08/29/24 NMR

Question 1

what are the 4 pieces of info on an NMR spectra

Answer 1

chemical shift, number of signals, multiplicity, area

Question 2

explain the process of NMR and how we get a signal from it

Answer 2

1. input energy (called Absorption) through the form of radio waves
   this causes the spin state to flip from the alpha spin state to the (higher energy) beta spin state
2. remove the energy
   - this causes relaxation, and the proton will realign itself with the magnetic field, causing an emission of radio waves that matches the amount put in

Question 3

when the applied external magnetic field is zero, what is the energy difference between the alpha and beta spin states

Answer 3

it is zero, there is no difference

Question 4

how much energy is emitted when we remove energy from the NMR

Answer 4

it emits the same amount of radio waves that we put in through relaxation

Question 5

what type of isotope gives an NMR?

Answer 5

atoms with an odd number of neutrons due to their ability to have 2 spin states

atoms with even numbers of neutrons do not have 2 spin states and are IR inactive

Question 6

how does magnet strength matter in NMR?

Answer 6

larger magnets can provide a larger and better spectra (better detection limits)

Question 7

how many signals does a NMR see for protons in the same chemical environment?

Answer 7

it considers it as 1 type of proton so there is only one signal

Question 8

what affects the chemical shift?

Answer 8

if a proton is shielded, it has a lower experienced magnetic field so it has a lower chemical shift (upfield)

if a proton is deshielded, it has a higher experienced magnetic field so it has a higher chemical shift (downfield)

Question 9

what is shielding and deshielding

Answer 9

in the presence of an electronegative atom or an electron withdrawing group, the proton is deshielded as that group pulls electron density away from it

this means it experiences a higher magnetic field and has a larger ∆E so it has a higher chemical shift

Question 10

what is the n+1 rule

Answer 10

it predicts how many peaks/spiltting will occur per signal

n is the number of vicinal hydrogens

Question 11

what is a vicinal hydrogen

Answer 11

it is a hydrogen that is 3 bonds away

Question 12

what is a geminal hydrogen

Answer 12

it is a hydrogen that is 2 bonds away

Question 13

what is tetramethylsilane (TMS)

Answer 13

it is the calibration standard for NMR @ 0.0 ppm

Question 14

what is a homotopic hydrogen

Answer 14

hydrogens that are chemically equivalent will give rise to the same NMR signal, they are said to be chemically shift equivalent

Question 15

what is the substitution rule

Answer 15

it was when you replace the suspected protons with an atom that is not on the molecule and compare the resulting compounds, if they are the same, the protons are equivalent, if they are different, the protons are not chemically equivalent and will give rise to different signals

Question 16

if the NMR picks up multiple protons as the same type, is there spilting?

Answer 16

no, there is only splitting from protons that are not chemically equivalent

Question 17

what are heterotopic hydrogens

Answer 17

they are hydrogens that are not in the same chemical environment