Organic Spectra Mass Spectrometry H. D. Roth THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth Mass Spectrometry Mass spectrometry is an analytical technique (not a spectroscopic technique; no photon is absorbed) based on the determination of the mass-tocharge ratios, m/z, of ions in the gas phase. The experiment has three stages: 1) evaporation 2) ionization 3) mass analysis 1. Introduction of the sample The experiment works only if the sample can be introduced into the apparatus. In many cases (direct probe) evaporation occurs instantly in the heated inlet given the high vacuum of the ionization chamber. Alternatively a leak valve can be used. The output of a GC (GC/MS, vide infra) can be introduced via a membrane or by a jet separator as inlet. High molecularweight or ionic samples can be introduced by “electro-spray”. 1 Organic Spectra Mass Spectrometry H. D. Roth 2. Ionization We will consider three different ways to ionize target molecules: a) ionization by electron impact using 70 MeV electrons neutral molecule diamagnetic even electron (EE) M + e– → M+ + 2e– molecular ion odd electron ion (OE) The molecular ion has a positive charge and one unpaired electron; it is a radical cation, written as M•+ (“M dot plus”). b) ionization by electron attachment neutral molecule M + e– → M – negative ion The ions resulting from electron attachment have a negative charge and one unpaired electron; they are radical anions, written as M•– (“M dot minus”). c1) chemical ionization (conversion to an ion by proton transfer) CH4 + e– → CH4+ + 2e– CH4+ + M → MH+ + •CH3 Chemical ionization is an excellent method to determine the mass of the parent molecule. c2) chemical ionization (conversion to an ion by electron transfer) M1 + e– → M1+ + 2e– M 1+ + M 2 → M 2+ + M 1 In this version of chemical ionization a relatively high-energy molecular ion is generated; electron transfer from a molecule to this species generates the molecular ion of that molecule. 2 Organic Spectra Mass Spectrometry H. D. Roth 3. Mass Analysis For the purpose of mass analysis the positive ions generated by electron impact (2a) or chemical ionization (2c) are accelerated by a series of electric field gradients and forced into a circular path (trajectory) by a magnetic field (remember the effect of magnetic fields on charges); the curvature of the trajectory is determined by the magnetic field strength (variable) and by m/z. Neutral molecules (or neutral fragments formed coincidentally) are not affected by electric or magnetic fields and, therefore, cannot be detected. The analysis of negative ions (method 2b) requires a separate, dedicated spectrometer (Negative Ion MS). Molecules bearing electron withdrawing groups (–CN or –NO2) are good targets for negative ion MS. NC CN NO2 NO2 The resolution (M/ΔM) of a mass spectrometer is the ability to recognize ions of different values of m/z at 10% overlap. High resolution, M/ΔM = 50,000 (e.g., Δm = 0.002, M = 100), allows determination of the exact mass to the 2nd or 3rd decimal and may reveal the exact composition of a sample compound. Exact masses of some nuclei: 1 H = 1.00783 12 C = 12.000 (Standard) 14 16 O = 15.9949 N = 14.0031 Peak of Mass 30 NO 14.0031 H2C=0 2.01566 12.0000 15.9949 15.9949 29.9980 30.01056 The ΔM of 0.0113 requires a resolution of ~8000 to differentiate between these ions, readily accomplished in a high-resolution MS. 3 Organic Spectra Mass Spectrometry Peak of Mass 60 C 2H 8N 2 CH2 NH2 CH2 H. D. Roth NH2 24.0000 8.0635 CH3 28.0062 60.0697 C 3H 8O CH2 CH2 OH 36.0000 8.0635 