A-LEVEL CHEMISTRY, MODERN ANALYTICAL TECHNIQUES II. Flashcards
MASS SPECTROMETRY USE.
MASS SPECTROMETRY IS USED TO FIND THE RELATIVE MOLECULAR MASS, Mr, OF A COMPOUND.
MASS SPECTROMETRY, PEAK.
PEAKS SHOS FRAGMENTS OF THE ORIGINAL MOLECULE.
THE LAST PEAK IS THE M+ PEAK OR THE MOLECULAR ION PEAK.
THIS IS THE SAME AS THE REALTIVE MOLECULAR MASS OF THE MOLECULE.
m/z.
m/z IS JUST THE MASS OF A FRAGMENT DIVIDED BY CHARGE.
AS MOST HAVE A +1 CHARGE THIS THE SAME AS THE FRAGMENT MASS.
HIGH RESOLUTION MASS SPECTROMETRY.
HIGH RESOLUTION MASS SPECTROMETRY IS USEFUL WHEN IDENTIFYING DIFFERENT MOLECULES WITH THE SAME MOLECULAR MASS ROUNDED TO THE NEAREST WHOLE NUMBER.
HIGH RESOLUTION MASS SPECTROMETERS MEASURE THE REALTIVE MASS TO SEVERAL DECIMAL PLACES UNLIKE THE STANDARD LOW RESOLUTION WHICH MAY ONLY BE ABLE TO MEASURE RELATIVE MASS TO THE NEAREST WHOLE NUMBER.
NMR MEANING.
NUCLEAR MAGNETIC RESONANCE.
NMR SPECTROSCOPY USE.
NUCLEAR MAGNETIC RESONANCE, NMR, SPECTROSCOPY IS USED TO HELP TO DETERMINE THE STRUCTURE OF A MOLECULE.
NMR TYPE.
THERE ARE TWO,2, MAIN TYPES OF NMR:
13C NMR:
TELLS YOU INFORMATION ON HOW CARBON ATOMS ARE ARRNAGED.
HIGH RESOLUTION 1H NMR:
TELLS US INFORMATION ON HOW HYDROGEN ATOMS ARE ARRANGED.
NMR SPECTROSCOPY, ODD NUMBER OF NUCLEONS.
IF AN ATOMIC NUCLEUS HAS AN ADD NUMBER OF NUCLEOSN, NEUTRONS AND PROTONS, THEN IT HAS A NUCLEAR SPIN.
THIS NUCLEAR SPIN CREATES A WEAK MAGNETIC FIELD.
NMR DETECTS HOW THESE MAGNETIC FIELDS ARE AFFECTED BY A LARGER EXTERNAL MAGNETIC FIELD.
NUCLEAR SPIN, HYDROGEN AND CARBON.
HYDROGEN HAS ONE,1, PROTON SO DOES HAVE A NUCLEAR SPIN.
CARBON NORMALLY HAS SIX,6, PROTONS AND SIX,6, NEUTRONS HOWEVER APPROXIMATELY 1% OF CARBONS ARE 13C WHICH HAS SEVEN,7, NEUTRONS AND DOES HAVE A NUCLEAR SPIN.
NUCLE SPIN.
NUCLEI SPIN IN RANDOM DIRECTIONS HOWEVER WHEN AN EXTERNAL MAGNETIC FIELD IS APPLIED THEY ALIGN IN TWO,2, DIRECTIONS.
WHEN WE APPLY AN EXTERNAL MAGNETIC FIELD THE NUCLEI SPIN EITHER IN THE DIRECTION OF THE EXTERNAL MAGNETIXC FIELD OR AGANIST IT.
THOSE THAT SPIN IN THE DIRECTION OF THE MAGNETIC FIELD HAVE LOWER ENERGY.
NMR FIRES OUT RADIO WAVES.
AT A SPECIFIC FREQUENCY THE NUCLEI THAT ARE ALIGNED WITH THE MAGNETIC FIELD ABSORB THE ENRGY AND FLIP TO THE HIGHER LEVEL, AGAINST THE MAGNETIC FIELD.
THOSE WITH HIGHER ENRGY CAN ALSO DROP TO LOWER ENERGY AND EMIT RADIO WAVES.
INITIALLY, THERE ARE MORE NUCLEI ALIGNED WITH THE MAGNETIC FIELD SO OVERALL MORE ENERGY IS ABSORBED THAN EMITTED.
NMR MEASURES THE AMOUNT OF ENERGY ABSORBED.
