Transcript Structure Determination with Spectroscopy

Structure Determination by
Spectroscopy

Mass spectroscopy

Ultraviolet-visible spectroscopy

Infrared spectroscopy

Nuclear magnetic resonance spectroscopy
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Mass Spectroscopy
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Mass spec gives information about the molecular weight, and thus
the formula, of a molecule.
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A sample is vaporized and bombarded with high energy electrons. The
impact ejects an electron from the sample to give a radical cation.
A-B

[A.B]+.
+ e-
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The cation is detected and recorded as the M+ (molecular cation) peak,
usually the highest mass peak in the spectrum.
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The M+ peak gives the molecular weight of the compound.
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The mass / charge (m/z) ratio is always an even
number except when the molecule contains an odd
number of Nitrogen atoms.
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Mass Spectroscopy
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Isotopes have different atomic weights and
so can be separated by the spectrometer.
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Halogens can be identified by their isotope
ratios.
 35Cl
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and 37Cl in a 3:1 ratio
79Br and 81Br in a 1:1 ratio
127I is the natural isotope
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Mass Spectroscopy

The radical cation can fragment to a radical
(no charge) and a cation.
[A.B]+.  A + B+
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or A+ + B
Only the cations are detected in the mass spectrometer.
The most intense peak is called the “Base Peak”, which is
arbitrarily set to 100% abundance; all other peaks are
reported as percentages of abundance of “Base Peak.”
Different groups of atoms will fragment in characteristic
ways.
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Interaction of electromagnetic
radiation energy and matter

When EMR is directed at a substance, the
radiation can be:

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Absorbed
Transmitted
Reflected
depending on the frequency (or wavelength or energy) of
the radiation and the structure of the substance.
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Electromagnetic Radiation
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Mathematical Relationships
c = 
 = Frequency (Hz)
E = h
E = hc / 
c = Velocity of Light
(3 x 1010 cm/sec)
 = Wavelength (cm)
h = Planck’s Constant
(6.62 x 10-27 erg-sec)
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Interaction of electromagnetic
radiation energy and matter
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Molecules exist only in discrete states that
correspond to discrete energy content.
The EMR energy that is absorbed is
quantized and brings about certain specific
changes in the molecule.
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electronic transitions (UV-vis)
vibrations (IR)
rotations (IR)
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Interaction of electromagnetic
radiation energy and matter
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Exact energies absorbed by a molecule are
highly characteristic of the structure and
are unique for each compound.
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spectroscopic “fingerprint”
Similar functional groups absorb similar
energies regardless of the structure of the
rest of the compound.
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UV-visible Spectroscopy
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Ultraviolet: 200 nm – 400 nm
Visible: 400 nm – 800 nm
Most organic molecules and functional groups do not absorb
energy in the UV-visible part of the EMR spectrum and thus,
absorption spectroscopy in the ultraviolet-visible range is of
limited utility.
When a molecule does absorb in the UV-vis, the energy
transitions that occur are between electronic energy levels of
valence electrons, that is, electrons in orbitals of lower
energy are excited to orbitals of higher energy.
Energy differences generally of 30 –150 kcal/mole
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UV-visible Spectroscopy

The ground state of an organic molecule can contain valence
electrons in three principal types of molecular orbitals:
 (sigma)
 (pi)
C:H
C::C
n (nonbonding)
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UV-visible Spectroscopy
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Electrons in sigma bonds (single bonds) are too tightly bound
to be promoted to a higher energy level by UV-visible
radiation.
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alkanes, alcohols, alkyl halides, simple alkenes do not absorb in the UV
Electrons in pi bonds and non-bonding orbitals are more
loosely held and can be more easily promoted.
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Conjugation of pi bonds lowers the energy of the radiation that is
absorbed by a molecule.
Conjugated unsaturated systems are molecules with two or more
double or triple bonds each alternating with a single bond.
If a molecule does not absorb in the UV, then it does not contain a
conjugated system of alternating double bonds or a carbonyl group.
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Infrared Spectroscopy
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Infrared
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Almost all organic compounds absorb in this
region between the visible and radiowaves
800 nm (12,500 cm-1) to 107 nm (1.0 cm-1)
Area of greatest interest in organic
chemistry is the vibrational portion
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2,500 nm (4,000 cm-1) to 15,000 nm (~700 cm-1)
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Infrared Spectroscopy

Radiation in the vibrational infrared region is
expressed in frequency units called wave numbers,
which are the reciprocal of the wavelength ()
expressed in centimeters.
 (cm-1)

= 1 /  (cm)
(cm-1) = (nm-1) x 107
Wave numbers can be converted to energy by
multiplying by hc. Thus wave numbers are
proportional to energy.
E  hc /   νhc
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Infrared Spectroscopy
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Molecular Vibrations
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Absorption of infrared radiation corresponds to energy
changes on the order of 8-40 kJ/mole (2-10 kcal/mol)
The frequencies in this energy range correspond to the
stretching and bending frequencies of covalent bonds,
that is, changes in bond length and in bond angle,
respectively.
Two uses for IR:
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IR spectra can be used to distinguish one compound from
another (“fingerprint”)
Information about the functional groups present in a
compound
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Infrared Spectroscopy
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Alkane
Decane
CH3(CH2)8CH3
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Aromatic
Isopropylbenzene
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Alkyne
1-Pentyne
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Alcohol
3-Heptanol
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Amine
Benzylamine
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Ketone
3-Hexanone
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Aldehyde
Hexanal
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Carboxylic acid
Proprionic acid
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Ester
Methyl benzoate
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Ether
Methyl phenyl ether
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Nitrile
Butyronitrile
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Nitro compound
Nitrobenzene
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Summary
• Identify functional groups that are present
or absent, using Pavia’s sections
• Do not over-analyze an IR spectrum –
there is usually complementary
information from other sources to identify
the compound
• Not every peak can be identified, so don’t
try
• Look at lots of examples!
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