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Today’s Lecture 5) Wed, Oct 8: Product operators I (tools to simplify the quantum mechanics) a. RF pulses b. Chemical shift Download Mathematica Player and Bloch Equation demo http://demonstrations.wolfram.com/MagneticResonanceAndBlochEquations/

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Today’s Lecture

5) Wed, Oct 8: Product operators I (tools to simplify the quantum mechanics)

a. RF pulses

b. Chemical shift

http://demonstrations.wolfram.com/MagneticResonanceAndBlochEquations/

NMR is all rotations.

Vector representations of Mx, My, and Mz

x

y

Notice that the coordinate system satisfies the “right hand rule”. If you point your right thumb along the z-axis, you fingers will close from x to y.

z

x

y

z

x

y

z

Rotation Matrices

These are 3D rotation matrices. When they act on a vector, they rotate the vector around the axis that defines the vector (Rx around x; Ry around y; Rz around z).

What is a rotation?

Here is an example of a rotation of Mz around the x-axis by an angle f:

Notice that if f=0, the vector stays the same. If f=p/2, the vector is rotated to the –y axis.

General Rule of Rotations

Every rotation in 3D space leads to 2 terms. The first term is pointing in the same direction and is multiplied by Cos(f) where f is the rotation angle. The second term is along the axis that the vector is rotated into and is multiplied by Sin(f).

For example, in the rotation above, a rotation of f around the x-axis (because it is the Rx rotation matrix) of Mz produces Mz*Cos(f)-My*Sin(f).

For example, the rotation we just saw:

becomes:

Product Operator Rules

• The thing above the arrow is the operator. The one shown above is a f pulse along the positive x-axis. The nucleus that we are worrying about is given by a capital letter like “I”. The orientation of the magnetization is given by the subscript. If we just have one nuclear spin to worry about (e.g. water), then there are 3 spatial orientations for that nucleus:
• Iz (pointing along the z-axis)
• Ix (pointing along x)
• Iy (pointing along y)

Product Operator Rules

Just like a pulse that produces a torque, operators rotate around a given axis but do not act on things along the axis of rotation. Corresponding to B1 fields along the x and y axes, we have 2 operators for pulses:

Pulse along the x-axis by f degrees

Operators have hats.

Pulse along the y-axis by f degrees

What is a rotation around the z-axis?

Chemical shift! The chemical shift operator works exactly like a pulse operator, but it only acts along the z-axis.

The only difference between the convention for a pulse and chemical shift is that we put in the frequency (W) times time for the shift and only the rotation angle for the pulse.

1D NMR experiment

a

a

b

c

(+ relaxation)

a

b

c

Product Operator I Summary

• Operators act on magnetization vectors and cause rotations.
• Rotations always have the starting term multiplied by cosine and the resulting term multiplied by sine of the angle.
• We have discussed two kinds of operators, pulse and chemical shift. These both do “normal” rotations in 3D and leave the magnetization unaltered.
• Operators can act sequentially (e.g. pulse followed by evolution). This becomes critical in analyzing 2D or multiple pulse experiments.

Next Lecture

6) Fri, Oct 10: Product operators II

a. Scalar (J) coupling

b. Multiple pulse experiments