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?

Verified step by step guidance

1. Identify the formula for chemical shift: $\delta =\frac{\text{Observed shift in Hz}}{\text{Spectrometer frequency in MHz}}$.
2. Calculate the chemical shift for the protons in iodomethane using the 300-MHz spectrometer: $\delta =\frac{650\text{ Hz}}{300\text{ MHz}}$.
3. Use the chemical shift calculated in part (a) to find the chemical shift in a 60-MHz spectrometer. The chemical shift $\delta$ remains constant regardless of the spectrometer frequency.
4. Calculate the observed shift in Hz for the 60-MHz spectrometer using the chemical shift: $\text{Observed shift in Hz}=\delta \times 60\text{ MHz}$.
5. Summarize the results: (a) Chemical shift in ppm, (b) Chemical shift in ppm for 60-MHz, (c) Observed shift in Hz for 60-MHz.

Key Concepts

Chemical Shift

Chemical shift is a key concept in NMR spectroscopy that refers to the resonance frequency of a nucleus relative to a standard reference compound, typically tetramethylsilane (TMS) for protons. It is measured in parts per million (ppm) and indicates the electronic environment surrounding the nucleus, which affects its magnetic field and thus its absorption frequency.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique used to determine the structure of organic compounds. It exploits the magnetic properties of certain nuclei, such as protons, when placed in a magnetic field. The frequency at which these nuclei resonate provides information about their chemical environment, allowing chemists to deduce structural information about the molecule.

Frequency and Magnetic Field Relationship

In NMR, the frequency at which nuclei resonate is directly proportional to the strength of the magnetic field applied. This relationship means that as the magnetic field strength increases (as in a higher MHz spectrometer), the frequency of resonance also increases. Consequently, the chemical shift in hertz can vary depending on the spectrometer's operating frequency, necessitating adjustments when comparing results from different instruments.