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High-level electronic structure calculations were conducted to examine the bonding and spectroscopic properties of the UO 0/± and UF 0/± diatomic molecules. The low-lying Ω states were described by using multireference SO-CASPT2 calculations. The adiabatic electronic affinity (AEA), adiabatic ionization energy (IE), and bond dissociation energy (BDE) were calculated at the Feller-Peterson-Dixon (FPD) level. The ground state of UO is predicted to be 5 I 4 , and that of UF is 4 I 9/2 . The calculated AEAs of UO and UF are 1.123 and 0.453 eV, respectively, and the corresponding IEs are 5.976 and 6.278 eV. The BDE of UO (749.5 kJ/mol) is predicted to be considerably higher than that of UF (627.2 kJ/mol), and both are higher than those predicted for UB, UC, and UN. NBO calculations show strong ionic character for the ground states of UO and UF and bond orders that range from 2 to 3 and from 1 to 2, respectively. Comparisons of the calculated properties to those of the series comprising UB, UC, and UN diatomic molecules are given.

In this work, properties related to antioxidant-potential mechanisms (such as the bond dissociation enthalpy, BDE, for the homolytic cleavage of the O-H bond and ionization energies, IEs) were determined for phenol, pyrocatechol, and gallic acid (GA). Both the protonated and deprotonated forms of GA were investigated. The Feller-Peterson-Dixon (FPD) composite method was employed with a variety of computational approaches, i.e., density functional theory, Möller-Plesset perturbation theory, and coupled-cluster-based methods, in combination with large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. FPD results were compared to experimental and computational data available in the literature, presenting good agreement. For example, the FPD BDE (298 K) obtained for phenol, which was based on valence-correlated MP2/CBS calculations with contributions from correlating all electrons, was determined to be 87.56 kcal/mol, a value that is 0.42 kcal/mol lower than the result obtained in the most recent experiments, 87.98 ± 0.62. Calibration against coupled-cluster calculations was also carried out for phenol. We expect that the outcomes gathered here may help in establishing a general protocol for computational chemists that are interested in determining antioxidant-related properties for phenolic compounds with considerable accuracy as well as to motivate future IE measurements (particularly for GA) to be accomplished in the near future.

The atomization enthalpies of the U(VI) species UF 6 and the uranium oxyhalides UO 2 X 2 (X = F, Cl, Br, I, and At) were calculated using a composite relativistic Feller-Peterson-Dixon (FPD) approach based on scalar relativistic DKH3-CCSD(T) with extrapolations to the CBS limit. The inherent multideterminant nature of the U atom was mitigated by utilizing the singly charged atomic cation in all calculations with correction back to the neutral asymptote via the accurate ionization energy of the U atom. The effects of SO coupling were recovered using full 4-component CCSD(T) with contributions due to the Gaunt Hamiltonian calculated using Dirac-Hartree-Fock. The final atomization enthalpy for UF 6 (752.2 kcal/mol) was within 2.5 kcal/mol of the experimental value, but unfortunately the latter carries a ±2.4 kcal/mol uncertainty that is predominantly due to the experimental uncertainty in the formation enthalpy of the U atom. The analogous value for UO 2 F 2 (607.6 kcal/mol) was in nearly exact agreement with the experiment, but the latter has a stated experimental uncertainty of ±4.3 kcal/mol. The FPD atomization enthalpy for UO 2 Cl 2 (540.4 kcal/mol) was within the experimental error limit of ±5.5 kcal/mol. FPD atomization energies for the non-U-containing molecules (used for reaction enthalpies) H 2 O and HX (X = F, Cl, Br, I, and At) were within at most 0.3 kcal/mol of their experimental values where available. The FPD atomization enthalpies, together with FPD reaction enthalpies for two different reactions, were used to determine heats of formation for all species of this work, with estimated uncertainties of ±4 kcal/mol. The calculated heat of formation for UF 6 (-511.0 kcal/mol) is within 2.5 kcal/mol of the accurately known (±0.45 kcal/mol) experimental value.

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAu n - ( n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu 6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAu n clusters ( n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UF n species, but significant deviations are seen starting with UAu 5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu 6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu 7 , only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

The thermochemistry of halocarbon species containing iodine and bromine is examined through an extensive interplay between new Feller-Peterson-Dixon (FPD) style composite methods and a detailed analysis of all available experimental and theoretical determinations using the thermochemical network that underlies the Active Thermochemical Tables (ATcT). From the computational viewpoint, a slower convergence of the components of composite thermochemistry methods is observed relative to species that solely contain first row elements, leading to a higher computational expense for achieving comparable levels of accuracy. Potential systematic sources of computational uncertainty are investigated, and, not surprisingly, spin-orbit coupling is found to be a critical component, particularly for iodine containing molecular species. The ATcT analysis of available experimental and theoretical determinations indicates that prior theoretical determinations have significantly larger uncertainties than originally reported, particularly in cases where molecular spin-orbit effects were ignored. Accurate and reliable heats of formation are reported for 38 halogen containing systems, based on combining the current computations with previous experimental and theoretical work via the ATcT approach.

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC - , and UC + molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller-Peterson-Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC - and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN + , the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC 0/+/- molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f 2 7s 1 ), UC - (5f 2 7s 2 ), and UC + (5f 2 7s 0 ). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

The results of calculations of the properties of the anion UN - including electron detachment are described, which further expand our knowledge of this diatomic molecule. High-level electronic structure calculations were conducted for the UN and UN - diatomic molecules and compared to photoelectron spectroscopy measurements. The low-lying Ω states were obtained using multireference CASPT2 including spin-orbit effects up to ∼20,000 cm -1 . At the Feller-Peterson-Dixon (FPD) level, the adiabatic electron affinity (AEA) of UN is estimated to be 1.402 eV and the vertical detachment energy (VDE) is 1.423 eV. The assignment of the UN excited states shows good agreement with the experimental results with a VDE of 1.424 eV. An Ω = 4 ground state was obtained for UN - which is mainly associated with the 3 H ΛS state. Thermochemical calculations estimate a bond dissociation energy (BDE) for UN - (U - + N) of 665.9 kJ/mol, ∼15% larger than that of UN and UN + . The NBO analysis reveals U-N triple bonds for the UN, UN - , and UN + species.