15.9949 60.0584 The ΔM of 0.01256 requires a resolution of ~9000 to differentiate between these ions, readily accomplished in a high-resolution MS. Other suitable examples include series such as a) C7H14, C6H10O, C5H6O2, C5H10N2; b) NO, H2C=0, H2N2; or c) C2H8N2, C3H8O, C2O2. For this course we will limit our discussion essentially to: a) electron impact MS b) positive ion MS and c) low resolution MS. Fragmentation of molecular ions Many molecular ions, M+ (or M•+), are not stable under the conditions of their formation in a mass spectrometer – they break into smaller fragments by breaking weak bonds; each fragmentation forms one neutral and one positively charged fragment (vide infra). Depending on the nature of the initially ionized species chemical ionization can lead to different fragmentation patterns. Species like CH4+ or CH3OH+ proceed by soft collision, resulting in few fragment ions. In contrast N2+ as the initially ionized species results in hard collisions, producing many fragment ions. 4 Organic Spectra Mass Spectrometry H. D. Roth The peak with the highest m/z is the molecular ion; using electron impact ionization (EI) no ion can have a mass larger than M•+. Due to the very low concentration of ions and molecules in a high vacuum reactions between two ions (ion–ion reactions) or with a neutral molecule (ion– molecule reactions) are highly improbable. The largest peak in the spectrum is called the base peak; it is assigned the intensity 100 (%); the intensity of the other peaks is given as a percentage relative to the base peak. 100 M•+ The abundance of the M•+ peak is determined by the depth of its potential well: molecular ions with a deep well are stable, giving rise to prominent M•+ peak. In contrast, molecular ions with a shallow potential well are unstable, causing the M•+ peak to be small or negligible. Fairly stable less stable prominent M•+ weaker M•+ highly unstable little or no M•+ only F1+ + F2• or F1•+ + F2 5 Organic Spectra Mass Spectrometry H. D. Roth In fact, some compounds have dissociative molecular ions, which fragment rapidly; in such cases the ion with highest mass is not the molecular ion. Substrates whose molecular ions readily dissociate include: a) haloalkanes, especially 3° ones Br •+ + Br + Br• b) acetals O R O R' e– –2e– O R .+ O O + O R R' O F•+ O + R M•+ – mass of R'• c) alcohols, R–OH typically give very poor M•+ peaks; ions of M–18 (loss of H2O) are prominent fragment ions. Criteria for a parent peak (molecular ion) 1. Even mass, unless the ion has an odd number (1, 3 …) of N atoms NH2 NO2 NH2 N NH2 60 123 94 2. The molecular ion peak is accompanied by (M+1) and (M+2) peaks because of isotopes; (M + 1) and (M + 2) peaks can be significant and they are useful for recognizing structural elements). 3. the molecular ion peak is accompanied by fragment ion peaks due to loss of typical small fragments (M–18, loss of H2O indicates R–OH). The role of isotopes in MS (recognizing the presence of isotopes) All mass spectra of carbon-containing compounds have small peaks one mass number [(M+1)•+] higher than the molecular ion peak. These peaks are due to the small fraction of molecules containing one 13C atom. 6 Organic Spectra Mass Spectrometry H. D. Roth Compounds with 1, 2, 3 . . . n carbon atoms have 1.1, 2.2, 3.3 . . . nx1.1% of the (M+1)+ peak. Nitrogen containing compounds have an insignificant contribution from 15N; the probabilities are additive. Only 1 13C is present; (M + 2) peaks due to the presence of 2 13C or 2 15N are negligible (2nd 13C: 0.011 x 0.011). The