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Year 3 - Chemistry > Option: MolecSpec > Flashcards

Option: MolecSpec Flashcards

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1
Q

What are the two forms of shielding?

A

Diamagnetic - opposes applied mag field
Paramagnetic - augments applied mag field

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2
Q

What is the dependence of σp?

A

σp α (-1/ΔE)(1/r3)

depends on effects of mag field mixing excited elec states into gs wavefn

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3
Q

What effect does isotope change NMR signal?

A

Shifts the signal most when light elements changed

As alters vib freq, elec population, etc.

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4
Q

What is a quadrupolar nuclei?

A

Nucleus with spin I > 1/2 posses an elec quadrupole moment
This interacts with local elec field gradients, which gives fast relaxation and v broad signals

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5
Q

What occurs to quadrupolar nuclei in high symm?

A

Sharper resonances than expected
This is because the elec field gradient has cancelled out

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6
Q

What is the coupling between quadrupolar and non-quadrupolar nuclei?

A

Coupling to quadrupolar nuc not resolved due to fast relaxation
(except in high symm when there is 2I+1 with equal intensities)

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7
Q

What is the binomial splitting when I=1 or I=3/2

A

When I=1 then add three numbers (2 when I=1/2)

When I=3/2 then add fours numbers

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8
Q

What is the NOE experiement?

A

One or more nuclei irradiated selectively
Intensity changes are seen as a result of dipolar relaxation (dipole-dipole coupling with r-6)
Nearby nuclei then also seen in spectrum

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9
Q

How can NMR distinguish hydrogen complexes?

A

1J is 25-34 Hz when metal coords to a H2 bond in η2 manner

2J is 5 Hz when there are H-M-H (two separate ligands)

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10
Q

Why is there faster relaxation in molecular hydrogen (η2) complexes?

A

H v close in proximity and so dipole-dipole relaxation occurs

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11
Q

What are the stages in which you collect a 2D spectra?

A
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12
Q

What are the pulses in the COSY (correlated spec) experiment?

A
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13
Q

What happens to a boron cluster when the spectra is H-decoupled?

A

No coupling between Bs which have a H bridge between them

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14
Q

What occurs due to magnetization in NOESY and EXSY?

A

NOESY - magnetization transferred between neighborough spins via NOE (dipolar coupling)

EXSY - magnetization transferred between sites by chemical exchange

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15
Q

What are the pulses required for NOESY and EXSY?

A

Magnetization transferred during τm

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16
Q

Why does solid state generally give broad lines?

A

Dipolar coupling
Long T1 relax times
Chemical shift anisotropy

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17
Q

How is dipolar coupling different in solid state?

A

Not averaged by molec tumbling like in solution

Broadens signal

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18
Q

How does long T1 relaxation times effect solid-state NMR?

A

Means pulse repetition rates slower, giving longer accumulation times and poor signal to noise

Also less scans possible

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19
Q

How does chemical shift anisotropy effect solid state NMR?

A

Chemical shielding depends on orientation, not averaged in solid state
This leads to line broadening

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20
Q

How can you get sharp peaks in solid state NMR?

A

Prob nucleus w/ low abundance so little homonuclear dipolar coupling (heteronuc removed with decoupling techniques)

Magic angle spinning (MAS) techniques

21
Q

What is magic angle spinning technique (MAS)?

A

Solid state sample spun at 54.44° which averages broadening with (3cos2θ -1) dependence so sharper lines

A few with this dependence as dipolar coupling & shift broadening reduced

22
Q

How does rate of MAS change peaks?

A

Slower - resonance with side bands separated by spinning freq with intensity reflecting profile of anisotropy

Faster - single isotropic average peak

23
Q

What must you use when you have spin-dilute solid state NMR?

A

Cross polarisation
Gives better signal to noise ratio

24
Q

What is the process of cross polarization when solid-state spin dilute nuclei?

A

Magnetization transferred from abundant spin I with high gyromag to a dilute low gyromag nucleus

Enhancement of the dilute nucleus occurs

Faster pulse repetition rates then possible based on the T1 of the abundant nuc

25
Q

What is observed when MAS used with cross polarization and high power decoupling?

A

Spinning side band artifact resonances are observed (from MAS)

Only observed when other techniques are also employed as weak

26
Q

Why can you not do X-ray crystallography of Zeolites?

