Background The impact of a stream of high energy electrons causes the molecule to lose an electron...
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Background
• The impact of a stream of high energy electrons causes the molecule to lose an electron forming a radical cation.– A species with a positive charge and one unpaired
electron
+ e-C HH
HH H
HH
HC + 2 e-
Molecular ion (M+)
m/z = 16
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Background
• Mass spectrum of ethanol (MW = 46)
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)
M+
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Background
• The cations that are formed are separated by magnetic deflection.
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Background
• Only cations are detected.– Radicals are “invisible” in MS.
• The amount of deflection observed depends on the mass to charge ratio (m/z).– Most cations formed have a charge of +1 so the
amount of deflection observed is usually dependent on the mass of the ion.
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Background
• The resulting mass spectrum is a graph of the mass of each cation vs. its relative abundance.
• The peaks are assigned an abundance as a percentage of the base peak. – the most intense peak in the spectrum
• The base peak is not necessarily the same as the parent ion peak.
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Easily Recognized Elements in MS Bromine:
M+ ~ M+2 (50.5% 79Br/49.5% 81Br)
2-bromopropane
M+ ~ M+2
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)
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Easily Recognized Elements in MS• Chlorine:
– M+2 is ~ 1/3 as large as M+
Cl
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/2/09)
M+2
M+
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Easily Recognized Elements in MS• Iodine
– I+ at 127– Large gap
Large gap
I+
M+
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/2/09)
I CH2CN
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Fragmentation Patterns
• Alkanes– Fragmentation often splits off simple alkyl groups:
• Loss of methyl M+ - 15• Loss of ethyl M+ - 29• Loss of propyl M+ - 43• Loss of butyl M+ - 57
– Branched alkanes tend to fragment forming the most stable carbocations.
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Fragmentation Patterns• Mass spectrum of 2-methylpentane
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Fragmentation Patterns• Alkenes:
– Fragmentation typically forms resonance stabilized allylic carbocations
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Fragmentation Patterns• Aromatics:
– Fragment at the benzylic carbon, forming a resonance stabilized benzylic carbocation (which rearranges to the tropylium ion)
M+
CHH
CH BrH
CH
H
or
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Fragmentation PatternsAromatics may also have a peak at m/z = 77 for the benzene ring.
NO2
77M+ = 123
77
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Fragmentation Patterns
H O CHCH3
MS of diethylether (CH3CH2OCH2CH3)
CH3CH2O CH2H O CH2
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Frgamentation Patterns
M+ = 136
CO
O CH3
105
77 105
77
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/28/09)
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Where in the spectrum are these transitions?
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The UV Absorption process• * and * transitions: high-energy, accessible in vacuum UV (max
<150 nm). Not usually observed in molecular UV-Vis.• n * and * transitions: non-bonding electrons (lone pairs),
wavelength (max) in the 150-250 nm region. • n * and * transitions: most common transitions observed in
organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm.
• Any of these require that incoming photons match in energy the gap corrresponding to a transition from ground to excited state.
• Energies correspond to a 1-photon of 300 nm light are ca. 95 kcal/mol
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What are the nature of these absorptions?
Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W):
HOMO u bonding molecular orbital LUMO g antibonding molecular orbital
h 170nm photon
Example for a simple enone
ππ
nππ
n
π*
ππ
nπ*π*
π*π*
π*π* π*
π*
-*; max=218 =11,000
n-*; max=320 =100
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How Do UV spectrometers work?
Two photomultiplier inputs, differential voltage drives amplifier.
Matched quartz cuvettes
Sample in solution at ca. 10-5 M.
System protects PM tube from stray light
D2 lamp-UV
Tungsten lamp-Vis
Double Beam makes it a difference technique
Rotates, to achieve scan
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Solvents for UV (showing high energy cutoffs)
Water 205
CH3CN 210
C6H12 210
Ether 210
EtOH 210
Hexane 210
MeOH 210
Dioxane 220
THF 220
CH2Cl2 235
CHCl3 245
CCl4 265
benzene 280
Acetone 300
Various buffers for HPLC, check before using.
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Organic compounds (many of them) have UV spectra
From Skoog and West et al. Ch 14
One thing is clear
Uvs can be very non-specific
Its hard to interpret except at a cursory level, and to say that the spectrum is consistent with the structure
Each band can be a superposition of many transitions
Generally we don’t assign the particular transitions.
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Beer-Lambert Law
Linear absorbance with increased concentration--directly proportional
Makes UV useful for quantitative analysis and in HPLC detectors
Above a certain concentration the linearity curves down, loses direct proportionality--Due to molecular associations at higher concentrations. Must demonstrate linearity in validating response in an analytical procedure.
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Polyenes, and Unsaturated Carbonyl groups;an Empirical triumph
R.B. Woodward, L.F. Fieser and others
Predict max for π* in extended conjugation systems to within ca. 2-3 nm.
Homoannular, base 253 nm
heteroannular, base 214 nm
Acyclic, base 217 nm
Attached group increment, nm
Extend conjugation +30
Addn exocyclic DB +5
Alkyl +5
O-Acyl 0
S-alkyl +30
O-alkyl +6
NR2 +60
Cl, Br +5
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Some Worked Examples
O
Base value 217 2 x alkyl subst. 10 exo DB 5 total 232 Obs. 237
Base value 214 3 x alkyl subst. 30 exo DB 5 total 234 Obs. 235
Base value 215 2 ß alkyl subst. 24 total 239 Obs. 237
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Distinguish Isomers!
HO2C
HO2C
Base value 214 4 x alkyl subst. 20 exo DB 5 total 239 Obs. 238
Base value 253 4 x alkyl subst. 20 total 273 Obs. 273
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Absorbing species
• Electronic transitions– , , and n electrons– d and f electrons– Charge transfer reactions
• , , and n (non-bonding) electrons
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Sigma and Pi orbitals
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Electron transitions
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Transitions• ->*
– UV photon required, high energy• Methane at 125 nm• Ethane at 135 nm
• n-> *– Saturated compounds with unshared e-
• Absorption between 150 nm to 250 nm• between 100 and 3000 L cm-1 mol-1
• Shifts to shorter wavelengths with polar solvents– Minimum accessibility
– Halogens, N, O, S
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Transitions
• n->*, ->*– Organic compounds, wavelengths 200 to 700 nm– Requires unsaturated groups
• n->* low (10 to 100)– Shorter wavelengths
• ->* higher (1000 to 10000)