Spectroscopy Flashcards
What are the 4 types of spectroscopy?
Mass spectrometry (MS)
Infrared spectroscopy (IR)
UV-Visible spectroscopy (UV-VIS)
Nuclear magnetic resonance spectroscopy
What is mass spectrometry?
Examination of ions (fragments) derived from molecules that allow the calculation of molecular mass
What is infrared spectroscopy?
Determines the presence or absence of particular functional groups in a molecule
What is UV-Vis spectroscopy?
Determines the presence of pi-bond electrons in a molecule as well as the presence of any conjugation in the structure of the compound
What is NMR?
Provides information about the carbon and hydrogen framework of a molecule
What is molecular spectroscopy?
IR, UV-VIS, NMR - depend on the interaction of molecules with radiation of specific energy
What is the relationship between energy and frequency? Provide an equation
E= h x v
where
h is plancks constant
v is frequency of radiation
Hence energy is directly proportional to frequency of radiation
What is the relationship between energy and wavelength? Provide an equation
c = v x lambda
where
c is speed of light
v is frequency of radiation
lambda is wavelength
therefore sub into E = hv
E = hc/lambda
Therefore energy is inversely proportional to wavelength of radiation
How does mass spectrometry work?
A molecule is bombarded with high energy electron beam -> this causes expulsion of an electron from the molecule creating a positively charged molecular ion (radical cation)
The radical cation formed is fragmented forming other positive ions and neutral fragments
A plot of relative ion abundance vs m/z forms the mass spectrum (mass of ion/charge of ion (+1))
How is the molecular mass of the molecule determined from the mass spectra?
The molecular ion (radical cation) must be the ion of the highest m/z hence the highest m/z value corresponds to the molecular mass of the parent ion
How does infrared spectroscopy work?
Absorption occurs when the infrared radiation frequency matches the frequency of bond vibration (specific to the type of bond)
How is the functional groups determined from IR spectra?
The relative masses of atoms involved and bond strengths determine the position of IR absorption displayed on spectrum.
Hence specific bonds will show absorption at specific ranges of the spectra
What are the strengths and weaknesses of MS
Only information about molecular mass - cannot determine between isomers
What are the strengths and weaknesses of IR spectroscopy?
Fast identification of functional groups
Compounds containing same functional groups will show very similar spectra
No indication on number of functional groups present
How does UV-Visible spectroscopy work?
UV-VIS radiation raises electrons in molecules (with pi bonds) from lower energy bonding and non-bonding molecular orbitals to higher energy anti-bonding molecular orbitals
The wavelength at which maximum absorption occurs is considered