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UV–Vis Spectroscopy: Woodward–Fieser Rules01:29

UV–Vis Spectroscopy: Woodward–Fieser Rules

UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the contributions...
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A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
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Structure of Amines

The hybridized nitrogen atom in amines possesses a lone pair of electrons and is bound to three substituents with a bond angle of around 108°, which is less than the tetrahedral angle of 109.5°. However, the C–N–H bond angle is slightly larger at 112°, with a carbon–nitrogen bond length of 147 pm. This carbon–nitrogen bond length of of amines is longer than the carbon–oxygen bond of alcohols (143 pm) but shorter than alkanes’ carbon–carbon bond (154 pm). These aspects are illustrated in Figure...
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Structure of Conjugated Dienes

Introduction
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Solution structure of some lambda(3) Iodanes: an (17)O NMR and DFT study.

Francesca Mocci1, Gianluca Uccheddu, Angelo Frongia

  • 1Dipartimento di Scienze Chimiche, Università di Cagliari, Complesso Universitario, S.S. 554, Bivio per Sestu, I-09042 Monserrato (CA), Italy. francy@dsc.unica.it

The Journal of Organic Chemistry
|May 1, 2007
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Bis(acyloxy)iodoarenes and benzoiodoxolones maintain their T-shaped structure in solution but undergo a dynamic iodine shift. This dynamic process, studied via 17O NMR and DFT, explains spectral differences between these hypervalent iodine compounds.

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Area of Science:

  • Organic Chemistry
  • Spectroscopy
  • Computational Chemistry

Background:

  • Hypervalent iodine compounds, specifically lambda(3) iodanes like bis(acyloxy)iodoarenes and benzoiodoxolones, are important synthetic intermediates.
  • Understanding their solution-state structure and dynamics is crucial for predicting reactivity and designing new transformations.
  • Previous studies often focused on solid-state structures, leaving solution behavior less characterized.

Purpose of the Study:

  • To elucidate the solution-state structure and dynamics of I-O bonded bis(acyloxy)iodoarenes and benzoiodoxolones.
  • To investigate the potential for dynamic processes, such as sigmatropic shifts, in these hypervalent iodine species.
  • To correlate observed spectroscopic features with computational findings.

Main Methods:

  • 17O Nuclear Magnetic Resonance (NMR) spectroscopy was employed to probe the electronic environment of oxygen atoms.
  • Density Functional Theory (DFT) calculations, utilizing the PBE0 functional and a specific basis set (LANL2DZ with polarization and diffuse functions), were performed.
  • A combined experimental and computational approach was used to analyze the structural and dynamic properties.

Main Results:

  • Both bis(acyloxy)iodoarenes and benzoiodoxolones retain their characteristic "T-shaped" geometry in chloroform solution.
  • A degenerate [1,3] sigmatropic shift of the iodine atom between the two acyloxy oxygen atoms was identified in solution.
  • The energy barrier for this sigmatropic shift was found to differ between the two compound classes, leading to distinct 17O NMR spectra at room temperature.

Conclusions:

  • The investigated lambda(3) iodanes exhibit dynamic behavior in solution, characterized by a [1,3] sigmatropic iodine shift.
  • The differing energy barriers for this dynamic process are directly responsible for the observed variations in 17O NMR spectra.
  • This study provides a comprehensive understanding of the solution-state structure and dynamics of these important hypervalent iodine compounds.