CHT-314 Week 3 PDF

12 Sep 2024 ELECTROMAGNETIC SPECTRUM...

12 Sep 2024 ELECTROMAGNETIC SPECTRUM VISIBLE REGION SPECTRUM The boundaries describing the electromagnetic spectrum are not rigid, and an overlap between spectral regions is possible. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INTERACTION OF LIGHT WITH MATTER When we view an object (by reflected light), the object absorbs all the component of white light except those which reach our eye. A similar thing happens when we look at light which has pass through a colored filter, certain parts of white light passed through, some are reflected and those which neither passed through nor reflected are absorbed. The energy of the light wave is transferred to the material which absorbed it and is converted into heat energy. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INTERACTION OF LIGHT WITH MATTER INTERACTION OF LIGHT WITH MATTER Absorption of electromagnetic radiation will cause molecular changes which may be classified as: The energy absorbed can cause the excitation of atoms and molecules, but the atom or molecule cannot remain in 1. Rotational - rotation of chemical dipole, caused by absorption the excited state and it has to go back to its ground state. of IR radiation with wavelength range from 100-10,000∝. It can do so by losing the energy that it has absorbed. 2. Vibrational – change in the average distance separating the atoms in the molecule, caused by absorption of IR radiation The favored deactivation process is the route that is most with wavelength range from 1.5-100m. rapid and spends less time in the excited state. 3. Electronic – change in the energy of electrons distributed about the atoms in the molecules or the transfer of electrons from the ground state to an excited state. This is caused by the absorption of visible and UV radiation with wavelength range from 150- 1500 nm. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 1  12 Sep 2024 INTERACTION OF LIGHT WITH MATTER INTERACTION OF LIGHT WITH MATTER There are various ways where in atoms and molecules can loss the There are various ways where in atoms and molecules can energy that it has absorbed; loss the energy that it has absorbed; Phosphorescence - is a type of photoluminescence, it Vibrational relaxation – molecules loss energy by releases the stored energy slowly over time. emission of heat in which the excited molecules transfers The molecule is "charged" by exposing it to light. Then the internal energy to other molecule by collision. energy is stored for a period of time and slowly released. Because this release occurs slowly, a phosphorescent A molecule maybe to promoted to several vibrational levels material appears to glow in the dark. during the electronic excitation process. In phosphorescence, light is absorbed by a material, bumping Collision of molecules with the excited species and solvent up the energy levels of electrons into an excited state. leads to rapid energy transfer and a slight increase in temperature of the solvent. Transitions to a lower and more stable energy state take time, but when they occur, light is released. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INTERACTION OF LIGHT WITH MATTER INTERACTION OF LIGHT WITH MATTER Unlike phosphorescent products, fluorescent pigments stop Fluorescence – photon emission from a singlet state (an electronic glowing once the light source is removed. state in which all electrons are spin paired). Although named for the green glow of the element phosphorus, In fluorescence, a surface absorbs and re-emits a photon almost phosphorus actually glows because of oxidation it is not instantly (about 10 nanoseconds). phosphorescent. Photoluminescence is quick because the energy of the absorbed photons matches energy states and allowed transitions of the material. Phosphorescence lasts much longer (milliseconds up to days) because the absorbed electron crosses into an excited state with higher spin multiplicity. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INTERACTION OF LIGHT WITH MATTER INTERACTION OF LIGHT WITH MATTER If enough light is absorbed, the stored and released light becomes sufficiently significant for material to appear to "glow in the dark." For this reason, phosphorescent materials, like fluorescent materials, appear very bright under a black (ultraviolet) light. Figure: Spin in ground and excited state. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 2  12 Sep 2024 MEASURING PHOTONS AS A SIGNAL MEASURING PHOTONS AS A SIGNAL Spectroscopy is possible only if the photon’s interaction with the sample leads to a change in one or more of these characteristic The source of the energetic state depends on the properties. photon’s energy. The electromagnetic spectrum shows that absorbing a photon of visible light causes a valence electron In absorption spectroscopy the energy carried by a photon is in the analyte to move to a higher-energy level. When an absorbed by the analyte, promoting the analyte from a lower-energy analyte absorbs infrared radiation, on the other hand, one of state to a higher-energy, or excited, state (Figure). its chemical bonds experiences a change in vibrational energy. The intensity of photons passing through a sample containing the analyte is attenuated because of absorption.a Simplified energy level diagram showing absorption of a photon. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry MEASURING PHOTONS AS A SIGNAL MEASURING PHOTONS AS A SIGNAL The measurement of this attenuation, which we call absorbance, serves as our signal. Note that the energy levels in Figure have well-defined values Emission of a photon occurs when an analyte in a higher- (i.e., they are quantized). energy state returns to a lower energy state (Figure). Absorption only occurs when the photon’s energy matches the difference in energy, ∆E, between two energy levels. A plot of absorbance as a function of the photon’s energy is called an absorbance spectrum (Figure). Ultraviolet/visible absorption spectrum for methylene blue. Simplified energy level diagram showing emission of a photon. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry MEASURING PHOTONS AS A SIGNAL SUMMARY Photons carry discrete amounts of energy called quanta which can be transferred to atoms and molecules when photons are absorbed. Depending on the frequency of the electromagnetic radiation, chemists can probe different parts of an atom or molecule's structure using different kinds of spectroscopy. Photons in the UV or visible ranges of the EM spectrum can have sufficient energy to excite electrons. Once those electrons relax back to their ground states, photons will be emitted, and the atom or molecule will give off visible light of specific frequencies. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 3  12 Sep 2024 SUMMARY These atomic emission spectra can be used (often informally by using a flame test) to gain insight into the electronic structure and identity of an element. SPECTROSCOPY Atoms and molecules can also absorb and emit lower frequency, METHODS IR radiation. IR absorption spectra are useful to chemists because they indicate the chemical structure of a molecule, and the types of bonds it contains. Lastly, spectroscopy can also be used in the laboratory to determine the concentrations of unknown solutions, using the Beer-Lambert Law. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry SPECTROSCOPIC MEASUREMENTS EMISSION 1. Emission 2. Absorption 3. Chemiluminescence 4. Photoluminescence Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry EMISSION EMISSION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 4  12 Sep 2024 ABSORPTION CHEMILUMINESCENCE Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry PHOTOLUMINESCENCE Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry SCATTERING OF RADIATION ABSORPTION OF RADIATION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 5  12 Sep 2024 TRANSMITTANCE vs. ABSORBANCE SPECTRAL PLOT Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry BEER’S LAW BEER’S LAW AND MIXTURES Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry LIMITATION OF BEER’S LAW LIMITATION OF BEER’S LAW Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 6  12 Sep 2024 LIMITATION OF BEER’S LAW LIMITATION OF BEER’S LAW Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry LIMITATION OF BEER’S LAW ABSORPTION SPECTRA 1. Atomic Absorption 2. Molecular Absorption Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry ATOMIC ABSORPTION ELECTRONIC ABSORPTION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 7  12 Sep 2024 MOLECULAR ABSORPTION MOLECULAR ABSORPTION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry VIBRATIONAL TRANSITION ROTATIONAL TRANSITION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry EMISSION OF RADIATION LINE SPECTRA Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 8  12 Sep 2024 BAND SPECTRA CONTINUOUS SPECTRA Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry VISIBLE SPECTROSCOPY VISIBLE SPECTROSCOPY Visible spectroscopy is also called colorimetry, it refers to the quantitative This is a study of the absorption of visible radiation within the determination of colored substance based on its ability to absorb light. wavelength range of 400 nm to 800 nm by any colored substance. The name spectrometry is given to analytical method carried out at a Every colored substance absorbs light radiation of different wavelengths. given spectral wavelength. When we plot a graph of the wavelength of radiation vs. absorbance, a However, spectrometric method are not limited to the use of visible light curve is derived which shows the wavelength at which maximum but includes those which employ radiant energy in other region of the absorption takes place. electromagnetic spectrum. This is termed as “λmax” and is unique for every compound. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry VISIBLE SPECTROSCOPY VISIBLE SPECTROSCOPY INSTRUMENTATION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 9  12 Sep 2024 VISIBLE SPECTROSCOPY VISIBLE SPECTROSCOPY INSTRUMENTATION INSTRUMENTATION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry ULTRAVIOLET SPECTROSCOPY UV SPECTROSCOPY UV spectroscopy is a type of absorption spectroscopy in which light of ultraviolet region (200-400 nm) is absorbed by the molecule. All molecules can absorb radiation in the UV or visible because they contain electrons both shared and unshared which can be excited to higher energy levels, thereby causing electronic transition. The wavelength of absorption and intensity are dependent on the type of group responsible for the absorption and the wavelength is also considered as a measure of the energy required for transition. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry UV SPECTROSCOPY ULTRAVIOLET SPECTROSCOPY The most favored transition is from the highest occupied molecular Molecules containing π-electrons or non-bonding electrons (n- orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). electrons) can absorb energy in the form of ultraviolet light to excite these electrons to higher anti-bonding molecular orbitals. The lowest energy occupied molecular orbitals are s orbital, which correspond to sigma bond (σ). The more easily excited the electrons, the longer the wavelength of light it can absorb. The p orbitals are at somewhat higher energy levels, the nonbonding orbitals(n) with unshared pair of electrons lie at higher energy levels. There are four possible types of transitions (π–π*, n–π*, σ–σ*, and n– σ*), and they can be ordered as follows: σ–σ* > n–σ* > π–π* > n–π* The unoccupied or antibonding orbitals (π* and σ *) are the highest energy occupied orbitals. