Download - Practical Solid-State NMR [More Than One Answer May Apply In

8/14/2019 Practical Solid-State NMR [More Than One Answer May Apply In

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Prac tica l Solid- state NMR

[More than one a nswer may ap ply in eac h ca se]

1. Adamantane is useful as a setup sample for the following situation(s):

A Chec king the shimm ing

B Referencing 13C shifts

C Ad justing the ma gic ang le

D Ca lib rat ion of samp le tem perature

E Sett ing up c ross-polarisa tion a t low sp in rates (say < 6 kHz)

F Sett ing up c ross-polarisa tion a t high sp in rates (say > 12 kHz)

2. The a mo unt o f noise in the spec trum is muc h higher tha n norma l. The signal

leve l is mo re o r less unc hange d , so w ha t c ould the prob lem(s) be?

A Pulse durations a re mis-ca lib rate d

B Amp lifiers ma y not be be ing b lanked during ac quisition

C Amp lifiers ma y be switched off / tripp ed out

D Filte rs may be missing

E Prob e is incorrec tly tuned

F The sam ple ma y have lea ked out

3. The intensity o f the NMR signa l is muc h lower tha n expec ted (while noise leve l is

normal). What could the prob lem(s) be ?

A Pulse durations a re mis-ca lib rate d

B Amp lifiers ma y not be be ing b lanked during ac quisition

C Amp lifiers ma y be switched off / tripp ed out

D Filte rs may be missing

E Prob e is incorrec tly tuned

F The sam ple ma y have lea ked out

4. Close e xamination o f a poo r qua lity spe c trum show s tha t a ll the lines have a

simila r irreg ula r and relatively b roa d linesha pe . What c ould the p rob lem(s) be ?

A The de c oup ling ma y have been mis-set

B The ma gic ang le ma y be mis-ad justed

C The shimm ing ma y be po or

D The samp les ma y be am orpho us

E The samp le ma y be hete rog ene ous

5. Close e xamination o f a poo r qua lity spec trum show s tha t the ind ividua l lines a re

rela tively b roa d , but with a regular linesha pe (~Lorenztian or Ga ussian). What

c ould the p rob lem (s) be ?

A The de c oup ling ma y have been mis-set

B The ma gic ang le ma y be mis-ad justed

C The shimm ing ma y be po orD The samp les ma y be am orpho us

E Th l b h t


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6. The spinning ap pea rs to b e stab le (a s judge d from the sp in speed rea dout), but

the sp inning spee d is we ll short of its ma ximum whe n the ma ximum ga s

pressures for the prob e a re b eing used . What c ould b e the cause(s)?

A The d rive tip/ c ap is wo rn

B There ma y be a ga s lea k

C The spinning mod ule, e.g. the air be a rings, ma y be dama ged

D The sam ple ma y be ba dly pa c ked

E The ma rk could be bad ly drawn / dama ged

7. The spec trom ete r is set up for a single-channel expe rime nt w ith the c orrec t

expe rime nta l pa ram ete rs, but the user has om itted to c hec k the tuning of the

p rob e. What ma y be the result(s)?

A The amp lifiers c ould overloa d or even be da ma ge d

B The probe may hea t up and start to arc

C The am ount of noise in the spec trum ma y inc rea se

D The pea ks in the spe c trum may be broad ened

E The sensitivity ma y be reduc ed

[What wo uld hap pe n in a do uble-resona nce experiment if the1H c hannel was

ba dly tuned?]

8. You e nc ounte r the fo llow ing expe rimenta l issues . Using the sc ale b elow to

indica te how co ncerned you would be ab out eac h situation: Very (stop and fix

the p rob lem imm ed iate ly), Moderately (should resolve the p rob lem b efo re

continuing), Slightly (worth noting in a log-bo ok) or Not at all.

i It is impossible to c a lib ra te a reasonab le 90/ 180 pulse duration

ii The 90 pulse durat ions a re ~20% shorter than p reviously ob served

iii Bursts of no ise a re appea ring in the NMR signa l (FID)

iv The rotor is ma king an odd grind ing noise

v The rotor see ms to be sp inning sta b ly, but the sp in read -out is a ll over the p lac e

vi The qua lity of my spec tra is muc h wo rse than previously ob served

vii The tuning is poo r, but just on my samp le

[There isn t nec essa rily a right answe r in ea c h c ase, but you should b e a b le to

justify the one you give!]

