Nuclear Magnetic Resonance StSpectroscopy 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 DdNlSiDepends on Nuclear Spin ItIsotope spin Nucleus Spins Abundance 1H ±½ 99. 9% +½ 12C 0 98.9% 13C ±½ 11%1.1% 19F ±½ 100% -½ 31P ±½ 100% We’ll be talking about spectroscopy of 1H (and later 13C). Each 1H Nucleus Has Its Own Spin 1 We’ll consider H nuclei in CH3OH as an example. In the absence of an -½ applie d fie ld, nuc le i are randomly oriented. C O +½ +½ -½ Nuclear Spins Are Aligned By An Applied Magnetic Field In the absence of an Bo -½ applied field, nuclei are randomly oriented. C O In the presence of an applied field Bo, nuclei are +½ oriented by the field. +½ + spins align with the field, -½ - spins align against the field. Overall, Applied Field Leads to Bulk SiSpin M agne tititization H Being aligned with field is more C stable than against field. O Bo H H So, more spins align with the field. H C O Boltzmann population H H distribution H H C O H C O H H H spin = -½ H C O H E H H C O H spiin = +½ H Overall, Applied Field Leads to Bulk SiSpin M agne tititization H Being aligned with field is more C stable than against field. O Bo H H So, more spins align with the field, and sum of spins also aligns with H the field. C O H H H H C O H C O H M H H H = bulk C O magnetization H H H C O H H Spins, And Overall Spin, Precess (Wbbl)iA(Wobble) in Appli lidMed Magneti tiFildc Field Bo Again, using CH3OH as an example, CO M bulk magnetization All nuclei wobble at a characteristic frequency. Spins, And Overall Spin, Precess (Wbbl)iA(Wobble) in Appli lidMed Magneti tiFildc Field Bo Nuclei precess at the γBo Larmor frequency. ν = o 2π M 4.73 T 9.46 T 11.75 T Bo bulk 1 νo ( H) 200 MHz 400 MHz 500 MHz magnetization 13 νo ( C) 50 MHz 75 MHz 125 MHz (Precession is fast.) Big Fields Means Big Magnets Interior of a 4.73 T magg(net (on display in NMR Facility, Smith Installation of 16.45 T magnet in Hall) Hasselmo Hall. An NMR Facility An NMR Spectrometer “Listens” to Frequency of Nuclear Precession Bo xy M time FiFourier Transform M (radiofrequency detector coil) bulk | magnetization xy |M frequency An NMR Spectrum No surprise: NMR spectrum of | y CH OH shows the presence of x 3 1H nuclei at Larmor frequency. |M frequency: 201 200 199 MHz MHz MHz but what if we look closer?? Not all 1H nuclei wobble at the exact same frequency; Differences in frequency reflect differences in magnetic environment. 200.002 200.001 200.000 MHz MHz MHz “Shielding” Influences Proton Frequency NMR spectrum of CH OH: 3 Magnetic field induces - B circulation of e cloud. H o C H O C H Circulation generates an H H opposing magnetic field. So, magnetic field and frequency are lower than one might expect. O Electron-withdrawing H oxygen means • Smaller e- cloud on H 200. 002 200. 001 200. 000 • LiLess opposing MHz MHz MHz magnetic field • Higher frequency “Shielding” Influences Proton Frequency Electron cloud B H H “shields” 1H o nucleus. “downfield”: frequency “upfield”: Lower magnetic field strength would Higher magnetic field strength would be be required to achieve the same required to counteract shielding electrons frequency. and achieve the same frequency. Chemical Shift: A Proportional Horizontal Axis Problem: Differences in frequency depend on spectrometer field strength, vary from instrument to instrument. Solution: Define an absolute scale independent of spectrometer frequency, call e d “c hem ica l s hift”. frequency Frequency defined as = 0 ppm 200.002200.001 200.000 MHz CH3 H 20001000 0 Hz (diff.) H C Si C 3 H H 105 0 ppm CH3 tetramethylsilane (TMS) Chemical Shift: A Proportional Horizontal Axis 1H NMR, 300 MHz, 1H decoupled = 3.4 ppm = 4.8 ppm CH3OH protons have these chemical shifts (ppm values), regardless of instrument they are measured on. Different Types of Protons Have Characteristic Chemical Shifts A better resource: http://www.chem.wisc.edu/areas/reich/Handouts/nmr-h/hdata.htm Chemical Shift: Multiple Bonds and IdInduce dCd Curren t In presence of applied field B 0 , “Ring current” reinforces applied H B0 field; deshields H aromatic protons ( = 656.5-858.5 ppm ). Induced current is H H less effective in olefins, still Acetylene protons H H dhilddeshields, blbut less are shielded ( = 4-6 ppm). ( = 1.5-3 ppm)..