Hello, welcome to module 1 of the course on Application of Spectroscopic Methods in

Molecular Structure Determination. In this module, you will ask questions about what

is spectroscopy? What is a spectrum? What is electromagnetic radiation? What are the

various regions of the electromagnetic radiations and when matter is interacting with electromagnetic

radiation, what are the changes that take place in the matter? What is the nature of

light? What is absorbance? What is transmission? All these terminologies that are used in spectroscopy,

we will deal within this general module in this particular presentation.

Now, spectroscopy in the broadest term is defined as the study of interaction of electromagnetic

radiation with matter and when we talk about matter we are dealing with atoms and molecules.

So, essentially the interaction of atoms and molecules with electromagnetic radiation is

what constitutes the term spectroscopy. Spectroscopy is a study of interaction of electromagnetic

radiations with matter as a function of frequency because whenever we record a spectrum, we

are essentially scanning various frequencies and recording the response of the material

that we study for the various frequencies of the electromagnetic radiation. Spectroscopy

is the study of the exchange of energy between electromagnetic radiation and matter. Here,

we are talking about the electromagnetic radiation being the source of energy, the energy of

the photon of the electromagnetic radiation gets transferred on to the atoms and molecule

and the changes that occur in the atoms and molecule as result of the absorption of energy

is what is recorded as a spectrum.

Here is an electromagnetic spectrum dealing with various frequencies and various energies

of the photons and the different regions of electromagnetic spectrum. The regions of the

electromagnetic spectrum are labeled here as radio waves, micro waves, infrared waves,

visible region of the electromagnetic spectrum, ultraviolet region of the electromagnetic

spectrum, the soft and the hard x-rays. Finally, the most powerful energetic gamma rays in

the right-hand side of the spectrum. Now, the top scale corresponds to wavelength which

is expressed in meters which is a unit of length. Just to understand what is this unit

of length actually means in terms of the dimensions of commonly seen objects, we can for example;

correlate the wavelength of radio frequency region corresponding to something like 10

meters to about 1000 meters corresponding to the size of a building or size of a stadium

and so on. On the other hand, if you look at the size of a molecule which is of the

order of a pico meter or so. We are talking about the dimensions of the wavelength of

the x-rays or the gamma rays in this particular region. Suppose, if we take a tiny dot on

a piece of paper which corresponds to about a millimeter or less than a millimeter that

corresponds to the micro wave radiation region of the electromagnetic spectrum. So, just

sort of gives you a perspective of what this unit of dimension in terms of the length unit

corresponds to commonly observed objects on our day to day life. In the bottom, the frequency

scale is mentioned. Frequency units are waves per seconds, in other words per second is

the unit that is normally used for the frequency scale. As you can see here the frequencies

starts from 10 to the power 6 per second to about 10 to the power 20 per second in the

highest frequency region which is a gamma ray region. As the frequency increases the

energy content also increases because frequency is directly proportional to energy in terms

of E is equal to h nu, where nu is the frequency of the electromagnetic radiation.

The last scale corresponds to energy per photon of the electromagnetic region, spectral regions.

For example, we are talking about something like 10 to the power minus 9 electron volt.

Electron volt is the energy unit that in which this is this particular scale is expressed

10 to the power minus 9 corresponds about to a nano electron volt is what we are talking

about a very low energy in terms of the energy content of a photon of a radio frequency wave.

On the other hand, if you come to the gamma rays it is about 10 to the power 6, one mega

electro volt is what we are talking about and this is a high energy radiation, normally

we call it as ionizing radiation because they ionize substances through which they pass

through. So, we have a wide range of energy content

per photon of the various regions of the electromagnetic radiation depending upon what kind of a radiation

that is being used. Different processes take place in atoms and molecules and the response

of the atom and molecules for the various frequency regions is what is recorded as a

spectrum.

In the electromagnetic spectrum, there is a very small portion what is known as the

visible portion of the electromagnetic radiation and this is the only portion to which human

eye is sensitive. In other words, we will be able to perceive only the colors of this

particular region and not see the colors or not see the other regions like for example,

the ultraviolet or the infrared spectroscopy. They are invisible to the human eye. Human

eye is sensitive to only about 380 or 390 nanometer, which is the violet region of the

electromagnetic spectrum to about 760 or 780 nanometers, which is the red region of the

electromagnetic spectrum. This is very important to know because when we talk about the UV

visible spectroscopy, you will be dealing with this particular spectral region in the

UV visible spectroscopic technique.