9. 3-metho xybenzoic ac id (C8H8O3) g ives the 13C CPMAS spec trum shown in figure

1, ac quired at 75.4 MHz and a t a sp in-rat e o f 5.5 kHz. Ac c ount fo r the fa c t tha t

the re are tw elve resolvab le signa ls.

A The spec trum c onta ins sp inning sideb ands

B The sample is a mixture of two polymo rphs

C There are two m olec ules in the c rysta llog raphic a symm etric unit

D The spec trum is exhibiting c oup ling to H

E The sample is imp ure


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10. In questions 10 - 12 a 13C CPMAS spec trum from the same sam p le tha t gave

figure 1 is show n. In ea c h c ase the re is som ething w rong with the experime nt.

What is wrong w ith the experime nt that p rod uced figure 2?

A The rec yc le delay is too short

B There is a sp ike (one e rroneo us point) in the FID

C The ac quisition time is too short

D The FID ha s a la rge DC offset

E The receiver ga in is too high

F The first point of the FID is d isto rted

G The spec tral wid th is too large

11. What is wrong w ith the experime nt that p rod uced figure 3?


B There is a sp ike (one e rroneo us point) in the FIDC The ac quisition time is too short





12. What is wrong w ith the experime nt that p rod uced figure 4?


B There is a sp ike (one e rroneo us point) in the FID

C The ac quisition time is too short





13. Figure 5 sho ws a 119Sn spec trum. There is a single tin spec ies in the samp le.

Whic h line is the c entreband ?

A The central line (with eq ua l numb ers of sideb ands on either side)

B The most inte nse line

C The line a t the centre-of-gravity of the pa ttern

D The line nea rest 0 ppm

E Ca nnot be certa in from this spec trum


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14. You ha ve a mixture of m aterials (X and Y). They e ac h c onta in a single

phosphorus environme nt, g ive simila r linew idths, ha ve simila r c ross-po larisa tion

be hav iour and there is mo re o f X than Y. You know tha t for X T1(H) = 2 s and

T1(P) = 50 s and for Y T1(H) = 1.5 s and T1(P) = 20 s. Whic h rec yc le should you

c hoo se to g et the best signa l-to-noise rat io from a 31P CPMAS expe riment?

A 10 s

B 7.5 s

C 250 s

D 100 s

E 2.4 s

15. You no w w ant to rec ord a d irec t-po larisat ion M AS spec trum from the mixture (in

question 14) so tha t you c an integ rate the signal intensities and d irec tly

dete rmine the ratio of X to Y. Whic h rec yc le should you use to rec ord the

spectrum?

A 10 s

B 7.5 s

C 250 s

D 100 s

E 2.4 s

16. You a re c a rrying out a 15N CPMAS experime nt w ith high p ow er 1H dec oup ling

on a rigid p olymer. The d uty cyc le for the expe riment c an b e roughly defined

as (co nta c t + ac qu isition time s)/ (co nta c t + ac quisition time s + rec ycle d elay).

Whic h of t he fo llowing c ond itions a re sa fe to use?

A c onta c t 1 ms, ac quisition 20 ms, rec ycle 1 s [d uty c ycle 0.02]B c onta c t 1 ms, ac quisition 50 ms, rec yc le 0.2 s [d uty c yc le 0.20]

C c onta c t 20 ms, ac quisition 70 ms, rec yc le 0.2 s [d uty cyc le 0.31]

D c onta c t 5 ms, ac quisition 10 ms, rec yc le 0.2 s [d uty c yc le 0.07]

E c onta c t 1 ms, ac quisition 2 s, recyc le 30 s [d uty c yc le 0.06]

17. You wa nt to c arry out a 13C DPMAS expe rime nt on a rubbe ry po lymer. Whic h

of the follow ing c ond itions are sa fe to use?

