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

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)

• Analytical techniques • Help determine structure• Destroy little or no sample• Light absorbed by the sample is

measured as wavelength varies• Types:

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•+ + e-

M•+ → 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

151414

Mass Spectrum of Hexane

15141414

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

81Br

CH3 CH CH2 CH3

I

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

CH3 CH

CH3

CH3 CH2 CH

CH3

CH3 CH3 CH2 CH2CH3 CH2 CH3

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• Examples: C C

H

H

Ha

b

c

OH

H

H

H

a

b

c

d

CH3

H Cl

H H

Cl

a b

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