Shapes & Structures Of Molecules Part 1 Flashcards
What is important to note when drawing tetrahedral structure of molecules?
If you are to draw 2 lines at right angles, one line should rest on top of the two bonds in the plane of the paper, and the other should separate the remaining two bonds (pie,wedge). Thus there is one bond in each quadrant.
Multiple tetrahedral centres should also be drawn from the same perspective.
Me
Methyl, -CH3
Et
Ethyl, -CH2CH3
nPr
Normal propyl, -CH2CH2CH3
iPr
isopropyl, -CH(CH3)2
nBu
Normal butyl, -(CH2)3CH3
tBu
Tertiary butyl, -C(CH3)3
Ac
Acetyl, CH3O-
Ph
Phenyl, -C6H5
R
Any alkyl group
Ar
Any aromatic group
What is the trivial name for methylbenzene?
Toluene
What is diffraction?
When a beam of light or wave system passes through a narrow aperture or across an edge, and spreads out as a result.
Describe how X-ray crystallography produces a diffraction pattern?
The wavelength of X-rays is typically the same size as the spacing between each ‘scatterer’ in a compound (atoms’ electrons). Thus a beam of X-rays focussed onto a crystals is diffracted as an X-ray striking an electron produces secondary spherical waves emanating from the electron.
What is the typical wavelength of an X-ray?
~1 angstrom
What are X-rays?
A form of EM radiation composed of photons.
How is an electron density map produced from diffraction patterns?
Diffraction patterns are collected at different orientations by mounting the crystal on a goniometer.
A 3D picture of electron density (contour plot) can be built by measuring the angles and intensities of these diffracted beams in the various orientations and mathematical Fourier transform.
How is the crystal structure determined from the electron density map?
Regions of high electron density represent the positions of atoms.
The bigger the electron cloud, the higher the atomic number of the element (i.e. hydrogen atoms tend not to show up).
What do atomic displacement parameters show?
The uncertainty in the position of atoms in the crystal structure, related to the fact that atoms oscillate around a mean position.
This uncertainty is reduced by streaming a jet of cold nitrogen onto the crystal to reduce vibrations.
What information is given in the crystal structure?
Mean positions of atoms
Bond length
Bond angles
Unit cell (repeating structure) of crystal.
Ads of x Ray crystallography?
Gives location of atoms
Disads of x ray crystallography?
Hard to locate H atoms.
Good quality crystals necessary.
Why is a regular crystalline structure necessary in X-ray crystallography?
Atoms of the same type in the same place will result in CONSTRUCTIVE interference of diffracted photons, rather than destructive interference, and so a more regular picture of photon intensities.
If atoms were not in the same positions fuzzy patterns would be produced with poor resolution- difficult to interpret.
Techniques used to ionise a sample for weighing via mass spectrometry?
ELECTRON IMPACT (dated)- high energy electrons fired at vaporised sample, knocking out electron and producing M+. ELECTROSPRAY (gentle, effective)- sample forced through fine hollow needle connected to +ve terminal of high voltage supply, forming charged aerosol. Solvent evaporates in vacuum leaving charged molecules (e.g. MH+, MNa+, MeO-, CH3COO-).
What is the molar mass of an electron?
0.00055 g/mol
Why might the mass spectrum of a single molecule contain more than one peak?
Isotopes
Fragmentation (i.e. particularly macromolecules).
How can the fragmentation pattern of a molecule be used to find out the identity and structure of a molecule?
Complexity is unique to the compound, so like a fingerprint.
MS/MS- certain fragmented ions can be isolated and seeing how these parent ions fragment into daughter ions (can work backwards, seeing which parent ions might fragment to form a particular daughter ion).
Ads of mass spec?
Gives molecular formula for a compound.
Analysis of mixtures.
Only small sample needed (few million molecules).
Very sensitive (measure to 1 part per 10million).
Disads of mass spec?
Difficult to interpret for unknown molecules.
What does it mean to say that the energy levels of electrons are quantised?
An electron in an atom can only have certain energies; to promote it to another level requires a fixed amount of energy.
Of the translational, vibrational and rotational energies possessed by a molecule, which of those are quantised?
Vibrational & rotational.
Thus can be promoted between these by absorbing light energy, the different given by deltaE = h*v.
Different molecules absorb different frequencies (v)- the basis of spectroscopic techniques.
What type of transition is caused when a molecule absorbs light of the radio frequency?
Transitions in the alignments of nuclear spin
What type of transition is caused when a molecule absorbs light of the microwave frequency?
Rotational
What type of transition is caused when a molecule absorbs light of the infrared frequency?
Vibrational
Which frequencies of light cause electronic transitions within a molecule?
Visible light
UV
X-rays
What type of transition is caused when a molecule absorbs light of the gamma frequency?
Electronic
Those concerning fundamental nuclear processes
When it is said that electrons/some nuclei possess spin, what is this actually referring to?
One of the two types of angular momentum (the other being orbital angular momentum).
What happens when nuclei with spin are placed in a strong magnetic field?
Nuclei with spin have a very weak local magnetic field which interacts with the external one. This gives rise to a set of nuclear spin energy levels (ie. spin up/parallel, spin down/antiparallel). Most nuclei will align with the magnetic field; few will be in the higher energy, antiparallel state.
In NMR, low frequency radio waves cause nucleic transitions between these energy levels resulting in a signal as nuclei relaxing back down emit photons.
How can the number of different energy levels of a nucleus placed in a magnetic field be determined from its nuclear spin quantum number, I?
2I + 1