A ac qu isition 80 ms, rec ycle 0.5 s, full po we r CW dec oup ling [d uty cyc le 0.14]

B ac quisition 200 ms, rec yc le 0.5 s, full pow er CW dec oup ling [d uty c yc le 0.29]C ac quisition 200 ms, rec yc le 0.5 s, 30% CW dec oup ling po we r [ duty c yc le 0.29]

D ac quisition 200 ms, recyc le 0.5 s, full pow er TPPM de c oup ling [d uty cyc le 0.29]

E ac quisition 100 ms, rec yc le 2 s, full pow er TPPM de c oup ling [d uty cyc le 0.05]

18. Figure 6 sho ws a 23Na MAS spec trum. There is one sod ium env ironm ent in the

samp le. What informat ion doe s the spec trum give you?

A The sod ium is expe rienc ing dipo la r c oup ling to ano ther nuc leus

B The sod ium env ironme nt ha s c ub ic symm etry

C The sod ium env ironm ent is asymm etricD The magic-a ng le is mis-set


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19. For the spec trum show n in figure 6 which p osition , marked by the numbers, is

mo st likely to be c losest to the isot rop ic c hemic a l shift?

A 1

B 2

C 3

D 4E 5

20. You ha ve the follow ing p rob es, with the at tributes listed , ava ilab le to you.

Probe Samp le

volume /l

Maximum

sp in-ra te / kHz

Shortest

achievable

90 pulse / s

A 450 7 4

B 160 12 3

C 52 18 2D 20 25 1.7

E 150 sta tic 1.5

Choo se the ap prop riate probe (s) for eac h of these c ases

i You a re investiga ting the effect of hydroge n bond ing on the 15N chem ica l shift in

an unlabe lled organic m ate rial of whic h you have 1 g.

ii You wa nt to rec ord a 13C CPMAS spec trum to investiga te polymorphism in a n

unlab elled organic m ate rial of which you ha ve 50 mg .

iii You are studying a lab elled organic molecule and nee d a 13C spec trum w ith the

ma ximum amo unt of resolution. You ha ve 0.5 g o f sam ple.iv You are studying a mo tional p roc ess in a deute rium labe lled sam p le. You have

0.2 g o f samp le.

v You need a qua ntitative 29Si spec trum from a deriva tised silica te samp le of whic h

you ha ve 0.4 g.

vi You are investiga ting the numb er of tin sites in the c rysta llog raphic asymm et ric

unit of an inorga nic c om pound . Relate d m ateria ls c onta in tin sites tha t have

shielding a nisot rop ies of ~ 1000 ppm. You ha ve 1 g of samp le.

21. Whic h of the follow ing should you not d o in an NMR lab orato ry?

A Use a mo b ile phone

B Ea t or d rink

C Try and rep air an elec tric a l fault in the spec trometer

D Allow anyone unfam ilia r with ma gne ts into the roo m

E Use the CD drive to listen to music with hea dphones


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C/ppm

406080100120140160180

Prac tical Solid-

state NMR

Figure 1

Figure 2


7/13

C/ppm

406080100120140160180

Figure 3

Figure 4


8/13

Sn / ppm

-600-400-2000200400600

Figure 5

Na / ppm

-30-25-20-15-10-50

15

2

4

3

Figure 6


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Practical solid-state NMR (answers)

1. Use of a dam anta ne a s a set-up sam ple: ABE

A Yes. The intrinsic 13C linew idths in adama nta ne a re ve ry narrow (


10/13

to b e fairly ba d b efore it has a no ticea b le effec t on solid-sta te spe c tra). Most other

expe rimenta l problem s (and am orphous sam p les) tend to result in smo oth, if broad ,

lineshapes.

5. Broad but smo oth lineshapes:ABD

A Yes (e.g. p roton transmitte r is significantly off resona nc e).

B Yes. A sca led po wd er pa tte rn w ill be ob served if the a nisotrop y to mis-set ra tio is suffic iently

high, but the mis-set w ould no rma lly have to be very large for this to be ob vious.

