Mass Spectroscopy and UV spectroscopy Flashcards
What is radical cation formation?
When a molecule loses one electron to gain a positive charge and a lone electron
Other mass fragments can be formed when
_________ or __________ are broken during ionization.
Mass spectroscopy only detects _______ __________
C—C
C—H
Positive fragments
Explain the separation of ions
- beam of electrons causes molecules to ionize and fragment
- mixture of ions ACCELERATED through a magnetic field where lighter ions bend too much and heavier bend too little
- Varying of magnetic field spectrometer plots the abundance of ions of each mass
Contrast what occurs to lighter and heavier ions when accelerating and passing through the magnetic field
Lighter ions- curves too much
Heavier ions- curves too little
The exact radius of curvature depends on its _____-______-_____ _______ symbolized by
Mass-to-charge ration
m/z
What does the exact curvature of an ion’s path depend on?
Mass-to-charge ratio(m/z)
What are the meanings in the symbol of m/z?
Mass-m
Z- charge
Why do we consider the path to be curved by ions to be only dependent on mass despite m/z?
Most have a charge of +1, considering m/z, then
m/1=m
How do we identify the base peak?
This is the tallest peak, defined to be 100%
What is significant about the base peak?
It is the most abundant fragment and so causes the strongest signal
What is the significance of the base peak being set to 100%?
Set to 100% by the software and other fragments are measured against it, the % abundance of all peaks are relative to the base peak
What is the molecular ion?
The ion with the mass of the original molecule and the peak with the highest mass
The fragmentation pattern of the molecule depends on ….
The structure of the compound
Why do the cleavage of bonds not give a 50-50 mix of the fragments ?
Fragments may have varying levels of stability
Why is it possible to “chop up” a molecular compound and predict the peaks that will appear on the mass spectrum?
Specific groupings of atoms often give specific fragmentation patterns. Hence establishing the pattern for an unknown compound for an unknown compound can often lead to its structure
Contrast how different types of double bonds react to UV absorption
Double bonds absorb UV radiation
Isolated double bonds absorb weakly
Conjugated double bonds and aromatic systems absorb strongly in the UV energy region
A compound of a _________ chain of conjugated double bonds absorbs light at a ___________ wavelength. Therefore…
Longer
Longer
Length of chain of conjugated double bonds and absorption length of light wavelength are directly proportional
Briefly state the inner workings of UV spectroscopy
In UV spectroscopy, a sample is irradiated with a continuous source of UV radiation of varying wavelengths.
Give a detailed explanation of of how UV spectroscopy works?
UV radiation is absorbed, when UV wavelength matches the difference of energy between an occupied molecular orbital and unoccupied molecular orbital
The absorption will cause the electron to move from the occupied orbital/ground state to the previously occupied orbital/excited state
If no UV energy is absorbed, what does this indicate?
The amount of UV radiation emitted by the source is equal to the amount of radiation detected by the detector
Energy associated with the UV portion of the electromagnetic spectrum coincides with the _______ ___ ___________ _______ between molecular orbitals
Difference in energy levels
In UV spectroscopy, where is the sample placed?
Between the UV source and a detector
What is the indication that UV energy is absorbed by a sample when a sample is placed between a UV source and a detector?
There is a difference between the UV radiated and the UV detected
Give the differences between IR spectra and UV spectra
- IR has many peaks while UV spectra often has a single broad peak
- IR spectra has the base line of zero absorption at the top of the chart and a valley indicates absorption while UV spectra has the baseline at the bottom of the chart so that a peak indicates the absorption
UV spectra has a single broad peak at the top, the wavelength at the top is denoted by…
Lambda symbol-max
How can Beer’s law be used to calculate sample absorbance?
Absorbance=molar absorbitivity(E) x concentration of a sample(c) x length of the light path(sample path length)
Define each symbol in Beer’s law to calculate absorption and their units
E= molar absorptivity of the molecule of the molecule- L/(mole x cm)
c= concentration of a solution of a sample- mol/L
L= length of the light path- cm
How can Beer’s law be rearranged to calculate concentration of a sample?
c= A/(E x L)
The _________ the molar absorptivity, the ________ amount of radiation absorbed by the compound
Higher
Greater
What does the wavelength of radiation needed to raise the energy of a pi electron depend on when in a conjugated molecule?
Depends on the nature of the molecules electron system. One of the most important factors is the extent of the conjugation system
Due to the extent of conjugation being an important factor in the wavelength needed to raise the energy of a pi electron, UV spectrum of an unknown substance we can…
Deduce structural information about the nature any electron system present in a molecule
As the extent of conjugation increases, the energy required for electronic transition ____________. So they are _________ ________________
Decreases
Inversely proportional
Give the expected increasing order of peak absorbance in the following molecules:
Buta-1,3-diene
Octa-1,3,5,7-tetraene
Hexa-1,3,5-triene
- but a-1,3-diene
- Hexa-1,3,5-triene
- Octa-1,3,5,7-tetraene
Give the peak absorption of each of the following:
Octa-1,3,5,7-tetraene
Hexa-1,3,5-triene
Buta-1,3-diene
Buta-1,3-diene = 217nm
Hexa-1,3,5-diene= 258 nm
Octa-1,3,5,7-tetraene= 290 nm
What is the ultraviolet maxima of benzene?
203 nm
What is the absorption maxima of But-3-en 2-one?
219 nm
What is the absorption maxima of Octa-1,3,5,7-tetraene?
290nm
What is the absorption maxima of hexa-1,3,5-triene?
258nm
What is the absorption maxima of cyclohexa-1,3-diene?
256nm
What is the absorption maxima of 2-methylbuta-1,3-diene?
220nm
Who discovered Mass spectroscopy?
J. J. Thompson
What part of the mass spectrometer deal with resolving ions into their characteristics mass components according to their mass-to-charge ratio?
Analyzer
What is the main use of mass spectrometer?
To determine the relative mass of atoms
Describe UV-VIS spectroscopy
For molecules containing pi or non-bonding electrons, the larger the energy difference between a molecule’s Highest energy Occupied Molecular Orbital (HOMO) and it’s Lowest energy Occupied Molecular Orbital, the lower the wavelength the compound absorbs
