solve part d 20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded v
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solve part d
20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded views shift area 0.93 3.00 1.56 2.00 3.17 1.00 3.58 2.00 (b) TMS CH3 CH2CH2OH (d) (c) (a) 0 ppm Chemical shift (6) a) Match the protons in the molecule to the corresponding peaks in the spectrum. Explain how you come to your conclusions. (base your explanation on the spin-spin splitting and your understanding of chemical shifts). (6 pts) b) Explain why the peak at 3.58 6 is shifted farthest downfield? (4 pts) c) What is the purpose of the TMS peak? (4 pts) d) In NMR why, in terms of magnet strength, is bigger better? (4 pts)Explanation / Answer
(d) There are two reasons why a bigger (or more correctly a higher) magnetic field strength is better in NMR spectroscopy.
(i) Higher the magnetic field strength, the better is the resolution of the peaks. By resolution, we mean the separation between the peaks on the NMR plot. Chemical shift is reported in (ppm) and we know that
= (sample – TMS)/(Spectrophotometer frequency in MHz)
In case of 1H NMR spectroscopy, we have, TMS = 0. Therefore,
= sample(in Hz)/Spectrophotometer frequency in MHz
Consider a resonance at 1 ppm and we have two NMR instruments with magnetic fields 300 MHz and 800 MHz.
1 ppm = 1*10-6 = sample/(300*106 Hz)
===> sample = 300 Hz
Again, on an 800 MHz instrument,
1 ppm = 1.0*10-6 = sample/(800*106 Hz)
===> sample = 800 Hz
Thus, from the above example, we can loosely say that in the scale, 1 ppm is spread over 1 300 Hz spectral window on a 300 MHz instrument while on a 800 MHz instrument, this spacing or spectral window is 800 Hz. Obviously, the higher the window (we think of this as spacing), the better will be the separation between the peaks and hence the better will be the resolution.
(ii) Higher resolution also leads to better sensitivity. We know that in presence of a magnetic field, the proton can assume two possible orientations: aligned with the field or opposite to the field. The arrangement in which the proton is aligned with the external field is the low energy state and the one where the proton is opposed is the high energy state. Offcourse, molecules tend to occupy the low energy state more. The energy difference between these two states, E is directly proportional to the applied field strength. As the field strength increases, the energy difference increases and more molecules occupy the low energy state. Thus there is greater separation between the two states and this improves sensitivity of the measurement.
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