13C NMR Spectroscopy

Transcription

1 13 C NMR Spectroscopy

2 Introduction Nuclear magnetic resonance spectroscopy (NMR) is the most powerful tool available for structural determination. A nucleus with an odd number of protons, an odd number of neutrons, or both, has a nuclear spin that can be observed by the NMR spectrometer. NMR active nuclei include: 1 H, 13 C, 19 F, and 31 P. Remember a spinning nucleus generates a magnetic field (magnetic moment). In the absence of an external magnetic field, proton magnetic moments have random orientations. However, in the presence of an external magnetic field, the magnetic moment is aligned either with or against the external field. 2

3 Introduction The stronger the magnetic field, the greater the energy difference between the two spin states, resulting in a greater population difference between the two states greater sensitivity. 3

4 External magnetic field Nuclear Spin Energy Levels A photon of light with the right amount of energy (radiofrequency, rf) can be absorbed and cause the spinning proton to flip. E 2 E 2 h absorption of energy E 1 E 1 The nuclei undergo a spin flip, and the nuclei are said to be in resonance. This absorption of energy leads to the NMR signal 4

5 Nuclear Spin Energy Levels If the two states become equally populated, then no net spin transitions occur and no signal is produced. This is called saturation. The frequency of EM radiation necessary for resonance depends on the strength of the magnetic field and on the chemical environment of the nucleus. Fortunately, protons (in 1 H NMR) in molecules usually experience different chemical environments (i.e. are shielded to varying extents). 5

6 1 H NMR Spectroscopy Therefore, different frequencies are required to bring different protons into resonance. Consider CH 3 OH: Deshielded, senses higher effective magnetic field so comes into resonance at a higher frequency. H H O C H H Shielded, senses a smaller effective magnetic field so comes into resonance at a lower frequency. 6

7 1 H NMR Spectroscopy A 1 H NMR spectrum provides the following information: 1. The # of different types of H number of basic groups of signals. 2. The relative numbers of different types of H 3. The electronic environment of the different types of H 4. The number of hydrogen neighbors a proton has 7

8 Simple Correlation Table of 1 H Chemical Shifts * See text and Lab manual for more extensive tables 8

9 Why Carbon ( 13 C) NMR Spectroscopy Some organic compounds have few C-H bonds: Others have very similar 1 H NMR spectra: HO HO C C O C C C O O O O H 3 C CH 3 H 3 C H H H H CH 3 O O 2,6-dimethylbenzoquinone 2,5-dimethylbenzoquinone 9

10 Carbon ( 13 C) NMR vs 1 H NMR The 13 C nucleus can also undergo nuclear magnetic resonance. 13 C NMR vs 1 H NMR : 12 C, the most abundant isotope of carbon, does NOT exhibit NMR behavior. Why? 13 C, only natural abundance, does exhibit NMR behavior. Due to low abundance, 13 C- 13 C coupling is usually not observed. Chemical shift ranges are much larger Integration in 13 C NMR is NOT reliable due to variable relaxation times from C to C. Also Nuclear Overhauser Effect - the intensity of the C signal increases as the number of attached protons increases. Not uniform however. 10

11 Fourier Transform (FT) spectroscopy The magnetic moment of the 13 C nucleus is about 1/4 that of the H nucleus resulting in lower sensitivity. The low natural abudance and small magnetic moment of the 13 C isotope results in the 13 C nucleus being about less sensitive than the 1 H nucleus to NMR phenomena. Consequently, much longer acquisition times were required. The development of Fourier transform (FT) spectroscopy has made 13 C NMR acquisition routine. The old way of acquiring NMR was to apply a constant magnetic field to the sample and scan the range of frequencies = continuous wave (CW) NMR. With FT-NMR the data is collected all at once by exciting the sample with an RF pulse (typically only a few microseconds long) which covers all the resonance frequencies, and thus changes the orientation of all the protons. 11

12 Intensity of signal Fourier Transform (FT) spectroscopy After the pulse has stopped, the decay of the signal from the sample is measured. The decaying sine wave called a free induction decay (FID): O C H 3 C CH 3 Fourier Transform Time (s) Frequency A Fourier transform converts the intensity vs time data into intensity vs frequency information. 12

13 Fourier Transform (FT) spectroscopy Fourier Transform 13

14 Chemical Shifts in 13 C NMR Two simple ideas will make interpretation of 13 C NMR spectra easier: 1. Hybridization of the C atom determines the chemical shift: sp 3 hybridized carbons have chemical shift values. sp 2 hybridized carbons have chemical shift values. 2. The presence of an EN element near a C atom will cause its chemical shift to move. 14

15 Simple Correlation Table of 13 C chemical shifts See text (p. 593) and Lab manual (p. 60) for more extensive tables 15

16 Coupling in Carbon NMR The low abundance of 13 C makes C-C coupling very rare. However, 13 C-H coupling is common. N+1 rule still applies: Coupling constants are large ~ Hz for directly attached H s. 16

17 Coupling in Carbon NMR Spectra which show 13 C-H coupling are called protoncoupled spectra. However, extensive 13 C-H coupling often produces splitting patterns that are difficult to interpret. To simply 13 C NMR spectra, often recorded using broad band proton decoupling. Therefore each carbon signal appears as a singlet, because C-H splitting has been eliminated. Spectra recorded in the broad band proton decoupling mode give the number of unique carbon atoms in a molecule. 17

18 Proton-coupled vs Proton-decoupled 13 C NMR Spectra 18

19 Interpreting 13 C NMR Spectra CH 3 H 3 C C CH 2 CH 3 CH 3 CDCl 3 solvent TMS 19

20 Interpreting 13 C NMR Spectra O CH 3 C O CH 2 CH 3 20

21 Interpreting 13 C NMR Spectra O CH 3 21

22 Interpreting 13 C NMR Spectra O O H 3 C CH 3 H 3 C H H H H CH 3 O O 22

23 Mass Spectrometry

24 Basic Principles Mass Spectroscopy (MS) is a destructive analytical technique for measuring the ( ) of ions in the gas phase. This allows accurate determination of the of a molecule. Structural information is also gained. Molecular Formula determination is sometimes possible. While the method is destructive, only very small amounts (1 mg or less) is required. 24

25 Basic Principles MS does not involve the absorption or emission of light. A mass spectrometer is designed to do 3 things: 1. Convert a neutral molecule, M, into positive (or negative) ions usually by bombardment with a beam of high energy electrons. M + e M + 2e ev in energy 1 ev = 23 kcal/mol 2. Separate the ions based on mass (mass-to-charge ratio, ). 3. Measure the relative abundance of each ion. 25

26 Schematic of Mass Spectrometer First the sample is vaporized under vacuum. A beam of electrons bombards Electron the molecules Impact in the gas phase causing ionization and formation Ionization of radical Source cations. ~70 Volts Electron Collector (Trap) Electron impact Ionization source Repeller + Neutral Molecules + Inlet _ Positive Ions + to Analyzer e - e - e - _ Electrons Filament Extraction Plate 26

27 Schematic of Mass Spectrometer The radical cations fragment further after ionization owing to the large amount of energy transferred by the electron beam. Some fragments carry a positive charge, others are neutral: CH 4 - e H H C H m/z = H Molecular ion [CH 3 ] + H m/z = [CH 2 ] m/z = + 2H Only the positively charged fragments are accelerated into the analyzer tube. 27

28 Schematic of Mass Spectrometer Magnetic Sector Mass Analyzer The analyzer tube is surrounded by a magnet whose magnetic field deflects the positively charge fragments in a curved path. ion trajectory not in register (too light) ion trajectory in register S Ion Source N Electromagnet ion trajectory not in register (too heavy) Detector The amount of deflection depends on m/z. 28

29 Basis of Fragment Separation Fragments with smaller m/z value are deflected than a larger m/z value. Since z is usually, the fragments are sorted by mass. By varying the magnetic field, cations of different masses are sorted and counted by a detector. The more stable the fragment the more likely it will make it to the detector. The masses are graphed or tabulated according to their relative abundance = The Mass Spectrum. 29

30 The Mass Spectrum of Methane m/z Intensity C12 H+ [C 12 ] +. [C 12 H 2 ] +. M + = 15 C 12 H m/z Base peak M + = 16 Molecular ion [C 12 H 4 ] +. 30

31 Relative abundance, % Isotopes Most elements common to organic compounds are mixtures of isotopes. The existence of atomic isotopes in nature accounts for the appearance of M+1 and M+2 peaks in a mass spectrum. Organic compounds containing only C, H, O, and N usually have relatively small M+1 and M+2 peaks. M+ M+ M+1 + M+1 + M+2 + m/z C 6 H 12 m/z C 20 H 42 31

