Question

Evidence for restricted rotation around amide CO-N bonds comes from NMR studies. At room temperature, the \({ }^{1} \mathrm{H}\) NMR spectrum of \(N, N\) dimethylformamide shows three peaks: \(2.9 \delta\) (singlet, \(3 \mathrm{H}\) ), \(3.0 \delta\) (singlet, \(3 \mathrm{H}\) ), and \(8.0 \delta\) (singlet, \(1 \mathrm{H}\) ). As the temperature is raised, however, the two singlets at \(2.9 \delta\) and \(3.0 \delta\) slowly merge. At \(180^{\circ} \mathrm{C},\) the \({ }^{1} \mathrm{H}\) NMR spectrum shows only two peaks: \(2.95 \delta\) (singlet, \(6 \mathrm{H}\) ) and \(8.0 \delta\) (singlet, \(1 \mathrm{H}\) ). Explain this temperature-dependent behavior.

Short answer

The merging peaks are due to increased rotation around the CO-N bond at higher temperatures. At lower temperatures, restricted rotation presents differing environments for methyl groups.

Step-by-step solution

Understanding the NMR Peaks at Room Temperature

At room temperature, the NMR spectrum of N,N-dimethylformamide has three peaks: at \(2.9\,\delta\), \(3.0\,\delta\), and \(8.0\,\delta\). The peaks at \(2.9\,\delta\) and \(3.0\,\delta\) correspond to the methyl groups attached to the nitrogen atom, both giving singlets of \(3\,\mathrm{H}\). The peak at \(8.0\,\delta\) is due to the formyl hydrogen (\(1\, \mathrm{H}\)). The presence of two different peaks for the methyl groups indicates that they are in different environments due to restricted rotation around the amide \(\text{CO–N}\) bond.

Observing Temperature Changes on NMR Spectrum

As the temperature is increased, the two peaks at \(2.9\,\delta\) and \(3.0\,\delta\) start to merge. At \(180^{\circ} \mathrm{C}\), these peaks coalesce into a single peak located at \(2.95\,\delta\). This final spectrum shows just two peaks: \(2.95\,\delta\) and \(8.0\,\delta\). This indicates that at higher temperatures, the restriction on the rotation around the \(\text{CO–N}\) bond is reduced, allowing the methyl groups to "see" equivalent environments.

Explanation of Restricted vs. Free Rotation

The different chemical shifts for the two methyl groups at room temperature are due to the restricted rotation around the \(\text{CO–N}\) bond in the amide linkage. This amide bond has partial double-bond character due to resonance, making rotation at lower temperatures slow on the NMR timescale; hence the two methyl groups appear non-equivalent. As temperature increases, the increased thermal energy allows the rotation to occur rapidly on the NMR timescale, making the environments of the methyl groups indistinguishable and resulting in a single peak.

Key concepts

Restricted Rotation

In molecules with an amide bond such as N,N-dimethylformamide, restricted rotation around the bond occurs due to resonance. Normally, bonds can rotate freely, but the presence of double-bond character makes this difficult. This partial double-bond character arises from resonance between the carbonyl oxygen and the nitrogen atom, which creates a more rigid structure.

• At lower temperatures, energy levels are insufficient to overcome this rigidity, resulting in a slow rotation on the NMR timescale.
• This slow rotation causes atoms like the methyl groups to appear in different chemical environments.

As temperature increases, however, the added thermal energy allows more rapid rotation, and the differences in chemical environments disappear.

Amide Bond

The amide bond is a link between a carbonyl group and a nitrogen atom. It is a key feature in many organic molecules and is crucial in determining the molecule's properties. The bond possesses partial double-bond character due to resonance.

• This partial double-bond character is due to the overlap of the lone pair on nitrogen with the π orbitals of the carbonyl group.
• Such overlap stabilizes the structure but also increases the rigidity of the bond, restricting rotation.

This restriction is significant in spectroscopy because it often leads to distinct NMR peaks for groups attached to either side of the bond.

Chemical Shifts

Chemical shifts in NMR describe how the local environment of nuclei affects their resonance frequency. Molecules like N,N-dimethylformamide exhibit chemical shifts due to different environments around the nuclei.

• In this case, the two methyl groups attached to nitrogen show slightly different shifts at room temperature ( \(2.9 \,\delta\) and \(3.0 \,\delta\)) because of the restricted rotation.
• As temperature increases, these shifts merge, indicating the environments become equivalent when the restricted rotation decreases.

Understanding chemical shifts is crucial for interpreting NMR spectra and identifying different groups within a molecule.

Resonance

Resonance in the amide bond plays a pivotal role in its behavior. It allows the nitrogen lone pair to delocalize into the carbonyl group, forming a resonance structure. This delocalization results in a partial double-bond character that restricts rotation.

• Resonance structures are necessary to describe the actual hybrid that depicts the bonding in the amide linkage.
• The resonance leads to an increased stability but also causes a higher energy barrier for rotation, as seen in the NMR spectra.

As temperature increases, resonance's effect on restricted rotation decreases, allowing the chemical environments of groups across the amide bond to become more equivalent.

Methyl Groups

Methyl groups are simple hydrocarbon groups important in organic chemistry. In N,N-dimethylformamide, two methyl groups are attached to nitrogen.

• These groups usually appear as peaks in an NMR spectrum, and their chemical shifts can provide valuable information about their chemical environment.
• At room temperature, restricted rotation around the amide bond makes these two methyl groups experience different environments, leading to distinctive peaks.
• Upon increasing temperature, these environments become equivalent, so the peaks merge into one in the NMR spectrum.

Methyl groups serve as excellent indicators of molecular dynamics, especially rotational freedom and neighboring group effects.