ENERGY ABSORBED BY THE NUCLEI IN NMR SPECTROSCOPY.
THE ENERGY ABSORBED BY THE NUCLEI IS DEPENDENT ON THE ENVIRONMENT IT IS IN.
A NUCLEUS CAN BE SHIELDED FROM AN EXTERNAL MAGNETIC FIELD FROM ELECTRONS SURROUNDING THE NUCLEUS.
ATOMS AND GROUPS OF ATOMS ADJACENT TO THE NUCLEUS AFFECT HE LEVEL OF ELECTRON SHIELDING.
FOR EXAMPLE, AN ELECTRONEGATIVE ELEMENT SUCH AS OXYGEN NEAR A CARBON ATOM WILL REDUCE THE ELECTRON SHIELDING ON THE CARBON ATOM.
THE MAGNETIC WILL BE FELT BY THE NUCLEI DIFFERENTLY DEPENDIG ON THE ENVIRONMENT IT IS IN AS THEY ABSORB DIFFERENT AMOUNTS OF ENERGY AND VARIOUS FREQUENCIES.
IT IS THIS DIFFERENCE THAT NMR SPECTROSCOPY PIKS UP.
THE ENVIRONMENT IS DETERMIEND BY THE GROUPS OF ATOMS THAT EXIST NEAR TO THE NUCLEI BEING EXAMINED.
WE LOOK ALONG THE FULL CHAIN NOT JUST THE ATOM IMMEDIATELY BONDED TO ATOM BEING EXAMINED.
FOR AN ATOM TO BE IN THE SAME ENVIRONMENT IT MUST BE BONDED TO AN ATOM OR GROUPS OF ATOMS THAT ARE IDENTICAL.
TETRAMETHYLSILANE.
TETRAMETHYLSILANE, TMS, IS A CHEMICAL USED AS A STANDARD WHEN LOOKING AT CHEMICAL SHIFT IN NMR SPECTRA.
AS NUCLEI ABSORB DIFFERENT AMOUNTS OF ENERGY AT DIFFERENT FREQUENCIES IT IS DIFFICULT TO MEASURE THE MAGNITUE OF THESE WITHOUT A REFERENCE OR STANDARD CHEMICAL TO MEASURE AGAINST.
THE STANDARD WE USE IS
TETRAMETHYLSALINE, TMS.
TMS HAS TWELVE, 12, HYDROGENS ALL IN IDENTICAL ENVIRONMENTS.
THIS WILL PRODUCE A LARGE, SNIGLE PEAAK WELL AWAY FROM PEAKS.
IT IS ALSO INERT, NON-TOXIC AND VOLATILE SO EASY TO REMOVE FROM YOUR SAMPLE.
THE CHEMICAL SHIFT.
WE MEASURE CHEMICAAL SHIFT IN PARTS ER MILLION, ppm, AS BEACAUSE IT IS USED AS A STANDARD TO MEASURE WE ASSIGN THIS PEAK AS HAVING CHEMICAL SHIST EQUALLING ZERO,0.
13C NMR SPECTROSCOPY.
13C NMR SPECTROSCOPY TELLS US HOW MANY DIFFERENT CARBON ENVIRONMENTS THERE ARE IN THE SAMPLE BEING TESTED.
THE PEAKS ON A 13C NMR SPECTRUM TELLS US THE NUMBER OF DIFFERENT CARBON ENVIRONMENTS.
FOR EXAMPLE:
CHLOROETHANE.
THE CARBON ATTACHED TO THE CHLORINE IS CLOSEST TO THIS ELECTRONEGATIVE CHLORINE.
THIS MEANS THE ELCTRON SHEILDING IS LOWER AND THE CHEMICAL SHIFT IS HIGHER.
THE OTHER CARBON IS FURTHER AWAY FROM CHLORINE.
THIS MEANS THE ELECTRON SHIELDING IS HIGHER AND THE CHEMICAL SHIFT IS LOWER.
EACH CARBON MOLECULE IS IN A DIFFERENT ENVIRONMENT AS THERE IS A DIFFERENT AMOUNT OF ELECTRON SHIELDING.
WE SEE TWO,2, PEAKS ASSOCIATED WITH CHLOROETHANE.
13C NMR SPECTROSCOPY, CYCLIN COMPOUNDS.
CYCLIC COMPOUNDS ARE MORE DIFFICULT TO PREDICT THEIR SPECTRA.
IN THESE CASES WE LOOK FOR SYMMETY TO WHICH ATOMS ARE IN DIFFERENT ENVIRONMENTS.
