CHAPTER 12 INFRARED SPECTROSCOPY. and MASS SPECTROSCOPY

Transcription

1 KOT 222 ORGANIC CHEMISTRY II CHAPTER 12 INFRARED SPECTROSCOPY and MASS SPECTROSCOPY

2 Part I Infrared Spectroscopy

3 What is Spectroscopy? Spectroscopy is the study of the interaction of matter and electromagnetic radiation. It is an analytical technique used to determine the organic structures. It works with small samples and destroy little or no sample. Absorption Spectroscopy: Involves the measurement of the amount of light absorbed by a compound as a function of the wavelength of light.

4 Spectroscopic Techniques Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen /carbon atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns.

5 Electromagnetic Radiation Radiant energy that displays wave properties. Frequency and wavelength are inversely proportional.

6 Electromagnetic radiation has wave-like properties which travel as photons. c = speed of light (3x10 10 cm/sec) h = Planck s constant, 6.62 x kj sec ν= frequency (Hz) λ = wavelength (cm)

7 Just below red in the visible region. The Infrared Region The infrared spectrometers usually operate at the wavelengths between μm. More common units are wavenumbers ( ν), or cm -1, the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and energy.

8 Molecular Vibrations The covalent bonds in molecules are constantly vibrating. A bond vibrates with both stretching and bending motions. Each stretching and bending vibration of a bond occurs with a characteristic frequency. Factors affecting the stretching vibration: Masses of the atoms Stiffness of the bond

9 Masses of the atoms: Heavier atoms vibrate slowly than lighter ones. Frequency decreases with the increasing atomic weight. Stiffness of the bonds: Stronger bonds usually vibrate faster than weaker bonds. Frequency increases with increasing bond energy.

10 Vibrational Modes Nonlinear molecule with n atoms usually has 3n -6fundamental vibrational modes. Water: H 2 O 3(3) 6 = 3 Methanol: CH 3 OH 3(6) 6 = 12 Ethanol: CH 3 CH 2 OH 3(9) 6 = 21 No. of vibrational modes no. of peaks in IR spectrum

11 IR Spectrum of Methanol cm -1, where the functional groups absorb. Fingerprint Region ( cm -1 ): contains many absorptions caused by complex vibrations. unique for different compounds

12 IR-Active and IR-Inactive IR absorptions only happen when the vibrations of bonds caused changes in their dipole moments. A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule (internal alkyne) will absorb weakly or not at all IR inactive.

13 Infrared Spectrometer It measures the frequencies of infrared light absorbed by a compound. Types: 1) Dispersive infrared spectrometer. 2) Fourier transform infrared spectrometer (FT-IR).

14 Dispersive infrared spectrometer Reference beam Allow only one frequency of light to enter the detector at a time Detect the difference in the intensity of light in both beams Sample beam 100 % = no absorption 0 % = absorb of the light

15 Fourier transform infrared spectrometer, FT-IR Has better sensitivity. Less energy is needed from source. Completes a scan in 1-2 seconds. Takes several scans and averages them. Has a laser beam that keeps the instrument accurately calibrated.

16 Interferogram The interferogram at the right displays the interference pattern and contains all of the spectrum information. A Fourier transform converts the time domain to the frequency domain with absorption as a function of frequency.

17 IR Spectroscopy of Hydrocarbons Two common bondings in hydrocarbons: Carbon carbon bond. Carbon hydrogen bond. H H H H H H C C C C C H H H H pentene

18 Carbon-Carbon Bond Stretching Three types of carbon-carbon bonds: C C single bond C = C double bond C C triple bond Each type gives different absorption band at different wavenumber depends on the bond s stiffness. C-C 1200 cm -1 (weak) C=C 1660 cm -1 C C <2200 cm -1 (weak or absent for internal alkyne)

19 C = C double bond Observable stretching absorptions in the region of cm -1. Specific frequency depends on whether there is another double bond nearby. Overlapping of pi bonds Isolated; 1645 cm -1 Conjugated; 1620 cm cm -1 isolated C=C cm -1 conjugated C=C cm -1 aromatic C=C approx cm -1

20 Carbon-Hydrogen Bond Stretching Alkanes, alkenes, and alkynes also have characteristic C-H stretching frequencies. Amount of s character in the carbon orbital used to form the C-H bond determine the absorption frequencies.

21 IR Spectrum of Alkane C-H bending Absorption band for C-C (1200 cm -1 ) is weak due to small dipole moment.

22 IR Spectrum of Alkene The most important absorptions in the 1-hexene are the C=C stretch at 1642 cm -1, and the unsaturated =C-H stretch at 3080 cm -1.

23 IR Spectrum of Alkyne Internal alkyne: No C-H bond. Disubstituted C C has very small dipole moment.