Mass Spectroscopy &NMR Spectroscopy Big Picture
What information is containedin each type of spectroscopy? O Mass Spectroscopy O Molecular weight from molecular ion peak O Molecular formula from molecular weight, M+1, M+2 peaks O Functional group and hydrocarbon branches from fragmentation patters
What information is containedin each type of spectroscopy? O 13C NMR O Number of signals = types of chemically nonequivalent carbons O Chemical shift O Alkane, alkene, aromatic carbons O Presence (or absence) of bonded electron withdrawing group (electronegative atom)
What information is containedin each type of spectroscopy? O 1H (proton) NMR O Number of signals = types of chemically nonequivalent hydrogens O Chemical shift O Alkane, alkene, aromatic hydrogens O Presence (or absence) of nearby electron withdrawing group (electronegative atom) O Integration – number of hydrogens which give rise to each signal O Splitting patterns – number of hydrogens on adjacent carbons
Mass Spectra: Information from Molecular IonsO Check M+ O Odd number – contains odd number N O Even number – contains combination of C, H, and possibly OO Check M, M+2 peaks O Two peaks of equal intensity Br present O Two peaks, M+2 about 1/3 of M Cl present
Mass Spectra: Molecular Formula DeterminationO Rule of 13 can be used for hydrocarbons (see class notes). Before applying rule of 13 subtract out O If N, Br or Cl present (previous slide), subtract mass of lower molecular weight isotope (halogens) from M peak value O If IR is available, check for presence of O functional groups and subtract O or O2 (carboxylic acid)O Given relative abundance of M, M+1 peaks, use 13C abundance of 1.10% to determine number of C’s in molecule (see class notes)
Mass Spectra: Fragmentation Patterns O Hydrocarbons will fragment at branches and favor more stable fragment (radical) patternsCH3-CH2-CH2-CH2-CH2-CH3 CH3CH2 CH3CH2CH2 CH3CH2CH2CH2
Mass Spectra: Fragmentation Patterns H3C CH2 CH3 HC CH2 CH3 CH3CH2CH2 And (CH3)2CH M-15 (CH3)
Mass Spectra: Fragmentation Patterns CH3H3C C CH3 CH3CH2 CH2 H3C (CH3)3C M-15 (CH3)
13C NMRO How many different signals? O Peak at 77 ppm is CDCl3 solvent O # signals = how many chemically different carbonsO Chemical shift O High C=O (150-220 ppm) O Mid alkene, aromatic, nitrile O Low O 15-60 ppm sp3 C O 20-90 ppm sp3 C bonded to electronegative atom
1 H (Proton) NMRO How many different signals? Gives how many types of chemically nonequivalent H’s
1 H (Proton) NMR Chemical ShiftO High (9-12 ppm) aldehyde or carboxylic acid HO Mid O Aromatic H 6-8.5 ppm O Phenol H 4.5-7.5 ppm O Alkene H 4-6.5 ppmO Low O H on sp3 C 0.9 – 1.8 ppm O H on sp3 C next to C=O or C=C (alkene or aromatic), alkyne H 2-3 ppm O H on sp3 C bonded to halogen, N or O 3.0-4.2 ppm O NH or OH 1-6 ppm
1 H (Proton) NMRO Integration (area under peaks) O Gives relative number of H giving rise to each signal O When done manually, results may be a bit offO Splitting patterns O Split into n+1 peaks, n= number of H on adjacent carbons O More than 4 peaks may be hard to interpret correctly
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