A

Al and Si have similar e- densities so cannot determine how much is one or the other

27
Q

What is Lowenstein’s rule for zeolites?

A

No Al-O-Al linkages can occur in a zeolite

28
Q

How does doping zeolites with N effect the shift?

A

As amount of N increases then peak observed which is more +ve

Also better resolution as improved crystallinity

29
Q

What occurs to sub at zeolite for high T?

A

More sub at high temp

30
Q

How long does a NMR experiment take?

A

102 - 10-9 s

Threfore compared to most analytical is v slow

31
Q

What is the berry pseudorotation mechanism?

A
32
Q

What is the cramer rotation of alkenes?

A

Coord alkenes will rotate at a certain temp

Means at low T the protons have separate peaks

33
Q

What is the rate of exchange at the coalescence point?

A

kex = πδv/Sqrt[2]

where δv is the change in freq between the nuclei in different sites

34
Q

What can you use kex in NMR?

A

Analysis using arrgenius plot

Analysis in terms of Eyring transition-crowd theory (ΔG‡)

35
Q

Why is there broadening with/without exchange?

A

Linewidths larger when there is exchange present

Short lifetime means energy of different forms less well defined so broadening
linewidth α 1/k, so as exchange faster then more defined

36
Q

How can you find volume of activation (ΔV‡) from NMR?

A

Variable pressure NMR

If as P increases then rate decreases then a +ve ΔV‡

If as P decreases then rate increases then a -ve ΔV‡

37
Q

How does extent of line-broadening in T-dependent NMR show the mechanism?

A

All resonances broaden @ same rate as T increases if in same environment
As equally as likely

i.e. if some protons exchangable and others not then the one which are not exchange slower so are less broad

38
Q

What is a magnetization transfer experiment and when is it used?

A

Nuclei at an exchanging site selectively excited, which is transferred to another site by chemical exchange (e.g. NOESY)

Used when there is a system in the slow exchange regime

39
Q

What is seen in NMR spectra for mag transfer experiments?

A

When transfer then mag transfer occurs, so only radiated peak and the other site is irradiated

e.g. if H2 & H3 transfer, then mag transfer occurs so signals are seen at the 2 and 3

40
Q

What is the structure of a 2D exchange spectroscopy?

A

π/2 pulses are 90 degree

τm is the magnetization time, and this is when different things occur

41
Q

What is the process of NOESY and EXSY?

A

NOESY - during τm mag transferred between neighbouring spins via NOE

EXSY - during τm mag transferred between sites by chem exchange

42
Q

How can EXSY be used to find rates of exchange?

A

If strong cross-peaks in EXSY then fast exchange

43
Q

When are crosspeaks in EXSY seen if multiple atoms different?

A

Only crosspeaks when only 1atom different (halogen)

e.g. BBr3, BClBr2, BCl2Br, BCl3

44
Q

How can you react a non-reversible reaction?

A

Know NMR signals of intermediates

Observe peaks at different times during the reaction

45
Q

How does isotopic sub change NMR spectra?

A

Small changes in chemical shifts as sub alters bond vib frequencies and so small changes in elec distributions and extent of shielding

Larger effects with lighter elements (H/D have largest % change)

46
Q

How can you work out the Sn splitting in NMR of SnH3-nDn?

A

SnH3 - quartet

SnH3 - 7 peaks in modified pascals triangle

SnH2D - triplet from H and then D causes each line to split into 3

SnHD2 - doublet from H and then 2xD causes quintet (I=1)

47
Q

How can you find the intensities of different peaks in (XeF6)4?

A

Use prob of active 129Xe to find intensity of peaks

Numbers add up to 4 due to 4xXe

48
Q

How do oxolate ligands compare to thiolate equivalents in terms of shift?

A

Thiolate equivalents have lower chemical shifts

σp α (-1/ΔE) <1/r3>

This is because larger distance between nucleus and surrounding e- (more covalency and larger orbital size)

49
Q

How can ring whizzing happen in Cp ligands?

A

1,2 shift - C2/4/5 change environment but 3 stays in same. C2/5 broadened more than C3/4

1,3 shift - C5 stays in same environment after one shift but C2/3/4 all change environment. C3/4 broadened more than C2/5

Year 3 - Chemistry (12 decks)

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