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-1-ORGANIC SPECTROSCOPY NOTES
Basics of Spectroscopy
UV/vis, IR and NMR are all types of Absorption Spectroscopy, where EM radiation
corresponding to exactly the energy of specific excitations in molecules is absorbed.
This is then measured by a spectrometer. These always consist of:
• The radiation source.
• The monochromator (continuous recording instruments).
• The sample container.
• The frequency analyser.
• The detector.
• The recorder.
Continuous recording implies scanning through a range of appropriate frequencies and
producing the difference in detected intensities (relative to a sample) in a spectrum.
An alternative procedure is to use a Fourier Transform spectrometer. The sample in this case
is subjected to a broad pulse of radiation covering the whole frequency range. The absorbed
frequencies which are re-emitted as the excited molecules decay back to their ground states
are recorded together and the complex signal is then broken down to its component
frequencies by subsequent computer assisted manipulation. This can improve sensitivity and
is usually faster.
Mass Spectrometry is completely different from the above, however. It doesn’t involve
absorption of EM radiation, and is hence known as spectrometry as opposed to spectroscopy.
It analyses sample molecules one at a time rather than the bulk sample. Its primary use is
obtaining the molecular weight of quite large and complicated molecules.
Ultraviolet/Visible Spectroscopy
Rarely used as a first analytical tool now, as it doesn’t show much information. Organic
molecules always absorb UV radiation in the region below about 150nm, as this corresponds
to the σ framework. Solvents typically absorb up to 200nm. Hence, scan at 200-400nm to
avoid these.
Lone pairs (n) and π-electrons are more mobile, as they require less energy for excitation to
their antibonding orbitals, and so we see conjugated systems at longer frequencies, e.g.
butadiene πÆπ* corresponds to 217nm.
It is conjugated systems, such as polyenes and poly-ynes (referred to as chromophores), that
give rise to diagnostic absorptions in the UV-visible region of the EM spectrum. For
unsaturated systems the most commonly observed transition is π−π* and n-π*.
The spectrometer shows A vs. λ, where A = log (Io/I) = εcl (the Beer-Lambert Law). This
comes from:
Beer’s Law – absorption is proportional to the number of molecules.
Lambert’s Law – Io/I is constant for our range of Io.
ε is the extinction coefficient, c is the concentration, and l is the length, usually 1.
It is possible to find λmax and εmax from each peak on the graph.
Examples:
Allene Not conjugated, not seen.
trans-PhHC=CHPh Planar, p overlap.
λmax = 296nm, ε = 31600 cm2 mol-1
cis-PhHC=CHPh Twist Î p not fully overlapped. Hence,
λmax = 280nm, ε = 4000 cm2 mol-1
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -2-
Woodward’s Rules can be used to predict λmax (based on steroids). Conjugation increases the
wavelength, corresponding to a smaller energy of transition between π−π*. So phenol is basic
in solutions shows a λmax = 298nm compared to only 255nm in acid. This is due to the
negative charge on the oxygen in the phenoxide ion being fully conjugated with the ring.
UV Spectra functional groups are hence:
Dienes λmax ~ 230nm
α,β-unsaturated ketones λmax ~ 220nm
Phenols
Benzenes
Can hence see why it is not that much use as a first analytical tool!
Infrared Spectroscopy
Spectrometer
Theory
From simple harmonic motion, it is noted that frequency of oscillation increases with the
stiffness of the bond, and decreases with the reduced mass. A bond will absorb at the energy
corresponding to one of its resonant frequencies of oscillation, therefore the absorbed
frequency is proportional to the inverse square root of k. k is the force constant, which is
characteristic of the bond.
Molecular vibrations occur in the range 2500 to 15400nm. Wavenumber of absorption is then
plotted against transmission.
The peak width is related to ε, which is proportional to the dipole change squared. Thus,
broad peaks are usually a result of H-bonding or increased concentration. This can be useful,
e.g.
Peak appearance can also vary for different functional groups. For example, an alcohol OH
gives a smooth broad peak, while the carboxylic OH (which is in the same frequency range)
has a wavering line instead.
Carbonyls
Influences on position:
R-CO-X Increasing electronegativity of X increases
wavenumber.
α,β-unsaturated ketone Decreases wavenumber by 15-40cm-1,
except in amides.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
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