Unit 4 Flashcards
Red wavelengths?
650-700nm
Orange wavelengths?
600-650nm
Yellow wavelengths?
550-600nm
Green wavelengths?
500-550nm
Blue wavelengths?
450-500nm
Purple wavelengths?
400-450nm
Resonance
radiation must have the same energy as a transition between two states
What kind of atomic/molecular transition is involved in UV-visible spectroscopy?
transitions of valence electrons
What kind of atomic/molecular transition is involved in infrared spectroscopy?
vibration transitions
What are ways in which an atom or molecule can become excited?
- thermal energy
- absorption of a photon
- chemical reaction
Between which molecular orbitals do molecules energetically favour excitation?
HOMO to LUMO
Ground singlet state (S0)
- totally spin quantum number = 0
- two electrons of opposite spin are in HOMO
1st excited singlet state (S1)
- total spin quantum number = 0
- one electron is located in HOMO and a spin paired electron is in the LUMO
How and why does an excited molecule return to ground state?
- excited state is transient
- via production of heat, light, or energy transfer to another molecule
What are ground state to excited state transition energies dependent on?
- molecular structure
- intermolecular interactions
- local environment
Vapour phase
- molecules can rotate and vibrate freely
- vibration and rotational fine structure observed
Dissolved in hexane
- rotational freedom is lost
- collisions with solvent broaden the vibrational transitions
Dissolved in water
- stronger interactions with solvent broaden transitions further
- obliteration of all fine structure
Chromophore
part of a molecular entity responsible for an absorption band and the approximate location of the corresponding electronic transiition
Provide an example of a chromophore
-aromatics
-alkenes
-alkynes
-carbonyls
(any unsaturated bonds)
What can be done to decrease transition energies?
More conjugation of unsaturated bonds
Effects of conjugation
- shifts absorption maximum to longer wavelengths
- absorptivity doubles with each conjugated double bond
Bathochromic
shift to longer wavelength
Hypsochromic
shift to shorter wavelength
Hyperchromic
shift to greater absorbance
Hypochromic
shift to lower absorbance
Auxochrome
chemical group that is attached to a chromophore and modifies its light absorption properties by altering the energies of the MOs
Transmittance
- the ration of the power (P) of the transmitted beam to the incident beam (P0)
- often expressed as %
Absorbance
- negative logarithm of transmittance
- number of orders of magnitude by which transmittance is decreased
Blank measurement
- account for non-absorption losses of light
- measured using only the solvent and the same/matched cell
Beer-Lambert Law
A=EbC (define!)
Spectroscopy
the study of how physical systems interact with or produce electromagnetic (EM) radiation
Spectrometry
the measurement of EM radiation to obtain information about systems and their components
Spectrophotometry
describe measurements made with ultraviolet (UV), visible, and infrared (IR) light
Molecular spectrophotometry
- method of molecular analysis
- samples comprise a collection molecules
Atomic absorption spectrophotometry
- method of elemental analysis
- isolated atoms do not usually exist in samples
- must generate an atomic population with energy input
Block diagram of spectrophotometer
light source > wavelength selector > sample cell > photodetector > electronics,computer
Light sources
- tungsten halogen lamp
- deuterium lamp
- light emitting diode
- laser
Wavelength selectors
- diffraction grating
- prism
- filters
Sample cell
- quartz, glass, plastic
- macro, micro, sub-micro
Photodetector
- photomultiplier tube (PMT)
- photodiode (array)
- charge-coupled device (CCD)