High-level electronic structure calculations of the low-lying energy electronic states for ThH, ThH - , and ThH + are reported and compared to experimental measurements. The inclusion of spin-orbit coupling is critical to predict the ground-state ordering as inclusion of spin-orbit switches the coupled-cluster CCSD(T) ordering of the two lowest energy states for ThH and ThH + . At the multireference spin-orbit SO-CASPT2 level, the ground states of ThH, ThH - , and ThH + are predicted to be the 2 Δ 3/2 , 3 Φ 2 , and 3 Δ 1 states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to be 0.832 eV in comparison to an experimental value of 0.87 ± 0.02 eV. The observed ThH - photoelectron spectrum has many transitions, which approximately correlate with excitations of Th + and/or Th. The adiabatic ionization energy of ThH including spin-orbit corrections is calculated to be 6.181 eV. The natural bond orbital results are consistent with a significant contribution of the Th + H - ionic configuration to the bonding in ThH. The bond dissociation energies for ThH, ThH - , and ThH + using the Feller-Peterson-Dixon approach were calculated to be similar for all three molecules and lie between 259 and 280 kJ/mol.

Accurate Pt-An bond dissociation enthalpies (BDEs) for PtAnO n + (An = U, Np, Pu and n = 0-2) and the corresponding enthalpies for the Pt + OAnO n + substitution reactions have been studied for the first time using an accurate composite coupled cluster approach. Analogous O-AnO n + bond dissociation enthalpies are also presented. To make the study possible, new correlation consistent basis sets optimized using the all-electron third-order Douglas-Kroll-Hess (DKH3) scalar relativistic Hamiltonian are developed and reported for Pt and Au, with accompanying benchmark calculations of their atomic ionization potentials to demonstrate the effectiveness of the new basis sets. For the charged PtAnO n + species ( n = 1, 2), a low-spin state (LSS) for which the Pt-An σ bond is doubly occupied is studied together with a high-spin state (HSS) obtained by unpairing the σ bond orbital and placing one electron into the An 5f shell. The relative energies of the two spin states have been compared and qualitatively assessed via natural population and natural bond analyses. The enthalpies for the Pt substitution reactions, i.e., Pt + OAnO n + → PtAnO n + + O, are calculated to range from about 14-62 kcal/mol, and the Pt-AnO n + bond dissociation enthalpies range from about 78-149 kcal/mol for the ground electronic states. For the PtAnO + species, the LSSs were all predicted to be the ground state, whereas the PtAnO 2+ molecules all favored the HSSs. The prediction for PtUO 2+ is consistent with previous theoretical findings. The natural bond orbital analyses indicate a triple bond between An and O, with a double to quadruple bond between the An and Pt.

Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, H 2 Y, HX, Y 2 , and X 2 for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se 2 , where the AoC-DHF value was used due to strong multireference effects in Se 2 for the SO calculations. The calculated results show that the At 2 BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. H 2 Po is not predicted to be stable to dehydrogenation to Po + H 2 in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H 2 SO 4 . The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.

We present a benchmark investigation on the O-H bond dissociation enthalpies (BDEs) and ionization potential (IP) for gallic acid (GA), a widely known polyphenolic antioxidant. These properties were determined in the gas-phase and in water through the use of density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), coupled-cluster with single and double excitations (CCSD), and coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)). The 6-311++G(df,p), cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were used. Regarding DFT functionals, the M06-2X provided the best agreement for the BDEs when compared to the corresponding CCSD(T)/aug-cc-pVTZ results; M06-2X was also found to be the most suitable for probing the IP for the protonated forms of GA while LC-ωPBE was the most reliable in the case of deprotonated GA. Given that these properties represent important descriptors for examining mechanisms related to the antioxidant potential of a given polyphenol, we hope that the present work can serve as a guide for computational chemists venturing in the field.

Ionization potentials (IPs) for MO 3 and MO 2 for M = U, Mo, W, and Nd have been predicted using the Feller-Peterson-Dixon (FPD) approach at the coupled cluster CCSD(T)/complete basis set level including additional corrections. The additional corrections are mostly small, with spin-orbit effects contributing less than 0.05 eV, except for NdO 2 where the correction lowers the IP by 0.26 eV. The IPs for UO 3 and UO 2 are calculated to be 9.59 and 6.09 eV, respectively. The calculated IPs for MoO 3 and WO 3 are very similar, 11.13 and 11.11 eV, respectively, and MoO 2 and WO 2 are 8.51 and 8.79 eV, respectively. MoO 2 has a triplet ground state, whereas WO 2 has a singlet ground state. The calculated IP for NdO 2 is 7.90 eV. NdO 3 does not achieve a high +VI formal oxidation state on the lanthanide and has an IP of 7.80 eV. These calculated IPs are expected to have error bars of ±0.04 eV.

Many-body potentials up to fourth order are constructed using nonrelativistic, scalar-relativistic, and relativistic coupled-cluster theory to accurately describe the interaction between superheavy oganesson atoms. The obtained distance-dependent energy values were fitted to extended two-body Lennard-Jones and three-body Axilrod-Teller-Muto potentials, with the fourth-order term treated through a classical long-range Drude dipole interaction model. From these interaction potentials, spectroscopic constants for the oganesson dimer and solid-state properties were obtained. Furthermore, these high-level results are compared to scalar-relativistic and two-component plane-wave DFT calculations based on a tailor-made projector augmented wave pseudopotential (PAW-PP) and newly derived parameters for Grimme's dispersion correction. It is shown that the functionals PBE-D3(BJ), PBEsol, and in particular SCAN provide excellent agreement with the many-body reference for solid oganesson. Finally, the results for oganesson are compared and related to the lighter rare gas elements, and periodic trends are discussed.

