Nuclear Magnetic Resonance Spectroscopy Features: Used to identify products of reactions Also gives information about chemical environment, connectivity and bonding of nuclei Requirements: Pure or mostly pure sample of material (not useful for analyzing mixtures) > 1 mg material Nuclear Magnetic Resonance Spectroscopy Depends on Nuclear Spin Isotope Nucleus Spins Abundance spin 1 ±½ 99.9% 9% +½ 12 0 98.9% 13 ±½ 11% 1.1% 19 F ±½ 100% -½ 31 P ±½ 100% We ll be talking about spectroscopy of 1 We ll be talking about spectroscopy of (and later 13 ).
Each 1 Nucleus as Its wn Spin We ll consider 1 nuclei in 3 as an example. -½½ In the absence of an applied field, nuclei are randomly oriented. +½ -½ +½ Nuclear Spins Are Aligned By An Applied Magnetic Field In the absence of an -½½ applied field, nuclei are randomly oriented. +½ In the presence of an applied field, nuclei are oriented by the field. -½ +½ + spins align with the field, - spins align against the field.
verall, Applied Field Leads to Bulk Spin Magnetization Being aligned with field is more stable than against field. So, more spins align with the field. Boltzmann population distribution E spin = -½ spin = +½½ verall, Applied Field Leads to Bulk Spin Magnetization Being aligned with field is more B stable than against field. o So, more spins align with the field, and sum of spins also aligns with the field. = M bulk magnetization
Spins, And verall Spin, Precess (Wobble) in Applied Magnetic Field Again, using 3 as an example, M bulk magnetization All nuclei wobble at a characteristic frequency. Spins, And verall Spin, Precess (Wobble) in Applied Magnetic Field Nuclei precess at the ν o = γ Larmor frequency. 2π M bulk magnetization 4.73 T 9.46 T 11.75 T o ν o ( 1 ) ν o ( 13 ) 200 Mz 50 Mz 400 Mz 75 Mz 500 Mz 125 Mz (Precession is fast.)
Big Fields Means Big Magnets Interior of a 4.73 T magnet (on display in NMR Facility, Smith all) Installation of 16.45 T magnet in asselmo all. An NMR Facility
An NMR Spectrometer Listens to Frequency of Nuclear Precession M xy time M (radiofrequency detector coil) Fourier Transform bulk magnetization M xy frequency An NMR Spectrum xy M x No surprise: NMR spectrum of 3 shows the presence of 1 nuclei at Larmor frequency. frequency: 201 200 Mz Mz 199 Mz but what if we look closer?? Not all 1 nuclei wobble at the exact same frequency; 200.002 Mz 200.001 Mz 200.000 Mz Differences in frequency reflect differences in magnetic environment.
Shielding Influences Proton Frequency NMR spectrum of 3 : Magnetic field induces circulation of e - cloud. irculation generates an opposing magnetic field. So, magnetic field and frequency are lower than one might expect. 200.002002 200.001001 200.000000 Mz Mz Mz Electron-withdrawing oxygen means Smaller e - cloud on Less opposing magnetic field igher frequency Shielding Influences Proton Frequency Electron cloud shields 1 nucleus. downfield : frequency upfield : Lower magnetic field strength would be required to achieve the same frequency. igher magnetic field strength would be required to counteract shielding electrons and achieve the same frequency.
hemical Shift: A Proportional orizontal Axis Problem: Solution: Differences in frequency depend on spectrometer field strength, vary from instrument to instrument. Define an absolute scale independent of spectrometer frequency, called chemical shift. frequency Frequency defined as = 0 ppm 200.002 200.001 200.000 Mz 2000 1000 0 z (diff.) 3 10 5 0 ppm 3 3 Si 3 tetramethylsilane (TMS) hemical Shift: A Proportional orizontal Axis 1 NMR, 300 Mz, 1 decoupled = 3.4 ppm = 4.8 ppm 3 protons have these chemical shifts (ppm values), regardless of instrument they are measured on.
Different Types of Protons ave haracteristic hemical Shifts A better resource: http://www.chem.wisc.edu/areas/reich/andouts/nmr-h/hdata.htm hemical Shift: Multiple Bonds and Induced d urrent B In presence of applied field B 0, Ring current reinforces applied B 0 field; deshields aromatic protons ( = 6585 6.5-8.5 ppm). Induced current is less effective in olefins, still deshields, but less ( = 4-6 ppm). Acetylene protons are shielded ( = 1.5-3 ppm).