1H NMR Spectra

Question 1

chemical shifts are 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…

Answer 2

the lower its δ

Question 3

how is a proton deshielded

Answer 3

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 effect does deshielding a proton have

Answer 4

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 values.

Question 5

Exact frequency emitted by a nucleus depends on what

Answer 5

its chemical environment

Question 6

methyl group protons have chemical shifts of what

Answer 6

0.80-1.4ppm

Question 7

aromatic protons have chemical shifts of what

Answer 7

δ ~ 7-8 ppm

Question 8

frequencies differ for each nucleus unless what

Answer 8

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

Question 9

resonance frequencies are converted to what

Answer 9

chemical shifts, δ

Question 10

what are chemical shifts defined as

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

see powerpoint for

Answer 11

chemical shift equation

Question 12

see powerpoint for

Answer 12

examples of chemical shifts and spectrums

Question 13

what is an advantage of NMR

Answer 13

spin-spin coupling

Question 14

what is spin-spin coupling

Answer 14

The spin of protons adjacent to a particular proton split the original proton signal

Question 15

see powerpoint for

Answer 15

spin-spin coupling example

Question 16

splitting in spin-spin coupling follow what tradition

Answer 16

Pascal’s Triangle

Question 17

see powerpoint for

Answer 17

multiplets and pascals triangle table

Question 18

what is a neighbouring molecule classed as

Answer 18

no more than three bonds away

Question 19

n-equivalent neighbouring hydrogens will split a 1H signal into what

Answer 19

(n + 1) Pascal pattern

Question 20

what happens if the neighbouring protons are not equivalent

Answer 20

n must be equivalent neighbouring hydrogens to give rise to a Pascal splitting pattern. If the neighbouring protons are not equivalent, then you will see a complex pattern (known as a complex multiplet).

Question 21

splitting of alcohol hydrogens

Answer 21

alcohol hydrogens (–OH) usually 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 22

what happens when spectra of alcohols are acquired in aqueous solution?

Answer 22

we do not see the –OH function proton signal because of exchange with NMR-inactive deuterium in 10% (v/v) D2O added.

Question 23

n+1 rule

Answer 23

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 24

see powerpoint for

Answer 24

example of n+1 rule

Question 25

give an application of NMR

Answer 25

quantitative drug analysis

Question 26

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 an equation (see powerpoint for equation)

Question 27

see powerpoint for

Answer 27

NMR case study