32 Structure determination

Question 1

Nuclear magnetic resonance

Answer 1

Nuclear magnetic resonance (NMR) gives information about the position of 13C or 1H atoms in a molecule.

Question 2

simpler spectra

Answer 2

13C NMR gives simpler spectra than 1H NMR

Question 3

use of deuterated solvents or CCl4

Answer 3

1H NMR spectra are obtained using samples dissolved in deuterated solvents or CCl4

Question 4

tetramethylsilane

Answer 4

(TMS)

Question 5

what is nuclear magnetic resonance spectroscopy

carbon-13

Answer 5

magnetic field applied to the sample, which is surrounded by a source of radio waves and a radio receiver. This generates an energy change in the nuclei of atoms in the sample that can be detected. electromagnetic energy is emitted, which can be interpreted by a computer
(greater electron density around carbon-13 and smaller magnetic field felt by nucleus and so lower the frequency at which it resonates)

Question 6

why use carbon 13?

Answer 6

carbon 13 have nuclear spin

Question 7

magnetic field and carbon

Answer 7

not all carbon 13 atoms in a molecule restate at exactly the same magnetic field strength. Carbon atoms in different functional groups feel the magnetic fields differently because all nuclei are shielded from external magnetic field by the electrons that surround them.
greater electron density = smaller magnetic field felt by the carbon

Question 8

chemical shift

Answer 8

units = parts per million (ppm)

chemical shift is related to the difference in frequency between the resonating nucleus and that of TMS

Question 9

interpreting spectra of carbon

Answer 9

height of peaks in NMR are not significant

Question 10

Proton NMR

Answer 10

H nucleus is being looked at
hydrogen atoms attached to different functional groups that feel the magnetic field different, because all nuclei are shielded from the external magnetic field by the electrons that surrounded by them
nuclei with more electrons around therm are better shielded

Question 11

What happens if a hydrogen has identical environments ?

Answer 11

only one chemical shift value

e.g. methane

Question 12

hydrogen chemical shift

Answer 12

in general, the further away a hydrogen atom is from an electronegative atom the smaller its chemical shift

Question 13

integration trace

Answer 13

In proton NMR spectra, the area of each peak is related to the number of hydrogen atoms producing it.

Question 14

TMS

Answer 14

the chemical shifts are measured by reference to a standard which is TMS
liquid 
calibrate the spectrum 
inert
non-toxic 
easy to remove form sample

Question 15

spin-spin coupling (spin-spin splitting)

Answer 15

it happens because the applied magnetic field felt by any hydrogen atom is affected by the magnetic field of the hydrogen atoms on the neighbouring carbon atoms. This spin-spin splitting gives information about the neighbouring hydrogen atoms.

Question 16

n+1 rule

Answer 16

n hydrogen on an adjacent carbon atom will split a peak into n+ 1 smaller peaks

Question 17

solvents of H NMR

Answer 17

solvents must not contain any hydrogen atoms 
commonly used solvent 
tetrachachloromethane (CCl4)
Deuterium 
CDCl3
D2O
C6D6

Question 18

What does NMR mean?

Answer 18

nuclear magnetic resonance spectroscopy

find the structure of complete molecules

Question 19

there isn’t a lot of carbon-13 so how does it work

Answer 19

modern instruments are sensitive enough

Question 20

nuclear spin

Answer 20

electron, proton and neutrons all have spin

odd number of electrons, the unpaired nucleon produces a small nuclear spin which generates a magnetic field

Question 21

Resonance

Answer 21

nuclei in low energy spin states can jump the energy gap when given the exact amount of energy ( excitation)

when excited the nucleus will drop back to its original energy state and emit the same amount of energy in from of radiation (relaxation)

cycle of excitation and relaxation is called resonance