What is spectroscopy?

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From the course by University of Manchester

Introduction to Molecular Spectroscopy

138 ratings

University of Manchester

Introduction to Molecular Spectroscopy

138 ratings

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.

From the lesson

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

Meet the Instructors

Patrick J O'Malley, D.Sc
Reader
Chemistry

So, first question you get is, this course is mainly on spectroscopy.

So what is spectroscopy?

So as I said here, it's a study of the interaction of matter and

electromagnetic radiation.

So we know what the matter is, it's your chemical or your biochemical samples.

And what is this electromagnetic radiation?

So, the current area of electromagnetic radiation is that,

it's a, [COUGH] it has both the properties of waves and particles.

So first of all,

we're going to look at this more into the wave side of electromagnetic radiation.

Because before quantum theory was developed,

people thought that electromagnetic radiation was wavelike.

So in some ways it's the most intuitive way to look at it.

But a lot of the spectroscopic work that you do,

you see that as well as behaving like a wave, electromagnetic radiation,

in various forms, can behave like a particle as well.

So you also need to understand a bit about that.

And that then you dove in quite deeply into quantum theory.

But we're going to skip through that a little bit at this stage.

We're not going to delve into quantum theory too much,

cuz that would be the basis of a whole electro-series.

But we're going to touch on it and mention the main points.

Because without quantum theory, you can't really explain spectroscopy.

Because spectroscopy is basically transitions between different energy

levels.

And these energy levels arise because you have quantum effects,

quantum mechanical effects.

The whole spectroscopy is based on quantum mechanics.

Okay, so the simplest example of electromagnetic

radiation that we're all familiar with is Visible Light.

We're all familiar with visible light.

So here we have the whole electromagnetic spectrum.

And as you can see the top wave there, you can see that I've written down

increasing energy going to the left.

And then on the call increasing wavelength going to the right.

And then below that you have the different classifications.

You have all the different types of electromagnetic radiation you have.

You have a very high energy called Gamma Rays and

then you have X-rays, and then you have the Ultra Violet Region.

And then you have that thin little piece there in the middle.

Sorry.

So you have this thin piece here in the middle and that's the visible region.

And as you all know from your school days you can break the visible region

into it's different colors using a prism or the rainbow effect.

And then [COUGH] physically what that means is that these

regions have different energies.

Say you have the higher energy low wavelength regions

near the blue and then you go up towards the red.

And it's gonna be in this stage to,

we have different ways of classifying electromagnetic radiation.

And you should now start to get in your head from that picture at the bottom

there, that electromagnetic radiation runs from

about 400 nanometers up to 700 nanometers.

Okay?

So that's the region of the visible region of the spectrum.

And then the [COUGH] visible region lies in between the ultraviolet here,

and the infrared.

And then as you go down to lower wavelengths or, sorry,

to higher wavelengths, lower energy.

You have the radio waves.

So when we talk about electromagnetic spectrum, we're looking at,

in terms of the wavelengths, we're looking at wavelengths of different lengths.

And these then correspond to different energy.

And this relationship between the wavelength and

the energy, we'll be going into in a few minutes.

Okay, so in our, the spectroscopy we cover in this course,

we'll be looking at the UV visible region.

So we looking at first.

And then we'll be looking at the transitions by irradiating matter or

molecules with that part of the spectrum,

we'll generate certain types of transitions.

Then in the infrared region we induce other types of transitions,

and then when we go down to radio waves, we're in the NMR region.

Okay, [COUGH] so

this is just a summary, basically of the different regions again.

So you have the gamma rays, and

these are emitted from the nucleus of radioactive elements.

They're very harmful because they're high energy.

X-rays, again, are harmful.

Okay, if you use it in small doses.

And we all know that UV light, if we get too much light UV from the Sun,

then it's a high energy radiation.

So basically if it's a high energy radiation,

it's strong enough to break bonds.

Cuz that's what we see about UV visible spectroscopy where it's sighting

electronic transitions.

And when you exactly find transitions you can cause breakage of bonds and

that can be damaging.

Then you have the Visible Light region, the one that we can see.

Then you have Infrared Radiation, infrared really is heat.

And then you have the Radio Waves, the lowest ones.

Radio TV, [COUGH] and for our purposes here, use them in NMR, or

in magnetic resonance imaging, which is NMR as well.

Okay.

So here's another look at electromagnetic radiation and it's composed,

what we show here is it's composed of two ways if you like.

You have what they call an electronic,

Electric Field part and you have a Magnetic Field part.

And the waves as you tried to illustrate here are perpendicular.

You can see that they're perpendicular to each other.

So it's two components, Electric Field and a Magnetic Field part.

And a given point in space experiences this disturbance from this radiation.

Just down here just to see,

I mean we're all familiar with wave length like water wave and so forth.

But just select electromagnetic radiation.

They're defined by how it disturbs the mediums.

So the water is disturbed but

these electromagnetic radiation the waves will pass through a vacuum.

Okay, so they don't need a medium to generate.

So what I want you to get through is,

we're going to be talking about these different electromagnetic radiations and

it's good to have an understanding of what it is.

All right, so to get a bit more into the details now.

So you have this nice wave, it could be a Sine wave or

a Cosine wave and you need to be able to define the parts of a wave.

So the way we characterize waves is we characterize

what is new the frequency of the wave.

So that's the number of crests or the number of cycles.

So you'd sit at this point here say or you'd start off with this point here and

then you'd wait until that point is reached again by the wave.

So you basically wait until that point has come across it.

>> [COUGH] >> So the time it would take for

that to happen, [COUGH] that's will give you

the frequency of that wave, how often that occurs within a given time.

And then you have the next one we talk about is the wavelength.

And the wavelength, the easiest way to define,

it's the distance between two identical parts on the wave.

So these two bits I've marked here would give you the wavelength.

Or say you could take it here, that would be a wavelength as well,

or any other point on the wave like that.

So you have the frequency, you have the wavelengths,

you have the velocity of the wave.

And then lastly for all waves is the same, it's the speed of light.

So electromagnetic radiation travels at the speed of light,

which is 2.998 here by 10 to the 8 meters per second.

And then you need to know how to relate these,

so you define C the velocity as lambda times the frequency.

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