A1 NMR Spectroscopy

Question 1

what does chemical shift tell you

Answer 1

gives information on the nature of the chemical environment of protons and the number of distinct environments

Question 2

what does integral tell you

Answer 2

information on how many protons are in each environment

Question 3

what does coupling patterns tell you

Answer 3

information as to which protons are in each environment

Question 4

what does coupling constant tell you

Answer 4

the molecular shape

Question 5

explain the theory behind NMR Spec

Answer 5

a hydrogen nucleus has an unpaired nuclear spin therefor has a charge and behaves like a small spinning magnet. In nmr we introduce an external magnetic field and if the spin is aligned with the magnetic field (spin up) it is lower energy than if it is opposed (spin down) and this can be observed by watching the absorption or emission of radiation. The energy gap between the 2 states gets bigger as the applied field gets bigger.

Question 6

what should spin/quantum number be for NMR

Answer 6

> 0

Question 7

is it NMR active if there are an even number of protons and neutrons

Answer 7

no as I=0

Question 8

is NMR active if there is an unpaired proton or an unpaired neutron

Answer 8

yes as I=0.5

Question 9

is 1^H NMR active

Answer 9

yes as 1-1=0

Question 10

is 16^O NMR active

Answer 10

no as 16-8=8

Question 11

what is a larmor frequency

Answer 11

the energy gap between nuclear spin states which correlates to a radio frequency

Question 12

what makes an isotope good for NMR

Answer 12

spin, abundance and sensitivity

Question 13

what is the usual 𝐼 for an isotope used in NMR

Answer 13

𝐼 = 1/2

Question 14

where on the electromagnetic scale do NMR frequencies lie

Answer 14

radio waves

Question 15

what type of NMR can be used on larger molecules such as proteins

Answer 15

2D NMR

Question 16

why are multiple signals seen in NMR

Answer 16

because protons experience slightly different magnetic fields depending on their chemical environment so each proton comes to resonance at slightly different frequencies

Question 17

why do protons in different positions experience different chemical environments

Answer 17

because they are surrounded by electron density which will create their own small magnetic field which opposes the applied magnetic field

Question 18

what is the actual magnetic field of a molecule in NMR made up of

Answer 18

applied magnetic field - shielding from surrounding electrons

Question 19

what happens to the frequency detected if electron density is removed from a molecule

Answer 19

removing electron density removes shielding so the overall field felt by the nucleus is stronger and higher energy radiation is required to get the nucleus to come to resonance so the frequency increases.

Question 20

what unit in ppm (chemical shift)

Answer 20

it is a relative scale of normalised values based on the standard tetramethylsilane (TMS/Me4Si)

Question 21

would more electronegative atoms in a molecule have a higher or lower frequency

Answer 21

elements that are more electronegative than carbon attract more than their share of the electrons in the bond therefore the carbon they are attached to is slightly deshielded and comes to resonance at a higher frequency

Question 22

what is the order of the chemical shift of CH, CH2, CH3 and why

Answer 22

CH>CH2>CH3 because carbon is more electronegative than hydrogen so replacing a hydrogen for a carbon causes a small deshielding

Question 23

why do pi bonds have higher frequency than sigma bonds

Answer 23

because the electrons create their own magnetic field and as they are held less tightly this magnetic field is larger and as the pi bond passes the proton it adds to the applied field so they experience a greater magnetic field and come to resonance at a higher chemical shift

Question 24

do aldehydes have a high or low chemical shift and why

Answer 24

high because the electronegative deshielding adds to the anisotropic deshielding

Question 25

what chemical shift do alkanes have

Answer 25

1-1.5ppm

Question 26

what chemical shift do amines have

Answer 26

2-3ppm

Question 27

what chemical shift do alcohols have

Answer 27

3.2-4ppm

Question 28

what chemical shift do esters have

Answer 28

4-5ppm

Question 29

what chemical shift do chloroalkanes have

Answer 29

3-4ppm

Question 30

what chemical shift do alkenes have

Answer 30

5-6ppm

Question 31

what chemical shift do arenes have

Answer 31

7-8ppm

Question 32

what chemical shift do aldehydes have

Answer 32

9-10ppm

Question 33

what chemical shift do allylics have

Answer 33

2-2.5ppm

Question 34

what chemical shift do benzylics have

Answer 34

2.5-3ppm

Question 35

what chemical shift do alpha-carbonyl have

Answer 35

2-3ppm

Question 36

what is the D2O shake and why is it done

Answer 36

it is done to remove OH and NH group signals from the spectrum as they vary hugely on H-bonding so can be hard to assign. therefore you add a drop of deuterium to the sample and the more acidic protons will exchange with deuterium and since deuterium has very different larmor frequency the signals wont be observed

Question 37

what does ppm stand for

Answer 37

parts per million

Question 38

what does the area under an NMR signal relate to

Answer 38

the number of protons in that environment

Question 39

how are the effects of the proton nuclei's magnetic field transmitted

Answer 39

through sigma bonds

Question 40

how many bonds away does coupling usually occcur

Answer 40

no more than 3 bonds

Question 41

how are couplets formed

Answer 41

when Ha spins against Hb but with the applied field

Question 42

do identical sets of protons couple to each other

Answer 42

no only if they are in different chemical environments

Question 43

what is pascals triangle

Answer 43

shows the relative intensities of the component parts of the mutiplet

Question 44

how are coupling constants measured and what are they measured in

Answer 44

distance between adjacent lines (measured in hertz)

Question 45

how do you convert ppm to Hz

Answer 45

multiply the operating frequency of the instrument in MHz

Question 46

why is Hz used instead of ppm when measuring coupling constants

Answer 46

because the coupling constant is an absolute no matter what the field strength of the machine therefor if ppm was used you would get different values on different machines

Question 47

what are coupling constants determined by

Answer 47

dihedral angle between C-H bonds and electronegativity of substituents

Question 48

what is the coupling constant for cis protons

Answer 48

10-12Hz

Question 49

what is the coupling constant for trans protons

Answer 49

14-18Hz

Question 50

what is the coupling constant for geminal protons

Answer 50

0-2Hz however their couplings are very small and tend to 0 so may not be observed

Question 51

what are vinyl compounds

Answer 51

compounds where some protons have near neighbours that are not equivalent and they have non-equivalent protons on the same carbon (all doublets)

Question 52

which isotopes of which elements are most useful for studying protein structure by NMR spectroscopy

Answer 52

1H, 13C, 15N

Question 53

explain why chloromethane has a higher chemical shift than tert-butyl chloride

Answer 53

the electronegative chlorine inductively reduces electron density around neighboring nuclei, it deshields them. This effect reduces with the distance so the proton 2 bonds away from the Cl will experience significantly more deshielding than those 3 bonds away in tert-butylchloride