15E.5

Question 1

what is nuclear magnetic resonance spectroscopy

Answer 1

it is a technique used to find the structures of organic compounds. it depends on the ability of a nuclei to resonate in magnetic field

Question 2

nucleons have

Answer 2

opposite spin

Question 3

the nucleons with an even number there spin will

Answer 3

cancel out

Question 4

the nucleons with an odd number there spin will

Answer 4

most of the spin will cancel out other than the odd ones, causing the nucleus to have a residual spin

Question 5

residual spins cause

Answer 5

a magnetic field, in which the nuclei can be imagined as a tiny magnet

Question 6

when magnetic nuclei is affected by external magnetic field what will happen

Answer 6

the nuclei will either line up, or oppose the magnetic field

Question 7

as the spins of the nuclei ‘flip’ between lining up and opposing the external magnetic field

Answer 7

they (the nuclei) can absorb electromagnetic radiation as there is an energy difference between the two states (opposing and lining)

Question 8

the nuclei of the two adjacent atoms in a molecule can

Answer 8

influence each other as well as there electron can have an effect, this means that the electromagnetic radiation is absorbed differently by different atoms in a molecule, and these are detected as signals at different frequencies

Question 9

for an NMR graph the vertical axis is labelled as

Answer 9

absorption (the absorption of radio frequency energy) (no units)

Question 10

for an NMR graph the horizontal axis is labelled as

Answer 10

chemical shift ( sometimes delta ) and measured in ppm (parts per million

Question 11

for a atom to show up in the NMR it must have a

Answer 11

odd number of nucleons so a residual spin

Question 12

what is chemical shift (of a proton or a group of protons)

Answer 12

it is a number (measured in ppm) that indicates the behavior of the proton in a magnetic field compared with tetramethylsilane. it can be used to identify the chemical environment of the carbon atoms or of the hydrogen atoms (protons) attached to it

Question 13

what is a 13C NMR

Answer 13

the use of NMR to detect 13C nuclei within the molecules of a substance, in order to determine the structure

Question 14

what is a proton NMR

Answer 14

the use of NMR to detect 1H nuclei within the molecules of a substance, in order to determine the structure

Question 15

the organic compounds that are being analyzed by NMR need to be

Answer 15

dissolved in solvent

Question 16

why cant we use all solvents for NMR

Answer 16

because most solvents contain carbon and hydrogen atoms that will produce a signal in the NMR graph interfering with the signals being produced by the compound being analyzed

Question 17

what solvent do we use in NMR

Answer 17

CDCl3 and D stands for deuterium

Question 18

why do we use CDCl3 as a solvent in NMR

Answer 18

because Deuterium is made from 2H isotope of hydrogen so it has one proton and one neutron so there spin cancels out meaning they wont have a produce a signal that would interfere with the compounds signal, and the carbon only produces one signal that can be easily removed from the spectrum

Question 19

what is tetramethylsilane TMS used for

Answer 19

to be a reference standard that give a prominent signal since carbon and hydrogen atoms in different chemicals environments will give different signals

Question 20

how does TMS look like

Answer 20

page 143

Question 21

what is silane

Answer 21

SiH4

Question 22

why is TMS suitable

Answer 22

1. it has 12 hydrogen atoms, all joined in the same way in a symmetrical argument , so it produces a single strong signal
2. its chemically unreactive, so it wont react with most organic compounds

Question 23

each 13C and 1H atom in a compound form

Answer 23

a signal at a characteristic chemical shift, depending on the other atoms attached to it

Question 24

What is TMS chemical shift

Answer 24

0 and all other atoms have values related to this

Question 25

chemical shift can be thought of as

Answer 25

resonant frequency of an atom