Organic Spectroscopy. UV - Ultraviolet-Visible Spectroscopy. !! nm. Methods for structure determination of organic compounds:

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1 Organic Spectroscopy Methods for structure determination of organic compounds: X-ray rystallography rystall structures Mass spectroscopy Molecular formula UV UltraViolet radiation Electron exitation IR InfraRed Vibration excitation NMR Nuclear Magnetic Resonance Nuclear spin excitation E 2 E 1 E Etot = Eelec + Evib + Erot + Ekin h" E = h" v = c / # E = h c / # UV - Ultraviolet-Visible Spectroscopy onjugated compounds absorb energy in the ultraviolet (UV) and visible (Vis) regions on the electromagnetic spectrum nm Lambert-Beer's law : A = " l A is the observed absorbance, " Molar absorptivity Is the intensity of the absorption 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 Overview of methods: Ethyl propenoat Use of UV Absorbance above 210 gives indication of conjugated systems oncentration measurements through calibration curve

2 Absorption Maxima for Nonconjugated and onjugated 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 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 Ethene has l max at 171 nm and 1,3-butadiene has l max 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 #-arotene has 11 conjugated double bonds and an absorbance maximum at 497 nm which is in the blue-green region of the visible spectrum #-arotene is perceived as red-orange, the complementary color of blue-green See more examples in Tab 13-2 (p. 412)

3 The molecule is commonly fragmentated into stable carbocations at branching points. IR - Infrared spectroscopy Vibration transitions: Streching Each Isotope of an atom gives a specific peak according to its abundance. ommon example with different isotope distrubution from unity: 79 Br / 81 Br is 1:1 Mw = 80 g/mole and 35 l / 37 l is 3:1 Mw= g/mole and Bending IR can be used for determination of functional groups in organic compounds. Important IR absorptions of fundamental bonding groups E 4000 cm cm -1 O-H N-H H H All vibrations streching unless noted. H N O O H O O H Bend Ar-H Bend Fingerprint area Bending vibrations Similar summary is presented in Fig 13,7 For more specified values see Table 13,3 (p. 416) Stretching vibrations: cm -1 Bendning vibrations: 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. arbonyl, =O Dipole moment must be changed if IR-radiation should be absorbed. H 3 H 3 H 3 H No absorbance Absorbance

4 Interpreting IR Spectra Example: 1-hexene Example: 1- hexyne Generally only certain peaks are interpreted in the IR -Those peaks that are large and above 1400 cm-1 are most valuable Hydrocarbons -H bonds where the carbon has more s character are shorter, stronger and stiffer and thus vibrate at higher frequency -H bonds at sp centers appear at cm-1 -H bonds at sp2 centers appear at about 3080 cm-1 -H bonds at sp3 centers appear at about cm bond stretching frequencies are only useful for multiple bonds - double bonds give peaks at cm-1 - triple bonds give peaks at cm-1 Example: octane Aromatic ompounds - The - bond stretching gives a set of characteristic sharp peaks between cm -1. Substitution pattern can be analyzed by peaks at cm -1 Example: Methyl benzene

5 Amines Functional Groups: The carbonyl group gives a strong peak which occurs round 1700 cm-1 Very dilute solution of 1o and 2o amines give sharp peaks at 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 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 cm-1. See the spectra above of ethanol as an example.. arboxylic Acids The carbonyl peak at cm-1 is very characteristic The presence of both carbonyl and O-H stretching peaks ( cm-1, very broad due to extensive hydrogen bonding) is a good proof of the presence of a carboxylic acid Example: propanic acid

6 Övning: A B Identifiera IR-spektra med rätt förening: Table IR