Introduction to infrared spectroscopy | Spectroscopy | Organic chemistry | Khan Academy

Summary

This script explains the concept of molecular vibrations using infrared (IR) spectroscopy. It likens molecular bonds to springs, demonstrating how they stretch and contract when energy from IR light is absorbed. The script delves into how these vibrations appear on an IR spectrum, with a focus on transmittance and wave numbers. It distinguishes between the diagnostic region, useful for identifying functional groups, and the fingerprint region, unique to each molecule. The aim is to help viewers understand the structure of molecules through IR spectroscopy.

Takeaways

• 🌐 Infrared light can be absorbed by molecules, causing bonds to stretch, a phenomenon known as stretching vibration.
• 🔗 The stretching vibration of a chemical bond is analogous to the oscillation of a spring, with the bond modeled as such for simplicity.
• 📊 An infrared (IR) spectrum is used to identify which frequencies are absorbed by a compound, indicating the presence of specific bonds.
• 📉 A frequency with 100 percent transmittance on an IR spectrum means no absorption occurred, suggesting that frequency is not relevant to the compound's bonds.
• 📈 Conversely, less than 100 percent transmittance indicates that some light was absorbed, corresponding to a stretching vibration of a bond.
• 🔢 Wave number, measured in reciprocal centimeters, is defined as one over the wavelength of light and is used to analyze IR spectra.
• 🔄 The relationship between wave number, wavelength, and frequency is fundamental, with frequency being directly proportional to wave number.
• 🔍 The diagnostic region of an IR spectrum is useful for identifying functional groups in a molecule, as different groups absorb different frequencies.
• 👁‍🗨 The fingerprint region of the IR spectrum is more complex and unique to each molecule, serving as a molecular identifier.
• 🔬 Understanding the location, intensity, and shape of signals in the IR spectrum is crucial for analyzing and identifying molecular structures.

Q & A

What happens when infrared light is shone on a molecule?

When infrared light is shone on a molecule, it's possible for the molecule to absorb energy from the light, which can cause a bond to stretch, a process known as a stretching vibration.

How is the stretching vibration of a bond compared to a spring?

The stretching vibration of a bond is compared to the oscillation of a spring, where the bond can be thought of as a spring with two masses (atoms) on either end that can stretch and contract when energy is applied.

What is the significance of the bond between carbon and hydrogen in the context of the video?

In the video, the bond between carbon and hydrogen is used as an example to model the concept of a bond as a spring, illustrating how energy can cause this bond to stretch and oscillate.

How does the infrared spectrum help in identifying molecular structures?

The infrared spectrum helps in identifying molecular structures by showing which frequencies of light are absorbed by a compound. These absorptions correspond to specific bond vibrations, which can indicate the presence of certain functional groups.

What is the meaning of 'percent transmittance' in the context of an infrared spectrum?

In the context of an infrared spectrum, 'percent transmittance' refers to the percentage of light that passes through a sample without being absorbed. A transmittance of 100% means no absorption occurred at that frequency.

How is wave number defined and how does it relate to the frequency of light?

Wave number is defined as one over the wavelength in centimeters. It is inversely proportional to the wavelength and directly proportional to the frequency of light, as shown by the relationship: frequency = wave number × speed of light.

What is the diagnostic region in an infrared spectrum and why is it important?

The diagnostic region in an infrared spectrum is the left side of the spectrum, typically below 1,500 wave numbers. It is important because signals in this region can be diagnostic for certain functional groups, helping to identify the structure of the molecule.

What is the fingerprint region in an infrared spectrum and how does it differ from the diagnostic region?

The fingerprint region is the right side of the infrared spectrum, above 1,500 wave numbers. It is more complex and harder to interpret than the diagnostic region, but it is unique to each molecule, acting like a molecular fingerprint for identification purposes.

Why is the location of a signal in an infrared spectrum significant?

The location of a signal in an infrared spectrum is significant because it corresponds to a specific wave number, which can be used to identify the type of bond or functional group that is absorbing the energy.

What additional aspects of a signal in an infrared spectrum, besides location, are important to consider?

Besides the location, the intensity and shape of a signal in an infrared spectrum are also important. These can provide further information about the strength of the bond vibrations and the complexity of the molecular structure.

Outlines

00:00

💡 Molecular Absorption and Stretching Vibrations

This section introduces the concept of molecular absorption of energy through infrared light. It explains how this energy causes bonds, like the carbon-hydrogen bond, to stretch in a manner similar to the oscillation of a spring. Using a carbon-hydrogen bond as an example, the process of stretching and contracting is likened to pulling masses on either end of a spring. The bond’s stretching vibration is explained as a form of energy absorption, modeled through the concept of infrared spectroscopy.