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 10  12 Sep 2024 ULTRAVIOLET SPECTROSCOPY ULTRAVIOLET SPECTROSCOPY Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry ULTRAVIOLET SPECTROSCOPY ULTRAVIOLET SPECTROSCOPY 1. σ → σ* transition σ electron from orbital is excited to corresponding anti-bonding σ* the energy required is large for this transition. Methane (CH4) has C-H bond only and can undergo σ → σ* transition and shows absorbance maxima at 125 nm. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry ULTRAVIOLET SPECTROSCOPY ULTRAVIOLET SPECTROSCOPY 2. π → π* transition 3. n → σ* transition π electron in a bonding orbital is excited to corresponding anti-bonding Saturated compounds containing atoms with lone pair of electrons like orbital π* O, N, S and halogens are capable of n → σ* transition Compounds containing multiple bonds like alkenes, alkynes, carbonyl, These transition usually requires less energy than σ → σ* transitions. nitriles aromatic compounds, etc undergo π → π* transition The number of organic functional groups with n → σ* peaks in UV e.g. Alkenes generally absorb in the region 170 to 205 nm region is small (150-250 nm) Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 11  12 Sep 2024 ULTRAVIOLET SPECTROSCOPY ULTRAVIOLET SPECTROSCOPY 4. n → π* transition Therefore in order to absorb light in the region from 200-800 nm, the An electron from non-bonding orbital is promoted to anti-bonding π* molecule must contain either π bonds or atoms with non-bonding orbitals. orbital A non-bonding orbital is a lone pair on atoms such as; oxygen, nitrogen Compounds containing double bond involving hetero atoms (C=O, C≡N, or a halogen. N=O) undergo such transitions This require minimum energy and show absorption at around 300 nm. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry ULTRAVIOLET INSTRUMENTATION INFRARED SPECTROSCOPY IR spectroscopy (which is short for infrared spectroscopy) deals with the infrared region of the electromagnetic spectrum, i.e. light having a longer wavelength and a lower frequency than visible light. Infrared Spectroscopy generally refers to the analysis of the interaction of a molecule with infrared light. This techniques when coupled with intensity measurement can be used for quantitative analysis. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INFRARED SPECTROSCOPY INFRARED SPECTROSCOPY When this electromagnetic wave interacts with a covalent bond which has an A single atom cannot absorb IR radiation, as it has no bonds. electrical dipole, the energy is absorbed and the bond will start oscillating back and forth. For a symmetric molecule such as hydrogen (H-H), there is no change Any such vibration that causes a change in the dipole of the molecule should absorb in dipole when the molecule vibrates (stretches and contracts), so IR radiation. again it will not absorb IR. An IR spectrophotometer is used to perform this analysis. But for an asymmetric diatomic molecule such as H-F, the dipole moment will change as the bond stretches or contracts, so HF does Older instruments slowly scan through the whole IR spectrum, but modern absorb IR radiation. machines use the Fourier Transform method, where the entire spectrum is analyzed with a single pulse from an IR laser. A computer is used to convert the signal to a readable form. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 12  12 Sep 2024 INFRARED SPECTROSCOPY INFRARED SPECTROSCOPY The first necessary condition for a molecule to absorb infrared light is Vibrational Motion- Vibrational modes are often given descriptive names, that the molecule must have a vibration during which the change in such as dipole moment with respect to distance is non-zero. Stretching- vibration or oscillation along the line of the bond. Molecules can have three modes of movement; vibration, rotation, and - distance between the two atoms increase or decreases, but atom translation. remain in the same bond axis, eg A -B where A and B represent the two atom and the double arrow as the stretched bond. A molecule can vibrate in many ways, and each way is called a vibrational mode. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INFRARED SPECTROSCOPY INFRARED SPECTROSCOPY a. Symmetrical Stretching - Bending- vibration or oscillation not along the line of the bond. the two atoms either move In which the position of the atom changes relatives to original bond axis. towards or away from the central atom in unison, by which either altering the interatomic distance or causing no change in valence angle. b. Asymmetrical stretching - one atom approaches the central atom while the other moves away from the central atom. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INFRARED SPECTROSCOPY INFRARED SPECTROSCOPY In plane bending vibration Out of plane bending a. Scissoring - two atoms vibration connected to a central a. Wagging – two atom either move toward atoms move to one or away from each other side of the plane. They with certain deformation move up and down the of the valence angle. plane. b. Rocking - In this b. Twisting – one atom instance, the structural moves above the plane unit swings back and and another atom forth in the plane of the moves below the plane. molecule. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 13  12 Sep 2024 INFRARED SPECTROSCOPY INFRARED SPECTROSCOPY Rotational Motion Translational motion is the When a molecule is irradiated with photons of light it may absorb the motion by which a molecule radiation and undergo an energy transition. The energy of the transition shifts from one point in space must be equivalent to the energy of the photon of light absorbed given to another. by: E=hν. Rotation of atoms is important in infrared study of molecules because changes in the rotational state affect the molecules vibrational fine structure. Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry INFRARED INSTRUMENTATION Seduco, N.A./ Technological University of the Philippines Visayas- Department of Chemistry 14