NMR VL2 chemical shift definition of the ppm scale

Summary

This video explains how to report and compare chemical shifts in NMR spectroscopy, focusing on the challenges posed by different magnetic field strengths. It introduces the concept of the PPM (parts per million) scale, which standardizes resonance frequencies for easy comparison across different instruments. The video discusses the role of TMS (tetramethylsilane) as the standard chemical shift reference and explains the mathematics behind the PPM calculation. It also covers historical NMR data, highlighting the shift from the outdated 'Tow' scale to PPM. Finally, it explores how different nuclei have varying chemical shift ranges, making NMR data more accessible and comparable.

Takeaways

• 😀 NMR resonance frequencies vary with the magnetic field strength, making direct comparison across instruments difficult.
• 😀 Early NMR instruments had low field strengths (around 1.4 Tesla), producing low resonance frequencies that were hard to interpret.
• 😀 Differences in shielding between compounds cause small differences in resonance frequencies, which are measurable but subtle.
• 😀 To standardize NMR data, the PPM (parts per million) scale was introduced, allowing chemical shifts to be compared across different instruments and field strengths.
• 😀 The PPM scale is calculated by taking the difference between the sample's resonance frequency and a standard's frequency, dividing by the standard, and multiplying by one million.
• 😀 TMS (tetramethylsilane) is the standard for hydrogen, carbon, and silicon NMR, chosen for its high shielding, chemical inertness, volatility, and solubility.
• 😀 For aqueous solutions, other standards like DSS or TSP are used, typically focusing on specific signals from hydrophilic groups.
• 😀 Using the PPM scale, chemical shifts become consistent regardless of instrument or field strength, simplifying comparison of NMR data from different sources.
• 😀 Typical chemical shift ranges vary by nucleus: hydrogen is usually 0–10 PPM, carbon-13 is 0–200 PPM, and heavier elements can have even larger ranges.
• 😀 Very old literature may report chemical shifts using the 'τ scale' (τ = 10 – δ), which is uncommon in modern NMR reporting.

Q & A

Why is it difficult to compare NMR data recorded on instruments with different magnetic field strengths?

Because the resonance frequency of nuclei depends directly on the magnetic field strength. Different instruments produce different resonance frequencies for the same sample, making raw data incomparable.

What is the purpose of the PPM (parts per million) scale in NMR spectroscopy?

The PPM scale normalizes chemical shifts, allowing resonance frequencies to be compared across instruments with different magnetic field strengths. It expresses the relative difference between a sample's resonance frequency and a standard.

How is the chemical shift in PPM calculated?

The chemical shift δ in PPM is calculated using the formula: δ(PPM) = ((ν_sample − ν_standard) / ν_standard) × 10^6, where ν_sample is the resonance frequency of the sample and ν_standard is the resonance frequency of the reference standard.

Why is TMS (tetramethylsilane) commonly used as a reference standard in NMR?

TMS is chemically inert, highly shielded, soluble in most organic solvents, volatile (easy to remove), and provides signals at a very high shielding level, ensuring most chemical shifts remain positive.

Which compounds are used as chemical shift standards in aqueous solutions?

DSS (sodium 2,2-dimethyl-2-silapentane-5-sulfonate) and TSP (sodium 3-(trimethylsilyl)propionate) are used as standards in aqueous solutions, with signals from their hydrophilic groups serving as the reference.

What are the typical PPM ranges for hydrogen and carbon-13 nuclei in NMR?

Hydrogen (¹H) chemical shifts usually range from 0 to 10 PPM, while carbon-13 (¹³C) chemical shifts typically range from 0 to over 200 PPM. Heavier nuclei generally have even broader ranges.

How does the PPM scale help when reading older literature?

Older literature may have been recorded on instruments with different field strengths. Using the PPM scale allows modern chemists to directly compare and reproduce these results without recalculating resonance frequencies.

What is the τ-scale, and where might it be encountered?

The τ-scale is an old chemical shift scale defined as τ = 10 − δ. It is mostly found in historical chemistry papers and is rarely used in modern NMR practice.

Why are deuterated versions of chemical shift standards sometimes used?

Deuterated standards are used to prevent additional signals from other hydrogen atoms in the standard, which could interfere with the sample's NMR spectrum, making interpretation clearer.

How does increasing magnetic field strength affect raw resonance frequencies?

Higher magnetic field strengths increase the resonance frequencies proportionally. For example, trichloromethane might resonate around 60 MHz at 1.4 Tesla but around 600 MHz at 14 Tesla, illustrating the need for normalization via PPM.

Why is it advantageous for the reference standard to have the highest shielding?

Using a highly shielded standard ensures that most chemical shifts in the sample are positive, simplifying interpretation and maintaining consistency across different nuclei and solvents.

Can chemical shifts be directly transferred between different NMR instruments once expressed in PPM?

Yes. Once chemical shifts are expressed in PPM, they are largely independent of the magnetic field strength, allowing direct comparison between different instruments and published data.