intensity of the (M + 1) peak (in %) is I(M + 1) = I(M) x (1.1 x #13C)/100 + (0.36 x #15N)/100 The natural abundance of 2H, 15N, 17O, and 18O are too low to be recognized in a mass spectrum; 19F, 31P, and 127I are isotopically pure. However, bromine (50.54 % 79Br, 49.46 % 81Br), chlorine (75.53 % 35Cl, 24.47 % 37Cl) and sulfur (95 % 32S, 5 % 34S) have significant fractions of a second isotope, giving rise to significant and important (M + 2) peaks. Natural Abundances of Key Isotopes 35Cl 76; 37Cl 24; I(M + 2) = 32% M 79Br 51; 81Br 49; I(M + 2) = 96% M 32S 95; 34S 18O 19F 5; 0.2; I(M + 2) = 5% M I(M + 2) = 0.2% M 127I 100; 100. Presence of more than 1 Halogens or S M : (M + 2) : (M + 4) Cl 100 : 63 : 10 Br 52 : 100 : 48 S 100 : 5 : 0.2 (M+2) and (M+4) peaks can be significant and important. Using chlorine as an example: The first halogen forms M•+ : (M+2)•+ 76 : 24 in the ratio of 7 Organic Spectra Mass Spectrometry H. D. Roth Given the 76% of ion M•+, the second halogen will generate M•+ or (M+2) •+ in the ratio of 76x0.76 : 76x0.24 Given the 24% of ion (M+2) •+, the second halogen will generate (M+2)•+ or (M+4)•+ in the ratio of 24x0.76 : 24x0.24, giving total abundances of M•+ : (M+2)•+ : (M+4)•+ = (abuna)2 : 2(abuna)(abunb) : (abunb)2 Notable fragment ion peaks due to loss of small fragments: M•+ –1 M•+ –2 M•+ –15 loss of •H loss of •CH3 leaves EE M•+ –18 loss of H2O leaves OE leaves EE loss of H2 (rarely) leaves OE Mass Spectral Reactions - Fragmentations MS fragmentations can take two pathways: a) radical ion → OE free radical not observed + cation even # of electrons (EE) fragmentation of a radical cation (odd electron, OE) into a radical (OE) plus a cation (EE), where spin and charge are apportioned to separate fragments: M•+ → F1 • + F2 + OE OE EE Examples include the loss of CH3• (or R• in general). For most molecular ions this is the major pathway. b) fragmentation of a radical cation forming a neutral molecule (EE) plus a radical cation (OE): M•+ → OE F3 + F 4 •+ EE OE Examples include loss of H2 (rarely), H2O, CO, CO2, SO2. 8 Organic Spectra Mass Spectrometry H. D. Roth Fragmentation may identify functional groups; for example the molecular ion of butanol yields four major fragment ions (loss of H2O or α cleavage); please note also which fragment ions are NOT formed or are formed in very low yields. + •+ m/z = 43 [–31 - loss of •CH2OH] •+ m/z = 56 [–18 - loss of H2O] base peak OH +CH OH 2 + m/z = 74 m/z = 41 [–33 - loss of •CH3OH2] loss of •CH2OH + H2 m/z = 31 [–43 - loss of •CH2CH2CH3] Factors Determining the Course of MS Fragmentations Which bond(s) will break? Which fragments will be formed? Generally fragmentation at the more highly substituted carbon and/or leading to the most stable fragments will be preferred. 1. Energetic factors a) relative bond strengths (BDE) b) stability of the resulting ions or radical ions c) stability of the resulting radicals or neutrals 2. Kinetic factors a) 1 a) availability of a favorable cyclic transition state Bond Strength C–Cl C–Br C–I 81 68 51 CH3 CH3 CH–Br CH2-CH2-Br CH3 BDE 78 kcal mol-1 CH3—C–Br CH3 68 9 67 Organic Spectra Mass Spectrometry H. D. Roth b) Stability of the resulting ions or radical ions + > + + > increasing ease of fragmentation [The stability of the product ion follows the same trend as the BDE of the neutral molecule – the fragmentation of 3° bromides is very facile.] Thermochemical cycle BDEM A–B A• + ΔI ΔI BDEM•+ A–B• + •B ΔI A+ + • B or A• + B+ Although BDEM may be a good guide, it does not identify BDEM•+ unambiguously