Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

year 2 spec > nmr 2 > Flashcards

nmr 2 Flashcards

1
Q

what is resonance in nmr (short version)

A

when a spin flip occurs

from low energy and aligned with Bo (+1/2)

to high energy and opposing Bo (-1/2)

radiation is absorbed when this occurs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what does resonance depend on

A

the type of nuclei being studied
the strength of the applied magnetic field Bo.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

why does resonance depend on Bo

A

larger the Bo = the larger the energy gap between the high and low energy spin states

meaning more energy is needed to be absorbed for resonance to occur.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

are nuclei isolated in a vacuum

A

nope,, theyre surrounded by electrons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what do the electrons do to the nucleus

A

they shield it from the applied magnetic field Bo,,

reduces the total applied magnetic field that the nucleus experiences.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what magnetic field reaches the nucleus then

A

effective magnetic field reaches the nucleus.

effective magnetic field < applied magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what equation links magnetic field and frequency

A

v = y. Bo /2n

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

if the effective magnetic field is lower than the applied magnetic field,, what about frequency

A

the observed frequency of precession ‘v’,, will be smaller than the theoretical frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

why is effective magnetic field smaller than applied magnetic field.

why is observed frequency of precession//larmor frequency smaller than the theoretical frequency

A

bc electrons are charged and when they spin that create their own magnetic field.

this magnetic field protects the nucleus from the applied magnetic field.

a smaller effective magnetic field means a smaller frequency // speed of procession // larmor frequency.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

electron cloud affects the precession / larmor frequency and theeee

A

the chemical shift

curly partial positive looking thingy <3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

chemical shifts differ between

A

differ between different nuclei

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

chemical shifts are reported relative to aaaa

A

a standard

usually at 0ppm

theyre not acc 0ppm but we just say they are lols.

and other things are compared to the 0ppm compounds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

0ppm is usually on theee … side of the spectrum

A

on the right hand side of the spectrum

ppm increases going to the left

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

the larger the ppm value,,

A

the less shielded the nuclei
the larger chemical shift
more to the left hand side of the spectrum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

the smaller the ppm value

A

the closer to 0,, the rhs it is
the smaller the chemical shift
the more shielded it is.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what affects ppm

A

the e- density.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

larger ppm meansss

A

LHS
larger chemical shift
less shielded,, bc it undergoes resonance at more parts per million

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

a smaller ppm means

A

to the rhs
larger chemical shift
more shielded,, bc it undergoes resonance at less parts per minute

bc less effective Bo reaches the nuc bc of shielding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what else can affect a nucleus’ chemical shift // signal position

A

the spin of neighbouring nuclei!!

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

when 2 neighbouring nuclei have the same magnetism // same alignment what happens

A

local magnetic field is strengthened
faster precession
higher chemical shift

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

when 2 neighbouring nuclei have opposite magnetism // opposite alignment what happens

A

local magnetic field is weakened
slower precession
lower chemical shift

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

1 signal splits into how many peaks

A

2I + 1

I as in 👁️

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

what does each peak represent

A

a unique combo of nuclear spins

24
Q

what is I 👁️

A

spin quantum number of the main character nucleus.

25
Q

AX2,, whats special about the x’s

A

they must be the same isotope,, the same nucleus // equivalent nuclei
must have the same environment

26
Q

when main character nuc is coupled to 2 equivalent nuclei,, how many nucs are there in total

A

3

27
Q

when the main nuc is coupled to 2 equivalent nuclei,, how many diff combos are there + explain the spectrum

A

draw x-A-x
bc its AX2 bc its coupled to 2 equivalent nucs - so theyre both called X.

A must always have an up arrow,, so draw the up arrow for each A.
the X’s can either:
both align (strengthen Bo,, higher shift)
both oppose (weaken Bo, lower shift)
a mix of both : up + down or down + up.

one variation = short
2 variations = tall
one variation = short

so itll be 1:2:1 triplet 💗

28
Q

coupling to 3 equivalent nuclei AX3,, how many diff combos are there

A

jot down what we have
x x x A
A will always align 💗
X’s dont always align
4 together
1 vs 3 up
2 vs 2
3 vs 1 up
4 diff ones,, some have more combos tho
this gives a quartet: 1:3:3:1

29
Q

if all the couples nuclei are aligned in the same way,, will this have a higher or lower chemical shift

A

higher chemical shift as they will strengthen the magnetic field,, giving a faster precession // larmor frequency which will in turn increase ppm (parts per minute)

30
Q

coupled nuclei which have the same net magnetism will whatttt

A

have a larger ppm,,, bc they strengthen eachothers magnetic field,,, they feel a greater effective magnetic field due to the additive nature of their magnetic moments,, they therefore experience a greater magnetic field strength leading to a larger ppm and a larger larmor//precession frequency.

