1. Structure Determination: MS, IR, NMR (A review)
Dr. Sheppard
CHEM 2412
Fall 2014
McMurry (8th ed.) sections , , , ,
13.13

2. Spectroscopy Analytical techniques Help determine structure
Destroy little or no sample
Light absorbed by the sample is measured as wavelength varies
Types:
Mass spectrometry (MS)
Fragments the molecule and measures the masses
Infrared (IR) spectroscopy
Measures the bond vibration frequencies in a molecule and is used to determine the functional group
Nuclear magnetic resonance (NMR) spectroscopy
Number, type and connectivity of atoms in a molecule
Ultraviolet (UV) spectroscopy
Uses electron transitions to determine bonding patterns (conjugated p systems)

3. Mass Spectrometry

4. Mass Spectrometry Used with Gas Chromatography
Mixture of compounds separated by gas chromatography, then identified by mass spectrometry
Determines MW and provides information about structure
A beam of high-energy electrons breaks molecules into ions (fragments)
M → M•+ + e-
M•+ → A+ + X
A+ → B+ + Y
etc.
Ions are separated and detected; mass determined

5. The Mass Spectrum Plot relative abundance vs. mass-to-charge ratio
Base peak = strongest (most abundant/stable ion)
Molecular ion/parent peak (M+) = mass of compound 14 14 15

6. Mass Spectrum of Hexane14 14 14 15

7. Isotopes Present in their usual abundance
81Br
Isotopes
Present in their usual abundance
Hydrocarbons contain 1.1% 13C, so there will be a small M+1 peak
If S is present, M+2 will be 4% of M+
If Cl is present, M+2 is one-third of M+
If Br is present, M+2 is equal to M+
If I is present, peak at 127; large gap

8. Infrared Spectroscopy

9. IR Spectroscopy Units are wavenumbers (4000-400 cm-1)
Measures molecular vibrations
No two molecules will give exactly the same IR spectrum (except enantiomers)

10. IR Spectrum Simple stretching: 1500-4000 cm-1
Baseline
Absorbance/Peak
Simple stretching: cm-1
Complex vibrations: cm-1
The “fingerprint region”
Interpretation:
Looking for presence/absence of functional groups
Correlation tables
Polar bonds is usually the most IR-active

11. IR Correlation Table (McMurry 8th ed.)

12. Hexane , Hexene and Hexyne Spectra

13. Alcohol and Amine Spectra

14. Carbonyl Spectra

15. NMR

16. NMR Most powerful technique for structure determination
Number and type of atoms in a molecule
Connectivity of atoms
Used to study a wide variety of nuclei:
1H, 13C, 15N, 19F, 31P, etc.
Radio-frequency radiation used to transition between energy states (nuclear spin)
Spinning nucleus acts as a bar magnet
Aligns with or against external field
Absorption of light causes spin flip
“Resonance”
Measured by spectrometer

17. Nuclei in a Molecule
Depending on their chemical environment, atoms in a molecule are shielded by different amounts
Chemically equivalent nuclei
Interchanged through bond rotation or element of symmetry
Have same absorption
Chemically different nuclei have different absorption

18. Chemical Shifts (d scale, in ppm)

19. TMS = Reference Compound
NMR Spectra
TMS = Reference Compound

20. 13C-NMR Signal = one sharp line for each different type of carbon
The number of different signals indicates the number of different kinds of carbon
The chemical shift indicates the functional group
Used to support 1H-NMR analysis

21. 1H-NMR More info than 13C-NMR The number of signals
How many different kinds of protons are present
The location (chemical shift) of the signals
Is the proton shielded or deshielded
The intensity (integration) of the signal
The number of protons of that type
Signal splitting (multiplicity)
The number of protons on adjacent atoms

22. 1H-NMR Number of Signals
One signal for each type of H in a molecule

23. 1H-NMR Chemical Shifts More shielded = upfield (to the right)
Less shielded = downfield (to the left)

24. 1H-NMR Integration
The signal intensity (area under signal) is proportional to the number of protons giving rise to that signal
Shown by integration line
Height of vertical line ≈ area under peak ≈ # H’s in set
Measure height with ruler or look at graph paper
Ratio of height = ratio of hydrogens

25. 1H-NMR Spin-Spin Splitting
Signals can be split into multiple peaks
The (n+1) rule:
A signal is split by n neighboring protons, into (n + 1) peaks

26. 1H-NMR Spin-Spin Splitting
Coupling constants (J)
Distance between the peaks of a split signal
Measured in Hz (usually 0-18)
Gives info on type of H

27. Stereochemical Nonequivalence
Usually, two protons on the same carbon are equivalent and do not split each other
If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other
Results in more complex splitting patterns
Examples:

28. Solving NMR Problems Given: Goal: Determine structure
1H-NMR
Molecular formula (typically)
IR (sometimes)
13C-NMR (sometimes)
Goal: Determine structure
If the molecular formula is known:
Determine the number of elements of unsaturation
Sum of number of rings + p bonds
Index of Hydrogen Deficiency IHD = C – ½(H + X) + ½N + 1
Example: C5H3N2O2Cl IHD = 5 – ½(3 + 1) + ½(2) + 1 = 5