Molecule structure determination

Example C7H7NO

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

Infrared spectroscopy

Wavelengths, Frequencies, and Energies of Some Regions of the Electromagnetic Spectrum

λcE h h= n =

• 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

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

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

×= =

´

Infrared Absorption Spectroscopy

h

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

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

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

=

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 = =´

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

• 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

• 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

Interpreting Infrared Spectra

Interpreting Infrared Spectra

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

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

Explain the following comparisons of C=O stretching frequencies:

Identifying Functional Groups Using IR Spectroscopy

Identifying Functional Groups Using IR Spectroscopy

Identify the functional groups in each of the following IR spectra:

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

• 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

OH1-Hexanol Nonane