Chapter 17 Spectroscopy I
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What is the process of mass spectrometry?
The compound loses an electron to form a 1+ ion The compound fragments The mass spectrometer detects the mass/charge, which gives the molecular mass of the compound
What is the M+ peak?
The peak the (furthest) right of the mass spec graph, giving the molecular mass of the compound
What is important to remember when writing down mass spec fragments?
They are ions so have + charges
What is the M+1 peak?
The peaker one more than the M+ peak due to carbon 13 isotopes Can have several peaks depending on what other compounds are present e.g chlorine has two isotopes 35 and 37
How does a mass spectrometer physically work?
Ionisation- forming 1+ ions, bombarding with electrons Acceleration- to same KE Deflection- by a magnetic field depending on mass Detection of IONS only
What are the uses of Mass Spectrometers?
Identifying unknown compounds Determine isotope abundances Gain further information about chemical structures Detecting banned substances such as steroids Analysing molecules in space
Show a possible fragmentation of CH3CH2CH3+
CH3CH2+ + CH3’ Only the ion is detected, not the radical
How do you calculate the formula from percentage compositions?
Divide by molar mass Divide moles by the lowest moles Multiple if necessary to whole numbers or round, then multiple to molecular mass given my mass spec
How does IR affect covalent bonds?
The covalent bonds absorb IR and vibrate more They can stretch- changing the distance between the atomic centres or bend- change the bond angle
What factors affect how much a bond vibrates?
The mass of the atoms- heavier atoms vibrate more slowly Strength of the bond- stronger bonds vibrate faster
How does the greenhouse effect work?
UV from the sign passes through atmosphere and absorbed. Some of it is re-emitted as IR Longer wavelength IR has the same frequency as the natural frequency of bonds of greenhouse gases so absorbed, THE BONDS ABSORB IR vibrate more Re-emitted as radiation increasing global temperatures
What region is the fingerprint region?
Below 1500cm-1
How does IR spectroscopy work?
Beam of IR radiation, 200-4000cm-1 fired through sample Emerging beam of light analysed Computer plots transmittance against wave number
What are common application of IR?
Breathalysers to detect ethanol, alcohol in breath Detecting vehicle emissions Compared against known graphs
What factors affect the greenhouse effect?
The concentration/amount of greenhouse gases The ability to absorb IR/ Natural frequency Residence time of greenhouse gases
Can peaks of IR merge together?
Yes If the peak is from 2500 to 3500 then it has a regular OH group and and a OH group from the carboxyl group
What does a secondary carbocation in mass spec mean about its structure?
It’s branched
Can an alcohol peak be distinct?
Yes, not always broad Especially when carboxylic acids are present. Or there is a mass broad peak for both
What things 3 things will change in the IR for ethanol forming ethanol?
The peak at 3200-3600 for the O-H bond will disappears There will be an additional peak between 1620-1820 for C=O The fingerprint region will differ