An examination of the nature of localized molecular orbitals and their value in understanding various phenomena that occur in organic chemistry

Stewart, James J. P.

doi:10.1007/s00894-018-3880-8

Cited by 20 publications

(8 citation statements)

References 58 publications

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“…Although MO population analysis is demonstrated to be able to explain the observable trends in the experimental XAS and VtC XES data, the values extracted from this mathematical procedure can vary dramatically based on the type of orbital localization method and should therefore be used qualitatively rather than quantitatively. 65,66 The results of the XAS and VtC XES studies, in combination with (TD)DFT calculations, are in strong support of assignment of the formally Cu(III) systems as low-spin d 8 ions, characterized by highly covalent bonding networks and high energy pre-edge and rising edge Cu K-edge XAS and VtC features.…”

Section: ■ Conclusionmentioning

confidence: 99%

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

et al. 2022

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A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Z eff , concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF 3 ) 4 ] − ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Z eff . These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ 2,5 emission features with increasing formal oxidation state within the [Cu n +(NHC 2 )] n+ series. The newly synthesized Cu(III) cation [Cu III (NHC 4 )] 3+ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF 3 ) 4 ] − , supporting a physical oxidation state assignment of low-spin d 8 Cu(III) for [Cu(CF 3 ) 4 ] − . Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.

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Section: ■ Conclusionmentioning

confidence: 99%

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

et al. 2022

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“…1 J C1′,H1′ and 1 J C1′,C2′ are expected to depend on ϕ, based on anticipated vicinal lone-pair effects on C–H and C–C bond lengths caused by rotation of the C1′–O1′ bond (ϕ). The C1′–H1′ and C1′–C2′ bond lengths are affected by n →σ* donation when a lone-pair orbital on O1′ is anti to the bond, in general leading to bond elongation , (in the ensuing discussion, oxygen atoms are assumed to bear two sp 3 -hybridized lone-pair orbitals although other lone-pair arrangements may pertain − ). Presumably the percent s -character of both bonds is greater (shorter bonds) for geometries in which both O1′ lone-pairs are gauche to them.…”

Section: Resultsmentioning

confidence: 99%

Nonconventional NMR Spin-Coupling Constants in Oligosaccharide Conformational Modeling: Structural Dependencies Determined from Density Functional Theory Calculations

2022

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Nonconventional NMR spin-coupling constants were investigated to determine their potential as conformational constraints in MA’AT modeling of the O -glycosidic linkages of oligosaccharides. Four ( 1 J C1′,H1′ , 1 J C1′,C2′ , 2 J C1′,H2′ , and 2 J C2′,H1′ ) and eight ( 1 J C4,H4 , 1 J C3,C4 , 1 J C4,C5 , 2 J C3,H4 , 2 J C4,H3 , 2 J C5,H4 , 2 J C4,H5 , and 2 J C3,C5 ) spin-couplings in methyl β- d -galactopyranosyl-(1→4)-β- d -glucopyranoside (methyl β-lactoside) were calculated using density functional theory (DFT) to determine their dependencies on O -glycosidic linkage C–O torsion angles, ϕ and ψ, respectively. Long-range 4 J H1′,H4 was also examined as a potential conformational constraint of either ϕ or ψ. Secondary effects of exocyclic (hydroxyl) C–O bond rotation within or proximal to these coupling pathways were investigated. Based on the findings of methyl β-lactoside, analogous J -couplings were studied in five additional two-bond O -glycosidic linkages [βGlcNAc-(1→4)-βMan, 2-deoxy-βGlc-(1→4)-βGlc, αMan-(1→3)-βMan, αMan-(1→2)-αMan, and βGlcNAc(1→2)-αMan] to determine whether the coupling behaviors observed in methyl β-lactoside were more broadly observed. Of the 13 nonconventional J -couplings studied, 7 exhibit properties that may be useful in future MA ’ AT modeling of O -glycosidic linkages, none of which involve coupling pathways that include the linkage C–O bonds. The findings also provide new insights into the general effects of exocyclic C–O bond conformation on the magnitude of experimental spin-couplings in saccharides and other hydroxyl-containing molecules.

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“…We chose to employ intrinsic bond orbitals (IBOs) to generate localized bond orbitals. − They are physically equivalent to the canonical molecular orbitals but give an intuitively understandable picture of bonding situations that has been shown to describe both the Lewis contributions and the relevant delocalization contributions well. − …”

Section: Resultsmentioning

confidence: 99%

Sesquicarbene Complexes: Bonding at the Interface Between M–C Single Bonds and M═C Double Bonds

2020

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Allylic dimetalated complexes [M 2 C(vinyl)] + (M = Au(IPr) and Cr(CO) 5 −) incorporate a new coordination mode of carbon. Digold complexes of this type have recently been detected experimentally. The intrinsic bond orbitals, partial charges, and structural parameters of the gold complexes and of chromium analogs were studied computationally and compared to those of the respective monometalated species and hydrocarbons. This showed that such digold complexes have a carbene character at both Au−C bonds comparable to typical carbene complexes of gold. Dichromium complexes with their stronger π-backdonation compete for interaction with carbon's π-orbital; each of the chromium atoms partakes in double bonding that is significant but weaker than that in the carbene analogs. Containing two M−C bonds on the interface between single and double bonds, these bridged complexes can be conceived as "sesquicarbene complexes". The π-system acted in a very adaptive manner and employed additional stabilization of the vinyl system only where needed. Significant carbene character is found simultaneously in both M−C bonds at the same carbon center. The discovery of these complexes with relatively strong double bond character between one carbon and two metal atoms could bring unusual single-carbon-centered organometallic cascade reactions to the horizon.

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An examination of the nature of localized molecular orbitals and their value in understanding various phenomena that occur in organic chemistry

Cited by 20 publications

References 58 publications

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

Nonconventional NMR Spin-Coupling Constants in Oligosaccharide Conformational Modeling: Structural Dependencies Determined from Density Functional Theory Calculations

Sesquicarbene Complexes: Bonding at the Interface Between M–C Single Bonds and M═C Double Bonds

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