Intro into Spectroscopy 2 Flashcards
When analysis organic compounds using spectroscopy, how do you prepare the compounds
- 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
As the number of atoms in a compound increases, what happens to the number of normal modes
The number of normal modes rapidly becomes very large and the IR spectrum become highly complex
What is the fingerprint region
a region which contain other single bond stretches, involving a pair of heavy atoms
As well as other vibrational modes, including bending vibrations
The presence of stetching bands at particular frequencies can be rationalised by
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
Higher freqency virbations occur when
(more to the left on an IR)
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
Why is the O-H peak really broad
Due to hydrogen bonding
Why do carbon triple bonds give weak absoptions but carbon-nitrogen triple bond give a intense peak
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
Why do carbonyl groups also have intense peaks
Because they have large dipole moments too
Ring strain in cyclic ketones, does what to the frequency of the peaks formed
increases the absorption frequencies since it makes the ring stiffer
(Smaller the ring, the more to the left the peak is)
Changes in the carbonyl (C=O) stretching frequnecies allows the different carbonyl functional groups to be indentified
Why
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)
Changes in the carbonyl (C=O) stretching frequnecies allows the different carbonyl functional groups to be indentified
Why
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)
The energy seperation between molecular orbitals are usually sufficiently high enough, that what radiation is required to cause electronic transition
ultra-violet
or visible electromagnetic radiation
The lowest energy (longest wavelength) transition in any molecule involves promoting an electron between which two molecular orbtials
Promoting an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)
The lowest energy (longest wavelength) transition in any molecule involves promoting an electron between which two molecular orbtials
Promoting an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)
Increasing the number of conjugated double bonds in a molecule does what to the energy required for electronic transition
Increases the wavelength
Decreases the energy of the electronic transition
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…
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
Moving from a relax to compressed state and a relaxed to stretched state, what does the energy curve look like for a sping
The energy curve has the space of a parabola
How do we work out the reduced mass of a molecule
where m₁ is the mass of one atom and m₂ is the mass of the other
If you have one lighter atom and one heavier atom, what is the movement like of both atoms
the lighter atom moves much more than the heavy one.
What is the equation for the fundamental vibrational wavenumber
where c = 2.988 x 10¹⁰ cm S¯¹(speed of light)
k = force constant (N m¯¹)
μ (in kg per molecule)
In infra-red spectroscopy, how do the molecule move to high vibrational energy levels
Certain frequencies of IR radiation are absorbed
Each vibrational energy level is labelled with a…
Within the harmonic oscillator approximation the vibrational energy levels are …
… 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
Why is the lowest vibrational energy level (ground state) not zero
molecules are always vibrating
Why is the lowest vibrational energy level (ground state) not zero
molecules are always vibrating
Why would you see IR spectra are represented as transmission spectra
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
Which transitions are called fundamental
the first and second overtones
The fundamental vibrations is by far most intense
The complicated vibrations of polyatomic molecules can be resolved into…
… 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
Normal modes are vibrational degrees of freedom. How would you work out the nuber of normal modes for a linear molecule
3N - 5
(N being the number of atoms)
Normal modes are vibrational degrees of freedom
How would you work out the nuber of normal modes for a non-linear molecule
3N-6
(N being the number of atoms)
Do all normal modes get shown on a IR spectrum
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
How many normal modes are there in H₂O
3(3) - 6 = 3
The strength of an IR absorption can depend upon
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
