SB Lecture 1 Flashcards
What are Angstroms (Å)? What is one Angstrom in m?
Typically, Angstroms (Å) are the unit of distance used when discussing atoms and molecules.
1 mm = 1 x 10-3m
1nm = 1 x 10-9m
1 Å = 1 x10-10m
1 pm = 1 x 10-12m
What is an example of a “small” molecule?
Indigotin

What is the bond distance of a C=O bond?
1.24 Å
What is the bond distance of a C=C bond?
1.34 Å
What is the bond distance of a C–Caromatic bond?
1.39 Å
(Aromatic Hydrocarbon - Ex. Benzene)
What is the bond distance of a C-C bond?
1.43 and 1.46 Å
What is the bond distance of a N-H bond?
0.98 Å
(The H atom positions are usually estimated, not actually measured, in an X-ray crystal structure)
What are four ways we can measure molecular shapes?
- X-ray crystallography
- Scanning-Tunneling Microscopy (STM)
- Infra-red (IR) absorption spectroscopy
- Raman spectroscopy
What is a crystal?
In a crystal, the consecutive particles (atoms, ions, molecules) are arranged periodically in a regular, repeating pattern, forming a lattice
Is it possible to crystalize ionic solics? Give examples.
It is possible to crystallize ionic solids (e.g., NaCl), metals (e.g, Ni), molecular solids (e.g., indigotin), and proteins (e.g., insulin).
Define Diffraction
Diffraction is the apparent bending of waves (e.g., light) around very small objects and spreading out of waves that travel through small openings.
Define X-ray
X-rays are electromagnetic radiation with wavelengths of λ = 0.1 – 100 Å.
frequencies of v = 3x1016 - 3x1019 Hz
energies of E = 100 - 1x105 eV

Define Crystallography
Crystallography is the theory of spatially periodic, perfectly long-range ordered patterns. (e.g., atoms in a crystal)
This is a technique that requires a crystalline sample of the analyte (the species being analyzed).
List the two different techniques of X-ray crystallography
- Powder X-ray Diffraction (PXRD)
- Single crystal X-ray Diffraction
Explain the Powder X-ray Diffraction (PXRD) technique
Powder X-ray Diffraction (PXRD) – This technique relies on measuring the scattering angle of an incident beam of X-rays as they are “bounced” off the surface of a microcrystalline powder.

Explain the Single crystal X-ray Diffraction technique
- When X-rays are transmitted through a crystal, they are diffracted by the atoms (specifically by the electron cloud, not by the nucleus) and emerge according to a diffraction pattern.
- By rotating the crystal in the X-ray beam, it is possible to collect a complete set of diffraction patterns at each angle that they occur.

Why do protons (H atoms) normally not “show up” in a X-ray crystal structure?
- Too little electron density to diffract the X-ray!
In a single crystal X-ray Diffraction what helps determine which atoms are bonded to one another?
Describe the geometry in different states.
- Atom positions relative to one another in space help determine which atoms are bonded to one another, and by what type of bond.
- e.g., double bonds are shorter than single bonds, so the atoms will be closer together.
- The geometry of non-rigid molecules might be different in the solid state than in the liquid (or solution) state.
- e.g., rotation around a single bond can occur in solution, but the atom positions are “locked” in the solid state.
Describe Scanning Tunneling Microscopy (STM)
The STM tip is brought very close to the sample surface. As it scans over the surface, it passes through regions of high and low electron density. A voltage is applied across the tip and sample and thus a current flows as electrons tunnel through the vacuum between the surface and the tip.
higher e- density = higher current

What does STM reveal?
STM reveals the structure of a surface, atom by atom. It is especially useful for determining the structure/shape of a molecule or molecular array adsorbed to a surface.
What are examples of Planar Molecules (“Flat”)?
- Benzene (C6H6)
- Boron trifluoride, BF3
- Water, H2O
- Psoralen, C11H6O3
- a type of furanocoumarin
- a phtototoxin
- a mutagen

What are examples of Octahedral Molecules (C.N. = 6)?
What is an octahedron?
Tungsten hexacarbonyl W(CO)6
Hexachlorophosphate PCl6
Octahedron
- 8 faces
- 12 edges
- 6 apices

What are examples of Tetrahedral molecules? (C.N. =4)
What is a tetrahedron?
- Methane CH4
- Tetrachlorophosphonium PCl4+
- Nickel tetracarbonyl Ni(CO)4
- 4 faces
- 6 edges
- 4 apices

Examples of other shapes
Just knowledge*

What are examples of polymers?
- Polypropylene [-CH(CH3)CH2-]n
- CDN polymer banknotes
- Cellulose (C6H10O5)n

What are examples of molecular and ionic solids?
- Ice, H2O
- Table salt, sodium chloride, NaCl

What is VSEPR theory?
Valence Shell Electron Pair Repulsion Theory
