ionization and fragmentation Chapter 5. Mass spectrometry which fragmentations? mass and frequency, m/z and count rate Reading: Pavia Chapters 3 and 4 Don t need 3.3 B-D, 3.4 B-D Use the text to clarify what s covered on the slides.
1. General - not a spectroscopic technique (no absorption of radiation) spectroscopy A spectrometry count rate λ, ν, ν m/z count rate: usually in % : relative abundance m/z: mass-to-charge ratio, here z =1 and: sample is often destroyed through fragmentation
2. Spectrometer - simplest version: electron ionization, magnetic sector mass analyzer desired event: M e M 2 e from accelerator: E kin = ½mv 2 = ev = zv M M smaller m for equal z larger m for equal z pump M in the analyzer: mv r = eb 0 m m B 02 r = = 2 z e 2V
3. Mass spectrum - from electron ionization (EI) M e M 2 e - M : radical cation (or cation radical) : often incorrectly called molecular ion, M : often the last peak in the spectrum, largest m/z: gives nominal mass - most abundant fragment (relative 100%) is the base peak - other peak intensities given as % of base peak - fragment ions - isotope peaks molecular ion peak base peak M 6 12 6 1 M Figure 4.20 Figure 4.22
4. Molecular ion, M Difficulties: -M is not always the last peak in the mass spectrum -last m/z can be a) smaller than the molar mass : M is too unstable, fragments before it reaches the detector H.. - H 2. H C C R C C R - H. detected: M - 18 detected: M -1 can be more extreme than in these examples! M M Figure 4.27 Figure 4.18
4. Molecular ion, M, continued Difficulties: -M is not always the last peak in the mass spectrum -last m/z can be b1) greater than the molar mass, first case : isotopes of higher mass are present in M 12 C 13 C 35 Cl 37 Cl 99% 1% 76% 24% more abundant isotope M M M 1 peak about 10% of 136 abundance peak Figure 4.39 Figure 4.65
4. Molecular ion, M, continued - isotope peak patterns for Cl- and Br-containing ions Figure 3.19 1:1 1:2:1 still the more abundant isotope Br 1:1 is 100 to 98 Table 3.8 (Figure 4.63) 10:3 10:6:1 Br 2 1:2:1 is 51 to 100 to 49 or 100 to 195 to 95 Cl 10:3 is 100 to 33 Cl 2 10:6:1 is 100 to 65 to 11
4. Molecular ion, M, continued Difficulties: -M is not always the last peak in the mass spectrum -last m/z can be b2) greater than the molar mass, second case : M collects a proton: M 1 : sometimes for amines, hydrazines, azo compounds; carbonyls : odd, even though no N present x Figure 3.15
4. Molecular ion, M, continued Is it possible that this is the molecular ion peak? - check for first substantial fragment peak, simple fragmentation! M? -14u 53-15u M? 72 yes, possible: -15u, means lost C Figure 4.43.. C (72) 15u (57) unified atomic mass unit no, not possible: -14u, too small, not a logical fragment Figure 4.18
5. Fragmentation considerations General - M has lost an electron to give M - 2 possible scenarios for M: - no specific preference for electron loss (alkane) and e not localized.. ionization or still 2 e in σ bond... or or two localized notations cleavage both fragments are detected with equal probability - specific preference for electron loss (functional group).. cleavage.. or or. cleavage not detected not detected. detected detected important points: - ionization - and balance -H atom balance - mass balance
5. Fragmentation considerations continued a) Energy - weak bonds break more readily - fragmentations that lead to stabilized species are preferred - electron octet -resonance - charge on the most likely atom [R--CH 2 -CH 2 ] [R--CH 2 R-=CH 2 ] radical preferred R- or R--CH 2 -CH 2 but [Ar--CH 2 -CH 2 ] Ar- [Ar-CH 2 -CH 2 ] Ar-CH 2 resonance stabilization radical
5. Fragmentation considerations continued b) Ions with an odd or even number of electrons I. dd electron ions (E ) - possess an unpaired electron, such as M - eliminate radicals R or neutral, closed-shell species N (often small, stable molecules: E E EE R H 2, HCl, C, C 2, C 2 H 4, C 2 H 2 ) E E N CH. 3 CH. 3.. Chapter 4.2B Chapter 4.2E II. Even electron ions (EE ) Chapter 4.2C - possess all paired electrons, such as M - eliminate neutral, closed-shell species N EE EE N R-=CH 2 R =CH and balance! 2
5. Fragmentation considerations continued c) Ionization energy, IE Chapter 4.2A - follows from E E N - fragment with the smallest IE carries the charge (Stevenson s rule) CH. 3. preferred: conjugated system has the higher HM : easier to excite (UV-Vis) : easier to ionize.
5. Fragmentation considerations continued Important to remember: - ionization is the first event: fragmentations start from M, not from M - and balance - H atom balance: it might be good to draw all H not to forget any - mass balance -R and N are not detected because not charged, do not give signals: masses for R and N are listed differently: CH. 3. (96) (81) 15u, indicating that for this particular one-molecule fragmentation there is no signal with m/z 15
a) α-cleavage 6. Fragmentations M - for carbonyls R R - for R-X-R, X =, N, S R R (- for phenyl).. where CH 2 X is the functional group R. C.. C C. CH 2 (72) 15u (57) (43) 29u shorthand: C. or C 57 43. Which is more likely? Figure 4.43
6. Fragmentations continued a) α-cleavage continued decarbonylation - C (57) 28u CH 2 (29) 29 M C - C C (43) 28u (15) not observed C.. C C. CH 2 (72) 15u (57) (43) 29u or Figure 4.43
6. Fragmentations continued b) benzyl and allyl cleavage - for aromatic compounds - for unsaturated compounds Figure 4.26 R. Figure 4.13 M M. CH 2. C 3 H 7.. CH 2 (134) (91) 43u (70) (41) 29u... highly delocalized, very stable tropylium ion delocalized: allyl cation
b) benzyl cleavage continued 6. Fragmentations continued Figure 4.26 tropylium fragmentation Chapter 4.2D H H M (91) - C 2 H 2 C 3 - C 2 H 2 26u 26u (65) (39) 39 51 65 77 phenyl fragmentation. CH 2. C 3 H 7 (134) (91) 43u... H H - C 2 H 2 - C 2 H 2 C 2 H 26u 26u (77) (51) (25) not observed highly delocalized, very stable tropylium ion
6. Fragmentations continued c) retro-diels-alder cleavage - for 6-membered rings with double bonds M.. (68) 68u or. or is irrelevant: paths are identical (C 5 H 8 ) C CH 2 C CH 2 C CH 2 (136) 68u (68) Figure 4.16
6. Fragmentations continued c) retro-diels-alder cleavage continued M -56-56 M.. (56) 56u. or or is relevant! (192) 136u (136) preferred Figure 4.17
6. Fragmentations continued d) McLafferty rearrangement - for longer chains with double bonds (C= C=C C=N) γ-h shift β-cleavage M -28 X H γ. X H α β α β. -28 M if C=C is part of an aromatic ring, H. H. the aromaticity is destroyed! (102) 28u (74) or Figure 4.48
Chapter 4.5 6. Fragmentations continued e) onium rearrangement - fragmentation not from M - for compounds with X =, N, P, S and charge on X H H oxonium ion C 4 H 8-42 M. H. - C 3 H 6 (102) 15u (87) (45) 42u Figure 4.36