Intro into Spectroscopy 2

Question 1

When analysis organic compounds using spectroscopy, how do you prepare the compounds

Answer 1

• Grinding the organic solid in a viscous oil called nujol to make a mull
• This is put between two sodium chloride plates
• Placed in IR device

Question 2

As the number of atoms in a compound increases, what happens to the number of normal modes

Answer 2

The number of normal modes rapidly becomes very large and the IR spectrum become highly complex

Question 3

What is the fingerprint region

Answer 3

a region which contain other single bond stretches, involving a pair of heavy atoms
As well as other vibrational modes, including bending vibrations

Question 4

The presence of stetching bands at particular frequencies can be rationalised by

Answer 4

The size of the reduced mass of the atoms involved in the corresponding bonds
And the force constant, using the expression for the fundmental vibrational wavenumber

Question 5

Higher freqency virbations occur when
(more to the left on an IR)

Answer 5

the reduced mass is small, usually if a light atom such as hydrogen attached to a heavier atom
The force constant is large, so frequencies of triple bond stretches are greater than those of double bonds

Question 6

Why is the O-H peak really broad

Answer 6

Due to hydrogen bonding

Question 7

Why do carbon triple bonds give weak absoptions but carbon-nitrogen triple bond give a intense peak

Answer 7

Because there is onyl a small change in the dipole moment on stretching in carbon-carbon but because of larger dipole moment in carbon-nitrogen

Question 8

Why do carbonyl groups also have intense peaks

Answer 8

Because they have large dipole moments too

Question 9

Ring strain in cyclic ketones, does what to the frequency of the peaks formed

Answer 9

increases the absorption frequencies since it makes the ring stiffer
(Smaller the ring, the more to the left the peak is)

Question 10

Changes in the carbonyl (C=O) stretching frequnecies allows the different carbonyl functional groups to be indentified
Why

Answer 10

They are sensive to resonance and inductive effects - which decrease and increase the strength of the double bond respectively
In amide (C=O) appears at lower frequencies than for ketones (resonance)
In carboxylic acids and ester (C=O) absorption appear at higher frequencies (induction)

Question 10

Changes in the carbonyl (C=O) stretching frequnecies allows the different carbonyl functional groups to be indentified
Why

Answer 10

They are sensive to resonance and inductive effects - which decrease and increase the strength of the double bond respectively
In amide (C=O) appears at lower frequencies than for ketones (resonance)
In carboxylic acids and ester (C=O) absorption appear at higher frequencies (induction)

Question 11

The energy seperation between molecular orbitals are usually sufficiently high enough, that what radiation is required to cause electronic transition

Answer 11

ultra-violet
or visible electromagnetic radiation

Question 12

The lowest energy (longest wavelength) transition in any molecule involves promoting an electron between which two molecular orbtials

Answer 12

Promoting an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)

Question 13

The lowest energy (longest wavelength) transition in any molecule involves promoting an electron between which two molecular orbtials

Answer 13

Promoting an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)

Question 14

Increasing the number of conjugated double bonds in a molecule does what to the energy required for electronic transition

Answer 14

Increases the wavelength
Decreases the energy of the electronic transition

Question 15

The bond between two atoms in a diatomic molecule behaves, to some degree like a spring
The motion of a spring can be described how…

Answer 15

as a simply harmonic motion
The spring vibrates because there is a restoring force when it is displaced from equilibrium position
As the spring is stretched, the force pulls it back towards its equilibrium position; as the spring is compressed, the force pushes it back

Question 16

Moving from a relax to compressed state and a relaxed to stretched state, what does the energy curve look like for a sping

Answer 16

The energy curve has the space of a parabola

Question 17

How do we work out the reduced mass of a molecule

Answer 17

where m₁ is the mass of one atom and m₂ is the mass of the other

Question 18

If you have one lighter atom and one heavier atom, what is the movement like of both atoms

Answer 18

the lighter atom moves much more than the heavy one.

Question 19

What is the equation for the fundamental vibrational wavenumber

Answer 19

where c = 2.988 x 10¹⁰ cm S¯¹(speed of light)
k = force constant (N m¯¹)
μ (in kg per molecule)

Question 20

In infra-red spectroscopy, how do the molecule move to high vibrational energy levels

Answer 20

Certain frequencies of IR radiation are absorbed

Question 21

Each vibrational energy level is labelled with a…
Within the harmonic oscillator approximation the vibrational energy levels are …

Answer 21

… vibrational quantum number (v) which takes interger values from 0 upwards and determines the energy
… equally spaced with the separation between adjacent levels equal to the fundamental vibrational frequency

Question 22

Why is the lowest vibrational energy level (ground state) not zero

Answer 22

molecules are always vibrating

Question 23

Why is the lowest vibrational energy level (ground state) not zero

Answer 23

molecules are always vibrating

Question 24

Why would you see IR spectra are represented as transmission spectra

Answer 24

100% transmission means all the light passes straight throught the sample, and non is absorbed
If a certain frequency is absorbed, then a dip is shown in the trace
These correspond to the transition from ground state, to the first, second and third excited states

Question 25

Which transitions are called fundamental

Answer 25

the first and second overtones
The fundamental vibrations is by far most intense

Question 26

The complicated vibrations of polyatomic molecules can be resolved into…

Answer 26

… a set of simpler vibration modes, called normal modes
During a normal mode vibration the centre of mass of the molecule is fixed and the individual atoms all move at the same frequency, but with different amplitudes

Question 27

Normal modes are vibrational degrees of freedom. How would you work out the nuber of normal modes for a linear molecule

Answer 27

3N - 5
(N being the number of atoms)

Question 28

Normal modes are vibrational degrees of freedom
How would you work out the nuber of normal modes for a non-linear molecule

Answer 28

3N-6
(N being the number of atoms)

Question 29

Do all normal modes get shown on a IR spectrum

Answer 29

Bands associated with all three of the normal modes can be see in the IR spectrum
Absorption of a photon with the correct energy can simultaneously excite multiple normal modes producing a combination band

Question 30

How many normal modes are there in H₂O

Answer 30

3(3) - 6 = 3

Question 31

The strength of an IR absorption can depend upon

Answer 31

The extent of the electrical dipole moment of the molecule changes during vibration
This will mean that some stretches will not be seen in the spectrum because there is no overall electric dipole moment