lecture 10: spec instruments and applications Flashcards
(28 cards)
What are the different components of a double-beam spectrophotometer?
- light source: 2 lamps that produce UV and visible light
- grating: sends narrow band of wavelengths into monochromator
- monochromater: sends narrower band of wavelength onto the sample and reference
- beam chopper: rotating mirror, directs light to the sample, 30 times each second
- second chopper: directs trasmitted light to PMT detector
- PMT creates electric current proportional to irradiance
What are the advantages of continuum light source?
- broad emission over all wavelengths
- intense
- stable intensity over time
What is understood from blackbody radiation?
the light emitted from a heated object
What are the two light sources and what can they be used for?
- tungsten lamp: radiation from 320 to 2500nm, visible and near IR
- Deuterium arc lamp: emits radiation from 110 to 400 nm in UV
What is diffraction?
bending of light rays by a grating
What is refraction?
bending of light rays by a prism or lens
What is the difference between constructive and destructive interference?
Constructive: waves are in phase, reinforcing each other
Destructive: waves are out of phase, cancelling each other out
What is the function of detectors?
it produces an electric signal when it is struck by photons
What is PTM?
PhotoMultiplier Tube, it contains a photocathode and dynodes that emit a cascade of electrons when struck by an electron.
It is very sensitive, 500-1000nm
What is the difference between scanning and PDA specs?
scan: have higher resolution, measure higher absorbances, but one wavelength at a time
PDA: lower resolution, low absorbances
What is an isosbestic point?
It is a wavelength at which the absorbance of two or more chemical species is the same, regardless of their concentrations. It is used to monitor reactions, and determine equilibrium constants
What is a sigma bond?
the sigma bind is similar to s-orbital shape, it forms a single bond. When bonds are formed, energy is released and the system becomes stable.
What is the difference between antibonding and nonbonding
antibonding: the electrons are out-of phase (destructive interference), and so they have higher energy (destabilizes)
nonbonding; molecular orbitals that do not form any bonds, because the wavelengths dont interact
What is the lowest and highest energy occupied by the molecular orbitals of formaldehyde?
highest: nonbonding orbital n
highest energy is pi bonding orbital
lowest: pi antibonding orbital
What is a singlet state?
at 355 nm, photon promotes to singlet state, where unpaired electrons have opposite spin: So to S1
What is the triplet state?
at 397 nm, photon promotes to triplet state, where unpaired electrons have identical parallel spins. So to T1
What does Homo stand for?
Highest occupied molecular orbital (nonbonding orbital n)
What is the translational state of infrared photons?
induce transitions between vibrational states of motion of a molecule
What is the translational state of microwave photons?
induces transitions between rotational states of a molecule
What are the states of motion for a molecule that is linear?
3 states are translations, 2 states are rotations: 3n-5 states are vibrations
What are the states of motion for a molecule that is non-linear?
3 states are translations, 3 states are rotations, 3n-6 states are vibrations
What is internal conversion (IC)
is a non-radiative transition between states with the same spin (squiggle line)
What is intersystem crossing (ISC)
is a non-radiative transition between states with different spin (also squiggly line)
What is fluorescence?
the release of a photon by the radiative relaxation from a singlet state to another singlet state
