NMR Chemical Shift

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Questions and Answers

What is the range of the chemical shift of carbon in an NMR spectrum?

~0 - 220 ppm

~0 - 1000 ppm

~0 - 50 ppm

~0 - 200 ppm (correct)

What type of energy is absorbed in NMR spectroscopy?

Ultraviolet radiation

Infrared radiation

X-ray radiation

Radiofrequency radiation (correct)

What is the horizontal axis of an NMR spectrum referred to as?

Wavelength axis

Intensity axis

Frequency axis

Chemical shift axis (correct)

What is the purpose of tetramethylsilane (TMS) in NMR spectroscopy?

It is a reference compound Signup and view all the answers

What is the significance of the signals in an NMR spectrum?

They correspond to the carbon atoms in the molecule Signup and view all the answers

What is the property of certain isotopes of most elements that is utilized in NMR spectroscopy?

Angular momentum or spin Signup and view all the answers

What is the effect of the magnetic field generated by the spins of electron clouds on the applied magnetic field?

It can either partially cancel or partially augment the applied field Signup and view all the answers

Why do signals for different carbons appear at different positions in the NMR spectrum?

Because of differences in the local magnetic field experienced by each nucleus Signup and view all the answers

What is plotted in an NMR spectrum?

The frequency of the signal versus the intensity of the signal Signup and view all the answers

Why is the Hz scale not convenient to use in NMR spectroscopy?

Because it is proportional to the strength of the applied magnetic field Signup and view all the answers

What is the purpose of using a relative scale in NMR spectroscopy?

To allow for comparison of results from different instruments Signup and view all the answers

What happens to the frequencies of the signals in an NMR spectrum when the magnetic field strength is doubled?

They double Signup and view all the answers

Why was it not possible to add individual scans in older instruments?

Because of slight drifting of the magnetic field Signup and view all the answers

What is the purpose of the internal lock on the field in modern FT spectrometers?

To allow multiple scans to be added together Signup and view all the answers

What is generated by the short powerful radio frequency pulse in a FT instrument?

The entire desired frequency range Signup and view all the answers

What is the result of Fourier Transform on the time domain information?

The NMR spectrum Signup and view all the answers

Why is a delay introduced in the pulse sequence?

To allow the nuclei to relax back to the ground state Signup and view all the answers

What must the nuclei have done before repeating the pulse or signal intensity will be lost?

Relaxed back to the ground state Signup and view all the answers

Study Notes

13C Coupled Spectra

Solvent peak observed in 13C coupled spectra can be ignored, e.g., chloroform (CDCl3) shows a triplet peak at ~77 ppm.

NMR Spectra

NMR spectrometer output is an NMR spectrum (plural: spectra).
Horizontal axis (chemical shift) has units of parts per million (ppm) and ranges from ~0-200 ppm for carbon atoms.
0 ppm is an arbitrary assignment for the chemical shift of tetramethylsilane (TMS), a reference compound.
Signals in the spectrum correspond to carbon atoms in the molecule, while protons do not appear.
Each carbon atom is magnetically distinct and has a unique chemical shift.

How NMR Works

NMR spectroscopy involves absorption of energy to change energy level from ground state to excited state.
Energy absorbed is in the radiofrequency range.
Nuclei of certain isotopes of most elements have an intrinsic property of angular momentum or spin.

NMR Frequencies and Chemical Shift

Each nucleus experiences a slightly different local magnetic field due to electron clouds generating a magnetic field.
Local magnetic field can either partially cancel (shielding) or partially augment the applied field (deshielding).
Differences in local magnetic field result in kilohertz frequency differences from the irradiating frequency, causing signals to appear at different positions in the spectrum.

Chemical Shift Scale

Hz scale is not convenient to plot due to direct proportionality to magnetic field strength.
To remove this issue, a relative scale is used where chemical shift is relative to a reference compound (e.g., TMS).

Pulsed Fourier Transform (FT) Spectrometers

Modern FT spectrometers use an internal lock on the magnetic field, allowing multiple scans to be added together to increase signal to noise.
A short, powerful radiofrequency pulse excites all nuclei (e.g., carbon or proton) simultaneously.
Nuclei relax back to the ground state, producing a free induction decay (FID) signal, which contains frequency difference signals for all nuclei.
FID signal is an interferogram containing decay signals of all carbons present.
Sequence can be repeated many times, and FIDs added to increase signal to noise.
Nuclei must have fully relaxed before repeating the pulse to avoid signal intensity loss.

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Description

Learn about the effect of electron spins on local magnetic fields in NMR spectroscopy. Understand how it causes slight frequency differences and chemical shifts. Test your knowledge on NMR frequencies and chemical shifts.