SB Lecture 1

Question 1

What are Angstroms (Å)? What is one Angstrom in m?

Answer 1

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

Question 2

What is an example of a “small” molecule?

Answer 2

Indigotin

Question 3

What is the bond distance of a C=O bond?

Answer 3

1.24 Å

Question 4

What is the bond distance of a C=C bond?

Answer 4

1.34 Å

Question 5

What is the bond distance of a C–C_aromatic bond?

Answer 5

1.39 Å

(Aromatic Hydrocarbon - Ex. Benzene)

Question 6

What is the bond distance of a C-C bond?

Answer 6

1.43 and 1.46 Å

Question 7

What is the bond distance of a N-H bond?

Answer 7

0.98 Å

(The H atom positions are usually estimated, not actually measured, in an X-ray crystal structure)

Question 8

What are four ways we can measure molecular shapes?

Answer 8

1. X-ray crystallography
2. Scanning-Tunneling Microscopy (STM)
3. Infra-red (IR) absorption spectroscopy
4. Raman spectroscopy

Question 9

What is a crystal?

Answer 9

In a crystal, the consecutive particles (atoms, ions, molecules) are arranged periodically in a regular, repeating pattern, forming a lattice

Question 10

Is it possible to crystalize ionic solics? Give examples.

Answer 10

It is possible to crystallize ionic solids (e.g., NaCl), metals (e.g, Ni), molecular solids (e.g., indigotin), and proteins (e.g., insulin).

Question 11

Define Diffraction

Answer 11

Diffraction is the apparent bending of waves (e.g., light) around very small objects and spreading out of waves that travel through small openings.

Question 12

Define X-ray

Answer 12

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

Question 13

Define Crystallography

Answer 13

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).

Question 14

List the two different techniques of X-ray crystallography

Answer 14

1. Powder X-ray Diffraction (PXRD)
2. Single crystal X-ray Diffraction

Question 15

Explain the Powder X-ray Diffraction (PXRD) technique

Answer 15

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.

Question 16

Explain the Single crystal X-ray Diffraction technique

Answer 16

• 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.

Question 17

Why do protons (H atoms) normally not “show up” in a X-ray crystal structure?

Answer 17

• Too little electron density to diffract the X-ray!

Question 18

In a single crystal X-ray Diffraction what helps determine which atoms are bonded to one another?

Describe the geometry in different states.

Answer 18

• 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.

Question 19

Describe Scanning Tunneling Microscopy (STM)

Answer 19

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

Question 20

What does STM reveal?

Answer 20

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.

Question 21

What are examples of Planar Molecules (“Flat”)?

Answer 21

• Benzene (C₆H₆)
• Boron trifluoride, BF₃
• Water, H₂O
• Psoralen, C₁₁H₆O₃
  
  • a type of furanocoumarin
  • a phtototoxin
  • a mutagen

Question 22

What are examples of Octahedral Molecules (C.N. = 6)?

What is an octahedron?

Answer 22

Tungsten hexacarbonyl W(CO)₆

Hexachlorophosphate PCl₆

Octahedron

• 8 faces
• 12 edges
• 6 apices

Question 23

What are examples of Tetrahedral molecules? (C.N. =4)

What is a tetrahedron?

Answer 23

• Methane CH₄
• Tetrachlorophosphonium PCl₄⁺
• Nickel tetracarbonyl Ni(CO)₄
• 4 faces
• 6 edges
• 4 apices

Question 24

Examples of other shapes

Just knowledge*

Answer 24

Question 25

What are examples of polymers?

Answer 25

* Polypropylene [-CH(CH₃)CH₂-]_n * CDN polymer banknotes * Cellulose (C₆H₁₀O₅)_n

Question 26

What are examples of molecular and ionic solids?

Answer 26

* Ice, H₂O * Table salt, sodium chloride, NaCl

Question 27

What is VSEPR theory?

Answer 27

Valence Shell Electron Pair Repulsion Theory