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Purdue CHM 26200 - Exam 1 Study Guide

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Chapter 12 Outline How do we see stuff that is colored? I. Electromagnetic Radiation - light and other forms of radiant energy a. Properties (wavelength, frequency (Hz)) -wavelength: the distance between consecutive peaks on a wave - frequency: the number of full cycles of a wave that pass a given point in a second - Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”) b. The electromagnetic spectrum c. Molecular spectroscopy - the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure - 3 types of spectroscopy - Measurement of light absorbed or emitted II. Infrared spectroscopy Vibrational energy levelsNuclear spin statesElectronic energy levelsInfraredRadio fequencyUltraviolet-visibleAbsorption ofElectromagneticRadiation Results in Transition BetweenRegion of theElectromagneticSpectrumNuclear magneticresonanceInfraredUltraviolet-visibleType of Spectroscopya. An IR spectrum i. Wavenumbers – “wavelengths per centimeters” cm-1 = 1/wavelength (in cm) ii. Proportional to frequency (and therefore energy since E = hv) b. Vibrations - atoms joined by covalent bonds undergo continual vibrations relative to each other - the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed - the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm-1 For a molecule to absorb IR radiation • the bond undergoing vibration must be polar and • its vibration must cause a periodic change in the bond dipole moment Covalent bonds which do not meet these criteria are said to be IR inactive • the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds i. Atoms held together by bond are not rigid and vibrate (like spring) with specific (quantized) frequencies ii. Molecules will absorb light if it has the same frequency as a vibration IF the bond that is vibrating is polar (bonds that do not affect dipole moment are IR inactive) iii. Most molecules have lots of different ways to vibrate (3n – 6 vibrations) - For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex - The simplest vibrational motions are bending and stretching iv. Types of vibrations 1. Stretching- symmetric - asymmetric - From this equation, we see that the position of a stretching vibration is proportional to the strength of the vibrating bond and is inversely proportional the masses of the atoms connected by the bond  The intensity of absorption depends primarily on the polarity of the vibrating bond 2. Bending - scissoring - rocking - wagging - twisting v. Factors affecting vibrational frequency 1. Hookes Law 2. Frequency goes down with mass 3. Frequency goes up with force constant, which depends on the strength of the bond (Stronger bonds vibrate faster) 4. Strength of absorption depends on dipole c. IR absorptions are characteristic for the type of bondd. IR spectroscopy is great for functionality detection - Organic functional groups have characteristic IR properties e. Hydrocarbons h. Since most organic molecules have hydrocarbon components, IR is not very useful for saturated components. Alkenes have good C-H stretchesi. Alkynes have a great CC stretch but it is weak (not very polar) j. OH stretches in alcohols are easily seen - Hydrogen bonding causes broadening - Ethers are tough, although C-O bonds can sometimes be evident m. Amine N-H stretches can sometimes be seen, but are easily mistakenn. C=O groups are really easy to seeo. Solving IR Problems i. Identify the functional group ii. Use C-H, O-H, and N-H bands to further classify iii. A few other useful stretches: 1. C=N 2250 (medium) 2. C=C 2100 – 2250 (weak)III. Mass spectrometry - Approach for “weighing” molecules to determine molecular weights - Can give information about structure - Can even be used for proteins and DNA! a. Mass spectrometer - Converts neutral atoms or molecules into ions (positive or negative) - Separate the ions based on their mass-to-charge (m/z) ratio o If charge = 1, then m/z is the mass of the ion - measure how many ions with each (m/z) - NEED TO THINK ABOUT THE STRUCTURE (Lewis structures) of IONS b. There are many different ways to ionize molecules, but the most common is electron ionization Net result: removing an electron - product is positively charged (a cation) - The ion formed by removing 1 electron from the molecule; yields molecular ion (tells us the molecular weight of the molecule) Molecular ion: a radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer- It doesn’t matter where the electron comes from; it’s from the highest energy occupied molecular orbital (HOMO) - Having a singly occupied orbital means it is also a radical c. Mass Spectrum - a plot of the relative abundance of ions versus their mass-to-charge ratio - Base peak – most intense ion, tallest peak (assigned an arbitrary intensity of 100) - electron ionization mass spectrometry (EI-MS) d. Other instrumental things - Different types of mass spectrometry use different ways to make ions and different ways to separate them • fast atom bombardment (FAB) • matrix-assisted laser desorption ionization (MALDI) • chemical ionization (CI) • electrospray ionization (ESI) e. Resolution – ability to distinguish m/z values a measure of how well a mass spectrometer separates ions of different mass • low resolution: refers to instruments capable of separating only ions that differ in nominal mass; that is ions that differ by at least 1 or more atomic mass units • high resolution: refers to instruments capable of separating ions that differ in mass by as little as 0.0001 atomic mass unit - “Isobaric” ions have the different formula but the same mass (at least at the integral level) - Exact masses will be different due to small differences in atomic masses and can be distinguished by high resolution f. Isotopes – most naturally occurring


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