Question

In a 300-MHz spectrometer, the protons in iodomethane absorb at a position 650 Hz downfield from TMS.
(a) What is the chemical shift of these protons?
(b) What is the chemical shift of the iodomethane protons in a 60-MHz spectrometer?
(c) How many hertz downfield from TMS would they absorb at 60 MHz?

Accepted Answer

To solve part (a), recall that the chemical shift (δ) is calculated using the formula: δ = (ν_sample - ν_TMS) / ν_spectrometer, where ν_sample is the frequency of the sample, ν_TMS is the frequency of TMS, and ν_spectrometer is the operating frequency of the spectrometer. Here, ν_sample - ν_TMS is given as 650 Hz, and ν_spectrometer is 300 MHz. Convert 300 MHz to Hz (1 MHz = $$10^6 Hz) $$and calculate δ in parts per million (ppm). For part (b), note that the chemical shift (δ) is independent of the spectrometer frequency. The value of δ calculated in part (a) will remain the same for the 60-MHz spectrometer. This is because δ is a ratio and does not depend on the absolute spectrometer frequency. To solve part (c), use the relationship between the chemical shift (δ) and the spectrometer frequency. The formula is: ν_sample - ν_TMS = δ × ν_spectrometer. Here, δ is the value calculated in part (a), and ν_spectrometer is 60 MHz (convert to Hz). Multiply δ by 60 MHz to find the number of hertz downfield from TMS at 60 MHz. Ensure that all units are consistent throughout the calculations. Convert MHz to Hz where necessary and express the final answers in the appropriate units (ppm for chemical shift and Hz for frequency differences). Review the concepts of chemical shift and its independence from spectrometer frequency to reinforce understanding. The chemical shift is a dimensionless quantity expressed in ppm, which standardizes NMR data across different spectrometers.

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Key Concepts

Chemical Shift

NMR Spectroscopy

Frequency and Magnetic Field Relationship