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

41 views2 pages
6 Feb 2017

For unlimited access to Class Notes, a Class+ subscription is required.

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
-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
-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
-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
-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
find more resources at
find more resources at
Unlock document

This preview shows half of the first page of the document.
Unlock all 2 pages and 3 million more documents.

Already have an account? Log in

Get access

$10 USD/m
Billed $120 USD annually
Homework Help
Class Notes
Textbook Notes
40 Verified Answers
Study Guides
1 Booster Class
$8 USD/m
Billed $96 USD annually
Homework Help
Class Notes
Textbook Notes
30 Verified Answers
Study Guides
1 Booster Class