Relationship between wavelength, frequency and energy
Energy is directly proportional to frequency, and inversely proportional to wavelength
Low energy photon
lower E, lower V, longer wavelength
High energy proton
Higher E, higher V, shorter wavelength
What does the visible region of light correspond to
Wavelengths between 400nm (violet) and 700 nm(red)
Infrared (IR)
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
H and C NMR
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.
Visible and UV radiation
Contain enough energy to promote valence e- to higher shells
Compare energy from IR radiation to visible and UV light
IR radiation is lower in energy and of a longer wavelength than visible and UV light
What happens when covalent molecules absorb IR light
- 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.
Do non polar covalent bonds such as O2 N2 Cl2 absorb IR?
No as they have no dipoles to change as they vibrate
What does the range of energies absorbed in IR depend on?
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.
Define wavenumber
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.
How is IR used for qualitative analysis
- 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.
How is IR used for quantitative analysis
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.
What two factors cause the many peaks in a spectrum
- 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
What is the process of mass spectroscopy
- Tje test sample is heated until it vaporises
- The vapour is bombarded with electrons, changing the particles to positive ions
- The positive ions are accelerated by an electric field so that they move through the apparatus very quickly
- The fast moving positive ions move between strong magnets that deflect them. The lighter particles are deflected more that the heavier ones.
- 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.
Sum up process of mass spec
vaporisation ionisation acceleration deflection detection
Which particles reach the detector in mass spec
Only the positive fragments reach the detector. the uncharged radicals are evacuated by the vacuum pump.
Explain how NMR affects spin energy
- 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.
What happens when a compound containing these nuclei is placed in a strong external magnetic field
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
Why is TMS the reference
- chemically intert
- All 1H are in the same chemical environment showing one peak
- Absorbs radio frequency well away from the most common molecules absorbed
What info does NMR reveal about the molecular structure?
- 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.
- The relative areas of the various signals can tell us how many protons there are of each chemically equivalent type
- 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
