Block 2 - Spectroscopy Flashcards
spectroscopy
study of electromagnetic radiation with matter
- allows most functional groups to be detected rapidly and often full determination of the structure of compounds
molecular spectroscopy
- 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
MS finds
mass of individual ions derived from compounds in gas phase
MS mechanism
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
MS limitation
- can’t distinguish between isomers
- when mass measure to nearest unit value, certain molecular formulae may give same m/z
IR finds
presence/absence of particular functional groups
IR mechanism
IR radiation absorption corresponds to vibrational excitation (increasing amplitude of vibration)
- absorption occurs when radiation frequency exactly matches frequency of bond vibration
IR benefits
- rapid identification of functional groups
- fingerprint: no two compounds have identical spectra
IR limitations
- same functional groups = similar spectra
- no indication of no. functional groups (length of peak is NOT representative of this - may be due to conc.)
UV-Vis finds
presence of pi electrons and conjugation
conjugation
pi - sigma - pi
sigma - sigma* in UV-Vis
big gap = high energy requirement so realistically never happens
- thus no absorption: light just passes through
C=C in UV-Vis
pi - pi*
- absorption at ~170nm - 180nm
C=O in UV-Vis
pi - pi*
- absorption at ~170nm - 180nm
n - pi* (lone e- pair on O)
- (weak) absorption at ~280nm
pi-pi* appears first on spectra
compounds with conjugated systems in UV-Vis
absorptions at > 200nm
- greater number of double bonds in conjugation = smaller the ∆E and larger the wavelength(max)
UV-Vis limitation
doesn’t show no. pi bonds (all appear under same bump)
conjugation and molar absorptivity
no effect thus conjugation has no effect on absorbance (doesn’t show how good/bad of an absorber it is)
MS, IR and UV-Vis give
no info with respect to hydrocarbon skeleton
NMR gives
info about carbon/hydrogen framework
greatest impact on structure determination
absence of strong magnetic field
random orientation of nuclei
nuclear spin
property that certain nuclei (e.g 1H and 13C) have that make them act like tiny bar magnets
presence of field
aligned nuclei either with (parallel) or against (antiparallel) the field
higher energy alignment
aligned against the magnetic field (energy required to keep it this way)
excitation from lower to higher energy spin state
energy flips the spin state of nuclei when ∆E = hv
TMS
tetramethylsilane - (CH3)4Si defined as 0∂ or ppm
proton decoupled spectra
no observation of splitting of signals which would otherwise be seen due to 13C-1H interaction
13C general chemical shift range
∂0-220
sp3 C chemical shift range
∂0-90
sp2 C chemical shift range
∂100-220
grey area
∂90-100 where you can’t tell whether it’s sp2 or sp3
where there’s symmetry
no. signals < no. carbons in molecular formula
- present in all monosubstituted benzene rings
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)
HNMR position of signals
shielded vs. deshielded
HNMR general chemical shift range
∂0-15 with most observed in ∂0-10 range
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
for H bonded to sp2 C
signals at higher ∂
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
HNMR splitting patterns
n+1 lines where n is the no. of vicinal/neighbouring H (3 bonds away)
splitting names
singlet, doublet, triplet, quartet, pentet etc.
monosubstituted - 1 signal
1 broad signal integration 5H
monosubstituted - 3 signals
- 1 doublet integration 2H
- 1 triplet integration 2H
- 1 triplet integration 1H
disubstituted - ortho - different substituents
no symmetry
- 1 doublet integration 1H x2
- 1 triplet integration 1H x2
disubstituted - ortho - same substituents
symmetry
- 1 doublet integration 2H
- 1 triplet integration 2H
disubstituted - meta - different substituents
no symmetry
- 1 singlet integration 1H
- 1 doublet integration 1H x2
- 1 triplet integration 1H
disubstituted - meta - same substituents
symmetry
- 1 singlet integration 1H
- 1 doublet integration 2H
- 1 triplet integration 1H
disubstituted - para
symmetry regardless of whether substituents are the same
- 2 signals: 1 doublet integration 2H x2
trisubstituents - all next to each other - all different
- 1 triplet integration 1H
- 1 doublet integration 1H
- 1 doublet integration 1H
trisubstituents - all next to each other - two the same
- 1 triplet integration 1H
- 1 doublet integration 2H
trisubstituents - alternating - all different
- 1 singlet integration 1H x3
trisubstituents - alternating - two the same
- 1 singlet integration 1H
- 1 singlet integration 2H
trisubstituents - two next to each other, one on other side
- 1 singlet integration 1H
- 1 doublet integration 1H x2
