Background The impact of a stream of high energy electrons causes the molecule to lose an electron...

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

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+

Background

• The cations that are formed are separated by magnetic deflection.

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.

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.

Easily Recognized Elements in MS Bromine:

M+ ~ M+2 (50.5% 79Br/49.5% 81Br)

2-bromopropane

M+ ~ M+2


Easily Recognized Elements in MS• Chlorine:

– M+2 is ~ 1/3 as large as M+

Cl


M+2

M+

Easily Recognized Elements in MS• Iodine

– I+ at 127– Large gap

Large gap

I+

M+


I CH2CN

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.

Fragmentation Patterns• Mass spectrum of 2-methylpentane

Fragmentation Patterns• Alkenes:

– Fragmentation typically forms resonance stabilized allylic carbocations

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

Fragmentation PatternsAromatics may also have a peak at m/z = 77 for the benzene ring.

NO2

77M+ = 123

77

Fragmentation Patterns

H O CHCH3

MS of diethylether (CH3CH2OCH2CH3)

CH3CH2O CH2H O CH2

Frgamentation Patterns

M+ = 136

CO

O CH3

105

77 105

77


Where in the spectrum are these transitions?

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

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

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

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.

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.

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.

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

Some Worked Examples

O

Base value 217 2 x alkyl subst. 10 exo DB 5 total 232 Obs. 237


Base value 215 2 ß alkyl subst. 24 total 239 Obs. 237

Distinguish Isomers!

HO2C

HO2C


Base value 253 4 x alkyl subst. 20 total 273 Obs. 273

Absorbing species

• Electronic transitions– , , and n electrons– d and f electrons– Charge transfer reactions

• , , and n (non-bonding) electrons

Sigma and Pi orbitals

Electron transitions

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

Transitions

• n->*, ->*– Organic compounds, wavelengths 200 to 700 nm– Requires unsaturated groups

• n->* low (10 to 100)– Shorter wavelengths

• ->* higher (1000 to 10000)

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