Chapter 13 Nuclear Magnetic Resonance Spectroscopy

Infrared Spectroscopy Mass Spectrometry

Nuclear Magnetic Resonance Spectroscopy

Maps the carbon-hydrogen framework NMR Spectroscopy Maps the carbon-hydrogen framework Probes molecular structure in greater detail than IR or MS A complete molecular structure can oftentimes be determined by NMR alone NMR has revolutionized organic chemistry since its introduction in the 1950’s Before the advent of NMR, structure elucidation may have taken months (or years!) 13.1 An Overview of Proton NMR Spectroscopy

NMR Machine

Sample + CDCl3 & (CH3)4Si

The NMR Spectrometer 13.11 The NMR Spectrometer

NMR Spectroscopy Nuclei with an odd # of protons and/or an odd # of neutrons exhibit magnetic properties called “spin” Nuclei with even numbers of BOTH P’s and N’s = no magnetic phenomena 1H and 13C are the most important of the spin active nuclei

Nuclear spin causes the nuclei to behave like a tiny magnets

Nuclear Spin 1H (and 13C) nuclei can have one of two spin states quantum numbers +½ or -½

Protons in a Magnetic Field 13.2 Physical Basis of NMR Spectroscopy

Protons in a Magnetic Field This absorption is nuclear magnetic resonance and is detected by an NMR spectrometer

The 1H NMR Spectrum Absorptions detect the protons in the molecule Separate resonance for each chemically non-equivalent set of nuclei The size (area) of a peak is proportional to the number of contributing protons

Chemical Shift The local magnetic field (Bp) “sensed” by a proton is different than the applied magnetic field (B0) This is due to the electrons around the proton which oppose the external field Bp = B0 – Be The reduction of the local field is called shielding

Chemical Shift Hence, the electronegativity of nearby atoms will affect the shielding around a proton Electronegative neighbors = deshielding H in question “feels” external magnetic field more Electropositive neighbors = increase in shielding H in question “feels” external magnetic field less

Chemical Shift

Chemical Shift and Structure Electronegativities of nearby groups is one of the most important factors in chemical shift

Chemical Shift The amount of alkyl substitution will also affect chemical shift

Chemical Shift A proton near more than one functional group will be affected by both