1 CHAPTER 13 Molecular Structure by Nuclear Magnetic Resonance (NMR)

2 Nuclear Magnetic Resonance Spectroscopy exploits a property called Nuclear Spin Any atomic nucleus that has an odd mass, an odd atomic number, or both is a nucleus that behaves like it is spinning. Just like for electrons, The 1 H nucleus has two spin states which in a magnetic field can have two different energies.

3 Nuclei that possess Spin Angular Momentum have quantized states whose energies are affected by an Applied Magnetic Field

4 The stronger the Applied Magnetic Field the larger the energy difference between the Spin States of the Nucleus 14,000 G (1.4 T) 60 MHz 70,500 G (7.0 T)300 MHz The actual energy difference is small. At 300 MHz, the energy difference for a proton is about 3 x kcal mol -1.

When the protons are placed in a homogenous magnetic field and a pulse of the correct energy is applied, the small excess of nuclei with the lower energy spin state absorb energy and the change in spin state is detected by the spectrometer 5 Image from www2.chemistry.msu.eduwww2.chemistry.msu.edu

6 When electromagnetic radiation having the same energy as the energy difference between the spin states strikes the nucleus, the electromagnetic radiation is absorbed and the slight excess of nuclei in the  state is reduced.

7 Figure 14.1 NMR Schematic

8 Proton nuclei in different electronic environments show different resonant frequencies. The resonant frequencies are plotted as relative frequencies called “chemical shifts” .

9 CHEMICAL SHIFT (  ): CHEMICAL SHIFT (  ): the shift in ppm of an NMR signal from the signal of TMS   = ( A – TMS ) ÷ Operating frequency in MHz Tetramethyl silane (TMS)  0.9

10 Why do different electronic environments lead to different signals? Shift to the RIGHT = “UPFIELD” = smaller E felt by nucleus = more shielded Shift to the LEFT = “DOWNFIELD” = larger E felt by nucleus= deshielded Lower freq = upfield e – density near the nucleus decreases the  E between its spin states so its resonant frequency decreases

e- SHEILD NUCLEI FROM EXTERNAL MAGNETIC FIELD Shielding: e – density near the nucleus decreases the  E between its spin states so its resonant frequency decreases 11

12 Features of 1 H NMR Spectrum: 1.Position of Signals on x-axis: Relative frequency of absorption: CHEMICAL SHIFT Info about electronic environment 2. Number of Signals Magnetic EQUIVALENCE Info about number of unique electronic environments (symmetry) 3. Size (Area) of signals: INTEGRATION Info about number of nuclei in each environment 4. Splitting Pattern of Signals Info about number of neighboring nuclei

13 Chemical Shift: Characteristic of neighboring Functional groups (e- shielding)

14 The 9 protons on the three methyl groups are all in the same electronic environment on average, but the methylene and hydroxyl protons are unique.

15 For a compound with the molecular formula C 4 H 10, how many isomers are there? How many chemically distinct environments are there in each case? Features of NMR Spectrum: Number of unique electronic environments ≤ number of chemically distinct proton sites on the molecule. How many distinct environments are there for the protons on each of the following molecules? 3 signals5 signals4 signals

The number of distinct NMR signals may alone allow one to determine the chemical structure given the molecular formula. Draw all isomers of C 2 H 6 O and tell how many unique electronic environments each has 16 3.3 1.2 3.6 4.7 Draw all isomers of C 4 H 10, and tell how many unique electronic environments are there in each case?

Both isomers of C 4 H 10 have two signals, but their relative intensities and shapes differ 17

18 Features of NMR Spectrum: Relative Size/Area of Signals AREA under an NMR peak is proportional to the number of equivalent nuclei contributing to the peak. Electronic integration gives relative number of H’s only. Use Molecular formula to detn absolute number of H’s responsible for each signal: Total #H A = (area A /area Tot ) (Total #H’s in formula) Total # H= 12 # H’s at 2.0 =[23/(23+67)]12 = 3

Features of NMR Spectrum: SHAPE of signals Multiplicity (#peaks to which each signal is split) depends on the number of H’s at adjacent carbon(s) Multiplicity = (#neighbors + 1) If multiplicity =2; # neighbors = 1 multiplicity =3, # neighbors =2 19

20 Features of an NMR spectrum: SHAPE Spin Coupling: Neighboring nuclei “split” NMR signals Usually n neighbors splits the signal into n+1 peaks Multiplicity = n+1 Which Isomer of C 5 H 10 O gives rise to this spectrum? Quartet means the nuclei responsible for this signal have 3 H’s “next door” Triplet => 2 neighbors