spectroscopic methods detail Flashcards
Mass spectrometry
measures the mass to charge ratio of an ion.
why do you see multiple peaks?
fragmentation of ion
what instrument do you use?
the sample enters an ion source, mass analyser then detector.
the detector is in a vacuum so as to avoid collisions between the ions which would decrease the resolution of the instrument
EI
Electron ionisation
what Is EI?
High energy electrons are aimed to gaseous sample.
this knocks of outer electrons of molecules that have a weaker electrostatic attraction between nucleus.
e.i lone pairs
disadvantage of EI?
Harsh.
Fragmentation
what happens to the positive ion
It is repelled out of mass analyser by the positive plate.
CI
Chemical Ionisation
CI
the reagent is ionised and collides with sample. This causes a transfer of protons
electrospray Ionisation
the solution of sample is added with an ionic compound.
it is sprayed through a fine capillary held at a potential of thousands of volts
this creates aerosol of charged droplets.
the solvent evaporates columbic explosion,
TOF
the electric field accelerates the ion with the same kinetic energy. The mass is determined by the formula 0.5mv^2
quadrupole mass analyser
4 parallel rods around 5-10cm.
opposite rod pairs are connected with the same polarity of DC voltage source. A radio frequency is also applied to the rods.
by controlling the radio and voltage only desired m/z can pass through as the trajectories change..
other m/z are neutralized
Fragmentation patterns / finger prints
x
quantized
to promote an electron to the next energy level it needs a fixed amount
energy levels
translational
rotational
vibrational
electronic
IR Spectroscopy
IR radiation is shone through a sample.
the molecule moves to a higher vibrational energy
how do energy levels go
fundamental.
first overtone
second overtone
IR in diatomics
IR photon is absorbed
molecule gains vibrational energy
In IR closely space sets of absorption are called
bands
what do bands show
energy levels
fundamental.
first overtone
second overtone
why is the 1st overtone greater than the 2nd
showa the spacing of vibrational energy
how can the vibration be described as
harmonic oscillator
as the bonds stretch and compress
the energy rises so there is a restoring force which tries to set the system to equilibrium length
frequency
o.5 pi sqrt kf / m
m reduced mass
m1m2/ m1 + m2
the strength of ir frequency
depends on the size of dipole change
if there is no dipole change present
no IR absorption for that vibration
the greater the bond strength
the greater the bond frequency
electrons have a property called
spin
nmr
detects the nuclei going back to equilibrium after transitioning from a higher energy level ftom radiowaves
what do we use for nmr
h, h2, c13
electron density
in a magnetic field electrons move in a way that sets up a magnetic field which imposes the orginal field.
this leads to a weaker mangnetic field.
the weaker the field
the smaller the energy difference in the spin states hence the smaller the frequency
anything that removes the electron density
shifts the resonance to a higher energy.
why does sp2 give higher shift
double bond
lower electron density
