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Mass Spectrometry Only Cations are Detected Ionization to Radical Cation Molecular Ion (m+) Glossary • Molecular ion - The ion obtained by the loss of one electron from the molecule (m+) • Base peak - The most intense peak in the MS, assigned 100% intensity • Radical cation - positively charged species with an odd number of electrons • Fragment ions - Lighter cations (and radical cations) formed by the decomposition of the molecular ion. These often correspond to stable carbcations. • m/z - mass to charge ratio Methanol Octane, m+ = 114 m-71 -71 (base) Base peak -57 -43 -29 -15 CH3CH2CH2CH2CH2CH2CH2CH3 m+ = 114 m-57 m-43 m-29 m+ Isooctane, no molecular ion CH3 CH3 CH3CCH2CHCH3 CH3 m+ = 114 loss of . (isobutyl) CH3 CH3C + CH3 m/z = 57 Decane 2-Methylpentane Effect of Branching in Hydrocarbons Isotopes • Mass spectrometers are capable of separating and detecting individual ions even those that only differ by a single atomic mass unit. • As a result molecules containing different isotopes can be distinguished. • This is most apparent when atoms such as bromine or chlorine are present (79Br : 81Br, intensity 1:1 and 35Cl : 37Cl, intensity 3:1) where peaks at "M" and "M+2" are obtained. • The intensity ratios in the isotope patterns are due to the natural abundance of the isotopes. • "M+1" peaks are seen due the the presence of 13C in the sample. Bromomethane 1-Bromopropane 2-Chloropropane (3-Chloropropyl)benzene CH2 CH2CH2Cl base peak m/e 91 (m-63) Chloroacetone O + m/z = 57 CH3CCH2 . - Cl O CH3CCH2Cl m+ = 92 m+2 = 94 (3:1 ratio) . - CH3 . + O CCH2Cl - CH2Cl O CH3C m/z = 77,79 + m/z = 43 (base) +. O 3-Pentanone + CH2CH3 C CH3CH2 . loss of CH3CH2C=O CH2CH3 m/z = 57 m/z = 86 m-29 base m+ +. O 2-Pentanone + CH2CH2CH3 C CH3 . loss of CH3C=O CH2CH2CH3 m/z = 43 m/z = 86 m-43 base m+ m-15 (E) 2-Hexene 2-methyl-2-pentene and 2-hexene 1-Butanol CH3CH2CH2CH2OH +. - H2O CH3CH2CH m+ = 74 + CH2OH m/z = 56 + CH2=OH m/z = 31 m-18 CH2 + . 1o Alcohols: Loss of H2O CH3 +. . +CH 2 loss of H Toluene + m/z = 92 m/z = 91 tropylium ion m-1 m+ + CH2CH2CH3 . + CH2 . loss of CH2CH3 Propylbenzene + m/z = 120 m/z = 91 m-29 m+ CH3 H3C C Isopropylbenzene H + +. CH3 CHCH3 loss of .CH 3 + m/z = 120 m/z = 105 m-15 m+ n-Butylbenzene m/e 91 CH2CH2CH2CH3 m/e 92 3-Methyl-1-penten-3-ol m/z = 71 OH m+ 4-Methyl-1-penten-3-ol m/z = 57 OH m+ McLafferty Rearrangement link to SDBS +. O H O H m+ = 86 H H +. H transfer from carbon results in loss of a neutral alkene O H +. + H m/z = 44 McLafferty Rearrangements in Alkyl Benzenes CH2 CH2 +. CH2 loss of CH 3CH=CH2 H CHCH3 H H m/e 92 m+ 134 - propyl . CH2 + m/e 91 +. Benzamide 2-Octanone H +. loss of . C6H13 + O O m+ = 128 m/z = 43 +. loss of pentene via McLafferty O H m/z = 58 2-Octanone H +. loss of . C6H13 + O O m+ = 128 m/z = 43 +. loss of pentene via McLafferty O H m/z = 58 Decanoic Acid p-Chloroacetophenone Methyl Octanoate 2,4-Dimethoxyacetophenone Cyclohexanone High Resolution Mass Spectrometry Determination of Molecular Formula CO N2 C2H4 all show m + at 28 CH2N exact mass CO 27.9949 N2 28.0062 C2H4 28.0312 CH2N 28.0187 Isotope Ratios Can Help to Determine Molecular Formula MF CO N2 C2H4 MW 28.0 28.0 28.0 Relative intensities (%) M M+1 M+2 100 1.12 0.2 100 0.76 ---100 2.23 0.01 Comparisons of Molecular Weights and Precise Masses MF C3H8O C2H8N2 C2H4O2 CH4N2O MW 60.1 60.1 60.1 60.1 exact mass 60.05754 60.06884 60.02112 60.03242 Determine the Formula fragment finder rel. intensity (%) Molecular mass m+1 m+2 110 111 112 100 6.96 0.60 exact mass = 110.0376 Determine the Formula rel. intensity (%) Molecular mass m+1 m+2 118 119 120 100 7.45 4.55 Subtract Sulfur’s contribution fragment finder rel. intensity (%) subtract sulfur (32) Molecular mass m+1 m+2 118 119 120 100 7.45 4.55 86 100 87 88 6.67 0.15 Determine the Molecular Formula rel. intensity (%) Molecular mass m+1 m+2 154 155 156 100 15.41 3.77 Compound gives four signals in the C-13 NMR spectrum rel. intensity (%) Molecular mass m+1 m+2 m+4 190 191 192 194 100 6.48 130.77 31.81