Solid state techniques and UV-Vis spectroscopy Flashcards
What is a crystal?
Solid material whose constituent atoms, molecules or ions are arranged in an ordered pattern extending in all three spatial dimensions
What is an amorphous crystal?
Non-crystalline material so has a lower lattice enthalpy and a much higher solubility
What is a polymorph crystal?
Has multiple crystal states so has very physical properties. Has a lower solubility.
What is differential scanning calorimetry (DSC)?
This technique measures the difference in the amount of energy (heat) required to increase the temperature of a sample and reference at a constant state. Any exothermic or endothermic event in the sample will change the energy requirement to maintain the temperature gradient.
What is Thermogravimetric analysis? (TGA)
Similar to DSC but includes an accurate assessment of the mass of the sample. Allows separation of mass neutral events (melting/change in form) from other events such as desolvation and or decomposition.
What is UV spectroscopy?
Radiation in the UV-Vis region of the electromagnetic spectrum has the correct energy to excite electrons in one orbital into an orbital of higher energy.
The spectrum shows that the frequency of light absorbed which is needed to promote an electron between the two electronic states
What does the Beer Lambert Law show?
It shows that UV-Vis allows direct measurement of concentration. This is because Path length is set by the spectrometer, therefore if we know the value of (e) for a compound then concentration is directly linked to absorbance (and hence transmitted light intensity).
What are the issues with using UV spec?
The wavelength of light directly links to the energetic gap between orbitals.
Simple molecules such as alkanes and isolated alkenes and carbonyls absorb weakly at short wavelengths.
What is a chromophore?
The parts of a molecule that absorbs light.
What type of molecules absorb more strongly and at higher wavelengths?
Conjugated molecules - those with alternating single and multiple bonds. Molecules with extended pi systems absorb more strongly at higher wavelengths
What is the most common wavelength used to evaluate drug molecules? (UV Vis)
254 nM (frequency of a benzene ring)
What is X-Ray Diffraction used for?
The ordered arrangement of heavy atoms within the unit cell creates ‘planes’ within the crystal due to the electron density of the atoms. The x-ray diffraction pattern of a single crystal is a composite of the diffraction from all of the planes within the crystal.
The pattern can therefore be used to understand the electron density pattern of the atoms in the crystal, the 3D arrangement and therefore the structure of the molecule.
What is XRPD?
A powder is used instead. The presence of defined planes within the sample still results in light scattering at a defined angle from the beam.
X-ray diffraction pattern is plotted as the angle of diffraction against intensity.
What is a key feature of fluorescent molecules?
They have both absorption and emission spectra
