Spectroscopy Molecules move Movement can be monitored with electromagnetic radiation, e.g. light

Energy of Light Visible light only a small portion of electromagnetic spectrum Short wavelength, high energy Fig Bailey and Bailey Fig Bailey and Bailey

Molecular Motions Stretch Twist Scissor Rock-n-roll Others

Relating Motion and Energy We will not be completely quantitative

Points to Make As you can see, stretching gets the most attention Short wavelength (high energy) portion called the functional group region ( cm -1 ) cm -1 referred to as “fingerprint region”

One Angle… In the following molecule, predict what molecular motions should be observed in an IR spectrum

One Angle… Where the bands should appear (looked up not memorized) CH stretch –CH 3 : 2962 and 2872 cm -1 –CH 2 : 2926 and 2853 cm -1 CH bend –CH 3 : 1375 and 1450 cm -1 –CH 2 : 1465 cm -1

The Other Angle… Using Fig 18.4, identify the following molecule. The formula is C 6 H 11

Ultraviolet (UV) Spectroscopy Very little useful information obtained One saving grace is that C-C bonds can be detected ~212 nm Another saving grace is that conjugated compounds can be detected

Nuclear Magnetic Resonance (NMR) Spectroscopy Much more information can be obtained Like IR, molecules should be relatively simple Equivalent hydrogen atoms Nuclei flip in a magnetic field of the correct strength Position in ppm (0 for TMS internal standard) Integration used to determine number of H’s Shifting of peaks Splitting of peaks

NMR Spectroscopy Problems A compound has the formula C 2 H 6 O and yields 1 peak by NMR spectroscopy. Propose a structure.

NMR Spectroscopy Problems The molecule shown below gives a singlet, a quartet, and a triplet in 3:2:3 ratio. At what ppm do these peaks show up as? (reference sheet)

NMR Spectroscopy Problems A compound has the formula C 9 H 10 O 2 and yields the following NMR spectrum. Propose a structure.

NMR Spectroscopy Problems The molecule shown below gives a singlet, a septet and a doublet in a 3:1:6 ratio. At what ppm should these peaks show up as? (reference sheet)

NMR Spectroscopy Problems The molecule shown below gives a triplet and a quartet in a 3:2 ratio. At what ppm do these peaks show up as? (reference sheet)

NMR Spectroscopy Problems A compound has the formula ___ and yields the following NMR spectrum. Propose a structure. C 5 H 10 O

Mass Spectroscopy Analogy of smashing a vase with a hammer, and determining what the vase looked like by putting the pieces back together Difficulty is that all bonds can potentially break, but not all bonds break equally

Important Mass Spectroscopy Terms Molecular ion- peak where molecule is not broken, just lost an e – (ABC + in this figure) Fragment ions- all other ions Base peak- most abundant peak in spectrum (a fragment ion)

Mass Spectroscopy Problems What should the molecular ion of CH 3 Br exist as? Below is the mass spectrum of 2,2-dimethylpropane. Indicate the identity of the major peaks

18.22a, 18.23f Write a structure consistent with the following data: A straight-chained ketone with the formula C 8 H 16 O (M=128) and major m/e ions at 29, 57, 71, and 99. Using the concepts in Table 18.4, predict the major ions in the mass spectra of the compound:

End of Material for Exam I