Generally, BDEM•+ < BDEM Fast mesolytic cleavages (the term “heterolytic” is not appropriate, because the cleavage yields a cation and a radical, not positive and negative ions) a) benzylic halides + CH2–X . + CH2 + X• EE b) allylic halides + X . + EE + X OE + X• OE . + + 10 Organic Spectra c) Mass Spectrometry H. D. Roth alcohols +• OH R• + C R H +O C H R' R' H OE EE In solution this EE species is accessible by protonation of an aldehyde. d) thiols – quite analogous to alcohols HS R• + C R e) OE H EE ethers R + O R R O R . OE 6 el 8 el O R + CH2 6 el CH2 8 el EE f) C R' R' H H +S •+ ketones +O O O R• C R +C R' C R' R' acylium ion – very stable g) Amines H H N •+ C R R' H H . R +N OE C R' H H EE iminium ion is very stable Summary: M•+ derived from halogen (X–) compounds cleave the C–X bond; M•+ containing O–, S–, N– cleave a bond next to the C–X bond; these reactions form EE ions and OE radicals (not detected). 11 Organic Spectra Mass Spectrometry H. D. Roth Secondary fragmentations of EE species major CH3-CH2-CH2+ CH3+ + CH2=CH2 EE EE minor CH3• + CH2=CH2•+ OE OE solution by 1,2-H shift followed by nucleophilic capture CH3-CH+-CH3 Secondary fragmentations occur mostly via the EE + EE pathway, as shown for ethyl isobutyl ether; they rarely form OE + OE fragments. + O CH2 H3C–H2C – • C3 H7 m/z 59 – C2H4 .+ O CH2 H3C–H2C i-C3H7 + HO CH2 m/z 102 + – • CH3 O CH2 – C4H8 i-C3H7 H2C m/z 59 m/z 87 Elimination reactions MS eliminations of an EE neutral species forming (OE) radical cations are the minor pathway. They occur as α,β-elimination with loss of H2, HCl, HBr, HI, H2O, H2S, CO, CO2, SO2, CH3OH, CH3-COOH. H2C–CH2 loss of + H Cl H-Cl CH3-HC–CH2 + H SH loss of H 2S 12 H2C=CH2 CH3-HC=CH2 Organic Spectra Mass Spectrometry H. D. Roth Some α,ε-eliminations are known CH2 —X loss of (CH)2 n H–X CH2 (CH2 )n CH2 CH2 —H The favored transition state has 5 (heavy) atoms (not counting the H atom) •+ H HO• + CH2=CH2 and/or •+ CH=CH2 H3C H3C Rearrangement of molecular ions Some fragment ions cannot be explained by the simple cleavage of bonds; they result from intramolecular rearrangements, such as migration of H• to (or abstraction of H• by) a heteroatom. McLafferty Rearrangement •+ O H C6H5 • OH + + •+ OH OH2 H • C6H5 C6H5 The McLafferty rearrangements occurs in molecular ions of various substrates, including: a) esters OCH3 OCH3 C C + O . C H C H . O+ H C6 H5 H HO + . H2 C H2 C C6 H5 13 C OCH3 CH C6 H5 Organic Spectra Mass Spectrometry H. D. Roth b) acetates CH 3 C .+ O O H 116 C H5 C2 H CH 3 C + O O .H 116 C H5 C2 H CH 2 +. HO C2 H5 (42) CH 3 (60) C O . + OH H2 C CH 48 C2 H5 . C 74 O O CH 3–C≡O+ 43 C2 H5 (73) c) alkylbenzenes .+ C H . H2 C + H H H C CH 3 H . CH 2 CH 3 + H H C H CH 3 d) phenyl ethers . +O .+ O CH 2 H H H CH 2 The McLafferty rearrangement/cleavage is always accompanied by the direct C–C cleavage forming an acylium ion, CH3–C≡O+ . O H H C6H5 C6H5 Both reactions are directly analogous to photochemical reactions. 14 Organic Spectra Mass Spectrometry H. D. Roth Photochemistry n,π* • . CH3-C =O . . O H H C6H5 C6H5 α-cleavage Norrish Ciamician OH C6H5 1,4-biradical leading to β-cleavage Mass Spectrometry . .+ CH3-C+=O OH+ O H H C6H5 C6H5 . C6H5 McLafferty rearrangement leading to β-cleavage α-cleavage The main difference between the (n,π*) excited state and the molecular ion is an (unpaired) electron in an antibonding (π*) orbital. A photoreaction forms intermediates with two singly occupied orbitals; the MS reaction generates intermediates with one singly-occupied and one empty orbital. LUMO π∗ n SOMO HOMO n,π* state molecule 15 molecular ion Organic Spectra Mass Spectrometry H. D. Roth Expulsion of Stable Neutral Molecules Loss of Alkene – Retro Diels-Alder Cleavage .