IR

Question 1

What does IR stand for?

Answer 1

Infrared Spectroscopy
SPECTROSCOPY

Question 2

What is IR spec?

Answer 2

• ## analysis of infrared light interacting with a molecule

Question 3

What does FTIR mean?

Answer 3

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.

Question 4

What is the fingerprint region?

Answer 4

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.

Question 5

What units are used

Answer 5

percentage
Transmittance and wavenumber

wavenumber = 1/ wavelength (cm)

Question 6

The science of why it absorbs

Answer 6

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.

Question 7

Explain bond stretches

Answer 7

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.

Question 8

Explain bond bending

Answer 8

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.

Question 9

Explain bond bending

Answer 9

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.

Question 10

What are the peaks called

Answer 10

Troughs

Question 11

Four influences of stretches

Answer 11

-Type of vibration
-Strength of the bond
-The masses of the atoms
-Electronegativity

Question 12

Effect 1: Type of vibration

Answer 12

stretching vibration occur typically at higher bending vibrations

Symmetric stretching, asymmetric stretching, scissoring, rocking

Question 13

How does Tolman electronic parameter work?

Answer 13

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.

Question 14

Simple relationship for CO and M

Answer 14

Lower wavenumber = worse donating L

Question 15

Effect 2: Strength of bond

Answer 15

Increase in bond order = higher frequency

Question 16

Effect 3: Masses of atoms

Answer 16

heavier atoms = lower frequency - slower vibrations - lower energy

Question 17

Effect 4: Electronegativity

Answer 17

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.

Question 18

Where do CO stretches usually come?

Answer 18

2144

Ni(CO)3L
2050-2100
PTBu3 PF3

carbonyl species - 1700-1800

Question 19

Where do CO2 stretch come?

Answer 19

2349 cm-1
asymmetric stretch

Question 20

How to calculate the number of degrees of freedom?

Answer 20

Linear Molecules: (3N - 5) degrees of freedom

Non-Linear molecules: (3N - 6) degrees of freedom

Question 21

What does IR require to see a trough?

Answer 21

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

Question 22

What is a dipole moment

Answer 22

dipole moment is a measure of the separation of positive and negative electrical charges within a system
electrical polarity of a system of charges