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
What is a conjugated pi-system?
When the molecule contains a pi bond - sigma bond - pi bond
What is the wavelength range of UV?
200-400nm
What is the wavelength range of Visible light?
400-800nm
What is an anti-bonding MO?
All bonding orbitals have an anti-bonding orbital
e.g. pi - bonding orbital
pi star - anti-bonding orbital
What are the types of non-conjugated functional groups that show absorption on UV-VIS spectra?
C=C 170-180nm
C=O 170-180nm 280nm
What absorptions are seen for C=C (non-conjugated system) and why?
The excitation of an electron from the pi bonding MO to the anti-bonding MO shows absorption at 170-180nm
What absorptions are seen for C=O (non-conjugated system) and why?
The excitation of an electron from the lone pair on oxygen to anti-bonding pi orbital shows weak absorption at 280nm
The usual absorption at 170-180nm from excitation of pi orbital electron is also seen.
Does UV-VIS provide info on the number of pi bonds?
No
What is the relationship between absorption and wavelength for conjugated systems compared to non-conjugated systems?
Absorption occurs at longer wavelengths for conjugated systems as the energy gap between the molecular orbitals are small
Usually >200nm
What is the relationship between number of double bonds in conjugation and wavelength?
The greater number of double bonds in conjugation = the smaller the gap in energy between molecular orbitals = larger the longest wavelength value
What is Beer’s Law
A = Ebc
where A is absorbance
b is path length (cm)
c is concentration (mol L-1)
E is molar absorptivity (L mol-1 cm-1)
Hence absorbance is directly proportional to concentration and path length
What is the relationship between amount of conjugation and molar absorptivity?
No relation
How does NMR work?
Nuclear magnetic resonance provides information about the carbon and hydrogen frameworks of molecules.
A magnetic field is applied to a number of nuclei causing excitation from a lower to higher energy spin state
What is the factor that affects the energy required for a flip in nuclei spin?
Shielding of protons due to electrons
What does shielding do to the chemical shift value?
Shielding decreases chemical shift
What is the chemical shift range for 13C NMR spectrum?
delta 0-220
What is the chemical shift range for sp3 hybridized carbon atoms?
0-90 delta
What is the chemical shift range for sp2 hybridised carbon atoms?
100-220 delta units
How does shielding change?
Shielding decreases when electronegative groups of atoms are bonded to the carbon as it pulls electrons away from the molecule
With proton NMR what are the 4 components of spectra?
Number of absorption signals
Position of absorption signals
Relative areas under absorption signals
Splitting pattern for absorption signals
What information does the number of absorption signals provide?
Number of H nuclei in different environment
What information does the position of absorption signals provide?
The amount of shielding of the proton nuclei
What information does the relative area under absorption signals provide?
The number of protons in the same environment
What does the splitting pattern for absorption signals provide?
The number of neighbouring hydrogens - each neighbouring proton will cause the signal to split once
