IR Flashcards
What does IR stand for?
Infrared Spectroscopy
SPECTROSCOPY
What is IR spec?
- ## analysis of infrared light interacting with a molecule
What does FTIR mean?
Fourier Transform Infrared Spectroscopy
originates from the fact that a Fourier transform (a mathematical process) is required to convert the raw data into the actual spectrum.
What is the fingerprint region?
The fingerprint region is the region to the right-hand side of the diagram (from about 1500 to 500 cm-1) usually contains a very complicated series of absorptions. These are mainly due to all manner of bending vibrations within the molecule.
What units are used
percentage
Transmittance and wavenumber
wavenumber = 1/ wavelength (cm)
The science of why it absorbs
Each frequency of light (including infra-red) has a certain energy. If a particular frequency is being absorbed as it passes through the compound being investigated, it must mean that its energy is being transferred to the compound.
Energies in infra-red radiation correspond to the energies involved in bond vibrations.
Explain bond stretches
In covalent bonds, atoms aren’t joined by rigid links - the two atoms are held together because both nuclei are attracted to the same pair of electrons.
The two nuclei can vibrate backwards and forwards - towards and away from each other - around an average position.
The energy involved in this vibration depends on things like the length of the bond and the mass of the atoms at either end.
That means that each different bond will vibrate in a different way, involving different amounts of energy.
Bonds are vibrating all the time, but if you shine exactly the right amount of energy on a bond, you can kick it into a higher state of vibration. The amount of energy it needs to do this will vary from bond to bond, and so each different bond will absorb a different frequency (and hence energy) of infra-red radiation.
Explain bond bending
bonds can also bend. The diagram shows the bending of the bonds in a water molecule. The effect of this, of course, is that the bond angle between the two hydrogen-oxygen bonds fluctuates slightly around its average value
Again, bonds will be vibrating like this all the time and, again, if you shine exactly the right amount of energy on the bond, you can kick it into a higher state of vibration.
Since the energies involved with the bending will be different for each kind of bond, each different bond will absorb a different frequency of infra-red radiation in order to make this jump from one state to a higher one.
Explain bond bending
bonds can also bend. The diagram shows the bending of the bonds in a water molecule. The effect of this, of course, is that the bond angle between the two hydrogen-oxygen bonds fluctuates slightly around its average value
Again, bonds will be vibrating like this all the time and, again, if you shine exactly the right amount of energy on the bond, you can kick it into a higher state of vibration.
Since the energies involved with the bending will be different for each kind of bond, each different bond will absorb a different frequency of infra-red radiation in order to make this jump from one state to a higher one.
What are the peaks called
Troughs
Four influences of stretches
-Type of vibration
-Strength of the bond
-The masses of the atoms
-Electronegativity
Effect 1: Type of vibration
stretching vibration occur typically at higher bending vibrations
Symmetric stretching, asymmetric stretching, scissoring, rocking
How does Tolman electronic parameter work?
measuring the frequency of the A1 C-O vibrational mode (ν(CO)) of a (pseudo)-C3v symmetric complex, [LNi(CO)3] by infrared spectroscopy, where L is the ligand of interest.
pon coordination of CO to a metal, ν(CO) typically decreases from 2143 cm−1 of free CO.
π backbonding: the metal forms a π bond with the carbonyl ligand by donating electrons through its d orbitals into the empty π* anti-bonding orbitals on CO. This interaction strengthens the metal-carbon bond but also weakens the carbon-oxygen bond, resulting in a lower vibrational frequency. If other ligands increase the density of π electrons on the metal, the C-O bond is weakened and ν(CO) decreases further; conversely, if other ligands compete with CO for π backbonding, ν(CO) increases.
Simple relationship for CO and M
Lower wavenumber = worse donating L
Effect 2: Strength of bond
Increase in bond order = higher frequency
Effect 3: Masses of atoms
heavier atoms = lower frequency - slower vibrations - lower energy
Effect 4: Electronegativity
electronegativity does not affect frequency, it does effect the intensity of the peak
This is because bond polarity effected absorption intensity. For example, C-C single and double bonds are either weak or moderate in intensity, while C-O single and double bonds are quite strong. This is because as infrared light causes stretching vibrations, bond dipoles will change. If a C=O double bond lengthens, the bond dipole will be stronger. This increases the intensity of the IR band.
Where do CO stretches usually come?
2144
Ni(CO)3L
2050-2100
PTBu3 PF3
carbonyl species - 1700-1800
Where do CO2 stretch come?
2349 cm-1
asymmetric stretch
How to calculate the number of degrees of freedom?
Linear Molecules: (3N - 5) degrees of freedom
Non-Linear molecules: (3N - 6) degrees of freedom
What does IR require to see a trough?
There must be a net change in dipole moment in a molecule as it stretches or rotates
he dipole moment is determined by the magnitude of the charge difference and the distance between the two centers of charge. As the molecule vibrates, there is a fluctuation in its dipole moment; this causes a field that interacts with the electric field associated with radiation.
O2 no
N2 no - no change in dipole
Br2 no
CO2 symmetric stretch - no net change in dipole
What is a dipole moment
dipole moment is a measure of the separation of positive and negative electrical charges within a system
electrical polarity of a system of charges
