molecule structure determination - chemistry...fm radio waves. molecular spectroscopy ... • slight...
TRANSCRIPT
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Molecule structure determination
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Example C7H7NO
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Timeline of available techniques Elemental analysis (combustion)Melting point, boiling pointChemical reactivity and degradation: derivatives
Ultraviolet-visible (UV-vis) - 1930s Woodward-Fieser Rules ca. 1941 Circular dichroism 1960s
Infrared (IR) - 1940s Functional groups, molecular fingerprints Raman spectroscopy 1960s
Mass spectrometry (MS) - 1950s Molecular weights, and observation of key fragments Characteristic reactivity ... molecular fingerprints
Nuclear magnetic resonance (NMR) - 1960s Fourier Transform NMR 1970s2-D correlation spectroscopies 1980s
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Infrared spectroscopy
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Wavelengths, Frequencies, and Energies of Some Regions of the Electromagnetic Spectrum
λcE h h= n =
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• Frequency is commonly reported in units of:
1. joules2. nm3. amu4. Hz5. m/z
• Which type of electromagnetic radiation possesses the highest energy?
1. IR light2. UV light3. Microwaves4. Visible light5. FM radio waves
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Molecular Spectroscopy• Study of which frequencies of radiation are absorbed or emitted by a
particular substance and the correlation of these frequencies with details of molecular structure
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Infrared (IR) Spectroscopy • Probes stretching and bending vibrations of organic molecules• Vibrational infrared region: Extends from 2.5×10−6 (2.5 μm) to 2.5×10−5
m (25 μm)• Wavenumber: Number of waves per centimeter, with units cm −1 (read reciprocal centimeters)• Infrared spectrum extends from 4000 to 400 cm −1 when expressed in frequencies
2110 m cm 4000 cm
2.5 10 mn
- -1¾-
-6
×= =
´
2110 m cm 400 cm
2.5 10 mn
- -1¾-
-5
×= =
´
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Infrared Absorption Spectroscopy
h
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Molecular Vibrations• Atoms joined by covalent bonds undergo continual vibrations relative to
each other• Energies associated are quantized, that is, within a molecule, only specific vibrational
energy levels are allowed• Energies associated with transitions between vibrational energy levels correspond to
frequencies in the infrared region• For a molecule to be infrared active, its vibration must cause a substantial
change in the bond dipole moment, and it is observed in the IR spectrum• Greater the bond dipole, greater the change in dipole moment caused by a vibration
• Covalent bonds that do not meet this criteria are said to be IR inactive • The C=C and C≡C bonds in symmetrically substituted alkenes and alkynes do not
absorb IR radiation as vibration does not result in a substantial bond dipole change
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Molecular Vibrations
• For a nonlinear molecule containing n atoms, 3n − 6 allowed fundamental vibrations exist• For even a relatively small molecule, a large number of vibrational energy levels exist
• Patterns of IR absorption are complex
• The simplest vibrational motions that give rise to absorption of IR radiation are bending and stretching
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Characteristic Absorption Patterns• Consider two covalently bonded atoms as two vibrating masses connected by a
spring• Total energy is proportional to the frequency of vibration• Frequency of a stretching vibration is given by an equation derived from Hooke’s law for a
vibrating spring
• K = Force constant of a bond, which is a measure of bond strength• Force constants for single, double, and triple bonds are approximately 5, 10, and 15 × 105
dynes/cm• μ = Reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms
• Hooke’s law predicts that the position of the absorption of a stretching vibration in an IR spectrum depends on:
• Strength of the vibrating bond• Masses of the atoms connected by the bond
• Stronger the bond is and lighter the atoms are, higher the frequency of the stretching vibration will be
• Intensity of absorption depends primarily on the change in dipole of the vibrating bond
4.12 Knµ
¾
=
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Infrared Stretching Frequencies of Selected Functional Groups
Calculate the stretching frequency in wavenumbers for a carbon-carbon double bondAssume that each carbon is the most abundant isotope, namely 12C
SolutionAssume a force constant of 10×105 dynes per centimeter for C=C The calculated frequency is 1682 cm−1, a value close to the experimental value of 1650 cm−1 5
110 10 4.12 1682 cm12 12/(12 + 12)
¾-´
n = =´
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Fingerprint Region
• Vibrations in the region 1500 to 400 cm−1 of IR spectra are complex and difficult to analyze but are characteristic for different molecules
• Slight variations in molecular structure and absorption patterns are most obvious in this region
• What functional group is most likely present if a compound shows IR absorption at these frequencies?
