STRUCTURE DETERMINATION:
MS, IR, NMR (A REVIEW)
Dr. Sheppard
CHEM 2412
Fall 2014
McMurry (8th ed.) sections 12.1-3, 12.5-8, 13.1-5, 13.7-11,
13.13
Spectroscopy
•
•
•
•
Analytical techniques
Help determine structure
Destroy little or no sample
Light absorbed by the sample is
measured as wavelength varies
• Types:
1.
Mass spectrometry (MS)
• Fragments the molecule and measures the masses
2.
Infrared (IR) spectroscopy
• Measures the bond vibration frequencies in a molecule and is used to
determine the functional group
3.
Nuclear magnetic resonance (NMR) spectroscopy
• Number, type and connectivity of atoms in a molecule
4.
Ultraviolet (UV) spectroscopy
• Uses electron transitions to determine bonding patterns (conjugated p systems)
MASS SPECTROMETRY
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•+ + eM•+ → A+ + X
A+ → B+ + Y
etc.
• Ions are separated and detected; mass determined
The Mass Spectrum
• Plot relative abundance vs. mass-to-charge ratio
• Charge = +1
• Base peak = strongest (most abundant/stable ion)
• Molecular ion/parent peak (M+) = mass of compound
14
14
15
Mass Spectrum of Hexane
14
14
14
15
Isotopes
81
Br
• 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
INFRARED
SPECTROSCOPY
IR Spectroscopy
• Units are wavenumbers (4000-400 cm-1)
• Measures molecular vibrations
• No two molecules will give exactly the same IR spectrum
(except enantiomers)
IR Spectrum
Baseline
Absorbance/
Peak
• Simple stretching: 1500-4000 cm-1
• Complex vibrations: 400-1500 cm-1
• The “fingerprint region”
• Interpretation:
• Looking for presence/absence of functional groups
• Correlation tables
• Polar bonds is usually the most IR-active
IR Correlation Table (McMurry 8th ed.)
Hexane , Hexene and Hexyne Spectra
Alcohol and Amine Spectra
Carbonyl Spectra
NMR
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
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
Chemical Shifts (d scale, in ppm)
NMR Spectra
TMS =
Reference
Compound
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
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
1H-NMR
Number of Signals
• One signal for each type of H in a molecule
2
4
3
1H-NMR
Chemical Shifts
• More shielded = upfield (to the right)
• Less shielded = downfield (to the left)
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
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
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
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
a
c
CH
c
H
H
a
H
• Examples:
OH
C C
3
Hb
d
H
H
b
H
Cl
aH
Hb
Cl
Solving NMR Problems
• Given:
• 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