Section B: spectroscopy Flashcards
describe the action of light as a wave
- electromagnetic (E) and magnetic (H) fields travel together in space
- they fluctuate in phase, both in time and along direction of travel
how can ultraviolet be detected?
by the blackening of Ag salts;
insects can ‘see’ UV light
how can infrared be detected?
using a thermometer
describe the theory of UV-visible spectroscopy
- absorption bands due to transitions between electronic energy levels
- transitions involving tightly-bonded electrons occur in the UV range (not visible to the human eye)
- transitions among valence electrons in unsaturated organic compounds can occur in the visible range, often in chromophores
- if an atom absorbs UV-visible light, the electron can be promoted to a higher energy orbit
what are some characteristics of chromophores?
- polyunsaturated systems
- polyaromatic systems
- systems containing C=O, C=S, C-P, etc.
- molecules containing transition metal ions
which electron transitions lie in the UV-vis spectra?
π-π* transitions lie in the UV range
π-σ* transitions usually lie at too high an energy to be observed in UV-vis spectra
how does the speed of light relate to the wavelength and frequency of light?
c = λ x v
how can the energy of a light particle (photon) be calculated?
E = h x v = (h x c) / λ
what is the speed of light?
2.998 x 10^8 m/s
which wavelengths does chlorophyll absorb at?
400-450 nm (blue-violet) and 600-700 nm (red) to reflect a green-yellow colour
what is Hooke’s Law?
F = - kx (restorative force)
where k is the force constant- related to bond strength
what units does k, the force constant, have?
N/m or kg/s^2
what is the relationship between the magnitude of the force constant and the strength of the bond?
the greater the force constant, the stronger the bond
which equation relates the force constant, the reduced mass and the frequency of absorption?
v(0) = 1/2π SQRT(k/μ)
explain the theory behind infrared spectroscopy
- atoms vibrate about their equilibrium positions in well-defined patterns
- the covalent bond can be modelled as a spring
- use of Hooke’s Law
- if the vibration causes a change in the electric dipole moment (μ0), this results in a fluctuating electric field that can interact with the infrared light
how is the reduced mass, μ, calculated?
μ= m1m2 / (m1 + m2) (x 1.67 x 10^-27 kg)
what is the mass of a proton?
1.67 x 10^-27 kg
what is the isotope substitution formula?
v0heavy / v0light = SQRT(μlight / μheavy)
describe the lack of absorption spectrum for homonuclear diatomic molecules
- no dipole moment
- μ0 does not chance during the vibration
- no absorption spectrum
can there be an absorption spectrum for molecules with no permanent dipole moment?
- yes, if they are more complication than homonuclear diatomic molecules
- some vibrations can cause a change in μ0
- these vibrations are IR-active
eg. CO2
what is the relationship between the magnitude of the dipole change and the IR absorption intensity?
if the dipole moment change is large, the IR absorption intensity is strong
which vibrations are IR-active for CO2?
asymmetric stretching ✓
bending ✓
symmetric stretching x
what is FTIR spectroscopy?
Fourier transform IR spectroscopy - simultaneous collection of data over a wide spectral range to obtain an infrared spectrum
what is the nature of CH2 and CH3 absorption peaks?
sharp peaks