05:00

📊 Interpreting IR Spectra and Transmittance

This section discusses how infrared spectroscopy works by analyzing how light interacts with a molecule. It explains the concept of transmittance in an IR spectrum, where 100% transmittance means no light is absorbed, and less than 100% indicates that energy is absorbed, resulting in bond stretching. The focus is on how specific frequencies of light are absorbed by bonds, and how this is represented in the IR spectrum. The relationship between transmittance and absorption is highlighted, with emphasis on using IR spectra to identify bonds in molecules.

📏 Wave Number and Its Relationship to Frequency and Wavelength

Here, the concept of wave number is introduced, which is the inverse of the wavelength in centimeters. The paragraph explains how to calculate the wave number by dividing one by the wavelength. Additionally, it shows how wave number is related to frequency by demonstrating the formula: frequency equals wave number times the speed of light. This calculation helps in understanding how wave number and frequency are linked, laying the groundwork for interpreting molecular vibrations and infrared spectra.

⚖️ Calculating Frequency Using Wave Number

This section continues the discussion on wave numbers and shows how to calculate frequency by multiplying the wave number by the speed of light. An example calculation demonstrates the units and how the wave number translates into frequency. The importance of this relationship for interpreting infrared spectra is emphasized, especially for analyzing the stretching vibrations of molecular bonds.

🔍 The Diagnostic and Fingerprint Regions in IR Spectra

This section introduces two key regions in an IR spectrum: the diagnostic region and the fingerprint region. The diagnostic region, located to the left of 1,500 wave numbers, is used to identify specific functional groups, such as triple bonds. The fingerprint region, to the right, is more complex and harder to interpret but is unique to each molecule, making it useful for distinguishing unknown compounds. The section stresses focusing on the diagnostic region for analyzing functional groups while mentioning that the fingerprint region can provide molecular 'fingerprints.'

📉 Interpreting Signal Location and Intensity in IR Spectra

In this section, the importance of the location, intensity, and shape of IR signals is explained. It highlights how the position of a signal on the spectrum, such as a signal appearing at around 2,100 wave numbers, corresponds to specific molecular bonds. The section also touches on the visual interpretation of hand-drawn IR spectra and emphasizes not worrying about exact scaling, but rather focusing on the relative position and significance of the signals in the spectrum.

🔬 Analyzing Bond Vibrations Using Classical Physics

The final section sets the stage for deeper analysis of bond vibrations, introducing the idea of modeling bonds as springs using concepts from classical physics. It teases future content where classical physics principles will be used to further explain molecular vibrations and how they relate to the absorption of energy in infrared spectra.

Keywords

💡Infrared light

Infrared light is a type of electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves. In the context of the video, infrared light is used to study molecules by causing them to absorb energy, which can lead to bond vibrations. This is a key concept as it sets the stage for understanding how molecular structures can be analyzed through infrared spectroscopy.

💡Stretching vibration

A stretching vibration refers to the movement of atoms within a molecule when they are pulled apart and then pushed back together, similar to a spring oscillating. The video uses the analogy of a spring to explain how bonds between atoms, such as the carbon-hydrogen bond, can stretch and contract. This is crucial for understanding how infrared light interacts with molecules and causes these vibrations.

💡Bond

A bond in chemistry is the force that holds two atoms together in a molecule. The video script discusses bonds, particularly the carbon-hydrogen bond, as springs that can stretch and vibrate when energy is applied. This concept is fundamental to the discussion of how infrared spectroscopy can be used to identify and study molecular structures.

💡IR spectrum

An IR spectrum is a graphical representation of how a molecule interacts with infrared light. The video explains that by shining a range of infrared frequencies through a compound, the absorbed frequencies can be observed in the IR spectrum. This is essential for identifying which frequencies correspond to stretching vibrations of specific bonds within the molecule.

💡Percent transmittance

Percent transmittance is a measure of how much light passes through a sample without being absorbed. In the video, it is used to explain that if a frequency has 100 percent transmittance, no energy at that frequency is absorbed by the molecule, indicating that the compound does not have a bond that vibrates at that frequency.

💡Wave number

Wave number is a unit of measurement used in spectroscopy to describe the frequency of light in terms of the reciprocal of the wavelength. The video explains that wave number is calculated as one over the wavelength in centimeters and is directly proportional to the frequency of light. This concept is important for interpreting IR spectra and understanding the relationship between wavelength, frequency, and the energy absorbed by molecules.

💡Speed of light

The speed of light is a fundamental constant in physics, approximately 3×10^10 centimeters per second. In the video, it is used in the calculation of frequency from wave number, demonstrating how the speed of light relates to the energy of the light absorbed by a molecule during infrared spectroscopy.

