IR Spectroscopy Flashcards
What is IR Spectroscopy?
a rapid and simple method for observing the functional group species present in an organic molecule
What is the IR spectrum plot?
a plot of the percentage of IR radiation that passes through the sample (% transmission) versus some function of the wavelength of the radiation related to covalent bonding
What are modern IR spectrometers based on?
Michelson interferometer
What is Raman spectroscopy?
provides information complementary to that obtained from IR spectroscopy
What are the quantum mechanical energy levels observed in IR spectroscopy?
molecular vibration
What are the two types of bond vibration and their definitions?
stretch: vibration or oscillation along the line of the bond (symmetric, asymmetric)
bend: vibration or oscillation not along the line of the bond (scissor, twist (oop), rock, wag)
What does photon absorption cause to increase?
the amplitude of the vibration, not the frequency
Why are wavenumbers used?
wavenumbers are directly proportional to the energy of transition being observed
What are the peaks on IR spectra?
Gaussian distributions of the average energy of a transition
How does the IR detect different bonds?
the potential energy stretching or bending vibrations of covalent bonds follow the model of the classic harmonic oscillator (Hooke’s Law)
Why does the application of the classical vibrational model fall apart?
- as two nuclei approach one another through bond vibration, potential energy increases to infinity, as two positive centers begin to repel one another
- At higher vibrational energy levels, the amplitude of displacement becomes so great that the overlapping orbitals of the two atoms involved in the bond, no longer interact and the bond dissociates
What is the y-axis of the IR spectrum?
Transmittance: the ratio of the amount of IR radiation transmitted by the sample to the intensity of the incident beam
What two primary factors is the intensity of an IR band affected by?
- whether the vibration is one of stretching or bending
- electronegativity difference of the atoms involved in the bond
What is the position of the spectral band dependent on?
bond strength and atomic size
What does the intensity of the peak result from?
the efficiency of the coupling (e.g. vibrations that have a large change in dipole moment create a larger electrical field with which a photon can couple more efficiently)
What are the ten factors that contribute to the position, intensity, and appearance of IR bands?
- symmetry
- mechanical coupling
- fermi resonance
- hydrogen bonding
- ring strain
- electronic effects
- constitutional isomerism
- stereoisomerism
- conformational isomerism
- tautomerism (dynamic isomerism)
What happens to the IR spectrum in cases of symmetry?
no IR band appears on the spectrum
What does coupling require?
that the vibration be of the same symmetry
No interaction is observed for mechanical coupling if what?
if coupling parts are separated by more than two bonds
Finish this sentence.
In a multi-atomic molecule, no vibration occurs…
without affecting the adjoining bonds
What is a fermi resonance?
a special case of mechanical coupling
What is fermi resonance often called?
accidental degeneracy
What happens if the overtone of a particular vibration coincides with that band from another vibration?
they can couple and cause a shift in group frequency and introduce extra bands
What can internal (intramolecular) H-bonding with carbonyl compounds do to a spectrum?
Lower the absorption frequency
What can intermolecular H-bonding do to a spectrum?
broaden IR bands due to the continuum of bond strengths that result from autoprotolysis
When can certain functional group frequencies be shifted?
if one of the atom’s hybridization is affected by the constraints of bond angle in ring systems
What would the presence of a halogen on the alpha carbon of a ketone (or electron w/d groups) do?
raise the observed frequency for the pi bond because due to electron w/d the carbon becomes more electron deficient, and the pi bond compensates by tightening
What would the contribution of one or more reactive resonance forms of an unsaturated system cause?
some loss of pi bond strength which is seen as a drop in observed frequency
What happens in extended conjugated systems?
some resonance contributors are “out-of-sync” and do not resonate with a group
What are the four parts of IR spectrometers?
- source/monochromator
- sample cell
- detector/amplifier
- output
What are some advantages of IR spectrometers?
- simple, easy to maintain
- last the life of the source and moving parts
What are some disadvantages of IR spectrometers?
- to cover the entire IR band of interest to chemists it is necessary to use two diffraction gratings
What is FT-IR?
- the laser source IR light is separated by a beam splitter, one component going to a fixed mirror, the other to a moving one and is reflected to the beam splitter
- the beam splitter recombines the two to a pattern of constructive and deconstructive interferences known as an interferogram
What is the resulting signal of FT-IR a plot of?
intensity vs. time
What are the three theoretical advantages of FT-IR’s?
Fellgett’s advantage: every point in the interferogram is information - all wavelengths are represented
Jacquinot’s advantage: the entire energy of the source is used - increasing signal-to-noise
Conne’s advantage: frequency precision - dispersive instruments can have errors in the ability to move slits and gratings reproducibly - FTIR is internally referenced from its own beam
What are Justik’s advantages and disadvantages for FTIR?
Advantages
1. single-beam instrument: collect a background (air has IR active molecules)
- fast: all frequencies are scanned simultaneously
- no referencing
- computer-based
Disadvantages
1. expensive relative to dispersive instruments and the components take more expertise and service calls to replace
What is FTIR-ATR?
Attenuated total reflectance (ATR) utilizes total internal reflection to generate an evanescent wave that penetrates the sample, providing valuable molecular information
Finish this sentence.
For ATR the measurement path length…
is independent of the thickness of the sample
Finish this sentence.
For ATR the sample path length…
is dependent on the depth of penetration of the infrared energy into the sample
What are the two major characteristics of an optical material that light energy passes through in ATR?
- it must be optically transparent to the frequency of the energy so that the sensor material absorbs little or none of the radiation
- the material has an index of refraction that is higher than that of the surrounding media, so that ATR acts as a waveguide, internally reflecting the light energy
What is the effective path length of the ATR sensor determined by?
multiplying the number of internal reflections by the depth of penetration of the evanescent wave
What are the experimental aspects of IR spectrometers?
sample size: typically the size of the beam
nondestructive: sample can be recovered with varying degrees of difficulty
liquid samples: best with thin film on ATR or salt plate
solid samples: best with KBr pellet to ATR