+ + .+ (extended, conjugated radical cations are more stable); The retro Diels-Alder cleavage occurs in structures containing a cyclohexene unit, including terpenes (e.g., limonene), .+ + . m/z 136 m/z 68 or in norbornene, . + .+ m/z 66 m/z 94 Ionone loses an isobutylene fragment, C4H8 (M•+ – 56) O O CH3 CH3 .+ 8 Cholesterol has 8 chiral centers and 2 stereoisomers. .+ HO obsd: C9H14O HO neutral C17H30 16 Organic Spectra Mass Spectrometry H. D. Roth Some bicyclic cyclohexene systems can undergo a retro Diels-Alder reaction without a reduction of m/z, + .+ . Cyclohexanol molecular ion shows an interesting cleavage-migrationcleavage sequence •+ HO H HO H + . H +O HO H + . CH3 H H . (–43) Cyclopentanone molecular ion shows a related cleavage-migrationcleavage sequence. Expulsion of Ketene a) from acetates .+ O CH2 C O R b) H from enol esters .+ O C O H O=C=CH2 (–42) .+ R–OH O=C=CH2 (–42) CH2 H O CH3 H 17 H .+ Organic Spectra Mass Spectrometry H. D. Roth c) from β-bromo-β-phenylpropionic acid O=C=CH2 O O H Br .+ C6H5 + O C C6H5 H H O O + C6H5 O H + H 18 O C6H5 Organic Spectra Mass Spectrometry H. D. Roth GCMS - a tool for following reactions In a GCMS experiment the eluent of a GC is partitioned so that a small fraction is introduced to the MS through an appropriate interface; mass spectra are taken repeatedly; spectra associated with a particular peak can then be examined. Schematic of the mass selective detection unit Example: conversion of a diketone into a bis-methylene compound by a Wittig reaction CH2 O O O C10H14O2 166 CH2 C11H16O 164 19 CH2 C12H18 162 Organic Spectra Mass Spectrometry H. D. Roth Common Fragment Ions (all EE) Mass 29 Structure + H–C≡O from aldehydes + 30 H2C=NH2 31 H2C=O–H 39 from 1° amines + from 1° alcohols cyclo-C3H3 + from cyclopropenes 41 CH2=CH-CH2 43 CH3-C≡O 47 49/51 59 91 CH2=SH CH2=C–Cl from allyl halides from ketones + from 1° thiols + from 1° chloroalk. + CH3–O-C≡O CH3-O-C-CHNH2 C6H5-CH2 or c-C7H7 105 + + || 88 Source + + from methyl esters O from methyl esters of amino acids from benzyl + compounds + C6H5–C≡O 20 from benzoyl cpds Organic Spectra Mass Spectrometry H. D. Roth Common Fragments Lost (OE or EE) Mass Fragment 1 15 18 H• CH3• H2O 19 26 F• •C≡N 27 28 29 HC≡N C=O or CH2=CH2 C2H5• or HC≡CH 35/37 36/38 41 42 Cl• HCl C3H5• allyl CH2=C=O 49/51 54 CH2=C–Cl• C4H6 59 CH3–O–C=O 79/81 77 78 Br• C6H5• C6H6 21 Organic Spectra Mass Spectrometry H. D. Roth MS SUMMARY • Obtain molecular ion • High resolution MS → exact mass → molecular composition and formula. • Readily recognized fragment ions provide structural information. • Mass differences are important as they identify unobserved neutral fragments • Final confirmation of structure by assigning fragment structures. [Presence of a fragment gives positive information. Failure to observe a fragment is just one more fact one needs to explain.] • Molecular ion (M•+) is largest for straight chains, decreases with increasing MW, and branching. • Cleavage occurs at 4° and 3° carbons (cation stability); largest R• eliminated. • Rings and aromatics favor M•+. • Double bonds and aromatics favor allylic and benzylic cleavages. • Cyclohexene derivatives undergo retro Diels-Alder reactions. • Halogen atoms are lost readily, particularly the high congeners. • C–C bonds next to heteroatoms are cleaved readily. • “Metastable” ions provide insights into fragmentation pathways. 22