rotational spectroscopy

Question 1

molecules in a gas are

Answer 1

free to rotate

Question 2

molecular rotational energy is

Answer 2

quantised

Question 3

the transitions between rotational energy levels correspond to what EMS regions

Answer 3

microwave and IR regions of the EMS

Question 4

the principle of rotational spec can be applied to neutral molecules and

Answer 4

radicals
ions

Question 5

rotational spec is normally measured in what phase

Answer 5

gas phase

or vapour

Question 6

why can’t solids or liquids have rotational spec

Answer 6

solid molecules can’t rotate
in liquids, molecules can rotate but will probably collide before rotating fully - they can’t rotate freely

Question 7

how can solid and liquid molecules have rotational spec

Answer 7

make a vapour out of them
rotational spec is very sensitive and therefore will probs pick up on it

Question 8

what is rotational spec used for

Answer 8

identifying and measuring molecular properties such as bond lengths

Question 9

what is rotational spectra depend on critically

Answer 9

the mass of the atoms

Question 10

in rotational spec, what does the graph look like

Answer 10

absorbance / transmittance: y axis wavenumber: x axis

intensity of abs increases then decreases

abs are regularly spaced out

Question 11

what happens if a solid or liquid absorb microwave radiation

Answer 11

no spectra
energy is distributed through the material due to collisions
converted into rotations and vibrations
it heats up 👩‍🍳

Question 12

what is the wavenumber equation

Answer 12

1 / wavelength

Question 13

what is wavenumber ( v-) proportionate to

Answer 13

frequency
energy

Question 14

normal units and SI units for wavenumber

Answer 14

cm-1
m-1

Question 15

wavenumber, frequency and speed of light equation

Answer 15

wavenumber = vc

Question 16

energy, planks, speed of light and wavenumber equation

Answer 16

energy = planks x speed of light x wavenumber

Question 17

in rotation of a diatomic molecule, what is r0

Answer 17

bond length
in m

Question 18

in rotation of a diatomic molecule what is the centre mass

Answer 18

the part of the bond where if u hold it, the ‘barbell’ will balance

Question 19

in rotation of a diatonic molecule, what is r1 and r2

Answer 19

r1 and r2: is the distance from the atoms to the centre of mass
in m

Question 20

in rotation of diatomic molecules, what is m1 and m2

Answer 20

the mass in kg of the atoms

Question 21

what is moment of inertia

Answer 21

rotation of a diatomic molecule around its centre of mass

Question 22

moment of inertia equation

Answer 22

I = (m1m2) / (m1 + m2) x r0 ^2

Question 23

what influences a molecules rotation

Answer 23

bond length and weight

heavy and long = harder to rotate

Question 24

actual moment of inertia equation

Answer 24

I = yr0^2

Question 25

what is y

Answer 25

more of a mirror image of y it’s reduced mass (m1m2) / m1 + m2

Question 26

a molecule can only rotate at what

Answer 26

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

Question 27

rotational energy equation

Answer 27

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)

Question 28

what shape does the wave function of J= 0 have

Answer 28

same as an s orbital

Question 29

degeneracy level of J issss

Answer 29

2J + 1

Question 30

simple rotational energy equation

Answer 30

B = ( h / 8🥧^2 x l ) where B is the rotational constant

Question 31

what is B in rotational energy equation and units of

Answer 31

rotational energy constant s-1

Question 32

how can rotational energies be represented in terms of rotational constants

Answer 32

Ej = hBj(j+1)

Question 33

gross selection rule for rotational spec

Answer 33

must have a permanent dipole this dipole must oscillate at an energy corresponding at the same frequency as the radiation

Question 34

specific selection rule for rotational spec

Answer 34

transitions can only occur between adjacent energy levels (adjacent j levels) 🔺j = +- 1

Question 35

how can we find the population of two rotational energy levels in rotational spec

Answer 35

use the boltzmann distribution

Question 36

the boltzmann distribution applied to rotational spec : what are the degeneracy of rotational levels

Answer 36

( 2J + 1)

Question 37

in rotational spectra, as the energy levels increase in energy, so do

Answer 37

the distances between rotational energy levels

Question 38

in rotational spec, when is EMR/energy absorbed

Answer 38

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

Question 39

what’s special between the peaks in rotational spec

Answer 39

the peaks are spaced out evenly x hB 12hB has more energy than 2hB tho

Question 40

rotational spectrum explanation

Answer 40

peaks are evenly spaced out at 2 hB apart larger hB values have more energy intensities if these peaks differ