Chemistry 341 spectroscopy lecture

Chemistry 341
Spectroscopy of Organic
Compounds


Modern Spectroscopic Methods


Revolutionized the study of Organic Chemistry



Determine the exact structure of small to
medium size molecules in a few minutes.



Nuclear Magnetic Resonance (NMR) and
Infrared Spectroscopy (IR) are particularly
powerful techniques which we will focus on.


Interaction of Light and Matter
The Physical Basis of Spectroscopy


Quantum properties of light (photons)



Quantum properties of matter (quantized

energy states).



Photons of light act as our “quantum
probes” at the molecular level giving us
back precise information about the energy
levels within molecules.


The Electromagnetic Spectrum


Continuous



Covers a wide range of wavelengths of
“light” from radio waves to gamma rays.



Wavelengths (λ) range from more than ten
meters to less than 10-12 meter.


The Electromagnetic Spectrum


Relationship Between Wavelength,

Frequency and Energy


Speed of light (c) is the same for all wavelengths.



Frequency (ν), the number of wavelengths per second, is
inversely proportional to wavelength:
ν = c/λ



Energy of a photon is directly proportional to frequency
and inversely proportional to wavelength:
E = hν = hc/λ
(where h = Plank’s constant)


Wavelength/Spectroscopy
Relationships
Spectral Region Photon Energy

Molecular
Energy
Changes

UV-Visible

~ 100 kcal/mole Electronic

Infrared (IR)

~ 10 kcal/mole

Bond vibrations

Radio

< 0.1 kcal/mol

Nuclear Spin
states in a
magnetic field


Spin of Atomic Nuclei


Spin 1/2 atoms:
mass number is odd.
examples: 1H and 13C.



Spin 1 atoms: mass number is even.
examples: 2H and 14N.



Spin 0 atoms: mass number is even.
examples: 12C and 16O.


Magnetic Properties of the Proton
Related to Spin


Energy States of Protons in a Magnetic
Field

∆ E = λ absorbed light
Applied
Magnetic
Field
H ext


Nuclear Magnetic Resonance
(NMR)


Nuclear – spin ½ nuclei (e.g. protons) behave as
tiny bar magnets.



Magnetic – a strong magnetic field causes a
small energy difference between + ½
and – ½ spin states.



Resonance – photons of radio waves can match
the exact energy difference between the + ½
and – ½ spin states resulting in absorption of
photons as the protons change spin states.


The NMR Experiment


The sample, dissolved in a suitable NMR solvent
(e.g. CDCl3 or CCl4) is placed in the strong
magnetic field of the NMR.



The sample is bombarded with a series of radio
frequency (Rf) pulses and absorption of the radio
waves is monitored.



The data is collected and manipulated on a
computer to obtain an NMR spectrum.


The NMR Spectrometer

The NMR Spectrometer


The NMR Spectrum






The vertical axis shows the intensity of Rf
absorption.
The horizontal axis shows relative energy at
which the absorption occurs (in units of parts per
million = ppm)
Tetramethylsilane (TMS) in included as a
standard zero point reference (0.00 ppm)
The area under any peak corresponds to the
number of hydrogens represented by that peak.


The NMR Spectrum


Chemical Shift (δ)


The chemical shift (δ) in units of ppm is defined as:

δ = distance from TMS (in hz)
radio frequency (in Mhz)


A standard notation is used to summarize NMR spectral
data. For example p-xylene:
δ 2.3 (6H, singlet)
δ 7.0 (4H, singlet)



Hydrogens in identical chemical environments
(equivalent hydrogens) have identical chemical shifts.


Shielding – The Reason for
Chemical Shift Differences


Circulation of electrons within molecular
orbitals results in local magnetic fields that
oppose the applied magnetic field.



The greater this “shielding” effect, the
greater the applied field needed to achieve
resonance, and the further to the right
(“upfield”) the NMR signal.

Structure Effects on Shielding


Electron donating groups increase the
electron density around nearby hydrogen
atoms resulting in increased shielding,
shifting peaks to the right.



Electron withdrawing groups decrease the
electron density around nearby hydrogen
atoms resulting in decreased shielding,
(deshielding) shifting peaks to the left.


Structure Effects on Shielding
The Deshielding effect of an electronegative
substituent can be seen in the NMR
spectrum of 1-Bromobutane
Br – CH2-CH2-CH2-CH3
δ (ppm):

3.4 1.8 1.5 0.9

No. of H’s:

2

2

2

3


Some Specific Structural Effects on
NMR Chemical Shift
Type of Hydrogen

δ (ppm

Alkyl (C – H)

0.8 – 1.7

Alkyl Halide (RCH2X)

3-4

Alkene (R2C=CH2)

4-6

Aromatic (e.g. benzene) 6 - 8
Carboxylic Acid
(RCOOH)

10 - 12

Spin-Spin Splitting


Non-equivalent hydrogens will almost always
have different chemical shifts.



When non-equivalent hydrogens are on adjacent
carbon atoms spin-spin splitting will occur due to
the hydrogens on one carbon feeling the
magnetic field from hydrogens on the adjacent
carbon.



The magnitude of the splitting between two
hydrogens (measured in Hz) is the coupling
constant, J.


Spin-Spin Splitting
Origin of the Doublet


Spin-Spin Splitting
Origin of the Triplet

Spin-Spin Splitting
Origin of the Quartet