CHEM 0310 Lecture Notes - Lecture 10: Nuclear Magnetic Resonance Spectroscopy, Electromagnetic Radiation, Chemical Shift

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6 Feb 2017
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Chapter 10: Nuclear Magnetic Resonance Spectroscopy
Physical and Chemical Tests
-Afte puifyig a opoud, e a’t e easoale etai of the idetity ad stutue of the
molecule
-Eleetal aalysis ill eeal the saple’s goss heial composition while chemical tests can help us
identify its functional group
-To differentiate between many alternative, a chemist makes use of a spectroscopy
Spectroscopy
-A technique for analyzing the structure of molecules, usually based on differences in how they absorb
electromagnetic radiation
-Nuclear spectroscopy (NMR) probes the structure in the vicinity of individual nuclei, particularly
hydrogens and carbons, and provides the most detailed information regarding the atomic connectivity
of a molecule
-Electromagnetic radiation can be described in the form of waves
-A wave is defined by its wavelength (λ o y its feuey ν; ν= /λ o =νλ
-C is defined as the speed of radiation or the speed of light = 3 x 1010 cms-1
-The absorption of quanta (electromagnetic energy) of radiation by a molecule brings about transitions
from its normal, ground state to an excited state
-A spectrometer records the absorption of this radiation
-Radiation of a specific wavelength range passes through the sample
-Baseline: absence of absorption; straight line
-Peak: deviation from baseline; sample absorbs electromagnetic radiation
-Spectrum: pattern of absorption of the sample
Hydrogen Nuclear Magnetic Resonance
-Each atomic nuclei can be thought of as spinning around an axis creating a nuclear spin
-The proton is positively charge, its spinning motion creates a magnetic field
-The proton may be viewed as a tiny cylindrical (bar) magnet floating freely in solution or in space
-When the proton is exposed to an external magnetic field of strength H0, two orientations are possible:
1. Aligned with H0 energetically favorable (alpha)
2. Against H0 higher in energy (beta)
-Irradiation of the sample at just the right frequency to bridge the difference in energy between the
alpha and beta states produces resonance a asoptio of eegy ΔE – as the alpha proton flips to
the beta spin state
-After excitation, the nuclei relax and return to their original states;
resonance has a continuous excitation and relaxation
-Increasing the magnetic field strength H0 makes alpha to beta spin
flip more difficult
-Hydrogen is not the only nucleus capable of magnetic resonance
(13C)
-The resonance frequency, which is characteristic of the nucleus
and its environment, is proportional to the strength of the external
magnetic field
The Proton Chemical Shift
-The position of an NMR absorption (chemical shift) depends on the electron density around the
hydrogen
-A free proton is essentially unperturbed by electrons however organic molecules contain covalently
bonded hydrogen nuclei, not free protons, and the electrons in these bonds affect NMR absorptions
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