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Related Concept Videos

NMR Spectroscopy of Benzene Derivatives01:37

NMR Spectroscopy of Benzene Derivatives

Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling constants depend...
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range. Consider...
Structure of Benzene: Kekulé Model01:07

Structure of Benzene: Kekulé Model

In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
Structure of Benzene: Molecular Orbital Model01:18

Structure of Benzene: Molecular Orbital Model

According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...

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Benz[cd]indol-2(1H)-one at 298 and 100 K.

Saeed I Khan1, Carolyn B Knobler, Emily F Maverick

  • 1Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA 90095-1569, USA.

Acta Crystallographica. Section C, Crystal Structure Communications
|January 7, 2012
PubMed
Summary
This summary is machine-generated.

Benz[cd]indol-2(1H)-one (naphtholactam) crystals exhibit disorder due to a 180° rotation of cis-lactam dimers. This molecular disorder influences crystal packing and amide bond length, as revealed by low-temperature X-ray diffraction.

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Area of Science:

  • Crystallography
  • Organic Chemistry
  • Solid-State Chemistry

Background:

  • Benz[cd]indol-2(1H)-one (naphtholactam) crystals are challenging for diffraction studies due to weak diffraction.
  • Previous room-temperature data collection in 1989 indicated crystal disorder.

Purpose of the Study:

  • To investigate the molecular structure and disorder in naphtholactam crystals.
  • To refine disorder models using advanced crystallographic techniques at low temperatures.

Main Methods:

  • Single-crystal X-ray diffraction data collection at 100 K using an area detector.
  • Refinement of two distinct disorder models involving cis-lactam dimer rotation.
  • Analysis of molecular geometry and crystal packing interactions.

Main Results:

  • The crystal disorder, characterized by a 180° rotation of centrosymmetric cis-lactam dimers, was confirmed at 100 K.
  • Two refined disorder models provided a detailed description of the molecular arrangement.
  • A C-N amide bond distance of 1.38 Å was observed, longer than typical saturated γ-lactams.
  • Opposing-dipole dimer interactions were identified as a significant cohesive packing force.

Conclusions:

  • The study elucidates the nature of crystal disorder in naphtholactam, attributed to dimer rotation.
  • Low-temperature diffraction data enabled accurate structural refinement and analysis of bonding.
  • Crystal packing forces, specifically dipole-dipole interactions, likely govern the observed molecular occupancy ratio.