Molecular SpectroscopySCES/P2250

Main Reference: Fundamentals of Molecular SpectroscopyColin N Banwell and Elain M McCashFourth EditionTata McGraw-Hill Publishing Company Limited

Reference book is imperative. Notes will refer to the book.

Proforma of Molecular Spectroscopy Course

Fakulti :Jabatan :

SainsKimia

Program Pengajian :

B.Sc. (Hons.)

Kod Kursus :Tajuk Kursus :Jam Kredit :

Prasyarat / Keperluan Minimum Kursus :

SCES2250Spektroskopi Molekul3

Kimia Fizik II

Objektif Kursus : Memberikan kefahaman tentang spektroskopi molekul sebagai alat utama untuk memahami struktur molekul dan sifat-sifatnya

Sinopsis Kandungan Kursus :

Spektroskopi putaran, getaran dan elektronik.Spektrum elektromagnet. Asas-asas spetroskopi. Spektrum putaran dan getaran molekul dwiatom dan poliatom. Kesan Raman; spektrum Raman putaran dan getaran. Keadaan elektron dalam molekul; teori kumpulan asas untuk spektroskopi, pencirian keadaan elektron (term symbols). Peralihan elektron, spektroskopi elektronik, momen dwikutub

peralihan. Pendafluor dan pendafosfor.

Spektroskopi resonans magnetSifat magnet elektron dan nucleus. Kelakuan electron dan nucleus dalam medan magnet: pengkuantuman momentum sudut, tenaga spin, taburan Boltzmann spin, pemagnetan makroskopik. Resonans magnet dan eksperimen. Parameter spectrum NMR: anjakan kimia, pengkupelan spin-spin dan masa relaksasi nucleus (T1 dan T2). Faedah medan magnet tinggi. Resonans dubel. Kesan relaksasi dan resonans dubel ke atas spectrum NMR karbon-13. Masa relaksasi T1 dan maklumat gerakan molekul. Kelakuan nukleus kuadrupol sebagai nukleus tak-magnet. . Skala-masa NMR; kesan fenomenon pertukaran ke atas spectrum NMR. Prinsip asas bagi NMR bagi keadaan pepejal, NMR dua-dimensi dan NMR mengimej.

Kaedah Penilaian :

Penilaian berterusan: 30% Peperiksaan: 70%

English Version

Faculty :Department :

ScienceChemistry

Course programme:

B.Sc. (Hons.)

Course code:Course title:Credit hours:

Pre-requisite:

SCES2250/2132/SCEP2132/SCES2235Molecular Spectroscopy3

Physical Chemistry II

Course objective:

To give a fundamental understanding of molecular spectroscopy as the most important tool in understanding molecular structure and its characteristics

Synopsis of course content:

Vibration, rotation and electronic spectroscopy.Electromagnetic spectrum. Fundamentals of spectroscopy. Rotational and vibrational spectra of diatomic and polyatomic molecules. The Raman effect; rotational and vibrational Raman spectra. Elementary group theory for spectroscopy. Electronic states in molecules and term symbols. Electronic transition, transition dipole moment and electronic spectra. Fluorescence and phosphorescence.

Magnetic resonance spectroscopy.Magnetic properties of the electron and nucleus: spin angular momentum and magnetic moment. Behavior of electron and nucleus in magnetic field: space quantization of angular momentum, spin energy, Boltzmann distribution and macroscopic magnetization. Magnetic resonance and experiment. Parameters in the NMR spectrum: chemical shift, spin-spin coupling and nuclear relaxation time (T1 and T2). Advantages of high magnetic field. Double resonance. Effect of nuclear relaxation and double resonance on carbon-13 NMR spectra. Relaxation time T1 and molecular motion. Behavior of quadrupolar nuclei as non-magnetic nuclei, NMR time-scale; effect of exchange phenomena on NMR spectra. Basic principles of solid-state NMR, two-dimensional NMR and NMR imaging.

Grading: Continuous assessment: 30% Examination: 70%

Other references:

1) Modern SpectroscopyJ. Michael HollasSecond EditionWiley

2) Molecular SpectroscopyJohn M BrownOxford Science Publications

3) Physical ChemistryP W AtkinsOxford

Other Spectroscopy/Physical Chemistry books available in the library

1 Introduction to Spectroscopy

1.1 Electromagnetic Spectrum

Molecular spectroscopy may be defined as the study of the interaction between electromagnetic waves (EMW) and matter (atoms or molecules).

When matter (molecules) absorb EMW, the molecules can undergo changes. These are broadly classified as: 1) rotation 2) vibration 3) redistribution of electrons.

The three types of changes occur when molecules absorb EMW of differing energies.

1.2 What is EMW? (pg. 1-3)

EMW of which visible light forms an obvious but very small part may be considered as a simple harmonic wave propagated from a source and traveling in a straight line except when refracted or reflected.

Diagram1.1: EMW

EMW applethttp://mutuslab.cs.uwindsor.ca/schurko/animations/emwave/emwave.htm

EMW consists of electric field and magnetic field parts; both waves are perpendicular to each other.

Q1: Write the equations of EMW:

Basic wave equation is c/ where is the wavelength in meters, c is the speed of light in ms-1 and is the frequency in Hz (cycles s-1). In spectroscopy, a widely used unit is the wavenumber (cm-1), giving a generalized connotation of energy, frequency etc. This is defined as:

ν=1/ λ

Using appropriate units for c (cms-1) and (cm), it would be easy to convert from Hz to wavenumber and vice versa. A bar above indicate units in cm-1

Q2: Convert – 133MHz to cm-1, 400cm-1 to Hz, 300nm to cm-1

1.3 EMW spectrum (pg. 5-9)

To answer the question concerning the types of interaction that will occur when molecules absorb EMW, we need to understand that EMW exists in different energies. This can be conveniently expressed in terms of an EM spectrum.

Visible light spectrum

Understand the EM spectrum presented (see a more lively EMW spectrum – in folder). Make sure you develop the sense of magnitude related to the spectrum. It is imperative that you understand, for example that EM in the microwave region, when absorbed by a molecule will cause the molecule to rotate.

Q3: Write brief notes on the EM spectrum and how it interacts with matter:

1.4 Quantization of Energy (pg. 3-5)

Initially matter was thought to be able to absorb energy in the form of EMW continuously, soaking up all the energy available over the whole spectrum of EMW. This was found to be incompatible with reality. Max Planck published a revolutionary paper suggesting that energy of an oscillator is discontinuous and change of energy can happen by means of a jump between two energy states.

Extending this idea, a molecule can also be thought to possess energy in quantized form – rotation, vibration, electronic.

Etotal = Erotation + Evibration + Eelectronic (ET = Er + Ev + Ee )

This is known as the Born Oppenheimer approximation.

Matter absorbs EM energy in a quantized manner. Best visualized via energy level diagram:

Diagram1.2 : Transitions between two energy levels

Upon absorption of EMW of a certain energy, a transition is said to occur. It is then said that the matter has absorbed a photon of EMW. This transition can be described by an equation suggested by Planck.

ΔE=hνs

Where E = En-Em.

νs is the frequency absorbed. This is in Hz. The symbol for frequency in

cm-1 is usually νs unless different symbols are noted to have units of cm-1.

If we plot a graph of intensity of absorption vs. frequency, we get a

spectrum with a peak at νs .

Three types of transitions are shown in diagram1.2. What are they?

Q4: Draw a spectrum with absorption at νs .

In spectroscopy we can envisage the quantized energy levels as depicted below:

Diagram1.3 : Magnitude of the different type of energy levels

The physics that deals with quantized energies of molecules is quantum mechanics. The basis of quantum mechanics is the Schrodinger equation. Solution to this equation basically gives two things. Energy (levels) (E) and wavefunctions (Ψ) associated with each energy levels.

As an example, lets jump straight into a real case – the hydrogen atom. A Schrodinger equation for the kinetic and potential energy at play within the atom which consists of an electron and a proton is set up. It is then solved to give energy levels and wavefunction. These wavefunctions are appropriately combined to result in atomic orbitals.

Thus, we have 1s, 2s, 3s, 2p etc. atomic orbitals (which are really combination of wavefunctions from the solutions of the schrodinger equation). Each orbital is identified with an energy level represented (or labelled) by its quantum number.

The details of this can be obtained from various Physical Chemistry texts.

Since hydrogen atom is an atom, the only energy it possesses is the electronic energy (besides the continuous translational energy). Absorption of EM in the region of UV/Vis will cause electrons to be excited from one level to another, corresponding to the energy absorbed.

A molecule, such as A-B on the other hand will have rotational and vibrational energy besides electronic energy.

Q5: Why do atoms lack rotation and vibrational energies?

