Predicting Molecules--More Realism, Please Hoffmann, Roald; Schleyer, Paul von Ragué; Schaefer, Henry F. III
Angewandte Chemie (International ed.),
September 8, 2008, Volume:
47, Issue:
38
Journal Article
Peer reviewed
To be or not to be: The terms “viable” and “fleeting,” rather than the imprecise term “stable,” are proposed to describe hypothetical molecules whose existence has been predicted by computational ...studies. A number of criteria that a molecule described as viable should fulfill are discussed.
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The synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)3 and ...MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)3. Both new species mediate the reduction of CO2 to CO following two‐electron reduction of the MnI center, as observed with preparative scale electrolysis and verified with 13CO2. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)3. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)3.
Catalytic manganese N‐heterocyclic carbene complexes have been synthesized and characterized: MnBrL(CO)3 (see scheme; L=N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine or N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine). Both species mediate the reduction of CO2 to CO following two‐electron reduction of the MnI center at a single potential, as observed with preparative scale electrolysis and verified with 13CO2.
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This survey of metal–metal (MM) bond distances in binuclear complexes of the first row 3d-block elements reviews experimental and computational research on a wide range of such systems. The metals ...surveyed are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc, representing the only comprehensive presentation of such results to date. Factors impacting MM bond lengths that are discussed here include (a) the formal MM bond order, (b) size of the metal ion present in the bimetallic core (M2) n+, (c) the metal oxidation state, (d) effects of ligand basicity, coordination mode and number, and (e) steric effects of bulky ligands. Correlations between experimental and computational findings are examined wherever possible, often yielding good agreement for MM bond lengths. The formal bond order provides a key basis for assessing experimental and computationally derived MM bond lengths. The effects of change in the metal upon MM bond length ranges in binuclear complexes suggest trends for single, double, triple, and quadruple MM bonds which are related to the available information on metal atomic radii. It emerges that while specific factors for a limited range of complexes are found to have their expected impact in many cases, the assessment of the net effect of these factors is challenging. The combination of experimental and computational results leads us to propose for the first time the ranges and “best” estimates for MM bond distances of all types (Ti–Ti through Zn–Zn, single through quintuple).
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4.
Stable Silicon(0) Compound with a Si=Si Double Bond Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong ...
Science (American Association for the Advancement of Science),
08/2008, Volume:
321, Issue:
5892
Journal Article
Peer reviewed
Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. ...Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl₄ {where L: is:CN(2,6-Pri₂-C₆H₃)CH₂ and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 ± 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.
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We present the working equations for a reduced-scaling method of evaluating the perturbative triples (T) energy in coupled-cluster theory, through the tensor hypercontraction (THC) of the triples ...amplitudes (t ijk abc ). Through our method, we can reduce the scaling of the (T) energy from the traditional O ( N 7 ) to a more modest O ( N 5 ) . We also discuss implementation details to aid future research, development, and software realization of this method. Additionally, we show that this method yields submillihartree (mEh) differences from CCSD(T) when evaluating absolute energies and sub-0.1 kcal/mol energy differences when evaluating relative energies. Finally, we demonstrate that this method converges to the true CCSD(T) energy through the systematic increasing of the rank or eigenvalue tolerance of the orthogonal projector, as well as exhibiting sublinear to linear error growth with respect to system size.
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Diarsenic as a Lewis acid: An N‐heterocyclic carbene stabilized diarsenic molecule L:AsAs:L (L:=:C{N(2,6‐iPr2C6H3)CH}2) (see scheme) has been prepared by potassium/graphite reduction of L:AsCl3. The ...nature of the bonding in this molecule was probed by DFT computations.