Potential energy surfaces for the group 11 trimers were generated at various levels of coupled-cluster theory to examine the effects of Jahn-Teller distortions. Our calculations show that the lowest-energy conformer for Cu 3 , Ag 3 , and Au 3 is the 2 B 2 (∼65° isomer) without spin-orbit corrections. Spin-orbit corrections have negligible contributions to the relative energies for the angle dependence of the potential energy surfaces for Cu 3 and Ag 3 . The inclusion of spin-orbit corrections for Au 3 makes the 2 B 2 (∼65°) and 2 A 1 (∼55°) states approximately degenerate. A novel 2 B 2 isomer of Au 3 at an obtuse angle of ∼125° was also characterized, providing evidence for bond angle isomerism on the same 2 B 2 potential energy surface. Spin-orbit corrections increase the barrier height between the 2 B 2 (65°) and 2 B 2 (125°) bond angle isomers of Au 3 . The calculated symmetric stretch vibrational frequencies are in good agreement with the available experimental values. All frequencies calculated for the Au 3 2 B 2 (∼125°) state are real, and there is at least one bound bending vibration for this state. Jahn-Teller parameters are also derived for each trimer.

Hydrogen adsorption on small group 4 metal oxide clusters for both the singlet and the first excited triplet states have been investigated by density functional theory and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The reaction starts with hydrogen physisorption on a metal center followed by formation of metal hydride/hydroxides due to splitting H 2 into H - and H + . The hydrogen physisorption energies are predicted to be -1 to -8 kcal/mol for the singlet and -1 to -26 kcal/mol for the triplet, respectively. The formation of metal hydride/hydroxides does not involve redox processes. Chemisorption leading to formation of metal hydride/hydroxides is exothermic by -10 to -50 kcal/mol for the singlet, and exothermic by up to -60 kcal/mol for the triplet. The predicted energy barriers are less than 20 kcal/mol. Formation of metal dihydroxides from the metal hydride/hydroxides is generally endothermic for the monomer and dimer and is exothermic for the trimer and tetramer. Formation of the dihydroxide is a proton coupled electron transfer (PCET) process. The singlet energy barriers for the H - → H + transfer process are predicted to be 35-60 kcal/mol, in comparison to triplet energy barriers of less than 15 kcal/mol for the H • → H + transfer process. For trimers and tetramers, there exist two different pathways: the first is a direct pathway with PCET to a terminal oxygen and the second is a two-step pathway with initial formation of a bridge OH group followed by a proton transfer to generate a terminal OH group. For the singlet, the two-step pathway is preferred for M = Ti and the direct pathway is more favorable for M = Zr and Hf. The two-step pathway is always preferred for the triplet as one-electron transfer is always more likely than two-electron transfer in the direct pathway.

Geometry parameters, vibrational frequencies, heats of formation, bond dissociation energies, cohesive energies, and selected fluoride affinities (difluorides) are predicted for the late alkaline earth (Sr, Ba, and Ra) oxides, fluorides, chlorides, and hydroxides at the coupled cluster theory CCSD(T) level. Additional corrections (scalar relativistic and pseudopotential corrections, vibrational zero-point energies, and atomic spin-orbit effects) were included to accurately calculate the total atomization energies and heats of formation following the Feller-Peterson-Dixon methodology. The calculated values are compared to the experimental data where available. In some cases, especially for Ra compounds, there are no experimental results, or the experimental energetics and geometries are not reliable or have very large error bars. All of the Sr, Ba, and Ra difluorides, dichlorides, and dihydroxides are bent structures with the OMO bond angles decreasing going down the group. The cohesive energies of bulk Be dihalides are predicted to be quite low, while those of Ra are relatively large. The fluoride affinities show that the difluorides are moderately strong Lewis acids and that such trifluorides may form under the appropriate experimental conditions.

A combined study of the vibrational spectroscopy of iodopentafluorobenzene by new Raman and Fourier transform infrared (FTIR) spectroscopies, over the spectral range 300 to 3200 cm -1 (Raman) and 50 to 3400 cm -1 (FTIR), with a state-of-the-art theoretical investigation is reported. This has enabled reliable identification of numerous fundamental, overtone, and combination band transitions in unprecedented detail. The theoretical analysis, beyond the double-harmonic approximation, is based on generalized second-order vibrational perturbation theory (GVPT2) with a hybrid coupled cluster/density functional theory (CC/DFT) approach. Anharmonic contributions to structural parameters, rotational constants, vibrational frequencies, and spectral intensities are incorporated. The procedures, of general applicability, enable rigorous comparison of theoretical methods with experimental results in vibrational spectroscopy.

Gas-phase acidities and aqueous solution pK a 's are predicted for MO 2 (OH) 2 , where the center atom M is a main Group 6, 16, and U atom using the Feller-Peterson-Dixon approach based on coupled cluster CCSD(T) calculations with additional corrections. The gas-phase acidities of the MO 2 (OH) 2 compounds are essentially the same for elements (M) of the same group, 304-310 kcal/mol at 298 K. All of the Group 6 compounds are 5-6 kcal/mol less acidic in the gas phase than H 2 SO 4 . The gas-phase acidity of UO 2 (OH) 2 is calculated to be up to 338.0 kcal/mol, ∼10% less acidic in the gas phase than the other MO 2 (OH) 2 acids. The most acidic molecule in aqueous solution is predicted to be H 2 SO 4 . Overall, for the Group 16 compounds, the pK a 's increase going down the group, with H 2 PoO 4 predicted to be slightly more acidic than nitric acid. H 2 CrO 4 is the most acidic of the Group 6 transition metal compounds. The aqueous acidities of H 2 MoO 4 and H 2 WO 4 are comparable and about 3 pK a units less acidic than H 2 CrO 4 and comparable in acidity to HNO 3 . H 2 UO 4 is not acidic at all in aqueous solution with a pK a near 20 pK a units and is also not predicted to readily undergo hydrolysis reactions.

