rotational spectroscopy Flashcards
molecules in a gas are
free to rotate
molecular rotational energy is
quantised
the transitions between rotational energy levels correspond to what EMS regions
microwave and IR regions of the EMS
the principle of rotational spec can be applied to neutral molecules and
radicals
ions
rotational spec is normally measured in what phase
gas phase
or vapour
why can’t solids or liquids have rotational spec
solid molecules can’t rotate
in liquids, molecules can rotate but will probably collide before rotating fully - they can’t rotate freely
how can solid and liquid molecules have rotational spec
make a vapour out of them
rotational spec is very sensitive and therefore will probs pick up on it
what is rotational spec used for
identifying and measuring molecular properties such as bond lengths
what is rotational spectra depend on critically
the mass of the atoms
in rotational spec, what does the graph look like
absorbance / transmittance: y axis wavenumber: x axis
intensity of abs increases then decreases
abs are regularly spaced out
what happens if a solid or liquid absorb microwave radiation
no spectra
energy is distributed through the material due to collisions
converted into rotations and vibrations
it heats up 👩🍳
what is the wavenumber equation
1 / wavelength
what is wavenumber ( v-) proportionate to
frequency
energy
normal units and SI units for wavenumber
cm-1
m-1
wavenumber, frequency and speed of light equation
wavenumber = vc
energy, planks, speed of light and wavenumber equation
energy = planks x speed of light x wavenumber
in rotation of a diatomic molecule, what is r0
bond length
in m
in rotation of a diatomic molecule what is the centre mass
the part of the bond where if u hold it, the ‘barbell’ will balance
in rotation of a diatonic molecule, what is r1 and r2
r1 and r2: is the distance from the atoms to the centre of mass
in m
in rotation of diatomic molecules, what is m1 and m2
the mass in kg of the atoms
what is moment of inertia
rotation of a diatomic molecule around its centre of mass
moment of inertia equation
I = (m1m2) / (m1 + m2) x r0 ^2
what influences a molecules rotation
bond length and weight
heavy and long = harder to rotate
actual moment of inertia equation
I = yr0^2
what is y
more of a mirror image of y
it’s reduced mass
(m1m2) / m1 + m2
a molecule can only rotate at what
certain quantised speeds
energy applied for it to rotate must correspond to the energy between energy levels
other energies that don’t match will not be absorbed
rotational energy equation
Erotation = Ej = (h^2 / 8🥧^2 l) x J(J+1)
EJ is the energy of the Jth level
J is the rotational quantum number (0,1,2,3 etc etc)
what shape does the wave function of J= 0 have
same as an s orbital
degeneracy level of J issss
2J + 1
simple rotational energy equation
B = ( h / 8🥧^2 x l )
where B is the rotational constant
what is B in rotational energy equation
and units of
rotational energy constant
s-1
how can rotational energies be represented in terms of rotational constants
Ej = hBj(j+1)
gross selection rule for rotational spec
must have a permanent dipole
this dipole must oscillate at an energy corresponding at the same frequency as the radiation
specific selection rule for rotational spec
transitions can only occur between adjacent energy levels (adjacent j levels)
🔺j = +- 1
how can we find the population of two rotational energy levels in rotational spec
use the boltzmann distribution
the boltzmann distribution applied to rotational spec : what are the degeneracy of rotational levels
( 2J + 1)
in rotational spectra, as the energy levels increase in energy, so do
the distances between rotational energy levels
in rotational spec, when is EMR/energy absorbed
when it’s equal to the energy gap between the next rotational energy level.
this can be seen by the rotational energy spectra lines getting taller then shorter
what’s special between the peaks in rotational spec
the peaks are spaced out evenly
x hB
12hB has more energy than 2hB tho
rotational spectrum explanation
peaks are evenly spaced out at 2 hB apart
larger hB values have more energy
intensities if these peaks differ
