Molekularne vibracije i IR (infra crvena spektroskopija)

Transcription

1 Molekularne vibracije i IR (infra crvena spektroskopija)

2 Vibracije What is a vibration in a molecule? Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds Can a vibration change the dipole moment of a molecule? (Shockwave animation) Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active. Symmetrical stretching/bending does not. Not IR active

3 What wavelength of electromagnetic radiation is involved in causing vibrations in molecules? Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of ,000 nm or µm) For a vibration at 4111 cm -1 (the stretch in H 2 ), how many vibrations occur in a second? 120 trillion vibration per second!!!! 120 x vibrations/sec or a vibration every 8 x seconds! WOW!!!!!

4 How does the mass influence the vibration? H 2 I 2 MM =2 g/mole MM =254 g/mole The greater the mass - the lower the wavenumber

5 How much movement occurs in the vibration of a C-C bond? stretching vibration 154 pm 10 pm For a C-C bond with a bond length of 154 pm, the variation is about 10 pm. bending vibration 4 o 10 pm For C-C-C bond angle a change of 4 o is typical. This moves a carbon atom about 10 pm.

6 Malo fizike electromagnetskog Energija (E) zračenja E = hν = hc/λ = hcν h je Planckova konstanta, c je brzina svjetlosti, ν je frekvencija the number of vibrations per second and λ is the wavelength Valni broj (ν ) Dan u cm -1 ν = 1/ λ Period (P) P = 1/ν Vrijeme titraja Energy, frequency, and wavenumber are directly proportional to each other.

7 Koje tipične vibracije imamo u pentanu? Pogledajmo IR spectar pentana.

8 IR spektar C-H istezanje Porast easing apsorpcije IR zračenja C-H savijanje C-C bending Increasing wavelength Increasing wavenumber (energy, frequency)

9 IR Spectra of chloroform and deuterochloroform Increasing absorbance Shift of peak due to replacement of H with D (2x mass) Mode of vibration C-H stretching C-H bending C-Cl stretching C-Cl bending CHCl 3 Calculated* * Spartan 02 AM1 minimization CHCl 3 Measured CDCl 3 Measured

10 Some results Calculated values using computational software give lower wave numbers Increasing mass of substituted atoms shifts wave numbers to lower values (Excel spreadsheet) Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)

11 Does the stretching energy have any relationship to the strength of the bond? Wavenumber (cm -1 ) Wavenumber vs. Bond Energy W = BE r 2 = Bond energy (kj/mole)

12 Let s examine the carbonyl group on three compounds formaldehyde phosgene acetone How does the C=O stretching energy compare for these three molecules? 2053 cm cm cm -1 The carbonyl group has a range of cm -1.

13 Functional group analysis in organic compounds Unlike atomic spectroscopy where sharp energy transitions occur due to well quantized electron transitions, molecular spectroscopy tends to show bands. Molecular vibrations are influenced by the surrounding groups!

14 savijanje Basic Functional Groups C-H O-H C=O C-O stretching O-H C-H C C C=C alkenes aromatic C-C WAVENUMBER (cm -1 )

15 Use of IR spectra Identification of functional groups on a molecule this is a very important tool in organic chemistry Spectral matching can be done by computer software and library spectra Since absorbance follows Beer s Law, can do quantitative analysis