Spectroscopy Breakdown
CH 202 Breakdown
UV/Vis:
Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals.
The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb.
Conjugation raises the energy of the bonding orbitals and lowers the energy of the antibonding molecular orbital.
This lowers the energy gap and therefore means less energy is required to excite electron from ground state to excited state.
Mass spectrometry: Mass spectrum gives the weight of a molecule
Mass spectrometer converts sample into ions, and that these ions can be further fragmented (broken down) into smaller molecules.
Ionisation can be achieved using various different techniques, electron ionisation, chemical ionisation and Fast atom bombardment.
When sample is ionised the analyser portion of the mass spectrometer seperates the molecules based on their mass to charge ratio.
A dector then detects the molecule weight of all the molecules present and a spectrum is obtained.
Base peak is the peak with the greatest intensity, and represents the fragment which appears in the greatest abundance (highest amount).
Although other fragments may still be present the base peak represents the most stable carbocation formed by the fragementation of the starting molecule.
Molecular ion will always be present and gives the relative molecular mass of the starting compound.
Know definitions of molecular ion, relative abundance etc.
Difference between Low resolution and High resolution mass spectrometry.
Gives you relative molecular weight of compound and also molecule composition.
Parts of molecule can fragment (break off) in mass spectrometry so can get various peaks.
Know stability of carbocations
Isotope effect:
Certain atoms have isotopes i.e 35Cl, 37Cl
Therefore can get two
UV/Vis:
Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals.
The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb.
Conjugation raises the energy of the bonding orbitals and lowers the energy of the antibonding molecular orbital.
This lowers the energy gap and therefore means less energy is required to excite electron from ground state to excited state.
Mass spectrometry: Mass spectrum gives the weight of a molecule
Mass spectrometer converts sample into ions, and that these ions can be further fragmented (broken down) into smaller molecules.
Ionisation can be achieved using various different techniques, electron ionisation, chemical ionisation and Fast atom bombardment.
When sample is ionised the analyser portion of the mass spectrometer seperates the molecules based on their mass to charge ratio.
A dector then detects the molecule weight of all the molecules present and a spectrum is obtained.
Base peak is the peak with the greatest intensity, and represents the fragment which appears in the greatest abundance (highest amount).
Although other fragments may still be present the base peak represents the most stable carbocation formed by the fragementation of the starting molecule.
Molecular ion will always be present and gives the relative molecular mass of the starting compound.
Know definitions of molecular ion, relative abundance etc.
Difference between Low resolution and High resolution mass spectrometry.
Gives you relative molecular weight of compound and also molecule composition.
Parts of molecule can fragment (break off) in mass spectrometry so can get various peaks.
Know stability of carbocations
Isotope effect:
Certain atoms have isotopes i.e 35Cl, 37Cl
Therefore can get two