncharge separated, zwitterion, deprotonated, doubly deprotonated), and α-ketoglutaric acid (neutral, deprotonated, doubly deprotonated).

Energetics of CO in Aqueous Solution.

Gas-phase and aqueous solution properties of neutral and anionic clusters of CO with 3, 4, and 8 explicit HO molecules are calculated at the coupled cluster (CCSD(T)) level plus a self-consistent reaction field. Anionic clusters with the radical electron density localized on the carbon of the CO molecule rather than localized on the HO molecules are more favorable energetically by 10-20 kcal/mol in the gas phase (Δ H(298 K)) and 20-30 kcal/mol in aqueous solution (Δ G(298 K)). The most favorable structures are those with the largest number of strong hydrogen bonds between the CO and the explicit HO molecules.

Experimental and Computational Study of the Gas-Phase Acidities of Acidic Di- and Tripeptides.

Gas-phase acidities (GA or Δ G) of acidic di- and tripeptides are determined for the first time. The peptides studied are composed of inert alanine (A) residues and one X residue of either aspartic acid (D) or glutamic acid (E): AX, XA, AAX, AXA, and XAA. Experimental GAs were measured by the thermokinetic method of deprotonation ion/molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer.