Chapter 19 Flashcards
19-1. Explain the difference in the way a CW and an FT-NMR experiment is performed.
In a continuous wave NMR experiment, the intensity of the absorption signal is monitored as the frequency of the source or the field strength of the magnet is scanned.
In a Fourier Transform NMR experiment, the analyte is subjected to periodic pulses of radio-frequency radiation. After each pulse, the decay of the emitted signal is monitored as a function of time. This free induction decay signal is then converted to a frequency
domain signal by a Fourier Transformation.
19-2. What are the advantages of an FT-NMR measurement over a CW measurement? What are the disadvantages?
Advantages of FT-NMR:
• Much greater sensitivity (decreased sample size requirements, and multi-nuclear capability)
• Increased analysis speed
• Higher resolution
• Improved S/N due to signal averaging capability
Disadvantages of FT-NMR: cost of stable high field magnets.
Define magnetic anisotropy
Magnetic anisotropy is a property of a molecule having magnetic properties that vary with molecular orientation.
Define the screening constant
The screening constant σ is a measure of the degree to which circulation of electrons around the nucleus reduce (or sometimes increase) the magnetic field felt by the nucleus. It is defined by the equation
σ = (B appl–B0)/B0 where B appl is the external field applied and B0 is the field felt by the nucleus
Define the chemical-shift parameter
The chemical shift parameter measures the shift in parts per million of the peak for a given nucleus from that of a reference (usually TMS). It is defined by Equations 19-18
and 19-19 . δ = (νr–νs/νr) x 106.
Define CW-NMR measurements
Continuous wave NMR measurements are performed by measuring the amplitude of the NMR signal as the radio frequency of the source is varied or the field strength of the magnet is scanned.
Define Larmor frequency
The Larmor frequency ν0 is the frequency of precession of a nucleus in an external field. It is given by ν0 =γB0
where γ is the magnetogyric ratio for the nucleus and
B0 is the magnetic field at the nucleus.
Define coupling constants
The coupling constant is the spacing in frequency units between the peaks produced by spin-spin splitting.
Define first-order NMR spectra
First-order NMR spectra are those in which the chemical shift between interacting groups Δν is large with respect to their coupling constant (Δν/J> 10).
19-6. A nucleus has a spin quantum number of 5/2. How many magnetic energy states does this nucleus have? What is the magnetic quantum number of each?
The number of magnetic energy states is given by 2I+ 1, where I is the spin quantum number. Thus, the number of energy states is 2(5/2) + 1 = 6, and the magnetic quantum number of each is +5/2, +3/2 , +1/2, –1/2, –3/2, and –5/2.
19-7. What is the absorption frequency in a 7.05-T magnetic field of (a) 1H. (b) 13C. (c) 19F, (d)31P?
ν0=γB0/2π
(a) For 1H, γ= 2.68×108 T-1s-1 and ν0 = 2.68x10 8T-1s-1 x 7.05 T / 2π = 300.7 MHz
19-8. What is the Larmor frequency for protons in magnetic fields of (a) 1.41 T (b) 4.69 T. (c) 7.05 T. (d) 11.7 T. (e) 18.8 T, and (I) 21.2 T?
ν0=γB0/2π
At 1.41 T, ν0= 2.68x10 8T-1s-1x 1.41 T / 2π = 60.1 MHz
At 4.69 T, ν0= 2.68x10 8T-1s-1x 4.69 T / 2π = 200 MHz
19-9. A resonance is displaced 90 Hz from TMS at a magnetic field strength of 1.41 T.
What will be the frequency difference at (a) 4.69 T. (b) 7.05 T. (c) and 18.8 T?
What will be the chemical shifts δ at these same magnetic field strengths?
The frequency difference is directly proportional to the magnetic field strength
At 4.69 T, Δν= 90 Hz×4.69/1.41 = 299 Hz
At 7.05 T, Δν= 90 Hz×7.05/1.41 = 450 Hz
At 18.8 T, Δν = 90 Hz×18.8/1.41 = 1200 Hz
But the chemical shift is independent of magnetic field strength
δ = (νr–νs/νr) x 106
δ = (90 Hz/60 MHz) x 106
= 1.5 ppm
19-12. What is the difference between longitudinal and transverse relaxation?
Longitudinal, or spin-lattice, relaxation arises from the complex magnetic fields that are generated by the rotational and vibrational motions of the host of other nuclei making up a sample. At least some of these generated magnetic fields must correspond in frequency and phase with that of the analyte nucleus and can thus convert it from the higher to the lower spin state.
Transverse, or spin-spin, relaxation, in contrast is brought about by interaction between neighboring nuclei having identical precession rates but different magnetic quantum states. Here, the nucleus in the lower spin state is excited while the excited nucleus relaxes.
Not net change in the spin state population occurs, but the average lifetime of a particular
excited nucleus is shortened.
19-13 Explain the source of an FID signal in FT-NMR.
The radio-frequency excitation pulse in FT NMR causes the sample magnetization vector to tip away from the direction of the external magnetic field. When the pulse terminates, the same magnetic moment rotates around the external field axis at the Larmor frequency.
This motion constitutes a radio-frequency signal that decays to zero as the excited nuclei relax. This decreasing signal is the free induction decay (FID) signal.
