Question

Is a nucleus that absorbs at \(6.50 \delta\) more shielded or less shielded than a nucleus that absorbs at \(3.20 \delta\) ? Does the nucleus that absorbs at \(6.50 \delta\) require a stronger applied field or a weaker applied field to come into resonance than the nucleus that absorbs at \(3.20 \delta ?\)

Short answer

At \(6.50 \, \delta\), the nucleus is less shielded and requires a weaker field.

Step-by-step solution

Understand Chemical Shifts

Chemical shift in NMR is often expressed in ppm (parts per million) and is denoted by \( \delta \). A higher \( \delta \) value indicates a more downfield shift, which is usually associated with less shielding of the nucleus by surrounding electrons.

Compare Chemical Shift Values

Compare the two chemical shift values: \(6.50 \, \delta\) and \(3.20 \, \delta\). Since \(6.50 \, \delta\) is higher than \(3.20 \, \delta\), this suggests the nucleus absorbing at \(6.50 \, \delta\) is less shielded.

Determine the Applied Field Strength

A nucleus that is less shielded requires a weaker applied magnetic field to reach resonance compared to a more shielded nucleus. Therefore, the nucleus at \(6.50 \, \delta\) requires a weaker applied field than the nucleus at \(3.20 \, \delta\).

Key concepts

Understanding Chemical Shift

Chemical shift is a fundamental concept in NMR (Nuclear Magnetic Resonance) spectroscopy. It represents the variation of the resonance frequency of a nucleus due to its electronic environment relative to a standard reference. Expressed in parts per million (ppm) and symbolized by \( \delta \), the chemical shift allows us to infer about the chemical structure of a molecule.

A higher \( \delta \) value suggests a more downfield resonance, which typically means that the nucleus is less shielded by its surrounding electrons. In essence, it reflects the amount of electron density surrounding the nucleus. Conversely, a lower \( \delta \) indicates an upfield resonance and typically hints at more electron density, meaning the nucleus is more shielded.

This difference in chemical shift can provide insights into the types of atoms or functional groups present in a molecule, aiding chemists in identifying molecular structures.

Shielding and Deshielding Effects

In NMR spectroscopy, shielding and deshielding describe how the electron cloud around a nucleus affects the magnetic environment it experiences. Shielding occurs when electrons circulate around a nucleus, creating a local magnetic field that opposes the external applied magnetic field. This electron-induced magnetic field reduces the net field experienced by the nucleus.

More shielding suggests that there are more electrons present. These electrons shield the nucleus from the external magnetic field, resulting in a lower chemical shift. This usually appears as an upfield signal in the NMR spectrum. Deshielding, on the other hand, happens when electron-withdrawing groups or other causes reduce electron density around the nucleus. This results in the nucleus experiencing a stronger external magnetic field, leading to a higher chemical shift value, observed as a downfield signal.

Therefore, comparing nuclei at \(6.50 \, \delta\) and \(3.20 \, \delta\), the nucleus at the higher \( \delta \) is more deshielded.

The Role of the Applied Magnetic Field

The applied magnetic field in NMR plays a crucial role in the resonance of nuclei. It influences which nuclei in a sample resonate and at what frequencies. When nuclei are less shielded, they require a weaker applied magnetic field to reach resonance compared to more shielded nuclei. This is because the net magnetic field they experience is stronger due to reduced electron density.

In practical terms, a nucleus absorbing at a higher \( \delta \) value (like \(6.50 \, \delta\)) experiences less electron shielding, making the external magnetic field feel stronger. Thus, a weaker applied field is needed for it to come into resonance. Conversely, a nucleus at a lower \( \delta \) value (such as \(3.20 \, \delta\)) is more shielded and requires a stronger applied field to achieve resonance.

Understanding these interactions helps chemists accurately interpret NMR spectra and deduce structural information about the compounds they study.