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Unit 2How 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’ designMetabolite analysis

Environmental analysis

How is molecular structure relevant in any of these areas?

Why do we

care?

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IUnit 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 InteractionsUsing spectroscopy to derive

structural information.

FOUR MAIN MODULES

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IStructure-Properties

1. How do we distinguish

substances?

2. How do we determine structure?

Learning Progression

3. How do we predict properties?

Particulate

Zoom I

nMacro

Molecular

Atomic

Electronic

Zoom

Out

Molecular

Particulate

Macro

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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

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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.

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The Challenge

Our ability to identify chemical substances in a given system and analyze their molecular structure has been greatly

enhanced by the analysis oflight-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?

AnalysisIs it here?

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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

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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 108 m/s

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IElectromagnetic Spectrum

Energy per photon

E = h • = h • c /

h = Planck’s constant = 6.6262 x 10-34 J•s

When light interacts with matter, energy is

transmitted in packets of energy called “photons.”

Energy Increases

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SpectroscopyThe analytical techniques based on the analysis of the EM radiation absorbed or emitted by a chemical

substance are called Spectroscopies.

Absorption Spectroscopy:

Ethanol (C2H6O)

Transmission

Data are represented in various ways

Ozone (O3)

Absorption

Spectrum

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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.

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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?

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ILet’s Explore

Let’s analyze these ideas using a simulation of the hydrogen atom:

Go to: http://www.chem.arizona.edu/chemt/C21/sim

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?

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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

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ILet’s Explore

Based on this model, what information about the structure of the atom is captured by the spectrophotometer?

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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

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Molecules can also be in different electronic states. They also have different vibrational, and

rotational energy states.

Molecules: Energy States

Eelect UV-visible

Evibr Infrared

Erot Microwave

E (KJ/mol)

Different Spectroscopies

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Let’s ThinkEstimate 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 = 6.6262 x 10-34 J•s

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Eelect UV-visible

Evibr Infrared

Erot Microwave

E (kJ/mol)

1000

10

10-2

Energy Scale

Breaking BondsExciting Electrons

Change in vibrational states

Change in rotational states

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IThe Water Case

UV IR MW

Why is water bluish? Microwave oven

~ 12 cm

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IRemote 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- EmissionStratosphere- MW

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ClOHNO3 HClH2OOO33 NO2CO

0

18

50

Km

MWIRUVEmission

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I Assess what you know

Let′s apply!

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ILet′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

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ILet′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?

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ILet′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

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Spectral Lines

(nm)

Ha, Hb, Hg 660, 480, 435

He 420

He+ 440

Na 580

Mg 518

Ca+ 854

0

10

20

30

40

50

60

70

80

400 450 500 550 600 650 700 750 800 850 900

Wavelength (nm)

Inte

ns

ity

0

50

100

150

200

250

300

400 450 500 550 600 650 700 750 800 850 900

Wavelength (nm)

Inte

nsi

ty

Let′s apply!

What can you say about the

temperature and composition of

these stars?

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Come up with one question about something you don’t understand at

this point?

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Analyzing Light-Matter Interactions

SummaryA 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.

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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 InteractionsSummary

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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?