Engineering Chemistry: Unit 1 Slides PDF

ENGINEERING CHEMISTRY Lata Pasupulety Department of Science and Humanities ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Module content: Interaction of electromagnetic radiation with matter Electromagnetic spectrum Born –Oppenheimer approximation Microwave spect...

ENGINEERING CHEMISTRY Lata Pasupulety Department of Science and Humanities ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Module content: Interaction of electromagnetic radiation with matter Electromagnetic spectrum Born –Oppenheimer approximation Microwave spectroscopy- diatomic rigid rotor model and the rotational spectrum IR spectroscopy- diatomic harmonic oscillator and anharmonic oscillator model Electronic spectroscopy- Vibrational coarse structure(Progressions), Franck Condon Principle ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Class content : Definition of Molecular spectroscopy Interaction of electromagnetic radiation with matter Comparison between atomic and molecular spectra Quantisation of energy Absorption and emission spectra Spectroscopic units ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Molecular spectroscopy is that branch of science which deals with the study of interaction of electromagnetic radiation with matter(molecules) Incident radiation → sample → collector → diffraction grating → analyser(detector) →recorder(spectrum) ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Some important features of electromagnetic radiations are: They have dual character i.e. particle character as well as wave character, for example, a beam of light is a stream of particles called photons moving through the space in the form of waves These waves are associated with electric and magnetic fields oscillating perpendicular to each other and also perpendicular to the direction of propagation All electromagnetic radiations travel with the velocity of light Source:https://byjus.com/physics /characteristics-of-em-waves/ ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Waves can be characterised by the following properties : http://www.chemistry.wustl.edu/ ~coursedev/Online%20tutorials/ Waves.htm Wavelength : The distance between two successive crests and troughs of the wave ; λ ; m , nm or Ao Frequency : The number of cycles completed in a certain amount of time ; ν ;Hz or s-1 Wave number : Number of complete waves or cycles contained in unit distance ; , cm-1 Energy of electromagnetic radiation is given by E=hν , where E is energy,h is Planck's constant,(h= 6.625 x 10-34 Js), and ν is frequency Wavelength is related to frequency by c=λν , where c is the speed of light, λ is wavelength, and ν is frequency Wave number is related to wavelength by = 1   ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Comparision between atomic spectra and molecular spectra Atomic spectra Due to electronic transitions from one atomic orbital to another Gives rise to line spectra Source:https://socratic.org/questions/what- happens-to-the-distance-between-energy- levels-at-higher-energy-levels Source:https://www.slideshare.net/mizakamaruzzaman/ phy-310-chapter-5  ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Molecular spectra Due to electronic transitions from the Highest occupied molecular orbital(HOMO) to Lowest unoccupied molecular orbital(LUMO) Gives rise to band spectra due to rotational and vibrational transitions possible in molecules along with electronic transitions Source:https://chem.libretexts.org/Courses/Pacific _Union_College/Quantum_Chemistry/13%3A_Mol Source:https://socratic.org/questions/what-are-the-molecular- ecular_Spectroscopy/13.06%3A_Electronic_Spectr orbital-configurations-for-n-2-n-2-2-n-2-n-2-and-n-2-2 a_Contain_Electronic%2C_Vibrational%2C_and_Ro tational_Information ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Quantisation of energy Energy levels in atoms or molecules are discrete or quantised Energy can be absorbed only in packets called quanta Frequency of light absorbed when there is a transition between two energy levels depends on the difference in energy between the two energy levels If an atom or molecule in ground state absorbs energy there exists a higher energy level corresponding to the frequency of light absorbed E2 E = E2 − E1 = h E1 ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Absorption and Emission spectrum Absorption spectrum is observed when an atom or molecule absorbs energy and moves from lower energy level to higher energy level Emission spectrum arises when molecule comes from higher energy level to lower energy level E = E2 − E1 = h E2 E1 ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Spectroscopic units E=hν where Energy is expressed in Joules 1 Wave number is related to wavelength by =  since c = λν , = c Therefore E = hc or = 𝐸 ℎ𝑐 The spectroscopic unit for energy of a radiation is cm-1 It is energy expressed in wave numbers It is for convenience of using small numerals e.g. 1 cm-1 = 1.99 x 10-23 J THANK YOU Lata Pasupulety Department of Science and Humanities [email protected] +91 80 6666 3333 Extn 759 ENGINEERING CHEMISTRY Lata Pasupulety Department of Science and Humanities ENGINEERING CHEMISTRY Module 1- Molecular spectroscopy Class Content : Electromagnetic spectrum Information obtained from each region of the spectrum ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Electromagnetic spectrum Source: https://www.radio2space.com/components-of-electromagnetic-spectrum/ ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Information obtained from different regions of electromagnetic spectrum Radiofrequency region Wavelength (10m-1cm) NMR, ESR Nuclear and electron spin reversal Microwave region Wavelength (1cm-100µm) Rotational spectroscopy Rotational levels Infra Red region Wavelength (100µm-1µm) Vibrational spectroscopy Vibrational levels ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Information obtained from different regions of electromagnetic spectrum Visible and Ultra-Violet Wavelength (1µm-10nm) region UV-Visible spectroscopy Electronic states Change in electronic distribution of valence electrons X-ray region Wavelength (10nm-100pm) X-ray spectroscopy Change in electronic distribution of inner electrons γ- ray region Wavelength (100pm-1pm) γ- ray spectroscopy Rearrangement of nuclear particles  ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Raman Spectroscopy: scattering of light When monochromatic radiation is passed through a transparent medium: Most of the scattered radiation consists of radiation of incident wavelength – Rayleigh scattering Some of the scattered radiation consists of radiation with different wavelength from incident wavelength – Raman scattering When the wavelength of scattered radiation is more than that of incident radiation – Stokes lines When the wavelength of scattered radiation is less than that of incident radiation – anti -Stokes lines https://www.edinst.com/blog/what-is-raman- spectroscopy/ ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Source:https://www.researchgate.net/figure/A- simplified-diagram-of-energy-transitions-for- Rayleigh-and-Raman-scattering_fig7_327321311 Exchange of energy between the molecules and radiation Homonuclear diatomic molecules which are microwave and Infra-red inactive are Raman active ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Information obtained from different regions of electromagnetic spectrum Source: http://photobiology.info/Visser-Rolinski.html THANK YOU Lata Pasupulety Department of Science and Humanities [email protected] +91 80 6666 3333 Extn 759 ENGINEERING CHEMISTRY Lata Pasupulety Department of Science and Humanities ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Class content: Born - Oppenheimer approximation Energy level diagram of a diatomic molecule Introduction to rotational spectroscopy ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Different types of energies possessed by a molecule Translational energy (Etrans): by virtue of translatory motion of the molecule Rotational energy (Erot) : by virtue of rotation of a molecule about its centre of gravity Vibrational energy (Evib): by virtue of periodic diaplacement of the atoms of a molecule about its equilibrium position Electronic energy(Eelec) : due to the different electronic arrangements in the molecule ENGINEERING CHEMISTRY Module I- Molecular Spectroscopy Born-Oppenheimer approximation Total energy of a molecule is the sum of translational, rotational, vibrational and electronic energies, i.e. E = Etrans + Erot + Evib + Eelec Translational energy is negligibly small. Hence Born-Oppenheimer approximation can be written as E = Erot + Evib + Eelec Electronic, vibrational and rotational energies of a molecule are completely independent of each other Because the electronic energy levels are much farther apart than the vibrational energy levels which are again much farther apart than the rotational energy levels Can be written as Erot