NMR Spectroscopy 6.3.2

practical uses of NMR

MRI scan

what isotopes are identified

¹³C

¹H

why is carbon 13 and a proton used in NMR

uneven number of nucleons which absorb radio waves

what shift to go for ?

always go for highest shift

what do the peaks show in carbon NMR

the number of different carbon environments 

what is the reference point in carbon NMR

Tetramethyl silane ( TMS)

why is TMS used in NMR

gives a single sharp peak

inert ( doesn’t react with sample)

volatile ( separated from sample easily)

TMS displayed formula

Si(CH₃)₄

what does carbon environment mean

number of carbon atoms attached to different atoms ( what the atom is attached to aswell)

when do methyl groups have the same carbon type

attached to same carbon

what type of solvent is used in Proton NMR

Deuterated solvent ( CDCl₃)

why are Deuterated solvents used ?

have a even number of nucleons so doesn’t produce signal

How to prepare a Proton NMR

molecule dissolved in Deuterated solvent ( CDCl₃)

with TMS as reference peak

what causes peak at O 

TMS reference peak

what is the Integrated Trace

gives information about relative proportion of each hydrogen type

what does Integrated trace show about hydrogen

Ratio of Hydrogens in each hydrogen Type

Integrated trace / R.P. A Image

number above peak

what causes proton splitting 

adjacent different proton types attached to carbon

How to work out splitting patterns of Proton type

Count number of adjacent proton in different environment = n

add 1 → n + 1 = splitting pattern

Splitting pattern of proton with no adjacent protons

1

n = 0

n + 1 = 1

peak splitting patterns names

peak splits into 1 → singlet

peak splits into 2 → Doublet

peak splits into 3 → Triplet

peak Splits into 4 → quartet

many peaks → Multiplet

Proton splitting ( IMAGE)

How to remove Alcohol OH and amine NH peaks

add D₂O → removes all NH and OH peaks 

what is the splitting pattern for OH and NH peak

always singlet

disadvantage of NH / OH peak

Mask other peaks

¹H NMR spectrum what to look for first

Always look for Benzene 

aldehyde 

carboxylic acid 

¹H NMR spectrum what does a triplet quartet anywhere on the spectrum show 

ethyl group → (CH₃CH₂)

always 3:2

¹H NMR spectrum what does benzene integration number show

the hydrogens on benzene 

other hydrogens been substituted 

¹H NMR spectrum how to identify benzene

Multiplet at 6-8 ppm

¹H NMR what does a single peak show

Hydrogen no adjacent hydrogens

¹H NMR what does a doublet peak show

there's a adjacent single hydrogen

¹H NMR what does a singlet (R.P.A of 9) show 

¹H NMR what does a singlet peak (R.P.A of 1) show

OH ( alcohol group)

¹H NMR spectrum what does a doublet (RPA 6) Multiplet (RPA 1) show

CH(CH₃)₂

What are the phases in TLC

Mobile phase

Stationary phase

TLC what is the stationary phase

silica

TLC what is the mobile phase

solvent

TLC what separates the molecules

Relative adsorption

TLC what do isomers produce

similar adsorption to stationary phase

large spots caused by merging smaller spots

Control variables in a TLC experiment

Same solvent

Same stationary phase

Control variables in Gas chromatogram

Same temperature

same pressure

same diameter of column

¹H NMR spectrum what does a triplet triplet RPA 2 mean

CH₂CH₂

¹H NMR spectrum what does a Doublet ( RPA 3)- Quartet ( RPA 1)

CHCH₃

¹H NMR what does a singlet (R.P.A of 6) show

C(CH₃)₂

What is the Mobile phase in Gas chromatography

helium

what is the stationary phase in Gas chromatography

liquids such as Large chain alkanes

Gas chromatography molecule and retention time relationship

More non polar = Longer retention time

( non-polar more soluble in alkanes)

Gas chromatography what does the peaks show

Number of compounds in the mixture

What do two molecules having similar retention times mean in Gas chromatography

Isomers

How to use Gas chromatography to work out concentrations

(ratio of compound (x) / ratio of known concentration ) x concentration

How to use Gas chromatography to work out concentrations (IMAGE)

How to use Calibration curve to calculate concentration

Gas chromatography if stationary phase is liquid how are molecules separated

By relative solubility

How does D₂O work

replace protons with D atoms which have even number of nucleons → produces no signal

difference between separation in gas chromatography and TLC

TLC → Relative adsorption

Gas → Relative solubility