3.3.15 nmr

what is nuclear magnetic resonance (nmr)?

a very powerful tool for identifying unknown compounds

what are the units of chemical shift (δ)?

ppm

what does nmr give information about?

about the position of ¹³C or ¹H atoms in a molecule

give the 3 steps of how nmr is carried out

1. sample must be dissolved in a solvent

2. a standard must be added + the peak is set to 0ppm chemical shift on the x-axis — all the other peaks’ shifts are measured relative to the TMS peak

3. then carry out either a carbon (¹³C) NMR or proton (¹H) NMR

what must the solvent not have any of for ¹H NMR? outline why

• it must not have any ¹H atoms present

• as they would produce a big signal + interfere with the spectrum of the compound you were analysing

what solvent is used if:

a. sample is polar?

b. sample is non-polar?

a. use CDCl₃ not CHCl₃

b. use CCl₄

explain why we use D (deuterium) instead of H in the solvent?

• the nucleus of H: 1 proton — odd number of nucleons so is magnetic - is NMR active - produces a signal - interferes with the spectrum of the compound you are analysing

• the nucleus of D: 1 proton + 1 neutron — even number of nucleons so is not magnetic - is not NMR active - does not produce a signal - does not interfere with the spectrum of the compound you are analysing

what is the standard that is added? + give its structure

tetramethylsilane (TMS)

outline 3 reasons why tetramethylsilane is added to all sample analysed

carbon-13 NMR is…...than H¹ NMR

less complex

what determines the shift with C-13 NMR?

the environment that each C atom is in will determine the shift

eg if there are 2 different carbon environments (like in ethanol) how many peaks will there be in the C-13 NMR?

2 peaks

what is the number of peaks in a C-13 NMR always equal to?

to the number of different carbon environments in the compound

what determines the shift of each peak in C-13 NMR?

the type of carbon

• example in image

however when will the number of peaks not equal the number of carbons in a compound?

if there is any symmetry in the molecule

eg how many environments + so peaks for this molecule?

• there are 4 environments so 4 peaks

• there are no branches, there is no symmetry so every carbon has a unique environment

example of symmetry in molecule

outline what will be produced if there are ever a number of of methyls on the same carbon

• they will be in the same environment due to symmetry on the tetrahedral C

• and so will give rise to 1 peak

A - 3

B - 2

give the number of C-13 NMR peaks for each isomer

P - 2

Q - 3

R - 4

in some exam questions, what will you have to deduce from a molecular formula + given number of C-13 NMR peaks

the structure of a compound

in other types of questions, what would you be required to suggest?

a chemical shift value for a given type of carbon atom

eg - giving chemical shift values

give the type of environment the labelled C is in + the range it will have a peak in

• benzene environment

• peak in the range 110-160ppm

give the type of environment the labelled C is in + the range it will have a peak in

• ketone environment

• peak in the range 190-220ppm

give the type of environment the labelled C is in + the range it will have a peak in

• peak in the range 25-60ppm