Name: 化學203.有機光譜學。第08講。NMR光譜學導論，第2部分。 (Chem 203. Organic Spectroscopy. Lecture 08. Introduction to NMR Spectroscopy, Part 2)
Uploaded: 2021-01-14T05:21:25.000Z
Duration: 54 min 27 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

All right, but we're going to talk about spins today.

So I want to continue our discussion of the concepts

and theory behind NMR spectroscopy and again,

this is not going to be about math or anything to that extent.

But we're going to be thinking very, very qualitatively.

All right, so when we last left things we had said

for a dipolar nucleus like a proton or C13.

and if you have an applied magnetic field there's a small

energy difference between the spin up state,

what we're calling the Alpha state and the spin down state,

so small unlike IR spectroscopy or electronic spectroscopy,

UV Vis spectroscopy where the energy differences are very big

and all of your molecules are in the ground state,

here there's only a miniscule number of nuclei

in the lower state more than the number in the upper state.

We said if there are -- if we take 2 million protons,

on the applied magnetic field, it will be 50 or 80

In turns out that difference in population is going

to be extremely important because it is only

that differential population that's going

in an XYZ coordinate frame, and I'll talk more

about NMR spectrometer in a second and how it works.

But imagine for a moment we have some coordinates,

so the X coordinate is coming out of the plane,

the Y coordinate is in the plane and the Z coordinate is pointing

up and we're going to have our applied magnetic field BNOT

These super conducting magnets are always vertical

because you've got this big pot of liquid helium surrounded

by a vacuum vessel, surrounded by liquid nitrogen,

that small differential of population with spin up is going

Now, in other words a way to think about this is for most

of our cases we're going to have one nucleus pointing up,

But for that small differential of access vectors you're going

to have some net magnetization along the Z-axis.

Now, the ay it works, when you apply a magnetic field is you

actually have those vectors that are processing around.

at that resonant frequency, at the Lamar Frequency

at 500 megahertz for 117,500 Gauss magnet.

So we actually can represent this by saying okay,

we've got spin sort of pointing in every which way,

So remember, remember this is only the differential population

that we're worried about because already for every one

where you have one up and you have an opposing one spin

down those vectors are going to cancel each other out.

Now the other thing is they're not quite on axis.

It's like a gyroscope if you've ever hung it from a string,

who's hung a gyroscope from a string as a kid?

it kind of hangs off axis and goes around like this.

But if you think about it, since those spins are not bunched

like this there's another one that's opposite it.

So in other words if were just like this you'd say,

oh there's a net magnetization along the Z axis

but also a net magnetization along the Y axis.

But there are other spins that are like this

out except the net magnetization along the Z axis.

All right, what I want you to imagine right now is

that we're going to place a coil along the X axis and we're going

because the quantum mechanical thing is going

But you have your net magnetization along the Z axis,

If I apply a force along the X axis, right hand rule and all

of that good stuff, we rotate our vector downward.

So after we apply a pulse, I'll just say a pulse.

when we apply an RF pulse our net magnetization moves along

to actually represent it I'll just say XYZ,