Spectroscopy 1

Question 1

Interaction of Light and Matter (1)

Answer 1

Light is made of photons.

We treat light as a wave, therefore electromagnetic radiation has both a frequency and a wavelength.

Question 2

Frequency

Answer 2

Frequency (v) is defined as the number of waves produced by a source each second. It is measured in Hz or s(^-1).

Question 3

Wavelength

Answer 3

Wavelength (λ) is defined as the distance between a point on one wave, and the same point on the next. It is usually measured in metres.

Question 4

Equations

Answer 4

Frequency (v) = Velocity (c)/Wavelength(λ)

Wavelength (λ) = Velocity (c)/Frequency (v)

c = speed of light in a vacuum = 3 x10(^8)

Question 5

Interaction of Light and Matter (2)

Answer 5

For infrared and Raman spectroscopy, the wavelength will be in the range of 400-100,000nm.

This covers the visible and infrared regions of the EM spectrum.

Question 6

Motion in Molecules (1)

Answer 6

There are different ways in which a molecule can move:
Translation - movement in 3 dimensions.
Rotation - movement about an axis.
Vibration - movement on a bond.

Question 7

Motion in Molecules (2)

Answer 7

All molecules have 3N degrees of freedom (dof), where N is the number of atoms.

Translation uses 3 dof.
Rotation uses 2 or 3 dof.
Vibration uses the remaining dof.

Question 8

Vibrational Modes

Answer 8

Linear molecules have 3N - 5 vibrational modes.

Non-linear molecules have 3N - 6 vibrational modes.

Question 9

Motion

Answer 9

Motion requires energy.
Translation requires the least amount of energy, it is purely kinetic.
Rotation requires more energy than translation, roughly equivalent to microwave wavelength.

Question 10

Vibration in Molecules (1)

Answer 10

Absorption of infrared radiation causes covalent bones within molecules to vibrate.

A molecule is only able to absorb radiation whose energy corresponds to the energy difference.

Question 11

Vibration in Molecules (2)

Answer 11

Most molecules have many covalent bonds, hence infrared and Raman spectra proving detailed information on molecular structure.

Bond vibrations that result in absorption of infrared radiation are either stretching or bending vibrations.

Question 12

Modes of Vibration in Molecules

Answer 12

Symmetric stretch
Asymmetric stretch
Twisting
Wagging
Scissoring
Rocking

Question 13

IR Active/Inactive (1)

Answer 13

Not all vibrations can be seen by IR spectroscopy, only those deemed “IR active” will be visible.

Gross selection rule:
A vibration must result in a change in the molecular dipole moment.

Specific selection rule:
The change in vibrational energy level must be Δν = ±1

Question 14

IR Active/Inactive (2)

Answer 14

Large changes in the dipole moment results in an intense absorption.

Small changes in the dipole moment results in a weaker absorption.

Question 15

Hooke’s Law (1)

Answer 15

By imagining a chemical bond to be a spring with a mass at each end, we can calculate the frequency of vibration using Hooke’s Law.

Question 16

Hooke’s Law (2)

Answer 16

The strength of a bond is given by k, while μ is the reduced mass.

Reduced mass is the mass at the centre of gravity, which is made up of a combination of the two masses at either end of the spring.

Question 17

IR Spectroscopy - Benefits

Answer 17

Generally little sample prep is required.
Can handle any state of sample.
Identifies functional groups.

Question 18

IR Spectroscopy - Limitations

Answer 18

Not usually quantitative.
Low sensitivity.
Cannot detect IR inactive impurities.