Having defined spectroscopy as a study of interaction of electromagnetic radiation with

matter, now we have to depending upon the region of the electromagnetic spectrum that

is used, what are the various processes that can take place in atoms and molecule? How

does one receive information in these processes? And what are the spectroscopic techniques

that corresponds to various wavelength regions of the electromagnetic spectrum?

This is actually defined in this particular table. If you look at column number one, the

region of the electromagnetic radiation, energy per photon is given starting from gamma rays

which are the powerful rays to the radio waves which of the least energetic rays that we

are dealing with in this particular table. Now, the processes that occur in molecules

and atoms corresponding to this particular energy regime is what is mentioned on this

particular column and the spectroscopic technique, corresponding spectroscopic techniques are

also given in red. Now, if we take for example, gamma rays, gamma rays are very powerful high

energy photons and they can cause transitions within the nucleus causing the change of nuclear

configuration and this essentially constitutes the Mossbauer spectroscopy.

On the other hand, if we come to microwave which are very, very low energy radiation

the transition among the rotational levels of a molecule is what is happening in the

case absorption of the microwave radiation and this essentially constitutes the rotational

spectroscopy of the electromagnetic spectrum corresponding to the microwave radiation is

called the rotational spectroscopy. Now, if we talk about radio wave frequency region

of the electromagnetic radiation then we are talking about change in the nuclear or the

electron spin in the presence of the external magnetic field. In the presence of an external

magnetic field, nuclear spins and electron spins have different energies, ground state

and excited state and the cause of spin change from the ground state to the excited state

is what is normally absorbed during the electron and the nuclear spin resonance electron spectroscopy.

NMR and ESR are the techniques which are responsible for this and we will deal with NMR in this

particular spectroscopic course in much more detail.

This particular slide tells us something about the responses upon interaction of electromagnetic

radiation. In other words, when a sample is being excited by electromagnetic radiation,

what are the processes that can occur in the sample? Now, let us consider a incident light

falling on the sample in this particular direction. I(0) is the intensity of the incident light

that is falling on this substance. The substance can absorb certain amount this light and then

transmit the remaining light into the other side, I is the intensity of the transmitted

light. Let us say for example, so I (0) minus I would

correspond to the absorbance or the intensity that is being absorbed by the sample itself.

Instead of absorbing the light the sample can simply scatter the light for example,

if it is scatters the light and if it is the light of the same wavelength then we call

it as Rayleigh scattering. If it scatters light of different wavelength, then we call

it as a Raman scattering. Raman spectroscopy is based on the scattering phenomena and we

deal with Raman spectroscopy along with our vibrational spectroscopy for the structural

elucidation purposes. Now, instead of the absorbed energy instead of being scattered

and so on, the molecule can also emit light for example, in the form of light emission

which corresponds to the emission spectroscopy. We have fluorescence spectroscopy and phosphorescence

spectroscopy as emission spectroscopic techniques corresponding to this. Now, let us define

certain terminologies that are used in the area of spectroscopy. The ratio of I by I(0),

in other words the intensity of transmitted light to the intensity of the initial light

is what is known as the transmittance or capital T is the symbol that is used for this particular

terminology. Now, logarithm scale of I(0) divided by I

[logI(0)/I] is what is known as the absorbance, capital A is what is used for as the symbol

for this particular term and absorbance transmittance are related to each other as you can see for

example, one is the logarithmic quantity which is logarithm of 1 by T (log(1/T) is what is

known as the absorbance. Now, the type of spectroscopic techniques that one can have

depends on the kind of phenomena that one observes. If the absorbance of the light is

what is being observed, then we call it as Absorption spectroscopy. If emission is what

is observed, then we call it as the emission spectroscopy. If scattering is what is being

observed then we call it as scattering spectroscopy, examples of Absorption spectroscopy are the

infrared, UV visible spectroscopy, rotational spectroscopy and so on. Emission spectroscopy

corresponds to fluorescence and phosphorescence spectroscopy and scattering spectroscopy for

example, corresponds to Raman spectroscopy.

