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Unformatted text preview: Textbook Assignment: Chapter 14- Mass Spec Homework (for credit): POW 2 posted (due 2/8) Today’s Topics: 13C NMR-intro and interpretation; Mass Spec Intro 13C-NMR Spectroscopy • Each nonequivalent 13C gives a different signal – a 13C signal is split by the 1H bonded to it according to the (n + 1) rule – coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine, so: • The most common mode of operation of a 13C-NMR spectrometer is a hydrogendecoupled mode, which results in singlets. • In a hydrogen-decoupled mode, a sample is irradiated with two different radio frequencies – one to excite all 13C nuclei – a second broad spectrum of frequencies to cause all hydrogens in the molecule to undergo rapid transitions between their nuclear spin states. • On the time scale of a 13C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled. – hydrogen-decoupled 13C-NMR spectrum of 1-bromobutane – hydrogen-decoupled 13C-NMR spectrum of Ethyl Acetate O O – hydrogen-decoupled 13C-NMR spectrum of Ethyl Benzene 2 peaks Chemical Shift - 13C-NMR Typ e of Carbon RCH3 RCH2 R R3 CH RCH2 I RCH2 Br RCH2 Cl R3 COH R3 COR RC CR R2 C=CR2 Chemical S hift (δ) 10-40 15-55 20-60 0-40 25-65 35-80 40-80 40-80 65-85 100-150 Type of Carb on CR O RCOR O RCNR2 O RCCOH O O RCH, RCR Chemical Sh ift (δ) 110-160 160 - 180 165 - 180 165 - 185 180 - 215 Chemical Shift - 13C-NMR Interpreting NMR Spectra • Alkanes – 1H-NMR signals appear in the range of δ 0.8-1.7...
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