Mass spectrometry and infrared spectrometry Flashcards
What can mass spectrometry be used to provide?
information about the structure of a molecule, it’s molecular mass, and its molecular formula
What happens during a mass spectra of ethanol vapour?
-electrons from an electron gun knock electrons off the molecules of ethanol
-molecules usually contain pairs of electrons so when an electron is removed, an unpaired electron remains leaving the ion with a positive charge (called a radical ion)
-ethanol molecules lose an ion:
C2H5OH —-> [C2H5OH]+*
-then the positively charged ethanol ions are accelerated by an electric field, pass through a drift area, and then they are detected
What symbol does the molecular ion have?
M+*
How do you find out the molecular mass (Mr)
the m/z value for the molecular ion peak
What can happen to molecular ions when they undergo fragmentation?
they form smaller ions and radicals
What is a problem with only having a particular mass for a sample?
there are frequently several possible nuclear formulae for a particular mass, and so we need to know the mass of the particular ion more accurately
How do we get a more accurate mass?
by using a high resolution mass spectrometer
How many decimal places can be accurate?
4 decimal places
What is infrared spectroscopy used for?
to identify the functional groups present in organic compounds
How does infrared spectroscopy identify functional groups?
-chemical bonds absorb infrared radiation of specific frequencies so a particular bond can be matched to a particular absorption range on the infrared spectrum
What happens when the molecules are exposed to infrared radiation?
it is absorbed by the bonds which makes them vibrate even more
What are the three ways bonds can move?
-symmetric stretching
-bending
-asymmetric stretching
What does the frequency of absorbed infrared radiation by different bonds depend on?`
-the bond strength
-the masses of the atoms
strong bonds between light atoms are high energy with large wavenumbers
weak bonds between heavy atoms are low energy with small wavenumbers
Which bonds stretch?
O-H/ C-H/ N-H
What does the energy corresponding to a bond stretch or bend correspond to?
a particular wavelength of electromagnetic radiation
How do you calculate the frequency/wavelength/wavenumber?
E = hv
v = c/λ
E-energy
h-planck constant
v-frequency (nu)
c-velocity of light (s-1)
λ-wavelength (m)
What is the unit of wavelength?
(cm-1)
Why are wavenumbers used?
they are numerically more convenient than frequency or wavelength
What are wavenumbers?
reciprocal wavelengths
1/λ cm-1
What happens in the infrared spectrometer?
-the organic sample is subject to a continuous range of infrared frequencies
-this sample will absorb infrared radiation corresponding to the energies of the bond stretches and bends in it’s molecules
-by recording the intensity of the remaining infrared radiation a spectrum can be plotted
-the infrared spectrum can be used to identify the functional groups and the compound
How do peaks/troughs appear on the spectrum?
when infrared radiation is absorbed by the bonds the transmission is reduced
What shape do C-H bonds make?
typically sharp, longer than C≡N
What shape do C≡N bonds make? (nitriles)
nitriles are very sharp, shorter than C-H
What shape do O-H bonds make? (alcohols/carboxylic acids)
broad (due to hydrogen bonding)
What shape do C=O bonds make? (aldehydes/ketones/carboxylic acids/esters)
sharp and very long
What is the fingerprint region?
-1500-400 cm-1
-a complex region of the spectrum that contains that contains many peaks/a wide variety of bond vibrations
What does the entire infrared spectrum of an organic compound provide?
a unique molecular fingerprint
How can scientists identify a compound in the fingerprint region?
-compare the unknown spectrum with spectra from an infrared data-base
-an exact match means that the scientist has identified the compound since the fingerprint region is unique to each compound
What does no match mean when comparing the fingerprint region?
no match means a unique fingerprint so the scientist may have made a new compound or it could be impure
Name the approximate wavelengths for-
C=O
O-H
carboxylic acids
C=O - 1700cm-1
O-H - 3300cm-1
carboxylic acids- 3000cm-1
