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 Molecular ion (M + ) m/z = 16

Background Mass spectrum of ethanol (MW = 46) SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09) M+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% 79 Br/49.5% 81 Br) 2-bromopropane M + ~ M+2 SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/1/09)

Easily Recognized Elements in MS Chlorine: – M+2 is ~ 1/3 as large as M + SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/2/09) M+2 M+M+

Easily Recognized Elements in MS Iodine – I + at 127 – Large gap Large gap I+I+ M+M+ SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/2/09)

Fragmentation Patterns Alkanes – Fragmentation often splits off simple alkyl groups: Loss of methyl M Loss of ethylM Loss of propylM Loss of butylM – 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+M+

Fragmentation Patterns Aromatics may also have a peak at m/z = 77 for the benzene ring. M + =

Fragmentation Patterns MS of diethylether (CH 3 CH 2 OCH 2 CH 3 )

Frgamentation Patterns M + = SDBSWeb : (National Institute of Advanced Industrial Science and Technology, 11/28/09)

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 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 = 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 π π n π π n π* π π n  -  *; 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 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) Water205 CH 3 C  N210 C 6 H Ether210 EtOH210 Hexane210 MeOH210 Dioxane220 THF220 CH 2 Cl CHCl CCl benzene280 Acetone300 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 groupincrement, 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 Base value217 2 x alkyl subst. 10 exo DB 5 total232 Obs.237 Base value214 3 x alkyl subst. 30 exo DB 5 total234 Obs.235 Base value215 2 ß alkyl subst. 24 total239 Obs.237

Distinguish Isomers! Base value214 4 x alkyl subst. 20 exo DB 5 total239 Obs.238 Base value253 4 x alkyl subst. 20 total273 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)