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NMR

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on 8 May 2014

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Transcript of NMR

Results of NMR
When several resonant frequencies are grouped over a range, a spectrum forms
Each peak represents a specific atom and the atoms it is bound to
Modeling NMR
In the lowest energy state, the torque tends to line up the magnetic moment with the magnetic field.
Modeling NMR
An accurate model for the physics behind NMR technology takes into account resonance frequency, magnetic moment, and the moment of inertia. Torque is experienced by a magnetic moment in a magnetic field of the expression:


What does NMR tell us?
NMR uses the magnetic properties of an atom's nuclei to show how atoms are bonded in a molecule
Essentials of NMR
Magnetic resonance can be used in spectroscopy because of the idea that, when placed in an external magnetic field, nuclei become aligned in a finite number of orientations. For a 1H, there are only two orientations: Along the field (Alpha) and against the field (Beta). Alpha is of lower energy, so more of the nuclei population are expected to be in alpha orientation.
Nuclear Magnetic Resonance Imaging (NMR)
How?
Place the molecule in a strong magnetic field
Causes the nuclei in the molecule to resonate at certain frequencies
Variations in the frequencies identify the atomic environment of the molecule
Magnetic Resonance and "Spin"
Resonance:
The tendency of a system to oscillate with greater amplitude at some frequencies than at others.
Resonant Frequencies:
The frequencies at which the response amplitude is a relative maximum
A system is able to store and easily transfer energy between two or more different storage modes (e.g., kinetic vs potential).
Resonance can be measured from all types of waves or vibrations.
Randomness and Orientation
Two Models
Classical Model

Quantum Model
Classical Model
Positive nuclei are thought to spin along a particular axis
Create small magnetic fields
Magnetic fields align with a stronger, applied magnetic field
Circular motion (green) is created by angular momentum and torque of the spinning nucleus
The external field strength is proportional to the precession rate, creating a resonant frequency that can be measured
Takes the half-spins of nuclei into consideration
Two spin directions (up or down) of equal energy
When an outside field is applied, up spin has lower energy
Up aligns with the magnetic field, while down opposes it
Energy gaps form between two spins
Lower energy nuclei absorb photons and become higher energy nuclei
The photon energy must equivocate to the energy gap
Energy can be equated to a particular frequency of electromagnetic radiation:
∆E = hν
∆E = energy gap; h = Plank's constant; ν = resonant frequency
Quantum Model
ν = γ B
B = magnetic field strength; γ = gyromagnetic ratio (ratio of magnetic dipole to angular momentum)
Acoustic Resonance
The tendency of an acoustic system to absorb more energy when it is forced or driven at a frequency that matches one of its own natural frequencies of vibration than it does at other frequencies.
Mechanical Resonance
The tendency of a mechanical system to respond at greater amplitude when the frequency of its oscillations matches the system's natural frequency of vibration than it does at other frequencies.
Resonance Disaster
Electron Spin Resonance
Guitar Top Resonance
Acoustic Levitation
Electromagnetic Resonance
Schumann Resonance
The Schumann resonances (SR) are a set of spectrum peaks in the extremely low frequency (ELF) portion of the Earth's electromagnetic field spectrum. Schumann resonances are global electromagnetic resonances, excited by lightning discharges in the cavity formed by the Earth's surface and the ionosphere.
Phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the magnetic field to produce absorption of the radiation.

Why did they sound different?
ESP is a technique for studying materials with unpaired electrons. The basic concepts of ESP are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei.
G-Factor Resonance of Paramagnetic Center
Schumann Resonant Frequency
Why Does This Work?
Think of the nuclei as tiny, weak magnets that can align with magnetic fields. With a current applied, keep in mind:

1. Electric currents have associated magnetic fields.

2.Magnetic fields can generate electric currents.

External field and frequency are proportional, shown by the expression:

Magnetogyric ratio, a constant:
Magnetic Moment
Moment of Inertia
Combined Model
This is similar in set up
Need a compass, bar magnet, current via solenoid with power supply (set voltmeter @ 1V), labquest, and Teslameter.
The interaction of an external magnetic field with an electron spin depends upon the magnetic moment associated with the spin.
A.B. Wood. A Textbok of Sounds. Bell and Sons. 1944.
http://faraday.physics.utoronto.ca/IYearLab/stwaves.pdf
Andreas Pohlkotter, et al. Resonant tuning fork detector for
electromagnetic radiation. 2009. Applied Optics, Vol 48 (4), 119-125.
Buxton, Richard. Introduction to functional magnetic resonance imaging:
principles and techniques. Cambridge University Press. 2009. Second
Edition.
Electron Spin Resonance. HyperPhysics. 2014. Web. http://
hyperphysics.phy-astr.gsu.edu/hbase/molecule/esr.html
Hornak, Joseph. The Basics of NMR. 2011. Web, 2014. http://
www.cis.rit.edu/htbooks/nmr/inside.htm
Reusch, William. Nuclear Magnetic Resonance Spectroscopy. 2014. http://
faraday.physics.utoronto.ca/IYearLab/stwaves.pdf
R.H. Randall. An introduction to acoustics. Addison-Wesley, 1951. http://
faraday.physics.utoronto.ca/IYearLab/stwaves.pdf
References
Application: MRI
Protons are capable of absorbing energy if exposed to EM energy at the frequency of oscillation. After absorbing energy, the nuclei release or reradiate this energy so they return to their initial state of equilibrium.

The reradiation/transmission of energy by the nuclei as they return to their initial state is what is observed as the NMRI signal. This “return” takes place over time is determined by the component of the nuclei parallel to the magnetic field (T1) and perpendicular to the magnetic field (T2).
T1, T2, and the density of the protons in a measured tissue determine the strength of the MRI signal. The human body is primarily fat and water, so the hydrogen nuclei composition is ~63%. One dimensional magnetic fields can be passed through various angles across 360° through tissue containing water with different densities to provide contrast for imaging.
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