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KOT 222 ORGANIC CHEMISTRY II CHAPTER 13 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Part I 1 NMR Spectroscopy ¾ The most powerful tool for organic structure determination. ¾ Small sample is needed and it does not harm the sample. ¾ Being used to study a wide variety of nuclei: 1H 13C 15N 19F 31P 2 Nuclear Spin ¾ A nucleus with an odd atomic number or an odd mass number has a nuclear spin. ¾ It has spin number (I) ≠ 0 and can absorb/emit electromagnetic radiation (RF for NMR). 3 Proton: ¾ The simplest nucleus with odd atomic number of 1. ¾ The nuclear spin of a proton will generates a small magnetic field called the magnetic moment (B). 4 ¾ In the absence of external magnetic field, proton magnetic moments have random orientations. ¾ When an external magnetic field is applied, each proton in a sample assumes the alpha or the beta-spin state. 5 ΔE and Magnet Strength • Energy difference is proportional to the magnetic field strength. • ΔE = hν = γ h B0 2π • Gyromagnetic ratio, γ, is a constant for each nucleus (26,753 s-1gauss-1 for H). • The strength of the magnetic field (B0) is proportional to the operating frequency (MHz). • In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. • For now, proton resonance frequencies occur in the radio-frequency (RF) region. 6 NMR Absorption ¾ A nucleus is “in resonance” when it is irradiated with RF photons having energy equal to the energy difference between the spin states. ¾ A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. The nuclei are said to be in resonance with the RF frequency. 7 Magnetic Shielding ¾ If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. ⇒ this happens to the naked protons. ¾ In organic compounds, the protons are surrounded by electrons that shield them from external magnetic field. ¾ Circulating electrons create an induced magnetic field that opposes the external magnetic field. Beffective = Bexternal - Bshielding 8 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. 9 Protons in a Molecules ¾ Depending on their chemical environment, protons in a molecule are shielded by different amounts. ¾ The presence of electronegativity atom will withdraw electron from the proton, hence it is deshielded. 10 NMR Spectrometer Emit a precise frequency The magnet with a sensitive controller produce a precise magnetic field. Measure the sample’s absorption of RF energy 11 NMR Spectrum 12 NMR Signals ¾ The number of signals shows how many different kinds of protons are present. ¾ The location of the signals shows how shielded or deshielded the proton is. ¾ The intensity of the signal shows the number of protons of that type. ¾ Signal splitting shows the number of protons on adjacent atoms. 13 The Chemical Shift ¾ Variations of the positions of NMR absorptions due to the electronic shielding and deshielding. ¾ It is a measure of how far the signal of the proton (sample) is from the reference signal. ¾ Most common reference compound is tetramethylsilane (TMS), (CH3)4Si. 14 CH3 H3C Si CH3 CH3 • Si is less electronegative than C. • Methyl groups withdraw electrons and their protons are shielded. • The TMS protons absorb at higher field compare to the most hydrogens bonded to other elements and its signal defined as zero. • Organic protons absorb downfield (to the left) of the TMS signal. 15 Chemical Shifts ¾ Measured in parts per million (ppm). ¾ Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (MHz). ¾ The chemical shift is independent of the operating frequency of the spectrometer ¾ Same value for 60, 100, or 300 MHz machine. ¾ Common scale used is the delta (δ) scale. 16 Delta Scale Each δ unit is 1 ppm difference from TMS 60 Hz at 60 MHz and 300 Hz at 300 MHz. 17 Location of Signals ¾ Depends on the shielding and deshielding effects on the protons. Methyl protons in alkane absorb at δ0.9 ¾ More electronegative substituent deshield more and give larger chemical shifts. 18 H H δ H γ H H β C C C H H H 0.9 1.3 1.7 α C Br H 3.4 ¾ Effect of an electronegative group on the chemical shift depends on its distance from the protons. ¾ The effect decreases with increasing distance. ¾ The effects can be negligible if the protons are separated from the electronegative group by four or more bonds. 19 ¾ Deshielding effect increases with more electronegative group. ¾ The deshielding effecs are nearly additive. 20 21 Characteristic Values of Chemical Shifts 22 23 Aromatic Protons, δ7-δ8 Along the ring axis, the induced field act to oppose the external field. Due to the deshielding effect at the edge of the ring, aromatic protons absorb at lower field 24 Spectrum of toluene It is the protons attached directly to the aromatic ring that will be the most deshielded. 25 Vinyl Protons, δ5-δ6 ¾ Similar deshielding effect to aromatic protons. ¾ The effect is weaker due to lack of large, effective ring of electrons as in the benzene. 26 Acetylenic Protons, δ2.5 Since the acetylenic protons are in the axis of the generated field that opposes the applied field, the acetylenic protons are shielded and will be found at higher fields than vinylic protons. 27 Aldehyde Proton, δ9-δ10 Electronegative oxygen atom The aldehyde proton is deshielded by the circulation of electrons in the pi bond. It is also deshielded by the electron-withdrawing effect of the carbonyl (C=O) group 28 O-H and N-H Signals ¾ Chemical shift depends on concentration. ¾ Hydrogen bonding in concentrated solutions deshield the protons, so signal is around δ3.5 for N-H and δ4.5 for O-H. ¾ Proton exchanges between the molecules broaden the peak. ¾ The protons pass through a variety of environments during this exchange, absorbing over a wider range of frequencies and field strengths. 29 Carboxylic Acid Proton, δ10+ ¾ Hydrogen bonding and oxygen atom strongly deshield the carboxylic acid proton. 30