Upon absorption of EM by a molecule, we ask three questions:

1) The frequency of absorption2) The line intensity3) The linewidth of the spectral line.

The frequency of the absorption can be read off the spectrum and the type of interaction depends on the value of the frequency (sense of magnitude? EM spectrum?)

Q6: Draw a single line spectrum and illustrate the spectrum with labels appropriate to the three questions:

Questions 2) and 3) will be answered below.

1.5 Spectral line intensity (pg. 18-20)

Spectral line intensity is determined basically by three factors:

a) Transition probability – A rigorous quantum mechanical treatment of EM-matter interaction will produce what is known as the transition probability. It is represented by:

Rnm=∫ψn∗μ⃗ψmdτ

Where ψnand ψ

m are wavefunctions of states n and m

respectively. μ⃗ is the electric dipole moment operator. The spectral line intensity is proportional to Rnm

2. If Rnm = 0 then there is no transition (forbidden), whereas if Rmn <> 0, there can be a transition (allowed). This is also known as the gross selection rule for a transition. The transition probability will be rationalized later for each type of transition; rotation, vibration and electronic.

b) Population of states – between two states, if the lower state is of higher population, and the upper one is lower in population, then the transition is very much favoured. Boltzmann distribution determines the relative population. Understand this properly. This will be discussed further for each type of interaction

Q7: Write the Boltzmann distribution population ratio of two states m and n

c) Concentration of sample or path length of sample – works within the Beer Lambert Law:

log Io/I = cl

Where I is the transmitted light intensity, Io is the incident light intensity, c is the concentration of sample, l is the path length and is molar absorption coefficient ( a function of frequency).Good applets on this can be obtained from :

http://www.chm.davidson.edu/ChemistryApplets/#Spectrophotometry

1.6 Spectral linewidth (pg. 17-18)

There are 3 factors that effect line broadening:

a) Natural linewidth – Heisenberg uncertainty principle. This is natural in all spectroscopic measurements.

Δν= 12πτ

There will always be broadening because can never be infinite. Broadening due to only one frequency – homogeneous spectral line

b) Doppler broadening – due to Doppler shift – based on Doppler effect

Diagram1.4 : Doppler effect

As an example: Sound waves emanating from an ambulance moving to the right. The perceived frequency is higher on the right, and lower on the left. Imagine molecules as source being stationary, or moving relative to the detector. Molecules move randomly – as a result, you get many different frequencies centred around a mid frequency, each frequency overlapping with each other forming a gaussian envelope – heterogeneous spectral line. Doppler effect applet-

http://www.phy.ntnu.edu.tw/java/Doppler/Doppler.html

c) Pressure broadening (collision broadening)

Due to continuous collision between molecules/atoms. This causes perturbation to the energy levels, thus broadening it – more severe in liquid. Can be reduced by taking spectrum in gas phase and lowering the pressure of gaseous samples

2 Representation of Spectra – practical aspects of spectroscopy (pg. 9-17)

There are basically 3 types of spectroscopy experiments:

1) Absorption2) Emission3) Scattering

Absorption spectrometer applet: http://artsci-ccwin.concordia.ca/facstaff/a-c/bird/c241/D2-part2.htmlWe shall only look into the first two at the moment.

2.1 Absorption

- UV/Vis, IR (optical) and microwave (non-optical)

Q8: Draw the complete diagram of a basic optical absorption experiment complete with appropriate labels and explanation of the labels.

2.2 Emission

- UV/Vis (optical)

Q9: Draw the complete diagram of a basic optical emission experiment complete with appropriate labels and explanation of the labels.

Diagram 1.4a – typical arrangement of an absorption spectrometer

Among the important parts of the spectrometer are:

1) Source – the types of sources will depend on the types of interaction desired (refer to handouts).

2) Slit – used to direct EMW to sample or to determine resolution. Unfortunately, slits also reduce the total intensity that could have been utilized.

3) Dispersing element or a grating – used to disperse EMW into its constituent frequencies. Used in conjunction with a slit (see handout). This part of the spectrometer is usually called a monochromator (below). An applet demonstrating the action of a grating can be found at:

http://www.mtholyoke.edu/~mpeterso/classes/phys301/laboratories/balmer.html

http://www.physics.uq.edu.au/people/mcintyre/applets/optics/grating.html

Diagram1.5 : Monochromator (action of a grating)

4) Sample holder – to hold sample – quartz and glass (UV/Vis), KBR, NaCl salt slabs (IR) – depending on the EMW region

5) Detector – types of which are determined by the different EMW sources – see handout

6) Display – displays the spectrum – computers, chart recorders.

