Question

The fluorine nmr spectrum of 1,2 -difluoro-tetrachloro-ethane, \(\mathrm{CFCI}_{2} \mathrm{CFCI}_{2}\) shows a single peak at room temperature, but at \(-120^{\circ}\) shows two peaks of unequal area. Explain.

Short answer

At room temperature, the rotation of the 1,2-difluoro-tetrachloro-ethane molecule averages out the local environments of the two fluorine atoms, making them indistinguishable and resulting in a single peak in the NMR spectrum. However, at -120°C, the reduced molecular motion causes the local environments of the two fluorine atoms to be recognized separately, leading to two peaks of unequal area. The unequal peak areas are due to the different electronic environments of the fluorine atoms caused by their neighboring chlorine atoms.

Step-by-step solution

Understanding the molecular structure of 1,2-difluoro-tetrachloro-ethane

The molecule is composed of a central ethane group, in which two carbons are bonded together. Each carbon is directly bonded to one fluorine atom and two chlorine atoms: \(\mathrm{CFCI}_{2} \mathrm{CFCI}_{2}\). The molecule has a mirror plane, resulting in a symmetric structure.

Analyzing fluorine NMR spectroscopy

Fluorine NMR spectroscopy is used to analyze the local electronic environment of fluorine atoms in a molecule. The chemical shift reflects the differences in the magnetic environment of each fluorine atom. In a symmetric molecule, fluorine atoms in similar environments will have the same chemical shift and contribute to a single peak.

Temperature effects on molecular motion

At room temperature, the ethane molecule can rotate freely around its carbon-carbon bond. This rotation averages the local environment of the two fluorine atoms, making them indistinguishable in NMR spectroscopy. Thus the spectrum will show only a single peak.

Temperature effects at -120°C

When the temperature is decreased to -120°C, the rotational motion of the molecule is significantly slowed down. As a result, the fluorine atoms' local environments are no longer averaged out, and the NMR spectrum will exhibit separate peaks for the individual fluorine atoms.

Explaining unequal peak areas

The two peaks have unequal areas because the local environment of each fluorine atom is not exactly the same in the molecule. Even though the molecule is symmetric in terms of geometry, each fluorine atom is attached to a carbon, and the carbon is bonded to two chlorine atoms. The fluorine nucleus interacts differently with the three neighboring chlorine atoms due to these different electronic environments. As a result, the two fluorine NMR signals will have different intensities. Overall, the behavior of 1,2-difluoro-tetrachloro-ethane in the fluorine NMR spectrum is influenced by the temperature. At room temperature, rotations average out the differences in the local environments of the fluorine atoms, resulting in a single peak. At -120°C, the slowed molecular rotation reveals the distinct environments of the two fluorine atoms, resulting in two peaks of unequal area.

Key concepts

NMR chemical shifts

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool used to determine the molecular structure of a compound. The chemical shift is a critical parameter in NMR spectroscopy, reflecting the resonance frequency of a nucleus relative to a standard reference compound. In fluorine NMR, chemical shifts provide insights into the electronic environment around fluorine nuclei in a molecule.

Each nucleus in an NMR-active isotope, like fluorine-19, can absorb electromagnetic radiation at a characteristic frequency when placed in a magnetic field. This frequency is affected by the electron cloud around the nucleus; electron-rich environments shield the nucleus and cause an upfield shift (lower ppm), while electron-poor environments deshield the nucleus and result in a downfield shift (higher ppm).

The number and position of peaks in an NMR spectrum can reveal how many distinct electronic environments there are. For example, in 1,2-difluoro-tetrachloro-ethane at room temperature, a single NMR peak suggests that both fluorine atoms are in equivalent electronic environments due to molecular symmetry and motion, which can lead to an averaging effect.

Molecular symmetry

Molecular symmetry plays a significant role in determining the number of peaks observed in NMR spectra. Symmetrical molecules, such as 1,2-difluoro-tetrachloro-ethane, tend to have fewer peaks because identical substituents experience similar electronic environments.

In terms of symmetry elements, 1,2-difluoro-tetrachloro-ethane possesses a mirror plane which bisects the molecule through the carbon-carbon bond, meaning that one half of the molecule is a reflection of the other. As a result, the two fluorine atoms are in mirrored environments, leading to identical chemical shifts under conditions where molecular rotation is not restricted.

However, if the symmetry is broken by any means, such as cooling the sample to significantly lower temperatures where molecular rotations are hindered, each distinct environment will result in a separate NMR peak. This is observed in the two peaks formed at -120°C in the fluorine NMR spectrum of 1,2-difluoro-tetrachloro-ethane, indicating that the molecule's symmetry does not average out at this temperature.

Temperature effects on NMR

Temperature plays a pivotal role in the behavior of molecules as observed by NMR spectroscopy. It influences molecular motion and can induce changes in the chemical shifts and splitting patterns of an NMR spectrum.

At higher temperatures, molecules have enough thermal energy to rotate rapidly around bonds, such as the carbon-carbon bond in 1,2-difluoro-tetrachloro-ethane. This rotational movement is fast on the NMR timescale, which causes an averaging of the local environments experienced by the fluorine atoms, resulting in a single peak in the NMR spectrum.

When the temperature is lowered, these rotational motions slow down and can even freeze, as is the case at -120°C for 1,2-difluoro-tetrachloro-ethane. Slowed molecular rotation prevents the averaging process, allowing the NMR instrument to distinguish between the slightly different environments of the two fluorine atoms, producing two distinct peaks in the spectrum.

Molecular rotation

Molecular rotation refers to the spinning of molecules around their bonds, a movement that can average out the small differences in the electronic environments of nuclei in symmetrical molecules. In NMR spectroscopy, this is particularly important as it can change the number of observed peaks due to the rapid averaging of chemically equivalent sites.

In the example of 1,2-difluoro-tetrachloro-ethane, at room temperature, the rapid rotation about the central carbon-carbon bond makes the two fluorine atoms equivalent on the timescale of the NMR experiment, which exhibits as a single peak. However, the cooling of this compound to -120°C restricts this rotation, causing the individual environments of the fluorine atoms to be observed separately on the NMR spectrum.

The degree to which rotation impacts an NMR spectrum depends on the temperature and the energy barriers to internal rotation. For molecules with higher barriers to rotation, even at room temperature, one might observe multiple peaks corresponding to different environments.