The reactions of lanthanides with H2O have been studied using density functional theory with the B3LYP functional. H2O forms an initial Lewis acid-base complex with the lanthanides exothermically with interaction energies from -2 to -20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H2LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from -25 to -75 kcal/mol. The formation of LnO + H2 for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H2LnO is mostly in the +III formal oxidation state with either Ln-O(-)/Ln-H(-) or Ln-(H2)(-)/Ln=O(2-) bonding interactions. A few of the H2LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with Ln=O(2-)/Ln-H(-) bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger Ln-O(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies.

The potential energy surfaces for the reactions of H2O with ThO2, PaO2(+), UO2(2+), and UO2(+) have been calculated at the coupled cluster CCSD(T) level extrapolated to the complete basis set limit with additional corrections including scalar relativistic and spin-orbit. The reactions proceed by the formation of an initial Lewis acid-base adduct (H2O)AnO2(0/+/2+) followed by a proton transfer to generate the dihydroxide AnO(OH)2(0/+/2+). The results are in excellent agreement with mass spectrometry experiments and prior calculations of hydrolysis reactions of the group 4 transition metal dioxides MO2. The differences in the energies of the stationary points on the potential energy surface are explained in terms of the charges on the system and the populations on the metal center. The use of an improved starting point for the coupled cluster CCSD(T) calculations based on density functional theory with the PW91 exchange-correlation functional or Brueckner orbitals is described. The importance of including second-order spin-orbit corrections for closed-shell molecules is also described. These improvements in the calculations are correlated with the 5f populations on the actinide.

Structures, vibrational frequencies, and heats of formation were predicted for MX4 and both singlet and triplet states of MX2 (M = group 4, group 14, Ce, and Th; X = F and Cl) using the Feller-Peterson-Dixon composite electronic structure approach based on coupled cluster CCSD(T) calculations extrapolated to the complete basis set limit with additional corrections including spin orbit effects. The spin-orbit corrections are not large but need to be included for chemical accuracy of ±1 kcal/mol. The singlet-triplet splittings were calculated for the dihalides and all compounds have singlet ground states except for the dihalides of Ti, Zr, and Ce which have triplet ground states. The calculated heats of formation are in good agreement with the available experimental data. Our predictions suggest that the experimental heats of formation need to be revised for a number of tetrahalides: TiF4, HfF4, PbF4, PbCl4, and ThCl4 as well as a number of dihalides: GeF2, SnF2, PbF2, TiF2, and TiCl2. The calculated heats of formation were used to predict various thermodynamic properties including average M-F and M-Cl bond dissociation energies and the reaction energies for MX2 + X2 → MX4. Edge inversion barriers were predicted. The calculated edge inversion barriers for the tetrafluorides show that the barriers for the group 14 tetrafluorides decrease with increasing atomic number, the group 4 barriers are ∼50 kcal/mol and CeF4 and ThF4 have inversion barriers of ∼25 kcal/mol.

Accurate bond dissociation energies (D0) are reported for different isotopologues of the highly anharmonic system ClHCl(-). The mass-independent equilibrium dissociation energy De was obtained by a composite method with frozen-core (fc) CCSD(T) as the basic contribution. Basis sets as large as aug-cc-pV8(+d)Z were employed, and extrapolation to the complete basis set (CBS) limit was carried out. Explicitly correlated calculations with the CCSD(T)-F12b method were also performed to support the conventionally calculated values. Core-core and core-valence correlation, scalar relativity, and higher-order correlation were considered as well. Two mass-dependent contributions, namely, the diagonal Born-Oppenheimer correction and the difference in zero-point energies between the complex and the HCl fragment, were then added in order to arrive at precise D0 values. Results for (35)ClH(35)Cl(-) and (35)ClD(35)Cl(-) are 23.81 and 23.63 kcal/mol, respectively, with estimated uncertainties of 0.05 kcal/mol. In contrast to FHF(-) ( Stein , C. ; Oswald , R. ; Sebald , P. ; Botschwina , P. ; Stoll , H. , Peterson , K. A. Mol. Phys. 2013 , 111 , 2647 - 2652 ), the D0 values of the bichloride species are larger than their De counterparts, which is an unusual situation in hydrogen-bonded systems.

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

High-quality static electric dipole polarizabilities have been determined for the ground states of the hard-sphere cations of U, Np, and Pu in the III and IV oxidation states. The polarizabilities have been calculated using the numerical finite field technique in a four-component relativistic framework. Methods including Fock-space coupled cluster (FSCC) and Kramers-restricted configuration interaction (KRCI) have been performed in order to account for electron correlation effects. Comparisons between polarizabilities calculated using Dirac-Hartree-Fock (DHF), FSCC, and KRCI methods have been made using both triple- and quadruple-ζ basis sets for U(4+). In addition to the ground state, this study also reports the polarizability data for the first two excited states of U(3+/4+), Np(3+/4+), and Pu(3+/4+) ions at different levels of theory. The values reported in this work are the most accurate to date calculations for the dipole polarizabilities of the hard-sphere tri- and tetravalent actinide ions and may serve as reference values, aiding in the calculation of various electronic and response properties (for example, intermolecular forces, optical properties, etc.) relevant to the nuclear fuel cycle and material science applications.

