LECTURE 16 ESTABLISHING AND USING MOLECULAR FORMULAS

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LECTURE 16 ESTABLISHING AND USING MOLECULAR
FORMULAS
MOLECULAR IONS INSIGHTS
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M ION ANALYSIS
M 1026.5
WHAT BEST FORMULA?
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M ION ANALYSIS
M 1026.5
WHAT BEST FORMULA?
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M ION ANALYSIS
M 1026.5
WHAT BEST FORMULA?
C53H86O19 J. Am. Chem. Soc. 2003, 125, 5296
C53H83O17Cl Org. Lett. 2004, 6, 2607.
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Spirasterellolide
C53H83O17Cl
Original Assignment
Revised Structure
HRCIMS m/z 1027.5805, D -3.8 mmu MF C58H86O19
(as methylspirasterellolide) (J. Am. Chem. Soc.
2003, 125, 5296)
1 C53H78 from DEPT experiment 2 LRESTOFMS m/z
1049 MNa (MeOH/CH2Cl2) 3 LRESTOFMS m/z 1054
MNa (MeOD) 4 ESHRTOFMS m/z 1049.52026
MNa (D -0.94 mmu) MF C58H83ClO17 (as
methylspirasterellolide) (Org. Lett. 2004, 6,
2607)
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C53H83O17Cl
5 exchangable protons
measured simulated
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EI-MS FOUR COMPOUNDS UNCOMPLICATED SPECTRA
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EI-MS FOUR COMPOUNDS UNCOMPLICATED SPECTRA
C2H2
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EI-MS FOUR COMPOUNDS UNCOMPLICATED SPECTRA
C2H2
HCN
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EI-MS FOUR COMPOUNDS UNCOMPLICATED SPECTRA
C2H2
HCN
C6H12O
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EI-MS FOUR COMPOUNDS UNCOMPLICATED SPECTRA
C2H2
HCN
C6H12O
C6H13N
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STRATEGIES TO ESTABLISH MOLECULAR FORMULA BY
EI-MS

• FIND M. PEAK
• DETERMINE MF FROM HIGHEST m/z PEAK
• Evaluate M. cluster peak ratios
• FIND BASE PEAK NOTE ODD/EVEN
• ANALYZE FOR HETERO ATOMS

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P257
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FIND MF FOR M/Z 320/322/324
320 322 324
Open book to pg 257
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MASS SPEC CALCULATOR ESTABLISHING THE MOLECULAR
FORMULA
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ISOTOPE CLUSTERS
MW 476 USE MOLECULAR ION CLUSTER (M1)/M ?
(1.1)(Cs) OBSERVE RATIO (M1)/M 24 ?
Csmax 22 USE MS CALC ? 41 FORMULAS TWO
CANDIDATES C16H32N18 C15H16N20
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M1 PEAK CAN BE SIZEABLE
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• C15 MAX (1.1 rule)
• MASS SPEC CALC 7 FORMULAS
• ATTRACTIVE FORMs C13H2209
• BUT M2 6 ?S1 (M2 4.4)
• with S1 10 formulas
• F1 (an inspired
  guess!)
• with F1 47 formulas

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• C15 MAX (1.1 rule)
• MASS SPEC CALC 7 FORMULAS
• ATTRACTIVE FORMs C13H2209
• BUT M2 6 ?S1 (M2 4.4)
• with S1 10 formulas
• F1 (an inspired
  guess!)
• with F1 47 formulas
• ISOTOPE CALC 100/16/6 C14H8F6S

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RULE OF 13
P239
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ESTIMATING MASS DEFECTS

• POSITIVE
• 1H 1.00783 ? high mass if large H count
• 14N 14.0031 ? not relevant unless count is big
• NEGATIVE
• 16O 15.9994
• 32S 31.9721
• CONCLUSIONS
• AT 0.1 amu ACCURATE MASS DEFECT MEASURE
• USEFUL AT 1kD

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ESTIMATING MASS DEFECTS
P240
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ESTIMATING MASS DEFECTS
P240
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COMBINE MS NMR
CASE WHERE m/z 154
P241
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CANDIDATE FORMULAS
P241
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56 CANDIDATES
.1344
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.1344
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3(CH3) 3(CH2) 2(CH) 2C C10H17 ?73
OH ? C10H17OH C10H18O
P241
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P241
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SUMMARY

• First inspection of EI-MS - use odd-even rule.
• All isotopes are simultaneously observed hence
  Cn x 1.1 is useful for peak height analysis.
• Estimating mass defect can be useful (see example
  Table 6.3).
• Using the rule of 13 or a MS-calculator is
  essential to consider all possible MF candidates.
  
• Get minimum C from 13C NMR
• Get minimum H from 13C NMR
• Get minimum H from 1H NMR.