a. 1705 cm−1
b. 2950 cm−1
• Solutiona. A C=O groupb. An aliphatic C—H group
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• Propanone and 2-propen-1-ol are constitutional isomers• Show how to distinguish between them by IR spectroscopy
• Solution• Only propanone shows strong absorption in the C=O stretching region, 1630–1820 cm−1
• Alternatively, only 2-propen-1-ol shows strong absorption in the O—H stretching region, 3200–3650 cm−1
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• The higher the wavenumber of a molecular vibration, the lower the energy of the infrared radiation needed to stimulate it
1. True2. False
• Infrared spectroscopy is based on _____ excitation
1. electronic 2. rotational3. nuclear4. vibrational
• What approximate frequency range is considered the fingerprint region in infrared spectroscopy?
1. 4000–3500 cm-1
2. 3500–3000 cm-1
3. 3000–2000 cm-1
4. 2000–1500 cm-1
5. 1500–500 cm-1
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Interpreting Infrared Spectra
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Interpreting Infrared Spectra
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Aldehydes and Ketones - Carbonyl Groups
• Position of C=O stretching vibration is sensitive to its molecular environment
• As ring size decreases and angle strain increases, absorption shifts to a higher frequency
• Presence of an adjacent C=C or benzene ring in conjugation with the carbonyl group shifts the C=O absorption to a lower frequency
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Factors That Determine IR Stretching Frequencies
• Why is it that the frequency of vibration is the same as the frequency of the IR light that is absorbed?
• Two main factors that determine the frequencies of IR absorption: • 1. the mass of the atoms
• 2. the strength of the bond
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Explain the following comparisons of C=O stretching frequencies:
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Identifying Functional Groups Using IR Spectroscopy
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Identifying Functional Groups Using IR Spectroscopy
Identify the functional groups in each of the following IR spectra:
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Steps in Solving Infrared Spectral Problems
• Step 1: Check the region around 3000 cm−1
• Absorption in this region is caused by C—H stretching
• Step 2: Check for a strong, broad band in the region of 3500 cm−1
• If present, the molecule contains an —OH group either of an alcohol or a carboxylic acid
• Functional group is an —OH group of an alcohol if there is no absorption around 1700 cm−1
• Functional group may be carboxyl if there is a peak around 1700 cm−1
• One or two peaks in the 3500 cm−1 region at somewhat lower frequency than for —OH may indicate a 2° or 1° amine, respectively
• Step 3: Check for a sharp peak in the region 1630–1820 cm−1
• If present, a C=O group is present and the peak will probably be the strongest peak in the spectrum
• If no peak is present in this region, no C=O is present
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• What functional groups are most likely present in a compound whose IR spectrum shows absorbances at 1648 and 2217cm-1?
1. Aldehyde and alkene2. Alkene and alcohol3. Alcohol and nitrile4. Alkyne and ketone5. Alkene and nitrile
• (R)-2-pentanol and (S)-2-pentanol give identical IR spectra
1. True2. False
• (E)-2-Butene and (Z)-2-butene give identical IR spectra
1. True2. False
• Which of the following compounds will have its carbonyl absorb at the lowest frequency in IR spectroscopy?
1. 2.
3. 4.
5.
O
O
O
H
O
O
O
O
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OH1-Hexanol Nonane