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year 2 spec > nmr 4 > Flashcards

nmr 4 Flashcards

1
Q

whats the problem with the most abundant carbon - C12

A

such high abundance but it has a spin quantum umber of i = 0

which means its invisible in nmr

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2
Q

what carbon can we use then

A

we can use the less abundant carbon - 13C

bc it has an i = 1/2

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3
Q

whats the problem with the 13C carbon that is nmr active bc it has i=1/2

A

its abundance = 1%

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4
Q

what is receptivity

A

sensitivity of the nucleus regardless of its abundance

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5
Q

is 13C more or less receptive than 1H

A

receptivity of 13C is much less than that of 1H

less detectedddd
less sensitive than 1H

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6
Q

overall sensitivity equation

A

receptivity x abundance

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7
Q

is the precession frequency of 13C less than 1H

A

yess the precession frequency is lesssss

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8
Q

is 13C nmr run alongside 1H nmr

A

yessss

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9
Q

similarites between 13C and 1H nmr

A

the same solvents are used
deuterated solvents are used (ones with Deuterium instead of H - although impurities are still seen).

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10
Q

deuterated solvent examples

A

C6D6

CDCl3

the C in these solvents is detected

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11
Q

does 13C couple to deuterium

A

yesss
they couple and coupling pattern can be seen using 2ni+1

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12
Q

if most of the deuterated solvent is deuterium,, the 13C signal will be in the

A

deutero solvent and not the protio solvent

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13
Q

when 13C couples to deuterium in CDCl3 where will the coupling signal be and what will it be

A

triplet at 77ppm.

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14
Q

when 13C couples to deuterium in CD2Cl2,, what type of peak will it be

A

13C couples to the 2 D

so we use 2 for n

so 2 x 2 x 1 + 1
which gives 5,, so theres a quintet

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15
Q

diff between 13C and 1H nmr spectra

A

13C has a larger range of chemical shift values

between 0 - 220 ppm

can be outside of this range tho.

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16
Q

what reference / standard is used for 13C

A

tetramethylsilane

TMS

a chem shift of 0

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17
Q

baseline of a spectra that is less sensitive

A

fuzzy,, kinda zigzag like

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18
Q

what do we expect 13C to be coupled to generally

A

1H
theyre normally bonded together in molecules
1H is very abundant
1H = i = 1/2

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19
Q

what spin system is C in a methyl group + explain

A

AX3

a = C
bonded to 3 H’s

gives a quartet (2ni+1)

this range is rlly big so we want to supress coupling for it to look nicer

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20
Q

what do we affect when we want to decouple smt

A

either change the population difference

make i less than 0 or make it 0 .

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21
Q

what coupling do we want to decouple

A

the non 13C nucleus by saturating it with ITS resonance frequency

normally saturating the 1H nucleus to supress 13C-1H coupling

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22
Q

if we want to saturate H,, what frequency must be supress it using

A

H,, we must use its resonance frequency.

if the nuclei is in an excited state,, it cannot couple to other things. this prevents coupling

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23
Q

what is proton decoupled spectra // nmr

A

where we irradiate the 1H to prevent stuff from coupling to it.

this removes X-H peaks

also called X[1H] nmrrrr

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24
Q

typical chem shift range 0-50

A

aliphatic

C-C=O
C-N
C- halogen

still heavily shielded

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25
Q

typical chem shift 5-100

A

CtripleC
C-O

26
Q

typical chem shift 100-150

A

benzene
C=C

27
Q

typical chem shift 150-200

A

C=O

little shielding

28
Q

shielding is independent offfff

A

independent of the nucleusss

the nucleus doesnt affect the shielding

the e- affect the shielding

29
Q

proton nmr shifts are XXXX than 13C chem shifta

A

they have a lower ppm range

30
Q

what can symmetry do the the total amount of signals seen

A

symmetry can reduce the amount of signals seen in a spectraaaa

31
Q

a spectrum can have low signal / noise after collection of data aka irridation has occurred,, how can we improve the spectrum quality

A

making multiple identical measurements and averaging the signal

scanning it a bunch of times

32
Q

for a less sensitive nuc we needddd

A

we need more scans for a less sensitive nuc

33
Q

how many scans does 1H need

A

8-16 scans to make an average and to improve the quality of the nmr spec

34
Q

how many scans des 13C need

A

13C is a less sensitive nuclei
it needs more scans

256-2000 scans

35
Q

between each scan what must we wait for

A

we must wait for the nuc to relax and for net magnetisation to be aligned with the z axis once again

36
Q

what nuc relaxes faster

A

the 1H nuc relaxes faster,, allowing less time between scans and therefor a faster time to get a an imoproved spectra

37
Q

does 13C nuc relax fast

A

nopeee
it relaxes slower meaning we need to do more scans bc its less sensitive and wait longer between the scan in order to give a strong signal in the spectra when it gets scanned.

