Organic Spectroscopy

Methods for structure determination of organic compounds:

X-ray Crystallography Crystall structures

Mass spectroscopy Molecular formula

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UV UltraViolet radiation Electron exitation

IR InfraRed Vibration excitation

NMR Nuclear Magnetic Resonance Nuclear spin excitation

E2

E1

!Eh" E = h"

v = c / #

E = h c / #

Etot = Eelec + Evib + Erot + Ekin

Overview of methods: Ethyl propenoat

UV - Ultraviolet-Visible Spectroscopy

Conjugated compounds absorb energy in the ultraviolet (UV) and visible (Vis) regions on the electromagnetic spectrum

! ! 200- 800 nm

Lambert-Beer's law : A = " · C · l

A is the observed absorbance, " Molar absorptivity Is the intensity of the absorption

C is the molar concentration of the sample and

l is length of the sample cell in centimeters

The wavelength of maximum absorption (!max) is reported in units of

nanometers (nm).

Example: UV absorption spectrum of 2,5-dimethyl-2,4-hexadiene

Use of UV

• Absorbance above 210 gives indication of conjugated systems

• Concentration measurements through calibration curve

Absorption Maxima for Nonconjugated and Conjugated Dienes In UV-Vis spectroscopy the electrons are excited from lower energy levels to higher ones

The electron is generally excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) Alkenes and nonconjugated dienes have absorptions below 200 nm because the energy difference between the HOMO and LUMO is large In conjugated dienes these energy levels are much closer together and the wavelengths of absorption are longer than 200 nm

Ethene has lmax at 171 nm and 1,3-butadiene has lmax at 217 nm The longer the conjugated system, the smaller the energy difference between the HOMO and the LUMO

A smaller energy gap results in longer !max in the ultraviolet -visible spectrum

#-Carotene has 11 conjugated double bonds and an absorbance maximum at 497

nm which is in the blue-green region of the visible spectrum

#-Carotene is perceived as red-orange, the complementary color of blue-green

See more examples in Tab 13-2 (p. 412)

MS - Mass Spectroscopy In mass spectroscopy (MS) a small quantity of a sample is vaporized and ionized to form charged particles – usually an electron is removed from each molecule to form charge ions. The charged particles are sorted according to their mass-to-charge ratio (m/z) and detected. All this is taken place in an instrument called a mass spectrometer. Scheme:

Example: Butanone

The molecule is commonly fragmentated into “stable” carbocations at branching points.

Each Isotope of an atom gives a specific peak according to its abundance. Common example with different isotope distrubution from unity: 79Br / 81Br is 1:1 Mw = 80 g/mole and 35Cl / 37Cl is 3:1 Mw= 35.45 g/mole

IR - Infrared spectroscopy

Vibration transitions:

Streching and Bending

IR can be used for determination of functional groups in organic compounds.

E

C O

650 cm-14000 cm-1 3000 2000 1000

O-H

N-H

C H

C H

C H C CC N

1600

C CC O

Fingerprint area Bending vibrations

C HBend Ar-HBend

All vibrations streching unless noted.

Important IR absorptions of fundamental bonding groups

C

O

O H

Similar summary is presented in Fig 13,7

For more specified values see Table 13,3 (p. 416)

Stretching vibrations:1000 - 4000 cm-1 Bendning vibrations: 600 - 1600 cm-1

The fingerprint area gives specified pattern of many peaks (bending vibrations) in the spectra for each organic molecule Strong dipoles gives generally strong absorbance, e.g. Carbonyl, C=O Dipole moment must be changed if IR-radiation should be absorbed.

CH3 C C CH3CH3 C C H

AbsorbanceNo absorbance

Interpreting IR Spectra

Generally only certain peaks are interpreted in the IR -Those peaks that are large and above 1400 cm-1 are most valuable

Hydrocarbons C-H bonds where the carbon has more s character are shorter, stronger and stiffer

and thus vibrate at higher frequency

! C-H bonds at sp centers appear at 3000-3100 cm-1

! C-H bonds at sp2 centers appear at about 3080 cm-1

! C-H bonds at sp3 centers appear at about 2800-3000 cm-1

!

- C-C bond stretching frequencies are only useful for multiple bonds

! C-C double bonds give peaks at 1620-1680 cm-1

! C-C triple bonds give peaks at 2100-2260 cm-1

Example: octane

Example: 1-hexene

Example: 1- hexyne

Aromatic Compounds

- The C-C bond stretching gives a set of characteristic sharp peaks between 1450-

1600 cm -1. Substitution pattern can be analyzed by peaks at 600-800 cm -1

Example: Methyl benzene

Functional Groups:

The carbonyl group gives

a strong peak which

occurs round 1700 cm-1

Alcohols and Phenols The O-H stretching absorption is very characteristic In concentrated solutions, the hydroxyl groups hydrogen bond to each other and a very broad and large peak occurs at 3200-3550 cm-1. See the spectra above of ethanol as an example..

Carboxylic Acids The carbonyl peak at 1710-1780 cm-1 is very characteristic The presence of both carbonyl and O-H stretching peaks (2500-3500 cm-1, very broad due to extensive hydrogen bonding) is a good proof of the presence of a carboxylic acid Example: propanic acid

Amines Very dilute solution of 1o and 2o amines give sharp peaks at 3300-3500 cm-1

for the N-H stretching ! 1o amines give two peaks and 2o amines give one peak ! 3o have no N-H bonds and do not absorb in this region

More concentrated solutions of amines have broader peaks Amides have amine N-H stretching peaks and a carbonyl peak Example: p-Methylaniline

Övning: Identifiera IR-spektra med rätt förening:

A

B

C

Table IR