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 agneti titization

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 agneti titization

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 IdInduced dC Current

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).