Explicitly correlated CCSD(T)-F12b calculations show that the lowest energy conformer of XeF(6) is the C(3v) structure with a stereoactive lone pair. The C(3v) structure is 1.08 kcal/mol below the C(2v) structure and 1.80 kcal/mol below the O(h) structure without vibrational corrections. The C(2v) conformer is a transition state connecting the different C(3v) minima on the pseudorotation potential energy surface, and the O(h) structure is a higher-order saddle point leading to the C(2v) transition states. The calculated vibrational frequencies for the C(3v) structure best fit the experimental frequencies. The calculated heats of formation for XeF(6), -62.1 ± 1.4 kcal/mol at 0 K and -64.0 ± 1.4 kcal/mol at 298 K, are the best available values and show that there are serious issues with the experimental values. The results show that the explicitly correlated CCSD(T)-F12b method can be used to address important electronic structure issues with smaller basis sets.

Heats of formation at 298.15 K were computed for cis- and trans-1,2-difluoroethylene, 1,1-difluoroethylene, difluoroethyne, and tetrafluoroethylene using conventional coupled cluster theory through perturbative triples and basis sets up through augmented 6ζ or 7ζ quality. As an independent check, calculations were also performed with the explicitly correlated F12b method and basis sets up through 4ζ quality. F12b calculations converge to the basis set limit much more rapidly than the conventional method. Both approaches were subsequently extrapolated to the complete basis set limit using a variety of simple formulas. After the 1-particle basis set limits were established, corrections were applied for core/valence, scalar relativistic, non-Born-Oppenheimer, and higher order correlation effects through explicit quadruple excitations. A final correction for the remaining error relative to full configuration interaction theory, the exact result for a given basis set, was included. Although this general approach produced excellent agreement with recently reported semiexperimental structures for the difluoroethylene compounds, the level of agreement with the available tabulated experimental heats of formation for 1,1-difluoroethylene is considerably poorer. The best theoretical ΔHf(0)(298K) estimates (with experimental values from tabulations given in parentheses) are 1,1-difluoroethylene = -84.1 ± 0.6 kcal/mol (-80.5 ± 0.2); cis-1,2-difluoroethylene = -74.4 ± 0.6 kcal/mol (-73.3 ± 1.2); trans-1,2-difluoroethylene = -73.5 ± 0.6 kcal/mol (-72.6 ± 1.2). In the other two cases, where Active Thermochemical Table values were available, the agreement was much better: C2F2 = 1.0 ± 0.3 (0.7 ± 0.4) and C2F4 = -161.3 ± 0.8 kcal/mol (-161.8 ± 0.2). Considering the demonstrated accuracy of the approach in previous studies for the three difluoroethylene equilibrium structures and its performance in predicting the enthalpies of formation of over 100 small-to-medium size chemical systems, we believe the magnitude of disagreement between theory and experiment for 1,1-difluoroethylene merits a re-examination of the experimental value.

Geometry parameters, frequencies, heats of formation, and bond dissociation energies are predicted for simple alkali metal compounds (hydrides, chlorides, fluorides, hydroxides and oxides) of Li, Na, and K from coupled cluster theory [CCSD(T)] calculations including core-valence correlation with the aug-cc-pwCVnZ basis set (n = D, T, Q, and 5). To accurately calculate the heats of formation, the following additional correction were included: scalar relativistic effects, atomic spin-orbit effects, and vibrational zero-point energies. For calibration purposes, the properties of some of the lithium compounds were predicted with iterative triple and quadruple excitations via CCSDT and CCSDTQ. The calculated geometry parameters, frequencies, heats of formation, and bond dissociation energies were compared with all available experimental measurements and are in excellent agreement with high-quality experimental data. High-level calculations are required to correctly predict that K(2)O is linear and that the ground state of KO is (2)Sigma(+), not (2)Pi, as in LiO and NaO. This reliable and consistent set of calculated thermodynamic data is appropriate for use in combustion and atmospheric simulations.

High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolated to the complete basis set limit have been made for the thermodynamics of the BrBrO(2), IIO(2), ClBrO(2), ClIO(2), and BrIO(2) isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO(2) isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO(2) and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O-Br bond with a bond dissociation energy (BDE) of 15.9 kcal/mol, and in BrBrO(2), it is the Br-Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O-Br bond with a BDE of 12.6 kcal/mol. Of the IIO(2) isomers, IIO(2) is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO(2) is the I-I bond with a BDE of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O-I bonds with values of 19.0 and 5.2 kcal/mol, respectively.

The heats of formation at 298.15 K and structures of five small oxygen fluoride molecules have been determined with coupled cluster theory incorporating at least through quadruple excitations. Because the wave functions of several of these systems exhibit strong multiconfiguration character, correlation recovery beyond CCSD(T) was found to be essential for achieving accurate results. Comparison is made with multireference configuration interaction properties where appropriate. The final DeltaH(f)(298 K) values obtained in this study are FO ((2)Pi(3/2)) = 26.5 +/- 0.2, FOO ((2)A'') = 6.4 +/- 0.7, OFO ((2)B(2)) = 125.0 +/- 0.3, FOF ((1)A(1)) = 5.9 +/- 0.3, and FOOF ((1)A) = 6.4 +/- 0.7 kcal/mol. For FO, FOO, and FOF, the theoretical DeltaH(f) values with their accompanying error bars easily fall within the experimental error bars. In the case of OFO the heat of formation has not been determined experimentally. The best current DeltaH(f) for FOOF lies outside the NIST-JANAF experimental error bars (4.59 +/- 0.5 kcal/mol) despite the use of very high levels of theory and the adoption of what is believed to be a conservative estimate of the theoretical uncertainty. Good agreement with experiment was found for the structures.

