Unit 11 Instrumentation. Mass, Infrared and NMR Spectroscopy

Transcription

1 Unit 11 Instrumentation Mass, Infrared and NMR Spectroscopy

2 Spectroscopic identification of organic compounds Qualitative analysis: presence but not quantity (i.e. PEDs) Quantitative analysis: quantity of substance (i.e. DUI) Structural analysis: arrangement of atoms (i.e. natural products) Infrared (IR) spectroscopy: bond identification Mass spectroscopy: atomic/molecular mass Nuclear magnetic resonance (NMR) spectroscopy: vital structural information

3 Mass Spectroscopy find the mass of individual atoms and relative abundance of different isotopes. find relative molecular mass of compounds clues about structure (fragmentation patterns)

4 Fragmentation Patterns electron from electron gun hits gaseous molecule. Molecule breaks up. The parent ion passes through, while other, lower molecular weight fragments are deflected. X(g) + e - X + (g) + 2e - by piecing together the fragments, it s possible to form a picture of the complete structure.

5 Fragmentation Patterns

6 Fragmentation Patterns

7 Fragmentation Patterns Use Table 28 in your data booklet to help identify fragments. You re expected to recognize the following, however: Don t forget to include the POSITIVE CHARGE on the ions detected by the instrument.

8 Example A molecule with an empirical formula CH 2 O has the simplified mass spectrum below. Deduce the molecular formula and give a possible structure of the compound.

9 Degree of Unsaturation/IHD IHD (index of hydrogen deficiency) - clue to structure once molecular formula is known. How much H 2 needed to make an alkane.

10 Infrared (IR) Spectroscopy IR radiation absorbed by certain bonds causing them to stretch or bend, giving information about bonds in a molecule. Frequency of radiation is often measured as number of waves per cm (cm -1 ), also called the wavenumber.

11 Infrared (IR) Spectroscopy bonds are like springs, vibrating according to bond strength and masses of the atoms. Light atoms vibrate at higher frequencies than heavier atoms and multiple bonds vibrate at higher frequencies than single bonds

12 Bond Excitation IR energy causes an induced dipole. The more polar the bond, the more it reacts to the IR radiation, the more intense the vibration of the bond (stretch or bend)

13 Stretching / bending in polyatomic molecules Water - stretching & bending as a whole 3 frequencies of vibration all of which are detectable Carbon dioxide - symmetrical linear molecule 4 modes of vibration (symmetric stretch undetectable)

14 Matching wavenumbers with bonds Different functional groups absorb IR radiation differently and distinctly. Chemists can use that information to identify different bonds. (Table 26 of IB data booklet)

15 Matching wavenumbers with bonds Different functional groups absorb IR radiation differently and distinctly. Chemists can use that information to identify different bonds. (Table 26 of IB data booklet)

16 Matching wavenumbers with bonds Different functional groups absorb IR radiation differently and distinctly. Chemists can use that information to identify different bonds. (Table 26 of IB data booklet)

17 Matching wavenumbers with bonds Different functional groups absorb IR radiation differently and distinctly. Chemists can use that information to identify different bonds. (Table 26 of IB data booklet)

18 Matching wavenumbers with bonds Different functional groups absorb IR radiation differently and distinctly. Chemists can use that information to identify different bonds. (Table 26 of IB data booklet) Hydrogen bonds!!

19 Sample IR Spectra Let s compare propanone (acetone) and ethanol

20 Sample IR Spectra Let s compare propanone (acetone) and ethanol

21 Unknown Comparisons The blue spectrum is pure heroin while the superimposed black spectrum is an unknown sample Forensics connection.

22 Nuclear Magnetic Resonance (NMR) Powerful technique for finding structure and shape of molecules Nuclei of atoms with odd #s of protons ( 1 H, 13 C, 19 F, 31 P) behave like tiny bar magnets. When placed in a magnetic field some will line up with and others against the field. Sample placed in an electromagnet, field strength is varied until nuclei flip (resonance), which can be detected.

23 NMR Non-invasive technique Can erase debit cards Anything ferromagnetic could be attracted to the powerful magnet Cooled with LN 2.

24 NMR Electrons shield nucleus (where the protons live) from the magnet. Different chemical environments for various protons then exist and produce different signals in the spectrum. Hydrogen nuclei (present in all organic molecules) give information about their position in the molecule. Measured against a standard, tetramethylsilane (TMS). Position of signal relative to the standard is the chemical shift. TMS is therefore assigned a shift of 0 ppm All of the protons are in the same chemical environment. Chemically inert.

25 Sample Shifts See Table 27 for a comprehensive list. *variance due to hydrogen bonding

26 Interpreting 1 H NMR spectra Sample spectrum of ethanal. Integration included to show # of protons attached to carbons.

27 Example The 1 H NMR spectrum of a compound which has the molecular formula C 3 H 8 O is shown here. C 3 H 8 O

28 C 3 H 8 O 1. Draw structural formulas and name 3 possible isomers 2. What is responsible for the peak at 0 ppm? 3. Identify the unknown compound from the number of peaks. 4. Identify the group responsible for the signal at 0.9 ppm.

29 C 3 H 8 O

30 Combining Analytical Techniques Deduce IHD and molecular formula (Mass Spec) Is a CH 3 group present? (Mass Spec) The infrared (IR) spectrum shows one absorption close to 2900 cm -1, but there is no absorption close to 1600 cm -1. State what can be deduced from this. Deduce molecular structure. ( 1 H NMR)

31 21.1 High-resolution 1 H NMR High-resolution reveals more about structure Single peaks (shown earlier) are split into smaller parts (multiple peaks) Effective magnetic field modified by the magnetic field produced by neighboring protons. Spin-Spin coupling! Splitting a result of potential values for energy difference between two nuclear energy levels for the CH 3 protons.

32 21.1 High-resolution 1 H NMR CHO proton split due to CH 3 protons. 2 possible orientations for each proton (2 3 combinations). 4 different local magnetic fields. 4 signals: 1, 3, 3, 1 intensities.

33 21.1 High-resolution 1 H NMR EXAMPLE: Predict the splitting pattern produced by a neighboring -CH 2 - group.

34 21.1 High-resolution 1 H NMR Splitting patterns can be deduced from Pascal s triangle. Protons bonded to the same atom don t interact with each other. Protons on non-adjacent carbon atoms do not generally interact with each other the O-H single peak in ethanol does not split unless the sample is pure (rapid exchange of protons between ethanol molecules)

35 Example #2 Empirical formula C 2 H 4 O a) Deduce the molecular formula b) Draw possible structures of molecules c) Use Table 27 to identify a structure which is consistent with the 1 H NMR and account for the number of peaks and the splitting patterns.

36 X-ray diffraction What s the most direct way to perceive an object? Shine a light on it! Visible light s way too big (large wavelength) to do that so.. We use X-rays (10-9 m) X-rays passing through a crystalline solid scatter in an orderly way A diffraction pattern results

37 X-ray diffraction Constructive interference: waves are in phase Destructive interference: waves are out of phase by 180 (cancel each other out)

38 X-ray diffraction By mapping electron density with monochromatic x-rays, a picture of the molecules structure can be shown. Sample must be in the solid state - only orderly structures give ordered diffraction patterns First applications were for inorganic crystals, but has now been expanded to organic molecules Anthracene