31
Q

multiplicity refers to…..

A

the number of peaks there is in the spectrum

32
Q

how do we find the multiplicity

A

2nI+1
n is the number of X’s
I is the spin of the X’s

AXn

33
Q

how do we figure out the intensity of these peaks

A

we need to use the lovely tree diagram yayaya 🌴

34
Q

5 peaks

A

quintet

35
Q

6 peaks

A

sextet

36
Q

7 peaks

A

septet

37
Q

when can nuclei couple

A

when they are connected via bondsss
dont have to be directly bonded together tho!!

38
Q

what does 3JHH mean

A

3 bond coupling between 2 H nuclei

theres 3 bonds between the 2 H’s.

39
Q

coupling constant issss

A

the gap between peak signals in nmr
measured in hz as this distance differs with magnet strength.

40
Q

the closer the bond coupling (bonds between the nuclei) theee …. the coupling constant

A

further apart the coupling constant

so if the nuclei are close together,, the coupling constant increases in length in hz

41
Q

if the coupling nuclei are further apart,, what happens to the coupling constant

A

the coupling constant gets smaller

42
Q

the value of nJXY gets whatt as the number of n increases

A

as n increases
the nuclei are further apart
meaning the coupling constant,, the nJXY value,, will decrease.

43
Q

when is the coupling constant too small to see

A

when the nuclei are too far apart – 3 to 4 bonds are normally the max <3

44
Q

coupling to non identical nuclei will give

A

different coupling constants (different gaps between the peaks)

45
Q

why is hz used for the coupling constant?

A

bc hz does not change with the strength of the magnetic field.

but ppm will change with the strength of the magnetic field.

coupling constants are constant

46
Q

equation for ppm

A

ppm = hz/Bo

47
Q

when are signals well defined

A

when using a higher Bo
bc of the ppm = hz/Bo equation

theyre more crispy and skinny
on a low Bo theyre more round and not as crispy.

48
Q

when I 👁️ is 1 what can ml be

A

-1, 0, +1

49
Q

how do we draw the stick diagram for I = 1

A

draw one line to start

then do 2I + 1
to give 3,, so u should end with 3 lines
so 3 peaks
each peak corresponds to one of the ml values
one for -1, one for 0 and one for +1.

50
Q

AX and AX2 defines what

A

the spin system
tells us which nuclei and how many are coupling

51
Q

what defines the signals appearance,, aka how the spectrums peaks will look like

A

u need the quantum numbers I and mI to see how many peaks yu have

u also need the spin system

52
Q

what is a spin active nuclei

A

a nuclei where I does not equal 0,, but more than 0

bc 2nI + 1 would give 1 if I =0
and 1 means no coupling
it gives a singlet

only one ml value aswell : 0

53
Q

conditions needed for coupling to occur

A
  • I > or must be 1/2 (2nI+1)
  • nucleus X must have a population difference between Na and Nb.
54
Q

if nuclei can couple to other nuclei if theyre I is 1/2 or more,, what does this mean about the spectrum

A

it means that there can be a bucnh of peaks and it can be overwhelming

we sometimes supress coupling to prevent this.

55
Q

whats it called when u supress coupling from occuring

A

decoupling.

56
Q

how can we decouple something

A

saturating nucleus with radio frequencies of nucleus X (to prevent nucleus x from having a difference in population)

measure nucleus A.

no coupling can occur bc nucleus x doesnt have a popukation difference and this is one of the things needed for coupling to occur.

57
Q

decoupling nucleus equation type thing and its explanation

A

A{X}

A is the observed nucleus
X is the saturated//irradated one that can no longer couple

u can only decouple one nucleus at a time

year 2 spec (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions