MS Interpretation II. Fragmentation

Transcription

1 MS Interpretation II Fragmentation

2 Ionization E Electron Ionization (EI): Even-electron neutrals yield odd-electron radical cations. M(EE) EI - 1e - M (E) Electron can come from anywhere. EI EI even electron - 1e - (n) - 1e - (π) odd electron EI even electron odd electron even electron - 1e - (σ) odd electron

3 Ionization E Electron can (and does) come from anywhere. - 1e - (n) EI EI - 1e - (π) EI - 1e - (σ)

4 Ionization E Electron can (and does) come from anywhere. - 1e - (n) EI EI - 1e - (π) EI - 1e - (σ) Likelihood of each of these depends on energy levels in molecular orbitals: vacuum level σ* π* n π σ IE n most likely IE π IE σ least likely

5 Ionization E Electron can (and does) come from anywhere. - 1e - (n) EI EI - 1e - (π) EI - 1e - (σ) Naturally, cannot distinguish these in mass spectrometer (all have m/z = 100). But fragmentation patterns will be different.

6 Ionization EE CI, MALDI and ESI: Even-electron neutrals yield even-electron cations. M(EE) H MH (EE) Like EI, ionization may occur at multiple places. H H Again, instrument cannot distinguish.

7 Fragmentation Mechanisms: EE EE ions don't fragment to form E Fragmentations more familiar and spectra generally less complex than E More frequent and varied rearrangements can make interpretation more difficult Spectra can be more sensitive than EI to small structural changes Sensitivity to experimental conditions incompatible with reference libraries

8 Fragmentation Mechanisms: EE

9 Fragmentation Mechanisms: EE Bond cleavage with charge migration

10 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2

11 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2 Cleavage with cyclization and migration

12 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2 Cleavage with cyclization and migration HN H N CR 2 HN CR 2 HN

13 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2 Cleavage with cyclization and migration HN H N CR 2 HN CR 2 HN Two-bond cleavage with charge retention

14 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2 Cleavage with cyclization and migration HN H N CR 2 HN CR 2 HN Two-bond cleavage with charge retention H H -H 2

15 Fragmentation Mechanisms: EE Bond cleavage with charge migration R-H2 R H2 Cleavage with cyclization and migration HN H N CR 2 HN CR 2 HN Two-bond cleavage with charge retention H H -H 2 ther fragmentations require MS/MS

16 Fragmentation Mechanisms in MS: E Electron Ionization: Fragmentation is always unimolecular. Two possible categories of fragmentation: parents daughters M (E) A (EE) B (E) charge migration (radical and charge part ways) M (E) A (E) B(EE) charge retention (neutral molecule is ejected)

17 Fragmentation Mechanisms in MS: E Electron Ionization: Fragmentation is always unimolecular. Two possible categories of fragmentation: parents daughters M (E) A (EE) B (E) charge migration (radical and charge part ways) M (E) A (E) B(EE) charge retention (neutral molecule is ejected) Important: nly daughter ions are detected by MS instrument. Released neutrals are only inferred.

18 Fragmentation Mechanisms: E

19 Fragmentation Mechanisms: E Direct dissociation

20 Fragmentation Mechanisms: E Direct dissociation R-R' R R'

21 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom

22 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R'

23 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R'

24 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage

25 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R'

26 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R' R-CH2-Y-R' R CH2-Y-R'

27 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R' R-CH2-Y-R' R CH2-Y-R' Two-bond cleavage

28 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R' R-CH2-Y-R' R CH2-Y-R' Two-bond cleavage Retro Diels-Alder

29 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R' R-CH2-Y-R' R CH2-Y-R' Two-bond cleavage Retro Diels-Alder Rearrangement

30 Fragmentation Mechanisms: E Direct dissociation R-R' R R' Cleavage adjacent to a heteroatom R-CH2-Y-R' R-CH2 Y-R' R-CH2-Y-R' R-CH2 Y-R' α-cleavage R-CH2-Y-R' R CH2=Y-R' R-CH2-Y-R' R CH2-Y-R' Two-bond cleavage Retro Diels-Alder Rearrangement McLafferty Rearrangement

31 Fragmentation Mechanisms in MS: Direct Cleavage EI (electron can come from any bond) Fragmentation (one-electron bond can break either way)

32 Fragmentation Mechanisms in MS: Direct Cleavage EI (electron can come from any bond) Fragmentation (one-electron bond can break either way)

33 Fragmentation Mechanisms in MS Relative Abundance m/z

34 Fragmentation Mechanisms in MS What governs which ions are predominant?

35 Fragmentation Mechanisms in MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are σ bonds

36 Fragmentation Mechanisms in MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are σ bonds. 2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.) Here, secondary cation/radical combination favored over primary

37 Fragmentation Mechanisms in MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are σ bonds. 2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.) Here, secondary cation/radical combination favored over primary

38 Fragmentation Mechanisms in MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are σ bonds. 2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.) Here, secondary cation/radical combination favored over primary. 3. In charge separation, cation stability is more important than radical stability. Here, masses 71, 43 favored over 15,

39 Fragmentation Mechanisms in MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are σ bonds. 2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.) Here, secondary cation/radical combination favored over primary. 3. In charge separation, cation stability is more important than radical stability. Here, masses 71, 43 favored over 15,

40 Fragmentation Mechanisms in MS Relative Abundance m/z

41 Fragmentation Mechanisms in MS: Direct Cleavage

42 Fragmentation Mechanisms in MS: Direct Cleavage

43 Fragmentation Mechanisms in MS: Direct Cleavage 57 71

44 Fragmentation Mechanisms in MS: Direct Cleavage Further fragmentation is common.

45 Fragmentation Mechanisms in MS: α-cleavage 56 41

46 Heteroatoms: Induced vs. Alpha Cleavage t-butyl ethyl ether 102

47 Heteroatoms: Induced vs. Alpha Cleavage t-butyl ethyl ether i

48 Heteroatoms: Induced vs. Alpha Cleavage t-butyl ethyl ether i 59 α H CH H

49 2-butyl ethyl ether 102

50 2-butyl ethyl ether i

51 2-butyl ethyl ether 45 i 57 α H 73 H

52 2-butyl ethyl ether 45 i 57 α H 73 H

53 Competition: i vs α Cleavage

54 Competition: i vs α Cleavage Induced cleavage is favored for larger heteroatoms

55 Competition: i vs α Cleavage Induced cleavage is favored for larger heteroatoms α-cleavage is favored for electron donating substituents

56 Competition: i vs α Cleavage Induced cleavage is favored for larger heteroatoms α-cleavage is favored for electron donating substituents Br,Cl < R,π bond,s, < N

57 butanethiol SH 41 90

58 butanethiol SH SH SH

59 butanethiol SH SH SH 57 SH 47 SH

60 butanethiol SH SH SH 57 SH 47 SH Both α- and i- cleavage are observed here 47

61 butylamine NH 2 73

62 butylamine NH 2 NH 2 NH

63 butylamine NH 2 NH 2 NH α-cleavage dominates spectrum here 73

64 E Fragments

65 E Fragments dd electron fragments are less common, especially at low mass

66 E Fragments dd electron fragments are less common, especially at low mass Even mass ions at low mass likely contain N

67 E Fragments dd electron fragments are less common, especially at low mass Even mass ions at low mass likely contain N These fragments arise from two-bond cleavages, or rearrangements

68 E Fragments dd electron fragments are less common, especially at low mass Even mass ions at low mass likely contain N These fragments arise from two-bond cleavages, or rearrangements Even mass E fragments are important ions, as they can be highly sensitive to structural changes

69 2-hexanone

70 2-hexanone

71 2-hexanone 43 H H H 43 H

72 2-hexanone 43 H H H 43 H E Product is from McLafferty rearrangement

73 2-methyl 3-pentanone

74 2-methyl 3-pentanone

75 2-methyl 3-pentanone McLafferty rearrangement is precluded here No major E 85

76 158

77

78 Charge stays with styrene fragment in this case 158

79

80

81 Charge stays with phenylbutadiene

82 Match the spectrum to the structure

83 Retro Diels-Alder reaction is observed

84 Retro Diels-Alder reaction is not observed X CH Aromatization via substitution dominates the spectrum

85 105 Match the Structure to the Spectrum Et Me Me

86 Et H 122

87 Me

88 Me