Infrared Spectroscopy Gives information about the functional groups in a molecule 1

Infrared Spectroscopy region of infrared that is most useful lies between 2.5-16 m (4000-625 cm-1) depends on transitions between vibrational energy states stretching bending 2

Fig. 13.30

Fig. 13.31 IR Spectrum of Hexane

Fig. 13.32

Fig. 13.33

Fig. 13.34

Fig. 13.35

Fig. 13.36

Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated  electron systems 1

Transitions between electron energy states gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm E = h 2

X-axis is wavelength in nm Conventions in UV-VIS X-axis is wavelength in nm Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity () max is the wavelength of maximum absorption and is related to electronic makeup of molecule,especially  electrons 3

UV Spectrum of cis,trans-1,3-cyclooctadiene max 230 nm max 2630 4

* Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller E), max shifts to longer wavelengths 5

Methyl groups on double bond cause max to shift to longer wavelengths CH3 C C H CH3 H H max 170 nm max 188 nm 5

Extending conjugation has a larger effect on max; shift is again to longer wavelengths max 170 nm max 217 nm 5

max 217 nm (conjugated diene) H C max 217 nm (conjugated diene) C H CH3 H3C max 263 nm conjugated triene plus two methyl groups 5

orange-red pigment in tomatoes Lycopene orange-red pigment in tomatoes max 505 nm 5

Mass Spectrometry mass spec is different because it is not related to electromagnetic radiation 1

Principles of Electron-Impact Mass Spectrometry Atom or molecule is hit by high-energy electron from an electron beam at 10ev e– e– beam + • forming a positively charged, odd-electron species called the molecular ion 2

Principles of Electron-Impact Mass Spectrometry Molecular ion passes between poles of a magnet and is deflected by magnetic field deflection depends on mass-to-charge ratio highest m/z deflected least lowest m/z deflected most + • 5

Principles of Electron-Impact Mass Spectrometry If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose. However, the molecular ion often fragments to a mixture of species of lower m/z. 6

Principles of Electron-Impact Mass Spectrometry The molecular ion dissociates to a cation and a radical. + • 2

Principles of Electron-Impact Mass Spectrometry The molecular ion dissociates to a cation and a radical. + • Usually several fragmentation pathways are available and a mixture of ions is produced. 2

Principles of Electron-Impact Mass Spectrometry mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass in mixture separation of peaks depends on relative mass + + + + + + 6

Principles of Electron-Impact Mass Spectrometry mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass separation of peaks depends on relative mass + + + + + + 6

Fig. 13.39

Some molecules undergo very little fragmentation Benzene is an example. The major peak corresponds to the molecular ion. m/z = 78 The largest peak is called the base peak and is reference to 100 to give relative intensity. 9

Isotopes in Chlorobenzene 35Cl 37Cl Isotopes in Chlorobenzene 11

Isotopic Clusters in Chlorobenzene + ion responsible for m/z 77 peak does not contain Cl 11

Alkanes undergo extensive fragmentation Decane, C10H22 The largest peak may not be the parent ion, or may not be visible at all. 13

Propylbenzene fragments mostly at the benzylic position 91 CH2—CH2CH3 120 14

Molecular Formula as a Clue to Structure 1

Molecular Weights One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula. We can gain some information about molecular formula from the molecular weight. Mass spectrometry makes it relatively easy to determine molecular weights. 6

The Nitrogen Rule A molecule with an odd number of nitrogens has an odd molecular weight. A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight. NH2 93 138 NH2 O2N 183 NH2 O2N NO2 6

Exact Molecular Weights CH3CO O CH3(CH2)5CH3 Heptane Cyclopropyl acetate Molecular formula C7H16 C5H8O2 Molecular weight 100 100 Exact mass 100.1253 100.0524 Mass spectrometry can measure exact masses. Therefore, mass spectrometry can be used to distinguish between molecular formulas. 6

Index of Hydrogen Deficiency Degree of Unsaturation relates molecular formulas to multiple bonds and rings For a molecular formula, CcHhNnOoXx, the degree of unsaturation can be calculated by: Degree = ½ (2c + 2 - h - x + n) 6

Rings versus Multiple Bonds Index of hydrogen deficiency tells us the sum of rings plus multiple bonds. Using catalytic hydrogenation, the number of multiple bonds can be determined. 6