C Unlikely (see question 4).

D Yes.

E Unlikely.

6. Spinning spe ed ha s p late au-ed : AB

A Yes.

B Yes (a leak w ill norma lly be ob vious from the noise!)

C Unlikely: da ma ge to the air be a rings (or roto r mo dule) is most likely to ca use ba d or

temperamental spinning behaviour.

D Unlikely: ag ain the roto r will tend to be hard to spin .

E No. A ba d ma rk c ould only lead to an errat ic spin rea d-out

7. Effect of p oo r tuning: AE

A Yes. Bad tuning mea ns that powe r will be reflec ted ba ck from the coil and into the am p lifiers

which ma y overheat (if not protec ted).

B No. Mis-tuning doe s not , nec essarily, a ffec t the RF pow er in the prob e so will not lea d to

increased heating. Never simply turn up t he p ow er leve l to g et the 90 leng th you w ant !

C No. Prob e tuning is unrelated to the amo unt of noise in the spe c trum

D No. Prob e mis-tuning canno t a ffect the appearanceof the spe c trum (in a single-c hanne l

experime nt), only its intensity. If the 1H c hannel wa s mis-tuned in a d oub le resona nc e

experiment then the lines might be broad ened .

E Yes. A ssuming that the tip ang les had been c orrec tly op timised in the pa st, they will no

longe r be co rrec t, and so the am ount of signa l will be reduc ed .

8. How are wo rried a re you?

i Moderately : An imp ossibly long 90 mea ns there is a funda menta l prob lem w hich need s to b eresolve d . It is not, nec essarily, dang erous.

ii Slightly: Drift in the c alib rat ion is not necessarily a p rob lem , but should b e logge d to allow

any long term change s to be spo tted . The previous c alibrat ion ma y have be en po or. If the

90 duration is c lose to the p rob e spec ific at ion, c hec k that the po we rs are not exce ssive.

iii Very : Noise b ursts in the FID are c aused by a rc ing - electric al b rea kdow n in the p rob e. Stop

immed iate ly and try to resolve the prob lem as you risk permane nt da ma ge to the p rob e.

iv Very : A smo othly spinning roto r prod uces a c lea n single-freq uenc y tone (if aud ible). A

grinding no ise ind ica tes tha t the roto r is sc rap ing a ga inst the a ir bea rings. Stop sp inning


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immed iately as this will be d ama ging the spinning module.

v Moderately/ slightly: Loo ks like a p rob lem w ith sp in rea d -out e .g. sp in mark is poo r,

tac hom ete r nee ds ad justme nt. This is not c ritic a l (sp in ra te c an be d ete rmined from sp inning

sidebands), unless a reliab le read out is req uired e.g . for sta b ilised sp in spee d con trol.

vi Moderately : Som eth ing is wrong . Diagnose and fix the p rob lem

vii Not at all: Som e samp les a re lossy e.g . ionic solutions, mea ning tha t the tuning will

nec essarily be p oo r. You ll nee d to sett le for longer 90s etc . for a given RF pow er. Don t be

temp ted to increase the RF am plitude to c omp ensate!

9. More signals than c a rbo ns in the mo lecule: C(B)

A This spec trum was ob ta ined with a sideband suppression expe riment so only low-intensity

residua l sideband s a re v isible, how eve r, the sp in-ra te is suc h tha t the sp inning sideband s a re

in the w rong positions to a c c ount fo r the extra signa ls (a sp in-ra te of 5.5 kHz a t 75.4 MHz is

~73 pp m)

B Possibly, but the po lymorph ra tio would need to be close to 50:50 to give ~1:1 pa irs

C Most likely. A numb er of the lines oc c ur in 1:1 pa irs. This ofte n indica tes tha t there are two

mo lec ules in the asymm et ric unit. Resolving pa irs for a ll the signa ls in a spec trum is relat ively

ra re. It is po ssible to ha ve m ore than two mo lecules in the asymme tric unit, so spec tra c an be

even more c omplica ted.

D No. This must be a 1H de c oup led spe c trum to give such na rrow lines. If it wasn t the lines

would b e severely broad ened through d ipo lar co upling.

