spectroscopic methods detail

Question 1

Mass spectrometry

Answer 1

measures the mass to charge ratio of an ion.

Question 2

why do you see multiple peaks?

Answer 2

fragmentation of ion

Question 3

what instrument do you use?

Answer 3

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

Question 4

EI

Answer 4

Electron ionisation

Question 5

what Is EI?

Answer 5

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

Question 6

disadvantage of EI?

Answer 6

Harsh.
Fragmentation

Question 7

what happens to the positive ion

Answer 7

It is repelled out of mass analyser by the positive plate.

Question 8

CI

Answer 8

Chemical Ionisation

Question 9

CI

Answer 9

the reagent is ionised and collides with sample. This causes a transfer of protons

Question 10

electrospray Ionisation

Answer 10

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,

Question 11

TOF

Answer 11

the electric field accelerates the ion with the same kinetic energy. The mass is determined by the formula 0.5mv^2

Question 12

quadrupole mass analyser

Answer 12

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

Question 13

Fragmentation patterns / finger prints

Answer 13

x

Question 14

quantized

Answer 14

to promote an electron to the next energy level it needs a fixed amount

Question 15

energy levels

Answer 15

translational
rotational
vibrational
electronic

Question 16

IR Spectroscopy

Answer 16

IR radiation is shone through a sample.
the molecule moves to a higher vibrational energy

Question 17

how do energy levels go

Answer 17

fundamental.
first overtone
second overtone

Question 18

IR in diatomics

Answer 18

IR photon is absorbed
molecule gains vibrational energy

Question 19

In IR closely space sets of absorption are called

Answer 19

bands

Question 20

what do bands show

Answer 20

energy levels
fundamental.
first overtone
second overtone

Question 21

why is the 1st overtone greater than the 2nd

Answer 21

showa the spacing of vibrational energy

Question 22

how can the vibration be described as

Answer 22

harmonic oscillator

Question 23

as the bonds stretch and compress

Answer 23

the energy rises so there is a restoring force which tries to set the system to equilibrium length

Question 24

frequency

Answer 24

o.5 pi sqrt kf / m

Question 25

m reduced mass

Answer 25

m1m2/ m1 + m2

Question 26

the strength of ir frequency

Answer 26

depends on the size of dipole change

Question 27

if there is no dipole change present

Answer 27

no IR absorption for that vibration

Question 28

the greater the bond strength

Answer 28

the greater the bond frequency

Question 29

electrons have a property called

Answer 29

spin

Question 30

nmr

Answer 30

detects the nuclei going back to equilibrium after transitioning from a higher energy level ftom radiowaves

Question 31

what do we use for nmr

Answer 31

h, h2, c13

Question 32

electron density

Answer 32

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.

Question 33

the weaker the field

Answer 33

the smaller the energy difference in the spin states hence the smaller the frequency

Question 34

anything that removes the electron density

Answer 34

shifts the resonance to a higher energy.

Question 35

why does sp2 give higher shift

Answer 35

double bond
lower electron density