The ^1 H-NMR and ^13 C-NMR spectra of both the anion and...

The ${ }^{1} \mathrm{H}-\mathrm{NMR}$ and ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectra of both the anion and cation derived from phenalene have been observed. The HMO pattern for phenalene is given below. The chemical shifts observed for the cation and anion are given below. $$ \begin{array}{lllllll} \hline & {{ }^{1} \mathrm{H}-\mathrm{NMR}} & &{{ }^{13} \mathrm{C}-\mathrm{NMR}} \\ \hline & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(9 \mathrm{~b}) & \mathrm{C}(3 \mathrm{a}) \\ {[\text { Cation }]} & 9.38 & 8.55 & 155.5 & 133.7 & 123.7 & 133.7 \\ {[\text { Anion }]} & 5.36 & 6.10 & 103.4 & 128.0 & 139.6 & 145.0 \\ \hline \end{array} $$ What conclusions can be drawn about electron distribution in the cation and anion from the NMR data and how does it relate to the HMO pattern?

The ${ }^{1} \mathrm{H}-\mathrm{NMR}$ and ${ }^{13} \mathrm{C}-\mathrm{NMR}$ spectra of both the anion and cation derived from phenalene have been observed. The HMO pattern for phenalene is given below.
The chemical shifts observed for the cation and anion are given below.
$$
\begin{array}{lllllll}
\hline & {{ }^{1} \mathrm{H}-\mathrm{NMR}} & &{{ }^{13} \mathrm{C}-\mathrm{NMR}} \\
\hline & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(9 \mathrm{~b}) & \mathrm{C}(3 \mathrm{a}) \\
{[\text { Cation }]} & 9.38 & 8.55 & 155.5 & 133.7 & 123.7 & 133.7 \\
{[\text { Anion }]} & 5.36 & 6.10 & 103.4 & 128.0 & 139.6 & 145.0 \\
\hline
\end{array}
$$
What conclusions can be drawn about electron distribution in the cation and anion from the NMR data and how does it relate to the HMO pattern?

Key Concepts

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy is a fundamental tool in organic chemistry that measures how the nuclei of atoms, typically hydrogen and carbon, interact with their electronic environment. The resonance frequencies (chemical shifts) of these nuclei are sensitive to local electron density, allowing chemists to infer details about bond structures, electron distribution, and molecular geometry. This technique is central for investigating differences in electron environments within a molecule or between related ionic species such as cations and anions.

Chemical Shift in NMR

The chemical shift observed in NMR spectra reflects the degree of shielding or deshielding experienced by a nucleus, which depends on the local electron density. Regions with high electron density generally cause increased shielding, translating to lower chemical shift values, whereas electron-deficient areas cause deshielding and higher chemical shifts. Analyzing the variations in chemical shift helps in understanding the distribution of electrons across the molecule, indicating regions of electron accumulation or depletion.

Electron Density Distribution

Electron density distribution defines how electrons are spread over a molecule, influencing its chemical behavior and reactivity. In aromatic and conjugated systems, variations in electron density can lead to differences in stability, reactivity, and spectroscopic properties. Differences in the electron distribution between cations and anions arise from changes in the number of electrons, affecting the magnetic environment around nuclei and thus their chemical shifts in NMR spectroscopy.

Hückel Molecular Orbital (HMO) Theory

Hückel Molecular Orbital theory is a simplified computational method used to understand the electronic structure of conjugated systems. It provides a way to predict the distribution and energy levels of ?-electrons in aromatic compounds by considering the overlap of p orbitals. The theory predicts which molecular orbitals are occupied and their nodal characteristics, which has direct implications for electron delocalization and electron density distribution within the molecule.

Relationship Between HMO Patterns and NMR Chemical Shifts

The HMO pattern of a molecule offers a theoretical framework for predicting the spatial distribution of electron density in conjugated systems. The calculated orbital coefficients and phases from HMO theory correlate with how electrons are delocalized, affecting the shielding effects observed in NMR measurements. By comparing the theoretical HMO electron distribution with experimental NMR data, conclusions can be drawn about how electron-rich or electron-poor regions in a molecule influence chemical shifts, thus linking quantum chemical predictions to observable spectroscopic properties.

Transcript

Please give Ace some feedback