ASR NMR Tutorial

1 Anderson Serangoon Junior College 2025 H3 Chemistry Nuclear Magnetic Resonance (NMR) Tutorial 1 (a) Outline in simple terms the principles of nuclear magnetic resonance. [N2008/2b(i)] In absence of magnetic field, the spin of the nucleus with odd proton number will oriented randomly. Once an external magnetic field is applied, they reorient their spin either aligned with the field or against the field. Orientation parallel to alignment of applied field is lower in energy that the one against the field. When nuclei are irradiated with radiowave, the lower energy nuclei absorbed the photon and flip to the orientation with a higher energy state. These nuclei are said to be in resonance. Nuclei in different chemical environment will have different energy gap between the two spin states and hence they will absorb the radiowave radiation at different frequency. (b) Explain the use of each of the following in 1H NMR spectroscopy:  TMS  Deuterated solvent  D2O 2015/2d(iii) TMS is used as an internal reference as it is volatile and inert. The four carbon atoms attached to silicon in TMS are all equivalent. Silicon is more electropositive than carbon. Hence, the carbon are fairly electron -rich (or shielded), which means they resonate at frequency a little less than that of most organic compounds. Hence, it produces a strong singlet peak at a lower frequency than most 1H absorptions in organic molecules, so its peak does not interfere with the other peaks. It will show a sharp singlet assigned δ = 0.The chemical shifts of protons are calculated using the difference between proton absorption frequency and that of TMS, divided by the irradiating frequency.   ( ( ) ( )) ( ) x TMS x TMS frequency Hz frequency Hz frequency MHz where x = chemical shift of proton x frequencyx = frequency of absorption of proton x frequencyTMS = frequency of absorption of the protons in TMS Deuterated solvent is used to dissolve the sample. Since the deuterium nuclei does not absorb in the same frequency range as 1H nuclei, the solvent does not complicate the NMR spectra. D2O allows identification of labile protons. Rapid exchange with deuterium occurs, removing absorption peaks of labile protons from the NMR spectra.

2 2 The compounds below are competitive inhibitors of acetylcholinesterase. They can be used in treating people with Alzheimer’s disease. One of the four compounds above has the following NMR spectrum in CDCl3 solution. δ 6.68 – 6.56 (m 2H) δ 3.30 (m 1H) 6.07 (d 1H) 3.07 (m 1H) 6.01 (m 1H) 2.69 (m 1H) 4.62 (m 1H) 2.48 (s 1H) 4.14 (m 1H) 2.41 (s 3H) 4.11 (d 1H) 2.17 – 1.97 (m 2H) 3.84 (s 3H) 1.59 (m 1H) 3.70 (d 1H) (a) Identify the compound responsible for the above spectrum, and assign four of the signals to particular protons in the molecule. Give your reasoning. Compound J