The performance of new relativistic energy-consistent pseudopotentials for the 5d elements, adjusted to atomic valence spectra from multiconfiguration Dirac-Hartree-Fock calculations (J. Chem. Phys. 2009, 130, 164108.), is examined in coupled cluster and multireference configuration interaction benchmark calculations for the diatomics HfO, TaO, WO, ReN, OsN, IrN, and Pt(2), with basis sets of up to quintuple-zeta quality. The final accuracy reached for the oxides and nitrides, with corrections for pseudopotential errors (contributions from 4f shell correlation, for example), is 0.3 pm for bond lengths and 17 cm(-1) (1.5%) for harmonic vibrational frequencies. The spectroscopic constants of the ground state of Pt(2) can be reproduced with deviations of 3 pm for the bond length and 1 cm(-1) for the vibrational frequency, without any correction for pseudopotential errors.

Using a newly developed relativistic pseudopotential and a series of correlation consistent basis sets for Hf, the ionization energy of HfO and the spectroscopic properties of the HfO(+) cation have been determined in coupled cluster calculations. After accounting for basis set incompleteness, outer-core correlation, the pseudopotential approximation, and higher order electron correlation effects, excellent agreement with recent experimental measurements is obtained.

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted from high level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set (CBS) limit for the H(1,2)O(m)S(n) (m, n = 0-3) compounds, as well as various radicals involved in different bond breaking processes. To achieve near chemical accuracy (+/-1.0 kcal/mol), additional corrections were added to the CBS binding energies based on the frozen core CCSD(T) energies including corrections for core-valence, scalar relativistic, and first-order atomic spin-orbit effects. Geometries were optimized up through the CCSD(T)/aV(T+d)Z level. Vibrational zero point energies were computed at the MP2/aV(T+d)Z level. The calculated heats of formation are in excellent agreement with the available experimental data and allow the prediction of adiabatic bond dissociation energies (BDEs) to within +/-1.0 kcal/mol. The decomposition mechanisms were largely determined by a preference to maintain a strong S=O bond in the dissociated products as opposed to O=O and S=S bonds, exactly matching the ordering of the BDEs in the diatomics. For the H(2)X(2) and H(2)X(3) systems, as well as the HX(3) radicals, the energetically favorable decomposition pathway leads to the formation of XH radicals and breaking the X-X bond as opposed to breaking the X-H bond. For the HX(2) radicals, however, the more thermodynamically favorable pathway leads to a breaking of the H-X bond and forming X(2) molecules.

Total atomization energies (TAEs) and normalized clustering energies (NCEs) of group IVB (MO(2))(n) (M = Ti, Zr, Hf) and VIB (MO(3))(n) (M = Cr, Mo, W) transition metal oxide clusters up to n = 4 were calculated at the coupled cluster [CCSD(T)] and density functional theory (DFT) levels. For all the clusters studied, the TAEs calculated at the CCSD(T) level were found to be strongly basis set dependent, whereas the NCEs were significantly less basis set dependent. Here we further develop an efficient strategy for calculating accurate thermodynamic properties of large clusters based on those of the cluster unit and the NCEs. The calculated TAEs, NCEs, and heats of formations for these clusters were compared with available experimental data. We also benchmarked the performance of popular DFT exchange-correlation functionals for the calculations of the TAEs and NCEs. The performance of many DFT functionals for the calculation of the TAEs strongly depends on the choice of the electronic state for the transition metal atom. Hybrid functionals were found to generally outperform pure functionals in the calculation of NCEs, and the PBE1PBE functional has the best performance with average deviations of approximately 1 kcal/mol for the dimers and approximately 2 kcal/mol for the trimers and tetramers. The benchmarked functionals all display gradual degradation in performance with increasing cluster size.

Multireference configuration interaction (MRCI) calculations of the Born-Oppenheimer diagonal correction (BODC) for H(3) were performed at 1397 symmetry-unique configurations using the Handy-Yamaguchi-Schaefer approach; isotopic substitution leads to 4041 symmetry-unique configurations for the DH(2) mass combination. These results were then fit to a functional form that permits calculation of the BODC for any combination of isotopes. Mean unsigned fitting errors on a test grid of configurations not included in the fitting process were 0.14, 0.12, and 0.65 cm(-1) for the H(3), DH(2), and MuH(2) isotopomers, respectively. This representation can be combined with any Born-Oppenheimer potential energy surface (PES) to yield Born-Huang (BH) PESs; herein, we choose the CCI potential energy surface, the uncertainties of which ( approximately 0.01 kcal/mol) are much smaller than the magnitude of the BODC. Fortran routines to evaluate these BH surfaces are provided. Variational transition state theory calculations are presented comparing thermal rate constants for reactions on the BO and BH surfaces to provide an initial estimate of the significance of the diagonal correction for the dynamics.

Electronic structure calculations at the coupled cluster (CCSD(T)) and density functional theory levels with relativistic effective core potentials and large basis sets were used to predict the isolated uranyl ion frequencies. The effects of anharmonicity and spin-orbit corrections on the harmonic frequencies were calculated. The anharmonic effects are larger than the spin-orbit corrections, but both are small. The anharmonic effects decreased all the frequencies, whereas the spin-orbit corrections increased the stretches and decreased the bend. Overall, these two corrections decreased the harmonic asymmetric stretch frequency by 6 cm-1, the symmetric stretch by 3 cm-1, and the bend by 3 cm-1. The best calculated values for UO22+ for the asymmetric stretch, symmetric stretch, and bend were 1113, 1032, and 174 cm-1, respectively. The separation between the asymmetric and the symmetric stretch band origins was predicted to be 81 cm-1, which is consistent with experimental trends for substituted uranyls in solution and in the solid state. The anharmonic vibrational frequencies of the isoelectronic ThO2 molecule also were calculated and compared to experiment to calibrate the UO22+ results.

