Some Generalizations Concerning the Magnetic Circular Dichroism of Substituted Benzenes

Foss, John G.; McCarville, M.E.

doi:10.1021/ja00977a006

Cited by 25 publications

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“…Then MCD will have the potential to become an additional structural tool in the hands of the organic chemist if simple rules for the signs can be worked out. An indication that this might be so was provided by the MCD spectra of substituted benzenes, the sign pattern in which is sensitive to the nature of the substituent [25]. In agreement with numerical PPP calculations [ 11,121, the sign of the first MCD band is negative for ?r-electron-donating (-E) substituents, and positive for ?r-electron-withdrawing ( + E ) substituents.…”

supporting

confidence: 56%

π-Electron systems: Recent developments in electronic spectroscopy and the PPP model

Michl

2009

Int. J. Quantum Chem.

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The PPP model gives good account of the recently measured MCD signs and polarizations of a a * transitions in hydrocarbons. The previously predicted mirror-image relation between the MCD spectra of two species paired in the sense of alternant symmetry has now been verified experimentally. Consideration of pairing properties suggests a classification of a-electron chromophores into "hard," whose MCD sign pattern is intrinsic and little affected by substituents, and "soft" (those paired with themselves), whose MCD sign pattern is dominated by the substituents and heteroatoms present. In many instances, prediction of the substituent-induced signs requires no more than an inspection of the Hiickel orbitals of the parent hydrocarbon. Agreement with experiment is very good for low-energy transitions.This contribution presents a summary of recent experimental work on conjugated ?r-electron systems performed in our laboratory and compares the results with expectations based on the semiempirical PPP model [ 11. Some related work done in other laboratories is mentioned briefly.It is sometimes considered fashionable to look down on semiempirical calculations and to view whatever agreement with experiment they achieve as a mere result of a posteriori adjustment of an excessive number of free parameters. It is therefore important to point out right at the beginning that no parameters were adjusted in any of the comparisons with experiment presented in the following, that many of the results are parameter independent, and that one of the experimental methods discussed (magnetic circular dichroism, MCD) was not even known to organic spectroscopists at the time when the PPP model was developed [ 11.The PPP model for a-electron states of a conjugated hydrocarbon is defined by postulating, first, a finite basis set of one-electron functions spanned by a set of orthonormal spin orbitals 1pu ) = lp )) a ) (a is H or -l,$), antisymmetric with respect to the molecular plane, and localized on the atomic centers in the conjugated system of the molecule, each of which carries a charge of + 1. The specification is then completed by postulating expressions for model operators. For the Hamiltonian operator,where it,, = Z~~P , is the site occupation number operator, and * P are the creation and annihilation operators of orbital lp), respectively, and the prime on a summation indicates the tight-binding approximation (neighbors only). The resonance integrals PPy and electron repulsion integrals yP,, are parameters of the model. The best values for these

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confidence: 56%

π-Electron systems: Recent developments in electronic spectroscopy and the PPP model

Michl

2009

Int. J. Quantum Chem.

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“…Of particular interest are the results for arylmethyl cations and anions, which can serve as models for isoelectronic even alternant hydrocarbons bearing 1T-electron withdrawing and 1Telectron donating conjugated substituents, respectively. Indeed, the sign of the B term of the lowest band of benzene is well known to differ for the two types of substitution, 15,21 in agreement with such an analogy. Similarly, inductive electron-donating and electron-withdrawing substituents should have opposite effects.…”

Section: Consequencesmentioning

confidence: 53%

Use of semiempirical models for calculation of B terms in MCD spectra. II. The Pariser-Parr-Pople (PPP) model: A consequence of the pairing symmetry in π systems with alternant skeletons

Michl

1974

The Journal of Chemical Physics

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“…It has been shown that MCD spectroscopy is complementary to UV–vis absorption spectroscopy, offering a better sensitivity to distinguish electronic transitions . Foss and McCarville , observed a correlation between the sign of the Faraday term and the direction of the electric transition dipole moment in the MCD spectra of toluene, naphthalene, anthracene, and tetracene. Further theoretical advancements in understanding the MCD response of polycyclic aromatic hydrocarbons were reported by Stephens and Larkindale …”

Section: Introductionmentioning

confidence: 99%

Magnetic Circular Dichroism of Naphthalene Derivatives: A Coupled Cluster Singles and Approximate Doubles and Time-Dependent Density Functional Theory Study

et al. 2020

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Some Generalizations Concerning the Magnetic Circular Dichroism of Substituted Benzenes

Cited by 25 publications

References 0 publications

π-Electron systems: Recent developments in electronic spectroscopy and the PPP model

π-Electron systems: Recent developments in electronic spectroscopy and the PPP model

Use of semiempirical models for calculation of B terms in MCD spectra. II. The Pariser-Parr-Pople (PPP) model: A consequence of the pairing symmetry in π systems with alternant skeletons

Magnetic Circular Dichroism of Naphthalene Derivatives: A Coupled Cluster Singles and Approximate Doubles and Time-Dependent Density Functional Theory Study

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