Nuclear Magnetic Resonance Spectroscopy Dr. Sheppard Chemistry 2412L

Introduction  NMR is the most powerful technique for organic structure determination  Number and type of atoms in a molecule  Connectivity of atoms  Used to study a wide variety of nuclei:  1 H  13 C  15 N, 19 F, 31 P  Radio-frequency radiation used to transition between energy states  30 – 900 MHz  Transition = nuclear spin

Nuclear Spin  A nucleus with an odd atomic number or an odd mass number has a nuclear spin  The spinning charged nucleus generates a magnetic field

External Magnetic Field  When placed in an external field, spinning nuclei act like bar magnets

Two Energy States  The magnetic fields of the spinning nuclei will align either with the external field, or against the field  A photon with the right amount of energy can be absorbed and cause the spinning nucleus to flip  Spin flip = resonance  Detected and recorded by the spectrometer as a signal

The NMR Spectrometer

Magnetic Shielding  If all nuclei absorbed the same amount of energy in a given magnetic field, not much information could be obtained  But nuclei are surrounded by electrons that shield them from the external field  Circulating electrons create an induced magnetic field that opposes the external magnetic field  Effective magnetic field

Shielded Nuclei  Magnetic field strength must be increased for a shielded nucleus to flip at the same frequency  Differences detected by machine, cause differences in signals (chemical shift,  )

Nuclei in a Molecule  Depending on their chemical environment, atoms in a molecule are shielded by different amounts  Chemically equivalent nuclei  Interchanged through bond rotation or element of symmetry  Have same absorption  Chemically different nuclei have different absorption

1 H-NMR Spectrum for Methanol

Tetramethylsilane  TMS is added to the sample  Since silicon is less electronegative than carbon, TMS protons are highly shielded  Signal defined as zero  Organic protons absorb downfield (to the left) of the TMS signal  Deuterated solvent signal

Chemical Shift  Measured in parts per million  Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz)  Same value for 60, 100, or 300 MHz machine  Called the delta (  ) scale

Delta Scale downfieldupfield

Location of Signals  More electronegative atoms deshield more and give larger shift values (downfield)  Effect decreases with distance  Additional electronegative atoms cause increase in chemical shift

Hydrogen and Carbon Chemical Shifts

NMR Spectra

13 C-NMR  12 C has no magnetic spin  13 C has a magnetic spin, but is only 1% of the carbon in a sample  Signals are weak, get lost in noise  Hundreds of spectra are taken, averaged  Signal = one sharp line for each different type of carbon

3-Pentanone  How many signals?  Chemical shifts:  sp 3 C upfield  sp, sp 2 C downfield  C adjacent to en atom downfield

2-Butanone  How many signals?  Chemical shifts?

How is 13 C-NMR useful for reactions we have studied?  Zaitsev vs. non-Zaitsev 7 signals5 signals

Interpreting 13 C-NMR  The number of different signals indicates the number of different kinds of carbon  The chemical shift indicates the functional group  Use to support 1 H-NMR analysis

1 H-NMR  More info than 13 C-NMR  The number of signals shows how many different kinds of protons are present  The location of the signals shows how shielded or deshielded the proton is  The intensity of the signal shows the number of protons of that type  Signal splitting shows the number of protons on adjacent atoms

1 H-NMR  Given a structure, how many signals are expected?  How many sets of H in each molecule? Isomers Same molecular formula Same IR stretches Different NMR

Another example:

Chemical shifts in 1 H-NMR  Info about type of H giving rise to signal  Strongly shielded = upfield (to the right)  Less shielded = downfield (to the left)  Most common shifts:  McMurry, Table 13-3

Typical Values

O-H and N-H Signals  Chemical shift depends on concentration  Hydrogen bonding in concentrated solutions deshield the protons, so signal is around  3.5 for N-H and  4.5 for O-H

Using chemical shifts  Given a structure, predict   Use to distinguish between two structures  Example:  Constitutional isomers  Each with 2 sets of H’s

Which isomer best fits this spectrum? or

Which isomer best fits this spectrum? or

Intensity of Signals  The area under each peak is proportional to the number of protons  Shown by integration line  Height  area under peak  # H’s in set  Measure height with ruler or look at graph paper  Ratio of height = ratio of hydrogens

So far…  Determine the number of sets of equivalent hydrogen atoms  Number of signals on spectrum  Determine the number of hydrogen atoms in each set  Integration line  Determine general information about adjacent groups  Chemical shift (  )

Next…  Determine specific information about adjacent groups  In particular, how many H atoms on the adjacent atoms  Signal splitting