The correlation consistent Composite Approach (ccCA), which has been shown to achieve chemical accuracy (+/-1 kcal mol-1) for a large benchmark set of main group and s-block metal compounds, is used to compute enthalpies of formation for a set of 17 3d transition metal species. The training set includes a variety of metals, ligands, and bonding types. Using the correlation consistent basis sets for the 3d transition metals, we find that gas-phase enthalpies of formation can be efficiently calculated for inorganic and organometallic molecules with ccCA. However, until the reliability of gas-phase transition metal thermochemistry is improved, both experimentally and theoretically, a large experimental training set where uncertainties are near +/-1 kcal mol-1 (akin to commonly used main group benchmarking sets) remains an ambitious goal. For now, an average deviation of +/-3 kcal mol-1 appears to be the initial goal of "chemical accuracy" for ab initio transition metal model chemistries. The ccCA is also compared to a more robust but relatively expensive composite approach primarily utilizing large basis set coupled cluster computations. For a smaller training set of eight molecules, ccCA has a mean absolute deviation (MAD) of 3.4 kcal mol-1 versus the large basis set coupled-cluster-based model chemistry, which has a MAD of 3.1 kcal mol-1. However, the agreement for transition metal complexes is more system dependent than observed in previous benchmark studies of composite methods and main group compounds.

The effects of aqueous solvation on the thermochemistry of reactions between mercury and small halogen molecules has been investigated by the microsolvation approach using ab initio and density functional theory (DFT) calculations. The structures, vibrational frequencies, and binding energies of 1, 2, and 3 water molecules with mercury-halide (HgBr2, HgBrCl, HgCl2, HgBr, and HgCl) and related mercury and halogen species (Br2, BrCl, Cl2, Cl, Hg, and Br) have been computed with second order Møller-Plesset perturbation theory (MP2) and the B3LYP density functional method. Accurate incremental water binding energies have been obtained at the complete basis set (CBS) limit using sequences of correlation consistent basis sets, including higher order correlation effects estimated from coupled cluster calculations. The resulting energetics were used to calculate the influence of water molecules on the thermochemistry of a number of reactions between mercury and small halogen-containing molecules. In general, the presence of water favors the formation of oxidized mercury halide species.

The family of correlation consistent polarized valence basis sets has been extended in order to account for core-core and core-valence correlation effects within the third-row, main group atoms gallium through krypton. Construction of the basis sets is similar to that of the atoms boron through argon, where either the difference between core-correlated and valence-only correlation energies were calculated via configuration interaction (CISD) computations on the ground electronic states of the atoms (named cc-pCVnZ) or the sets were optimized with respect to the core-valence correlation energy and a small weight of core-core correlation energy (cc-pwCVnZ). Due to the correlation of 3d orbitals, added shells of higher angular momentum exponents compared to the valence sets are necessary to describe the core region. The pattern of added core-correlating functions is (1s1p1d1f) for double-zeta, (2s2p2d2f1g) for triple-zeta, (3s3p3d3f2g1h) for quadruple-zeta, and (4s4p4d4f3g2h1i) for quintuple-zeta. Atomic and molecular results show good convergence to the CBS limit, with the cc-pwCVnZ sets showing improved convergence compared to the cc-pCVnZ ones for molecular core-valence correlation effects. After testing the basis sets on the homonuclear diatomics Ga2-Kr2 with coupled cluster wave functions, it is concluded that a treatment of core-valence correlation effects is essential for high-accuracy ab initio investigations of third-row-containing molecules. Though the basis sets are optimal for 3s3p3d correlation, preliminary atomic and molecular results show the basis sets to be efficient with respect to 3d-only correlation, and these potentially could be used with 3d-only correlation for more qualitative studies on larger species.

Two types of halogen...halide synthons are investigated on the basis of theoretical and crystallographic studies; the simple halogen...halide synthons and the charge assisted halogen...halide synthons. The former interactions were investigated theoretically (ab initio) by studying the energy of interaction of a halide anion with a halocarbon species as a function of Y...X- separation distance and the C-Y...X- angle in a series of complexes (R-Y...X-, R=methyl, phenyl, acetyl or pyridyl; Y=F, Cl, Br, or I; X-=F-, Cl-, Br-, or I-). The theoretical study of the latter interaction type was investigated in only one system, the [(4BP)Cl]2 dimer, (4BP=4-bromopyrdinium cation). Crystal structure determinations, to complement the latter theoretical calculations, were performed on 13 n-chloropyridinium and n-bromopyridinium halide salts (n=2-4). The theoretical and crystallographic studies indicate that these interactions are controlled by electrostatics and are characterized by linear C-Y...X- angles and separation distances less than the sum of van der Waals radius (rvdW) of the halogen atom and the ionic radii of the halide anion. The strength of these contacts from calculations varies from weak or absent, e.g., H3C-Cl...I-, to very strong, e.g., HCC-I...F- (energy of interaction ca. -153 kJ/mol). The strengths of these contacts are influenced by four factors: (a) the type of the halide anion; (b) the type of the halogen atom; (c) the hybridization of the ipso carbon; (d) the nature of the functional groups. The calculations also show that charge assisted halogen...halide synthons have a comparable strength to simple halogen...halide synthons. The nature of these contacts is explained on the basis of an electrostatic model.

