1. Structure Determination by Spectroscopy Mass spectroscopy Ultraviolet-visible spectroscopy Infrared spectroscopy Nuclear magnetic resonance spectroscopy 1

2. Mass Spectroscopy Mass spec gives information about the molecular weight, and thus the formula, of a molecule. A sample is vaporized and bombarded with high energy electrons. The impact ejects an electron from the sample to give a radical cation. A-B  [A. B] +. + e - The cation is detected and recorded as the M+ (molecular cation) peak, usually the highest mass peak in the spectrum. The M+ peak gives the molecular weight of the compound. The mass / charge (m/z) ratio is always an even number except when the molecule contains an odd number of Nitrogen atoms. 2

3. Mass Spectroscopy Isotopes have different atomic weights and so can be separated by the spectrometer. Halogens can be identified by their isotope ratios. 35 Cl and 37 Cl in a 3:1 ratio 79 Br and 81 Br in a 1:1 ratio 127 I is the natural isotope 3

4. Mass Spectroscopy The radical cation can fragment to a radical (no charge) and a cation. [A. B] +.  A + B + or A + + B Only the cations are detected in the mass spectrometer. The most intense peak is called the “Base Peak”, which is arbitrarily set to 100% abundance; all other peaks are reported as percentages of abundance of “Base Peak.” Different groups of atoms will fragment in characteristic ways. 4

5. Interaction of electromagnetic radiation energy and matter When EMR is directed at a substance, the radiation can be: Absorbed Transmitted Reflected depending on the frequency (or wavelength or energy) of the radiation and the structure of the substance. 5

6. Electromagnetic Radiation 6

7. Mathematical Relationships E = hc / E = h c = = Frequency (Hz) c = Velocity of Light (3 x 10 10 cm/sec) = Wavelength (cm)h = Planck’s Constant (6.62 x 10 -27 erg-sec) 7

8. Interaction of electromagnetic radiation energy and matter Molecules exist only in discrete states that correspond to discrete energy content. The EMR energy that is absorbed is quantized and brings about certain specific changes in the molecule. electronic transitions (UV-vis) vibrations (IR) rotations (IR) 8

9. Interaction of electromagnetic radiation energy and matter Exact energies absorbed by a molecule are highly characteristic of the structure and are unique for each compound. spectroscopic “fingerprint” Similar functional groups absorb similar energies regardless of the structure of the rest of the compound. 9

10. UV-visible Spectroscopy Ultraviolet: 200 nm – 400 nm Visible: 400 nm – 800 nm Most organic molecules and functional groups do not absorb energy in the UV-visible part of the EMR spectrum and thus, absorption spectroscopy in the ultraviolet-visible range is of limited utility. When a molecule does absorb in the UV-vis, the energy transitions that occur are between electronic energy levels of valence electrons, that is, electrons in orbitals of lower energy are excited to orbitals of higher energy. Energy differences generally of 30 –150 kcal/mole 10

11. UV-visible Spectroscopy The ground state of an organic molecule can contain valence electrons in three principal types of molecular orbitals: C::C C : H  (sigma)  (pi) n (non- bonding) 11

12. UV-visible Spectroscopy Electrons in sigma bonds (single bonds) are too tightly bound to be promoted to a higher energy level by UV-visible radiation. alkanes, alcohols, alkyl halides, simple alkenes do not absorb in the UV Electrons in pi bonds and non-bonding orbitals are more loosely held and can be more easily promoted. Conjugation of pi bonds lowers the energy of the radiation that is absorbed by a molecule. Conjugated unsaturated systems are molecules with two or more double or triple bonds each alternating with a single bond. If a molecule does not absorb in the UV, then it does not contain a conjugated system of alternating double bonds or a carbonyl group. 12

13. Infrared Spectroscopy Infrared Almost all organic compounds absorb in this region between the visible and radiowaves 800 nm (12,500 cm -1 ) to 10 7 nm (1.0 cm -1 ) Area of greatest interest in organic chemistry is the vibrational portion 2,500 nm (4,000 cm -1 ) to 15,000 nm (~700 cm -1 ) 13

14. Infrared Spectroscopy Radiation in the vibrational infrared region is expressed in frequency units called wave numbers, which are the reciprocal of the wavelength ( ) expressed in centimeters. (cm -1 ) = 1 / (cm) (cm -1 ) = (nm -1 ) x 10 7 Wave numbers can be converted to energy by multiplying by hc. Thus wave numbers are proportional to energy.  14

15. Infrared Spectroscopy Molecular Vibrations Absorption of infrared radiation corresponds to energy changes on the order of 8-40 kJ/mole (2-10 kcal/mol) The frequencies in this energy range correspond to the stretching and bending frequencies of covalent bonds, that is, changes in bond length and in bond angle, respectively. Two uses for IR: IR spectra can be used to distinguish one compound from another (“fingerprint”) Information about the functional groups present in a compound 15

16. Infrared Spectroscopy 16

17. Alkane Decane CH 3 (CH 2 ) 8 CH 3 17

18. Aromatic Isopropylbenzene 18

19. Alkyne 1-Pentyne 19

20. Alcohol 3-Heptanol 20

21. Amine Benzylamine 21

22. Ketone 3-Hexanone 22

23. Aldehyde Hexanal 23

24. Carboxylic acid Proprionic acid 24

25. Ester Methyl benzoate 25

26. Ether Methyl phenyl ether 26

27. Nitrile Butyronitrile 27

28. Nitro compound Nitrobenzene 28

29. Summary Identify functional groups that are present or absent, using Pavia’s sections Do not over-analyze an IR spectrum – there is usually complementary information from other sources to identify the compound Not every peak can be identified, so don’t try Look at lots of examples! 29