Determining The Molecular Formula

  1. Molecular Ion. Using mass spectrometry to determine the molecular ion allows us to identify possible molecular formulas. In the 70 eV EI spectrum, the highest m/z ion appears to be m/z 154. In the 13 eV EI spectrum, less fragmentation is observed because the ionization energy is much lower. The m/z 154 ion is significantly enhanced in this spectrum. This is is consistent with this being the molecular ion. And finally from the Chemical Ionization spectrum, the m/z 155 peak is consistent as an M+H adduct ion. The higher mass ions observed at m/z 171 and 205 could also be adducts of the molecular ion, so there may be some question about the identity of the molecuar ion. From the MS data it appears that the compound has a molecular weight of 154. Possible combinations of elements may be determined using computer programs or using tables available in several textbooks. Including:
    1. McLafferty, F.; Turecek, F. Interpretation of Mass Spectra;, 4th ed,; University Science: Mull Valey, CA, 1993; pp 355-359. Table A.7 Common elemental compositions of molecular ions.. This table lists the following possible molecular ions for m/z 154; C10H18O, C8H7ClO, C2ClF5, C8H10OS. Find a copy of the book and look at the table.

    2. Silverstein, R.; Webster, F. Spectrometric Identification of Organic Compounds; Wiley: New York, 1998; pp 45-65. Chapter 2, Appendix A. Formula Masses (FM) for Various Combinatinos of Carbon, Hydrogen, Nitrogen and Oxygen. This table is much more exhaustive, listing 23 combinations of C, H, N and O.

    3. The program MolForm lists 39 possiblities.

  2. Isotope abundance At this point it would be useful to reduce the number of possible formulas. One way to do this is by looking at the intensity of the isotope peaks in the mass spectrum. The intensity of the M+1 peak in an electron ionization experiment may be used to determine the abundance of 13C in the compound. From this, it is possible to determine the number of carbon atoms in the molecule. In the 70 eV EI spectrum the intensities are:
    154 3.904606416
    155 0.580805704
    So the intensity of the M+1 peak is 14.9%

    In the 13 eV EI mass spectrum the intensities are:
    154 78.30699837
    155 11.48840397
    So the intensity of the M+1 peak is 14.7%

    Since the natural abundance of 13C is 1.1%. To a first approximation the intensity of the M+1 peak is 1.1% per C in the molecule. This would give 13.3 C's in the molecule. Rounded to 13. Alternatively, we can use table 2.2 in McLafferty, which lists the relative abundance of isotopic peaks for carbon. With C13 the abundance would be 14.3%. With C14 it would be 15.4%. This is consitant with the compound containing 13 C's. However, this is obviously not consistent with a molecular ion at m/z 154. This piece of information is not consistant.

  3. MS Fragments Major fragments observed in the mass spectrum include:
    1. 139 - Loss of CH3
    2. 136 - Loss of H2O
    3. 121 - Loss of CH3 and H2O
    4. 111 - Loss of C3H7
    5. 98 - Loss of 56
    6. 84 - Loss of 70
    We'll come back to these to verify the final structure.

  4. Exact Mass. Using a high resolution mass spectrometer, the exact mass of the molecular ion is found to be 154.1331 m/z. Based upon the tables in Silverstein, and an uncertainty of +/- 0.005 m/z there is one possible formula (Take a look at this book). The programs MolForm, Elemental, and the calculator at the SIS Web site all return the same formula for m/z 154.1331 +/- 0.005: C10H18O (154.135765 ).

  5. Combustion Analysis. This verifies the high resolution mass spectrometry results. You should be able to verify our results by calculating the percent elemental composition.
    1. C = 78%
    2. H = 12%

  6. C-13 NMR. The C-13 spectrum (discussed latter) the peak at 202.7 is a strong indication of a carbonyl group (probably aldehyde) so there is a minimum of 1 oxygen atom.
  7. FTIR. The FTIR spectrum (discussed latter) shows a strong peak at 1722 cm-1. This is consistent with an aldehyde.
  8. C10H18O. All this information is consistent with the molecular formula C10H18O .