Direct Assessment of Electron Delocalization Using NMR Chemical Shifts

Steinmann, Stephan N.; Jana, DanielâF.; Wu, JudyâI.-C.; Schleyer, PaulâvonâR.; Mo, Yirong; Corminboeuf, ClÃ©mence

doi:10.1002/anie.200905390

Cited by 50 publications

(65 citation statements)

References 38 publications

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“…This phenomenon was confirmed recently by Corminboeuf and coworkers [47], who combined the BLW (Block-Localized Wave-function) scheme with the IGLO method and applied to benzene and cyclohexatriene. The BLW-IGLO study showed that deformation of benzene to a cyclohexatriene geometry with the C−C (1.527 Å) and C=C (1.330 Å) bond lengths close to those for ethane and ethene decreases moderately magnetic and energetic parameters of aromaticity.…”

Section: Original Homa Index (Ohoma)supporting

confidence: 59%

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

et al. 2010

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Abstract:The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono-and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index.

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Section: Original Homa Index (Ohoma)supporting

confidence: 59%

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

et al. 2010

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“…According to our aromaticity criterium BV(ELF  ) ≤ 0.2, benzene becomes "nonaromatic" at a bond alternant geometry with bond lengths of approximately 1.455 and 1.345 Å. These findings clearly fall in line with the observed strong calculated ring currents for constricted benzenes [64,65,67], such as the one of Siegel and co-workers [63], and with the NICS and energetic data of Stanger as well as Corminboeuf and colleagues [66,68]. Consequently, even when given the opportunity through the help of geometric distortions, an aromatic -electron system is not prone to distort, but in contrast adheres to a delocalized distribution which is not very different from that of the maximally aromatic equilibrium geometry.…”

Section: The Elf  Bifurcation Values and CC Bond Lengthssupporting

confidence: 81%

“…Indeed, such bond alternant benzenes retain a large portion of the cyclic delocalization energy of benzene, in line with earlier findings on conservation of aromaticity as observed in calculations of ring-currents, NICS, and energetic measures (e.g., hydrogenation energies) [64][65][66][67]. In particular, Corminboeuf, Schleyer, Mo and co-workers recently applied the block-localized wave function (BLW) scheme to compute NICS(0) zz , and found that a geometric distortion from D 6h benzene to a D 3h symmetric "1,3,5-cyclohexatriene" only reduced the NICS value from -36.3 to -28.2 [68]. However, effecting an "electronic" localization (blocking of -conjugation) through the BLW approach led to a NICS(0) zz of zero.…”

Section: Introductionmentioning

confidence: 99%

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

et al. 2010

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Abstract:The degree of -electron (de)localization and aromaticity of a series of polybenzenoid hydrocarbons (PBHs) has been analyzed through the π-contribution to the electron localization function (ELF π ), calculated at the B3LYP/6-311G(d,p) hybrid density functional theory level. The extent of -electron delocalization in the various hexagons of a PBH was determined through analysis of the bifurcation values of the ELF  basins (BV(ELF  )), the spans in the bifurcation values in each hexagon (ΔBV(ELF π )), and the ring-closure bifurcation values of the ELF π (RCBV(ELF π )). These computed results were compared to the qualitative description of local aromaticities of the different hexagons in terms of Clar structures with -sextets. Benzene, [18]annulene, and thirty two PBHs were analyzed at their equilibrium geometries, and benzene and triphenylene were also analyzed at bond length distorted structures. In general, the description of PBHs in terms of Clar valence structures is supported by the ELF  properties, although there are exceptions. For PBHs at their equilibrium geometries there is a clear sigmoidal relationship between the CC bond lengths and the amount of -electron (de)localization at these bonds, however, this relationship is lost for bond distorted geometries. In the latter cases, we specifically examined benzene in D 3h symmetric "1,3,5-cyclohexatriene" structures and triphenylene in eight different structures. From the distorted benzenes and triphenylenes it becomes clear OPEN ACCESSSymmetry 2010, 2 1654 that there is a distinct tendency for the -electron network to retain delocalization (aromaticity). The ELF  analysis thus reveals an antidistortive rather than a distortive behavior of the -electrons in these investigated compounds.

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“…There is strong diatropic current flowing along [8]CP 2-. The current susceptibility strength is 35 nA/T or about 6.5 nA/T stronger than in [6]CP.…”

Section: Introductionmentioning

confidence: 99%

Magnetically Induced Currents in [n]Cycloparaphenylenes, n = 6−11

2010

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We report calculations of the gauge-independent magnetically induced current densities in [n]cycloparaphenylenes ([n]CP), n = 6-11. In addition to the neutral [n]CPs, the dianion of [6]CP and the current densities of the corresponding metal complexes Li 2 [6]CP and Mg[6]CP are also investigated. By the ring current criterion, the [6]CP with 4n π electrons has a slight antiaromatic character, while [7]CP has (4n þ 2) π electrons and is weakly aromatic with a ring current susceptibility strength that is about 25% of the ring current of benzene. The larger neutral [n]CPs, n = 8-11, do not sustain any net ring current around the nanohoop and are essentially nonaromatic. The weak paramagnetic ring current susceptibility of [6]CP flows along a 4n π pathway on either edge of the phenylene rings. For the dianions, the ring current susceptibility strengths are 24-35 nA/T diatropic and thus the addition of two electrons induces an electron delocalization and an aromatic character of the nanohoops. The dilithium complex of [6]CP with (4n þ 2) π electrons is aromatic with a net ring current strength of 28 nA/T or 2.4 times the ring current strength of benzene, involving all 62 π electrons in the current pathway. The 1 H NMR chemical shieldings and the nucleus-independent chemical shifts correlate with the strengths of the magnetically induced currents. The aromatic [n]cycloparaphenylenes have a quinoid structure, whereas the weakly aromatic or nonaromatic ones are benzoidic.

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Direct Assessment of Electron Delocalization Using NMR Chemical Shifts

Cited by 50 publications

References 38 publications

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

Magnetically Induced Currents in [n]Cycloparaphenylenes, n = 6−11

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