1Lecture I-3:The Molecular Ion (McLafferty Chapter 3)CU- Boulder CHEM 5181Mass Spectrometry & ChromatographyProf. Jose-Luis JimenezFall 20072Issues for Start of Class• HW#5 due today• HW#4 graded– Better grades for wrong molecule, right arguments– Always explain WHYyou are concluding something• Schedule adjustments– See updated handout, hopefully no more changes•2ndMidterm on 13-Nov (interpretation)• Progress on projects?– Everything due Nov. 15– Summary due today3Clicker Question• With an error level of +/-15% or 1% absolute, whichever is greater• If [A+2]/[A] for the molecular ion is 2.3%A. The molecule cannot contain Cl and Br, but can contain S or SiB. The molecule cannot contain Cl, Br, and S, but can contain SiC. The molecule can contain 21 carbonsD. The molecule can contain 55 carbonsE. I don’t remember that stuff any more…4Standard Interpretation Procedure2. Using isotopic abundances (where possible) deduce the elemental composition of each peak in the spectrum; calculate rings plus double bonds (last day).3. Test molecular ion identity; must be the highest mass peak in spectrum, odd-electron ion, and give logical neutral losses. Check with CI or other soft ionization (TODAY).4. Mark ‘important’ ions: odd-electron and those of highest abundance, highest mass, and/or highest in a group of peaks (TODAY).5The Molecular Ion• Most valuable info of the mass spectrum– Molecular mass– Elemental composition–Fragmentsmust be consistent with it• Not always stable with EI– Careful about overinterpreting peak of highest m/z!–Usesoft-ionization such as CI in parallel• MS Definition:– m/z of the molecular ion is the peak that contains the most abundant isotope of all the elements involved (by convention)• Won’t always be most abundant peak?6Requirements for the Molecular Ion• Necessary but not sufficient conditions– It must be the ion of highest mass (isotope caveat) –It must be anodd-electron ion (for EI)– It must be capable of yielding the most important ions in the high-mass region by loss of logical neutral species• If candidate fails either test, cannot be MI• If candidate passes all tests, may or may not be MI7Odd-Electron Ions• For EI, molecule becomes ionized by loosing one electron– Must have an unpaired electron (so it’s a radical)McLafferty, Fig. 3.1-3.38Even Electron Ions• Even-electron ions:– All electrons on the outer shell are fully paired– Generally more stable– Often the more abundant fragment ionsCH4+⋅→ CH3+ + H⋅– Most ESI, CI (e.g. H+transfer) give even electron ions such as MH+, resulting in lower fragmentation9More on Odd- & Even-Electron Ions• If you can establish the elemental composition of the molecular ion, the rings-plus-double-bonds rule will show whether ion is odd or even-electron:– Even: integer + 1/2 RPDB– Odd: integer RPDB–CxHyNzOn:• Even or Odd?–C5H5N+–C7H5O+–H3O+RPDB = x – 1/2 y + 1/2 z + 110The Nitrogen Rule I• For most elements in organic compounds, there is a relationship between mass of the most abundant isotope and the valence– Both odd or both even– N is the exception11The Nitrogen Rule II• ‘Nitrogen Rule’: If a compound contains no (or even number of) N atoms, its molecular ion will be at an even mass numberAmmonia NH317Pyridine C5H5N7912The Nitrogen Rule III• N-Rule applies to all ions–Anodd-electron ion will be at an even mass number if it contains an even number of nitrogen atoms–An even-electron ion containing an even number ofnitrogen atoms will appear at an odd mass number13Nitrogen Rule Practice• Which of the following are OE+.and EE+?• Which have odd and even mass? Does it agree with N-Rule?–C2H4–C3H7O–C4H9N–C3H9–C4H8NO–C7H15ClBr–C3F10–C29F29–C3H9SiO14Which ions are odd vs. even?15Relative Importance of Peaks•OE +⋅have special mechanistic significance– Mark all important OE +⋅ions directly on the spectrum• Importance increases with–Intensity– m/z– Mass in the peak group– What about isotopic peaks (e.g. 13C)?16Clicker Question 1• Choose one of the following answersA. C7H12ClBr+is an odd electron ionB. H2O-is an odd electron ionC. C10H21+is an odd electron ionD. A, B, and CE. A and B17Imp. OE +⋅ions at low m/z: unlikely!• Important OE +⋅ions are even less likely at low m/z– Intense even mass peaks in that region usually have an odd-number of N10 20 30 40 50 60 70 80 90 100 11005010015 18304456587286101N18Corollary to the Nitrogen Rule• A scarcity of important even-mass ions, especially at lower m/z, indicates an even mass-molecular weight– The reverse is not always true!NeopentaneC5H12 : 72 amuMcLafferty, 1993. Fig. 3.119Logical Neutral Losses I• Only a certain number of low mass neutral fragments are commonly lost in decompositions of molecular ions• Small neutral fragments lost from the molecular ion are commonly those attached by a single bond– Mass losses of 4 to 14 and 21 to 25 that give important peaks are highly unlikely - WHY?20Logical Neutral Losses II• The presence of an ‘important’ ion separated from the highest mass ion by an anomalous mass or elemental formula will indicate that the latter ion is not the molecular ion!– E.g., if there is an abundant ion 5 mass units below the ion of highest m/z, can that be the molecular ion?– Last example?• Can the ion of highest mass (first) be the molecular ion if the following are the major ions of high mass?–C10H15O, C10H14O, C9H12O, C10H13, C8H10O –C10H14, C10H13, C9H11, C8H9, C7H8, C7H721Unknown 3.4•Elemental composition– A+2, A+1, O, H, A• Molecule?•RPDB?• Even/Odd e- ions?– Molec. ion?•N-Rule?• Neutral losses?22Unknown 3.5• Elemental composition– A+2, A+1, O, H, A• Molecule?•RPDB?• Even/Odd e- ions?– Molec. ion?•N-Rule?• Neutral losses?23Molecular Ion Abundance I• Abundance of molecular ion depends on:–Its stability (often not in spectrum)– The amount of energy needed to ionize the molecule• There is a correlation between those properties and the structure of the molecule–The magnitude of M+⋅provides an indication about the structure of the molecule24Molecular Ion Abundance II• In general the chemical stability of M+⋅parallels the stability of the molecule –M+⋅increases with ‘un-saturation’ and rings–M+⋅decreases with chain branching– Effect of MW is not as clear25Molecular Ion Abundance III• If less energy is needed to