Question

At room temperature, the nmr spectrum of 1 -ethylaziridine + CYCLOA shows the triplet-quartet of an ethyl group, and two other signals of equal peak area. When the temperature is raised to \(120^{\circ}\), the latter two signals merge into a single signal. How do you interpret these observations?

Short answer

In the given NMR spectrum at room temperature, the two additional signals with equal peak area correspond to hydrogens in CYCLOA with similar but distinct chemical environments. This distinction could be due to conformation or chemical exchange processes. Upon raising the temperature to 120°C, the rate of the exchange process or conformational change increases, leading to an averaging of the chemical shifts of these hydrogens and causing the two signals to merge into a single signal. This suggests that the compound CYCLOA undergoes conformational change or has exchangeable protons whose chemical environments are affected by temperature.

Step-by-step solution

Recall the Ethyl Group Signals in NMR

The ethyl group in NMR typically gives rise to a triplet-quartet pattern, which we observe in the given spectrum. This pattern is composed of a triplet signal (corresponding to the methylene group, -CH2-) and a quartet signal (corresponding to the methyl group, -CH3). The splitting pattern occurs due to the scalar coupling between the adjacent hydrogens.

Identify the other two signals

The NMR spectrum shows two other signals of equal peak area besides the triplet-quartet of the ethyl group. These signals likely arise from the hydrogens in the unknown compound CYCLOA. Since these signals have equal peak area, we can infer that the corresponding hydrogens are chemically equivalent or have very similar chemical environments.

Analyze the effect of raising the temperature

Raising the temperature to 120°C causes the two equal peak area signals to merge into a single signal. This suggests that increasing temperature removes the differences in the chemical environment of the hydrogens that were previously observed as two separate signals. This effect can happen due to the increasing rate of chemical exchange or conformational change of CYCLOA at higher temperatures. As the temperature increases, the speed of the exchange process or the transition between different conformations also increase, leading to an average chemical shift for the corresponding hydrogens.

Interpret the observations

Based on the information above, we can interpret these observations as follows: At room temperature, the two additional signals in the spectrum belong to the hydrogens in CYCLOA and have similar but distinct chemical environments. These differences could be due to the conformation or chemical exchange process. When the temperature is raised to 120°C, the rate of the exchange process or conformational change increases, causing the two signals to appear as one due to averaging of their chemical shifts. This observation implies that the unknown compound CYCLOA undergoes conformational change or has exchangeable protons whose chemical environments are affected by temperature.

Key concepts

Ethyl Group Signals

In NMR spectroscopy, the presence of ethyl groups is often identified through a characteristic triplet-quartet pattern. This pattern arises due to the scalar coupling between neighboring hydrogen atoms within the ethyl group.

• The triplet corresponds to the methylene group (-CH2-), as it is typically split into three peaks by the adjacent methyl group’s three hydrogens.
• The quartet, on the other hand, is formed by the coupling of the methyl group (-CH3-) and the methylene group’s hydrogens.

This distinct splitting pattern helps chemists identify and confirm the presence of ethyl groups within a molecule, confirming the structure of the given compound.

Chemical Shift

Chemical shift is a fundamental concept in NMR spectroscopy, referring to the resonant frequency of a nucleus relative to a standard in a magnetic field.

• This shift provides insights into the electronic environment surrounding a nucleus, helping identify the types of atoms or functional groups in a molecule.
• In the exercise, two additional signals of equal peak area suggest that despite being distinct, the hydrogens exhibit similar electronic environments at room temperature.

The merging of these signals at elevated temperatures indicates that these environments are affected by changes in temperature, an important observation in understanding molecule dynamics and functionality.

Conformational Change

Conformational change plays a critical role in determining how a molecule behaves under different conditions. Such changes involve the structural rearrangement of a molecule, often leading to a visible change in its NMR spectrum.

• In the context of CYCLOA, at room temperature, hydrogens appear in distinct environments, resulting in separate signals.
• However, as the temperature increases, the molecule experiences conformational movements that average out these environments, causing the signals to merge.

Understanding these changes can provide invaluable clues to the molecule’s structure and its potential interactions or reactivity under different conditions.

Temperature Effects on NMR

Temperature has a significant impact on the NMR spectra due to its influence on molecular dynamics and kinetics.

• At elevated temperatures, molecules experience faster motions, which can lead to averaging of chemical environments and alterations in chemical shift.
• In the case of CYCLOA, raising the temperature to 120°C induces a conformational transition that merges two hydrogen signals into one.

This effect highlights the importance of temperature in NMR studies, providing insights into the dynamic behavior and flexibility of molecules that might otherwise be missed at lower temperatures. Understanding these changes deepens the analysis of molecular structures and their reactions.