Chemistry 125: Lecture 5 X-Ray Diffraction Because light is scattered by charged particles of small mass, the electron distribution in molecules can be determined by x-ray diffraction. The roles of molecular pattern and crystal lattice repetition can be illustrated by shining a visible laser through diffraction masks to generate scattering patterns reminiscent of those encountered in X-ray studies of ordered solids. Synchronize when the speaker finishes saying, “…despite Earnshaw...” Synchrony can be adjusted by using the pause(||) and run(>) controls. For copyright notice see final page of this file

Despite Earnshaw, might there still be shared-pair bonds and lone pairs?

Scanning Probe Microscopies (AFM, STM, SNOM) are really powerful. Sharp points can resolve individual molecules and even atoms but not bonds

Lux In just the right light it is possible to perceive Boltwood’s foresight

A lonely architectural curiosity on Sterling Chemistry Laboratory at Yale University (1923)

Micrographia Robert Hooke (1665) “But Nature is not to be limited by our narrow comprehension; future improvements of glasses may yet further enlighten our understanding, and ocular inspection may demonstrate that which as yet we may think too extravagant either to feign or suppose.”

Interference upon Scattering Strong 400 nm Scattering No 800 nm Scattering Path Difference = 400 nm “Thickness” ~ 200 nm Oil = 1 l = 0.5 l Water Interference upon Scattering

Chris Incarvito’s New Toys

User Operated - CCD Detector ~$200K X-Ray Tube

~$350K Image Plate Curved Image Plate

"Seeing" Individual Molecules, Atoms, and Bonds? Problem:

Electromagnetic Waves What IS light? Electromagnetic Waves

In What Way is Light a Wave? Force on Charge at One Position Up Down Charged Particle Time

In What Way is Light a Wave? Force at Different Positions - OneTime Up Down Charged Particle Position

Accelerated Electrons “Scatter” Light direct beam Why don’t protons or other nuclei scatter light? Too heavy!

Interference of Ripples Angular Intensity Distribution at great distance depends on Scatterer the origin

But a lens for x-rays is hard to come by. By refocussing, a lens can reassemble the information from the scattered wave into an image of the scatterers. But a lens for x-rays is hard to come by. Be sure to read the webpage on x-ray diffraction.

"Seeing" Molecules, Atoms, Bonds Collectively by X-Ray Crystallography

Blurring Problem Blurring Problem from Motion and Defects Time Averaging Space Averaging in Diffraction (Cooperative Scattering) Mask from T. R. Welberry, by permission Advantage for SPM (Operates in Real Space)

In 1895 Röntgen Discovers X-Rays Shadow of Frau Röentgen’s hand (1896) In 1895 Röntgen Discovers X-Rays In 1912 Laue Invents X-Ray Diffraction CuSO4 Diffraction (1912) http://commons.wikimedia.org/wiki/Image:Wilhelm-Roentgen%27s-X-ray-photograph-of-his-wife%27s-hand.png http://commons.wikimedia.org/wiki/Image:Wilhelm_Conrad_Röntgen-cropped.JPG

Wm. Lawrence Bragg (1890-1971) Determined structure of ZnS from Laue's X-ray diffraction pattern (1912) Youngest Nobel Laureate (1915) Courtesy Dr. Stephen Bragg

B-DNA R. Franklin (1952) http://sandwalk.blogspot.com/2007/07/story-of-dna-part-2.html

Science, 11 August 2000

>100,000 atoms + hydrogens! 25 nm (250 Å)

What can X-ray diffraction show? Molecules? Atoms? Bonds? How does diffraction work? Like all light, X-rays are waves.

Wave Machines

Bragg Machine Breaks? in & out same phase http://www.eserc.stonybrook.edu/ProjectJava/Bragg/ in & out same phase Breaks? by permission, Konstantin Lukin

Two Scattering Directions are Always Exactly in Phase Specular perpendicular to scattering vector “scattering vector” Specular All electrons on a plane perpendicular to the scattering vector scatter in-phase at the specular angle ! Direct

Electrons-on-Evenly-Spaced-Planes Trick scattering vector 3 4 2 1

3 +4 +2 +1 10 3 2 4 1 Electrons-on-Evenly-Spaced-Planes Trick Total Electrons scattering vector 3 Suppose  & angle such that: +4 1l +2 2l +1 3l 10 3 2 4 1 Net in-phase scattering

Electrons-on-Evenly-Spaced-Planes Trick Total Electrons scattering vector 3 3 +4 -4 0.5l 1l 1.5l +2 +2 +1 -1 10 3 2 4 1 Net in-phase scattering Suppose first path difference is half a wavelength, because of change in  (or angle)

DIFFRACTION MASK (courtesy T. R. Welberry, Canberra) spacing = 10.8 cm ………………….. 633 nm View from Ceiling Q. What is the line spacing? 10.6 m DIFFRACTION MASK (courtesy T. R. Welberry, Canberra)

End of Lecture 5 Sept 12, 2008 Copyright © J. M. McBride 2009. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0). Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol . Third party materials may be subject to additional intellectual property notices, information, or restrictions.   The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0