NMR

Question 1

chemical shifts are explicitly determined by what

Answer 1

the degree to which a proton is shielded/deshielded by the surrounding nuclei

Question 2

the more shielded a proton is

Answer 2

the lower its δ

Question 3

what desheilds the proton

Answer 3

if a proton is attached to an electron withdrawing system such as an aromatic, a field opposing the applied magnetic field is created, which deshields the proton.

Question 4

what is the effect of deshielding the proton

Answer 4

effect of increasing the δ value of the proton adjacent to an aromatic function, i.e. it will resonate at higher frequency (also known as lower field), i.e. at a higher chemical shift value.

Question 5

frequency emitted by nuclear depends on what

Answer 5

its chemical environment

Question 6

chemical shift values are..

Answer 6

very sensitive; methyl group protons have chemical shifts of 0.80-1.4 ppm, whereas aromatic protons are observed at δ ~ 7-8 ppm

Question 7

frequencies

Answer 7

differ for each nucleus, unless they are chemically equivalent and in identical molecular environments i.e. tetramethylsilane, TMS, or its water-soluble analogue, tetra- deuterated trimethylsilylpropionate (TSP)

Question 8

resonance frequencies are converted to what

Answer 8

chemical shifts

Question 9

what do chemical shift values allow

Answer 9

results from different experiments to be readily compared – can be thought of as a ‘normalisation’

Question 10

chemical shift definition

Answer 10

the resonant frequency of a sample compared to that of a reference (tetramethylsilane usually used as has a δ (chemical shift) values of 0.00 ppm)

Question 11

absorbed frequency measured in

Answer 11

Hz

Question 12

spectrometer frequency measured in

Answer 12

MHz meaning there is a million fold difference in frequencies here hence the term ‘parts per million (PPM)’

Question 13

chemical shift (reported as parts per million shift downfield from the TMS standard) equation

Answer 13

chemical shift = ((frequency of absorbed electromagnetic radiation by sample nucleus in Hz)-(frequency of absorbed electromagnetic radiation by TMS standard in HZ))/Spectrometer frequency in MHz(this accounts for magnetic strength)

Question 14

examples of chemical shifts

Answer 14

look at ppt

Question 15

1H spectrum of methylacetate

Answer 15

• The 1H NMR spectrum of methylacetate has two signals
• Signal at ~ 3.7 ppm deshielded more in view of electronegative groups adjacent (-O-CH3 function)
• Signal at ~ 2.1 ppm attributable to H3C-CO- function.

Question 16

advantages spin spin coupling of ethyl acetate

Answer 16

Look on ppt to see graphs

• The spin of protons adjacent to a particular proton split the original proton signal
• The blue protons from the r.h.s.
-CH3 function on the ester are split three ways by the two red protons:
• For option 1, there are two equivalent alignments, where the effects of the red protons cancel each other out and don’t perturb the δ value of the original signal. This leads to the central line of the blue triplet with a 2 x (double) intensity
• Options 2 and 3 result in an equal and opposite split, hence the triplet observed
• The red protons are split into a quartet by the alcoholic ester terminal-CH3 protons

Question 17

splittings all follow what?

Answer 17

pascals triangle (on ppt)

Question 18

complications spin spin coupling of ethyl acetate

Answer 18

• The l.h.s. methyl function protons remain the ‘expected’ signal we observed for methyl acetate, since they are isolated from adjacent (r.h.s.) protons by the ester group

Question 19

Pascal’s triangle

Answer 19

determines the splitting pattern of each peak.

In general, n-equivalent neighboring hydrogens will split a 1H signal into an (n + 1) Pascal pattern.
“neighbouring” – no more than three bonds away

look at ppt

Question 20

N must be what to give rise to a pascal splitting pattern

Answer 20

equivalent neighbouring hydrogens

Question 21

if the neighbouring hydrogen s are not equivalent then..

Answer 21

then you will see a complex pattern (known as a complex multiplet).

Question 22

splitting pattern of OH

Answer 22

do not split neighboring hydrogen signals nor is it split. It is normally a broad singlet of relative integration 1 between 1 – 5.5 ppm (variable) when spectra are acquired in deuterated organic solvents.

Question 23

why don’t we see the OH function when spectra of alcohol are acquired in aqueous solution

Answer 23

because of exchange with NMR-inactive deuterium in 10% (v/v) D2O added.

Question 24

spin spin coupling

Answer 24

focusing on one group of magnetically-equivalent 1H nuclei in a particular functional group – Imagine that you are sitting on the carbon atom involved, i.e. a –CH3, -CH2 or –CH function.
- All you need do now is count the number of adjacent nuclei!

Question 25

Examples of spin spin coupling

Answer 25

look at ppt.

Question 26

NMR applications- quantitative drug analysis

Answer 26

Drugs can be quickly quantified by measuring suitable protons such as those in methyl groups – usually against the intense signal of those in t-butanol using the following equation: look at ppt