Now, let us ask this question what is a spectrum? A spectrum is essentially a plot of energy

in the x-axis and the response that is being received from the sample as y-axis The energy

is the energy of the electromagnetic radiation that is being applied in this particular case

and the response is the kind of response that one records in terms of whether it is an absorbance

or transmittance or emission intensity or scattering intensity is what is being recorded

on the y-axis and the peaks that are normally seen in the spectrum are essentially due to

the intensities of absorption or transmittance or intensities of emission or scattering is

what is plotted against the energy of the electromagnetic radiation.

Certain laws are governing the quantitative aspects of spectroscopy is important to understand.

One is called the Beer Lambert law, is a very basic law dealing with the quantitative correlation

between absorbance and the concentration. Now, this expression is what is known as the

Beer Lambert law, absorbance which is actually logarithmic ratio of I(0) by I is directly

proportional to the concentration. In other words, when light is passing through a medium

it depends on how many number of molecules that it encounters, absorbance will be accordingly

more or less. In other words, absorbance is directly proportional to the number of molecules

the light interacts on it is path. Now, the proportionality constant is what is known

as extinction coefficient or the molar absorptivity and extinction coefficient or the molar absorptivity

is a constant at a given wavelength for a given substance and this is what makes the

absorption spectroscopy a quantitative tool in order to find concentration of unknown

substances.

Now, let us have a look at the nature of light. Light can be described to have dual nature

that is both the nature of wave as well as particle. In the wave nature of light, it

is actually explained in the form of this particle diagram that is shown here. If x

is the direction of propagation of the light then we can define two fields, one is an electric

field another one is the magnetic field. The blue one is the magnetic field and the red

one is the electric field. Thus, you can see here the two fields, namely the electric field

and magnetic field lie orthogonal to each other. In other words, they are perpendicular

to each other and light can be expressed in the form of a sine wave consisting of two

sine waves, interacting sine waves for example, one corresponding to the electric field, the

other one corresponding to the magnetic field. The distance between the hump to the hump

is what is known as the Wavelength and this wavelength is what we referred in terms of

the electromagnetic spectrum that earlier we saw in terms of the defining the regions

of the electromagnetic spectrum with the different Wavelengths and the arrows that are shown

here are the vectors of the intensity or the amplitude of the electric field in this particular

case. The amplitude of the magnetic field is shown in the blue color. So, this is essentially

the wave nature of light is being explained in this particular manner and whenever molecules

interact with electromagnetic radiation in spectroscopy like for example, a electronic

spectroscopy or infrared spectroscopy or rotational spectroscopy it is supposed to be the electric

field that is interacting with the molecules in those kinds of spectroscopy. Unlike for

example, in the case of magnetic resonance spectroscopy it supposed to be the magnetic

field that is interacting with the magnetic nuclei. So, it is in the case of NMR and ESR

essentially the magnetic field is what is interacting with the molecules.

In order to explain the other nature namely the corpuscular nature of the light one proposes

that light consists of particles called photons with finite energy content. In other words,

E is equal to h x nu this is very famous expression which deals with the energy of photon which

corresponds to the h which is the Planck's constant and nu the frequency of the electromagnetic

radiation, which is correlated to the velocity of light in vacuum namely c for example, divided

by lambda. In other words, E is equal to h x nu which is equal to the h x c by lambda,

where lambda is the wavelength of the light that we are dealing with.

In order to calculate the amount of energy that is present in a photon and one can use

this expression very easily. Let us have an example of how much of a photon energy is

present in a 500-nanometer wavelength region which is the visible wavelength region for

example. So, all you have to do is plug in the Planck’s constant which is available

readily which is 6.626 into 10 to the power minus 34 joules per second and put in the

value of the velocity of the light which is about approximately 3 into 10 to the power

8 meters per second divided by 500 nanometers, which is 500 into 10 to the power minus 9

meters. Now, the units which are the time units they

cancel out each other. The distance unit namely the meters they cancel each other finally,

you are ending up with the energy which is joules in this particular case. So, the energy

content of a photon of a 500-nanometer wavelength light is about 3.98 into 10 to the power minus

19. Light of 297 nanometers which is in the ultraviolet region has much higher energy.

In fact, this is about 400 kilo joules per mole. This is per mole is what is expressed,

in other words, if you want to convert per photon energy into a per mole energy you have

to multiple this quantity by per photon quantity by Avogadro number which is 6.023 into 10

to the power 23. So, if you multiply the per photon energy by Avogadro number you get the

energy per mole of the corresponding wavelength.