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The classic SEAr mechanism of electrophilic aromatic substitution (EAS) reactions described in textbooks, monographs, and reviews comprises the obligatory formation of arenium ion intermediates (σ ...complexes) in a two-stage process. Our findings from several studies of EAS reactions challenge the generality of this mechanistic paradigm. This Account focuses on recent computational and experimental results for three types of EAS reactions: halogenation with molecular chlorine and bromine, nitration by mixed acid (mixture of nitric and sulfuric acids), and sulfonation with SO3. Our combined computational and experimental investigation of the chlorination of anisole with molecular chlorine in CCl4 found that addition–elimination pathways compete with the direct substitution processes. Detailed NMR investigation of the course of experimental anisole chlorination at varying temperatures revealed the formation of addition byproducts. Moreover, in the absence of Lewis acid catalysis, the direct halogenation processes do not involve arenium ion intermediates but instead proceed via concerted single transition states. We also obtained analogous results for the chlorination and bromination of several arenes in nonpolar solvents. We explored by theoretical computations and experimental spectroscopic studies the classic reaction of benzene nitration by mixed acid. The structure of the first intermediate in this process has been a subject of contradicting views. We have reported clear experimental UV/vis spectroscopic evidence for the formation of the first intermediate in this reaction. Our broader theoretical modeling of the process considers the effects of the medium as a bulk solvent but also the specific interactions of a H2SO4 solvent molecule with intermediates and transition states along the reaction path. In harmony with the obtained spectroscopic data, our computational results reveal that the structure of the initial π complex precludes the possibility of electronic charge transfer from the benzene π system to the nitronium unit. In contrast to usual interpretations, our computational results provide compelling evidence that in nonpolar, noncomplexing media and in the absence of catalysts, the mechanism of aromatic sulfonation with sulfur trioxide is concerted and does not involve the conventional σ-complex (Wheland) intermediates. Stable under such conditions, (SO3)2 dimers react with benzene much more readily than monomeric sulfur trioxide. In polar (complexing) media, the reaction follows the classic two-stage SEAr mechanism. Still, the rate-controlling transition state involves two SO3 molecules. The reactivity and regioselectivity in EAS reactions that follow the classic mechanistic scheme are quantified using a theoretically evaluated quantity, the electrophile affinity (Eα), which measures the stabilization energy associated with the formation of arenium ions. Examples of applications are provided.
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The chemical conversion of CO2 has been studied by numerous experimental groups. Particularly the use of rhenium tricarbonyl-based molecular catalysts has attracted interest owing to their ability to ...absorb light, store redox equivalents, and convert CO2 into higher-energy products. The mechanism by which these catalysts mediate reduction, particularly to CO and HCOO–, is poorly understood, and studies aimed at elucidating the reaction pathway have likely been hindered by the large number of species present in solution. Herein the mechanism for carbon monoxide production using rhenium tricarbonyl catalysts has been investigated using density functional theory. The investigation presented proceeds from the experimental work of Meyer’s group (J. Chem. Soc., Chem. Commun. 1985, 1414–1416) in DMSO and Fujita’s group (J. Am. Chem. Soc. 2003, 125, 11976–11987) in dry DMF. The latter work with a simplified reaction mixture, one that removes the photo-induced reduction step with a sacrificial donor, is used for validation of the proposed mechanism, which involves formation of a rhenium carboxylate dimer, Re(dmb)(CO)32(OCO), where dmb = 4,4′-dimethyl-2,2′-bipyridine. CO2 insertion into this species, and subsequent rearrangement, is proposed to yield CO and the carbonate-bridged Re(dmb)(CO)32(OCO2). Structures and energies for the proposed reaction path are presented and compared to previously published experimental observations.
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The design, synthesis, and assessment of a new manganese-centered catalyst for the electrochemical reduction of CO2 is described. The reported species, ...MnBr(6-(2-hydroxyphenol)-2,2′-bipyridine)(CO)3, includes a ligand framework with a phenolic proton in close proximity to the CO2 binding site, which allows for facile proton-assisted C–O bond cleavage. As a result of this modification, seven times the electrocatalytic current enhancement is observed compared to MnBr(2,2′-bipyridine)(CO)3. Moreover, reduction is possible at only 440 mV of overpotential. Theoretical computations suggest that the entropic contribution to the activation free energy is partially responsible for the increased catalytic activity. Experimental work, including voltammetry and product quantification from controlled potential electrolysis, suggests a key mechanistic role for the phenolic proton in the conversion of CO2 to CO.
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