Presentation on theme: "Chemistry 125: Lecture 5 Sept. 10, 2010 X-Ray Diffraction In the last 25 years various manifestations of Scanning Probe Microscopy, such as AFM, STM, and."— Presentation transcript:
Chemistry 125: Lecture 5 Sept. 10, 2010 X-Ray Diffraction In the last 25 years various manifestations of Scanning Probe Microscopy, such as AFM, STM, and SNOM, have enabled chemists to “feel” individual molecules and atoms. SPM techniques are not quite good enough yet to study how electrons are distributed in bonds. Because light is scattered predominantly by the charged particles with the smallest 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. For copyright notice see final page of this file
Despite Earnshaw, might there still be shared-pair bonds and lone pairs?
Gold coated Si Chip 1.4 mm Atomic Force Microscopy The smallest scale division is 20 m Hair Only one of these five cantilevers is used in any one experiment. They differ in stiffness and ability to twist.
Cantilever/Chip Holder Sensitive to < 1 molecule change in height !
AFM traces at 1 min intervals of part of the surface of a benzoin crystal dissolving in 95% water / 5% n-propanol. Each ledge is one “unit cell” (1.7 nm) high. The larger pit is 5.1 nm deep. 5 m (~600 molecules) fast slow slower Ledge Dissolution Rate
Scanning Tunneling Microscopy (1999) Geo. Flynn D. Yablon (Columbia Univ) Br O Br(CH 2 ) 11 COOH on graphite Graphite
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.”
Water Oil “Thickness” ~ 200 nm Path Difference = 400 nm = 0.5 Strong 400 nm Scattering No 800 nm Scattering = 1 Interference upon Scattering
Hooke: Observ. IX. Of the Colours observable in Muscovy Glass, and other thin Bodies. Confus’d Pulses of Light
In What Way is Light a Wave? Force on Charge at One Position Up Down 0 Time Charged Particle
Charged Particle In What Way is Light a Wave? Force at Different Positions - OneTime Up Down 0 Position
Accelerated Electrons “Scatter” Light Why don’t protons or other nuclei scatter light? Too heavy! direct beam
Interference of Ripples Angular Intensity Distribution at great distance depends on Scatterer Distribution at the origin
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 SPM “feels” them Individually
Blurring Problem Blurring Problem from Motion and Defects Time Averaging Space Averaging in Diffraction (Cooperative Scattering) Advantage for SPM (Operates in Real Space)
In 1895 Röntgen Discovers X-Rays Shadow of Frau Röentgen’s hand (1896) In 1912 Laue Invents X-Ray Diffraction CuSO 4 Diffraction (1912)
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
by permission, Konstantin Lukin Bragg Machine http://www.eserc.stonybrook.edu/ProjectJava/Bragg/ Breaks? in & out same phase
Direct Two Scattering Directions are Always Exactly in Phase “scattering vector” Specular perpendicular to scattering vector All electrons on a plane perpendicular to the scattering vector scatter in-phase at the specular angle ! Specular
10 scattering vector 3 +2 +4 +1 3 2 4 1 1 2 3 Net in-phase scattering Total Electrons Suppose & angle such that: Electrons-on-Evenly-Spaced-Planes Trick
10 scattering vector 3 +2 +4 +1 3 +2 -4 0 3 2 4 1 Suppose first path difference is half a wavelength, because of change in (or angle) Net in-phase scattering Total Electrons 0.5 1 1.5 Electrons-on-Evenly-Spaced-Planes Trick
View from Ceiling 10.6 m 633 nm DIFFRACTION MASK (courtesy T. R. Welberry, Canberra) ………………….. spot spacing = 10.8 cm Q. What is the line spacing?
To see and understand these diffraction images, study the course X-ray website: https://webspace.yale.edu/chem125/125/xray/diffract.html and particularly the section: https://webspace.yale.edu/chem125/125/xray/laserdiffraction.htm https://webspace.yale.edu/chem125/125/xray/diffract.html https://webspace.yale.edu/chem125/125/xray/laserdiffraction.htm