75 research outputs found
CO<sub>2</sub> Reduction Pathways on MnBr(N-C)(CO)<sub>3</sub> Electrocatalysts
MnBr(<i>N</i>-ethyl-<i>N</i>′-2-pyridylimidazol-2-ylidine)(CO)<sub>3</sub> reduces CO<sub>2</sub> 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<sup>+</sup> reduction. The phenolated complex shows barrierless CO<sub>2</sub> 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 PPH<sub>2</sub> 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 > OSiH<sub>3</sub> > OCH<sub>3</sub> > OPh > NH<sub>2</sub> > N(CH<sub>3</sub>)<sub>2</sub> > F > ONH<sub>2</sub> > OBH<sub>2</sub> > CH<sub>3</sub> > OOH > Ph > BF<sub>2</sub> > PH<sub>2</sub> > SiH<sub>3</sub> >
SH > HCO > Cl > CF<sub>3</sub> > Br > SiF<sub>3</sub> > NF<sub>2</sub> > NO<sub>2</sub> > CCH > OF > CN. Natural bond orbital
(NBO) analysis explains other qualitative bonding features, for example,
phosphorus–phosphorus bond orders as large as 2.5 for R<sub>2</sub>PP 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><sub>e</sub>(ClO) = 2.082, <i>r</i><sub>e</sub>(OO) = 1.208, and θ<sub>e</sub>(ClOO)
= 115.4°, in remarkable agreement with Endo’s semi-experimentally
determined values <i>r</i><sub>e</sub>(ClO) = 2.084(1), <i>r</i><sub>e</sub>(OO) = 1.206(2), and θ<sub>e</sub>(ClOO)
= 115.4(1)°. Moreover, we compute a Cl–O bond dissociation
energy of 4.77 kcal mol<sup>–1</sup>, which is likewise in
excellent agreement with the most recent experimental value of 4.69
± 0.10 kcal mol<sup>–1</sup>
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 [ReH<sub>9</sub>]<sup>2–</sup> Dianion: Molecular Structure and Vibrational Frequencies
The equilibrium geometries and vibrational
frequencies of the extraordinary
[ReH<sub>9</sub>]<sup>2–</sup> dianion (<i>D</i><sub>3<i>h</i></sub> 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 [ReH<sub>9</sub>]<sup>2–</sup> dianion, the M<sub>2</sub>ReH<sub>9</sub> (M
= Na, K) sandwich complexes (<i>D</i><sub>3<i>h</i></sub> 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 [ReH<sub>9</sub>]<sup>2–</sup> 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(C<sub>5</sub>H<sub>10</sub>)] 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 PhCOONR<sub>2</sub> (where −NR<sub>2</sub> = pypiridyl); and (iii) reductive elimination wherein the
−N(C<sub>5</sub>H<sub>10</sub>) substituent gets transferred
to the substrate. A heterobimetallic active species [Pd(μ-OAc)<sub>3</sub>Ag] 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 C<sub>2</sub>H<sub>3</sub> + H<sub>2</sub> → C<sub>2</sub>H<sub>4</sub> + 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 C<sub>2</sub>H<sub>3</sub> + H<sub>2</sub> → C<sub>2</sub>H<sub>4</sub> + 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<sup>–1</sup>, and the overall reaction is predicted to be exothermic by 5.65 kcal mol<sup>–1</sup>. 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<sup>–1</sup>, 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<sup>–1</sup>, 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)<sub>3</sub>-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
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