Question

All of the following are true conceming nmr spectroscopy EXCEPT: A. Protons are distinguished when they absorb magnetic energy at different field strengths. B. Downfield is to the left on an nmr spectrum. C. Functional groups are distinguished when they absorb magnetic energy at different field strengths. D. Delocalized electrons generate magnetic fields that can either shield or deshield nearby protons.

Short answer

Statement C is incorrect.

Step-by-step solution

- Understanding NMR Spectroscopy

NMR (Nuclear Magnetic Resonance) spectroscopy is a technique used to observe the local magnetic fields around atomic nuclei. It relies on the principles of nuclear magnetic resonance to provide information about the structure, dynamics, reaction state, and chemical environment of molecules.

- Evaluating Statement A

Statement A says that protons are distinguished when they absorb magnetic energy at different field strengths. This is a true statement because in NMR spectroscopy, different protons in the molecule absorb energy at varying magnetic field strengths, depending on their chemical environment.

- Evaluating Statement B

Statement B claims that downfield is to the left on an NMR spectrum. This is also a true statement. In an NMR spectrum, 'downfield' refers to signals that appear towards the left (higher chemical shift), while 'upfield' refers to signals towards the right (lower chemical shift).

- Evaluating Statement C

Statement C suggests that functional groups are distinguished when they absorb magnetic energy at different field strengths. This is not true. NMR distinguishes functional groups based on the chemical shift and splitting patterns related to the chemical environment of the protons, not the magnetic field strengths directly.

- Evaluating Statement D

Statement D asserts that delocalized electrons generate magnetic fields that can either shield or deshield nearby protons. This is indeed true, as the presence of delocalized electrons can affect the local magnetic field experienced by protons, either shielding or deshielding them and thus affecting their chemical shift.

- Identifying the Incorrect Statement

Based on the evaluations, Statements A, B, and D are true. Only Statement C is incorrect because functional groups in NMR are distinguished by chemical shifts and coupling patterns, not by the absorption of magnetic energy at different field strengths.

Key concepts

Chemical Shift

In NMR spectroscopy, the term 'chemical shift' refers to the position where a signal appears on the NMR spectrum. It’s denoted by the symbol δ.

The chemical shift provides information about the electronic environment surrounding a nucleus in a molecule. Protons experience different chemical shifts based on the atoms or groups adjacent to them.

Here’s what you need to know about chemical shifts:

• Chemical shifts are measured in parts per million (ppm).
• A higher chemical shift (downfield) means the proton is deshielded.
• A lower chemical shift (upfield) means the proton is shielded.

Knowing the chemical shift helps chemists identify the structural features of a molecule.

Shielding and Deshielding

Shielding and deshielding describe how the local electronic environment influences the magnetic field around a nucleus.

Shielding occurs when electrons around a proton generate a local magnetic field that opposes the external magnetic field. This makes the effective magnetic field experienced by the proton lower. As a result, the proton absorbs energy at a lower frequency, leading to an upfield shift.

Deshielding happens when the local electrons do not effectively counter the external magnetic field. This makes the effective magnetic field experienced by the proton higher. Consequently, the proton absorbs energy at a higher frequency, resulting in a downfield shift.

Key points to remember:

• Shielded protons appear upfield (lower δ value).
• Deshielded protons appear downfield (higher δ value).
• Electronegativity of nearby atoms can cause deshielding.
• Electron-donating groups can lead to shielding effects.

Magnetic Field Strength

Magnetic field strength is a critical factor in NMR spectroscopy. It determines the energy difference between different nuclear spin states.

Protons absorb energy at different magnetic field strengths depending on their chemical environment. This absorption leads to resonance and is recorded as an NMR signal.

Here’s a breakdown:

• Higher magnetic field strengths provide better resolution and separation of NMR signals.
• The frequency at which protons resonate is directly proportional to the magnetic field strength.
• Functional groups do not absorb energy strictly based on field strength; instead, it’s their chemical environment that dictates their NMR signature.

Understanding magnetic field strength helps in interpreting NMR spectra and distinguishing between different protons in a molecule.