2444-083111 - Everything is on-line...

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Everything is on-line http://orgchem.chem.uconn.edu/courses/testhome.html • old exams and quizzes • syllabus • all answer keys will be posted here To add the class - see me after class for a permission number Office: A-416 Phone: 486-2881 [email protected] Office hours: 10-1, MWF THIS IS THIS IS NOT NOT ON HUSKY-CT ON HUSKY-CT
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When a molecule is placed in the mass spectrometer: Ejection of an electron gives a radical cation The molecular ion (parent ion) H 3 C O CH 3 •• •• H 3 C O CH 3 •• H 3 C O CH 3 + mass = 58 mass = 58 - 1e - Molecular ion or Parent ion 70 eV High energy Mass of acetone – 1 e H 3 C O CH 3 •• •• H 3 C O CH 3 + mass = 58 mass = 58 - 1e - 70 eV H 3 C O CH 3 + + 1 2 3 mass = 43 mass = 15 Molecular ion fragments into daughter ions Ion Fragmentation: Daughter Ions
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m/z Abundance m 3 m 2 m 1 m=0 m=100 Separation of the ions, and detection leads to the mass spectrum Abundance is the amount of each ion detected, relative to the ion detected in greatest abundance (base ion = B) m/ z = mass/charge mass of each ion divided by the charge of that ion. If the charge is +1, then m/z= mass of the ion Mass Spectrum
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Mass spectrum for the ionization of acetone 0 10 20 30 40 50 60 70 m/z Relative Abundance H 3 C O CH 3 + H 3 C O + + CH 3 M – 15 (loss of methyl) It is difficult to identify the ions a priori , but we can take the parent ion (M or P) and look for the mass of lost fragments. Loss of 15 mass units, for example, corresponds to loss of a methyl group from the parent. MS of Acetone 58 43
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High Resolution Mass Spectrometry 5 • Detection of radical cation fragments in modern mass spectrometers has been refined to the point where it is possible to determine the mass of a fragment to 4-5 decimal places. • A molecular ion may be determined to 100.1251, or better. • With this level of accuracy, a molecular formula may be obtained directly from the molecular ion. • The values in the periodic table are the masses of the elements as found in Nature. • However, carbon in Nature is not just carbon 12 ( 12 C), but a mixture of three isotopes, 12 C, 13 C, and 14 C. • Since the mass spectrometer separates ions by mass, fragments containing different isotopes may be detected.
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High Resolution Mass Spectrometry 6 Isotopic abundance of common isotopes and Isotopic Masses 12 C 13 C 1 H 2 H 14 N 15 N 16 O 17 O 18 O 32 S 33 S 34 S 35 Cl 37 Cl 79 Br 81 Br I s o t o p e R e l a t i v e A b u n d a n c e ( % ) 100 1.1 100 0.106 100 0.38 100 0.14 0.20 100 0.78 4.40 100 32.5 100 98.0 I s o t o p e M a s s 12.00000 13.00336 1.00782 2.01410 14.00307 15.00010 15.99491 16.99913 17.99916 31.97207 32.9716 33.96786 34.96885 36.96590 78.91839 80.91642
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High Resolution Mass Spectrometry 7 • High resolution mass spectrometry provides a high level of accuracy for the molecular ion, but how is this translated to a formula?
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