Homework 7

Question 1

In NMR spectroscopy, what are the advantages of using a magnet with as great a field strength as possible?

Answer 1

1. If line width is constant, resolution improves with field strength
2. Sensitivity improves with field strength
3. as the ∆v/J ratio increases, spectral interpretation becomes easier

Question 2

How can spin spin splitting lines be differentiated from chemical shift lines?

Answer 2

by varying the magnetic field.

Spin spin splitting is independent of magnetic field strength, whereas chemical shifts increase with increases in field strength

Question 3

define magnetic anisotropy

Answer 3

property of a molecule having magnetic properties that vary with molecular orientation

Question 4

define screening constant

Answer 4

a measure of the degree to which circulation of electrons around the nucleus reduce (or sometimes increase) the magnetic field felt by the nucleus

Question 5

define chemical shift parameter

Answer 5

measures the shift in parts per million of the peak for a given nucleus from that of the reference (usually TMS)

Question 6

define CW-NMR measurements

Answer 6

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

Question 7

define Larmor frequency

Answer 7

frequency of a precession of a nucleus in an external field

Question 8

define coupling constants

Answer 8

spacing in frequency units produced by spin-spin splitting

Question 9

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?

Answer 9

2I + 1 where I is quantum spin number
2(5/2) + 1 = 6
+5/2, +3/2, +1/2, -1/2, -3/2, -5/2

Question 10

What is the difference between longitudinal and transverse relaxation?

Answer 10

Longitudinal: arises from complex magnetic fields generated by rotation and vibration of the host of other nuclei in a sample. At least some of these magnetic fields must correspond in frequency and phase with that of the analyte nucleus and thus can convert it from higher to lower spin state

Transverse: brought about by interaction between neighboring nuclei with identical precession rates but different magnetic quantum states. Nucleus in lower state is excited while excited nucleus relaxed. No net change in spin state population occurs, but avg lifetime of a particular excited nucleus is shortened