We should also consider the two types of operation in a spectrometer – single beam and double beam operation – this is done in order to compensate for the background which may affect the spectrum due to the presence of water vapour and carbon dioxide absorption. This is especially true for IR spectroscopy (pg. 91-93) – See brochure of Lambda series of spectrometers.

2.3 Signal to Noise Ratio (pg. 15)

Modern electronic instruments can’t escape from noise. The noise can come from the electronic cricuitry in the spectrometer or from the detector. Noise is usually described in termso fluctuation. To differentiate between signal and noise, we need the signal to be at least 3 or 4 times larger than noise.

Signal-to-noise ratio is an engineering term for the power ratio between a signal (meaningful information) and the background noise:

Because many signals have a very wide dynamic range, SNRs are often expressed in terms of the logarithmic decibel scale. In decibels, the SNR is 20 times the base-10 logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio:

where P is average power and A is RMS amplitude

There are various techniques to increase S/N ratio such as filtering and computer averaging (pg. 26).

2.4 Spectral resolution (pg. 16)

Often used as a mesure of the performance of a spectrometer.

Q10: Explain how the size of the output slits can affect the resolving power of a spectrometer. Use appropriate illustrations.

2.5 Fourier Transformation in Spectroscopy (pg. 20-26),(pg. 93-96)

Conventionally a spectrum is taken by sweeping over the whole range of frequencies within the spectral domain. Say, a spectrum from 1000cm-1 to 4000cm-1 is to be recorded. The frequency is swept smoothly from 1000 to 4000cm-1 (by rotating the grating in the monochromator). This takes a long time. Very inefficient. With FT technology, the whole spectrum can be taken simultaneously within the required region. To understand FT, first look at beat frequency. Visit:

http://www.ithacasciencezone.com/explrsci/dswmedia/tonebeat.htmhttp://physics.pingry.org/Explorations/Acoustics/Beats/

for audio demonstration of beat frequency.

300Hz

+ about 300Hz (say 301 Hz)gives….….a beat frequency.

..which is an interference pattern. Imagine interfering more than 2 frequencies, in fact imagine interfering hundreds of frequencies together. Conceptually, a combination of many different frequencies will result in a particular interference pattern. This pattern is in time domain (serial domain). You can’t tell what the different frequencies are…we have to convert this interference pattern into a frequency domain spectrum where interfering frequencies can be identified – FT!!

Note that an interferogram shows an oscillating signal decaying smoothly to zero. The oscillation is due to the beat pattern set up by all the superimposed, but slightly different sine waves emitted. The decay can be imagined by considering that initially, all waves in the package are in step but as time goes by they start to be out of phase that on average, they cancel out (pg.22-23)

Good applets to demonstrate time domain to frequency domain using FT..

http://storm.uni-mb.si/CoLoS/applets/fft/ftd.html

http://sepwww.stanford.edu/oldsep/hale/FftLab.html

By using an interferometer, we can take the spectrum instantaneously and by using FT, the interferogram is transformed into a frequency

domain spectrum. Read about the interferogram and understand how it works.

A typical arrangement of an FTIR is depicted below (diagram 1.6). How does it work?

Diagram 1.6 : FTIR

Demonstration of Michelson interferometer.

http://www.physics.uq.edu.au/people/mcintyre/applets/michelson/michel.html

Tutorial:

1) Convert 99.3MHz, 103.3MHz, 105.9MHz dan 2.4GHz into wavenumber (cm-1).

2) Which region of the EMW causes molecular rotation? What are the selection rules required in order to observe rotational spectrum?

3) A molecule X absorbs EMW at 3500cm-1. What happened to it?4) Electron redistribution in a molecule or atom occurs in the EMW

region of….5) Draw the diagram of a simple absorption spectrometer and

explain the workings of this apparatus6) What is the function of the first dispersion element of an

emission spectrometer (in the monochromator before EMW enters the sample?

7) Give example of the source of EMW for each identifiable region of the spectrum.

8) Write down the energies of a molecule based on the Born Oppenheimer approximation.

9) List down the factors that influence the intensity of a spectral line

10) Discuss the three types of spectral line broadening11) Discuss how an FTIR spectrometer works. 12) Explain why the natural linewidth in a rotational spectrum

is smaller that the natural linewidth of a UV spectrum?13) What is S/N ratio?14) What is Rmn?