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CHEM 110 > Block 2 - Spectroscopy > Flashcards

Block 2 - Spectroscopy Flashcards

(51 cards)

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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

Study These Flashcards
A

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

22
Q

presence of field

Study These Flashcards
A

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

23
Q

higher energy alignment

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A

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

24
Q

excitation from lower to higher energy spin state

Study These Flashcards
A

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

25
TMS
tetramethylsilane - (CH3)4Si defined as 0∂ or ppm
26
proton decoupled spectra
no observation of splitting of signals which would otherwise be seen due to 13C-1H interaction
27
13C general chemical shift range
∂0-220
28
sp3 C chemical shift range
∂0-90
29
sp2 C chemical shift range
∂100-220
30
grey area
∂90-100 where you can't tell whether it's sp2 or sp3
31
where there's symmetry
no. signals < no. carbons in molecular formula | - present in all monosubstituted benzene rings
32
HNMR no. signals
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
HNMR position of signals
shielded vs. deshielded
34
HNMR general chemical shift range
∂0-15 with most observed in ∂0-10 range
35
HNMR relationship between H and C
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
for H bonded to sp2 C
signals at higher ∂
37
HNMR relative areas
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
HNMR splitting patterns
n+1 lines where n is the no. of vicinal/neighbouring H (3 bonds away)
39
splitting names
singlet, doublet, triplet, quartet, pentet etc.
40
monosubstituted - 1 signal
1 broad signal integration 5H
41
monosubstituted - 3 signals
- 1 doublet integration 2H - 1 triplet integration 2H - 1 triplet integration 1H
42
disubstituted - ortho - different substituents
no symmetry - 1 doublet integration 1H x2 - 1 triplet integration 1H x2
43
disubstituted - ortho - same substituents
symmetry - 1 doublet integration 2H - 1 triplet integration 2H
44
disubstituted - meta - different substituents
no symmetry - 1 singlet integration 1H - 1 doublet integration 1H x2 - 1 triplet integration 1H
45
disubstituted - meta - same substituents
symmetry - 1 singlet integration 1H - 1 doublet integration 2H - 1 triplet integration 1H
46
disubstituted - para
symmetry regardless of whether substituents are the same | - 2 signals: 1 doublet integration 2H x2
47
trisubstituents - all next to each other - all different
- 1 triplet integration 1H - 1 doublet integration 1H - 1 doublet integration 1H
48
trisubstituents - all next to each other - two the same
- 1 triplet integration 1H | - 1 doublet integration 2H
49
trisubstituents - alternating - all different
- 1 singlet integration 1H x3
50
trisubstituents - alternating - two the same
- 1 singlet integration 1H | - 1 singlet integration 2H
51
trisubstituents - two next to each other, one on other side
- 1 singlet integration 1H | - 1 doublet integration 1H x2

CHEM 110 (4 decks)

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