HL Chemistry - Option A :Modern Analytical Chemistry NUCLEAR MAGNETIC RESONANCE
Different Types of NMR • Electron Spin Resonance (ESR) • 1-10 GHz (frequency) used in analyzing free radicals (unpaired electrons) • Magnetic Resonance Imaging (MRI) • 50-300 MHz (frequency) for diagnostic imaging of soft tissues (water detection) • NMR Spectroscopy (MRS) • 300-900 MHz (frequency) primarily used for compound ID and characterization
NMR in Everyday Life Magnetic Resonance Imaging
Explaining NMR UV/Vis spectroscopy Sample
Principles of NMR • Measures nuclear magnetism or changes in nuclear magnetism in a molecule • NMR spectroscopy measures the absorption of light (radio waves) due to changes in nuclear spin orientation • NMR only occurs when a sample is in a strong magnetic field • Different nuclei absorb at different energies (frequencies)
Bigger Magnets are Better Increasing magnetic field strength low frequency high frequency
Different Isotopes Absorb at Different Frequencies 2H 15N 13C 19F 1H 30 MHz 50 MHz 125 MHz 480 MHz 500 MHz low frequency high frequency
Which Elements or Molecules are NMR Active? • Any atom or element with an odd number of neutrons and/or an odd number of protons • Any molecule with NMR active atoms • 1H - 1 proton, no neutrons, AW = 1 • 13C - 6 protons, 7 neutrons, AW =13 • 15N - 7 protons, 8 neutrons, AW = 15 • 19F = 9 protons, 10 neutrons, AW = 19
NUCLEAR SPIN The nuclei of some atoms have a property called “SPIN”. These nuclei behave as if they were spinning. ….. we don’t know if they actually do spin! This is like the spin property of an electron, which can have two spins: +1/2 and -1/2 . Each spin-active nucleus has a number of spins defined by its spin quantum number, I. The spin quantum numbers of some common nuclei follow …..
Spin Quantum Numbers of Some Common Nuclei The most abundant isotopes of C and O do not have spin. Element1H 2H 12C 13C 14N 16O 17O 19F Nuclear Spin Quantum No1/2 1 0 1/2 1 0 5/21/2 ( I ) No. of Spin 2 3 0 2 3 0 6 2 States Elements with either odd mass or odd atomic number have the property of nuclear “spin”. The number of spin states is 2I + 1, whereI is the spin quantum number.
THE PROTON Although interest is increasing in other nuclei, particulary C-13, the hydrogen nucleus (proton) is studied most frequently, and we will devote our attention to it first.
NUCLEAR SPIN STATES - HYDROGEN NUCLEUS The spin of the positively charged nucleus generates a magnetic moment vector, m. m + + The two states are equivalent in energy in the absence of a magnetic or an electric field. m + 1/2 - 1/2 TWO SPIN STATES
THE “RESONANCE” PHENOMENON absorption of energy by the spinning nucleus
Nuclear Spin Energy Levels N -1/2 unaligned In a strong magnetic field (Bo) the two spin states differ in energy. +1/2 aligned Bo S
Absorption of Energy quantized Opposed -1/2 -1/2 DE DE = hn Radiofrequency +1/2 +1/2 Applied Field Bo Aligned
THE ENERGY SEPARATION DEPENDS ON Bo - 1/2 = kBo = hn DE degenerate at Bo = 0 + 1/2 Bo increasing magnetic field strength
The Larmor Equation!!! DE = kBo = hncan be transformed into gyromagnetic ratio g g n = 2p gB0 n = 2p frequency of the incoming radiation that will cause a transition Bo strength of the magnetic field g is a constant which is different for each atomic nucleus (H, C, N, etc)
A SECOND EFFECT OF A STRONG MAGNETIC FIELD WHEN A SPIN-ACTIVE HYDROGEN ATOM IS PLACED IN A STRONG MAGNETIC FIELD ….. IT BEGINS TO PRECESS (that means spin) OPERATION OF AN NMR SPECTROMETER DEPENDS ON THIS RESULT
N w Nuclei precess at frequency w when placed in a strong magnetic field. RADIOFREQUENCY 40 - 600 MHz hn NUCLEAR MAGNETIC RESONANCE If n = w then energy will be absorbed and the spin will invert. NMR S