💡Diagnostic region

The diagnostic region in an IR spectrum refers to the area that contains signals which can be used to identify specific functional groups within a molecule. The video highlights that signals in this region are diagnostic for certain functional groups, such as the presence of a triple bond, which can help in determining the molecular structure.

💡Fingerprint region

The fingerprint region of an IR spectrum is the area that contains a complex pattern of signals unique to each molecule. While it is more challenging to interpret than the diagnostic region, it is used to distinguish between different molecules due to its unique pattern. The video script mentions that this region is like a fingerprint for the molecule.

💡Functional group

A functional group in chemistry is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. The video script uses the example of a triple bond, which is indicative of a specific functional group, to illustrate how IR spectroscopy can be used to identify the presence of certain functional groups within a molecule.

Highlights

Infrared light can cause molecules to absorb energy, leading to stretching vibrations of bonds.

Stretching vibrations are likened to the oscillation of a spring, providing a model for understanding bond behavior.

The carbon-hydrogen bond is used as an example to illustrate the spring model of a molecular bond.

Infrared (IR) spectrum analysis allows for the identification of specific bond vibrations within a molecule.

Percent transmittance in an IR spectrum indicates which frequencies of light are absorbed by a compound.

A frequency of 3,000 or 4,000 is used to represent light frequencies in the context of IR spectrum analysis.

100 percent transmittance signifies that no light is absorbed at a particular frequency by the compound.

Less than 100 percent transmittance indicates that some light is absorbed, suggesting a stretching vibration of a bond.

The specific frequency absorbed by a molecule can be identified by the presence of a signal in the IR spectrum.

Wave number is defined as one over the wavelength in centimeters and is used to analyze light properties.

The relationship between wave number, frequency, and the speed of light is explored, with calculations provided.

The frequency of light is directly proportional to the wave number, a key concept in interpreting IR spectra.

The diagnostic region of an IR spectrum is highlighted as a key area for identifying functional groups in molecules.

The fingerprint region of an IR spectrum is described as unique to each molecule, useful for molecular identification.

The importance of signal location, intensity, and shape in the diagnostic region for understanding molecular structure is emphasized.

The video concludes with a预告 of upcoming content focusing on the classical physics of bonds as springs.

Transcripts

- [Voiceover] If you shine infrared light on a molecule,

it's possible for the molecule to absorb

energy from the light.

Energy from the light can cause a bond to stretch.

We call that a stretching vibration.

You can have other kinds but we're only going to

focus on stretching here.

The stretching vibration of a bond is like the oscillation

of a spring so you can think about a bond

as being like a spring.

Let's think about this bond right here.

The bond between the carbon and the hydrogen.

We're going to model that bond as a spring.

We're going to attempt to draw a spring in here.

So here's the spring.

Then let's put in the carbon on one side.

So we have the carbon on one side and

the hydrogen on the other side.

So the stretching vibration of the bond is

like the oscillation of the spring.

So if you had a spring and you had two masses

on either end of the spring, if you put some energy in,

you can stretch that spring.

So you could pull the carbon this way and

you could pull the hydrogen that way so it's like

the stretching of this bond here.

We know that springs also contract.

So then the spring can pull back in this direction

and you get an oscillation of the spring which is

once again how we model the stretching vibration of a bond.

Let's look at the IR spectrum for this molecule.

So we're talking about one octyne here.

If you shine a range of infrared frequencies through

a sample of this compound,

some of the frequencies are absorbed by the compound.

You can tell which frequencies are absorbed by

looking at your infrared spectrum here.

For right now, let's think about these numbers,

like 3,000 or 4,000.

Let's think about those as representing

frequencies of light.

Over here we have "% transmittance."

If you had 100 percent transmittance,

let me go ahead and draw a line up here.

So 100 percent transmittance, let's say we're talking about

this frequency of light.

I look at the frequency of light, I go up to here,

and I can see I have 100 percent transmittance.

100 percent transmittance means all that light was

transmitted through your sample.

If all the light went through your sample,

nothing was absorbed.

So this particular frequency was not

absorbed by your compound.

If you're talking about less than 100 percent transmittance,

let's say for this frequency right here.

So for this frequency, we have the signal here,

appearing at this frequency.

We don't have 100 percent transmittance so that means

not all of the light went through the compound.

Some of it was absorbed.

So this specific frequency was absorbed by the molecule.

That energy can cause a bond to stretch,

and we get a stretching vibration.

Actually this signal corresponds to the bond

that we've been talking about.

So this signal indicates the stretching

of this bond right here.

Let's think about wave number next.

