The nature and properties of electromagnetic radiation

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Introduction to Molecular Spectroscopy

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University of Manchester

Introduction to Molecular Spectroscopy

173 classificações

The course introduces the three key spectroscopic methods used by chemists and biochemists to analyse the molecular and electronic structure of atoms and molecules. These are UV/Visible , Infra-red (IR) and Nuclear Magnetic Resonance (NMR) spectroscopies. The content is presented using short focussed and interactive screencast presentations accompanied by formative quizzes to probe understanding of the key concepts presented. Numerous exercises are provided to facilitate mastery of each topic. A unique virtual spectroscopic laboratory is made available to enable students to measure and analyse spectra online. Assessment is via summative quizzes completed during the course period.

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Ultraviolet and Visible Spectroscopy

In this first week we introduce the electromagnetic spectrum and the origin of transitions giving rise to ultraviolet and visible (UV/Vis) spectra. You will learn that electronic transitions are caused by absorption of radiation in the UV/Vis region of the electromagnetic spectrum. The reason for the wavelength and intensity of bands will be described and the colour origin of certain compounds will be discussed. You will also be shown how UV/Vis spectroscopy is performed and you will be able to run and analyse your own spectra. As the final activity in this module you are given a link to view how to obtain a UV/Visible spectrum in the laboratory. Good luck, try and participate in the discussion forums to enhance your learning and don't forget to complete the end of week laboratory quiz which contributes to your final mark.

What is spectroscopy?9:50

The nature and properties of electromagnetic radiation6:42

Example of energy calculation10:48

Conheça os instrutores

Patrick J O'Malley, D.Sc
Reader
Chemistry

Right, so let's, we talked about the wave properties of electromagnetic radiation.

But as I say, until the 1920s,

people thought that that was the way you treated electromagnetic radiation.

But with the development of quantum theory,

people like Einstein and Planck, showed that you also need to

treat electromagnetic radiation as a particle or a photon.

Because of certain experiments where you can't explain, you hit electromagnetic

radiation on a certain material, and what happens can't be explained by waves.

You may have in school, if you did physics,

you may have done the photo electric effect, when you knock electrons

off a certain material, and you can't explain that, in terms of, the wave.

It's 3D, electromagnetic radiation is behaving like a particle.

[SOUND] It's physically knocking the electron out.

So to understand that, you have to also say that, in certain cases,

electromagnetic radiation behaves as a photon.

And Planck was the main proponent of that, and

he proposed this famous equation, which I assume you've all seen,

is that E=hv, where v, we've already defined as the frequency,

of the electromagnetic radiation, and h is Planck's constant.

6.626 by 10 to the minus 34 joules second.

Okay, so you should all, you don't know it already,

you should all be able to remember this fundamental equation.

So we're given Planck's constant there.

So now you need to also be able to, especially if make spectroscopy.

We find the difference from spectroscopy, sometimes we refer it to the wave length,

sometimes we refer it to the frequency.

Sometimes we refer it to something else called the wave number.

So you need to be able to [COUGH] know what these are,

and how to change between them.

All spectroscopists have to be able to do this without thinking, really.

So the first equation here,

you should remember, is E=hv, and

then we know that C is equal lambda times new so

that new is equal to c over lambda.

So, therefore, you can move on to this one here that e is equal h, hc over lambda.

So, again, if you haven't seen this before, you're not familiar with it,

you need to be.

So the energy of a photon wave is directly proportional to frequency.

Okay, so that should be obvious from this equation here.

E is equal to h and is it proportional to frequency, and h,

Planck's constant, is proportional to constant.

And the energy of the photon is inversely proportional to the wavelength.

And if you think back to the slide we had with electromagnetic radiation spread out,

the longer the wavelength, then the lower the energy.

And this comes from this relationship here,

you can see that E is proportion to one over the wave.

[COUGH] Okay, another way is you define it.

You define it as a joules is the SI unit of energy.

So you can have joules per fundamental particle or you can have,

sometimes we convert it to a more common way of using is kilojoules,

where kilo is 10 to the 3, joules per mol minus 1, express it in per mol.

So you need to be able to convert energy between these different forms.

And another way you'll often find in spectroscopy is this definition, here.

Here, we have, well here we have hc over lambda, and

here you have hc into 1 over lambda.

So that's the inverse of the wavelength, and

that's often denoted here by this nu bar.

So you have something like a nu and you have a bar over it, and

that's called the wave number.

And as I say here, in SI units, for length, we use meters.

So, therefore, mol bar, one over the wavelength, should be meters minus 1.

But you'll find in a lot of spectroscopy,

it's defined as centimeters minus 1.

Especially in new, move on to infrared spectroscopy,

you can see an infrared spectrum, it's usually defined in centimeters minus 1.

And usually, you might be wondering why, do they use different units for

different spectrosities?

Cuz sometimes it's just easier to understand, the way the region of infrared

has nice numbers, if you like, in centimeters minus 1.

So it's easy to follow variations in it, whereas,

in something like UV spectroscopy, sometimes they talk about nanometers.

Again, that's because it gives you nice numbers.

You don't want lots of exponential numbers,

cuz it's difficult to understand it.

But they, as the wave numbers v is minus 1 but new n,

you usually end up in centimeters.

And, again, sometimes students have a lot of trouble converting,

say from new meters minus 1, to centimeters minus one.

What you've got to remember here is, you have a 100 centimeters in a meter.

Most people get that, but

then it's going the other way around that people find difficult.

And you have to think of the number of ways in a given distance.

So here we have a [INAUDIBLE] per meter, so the number of waves in that

is going to be a lot bigger than a number of waves in a centimeter, per centimeter.

So, there's a 100 times more waves, if you like,

in a meter than there is in a centimeter.

So it's one way of thinking it, usually people tend to mess that up.

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