Resonance Frequencies of Selected Nuclei Isotope Abundance Bo (Tesla) Frequency(MHz) g(radians/Tesla) 1H 99.98% 1.00 42.6 267.53 1.41 60.0 2.35 100.0 7.05 300.0 2H 0.0156% 1.00 6.5 41.1 7.05 45.8 13C 1.108% 1.00 10.7 67.28 2.35 25.0 7.05 75.0 19F 100.0% 1.00 40.0 251.7 4:1
POPULATION AND SIGNAL STRENGTH The strength of the NMR signal depends on the Population Difference of the two spin states Radiation induces both upward and downward transitions. induced emission resonance For a net positive signal there must be an excess of spins in the lower state. excess population Saturation = equal populations = no signal
Absorbance Typical 1H NMR Spectrum
CLASSICAL INSTRUMENTATION typical before 1960 field is scanned
NMR Magnet Cross-Section Sample Bore Cryogens Magnet Coil Magnet Legs Probe
A Simplified 60 MHzNMR Spectrometer hn RF (60 MHz) Oscillator RF Detector absorption signal Recorder Transmitter Receiver MAGNET MAGNET ~ 1.41 Tesla (+/-) a few ppm N S Probe
hn Fortunately, different types of protons precess at different rates in the same magnetic field. Bo = 1.41 Tesla N EXAMPLE: 59.999995 MHz 59.999700 MHz To cause absorption of the incoming 60 MHz the magnetic field strength, Bo , must be increased to a different value for each type of proton. 59.999820 MHz 60 MHz S Differences are very small, in the parts per million range.
IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT SCANS FROM “LOW FIELD” TO “HIGH FIELD” LOW FIELD HIGH FIELD NMR CHART increasing Bo DOWNFIELD UPFIELD scan
NMR Spectrum of Phenylacetone NOTICE THAT EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF HYDROGEN THERE ARE
MODERN INSTRUMENTATION PULSED FOURIER TRANSFORM TECHNOLOGY FT-NMR requires a computer
A Modern NMR Instrument Radio Wave Transceiver
PULSED EXCITATION N n2 n1 BROADBAND RF PULSE contains a range of frequencies n3 (n1 ..... nn) S All types of hydrogen are excited simultaneously with the single RF pulse.
FREE INDUCTION DECAY ( relaxation ) n1 n2 n3 n1, n2, n3 have different half lifes
COMPOSITE FID “time domain“ spectrum n1 + n2 + n3 + ...... time
FOURIER TRANSFORM A mathematical technique that resolves a complex FID signal into the individual frequencies that add together to make it. ( Details not given here. ) DOMAINS ARE MATHEMATICAL TERMS converted to TIME DOMAIN FREQUENCY DOMAIN FID NMR SPECTRUM FT-NMR computer n1 + n2 + n3 + ...... COMPLEX SIGNAL Fourier Transform individual frequencies a mixture of frequencies decaying (with time) converted to a spectrum
The Composite FID is Transformed into a classical NMR Spectrum : “frequency domain” spectrum
Fourier Transformation iwt F(w) = f(t)e dt Converts from units of time to units of frequency
FT NMR Free Induction Decay FT NMR spectrum
COMPARISON OF CW AND FT TECHNIQUES
CONTINUOUS WAVE (CW) METHOD THE OLDER, CLASSICAL METHOD The magnetic field is “scanned” from a low field strength to a higher field strength while a constant beam of radiofrequency (continuous wave) is supplied at a fixed frequency (say 100 MHz). Using this method, it requires several minutes to plot an NMR spectrum. SLOW, HIGH NOISE LEVEL
PULSED FOURIER TRANSFORM (FT) METHOD FAST LOW NOISE THE NEWER COMPUTER-BASED METHOD Most protons relax (decay) from their excited states very quickly (within a second). The excitation pulse, the data collection (FID), and the computer-driven Fourier Transform (FT) take only a few seconds. The pulse and data collection cycles may be repeated every few seconds. Many repetitions can be performed in a very short time, leading to improved signal …..
IMPROVED SIGNAL-TO-NOISE RATIO By adding the signals from many pulses together, the signal strength may be increased above the noise level. signal enhanced signal noise 1st pulse 2nd pulse add many pulses noise is random and cancels out nth pulse etc.
NMR Spectrum of Phenylacetone Each different type of proton comes at a different place . You can tell how many different types of hydrogen there are in the molecule.