We just talked about percent transmittance,

let's think about wave number.

I've been calling all these things frequencies,

different frequencies of light.

Let's see how wave number relates to the frequency of light

and also the wavelength of light.

The definition of wave number, so a wave number,

here's the symbol for wave number.

The definition of wave number is it's equal to

one over the wavelength in centimeters.

So if we had a wavelength of light of .002 centimeters,

so let's go ahead and plug that in.

A wavelength of light of .002 centimeters,

what would be the wave number?

Let's get out the calculator here and let's do that math.

One divided by .002 is equal to 500.

So that's equal to 500.

Your units would be one over centimeters,

or you could write that, meaning the same thing.

So that's the wave number.

That's the wave number.

If you go over here, a wave number of 500,

you can think about this corresponding to a

particular wavelength of light.

Of course this also relates to frequency because we know

that wavelength and frequency are related to each other.

So let's get some more room down here.

We know that the wavelength times the frequency,

wavelength times the frequency,

so lambda times nu, is equal to the speed of light.

That's equal to c.

So if I wanted to solve for the frequency, solve for nu,

the frequency is equal to the speed of light divided by

lambda divided by the wavelength.

That's the same thing as one over lambda

times the speed of light.

One over lambda was our definition for wave number.

So you can say that the frequency of light is

equal to the wave number times the speed of light.

So let's go ahead and do that calculation here.

We talked about this as being a particular wavelength.

We found the wave number.

Let's plug that wave number into here and see what we get.

So the wave number was 500.

Units were one over centimeters.

Multiply that by the speed of light.

We need to have the speed of light in centimeters.

That's three times approximately,

three times 10 to the tenth centimeters per second

is the speed of light.

Notice what happens to the units.

The centimeters cancel.

Let's do that math.

Let's get some room over here.

We take the wave number and we multiply that by

the speed of light in centimeters.

So three times 10 to the tenth.

We get 1.5 times 10 to the thirteenth.

The frequency would be 1.5 times 10 to the thirteenth.

The units would be one over seconds or you

could use hertz for that.

A wave number corresponds to the wavelength,

And you can also get the frequency from that.

So we have, let me just rewrite this really quickly,

so frequency is equal to wave number

times the speed of light.

Wave number is equal to the frequency

divided by the speed of light.

We'll use this in a later video.

We'll come back to this idea.

For right now the frequency of light is directly

proportional to the wave number.

You can look at an infrared spectrum and,

go back up here.

You can look at an infrared spectrum and call this

down here, you can call this wave number,

you can refer to it as a frequency,

you can call it whatever you want as long as you

understand what's going on here.

Let's look more in detail at this infrared spectrum.

Let's draw a line at approximately

1,500 wave numbers right here.

The left side, the left side of that line,

so we've divided our spectrum into two regions.

The region on the left is called the diagnostic region.

So this is called, this is called the

diagnostic region of our spectrum.

It's because a signal in this region can be diagnostic

for a certain functional group.

For example, this signal right here,

if we go down here to the wave number,

that signal is at approximately 2,100

for this wave number here.

That's corresponding to the triple bond here.

This tells us a functional group.

This tells us that a functional group is present.

This triple bond is present.

It's diagnostic.

It helps you figure out the structure of the molecule.

You can figure out different functional groups

present in molecules using IR spectra.

So the right side, the right side of this line is

called the fingerprint region.

So this is the fingerprint region.

It's harder to interpret the fingerprint region.

It's much more complicated.

It's not as easy to see different signals.

It's extremely complicated,

but it is unique to each molecule.

So it's like a fingerprint for the molecule and so you

can match up IR spectra.

If you have an unknown you can look at the

fingerprint region, and again it's unique.

All these different lines are unique to that molecule.

So we have the diagnostic region and the fingerprint region.

We're not gonna deal much with the fingerprint region.

Maybe a little bit.

We're going to focus in on the diagnostic region.

We're gonna focus where the signals appear.

All right, so I look at the signal,

I go down to here, and I get a specific wave number.

So the location of the signal is pretty important.

I did want to point out that, if you look at what I used

for the wave number here, I changed how I did everything.

So the spacing is different.

It doesn't really matter.

I only did this to fit this video and to make it

work for this video.

I'm also gonna hand draw all my IR spectra,

so it's really not gonna be perfect.

The idea is not worry too much about what they

give you here for the scaling for the wave number.

Think about where the signals appear.

So the location of this signal, approximately

2,100 wave numbers.

You also want to think about the intensity of the signal,

and the shape of the signal.

We'll talk much more about those in later videos.

On the next video we need to develop this idea of

bonds as springs a little bit more.

So we'll think a little bit about some classical physics.