from revision 3 Flashcards
rotational spec isss
microwaves
moment of inertia
energy required to rotate an object
moment of inertia equation
sum of all atoms for :
(atom mass - kgmol-1 ) x (distance from central atom, m squared)
kg m2
types of rotor for rotational spec
spherical rotor
octahedral // tetrahedral shape
iz = iy = ix
symmetric:
ix = iy
linear ix = iy
rotate along the axis that needs the least rotation to become its symmetrical form (normally z - the one that joins atoms)
asymmetrical rotor
none are the same (diff atoms on each)
rotational, microwave spec selection rules
must have a permanent dipole to show a pure rotational spectrum
vib spec degrees of freedom
3 x number of atoms in molecule
3 translations (each cartesian axis)
3 rotations ( cart axis for non linear molecules)
2 rotations for linear molecules (no x axis rotation, stays the same)
non linear vibrational modes: 3N - 6 (3 rot, 3 trans)
linear vibrational modes:
3N - 5 (2 rot, 3 trans)
anharmonic oscillator description and explanation
based on real vibrations.
very steep at first: + nucs repel eachother
tails off and becomes constant: distance no longer affects their energies as theyre too far apart.
V0 to constant = bond dissociation energy = energy needed to break a bond:
hw(V+1/2)
as V increases, Vlevels get closer together until they reach a continuum at the constant bit (dissociation energy bit)
diff between harmonic and anharmonic oscialltor specific selection rule
harmonic: only go up by 1 V level
anharmonic: can go up many V levels - seen as an overtone (still not very probably/ common tho)
what affects where V=0 is
the mass aka reduced mass in the ‘w’ equation
larger = lower value bc it vibrates slower (lower w value)
lower v=0 means larger bond dissociation energy.
heavier atoms = more energy needed to break the bond.
u have energy in J, but they want it in cm-1, what do u do
use: E=hc wn
find wn
put c in the needed units
