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Hore: Nuclear Magnetic Resonance 2e

Chapter 5

Instructions

Answer the following questions and then press 'Submit' to get your score.

Question 1

If Δn(t)/Δneq in eqn 5.1 equals 0.5 at t = 1.40 s, what is the value of T1?

Question 2

Suppose that the 13C nuclei in a molecule in a 600 MHz spectrometer can be 100% polarized (p = 1). If T1 = 5.0 s, how long does it take for p to reach a value equal to twice the thermal equilibrium polarization at 298 K?
[The polarization relaxes exponentially: p(t) = [p(0) - peq]exp(-t/T1) + peq.]

Question 3

Which of the following statements is not a reason why spin-½ nuclei in small molecules in solution usually relax slowly?

Question 4

The rotational correlation time of a spherical object of radius r immersed in a liquid with viscosity η is given by the Stokes equation: τc = 4πηr3/3kBT. A small molecule with relative molecular mass Mr = 100 in water at 298 K has τc = 50 ps. Assuming that Mr is proportional to the molecular volume, estimate τc for a small protein with Mr = 8400.

Question 5

The T2 of an NMR line is 15 ms. Ignoring other sources of linebroadening, calculate its linewidth.

Question 6

A molecule has a rotational correlation time of 1 ns. At what magnetic field would protons in this molecule have the fastest spin-lattice relaxation? [Use eqns 5.3 and 5.4.]

Question 7

In the extreme narrowing limit, what is the 15N{1H} nuclear Overhauser enhancement in the absence of other relaxation mechanisms?

Question 8

Calculate the ratio T1/T2 for protons in a 900 MHz spectrometer when (i) τc = 50 ps and (ii) τc = 50 ns. [Use eqns 5.3, 5.4, and 5.11.]

Question 9

Which of the following statements most accurately explains why the 1H spectrum of 12CHCl3 is a singlet?

Question 10

Which of the following statements most accurately explains why the T1 of a nucleus is sometimes longer than its T2?