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CHEM 110 > Block 2 - Spectroscopy > Flashcards

Block 2 - Spectroscopy Flashcards

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1
Q

spectroscopy

A

study of electromagnetic radiation with matter

- allows most functional groups to be detected rapidly and often full determination of the structure of compounds

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2
Q

molecular spectroscopy

A
  • depend on interaction of molecules with radiation of specific energy
  • absorption of radiation of particular frequencies is characteristic of specific structural units (functional groups)
  • non-destructive with respect to sample
  • IR, UV-Vis, NMR
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3
Q

MS finds

A

mass of individual ions derived from compounds in gas phase

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4
Q

MS mechanism

A

electron impact: molecule bombarded with high energy electron beam
- ionised molecule (molecular ion) results from expulsion of (usually) one electron and thus is a radical cation
M = M+. + e-
M+. (fragmentation) = other +ve ions + neutral fragments

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5
Q

MS limitation

A
  • can’t distinguish between isomers

- when mass measure to nearest unit value, certain molecular formulae may give same m/z

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6
Q

IR finds

A

presence/absence of particular functional groups

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7
Q

IR mechanism

A

IR radiation absorption corresponds to vibrational excitation (increasing amplitude of vibration)
- absorption occurs when radiation frequency exactly matches frequency of bond vibration

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8
Q

IR benefits

A
  • rapid identification of functional groups

- fingerprint: no two compounds have identical spectra

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9
Q

IR limitations

A
  • same functional groups = similar spectra

- no indication of no. functional groups (length of peak is NOT representative of this - may be due to conc.)

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10
Q

UV-Vis finds

A

presence of pi electrons and conjugation

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11
Q

conjugation

A

pi - sigma - pi

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12
Q

sigma - sigma* in UV-Vis

A

big gap = high energy requirement so realistically never happens
- thus no absorption: light just passes through

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13
Q

C=C in UV-Vis

A

pi - pi*

- absorption at ~170nm - 180nm

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14
Q

C=O in UV-Vis

A

pi - pi*
- absorption at ~170nm - 180nm
n - pi* (lone e- pair on O)
- (weak) absorption at ~280nm

pi-pi* appears first on spectra

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15
Q

compounds with conjugated systems in UV-Vis

A

absorptions at > 200nm

- greater number of double bonds in conjugation = smaller the ∆E and larger the wavelength(max)

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16
Q

UV-Vis limitation

A

doesn’t show no. pi bonds (all appear under same bump)

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17
Q

conjugation and molar absorptivity

A

no effect thus conjugation has no effect on absorbance (doesn’t show how good/bad of an absorber it is)

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18
Q

MS, IR and UV-Vis give

A

no info with respect to hydrocarbon skeleton

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19
Q

NMR gives

A

info about carbon/hydrogen framework

greatest impact on structure determination

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20
Q

absence of strong magnetic field

A

random orientation of nuclei

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21
Q

nuclear spin

A

property that certain nuclei (e.g 1H and 13C) have that make them act like tiny bar magnets

22
Q

presence of field

A

aligned nuclei either with (parallel) or against (antiparallel) the field

23
Q

higher energy alignment

A

aligned against the magnetic field (energy required to keep it this way)

24
Q

excitation from lower to higher energy spin state

A

energy flips the spin state of nuclei when ∆E = hv

25
Q

TMS

A

tetramethylsilane - (CH3)4Si defined as 0∂ or ppm

26
Q

proton decoupled spectra

A

no observation of splitting of signals which would otherwise be seen due to 13C-1H interaction

27
Q

13C general chemical shift range

A

∂0-220

28
Q

sp3 C chemical shift range

A

∂0-90

29
Q

sp2 C chemical shift range

A

∂100-220

30
Q

grey area

A

∂90-100 where you can’t tell whether it’s sp2 or sp3

31
Q

where there’s symmetry

A

no. signals < no. carbons in molecular formula

- present in all monosubstituted benzene rings

32
Q

HNMR no. signals

A

indicate no. unique nuclei
- for monosubstituted benzene, technically there are 3 unique environ. but depending on the substituent 3 H’s may appear on top of each other giving rise to ONE signal (to confirm check how many protons gave rise to the peak)

33
Q

HNMR position of signals

A

shielded vs. deshielded

34
Q

HNMR general chemical shift range

A

∂0-15 with most observed in ∂0-10 range

35
Q

HNMR relationship between H and C

A

what happens to the sp3 C bonded to the H in terms of deshielding also happens to the H
- substituents containing double bonds deshield

36
Q

for H bonded to sp2 C

A

signals at higher ∂

37
Q

HNMR relative areas

A

proportional to no. H giving rise to the signal

  • if only one unique environ. = no relative area = no signal for comparison
  • if sum does not match molecular formula, multiply each by same integer
38
Q

HNMR splitting patterns

A

n+1 lines where n is the no. of vicinal/neighbouring H (3 bonds away)

39
Q

splitting names

A

singlet, doublet, triplet, quartet, pentet etc.

40
Q

monosubstituted - 1 signal

A

1 broad signal integration 5H

41
Q

monosubstituted - 3 signals

A
  • 1 doublet integration 2H
  • 1 triplet integration 2H
  • 1 triplet integration 1H
42
Q

disubstituted - ortho - different substituents

A

no symmetry

  • 1 doublet integration 1H x2
  • 1 triplet integration 1H x2
43
Q

disubstituted - ortho - same substituents

A

symmetry

  • 1 doublet integration 2H
  • 1 triplet integration 2H
44
Q

disubstituted - meta - different substituents

A

no symmetry

  • 1 singlet integration 1H
  • 1 doublet integration 1H x2
  • 1 triplet integration 1H
45
Q

disubstituted - meta - same substituents

A

symmetry

  • 1 singlet integration 1H
  • 1 doublet integration 2H
  • 1 triplet integration 1H
46
Q

disubstituted - para

A

symmetry regardless of whether substituents are the same

- 2 signals: 1 doublet integration 2H x2

47
Q

trisubstituents - all next to each other - all different

A
  • 1 triplet integration 1H
  • 1 doublet integration 1H
  • 1 doublet integration 1H
48
Q

trisubstituents - all next to each other - two the same

A
  • 1 triplet integration 1H

- 1 doublet integration 2H

49
Q

trisubstituents - alternating - all different

A
  • 1 singlet integration 1H x3
50
Q

trisubstituents - alternating - two the same

A
  • 1 singlet integration 1H

- 1 singlet integration 2H

51
Q

trisubstituents - two next to each other, one on other side

A
  • 1 singlet integration 1H

- 1 doublet integration 1H x2

CHEM 110 (4 decks)

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