Lecture 10 - IR Spectroscopy Flashcards
spectroscopy:
field of study of the interaction of matter and light
what can spectroscopy be used to do?
spectroscopy can be used to determine unknown molecular structures
what is light?
electromagnetic radiation that comes in various waves
types of electromagnetic radiation:
visible, x-ray and ultraviolet light
what does electromagnetic radiation propagate as and what is it categorised by?
EM radiation propagates as a wave motion through space and is categorized by its wavelength (l) or frequency (n)
light particles are called ________
photons
how can light exist?
although light can be described as a wave it also exhibits particle-like behaviour
the most common type of spectroscopy used for structure determination:
absorption spectroscopy
the amount of electromagnetic radiation absorbed by a sample is determined by?
a spectrometer or spectrophotometer
what does infrared spectroscopy provide information about?
infrared provides information about what functional groups are present
what does nuclear magnetic resonance provide information about?
nuclear magnetic resonance provides information on the number, connectivity and functional group environment of C & H
what information does UV-visible spectroscopy provide information about?
UV-visible provides information about the types of p-electron systems present
how is the light absorbed by a substance is recorded?
as a function of wavelength
the horizontal axis is the __________ the ________ of the wavelength
wave number
inverse
what are the conventional units of the infrared spectrum?
reciprocal cm or inverse cm (cm^-1)
what is the y axis of infrared spectroscopy?
percent transmittance
what are absorptions in the IR spectrum the result of?
they absorbances are as a result of bond vibrations
what must the radiation exactly match?
the frequency of radiation must exactly match the frequency of bond vibration
what do peaks correspond to in an IR spectrometry graph?
peaks corresponds to absorption of energy by the vibration of a particular bond or group
not all peaks can be interpreted but many are diagnostic
wavenumber for O-H, N-H & C-H:
3400-2800
wavenumber for C///N, C///C:
2250-2100
wavenumber for C=O, C=N, C=C:
1850-1600
wavenumber for C—C, C—O, C—N:
1600-1000
wavenumber for C—H:
1000-600
fingerprint region:
absorptions in this region are not usually interpreted, but can serve as a valuable molecular fingerprint
Factors That Determine IR Absorption Position:
(1) strength of bond: stronger bond = higher frequencies
(2) masses of atoms:lighter masses = higher frequencies
(3) type of vibration: stretching vibrations = higher frequencies than bending ones
three types of normal vibrational nodes:
symmetrical stretch, unsymmetrical stretch, unsymmetrical in-plan bend (rock)
Factors That Determine IR Absorption Intensity:
• overall peak intensity is related to the concentration of the sample
• relative peak intensity is additive: A large number of similar groups (e.g., alkyl) will increase the intensity of a given peak
• relative peak intensity is also due to the dipole moment
dipole moment is related to:
the charge separation and distance between two atoms
as the bond stretches the dipole…
… dipole increases
as the bond compresses the dipole….
… decreases
The electric field of a light wave cannot interact with a bond that…
has no dipole
bonds with no dipole will not absorb in IR
how do groups with large dipole interact with IR?
groups with large dipoles provide intense absorptions
Molecular vibrations that occur but do not give rise to IR absorption are said to be…
infrared-inactive
Any vibration that does give rise to an absorption is said to be…
infrared-active
IR Spectra of Alcohols and Ethers:
•O-H stretch (H-bonded): 3200-3400 cm-1
•O-H stretch (not H-bonded): 3600 cm-1
•C-O stretch: 1050-1200 cm-1 (ROH and ethers)
IR Spectroscopy of Aldehydes and Ketones:
Strong C=O stretch: 1700 cm-1
Conjugation with a p bond…
lowers the absorption frequency (typically by 20-30 cm-1)
The Infrared Spectrometer Machine:
•Most modern IR spectrometers are Fourier-transform spectrometers
•Liquid samples can be analyzed undiluted (neat), as a mineral oil dispersion (mull), or as a solution (CHCl3 or CH2Cl2 as solvent)
•Solid samples can be analyzed as a fused KBr pellet
•Our instruments are attenuated total reflectance (ATR) instruments
spectroscopy deals with:
the interaction of matter and electromagnetic radiation
IR spectroscopy deals with:
molecular vibrations
there needs to be a change in ______ ______ on vibration
dipole moment