Now, according to the quantum mechanics, energy levels are quantized. We are talking about

electronic energy levels or vibration energy levels or rotational energy levels they are

quantized. What is meant by quantization? The energy gap between the ground state and

the excited state is a finite one. When we say it is a finite one then, the energy difference

and the energy of the incident photon matches with the energy difference ground state and

the excited state one can expect the absorption of the light to take place. In other words,

E1 let us say ground state energy and E2 is the excited state energy of certain system

it could be a electronic energy or it could be vibrational energy or it could be rotational

energy as the case may be. Now, the difference in the energy is delta E and the delta E is

equal to h nu. So, when the nu corresponds the electromagnetic radiation of certain frequency

corresponds to the difference in the energy between the ground state and excited state,

absorption is going to take place. Otherwise, if there is a different frequency

which does not match this particular criterion of delta E, then the absorption will not take

place that is what is implied in this particular diagram.

Now, once you say there are two energy levels, there should be a population in this energy

level of molecules and there should be a population in this energy level in this molecule. One

can easily calculate the difference in the population or the ratio of the population

in the excited state to the ground state using the Maxwell-Boltzmann distribution. Maxwell-Boltzmann

distribution is expressed by this particular equation here, n2 by n1 is the ratio of the

number of molecules in the excited state n2, n1 is the number of molecules in the ground

state and this ratio corresponds to the delta E divided by kT in terms of exponential term

being added here. Now, in the Boltzmann distribution one can calculate the ratio of the excited

state to the ground state using this particular equation.

When you do that using the Maxwell-Boltzmann distribution expression, one can show that

at equilibrium we are talking about thermal equilibrium. Let us say at room temperature

less than one percent of the molecules are present in the excited state vibrational level

at room temperature that is we are talking about the vibrational spectroscopy. When we

say there is a ground state vibration and in the excited vibration state. Nearly 99.9

percent of the molecules do exist in the lower vibrational level, compared to the higher

vibrational level and when we come to electronic energy states when we talk about ground state

electronic state to the excited state electronic state because electronic states have much

higher energy gap practically all the molecules will be present in the ground state compare

to the excited state and this is very important to understand because this kind of population

difference is directly responsible for the sensitivity of the technique that we are talking

about.

Now, let us look at the NMR spectroscopy or ESR spectroscopy, where we are talking about

nuclear spin state, ground state and excited state. The energy difference between the two

states are extremely small compared to the energy states of the electronic spectroscopy

or the infrared spectroscopy. So, as a result of that the population difference is extremely

low only parts per million difference is present in the population difference is present between

the ground state and the excited state and that makes the NMR spectroscopy as the least

sensitive spectroscopy techniques that one can think of compare to for example, the electronic

spectroscopy or the vibrational spectroscopic technique. So, the population difference is

directly responsible for the sensitivity because the intensity of absorption is directly proportional

to the excess population that is present in the ground state in comparison to the excited

state. So, when the entire population is in the ground state this spectroscopic technique

will be very sensitive because we have a large number of molecules to excite from the ground

state to excited state. On the otherhand when you have already reached

equal population between the excited state and the ground state there is very little

excess population present in the ground state to be excited to the higher level. So, accordingly

the intensity of absorption will be low. So, the sensitivity will be consequently low in

the case of NMR and other spectroscopy techniques.

Now, let us define another parameter what is known as the natural spectral line width.

Let us say, if the energy levels are quantized then the absorption and the emission should

occur in a precise frequency. In other words, a monochromatic frequency is what would be

responsible for each absorption and emission wavelengths that we are talking about, but

usually this does not happen. There is a measurable width to any spectral line which is known

as the natural line width of the spectroscopy and this is defined as shown in this particular

diagram. This is essentially a lambda plotted against the relative intensity you can see

that the spectral line has a particular shape usually at Gaussian shape and the if we take

half the intensity and measure the width of the spectral line here, this is what is known

as the full-width half-maximum spectral width and this is what is normally known as the

natural spectral line width.

Now, why do we have a natural spectral line width instead of simply line kind of a spectrum?