E Not in the sense tha t an impurity is a sma ll amo unt of an unrelated sub sta nc e.

10. Trunca tion of the FID: C

C Yes. The w iggles a t the ba se of the peaks a re an indic ation that the signa l has not fully

dec ayed by the end of ac quisition time . Inc rea sing the a cquisition time w ill solve the

prob lem (but b e c areful with de coup ling p ow ers for long a c quisition time s)

G Not direc tly, although, de pe nding on the spe c trome ter software, c hang ing the spe ctral

width ma y auto ma tic ally c hang e the ac quisition time (e.g. halving the spe ctral width wo uld

doub le the ac qu isition time ).

11. Rec eiver overload : E

The irreg ular nature of the baseline is indica tive of a rec eiver overload . Here the re is too

muc h signa l from the sam ple a nd the full intensity of the signal c annot b e d igitally

rep resented. Bad c ases of a rc ing c an result in a spe c trum w ith a similar ap pea ranc e. Dips in

the b aseline a t the base o f a pea k in a spe c trum c onta in a sma ller number of lines a re a lso

symp tom atic o f a rec eiver ove rload . To solve, red uc e the rec eiver ga in or ad d a nattenua tor to the signa l path b efore the preamp . or take some sam ple out.

12. Sp ike in the FID: B

B Yes. O ne errone ous da ta point p rod uc es a single freq uenc y osc illation in the baseline. The

nea rer this is to the sta rt of the FID, the slower the osc illat ion.

D This would p rod uc e a cent ra l ( transmitte r ) sp ike in the spec trum.


12/13

F This p rod uc es a vertica l offset in the spec trum.

13. Determination of c entreb and : CE

A Not necessarily

B Som et imes but not a lwa ys

C Yes, the ce ntreb and is at the cent re-of-grav ity of the pa ttern. But can you tell which line it isfrom this spec trum? See E.

D Not nec essarily

E Yes. Ide ally you need a sec ond sp in-rate to be certain. The c entreba nd is a t the c entre-of-

g ravity of the pa tte rn. The sum o f all n I whe re n is the sideb and numb er (going outw ards

from 0 the c ent reb and ) and I its intensity shou ld be equa l for sidebands on e ither side o f the

c entreba nd. The c entreba nd is at 100 pp m in this spe c trum.

14. Choice o f rec ycle: E

A rec yc le of 1.2 times T1 will g ive the best S/ N in any given time. In a CP experime nt the

rec ycle is dete rmined by T1(H). As X is the dom inant c om pone nt 2.4 s is the best rec yc le to

use.

15. Choice o f rec ycle: C

If you wa nt a d irec t-polarisation expe riment to be q uantitative (without having to c a lc ulat e

c om pe nsat ion fa c tors for differing T1s) you need a rec ycle o f 5 time s the long est T1(P) (in this

c ase) in the sample. Here it is 250 s.

16. Duty cycle: AD

For safe ty, the duty c ycle should no t exce ed 0.2. Duty cyc le is a measure o f the rf on-time .Too high a nd yo u risk da ma ging the prob e (and hea ting the samp le). So A a nd D a re safe , B

is bo rde rline a nd C is not a cc ep table. For B and C , increasing the recycle w ill make the

experime nt sa fe. For E the d uty cyc le is low, bu t 2 s of a c quisition time with full pow er

dec oup ling is dangerous. You w ill ra rely (if eve r) need suc h a long t ime fo r a rigid solid, so

either red uce the a cquisition time or, if you rea lly do ne ed it, red uce the d ec oup ling p ow er.

17. Duty cycle: ACE

Aga in the dut y cycle should no t exceed 0.2, so A and E are sa fe. For B and D, the ac quisition

c onditions are not a c c ep tab le, but for C the d ec oup ling p owe r is turned do wn so these

c ond itions would be a c c ep tab le. Note that it doe s not matter that one d ec oup ling sche me

is TPPM and the other is CW the pow er used is the same . There might b e a c ase fo r usingTPPM if you ha ve turned d ow n the p ow er as it ma y be more e ffic ient tha n CW dec oup ling.

