nmr 4

Question 1

whats the problem with the most abundant carbon - C12

Answer 1

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

which means its invisible in nmr

Question 2

what carbon can we use then

Answer 2

we can use the less abundant carbon - 13C

bc it has an i = 1/2

Question 3

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

Answer 3

its abundance = 1%

Question 4

what is receptivity

Answer 4

sensitivity of the nucleus regardless of its abundance

Question 5

is 13C more or less receptive than 1H

Answer 5

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

less detectedddd
less sensitive than 1H

Question 6

overall sensitivity equation

Answer 6

receptivity x abundance

Question 7

is the precession frequency of 13C less than 1H

Answer 7

yess the precession frequency is lesssss

Question 8

is 13C nmr run alongside 1H nmr

Answer 8

yessss

Question 9

similarites between 13C and 1H nmr

Answer 9

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

Question 10

deuterated solvent examples

Answer 10

C6D6

CDCl3

the C in these solvents is detected

Question 11

does 13C couple to deuterium

Answer 11

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

Question 12

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

Answer 12

deutero solvent and not the protio solvent

Question 13

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

Answer 13

triplet at 77ppm.

Question 14

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

Answer 14

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

Question 15

diff between 13C and 1H nmr spectra

Answer 15

13C has a larger range of chemical shift values

between 0 - 220 ppm

can be outside of this range tho.

Question 16

what reference / standard is used for 13C

Answer 16

tetramethylsilane

TMS

a chem shift of 0

Question 17

baseline of a spectra that is less sensitive

Answer 17

fuzzy,, kinda zigzag like

Question 18

what do we expect 13C to be coupled to generally

Answer 18

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

Question 19

what spin system is C in a methyl group + explain

Answer 19

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

Question 20

what do we affect when we want to decouple smt

Answer 20

either change the population difference

make i less than 0 or make it 0 .

Question 21

what coupling do we want to decouple

Answer 21

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

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

Question 22

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

Answer 22

H,, we must use its resonance frequency.

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

Question 23

what is proton decoupled spectra // nmr

Answer 23

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

this removes X-H peaks

also called X[1H] nmrrrr

Question 24

typical chem shift range 0-50

Answer 24

aliphatic

C-C=O
C-N
C- halogen

still heavily shielded

Question 25

typical chem shift 5-100

Answer 25

CtripleC C-O

Question 26

typical chem shift 100-150

Answer 26

benzene C=C

Question 27

typical chem shift 150-200

Answer 27

C=O little shielding

Question 28

shielding is independent offfff

Answer 28

independent of the nucleusss the nucleus doesnt affect the shielding the e- affect the shielding

Question 29

proton nmr shifts are XXXX than 13C chem shifta

Answer 29

they have a lower ppm range

Question 30

what can symmetry do the the total amount of signals seen

Answer 30

symmetry can reduce the amount of signals seen in a spectraaaa

Question 31

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

Answer 31

making multiple identical measurements and averaging the signal scanning it a bunch of times

Question 32

for a less sensitive nuc we needddd

Answer 32

we need more scans for a less sensitive nuc

Question 33

how many scans does 1H need

Answer 33

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

Question 34

how many scans des 13C need

Answer 34

13C is a less sensitive nuclei it needs more scans 256-2000 scans

Question 35

between each scan what must we wait for

Answer 35

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

Question 36

what nuc relaxes faster

Answer 36

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

Question 37

does 13C nuc relax fast

Answer 37

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.

Question 38

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

Answer 38

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.

Question 39

the more protons a Carbon is bonded to...

Answer 39

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

Question 40

signal intensity trend

Answer 40

C< CH

Question 41

larger integration means

Answer 41

a taller peak // a stronger signal

Question 42

what does DEPT stand for

Answer 42

distortionless enhancement by polarisation transfer

Question 43

what should we think of when we think of DEPT

Answer 43

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

Question 44

which peaks in DEPT point down

Answer 44

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

Question 45

which peaks in DEPT point up

Answer 45

those with a odd number of protons odd 1H's.

Question 46

whats a quaternary carbon

Answer 46

one that isnt bonded to any H's

Question 47

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

Answer 47

they arent included in ittt theyre peaks are not visible.

Question 48

describe cool things about 19F

Answer 48

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

Question 49

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

Answer 49

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

Question 50

what does 19F normally couple to

Answer 50

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

Question 51

nmr range of 19F

Answer 51

+ 500 ---> -500ppm

Question 52

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

Answer 52

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

Question 53

29Si spin and abundance percentage

Answer 53

i = 1/2 abundance = 4.7%

Question 54

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

Answer 54

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

Question 55

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

Answer 55

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)

Question 56

if in 1H theres smt very abundant and smt not abundant but the not abundant is nmr active what will be seen

Answer 56

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)

Question 57

in the 1H and 29Si situation of silane,, what peaks will be seen 29Si : abundance = 4.7% and 93.3%

Answer 57

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.

Question 58

the satellites have what intensity

Answer 58

4.7% divided by 2 2.35 intensity for each satellite.

Question 59

describe the peaks seen in the Silane 1H nmr

Answer 59

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

Question 60

satellite coupling constants,, how do we do them

Answer 60

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

Question 61

what does superimposed mean

Answer 61

that the satellites are laid on top of the singlet peakkk the singlet peak is in the middle of the singletttt.