Intermolecular Forces and Potentials

Question 1

Van der Waals

Answer 1

non-covalent interactions

Question 2

force

Answer 2

vector

Question 3

Positive Q

Answer 3

attractive

Question 4

Negative Q

Answer 4

repulsive

Question 5

Multipole moment of heterodiatomic

Answer 5

quadrupole, accumulation of e- density between the nuclei

Question 6

monopole

Answer 6

point charge

Question 7

dipole

Answer 7

superposition of 2 monopoles with opposite charges

Question 8

quadrupole

Answer 8

superposition of 2 oppositely oriented dipoles

Question 9

Components of intermolecular potential

Answer 9

dipole A - dipole B, dipole A - quad B, quad A - dipole B, quad A - quad B

Question 10

Distance dependence of monopole-monopole

Answer 10

r-1

Question 11

Distance dependence of monopole-dipole

Answer 11

r-2

Question 12

Distance dependence of monopole-quadrupole

Answer 12

r-3

Question 13

Distance dependence of dipole-dipole

Answer 13

r-3

Question 14

Distance dependence of dipole-quadrupole

Answer 14

r-4

Question 15

Distance dependence of quadrupole-quadrupole

Answer 15

r-5

Question 16

Orientation of multipoles

Answer 16

orientations sampled with probability that reflects their energy ie. use boltzmann

Question 17

Dipole-dipole interactions

Answer 17

also called Keesom, attractive or repulsive depending on orientation, molecules don’t rotate freely at finite temp, attractive orientations more favoured than repulsive interactions

Question 18

Induction interactions

Answer 18

always attractive, permanent multipole of A induces permanent multipole on B, magnitude depends on magnitude of multipole of A and polarisability of B, not frustrated by thermal motion

Question 19

Dispersion interactions

Answer 19

also called London Dispersion, always attractive, instantaneous multipole of A correlate with instantaneous multipole of B, not frustrated by thermal motion

Question 20

Origin of dispersion interactions

Answer 20

electrons in constant motion so can have instantaneous dipole moment, this induces another dipole in adjacent atom giving an attractive potential

Question 21

Magnitude of polarisability volumes

Answer 21

similar in magnitude to actual molecular volumes, correlates with HOMO-LUMO separation in atoms/molecules, electron distribution distorted easily if LUMO and HOMO are close in energy

Question 22

Dipoles at low AC frequency

Answer 22

dipoles have time to re-orient themselves with oscillating electric field, can contribute to dielectric constant, use Debye equation to interpret temp dependence of molar polarisation

Question 23

Dipoles at high AC frequency

Answer 23

dipoles can’t keep up with oscillating freq, can’t contribute to dielectric const, molar polarisation independent of temp and use mossotti to determine polarisability

Question 24

Stark deceleration

Answer 24

molecule with permanent dipole approaches dipole and slows due to increasing field, reaches position at top of energy hill, fields switched to prevent molecule continuing down hill, voltage of first and second electrode swapped, molecule instantaneously arrives at bottom of hill and continues climbing but slower, repeated until desired velocity reached

Question 25

Repulsive wall contributions

Answer 25

nuclear-nuclear repulsion, electron-nucleus repulsion and electron-electron repulsion

Question 26

Valence repulsion

Answer 26

atoms with closed shells repel each other, valence repulsion due to overlap of orbitals of interacting molecules, overlap decays exp with increasing molecular distance

Question 27

which interactions are additive

Answer 27

electrostatic only