Spectroscopy

Question 1

Relationship between wavelength, frequency and energy

Answer 1

Energy is directly proportional to frequency, and inversely proportional to wavelength

Question 2

Low energy photon

Answer 2

lower E, lower V, longer wavelength

Question 3

High energy proton

Answer 3

Higher E, higher V, shorter wavelength

Question 4

What does the visible region of light correspond to

Answer 4

Wavelengths between 400nm (violet) and 700 nm(red)

Question 5

Infrared (IR)

Answer 5

uses the IR region which is of lower energy so cannot promote e- to higher shells but can promote stretching of covalent bonds in molecules. Provides info about the functional groups in a molecule

Question 6

H and C NMR

Answer 6

Uses radio waves to provide info about the structure of a molecule. Radio waves are too low to affect E- at all but can change the direction of the spin of nuclei in molecules.

Question 7

Visible and UV radiation

Answer 7

Contain enough energy to promote valence e- to higher shells

Question 8

Compare energy from IR radiation to visible and UV light

Answer 8

IR radiation is lower in energy and of a longer wavelength than visible and UV light

Question 9

What happens when covalent molecules absorb IR light

Answer 9

• The amount of energy that a molecule contains is quantised. The bonds within a molecule can only stretch or bend at specific frequencies.
• Covalent bonds (only dipoles) can undergo specific amounts of bending and stretching causing molecular vibration.
• Molecules will absorb a discrete energy (photon, wavelength from IR spectrum) to move from one vibrational energy level to the next.

Question 10

Do non polar covalent bonds such as O2 N2 Cl2 absorb IR?

Answer 10

No as they have no dipoles to change as they vibrate

Question 11

What does the range of energies absorbed in IR depend on?

Answer 11

Strength of the bonds(single vs double) and the mass of the atom attached by the bond. The higher the mass, the lower the energy absorbed.

Question 12

Define wavenumber

Answer 12

Inversely proportional to the wavelength. A bond that vibrates at a higher frequency absorbs IR radiation with a. high wavenumber and greater energy than a bond that vibrates at a lower frequency.

Question 13

How is IR used for qualitative analysis

Answer 13

• IR tells us about the presence or absence of a particular functional group. Wavenumber of each type of covalent bond absorb IR only in small regions of the spectrum
• Comparision of IR spectra can tell us about the structural similarities between two substances. Although each each of covalent bond has its own characteristic absorption frequencies, no two different molecules have precisely the same spectrum. Differences appear in the fingerprint region, less than 1400 which tells us whether 2 compounds are identical.

Question 14

How is IR used for quantitative analysis

Answer 14

A strong sharp peak in the spectrum is chosen.
The absorbance increases as the concentration of the molecule increases.
A calibration curve is constructed by using standard solutions of known concentration.
The concentration of the sample is found by comparing with the calibration curve.

Question 15

What two factors cause the many peaks in a spectrum

Answer 15

• the fragmentation of molecules into a large number of different positive ions
• the occurrence of different isotopes of the atoms that make up the molecules

Question 16

What is the process of mass spectroscopy

Answer 16

1. Tje test sample is heated until it vaporises
2. The vapour is bombarded with electrons, changing the particles to positive ions
3. The positive ions are accelerated by an electric field so that they move through the apparatus very quickly
4. The fast moving positive ions move between strong magnets that deflect them. The lighter particles are deflected more that the heavier ones.
5. As the deflected ions pass out of the magnetic field they are detected by an ion detector and the presence and number are recorded as a spectrum.

Question 17

Sum up process of mass spec

Answer 17

vaporisation ionisation acceleration deflection detection

Question 18

Which particles reach the detector in mass spec

Answer 18

Only the positive fragments reach the detector. the uncharged radicals are evacuated by the vacuum pump.

Question 19

Explain how NMR affects spin energy

Answer 19

• Sub atomic particles are spinning on their axis (up or down). In many nuclei the orientations of the spins of all nucleons are paired and cancels out. However in atoms with an odd number of nucleons (e.g. 1H 13C) the nucleus always has an overall spin.
• The nuclei of certain elements behave as if they were spinning charges. Any spinning charges creates a magnetic field and behaves as a tiny bar magnet.

Question 20

What happens when a compound containing these nuclei is placed in a strong external magnetic field

Answer 20

two orientations are allowed
-Most align with the magnetic field (ground state spin)
-those higher in energy state will be aligned against the magnetic field (Excited spin energy level)
Difference in E between the two states is in the radio region only

Question 21

Why is TMS the reference

Answer 21

1. chemically intert
2. All 1H are in the same chemical environment showing one peak
3. Absorbs radio frequency well away from the most common molecules absorbed

Question 22

What info does NMR reveal about the molecular structure?

Answer 22

1. The number of signals in the NMR spectrum and the chemical shift of each can tell us the number of different types of chemically equivalent protons within the molecule.
2. The relative areas of the various signals can tell us how many protons there are of each chemically equivalent type
3. The splitting pattern of each signal can tell us information about the number of neighbouring protons in the environment of each type of chemically equivalent proton