18. Quadrupolar bandshape: E

A No. This ba ndshape is c harac teristic of a sec ond order quad rupo lar interac tion.

B No. Suc h an env ironm ent would result in a na rrow , symm etric line.

C No. This wo uld give a d ifferent ba ndshap e.


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D Not nec essarily.

E Yes. This quadrup ola r band shape is indica tive of an axia lly symm etric environme nt. Fitting

the b and shape gives a qua d rupolar co upling c onstant of 1.09 MHz and an a symme try of 0.0.

19. Isotrop ic shift of a qua drupo lar ba ndshape : A

For a q uad rupo lar band shap e the isotrop ic chem ica l shift is not ne c essarily at a pe akma ximum or, indeed, within the b and shape . For this type of b andshap e it is roug hly at

position 1. You need to simulate the spec trum t o d ete rmine the isotrop ic shift a c curate ly.

Doing t ha t g ives a shift of -8.1 ppm in this case.

20. Choice of probe

i. A. Nitrog en-15 has a low na tura l ab unda nce (0.37%) so yo u need as much samp le in thespec trome ter as possible. Samp le volume is the key a ttribute he re, all other fac to rs a re

sec ond a ry. You ha ve eno ugh sam ple to fill the rotor for prob e A.

ii. C. Per mg of samp le, a p rob e w ith a sma ll rotor is mo re sensitive tha n a larger one. In thiscase, probe C will give a be tter result than using p rob e A or B even if you red uce this

samp le volume or use a n inert filler wh ich is best avoided anywa y. The sma ller probe w ill

have o ther adva ntage s highe r spin-rate and highe r dec oup ling field w hic h may

com pe nsate to som e extent for the lac k of sam ple.

iii. D. For ma ximum resolution you w ant the highest p ossib le de c oup ling field so the p rob e t hatcan give you the shortest 90 pulse is the o ne o f choice he re. High spee d sp inning might

also he lp. The lac k of sam ple vo lume will be o ffset b y the fac t that the sam p le is labe lled .

Sma ll volume p rob es tend to b e b ette r shimmed than large volume o nes as we ll.

iv. E. The sta tic 2H ba ndshape is useful for stud ying m otion .v. A. Spin-ra te a nd rf field a re no t key fea tures for 29Si experime nts. As a q uantitat ive

spec trum (direc t-polarisat ion) is req uired a long rec ycle ma y be need ed and as 29Si has a

low natural ab unda nc e (4.70%) we wa nt as muc h sam p le as possible in the spe c trom ete r.

vi. C. Spin-ra te is the issue here. The p rob ab le high shielding anisot rop y me ans the signa l willbe sp lit into a large manifold of sp inning sidebands. Sp inning fa st will red uc e the num ber of

side ba nds and should m ake the signa l more d etec tab le. The a bund anc e of 119Sn is 8.58%

so fa st spinning has to b e o ffset a ga inst the red uc tion in sam ple vo lume. Prob e C wo uld b e

a g ood c hoice , espe c ia lly at highe r ma gne tic fields. Prob e B might suffic e b ut if the lines

are b roa d 12 kHz might no t be fa st enough t o p rop erly sep arate the sideb and s. The low

sam ple volume for probe D might be a problem.

21. Things not t o d o in a lab .: ABCDE

A Best avoid ed (espec ially if it b rea ks any loc a l p rohib itions).

B Eat ing or d rinking shou ld never be done in any lab oratory.

C Elec tric al wo rk should only be ca rried out be qualified people. There are lethal voltag es

inside a spe c trom eter.

D Any visitor (wha tever the purpo se of the ir visit) should be mad e aware of the hazards -

pa rticularly of the ma gne tic field - before they are allow ed in.

E This is ofte n banned in dep a rtme nta l sa fety po lic ies. It is useful to be ab le to hea r a rotor

sp inning - in c ase it sud denly c rashes - and for the fire a larm of c ourse!

Download - Practical Solid-State NMR [More Than One Answer May Apply In

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