CO₂ Reduction Pathways on MnBr(N-C)(CO)₃ Electrocatalysts

MnBr­(<i>N</i>-ethyl-<i>N</i>′-2-pyridylimidazol-2-ylidine)­(CO)₃ reduces CO₂ to CO in the absence of strong acids. Herein, we employ density functional theory and domain based local pair natural orbital coupled cluster theory to perform the first mapping of the catalytic pathway for this catalyst and various derivatives. The benzimidazole-containing derivative proceeds along the same pathway as its parent complex, but with an increased barrier to H⁺ reduction. The phenolated complex shows barrierless CO₂ addition to the activated catalyst and facile C–O bond cleavage. All species exhibit a novel pyridine dissociation upon one-electron reduction of the tetracarbonyl species, but the active tricarbonyl catalysts can be regenerated with a small barrier. This novel step in the pathway presents a further consideration in the design of catalysts and provides insight into the potential degradation pathways of this catalyst

Reducing and Reversing the Diphosphene–Diphosphinylidene Energy Separation

The dependence of the relative energies of 116 diphosphene and diphosphinylidene compounds on the modification of their structures is studied theoretically. Optimized geometries and relative energies are reported for all structures. With the purpose of investigating the effects of various substituents on the parent PPH₂ and HPPH molecules, isodesmic reaction energies were obtained for single and double substitution. In the case of the substitution of both H atoms by lithoxy (OLi) or ONa groups is the diphosphinylidene type structure found to be lower in energy. For the lithoxy group, the energy difference amounts to 33 kcal/mol at CCSD­(T) cc-pVTZ level of theory. This result is explained through the natural population analyses, where a very favorable Coulombic attraction is found in the OLi substituted diphosphinylidene structure. The order of the effectiveness of the substituents in lowering the relative energy of the diphosphinylidene structure is OLi > ONa > OH > OSiH₃ > OCH₃ > OPh > NH₂ > N­(CH₃)₂ > F > ONH₂ > OBH₂ > CH₃ > OOH > Ph > BF₂ > PH₂ > SiH₃ > SH > HCO > Cl > CF₃ > Br > SiF₃ > NF₂ > NO₂ > CCH > OF > CN. Natural bond orbital (NBO) analysis explains other qualitative bonding features, for example, phosphorus–phosphorus bond orders as large as 2.5 for R₂PP structures and as small as 1.6 for RPPR structures

The Structure and Cl–O Dissociation Energy of the ClOO Radical: Finally, the Right Answers for the Right Reason

The chlorine peroxy radical (ClOO) has historically been a highly problematic system for theoretical studies. In particular, the erratic <i>ab initio</i> predictions of the Cl–O bond length reported in the literature thus far exhibit unacceptable errors with respect to the experimental structure. In light of the widespread disagreement observed, we present a careful and systematic investigation of the ClOO geometry toward the basis set and correlation limits of single reference <i>ab initio</i> theory, employing the cc-pVXZ (X = D, T, Q, 5, 6) basis sets extrapolated to the complete basis set limit and coupled cluster theory through single, double, triple, and perturbative quadruple excitations [CCSDT­(Q)]. We demonstrate a considerable sensitivity of the Cl–O bond length to both electron correlation and basis set size. The CCSDT­(Q)/CBS structure is found to be <i>r</i>_e(ClO) = 2.082, <i>r</i>_e(OO) = 1.208, and θ_e(ClOO) = 115.4°, in remarkable agreement with Endo’s semi-experimentally determined values <i>r</i>_e(ClO) = 2.084(1), <i>r</i>_e(OO) = 1.206(2), and θ_e(ClOO) = 115.4(1)°. Moreover, we compute a Cl–O bond dissociation energy of 4.77 kcal mol^–1, which is likewise in excellent agreement with the most recent experimental value of 4.69 ± 0.10 kcal mol^–1