32 Isotopes Element Most abundant isotope Less abundant isotope Relative abundance Hydrogen 1 H 2 H Carbon 12 C 13 C 1.08 Nitrogen 14 N 15 N 0.38 Oxygen 16 O 18 O 0.20 Sulfur 32 S 34 S 4.4 Chlorine 35 Cl 37 Cl 32.5 Bromine 79 Br 81 Br

33 Relative abundance, % Isotopes MS is particularly valuable for compounds which contain Cl and Br: If one S atom is present, M + 2 is ~ 4% of M +. If one Cl atom is present, M + 2 is ~ 33% of M +. If one Br atom is present, M + 2 is ~ to M +. M M M M M M M+ M+ M+ M+2 + M+2 + M m/z m/z m/z

34 Mass Spectrum with Chlorine Cl CH H 3 C CH 3 M+2 + M+ 34

35 Mass Spectrum with Bromine H 3 C CH 2 CH 2 Br M+ M

36 Isotopes Carbon Rule For compounds containing only C, H, and O, the following formula can be used to determine the number of carbons in the molecule: no.c s= relativeintensity of 1.1 M +1 peak Determine the molecular formula of the unknown organic compound whose mass spectral data is given in the table below: Peak Mass (m/z) Relative intensity M M M

37 Isotopes Nitrogen Rule: if a compound has: An odd number of nitrogen atoms, its molecular ion, M+, will be odd. Zero or an even number of nitrogen atoms, its molecular ion, M+, will be even. 37

38 Resolution Resolution: a measure of how well a mass spectrometer separates ions of different mass. Low resolution capable of distinguishing among ions of different nominal mass, i.e. different by at least one or more amu. High resolution capable of distinguishing among ions that differ in mass by as little as amu. For example: CO, N 2, and ethene all have a nominal mass of 28 amu. High resolution MS can distinguish these molecules. CO N 2 CH 2 =CH amu amu amu 38

39 Fragmentation Pathways Structural information is available from analysis of fragments formed by bond cleavages in the molecular ion, M +. In general, the molecular ion, M +, will fragment so as to form the most stable cationic fragment (usually a carbocation). In some cases, the M + peak is very small or absent. Occurs if the fragments are considerably more stable M +. 39

40 Mass Spectrum-Fragmentation Consider the mass spectrum of pentane: p text Fragmentation of the molecular ion often results: 40

41 Mass Spectrum of Pentane 41

42 Fragmentation of Alkanes 43 CH 3 CH 3 CH 2 CH 2 CH CH M + 1 CH 3 CH 3 CH 2 CH 2 + HC CH 3 m/z 43 CH 3 CH 3 CH 2 CH 2 CH CH 3 2 CH 3 CH 3 CH 2 CH 2 CH + CH 3 m/z 86 3 m/z 71 CH 3 CH 3 CH 2 + H 2 C CH CH 3 m/z 57 42

43 Compounds with Heteroatoms Molecules containing O, N, halogens, or other heteroatoms often undergo (adjacent to heteroatom). Driving force is resonance stabilized cations. 43

44 Fragmentation of Alcohols Alcohols common fragmentation is -cleavage and loss of H 2 O to give an M-18 peak. OH M-29 H 3 C C CH 2 CH 3 CH 3 M + = 88 (not observed) M-15 M M-18 44

45 Fragmentation of Amines + CH 2 =NH 2 m/z=30 H 3 C H 3 C CH CH 2 NH 2 M + = 73 Mass spectrum of isobutylamine 45

46 Fragmentation of Ketones 43 O H 3 C C CH 2 CH 3 57 M + = 72 Mass spectrum of 2-butanone 46

47 McLafferty Rearrangement If one of the alkyl groups attached to the carbonyl carbon of an aldehyde or ketone has a hydrogen, a cleavage known as a McLafferty rearrangement can occur. M 28 O Mass spectrum of butyraldehyde H M + =72 47

48 Aromatic Compounds Usually strong M+ peak. m/z =91 for tropylium ion and methylene spacings above 91 (105, 119, etc. for alkyl chains) often observed. m/z = 65 (C 5 H 5+ ), 77 (C 6 H 5+ ) are sometimes observed. CH 2 R CH -R 2 m/z 91 m/z 91 48

49 Aromatic Compounds 49

50 Common Fragments O CH 3 CH 3 CH 2 C H H 3 C m/z = O C HO O C CH 2 O C m/z =