A coupled cluster composite approach has been used to accurately determine the spectroscopic constants, bond dissociation energies, and heats of formation for the X1(2)II(3/2) states of the halogen oxides ClO, BrO, and IO, as well as their negative ions ClO-, BrO-, and IO-. After determining the frozen core, complete basis set (CBS) limit CCSD(T) values, corrections were added for core-valence correlation, relativistic effects (scalar and spin-orbit), the pseudopotential approximation (BrO and IO), iterative connected triple excitations (CCSDT), and iterative quadruples (CCSDTQ). The final ab initio equilibrium bond lengths and harmonic frequencies for ClO and BrO differ from their accurate experimental values by an average of just 0.0005 A and 0.8 cm-1, respectively. The bond length of IO is overestimated by 0.0047 A, presumably due to an underestimation of molecular spin-orbit coupling effects. Spectroscopic constants for the spin-orbit excited X2(2)III(1/2) states are also reported for each species. The predicted bond lengths and harmonic frequencies for the closed-shell anions are expected to be accurate to within about 0.001 A and 2 cm-1, respectively. The dissociation energies of the radicals have been determined by both direct calculation and through use of negative ion thermochemical cycles, which made use of a small amount of accurate experimental data. The resulting values of D0, 63.5, 55.8, and 54.2 kcal/mol for ClO, BrO, and IO, respectively, are the most accurate ab initio values to date, and those for ClO and BrO differ from their experimental values by just 0.1 kcal/mol. These dissociation energies lead to heats of formation, DeltaH(f) (298 K), of 24.2 +/- 0.3, 29.6 +/- 0.4, and 29.9 +/- 0.6 kcal/mol for ClO, BrO, and IO, respectively. Also, the final calculated electron affinities are all within 0.2 kcal/mol of their experimental values. Improved pseudopotential parameters for the iodine atom are also reported, together with revised correlation consistent basis sets for this atom.

Using a composite coupled cluster method employing sequences of correlation consistent basis sets for complete basis set (CBS) extrapolations and with explicit treatment of core-valence correlation and scalar and spin-orbit relativistic effects, the 0 K enthalpies of a wide range of cadmium-halide reactions, namely, Cd + (HCl, HBr, ClO, BrO, Cl2, BrCl, Br2) have been determined to an estimated accuracy of +/-1 kcal/mol. In addition, accurate equilibrium geometries, harmonic frequencies, and dissociation energies have been calculated at the same level of theory for all the diatomic (e.g., CdH, CdO, CdCl, CdBr) and triatomic (CdHCl, CdHBr, CdClO, CdBrO, CdCl2, CdBrCl, CdBr2) species involved in these reactions, some for the very first time. Like their mercury analogues, all of the abstraction reactions are predicted to be endothermic, while the insertion reactions are strongly exothermic with the formation of stable linear, Cd-centric complexes. With the exception of CdH and the reactions involving this species, the present results for the remaining Cd-containing systems are believed to be the most accurate to date.

Accurate 0 K enthalpies have been calculated for reactions of mercury with a series of small iodine-containing molecules (I2, IBr, ICl, and IO). The calculations have been carried out with the coupled cluster singles and doubles method with a perturbative correction for connected triple excitations [CCSD(T)] using sequences of correlation consistent basis sets and accurate relativistic pseudopotentials. Corrections have been included to account for core-valence correlation, spin-orbit coupling, scalar relativity, and the Lamb shift. In a few cases coupled cluster calculations with iterative triple (CCSDT) and quadruple (CCSDTQ) excitations have been carried out to estimate the effects of higher order electron correlation. The pseudopotential calculations have also been compared to all electron calculations using second- and third-order Douglas-Kroll-Hess Hamiltonians. In addition to the reaction enthalpies, heats of formation, bond lengths, and harmonic vibrational frequencies have been calculated for the stable triatomic products HgI2, HgIBr, HgICl, and HgIO. Accurate dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies have also been calculated for each of the diatomic molecules involved in this study (HgI, HgBr, HgCl, HgO, I2, IBr, ICl, and IO). The reported enthalpies are expected to have accuracies of 1 kcal/mol or better.

A global potential energy surface (PES) for the (1)A' ground state of HgBr(2) has been constructed in order to determine the rate constants for atmospherically important reactions involving mercury and bromine. The total energy of HgBr(2) was calculated by the multireference configuration interaction level of theory with series of correlation consistent basis sets up to quadruple-zeta quality with subsequent extrapolation to the complete basis set limit. An additive correction for spin-orbit coupling was also included. The global PES was represented piecewise by interpolating three separate parts of the surface with the reproducing kernel Hilbert space method and connecting them smoothly by switch functions. Quasiclassical trajectory calculations carried out on the surface yielded 298 K thermal rate constants of 3.89 x 10(-11) cm(3)/(mol.s) for the abstraction reaction HgBr + Br --> Hg + Br(2), 2.98 x 10(-11) cm(3)/(mol.s) for the recombination reaction Br + HgBr --> HgBr(2), and 3.97 x 10(-11) cm(3)/(mol.s) for the exchange reaction Br + HgBr --> BrHg + Br. The insertion reaction Hg + Br(2) --> HgBr(2) was found to have a high barrier of 27.2 kcal/mol and a very small rate constant of just 2.74 x 10(-31) cm(3)/(mol.s) determined by the microcanonical variational transition state theory method. The implications of the obtained results to the description of the mechanism of recently observed polar tropospheric mercury depletion events are briefly discussed.

The methyl cation affinities of the rare gases have been calculated at 0 and 298 K by using coupled cluster theory including noniterative, quasiperturbative triple excitations with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. To achieve near chemical accuracy (+/-1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies, based on frozen core coupled cluster theory energies, two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero-point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA in kcal/mol) at 298 K are the following: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6. Because of the importance of the MCA(N(2)) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH(3)N(2)(+) and CH(2)N(2). The calculated values for diazomethane at 298 K are: DeltaH(f)(CH(2)N(2)) = 65.3 kcal/mol, PA(CH(2)N(2)) = 211.9 kcal/mol, and MCA(N(2)) = 43.2 kcal/mol.