Positional selectivity in the interaction of toluene with nitronium ion

<p>Three alternative theoretical approaches – electrostatic potentials at the ring carbon atoms, Hirshfeld charges, and electrophile affinities – are employed in predicting the positional selectivity for the nitration of toluene with nitronium ion in dichloromethane medium. The theoretical estimates are compared with recent experimental data of Nieves-Quinones and Singleton. The computed electrostatic potential values at the ring carbon nuclei and the respective Hirshfeld charges predict excellently the regioselectivity of the reaction. The electrophile affinity approach, based on estimated energies of formation of the σ-complex intermediates for the <i>ortho</i>, <i>meta</i>, <i>para</i>, and <i>ipso</i> positions in the ring, also provides reasonable theoretical predictions of reactivity. The mechanism of the nitration reaction is also discussed.</p

The Remarkable [ReH₉]^2– Dianion: Molecular Structure and Vibrational Frequencies

The equilibrium geometries and vibrational frequencies of the extraordinary [ReH₉]^2– dianion (<i>D</i>_3<i>h</i> symmetry) are investigated using Hartree–Fock (HF) theory, coupled cluster theory with single and double excitations (CCSD), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD­(T)]. The new generation of energy-consistent relativistic pseudopotentials and correlation consistent basis sets [cc-pVXZ-PP (Re) and cc-pVXZ (H) (X = D, T, Q)] are used. Anharmonicity was considered using second-order vibrational perturbation theory. The predicted geometries and vibrational frequencies generally agree with experimental findings. In order to stabilize the [ReH₉]^2– dianion, the M₂ReH₉ (M = Na, K) sandwich complexes (<i>D</i>_3<i>h</i> symmetry) are studied at the CCSD­(T)/VTZ (VTZ = cc-pVTZ-PP (Re) and cc-pVTZ (H, Na, K)) level of theory. Compared to the [ReH₉]^2– dianion, the predicted vibrational frequencies involving Re–H stretching modes are improved, indicating the importance of considering counterions in electronically dense systems. The natural bond orbital analysis shows that each H atom only bonds with the Re center, and the 5d orbitals of Re and 1s orbitals of H are major factors for the covalent Re–H bonding

Palladium–Silver Cooperativity in an Aryl Amination Reaction through C–H Functionalization

The mechanism of palladium-acetate-catalyzed <i>ortho</i>-amination of <i>N</i>-arylbenzamides by using <i>O</i>-benzoyl hydroxylpiperidine [PhCOON­(C₅H₁₀)] has been examined by using DFT­(M06, B3LYP) computational methods. Particular emphasis is placed on the role of additives such as cesium fluoride and silver acetate. The lowest-energy pathway has been identified by carefully examining 15 or more configurationally different possibilities in each important step of the reaction. The key mechanistic events include (i) the aryl C–H activation of the substrate through a cyclometalation deprotonation; (ii) N–O activation of the reactant PhCOONR₂ (where −NR₂ = pypiridyl); and (iii) reductive elimination wherein the −N­(C₅H₁₀) substituent gets transferred to the substrate. A heterobimetallic active species [Pd­(μ-OAc)₃Ag] is identified as catalytically superior over the conventionally proposed monometallic palladium acetate. A cooperative interaction between Pd­(II) and Ag­(I) is found to offer additional stabilization to the transition states and intermediates, as compared to those devoid of such an interaction. The second additive, CsF, helps in the deprotonation of the amidic nitrogen as well as offers electrostatic stabilization to intermediates and transition states, thereby influencing the energetics of the reaction. Our findings clearly suggest that refined transition-state models inclusive of additives are highly desirable toward identifying the most preferred mechanistic pathways

