CHEM111 - mod. 2 BONDING THEORIES

Lec. 8, 9

what is VB theory used to understand?

structures, energies, reactivities of some chemical compounds

how can bond angles be predicted by VB theory?

geometry and where the orbitals are directed in space

lowest energy conformations of cyclohexane

chair and boat

what is different about cyclohexane in boat and chair formation (H atoms)

each H atom isn’t equivalent due to the movement out of the planar shape

axial

sticks out of plane

equatorial

lies within the plane

why is benzene more accurately represented by a delocalised pi bond?

when p orbitals overlap to form pi bonds, there’s an equal probability of a given orbital overlapping with its R or L neighbour

conjugated

alternating single and double bonds

hybrid orbitals in diamond

network of sp3 hybridised C, with each hybrid orbital overlapping with another

hybrid orbitals in graphite

each C has one singly occupied p orbital that’s delocalised

graphite structure

weakly held together sheets of C linked with strong C-C bonds in the plane

bond lengths get shorter as

bond energy increases

why is a sigma bond stronger than a pi bond?

pi bonds have orbitals overlapping above and below the plane of the molecule, this is insufficient overlap and doesn’t hold the atoms together strongly

why are double bonds more reactive?

because a double bond is made up of a sigma and a pi bond, the pi bond is weakly overlapping and so easy to break

what does a substitution attack from a nucleophile result in?

two possible enantiomers

orbitals involved in nucleophilic substitution reaction

the electrophile has an unoccupied p orbital (above and below the plane), the electrophile can donate its electron pair to either half of the p orbital

main limitation of VB theory

the structure of a molecule may work according to VB theory but experimentally it might deflect the magnetic field indicating unpaired electrons which isn’t supported by VB theory

what are the allowed states of a system in MO theory

the wavefunctions that are solutions to the Schrödinger equation

in molecular systems, molecular orbitals are

a hierarchy of available shapes

orbital spread of atomic orbitals

spread over whole atom

orbital spread of molecular orbitals

spread over whole molecule

what does the spread of molecular orbitals create?

a single set of orbitals for the entire molecule

why do the atoms bond according to MO theory?

the orbitals have regions of high electron density between atoms

semi-quantitative nature of MO theory

can get numerical values for energies

what does a potential energy curve show?

the energetics involved in the formation of a bond and the visualisation of the position of the electron along the curve

bonding molecular orbital

constructive interference — equal probability of the electron being in the orbital of either atom

antibonding molecular orbital

destructive interference — no chance of the electron being between the two nuclei, the atoms are pushed apart

why are bonding MO lower in energy than antibonding MOs

because there is a lowering in the total energy

when drawing an MO diagram, how are the electrons filled in?

according to the aufbau principle, Hund’s rule and Pauli exclusion principle

what is the * symbol used for in MO diagrams

antibonding orbitals

calculation for bond order

1/2[(# e in bonding MOs) - (# e in antibonding MOs)]

how do p orbitals overlap?

constructively and destructively forming sigma and pi MOs. 

the electronic configuration for molecules is the configuration for the

ground state

what does removing an electron from an antibonding orbital do?

stabilises the molecule

what is the effect of 2p orbitals being shielded more than 2s orbitals?

Zeff is less and the orbital energy is higher

which molecules have no s-p mixing

O2, F2

which molecules have s-p mixing?

N2, C2, B2

s-p mixing

the 2s orbital of one atom can overlap with a 2p orbital on the other atom due to the similar energies of the orbitals

what changes in the MO diagram when s-p mixing is present?

in the 2p MO, pi is lower energy than sigma. Antibonding orbitals remain the same

similarities between MO and VB theories

• use interference to predict where e might be bonding in the molecule

• use sigma and pi as labels

differences in overlap between VB and MO theories

VB has only constructive overlap of AO, while MO has both constructive and destructive overlap of AOs. 

filling up of orbitals in VB theory

AOs are filled then overlapped to form bonds

filling up of orbitals in MO theory

AOs are overlapped to form possible orbitals then filled. The bonds arise from specific density between atoms

treatment of bond/molecule in VB theory

bonds treated separately which results in electron localisation

treatment of bond/molecule in MO theory

molecule treated as a whole which allows delocalisation and single occupancy

strengths of MO theory (3)

• accurately predicts magnetic properties

• works well for “exotic” systems

• can be made quantitative (predicting reactivity)

weaknesses of MO theory (2)

• hard to do by hand for anything other than diatomics

• doesn’t easily give information about geometry

strengths of VB theory (2)

• easy information on geometry

• works well for organic systems

weaknesses of MO theory (3)

• can’t describe magnetic properties

• don’t get quantitative information on reactivity

• doesn’t work as well for “exotic” molecules

what theory do you use?

depends on what you want to know about the molecule