NMR spectroscopy

NMR stands for

nuclear magnetic resonance

integration shows

number of each proton in environment produced

the peak at 0.00 is the

TMS reference peak

scale position shows

the environment of protons studied/ in the peak

Identify a solvent in which J can be dissolved before obtaining its 1H n.m.r. spectrum.

CDCl

standard used to carbon NMR

C4H12Si

Give two reasons why C4H12Si is used as the standard in carbon NMR?

has one environment producing a single peak

non toxic or inert

low boiling point

all atoms have spin except those with a

even mass number

nuclei with spin behave like

bar magnets- they respond to an applied magnetic field

nuclei that align with a magnetic field have

relatively low energy state

nuclei that align against the magnetic field

have relatively high energy state

what is needed to move a nucleus to a higher energy state?

energy

the amount depends upon chemical environment of atom.

an NMR spectrum tells us what about a molecule?

-different non equivalent hydrogen atoms in molecule

-ratios of number of equivalent hydrogen atoms

-chemical environment of equivalent hydrogen atoms

- number of adjacent hydrogen atoms that are non equivalent

different peaks tell you

number of different non equivalent hydrogen atoms in molecule

area under the peak tells you

ratios of number of equivalent hydrogen atoms

the position of the peak tells you

chemical environment of equivalent hydrogen atoms

peak splitting in high resolution spectra

the number of adjacent hydrogen atoms that are non equivalent

The hydrogen atom in the OH group is attached to an oxygen atom, which is very electronegative. A hydrogen atom like this is...

deshielded.

The hydrogen atoms in the CH3 group are far from the oxygen atom, this means they are...

shielded.

deshielded

The peak it produces is shifted downfield in the spectrum.

what molecule is chosen for the reference peak?

TMS, tetramethylsilane.

why is tetramethylsilane used for the reference peak?

- has 12 equivalent hydrogen atoms, same chemical environment so produces a single intense peak

- its protons are highly shielded, NMR peak is upfield of most other peaks

- non toxic and chemically inert, doesn't react with sample

the rule for number of peaks produced by splitting

n + 1

number of neighbouring non-equivalent hydrogen atoms is known as

n

if n + 1 = 1

is a singlet

ratio of peak heights is 1

if n + 1 = 2

is a doublet

ratio of peak heights is 1:1

if n + 1 = 3

is a triplet

ratio of peak heights is 1:2:1

if n + 1 = 4

is a quartet

ratio of peak heights is 1:3:3:1

Give two other reasons why TMS is used as a standard in recording n.m.r. spectra. (2 marks)

one intense peak

Upfield to all other peaks

interfere with other peaks

Low bp

volatile

can easily be removed

Explain how infrared spectroscopy can be used to identify this isomeric alcohol. (2 marks)

Reference to the 'fingerprint region' or below 1500 (cm -1)

Match with or same as known sample/database spectra

N.m.r. spectra are recorded using samples in solution.

The 1H n.m.r. spectrum was recorded using a solution of atenolol in CDCl3

Suggest why CDCl3 and not CHCl3 was used as the solvent. (1 marks)

Solvent must be proton-free

Suggest why CDCl3 is a more effective solvent than CCl4 for polar molecules such as atenolol. (1 marks)

CDCl3 is polar

Suggest how you could show that the atenolol produced by reduction of a ketone was a racemate and not a single enantiomer. (2 marks)

plane polarised light

polarised light is not rotated or is unaffected

Suggest one advantage and one disadvantage of using a racemate rather than a single enantiomer in medicines. (2 marks)

cheaper medicine due to cost or difficulty of separation

may be side effects from one enantiomer in the mixture one enantiomer may be ineffective or double dose required

A compound is usually mixed with Si(CH3)4 and either CCl4 or CDCl3 before recording the compound's 1 H NMR spectrum. State why Si(CH3)4, CCl4 and CDCl3 are used in 1 H NMR spectroscopy. Explain how their properties make them suitable for use in 1 H NMR spectroscopy

1a CDCl3 or CCl4 solvent as reference / calibration / standard

Inert so unlikely to react with the sample allow

CCl4 or CDCl3 as solvent Both have no H atoms so give no signals in spectrum tied to

CCl4 non polar good solvent for non-polar organic molecules

CDCl3 polar covalent molecule good solvent for polar organic compounds

Signal in an area away from other typical H signals / peak upfield from others

Easy to remove / volatile / low bp