38
Q

how can we make a nucleus relax faster + increase signal strength

A

irridate // saturate the nuclei around the wanted nuclei

aka if we want to increase relaxation speed for C,, we would saturate the Hs its bonded to. - to decouple it from them

the more H’s bonded to the C, the larger the integration,, higher the peak.

39
Q

the more protons a Carbon is bonded to…

A

the more protons
= faster relaxation
= stronger signals
= larger intergration

40
Q

signal intensity trend

A

C< CH<CH2<CH3

41
Q

larger integration means

A

a taller peak // a stronger signal

42
Q

what does DEPT stand for

A

distortionless enhancement by polarisation transfer

43
Q

what should we think of when we think of DEPT

A

Cash is low = 13C
low = some peaks point down

44
Q

which peaks in DEPT point down

A

the ones with an even amount of protons.
even 1Hs.

45
Q

which peaks in DEPT point up

A

those with a odd number of protons

odd 1H’s.

46
Q

whats a quaternary carbon

A

one that isnt bonded to any H’s

47
Q

what happens to the quaternary C’s in the 13C nmr DEPT experiment

A

they arent included in ittt

theyre peaks are not visible.

48
Q

describe cool things about 19F

A

100% abundance
I = 1/2
its also very sensitive : less scans needed for a large intergration signal.

49
Q

does 19F behave similar to 1H in a coupling perspective

A

yes bc they both have i 1/2 and 100% abundance.

50
Q

what does 19F normally couple to

A

normally couples to 1H bc compounds with F normally contain H

51
Q

nmr range of 19F

A

+ 500 —> -500ppm

52
Q

can we use the 2ni + 1 rule for F coupling

A

yesss bc its 100% abundant and has an i=1/2 which is similar to 1H!!!!!

53
Q

29Si spin and abundance percentage

A

i = 1/2
abundance = 4.7%

54
Q

in a 1H nmr spectrum of a silane what bonds would be seen

A

Si-H
bonds

they form satellites bc the abundance is very low and the rest of Si is inactive due to being i=0.

so nmr would detect all nucs but only some will bc nmr active ::: if the nucs that are coupled to 1H are of low abundance,, satellites will be seen

55
Q

if Si is 4.7% abundant,, what % of the molecules will contain a 29Si nucleus

A

4.7% of the molecules will contain a 29Si nuccccc.

this will show a doublet bc
i =1/2
and its coupling to Si
2ni+1
2 x 1 x 1/2 + 1 = 2 (doublet peak)

56
Q

if in 1H theres smt very abundant and smt not abundant

but the not abundant is nmr active

what will be seen

A

satellites for the low abundant isotope thats nmr active

also a singlet for the 1H due to the high abundance isotope being nmr inactive.

2 x 1 x 0 + 1 = 1 (singlet)

57
Q

in the 1H and 29Si situation of silane,, what peaks will be seen

29Si : abundance = 4.7% and 93.3%

A

singlet for the 1H thats bonded to the 29Si thats inactive!!! 2 x 1 x 0 + 1 = 1
high intensity due to abundance of the inactive isotope!!

satellites with low intensity due to the nmr active low abundance isotope.

these peaks are superimposed

intensity corresponds to abundance of each isotope.

58
Q

the satellites have what intensity

A

4.7%

divided by 2

2.35 intensity for each satellite.

59
Q

describe the peaks seen in the Silane 1H nmr

A

satellites: 1 on the left of the singlet, 1 on the right of the singlet

60
Q

satellite coupling constants,, how do we do them

A

cc will be from one satellite to the other as the 3 peaks are not a triplet but a singlet and satellite.

61
Q

what does superimposed mean

A

that the satellites are laid on top of the singlet peakkk

the singlet peak is in the middle of the singletttt.

year 2 spec (8 decks)

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