Reaction Energetics for the Abstraction Process C₂H₃ + H₂ → C₂H₄ + H

The fundamentally important combustion reaction of vinyl radical with hydrogen has been studied in the laboratory by at least five experimental groups. Herein, the reaction C₂H₃ + H₂ → C₂H₄ + H has been examined using focal-point analysis. Molecular energies were determined from extrapolations to the complete basis-set limit using correlation-consistent basis sets (cc-pVTZ, cc-pVQZ, and cc-pV5Z) and coupled-cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples [CCSDT], and perturbative quadruples [CCSDT(Q)]. Reference geometries were optimized at the all-electron CCSD(T)/cc-pCVQZ level. Computed energies were also corrected for relativistic effects and the Born–Oppenheimer approximation. The activation energy for hydrogen abstraction is predicted to be 9.65 kcal mol^–1, and the overall reaction is predicted to be exothermic by 5.65 kcal mol^–1. Natural resonance theory (NRT) analysis was performed to verify the reaction pathway and describe bond-breaking and bond-forming events along the reaction coordinate

Structures, Bonding, and Energetics of Potential Triatomic Circumstellar Molecules Containing Group 15 and 16 Elements

The recent discovery of PN in the oxygen-rich shell of the supergiant star VY Canis Majoris points to the formation of several triatomic molecules involving oxygen, nitrogen, and phosphorus; these are also intriguing targets for main-group synthetic inorganic chemistry. In this research, high-level <i>ab initio</i> electronic structure computations were conducted on the potential circumstellar molecule OPN and several of its heavier group 15 and 16 congeners (SPN, SePN, TePN, OPP, OPAs, and OPSb). For each congener, four isomers were examined. Optimized geometries were obtained with coupled cluster theory [CCSD­(T)] using large Dunning basis sets [aug-cc-pVQZ, aug-cc-pV­(Q+d)­Z, and aug-cc-pVQZ-PP], and relative energies were determined at the complete basis set limit of CCSDT­(Q) from focal point analyses. The linear phosphorus-centered molecules were consistently the lowest in energy of the group 15 congeners by at least 6 kcal mol^–1, resulting from double–triple and single–double bond resonances within the molecule. The linear nitrogen-centered molecules were consistently the lowest in energy of the group 16 congeners by at least 5 kcal mol^–1, due to the electronegative central nitrogen atom encouraging electron delocalization throughout the molecule. For OPN, OPP, and SPN, anharmonic vibrational frequencies and vibrationally corrected rotational constants are predicted; good agreement with available experimental data is observed

Non-innocent Additives in a Palladium(II)-Catalyzed C–H Bond Activation Reaction: Insights into Multimetallic Active Catalysts

The role of a widely employed additive (AgOAc) in a palladium acetate-catalyzed <i>ortho</i>-C–H bond activation reaction has been examined using the M06 density functional theory. A new hetero-bimetallic Pd-(μ-OAc)₃-Ag is identified as the most likely active species. This finding could have far-reaching implications with respect to the notion of the active species in palladium catalysis in the presence of other metal salt additives

Can Density Cumulant Functional Theory Describe Static Correlation Effects?

We evaluate the performance of density cumulant functional theory (DCT) for capturing static correlation effects. In particular, we examine systems with significant multideterminant character of the electronic wave function, such as the beryllium dimer, diatomic carbon, <i>m</i>-benzyne, 2,6-pyridyne, twisted ethylene, as well as the barrier for double-bond migration in cyclobutadiene. We compute molecular properties of these systems using the ODC-12 and DC-12 variants of DCT and compare these results to multireference configuration interaction and multireference coupled-cluster theories, as well as single-reference coupled-cluster theory with single, double (CCSD), and perturbative triple excitations [CCSD­(T)]. For all systems the DCT methods show intermediate performance between that of CCSD and CCSD­(T), with significant improvement over the former method. In particular, for the beryllium dimer, <i>m</i>-benzyne, and 2,6-pyridyne, the ODC-12 method along with CCSD­(T) correctly predict the global minimum structures, while CCSD predictions fail qualitatively, underestimating the multireference effects. Our results suggest that the DC-12 and ODC-12 methods are capable of describing emerging static correlation effects but should be used cautiously when highly accurate results are required. Conveniently, the appearance of multireference effects in DCT can be diagnosed by analyzing the DCT natural orbital occupations, which are readily available at the end of the energy computation