Chemistry XXI Unit 2 How do we determine structure? The central goal of this unit is to help you develop ways of thinking that can be used to predict the atomic and molecular structure of substances. Crime investigation Drug action Space exploration Materials’ design Metabolite analysis Environmental analysis How is molecular structure relevant in any of these areas? Why do we care?

Chemistry XXI Unit 2 How do we determine structure? M4. Inferring Charge Distribution Analyzing the distribution of electrons in molecules. M3. Predicting Geometry Predicting the three dimensional geometry of molecules. M2. Looking for Patterns Deducing atom connectivity based on atomic structure. M1. Analyzing Light-Matter Interactions Using spectroscopy to derive structural information. FOUR MAIN MODULES

Chemistry XXI Structure-Properties 1. How do we distinguish substances? 2. How do we determine structure? Learning Progression 3. How do we predict properties? Particulate Zoom InZoom In Macro Molecular Atomic Electronic ZoomOutZoomOut Molecular ParticulateMacro

Chemistry XXI Integration To illustrate the power of chemical ideas and models in determining the molecular structure of matter we will focus our attention on substances that can affect our health and well being. How do they interact with light? Why is this so? How can we determine or predict their structure? Aspirin

Chemistry XXI Unit 2 How do we determine structure? Module 1: Analyzing Light-Matter Interactions Central goal: To use absorption and emission spectra to derive information about the structure of matter.

Chemistry XXI The Challenge Our ability to identify chemical substances in a given system and analyze their molecular structure has been greatly enhanced by the analysis of light-matter interactions. How can we use light to derive structural properties of chemical substances? How can we use light to detect, identify, and quantify the substances present in a system? Analysis Is it here?

Chemistry XXI Much of what we know about the structure of atoms and molecules is derived from the analysis of the interaction of light, electromagnetic (EM) radiation, with the substance of interest. In most cases, we analyze the type and amount of EM radiation that the substance either absorbs or emits when heated. Light-Matter Interactions

Chemistry XXI Light Properties The different types of EM radiation are characterized by their wavelength ( ), frequency ( ) or energy (E) (given one, we can calculate the others two).  *  = c c = Speed of light = 3.00 x 10 8 m/s

Chemistry XXI Electromagnetic Spectrum Energy per photon E = h = h c / h = Planck’s constant = x Js When light interacts with matter, energy is transmitted in packets of energy called “photons.” Energy Increases

Chemistry XXI Spectroscopy The analytical techniques based on the analysis of the EM radiation absorbed or emitted by a chemical substance are called Spectroscopies. Absorption Spectroscopy: Ethanol (C 2 H 6 O) Transmission Data are represented in various ways Ozone (O 3 ) Absorption Spectrum

Chemistry XXI The location of the absorption peaks vary from substance to substance. Selective Absorption Analysis of absorption of UV-visible radiation provides a means of detecting the presence of relevant species in the atmosphere.

Chemistry XXI Atoms and molecules are made of charged particles (electrons, protons) that can absorb or emit EM radiation. Experiments indicate that substances only absorb EM radiation of specific frequencies or energies. Let’s Think Propose a model to explain it?

Chemistry XXI Let’s Explore Let’s analyze these ideas using a simulation of the hydrogen atom: Go to: Light absorption Select  “Prediction, Schrödinger” “Show spectrometer,” “Show electron energy diagram.” Turn on the white light and carefully observe the behavior of the system. How do you explain what you see?

Chemistry XXI Isolated atoms can exist in different electronic energy states depending on how their electrons are distributed among different energy levels. Atoms: Energy States The absorption or emission of energy changes the structure of the electron density (probability regions). Energy n =1 n = 2

Chemistry XXI Let’s Explore Based on this model, what information about the structure of the atom is captured by the spectrophotometer?

Chemistry XXI Turn on the monochromatic light and observe the behavior of the system at different values of. Given that E = hc/ for EM radiation, calculate the energy difference between the energy levels labeled n = 1 and n = 2? Let’s Think

Chemistry XXI Molecules can also be in different electronic states. They also have different vibrational, and rotational energy states. Molecules: Energy States  E elect  UV-visible  E vibr  Infrared  E rot  Microwave E (KJ/mol) Different Spectroscopies

Chemistry XXI Let’s Think Estimate the order of magnitude, in kJ/mol, of the energy required to induce electronic, vibrational, and rotational transitions in molecules. UV ~ 100 nm IR ~ 10  m MW ~ 1 cm Energy per photon E = h c / h = x Js

Chemistry XXI  E elect  UV-visible  E vibr  Infrared  E rot  Microwave E (kJ/mol) Energy Scale Breaking Bonds Exciting Electrons Change in vibrational states Change in rotational states

Chemistry XXI The Water Case UVIRMW Why is water bluish? Microwave oven ~ 12 cm

Chemistry XXI Remote Sensing Spectroscopy is one of the main tools used in atmospheric sciences to study the composition and evolution of our atmosphere. NASA Aura Mission 4 Main Instruments: UV, IR, MW Absorption + Emission Spectrometers Detect/Quantify ~ 20 species Tropo/Stratosphere Carbon Monoxide (CO) Troposphere- Emission Stratosphere- MW

Chemistry XXI ClOHNO 3 HCl H2OH2O O3O3O3O3 NO 2 CO Km MWIRUVEmission

Chemistry XXI Assess what you know Let′s apply!

Chemistry XXI Let′s apply! What is in a Star? Astronomers use spectroscopic techniques to investigate the composition of stars. Stars, as many hot solids, emit EM radiation in a continuous range of wavelengths (. However, the intensity of this radiation is a function of and temperature (T). Black Body Radiation

Chemistry XXI Let′s apply! Star Spectrum This is the spectrum of our Sun taken at different altitudes, compared with the expected blackbody spectrum. What could cause the differences between the spectra?

Chemistry XXI Let′s apply! Photosphere Chemical elements in a star’s photosphere absorb specific radiation. The “dips” in the spectra can then be used to detect their presence in the star. H

Chemistry XXI Spectral Lines  (nm) H a, H b, H g 660, 480, 435 He420 He Na580 Mg518 Ca Let′s apply! What can you say about the temperature and composition of these stars?

Chemistry XXI Come up with one question about something you don’t understand at this point?

Chemistry XXI Analyzing Light-Matter Interactions Summary A variety of powerful analytical techniques to determine the atomic and electronic of chemical substances are based on the analysis of the types and amount of electromagnetic (EM) radiation that they absorb or emit (Spectroscopies). Atoms and molecules are made of charged particles that can interact with EM radiation and absorb energy.

Chemistry XXI Experiments indicate that substances only absorb EM radiation of specific frequencies. This explained by assuming that atoms and molecules, as well as their electrons and protons, can only exist in specific energy states (their energy is quantized). Different atoms or molecules absorb/emit EM radiation at specific frequencies and their absorption/emission spectra can be used to identify them or detect their presence in a given system. Analyzing Light-Matter Interactions Summary

Chemistry XXI For next class, Investigate how we can use Infrared Spectroscopy to investigate atom connectivity in a molecule. How can we use information about the infrared radiation absorbed by a substance to determine its molecular structure or identity?