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Unformatted text preview: Spectroscopy 1 Copyright, Arizona State University Copyright, Arizona State University Spectroscopy Molecular Structure Determination (more...) Instrument-based methods for determination of structure of organic molecules 1) Mass Spectrometry - yields molecular weight/elements/possible molecular formulas 2) Infrared Spectroscopy - yields functional groups 3) NMR Spectroscopy - very important, yields structure 1 Mass Spectrometry Gives molecular weight, some elemental information, possible molecular formulas (and degrees of unsaturation) schematic overview of the spectrometer... variable magnet heated sample accelerating electrode ion detector slit ionizing beam heavy ions light ions Example: Butane H 3 CCH 2 CH 2 CH 3 vaporize and ionize H 3 CCH 2 CH 2 CH 3 radical cation: mass = 58 butane: MW = 58 H 3 CCH 2 CH 2 CH 3 cation: 29 H 3 CCH 2 CH 2 CH 3 radical cation + + + + CH 2 CH 2 CH 3 H 3 C cation: 43 cation: mass = 15 fragmentation various positively charged ions are formed in the mass spectrometer the ion corresponding to the structure of interest, the "molecular ion", is a radical cation, and its mass is equal to the molecular weight of the structure the parent radical cation will also fragment to form cations of lower mass radical cation and cations are deflected in magnetic field and the ion mass is "selected" by variable magnet An Idealized mass spectrum of butane... ion current (relative # of ions) mass/charge = m/z ~ weight 58 15 29 43 "molecular ion" peak (not fragmented) mass spectrum is plot of ion current (i.e. # of ions) versus mass/charge ratio (m/z) because the charge is almost always positive one, m/z is basically the same as mass largest m/z value corresponds to "molecular ion", i.e. the unfragmented molecule - molecular weight molecular ion peak not necessarily the most "abundant" ("tallest"), but has largest m/z Spectroscopy 2 Copyright, Arizona State University 1.1 Important Information from Molecular Ion Peaks Example of : A compound containing C, H and O only (the molecular ion mass is an even number) m/z M + 1 peak at MW + 1, ca. 10% intensity O molecular ion peak @ e v e n number (74) MW = 74 M+1 peak due to molecules containing D, 13 C, etc. an even numbered molecular weight means that the molecular contains carbon and hydrogen, and possibly (but not necessarily) oxygen. There is no specific "signature" for oxygen in mass spectrometry Example of: A compound containing Br has TWO molecular ions, one for the molecules that contain 79Br, and one for the molecules that contain 81Br these two molecular ion peaks are separated by 2 mass units and have ca. 1:1 size ratio Br 81 MW = 138 (Br = 81) (equal size) Br atomic "weight" = 80 Br is ~ 50 :50 mixture of Br 79 and Br 81 m/z Br 79 MW = 136 (Br = 79) Example of: A compound containing Cl has TWO molecular ions, one for the molecules that contain 35Cl, and one for the molecules that contain 37Cl...
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