The finite lifetime of the excited state is what is responsible for the line width. In

other words, the finite lifetime of the excited state and the consequent uncertainty in the

excited state energy is what gives the line space for many of the spectroscopic techniques.

Now, the uncertainty principle is expressed in a different format in this particular case.

Normally, uncertainty principle is expressed in terms of momentum and position where you

can have either uncertainty in the momentum or uncertainty in position. In this particular

case, this is expressed as the energy and lifetime in other words this is could be the

uncertainty in the difference in the energies or uncertainty in the energy and this will

be the lifetime of the system. Now, when we talk about for example, lifetimes

of the excited state or the relaxation times, if you are talking about ultraviolet spectroscopy

for example, the relaxation times are the typically of the order of the pico seconds

or faster. So, there is a large uncertainty associated with a excited state energy level

and this uncertainty in the excited state energy level is what causes the line broadening

which leads to the natural line width of the UV visible spectroscopy. On the other hand,

if you look at NMR spectroscopy, where the relaxation times of the order of the seconds

much slower than the relaxation times of the electronic spectroscopy, the uncertainty in

the energy of the excited spin state is very, very low as the result of that you get sharp

peaks less than one hertz width natural line width what we get in the case of NMR spectroscopy.

Now, so far we have been talking about spectroscopic technique which have some commonality in terms

of either being absorbance or the emission kind of a spectroscopy, where electromagnetic

radiation is used as a source of energy for causing certain types of transitions. It could

be electronic transition, it could be spin transition or it could be vibrational transition

and so on. The commonality of all the spectroscopic that we talked about is very different when

we compare it with for example, Mass spectrometry.

First of all, mass spectrometry is not a spectroscopic technique at all. It is a spectrometry technique

there is no interaction between electromagnetic radiation and sample in mass spectrometry,

is a very special kind of technique. It is an extremely useful technique for molecular

structure determination that is why it is normally put under the spectroscopy many spectroscopy

books talk about mass spectrometry as well because it is useful in structural elucidation

problems. In mass spectrometry, we are talking about interaction between a sample that is

an atom or molecule and an electron which is usually high energy electron of the order

of 70 electron volts or 100 electrons volts, leading to ionization of the sample fragmentation

of the sample and so on. We will deal with the mass spectrometry slightly later and just

wanted to bring out the difference between the various spectroscopy and mass spectrometry

in terms of the, this spectroscopy being spectrometry being a special case and it comes under the

spectroscopy essentially because of it is useful not because of it is commonality with

the other spectroscopic methods.

Now, let us see what are the resources that we need to follow this particular program.

At least one or more books need to be referred to. This book which was published last year

that is in 2015 by Prof. D. N. Sathyanarayana, who was a former professor in the Department

of Inorganic and Physical Chemistry in Indian Institute of Science, Bangalore. This is a

very nice book, it deals with various spectroscopic techniques right from radio waves to gamma

rays for example, in other words he is covering the nearly the entire electromagnetic radiation

region in this particular book. It gives very fundamental aspects of the many of the spectroscopic

techniques with suitable examples and so on. I would recommend that you read this book

for clear understanding of the basics of these spectroscopic.

This is another book by name it is Spectroscopy by Pavia and others. There are four other

authors associated with this particular book. This is essentially a spectroscopy book with

emphasis on problem solving, in other words structural elucidation problem solving is

what is the emphasis that is shown in this spectroscopy book. It is an excellent text

book it is available in Indian edition should be possible for some of you to buy this book

and refer this particular book.

NMR Spectroscopy by Herald Gunther is a very famous text book for example, it is very lucidly

presented text book in the terms of the concepts and applications of NMR spectroscopy. So,

this spectroscopy book is also going to be very useful for this particular course.

This particular book namely Spectroscopic Identification of Organic Compounds by Silverstein

and Webster is actually used to be Silverstein and Basler. Now, it is called as Silverstein

and Webster in terms of the authors names and this is published in 2006. This is also

an excellent source of problem solving sessions in the spectroscopy.

Finally, the Organic Spectroscopy by William Kemp is also a good source of information

particularly, if you are dealing with organic structural which we will be doing in this

particular course.

Now, I would like to thank you for patient hearing. We will see you in the next module.

Thank you.