Intro into Spectroscopy Flashcards
What is the basic theory behind Spectroscopy
Spectroscopy probes the energy levels of molecules and their component atoms
Is the means that chemists use to determine molecular structure
Quantum Mechanics tells us …
… that the vibrational, rotational and translational contributions to the energies of molecules are quantised
This means the energy of these microscopic particles only take discrete values and the energies of individual molecules distributed across a set of discrete energy level
What is a transition
It is possible for an atom or molecule to change its energy by moving from one energy level to another
Moving to a higher energy level requires …
… absorption of energy
Moving to a lower energy level requires …
… emission of energy
The energy absorbed or emitted can come from
Electromagnetic radiation
When electromagnetic radiation causes a transition, it must be a …
Discrete amount of energy absorbed or emitted - known as a photon
The photon has a defined energy which is related to the frequency of the radiaition
What is the equation to work out the energy of a photon
E = hv
h = 6.626 x 10¯³⁴ J s (Planck’s constant)
v = frequency (s¯¹)
How does Spectroscopy relate to EM radiation ?
Why is this useful?
Spectroscopy is the analysis of the exact frequencies of the radiation absorbed or emitted by molecules
This allows us to measure the energy seperations between the levels involved in a transition and in turn calculated molecular parameters such as bond length or bond strength
The frequency range (spectrum) of electromagnetic radiation is
extends over 20 orders of magnitude, from radiowaves to gamma rays
How is wavelength (m) and frequency of radiation (Hz) related
c = 2.998 x 10⁸ m s¯¹
v = frequency (MHz) - radio part of EM spectrum
λ = wavelength (nm) - visible part of EM spectrum
Define wavelength (λ)
Is the distance from the top of one wave to the top of the next
Define wavenumber (ṽ)
What apart of the EM spectrum is it commonly used
the number of wave in one centimetre and is calculated as the reciprocal of wavelength
Wavenumbers commonly used when dealing with the IR part of the EM spectrum
What is the equation the related wavenumber (ṽ) to frequency (Hz)
c = 2.998 x 10¹⁰ cm s¯¹
v = frequency (Hz)
The separations between the different types of molecular energy levels are very different in magnitude
This means what in terms of transition energy
The energies required to cause transition are also very different
This means the corresponding photons come from separate parts of the electromagnetic spectrum and that different types of spectroscopy (which operate independently of one another) give information about different molecular parameters
What are the general features of an absorption spectrometer
Monochromator
Sample
Detector
Computer
What is the general difference between an absorption spectrum and transmission spectrum
Absorption spectrum: Peaks in the spectrim rise up from the baseline
Transmission spectrum: Peaks hang down from the baseline
What is the general difference between an absorption spectrum and transmission spectrum
Absorption spectrum: Peaks in the spectrim rise up from the baseline
Transmission spectrum: Peaks hang down from the baseline
When radiation is absorbed during spectroscopy, what happens to transmitted intensity as the sample length (L) and concentration [A] of the absorbing molecule increase
What is the name of this theory
Transmitted intensity decays
Beer-Lambert law
What is the Beer-Lambert law equation
I₀ = the incident intensity
I = Transmitted radiation
[A] = concentraion
ε = molar absorption coefficient (dm³ mol¯¹ cm¯¹)
L = sample length
The quantity ε (molar absorption coefficient) depends on what
The frequency (or wavelength as they are linked) and the nature of the solvent and the absorbing molecule
When incident radiation with an intensity of I₀ hits a sample in solution (with a concentration, c)
What will happen
Some radiation is absorbed and the rest emerges with a lower intensity I
Moving to a higher energy level requires the absorption of energy
How can this be measured
Moving to a lower energy level involves emission of energy
How can this be measured?
