Ch20B-TCF

Ch20B-TCF - Quantitative Chemical Analysis Quantitative...

Info iconThis preview shows pages 1–13. Sign up to view the full content.

View Full Document Right Arrow Icon
Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 20B: Fourier Transform Spectroscopy Fourier Transform Spectroscopy Part 2: Nuclear Magnetic Resonance Spectroscopy
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Nuclear Magnetic Resonance Spectroscopy ( NMR ) CH 3 CH 3 Si CH 3 CH 3 tetramethyl- silane (TMS) 500 MHz 1 H Spectrum of CH 3 CH 2 CH 2 CH 2 -O-CH=CH 2 62 MHz 13 C Spectrum of CH 3 CH 2 CH 2 CH 2 -O-CH=CH 2 Reference Standard 0 ppm
Background image of page 2
3 Chemical shifts are reproducible
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 Coupling constants are very characteristic of the types of protons that are coupling. Slides 2, 3, and 4 are for context ONLY. You will NOT be tested on them.
Background image of page 4
5 Fourier Transform Methodology – NMR Many nuclei possess a magnetic moment – rationalize by postulating a nuclear spin (spinning charge magnetic field) – nuclear spin states are quantized Nuclear Spin Quantum Number: 0 1 / 2 1 3 / 2 2 5 / 2 Number of Spin States: 0 2 3 4 5 6 … Nuclei with spin 1 / 2 have magnetic dipoles Nuclei with spin 1 or larger have magnetic quadrupoles – their NMR is more complicated and less useful.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
6 Some Examples of Magnetic Nuclei
Background image of page 6
7 Consider the hydrogen atom spin QN = ½ spin states are + ½ and ½ In an applied magnetic field, , the magnetic moment aligns with the field (“parallel”, + 1 / 2 ) or against the field (“antiparallel”, 1 / 2 ) 1 / 2 state is higher in energy + 1 / 2 state is lower in energy Energy difference is proportional to the size of Proportionality constant = γ h where γ is the 2 π magnetogyric ratio – a property of a given element Zeeman effect
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
8 Thus, E = γ h and E = ν, 2 π so ν o = γ 2 π This is the resonance condition Where ν o is the Larmor ( resonance ) frequency for an applied magnetic field of . For technologically accessible magnets, ν o is in the radio frequency range. Units of magnetic fields: – earth’s magnetic field is ~1 gauss – 1 Tesla = 10,000 gauss
Background image of page 8
9 The Resonance Condition and Chemical Shifts shielding H-H coupling = B electron + B B local o effective 2 π B γ = ν H ν o RF ν = B effective applied = B + B local Resonance is when and where Resonance ! Field at the nucleus and The effective field varies as the electronic (chemical) environment varies, so … Electron shielding is described as Chemical Shift
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
10 It is useful to think of resonance both as … a quantum phenomenon and as a classical phenomenon. (1) A Quantum Phenomenon At resonance, photons of RF (radio frequency) “light” are absorbed causing a spin flip .
Background image of page 10
11 Stimulated emission has exactly the same probability as excitation: At a given applied field, Boltzmann’s law describes the spin state populations at a given temperature:
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon