UV, IR, Mass Spec

Question 1

How is UV/Visible radiation absorbed by molecules?

Answer 1

They are absorbed by the bonding electrons in the organic molecule

the electrons are promoted to higher energy levels from ground state - only occurs when light of the correct energy/wavelength are used

Question 2

What does you need for a specific transformation?

Answer 2

for a specific transformation only light of a specific colour will make it happen

Question 3

Why is UV light damaging to living things?

Answer 3

strong chemical bonds have their electrons held tightly - require high energy, short wavelength to excite the electrons

in a single bond, electrons are excited away from the bonding molecular orbital - bond breaks and the molecule falls apart = damage to living cells

Question 4

Why is longer wavelength light preferred over shorter wavelength light?

Answer 4

shorter wavelength light has higher energy - excites electrons in single bonds resulting in the bond breaking and the molecule falling apart = sigma bond breaks

loner wavelength light has lower energy - excites electrons in the double bond which are more loosely held = temporary breaking of the pi bond while the sigma bond holds the molecule together

single bond = one sigma bond
double bond = one sigma and one pi bond
triple bond = one sigma and two pi bonds

Question 5

What kind of compounds does UV light look at?

Answer 5

molecules/compounds with doubles bonds especially conjugated systems

Question 6

What are conjugated systems?

Answer 6

known as Chromophores = includes aromatic ring systems
have alternating single and double bonds
conjugated systems form larger molecular orbitals - loner chain

the larger the orbital, the greater the amount and lower the energy wavelength of light is absorbed = more electrons to absorb them

Question 7

How is UV spectra measured?

Answer 7

sample are run a dilute solutions = 1mg per 100ml
solvent is usually ethanol or water
- must not use a solvent that absorbs light in the same region as the sample
solution is put in a silica cell
- glass or plastic will absorb some wavelengths of interest

light source is shone, monochromator splits the light into its constituent wavelengths which are rotated to allow different colours to pass through, sample absorbs the light, amount of light absorbed is detected

Question 8

What is lambda max in UV spectra measurements?

Answer 8

it is the wavelength at which maximum amount light is absorbed by the sample

a range of wavelengths must be scanned to find this

Question 9

What is the use of a reference cell in UV spectra?

Answer 9

many instruments have two beams
1 - for the sample
2 - for a reference cell containing just the solvent (no sample)

subtract the absorbance of the solvent from the absorbance of the sample and the solvent to get the sample absorbance alone

Question 10

What is the Beer-Lambert Law?

Answer 10

A = Ecl
A - absorbance 
E - molar coefficient or A( 1%, 1cm) 
c - concentration - M or g/100ml
l - length in cm 

If E is A (1%, 1cm) then c is measure in g/100ml

Question 11

What is UV spectroscopy used for?

Answer 11

measured in wavelength

to quantify the amount of drug in the preparation
a calibration curve can be made using solutions of known concentrations
this can be used to find the concentration of the solution

Question 12

What are the three energies a molecule can be divided into?

Answer 12

electronic - requires the most energy
vibrational
rotational - requires the least energy

100 : 1 : 0.01

Question 13

How do UV and IR affect the energy of a molecule?

Answer 13

UV has enough energy to change all of the energy states - vibrational and rotational overlap in most cases only electronic is seen

IR has less energy and can only bring about vibrational and rotational changes

middle IR radiation= 2500-20000 nm
UV = 200-700 nm

Question 14

How does IR affect molecules?

Answer 14

molecules which absorb IR contain asymmetrical bonds - different atoms joined
the bonds behave like springs with balls (atoms) attached
different energies can excite different bonds to move in certain ways

symmetric stretch
symmetric stretch
wagging
rocking

Question 15

What is the IR used for?

Answer 15

measure in wavenumber = 1/wavelength

identifying compounds - shows the functional groups in a compounds
rarely used quantitatively

known compounds can be identified by the distinct fingerprint their IR spectrum provides

Question 16

How is a IR sample/spectra prepare?

Answer 16

1 - liquid sample is run as a thin film between sodium chloride discs
2 - solid sample can be ground with liquid paraffin (nujol) to make a paste and sandwiched between sodium chloride discs
3 - solution is made and placed in a sodium chloride cell with a short path length. a matched cell with solvent only is used as a reference = removes bands due to solvent absoroption
4 - solid sample is ground with potassium bromide and the powdered mix is pressed at 10 tonnes to give a solid disc that can be used directly

sodium chloride does not absorb in the region of interest