Spectroscopy

What class of substances are a) attracted and b) repelled by a magnetic field

a) paramagnetic (at least 1 unpaired electron)

b) diamagnetic (no unpaired electrons)

Electrons pairs in diamagnetic materials (contribute to/oppose) homogenous magnetic field H₀

Electrons pairs in diamagnetic materials (contribute to/oppose) homogenous magnetic field H₀

What are the two forces that paramagnetism/effective magnetic moment arises from?

spin angular momentum

orbital angular momentum= movement between orbitals

Spin only formula

μ_eff = √n(n+2) μ_B

where n= number of unpaired electrons

What causes deviations from the spin only formula?

when the number of electrons in the d_yx, d_xz and d_yz orbitals is not the same

only significant for T terms where electrons can be arranged in 3 ways

What causes Zeeman splitting?

in an applied magnetic field the electrons with spin in the same direction as the field raise in energy and the electrons with opposite spin direction lower in energy

When is orbital contribution (L) to μ_eff relevant?

unequal occupancy of octahedral t_2g orbitals causing ability of electrons to transfer between d_yx, d_xz and d_yz, increasing magnetism

Curie Law

X_M= C/T

where C is the Curie constant

stipulates that thermal motion disrupts alignment of spin with H₀ causing decreased magnetism

If the Curie Constant is not known, what is an approximation of the Curie Law' magnetic sisceptibility?

X_MT≈ s(s+1)/2

A change in physical environment can trigger a switch between high and low spin configurations in complexes with how many d electrons?

4, 5, 6, 7

Curie-Weiss Paramagnets: formula and what the components mean

X_M= C/(T-θ)

where θ is the Curie-Weiss constant

• 0>θ= ferromagnetic (image; -c)

• θ<: antiferromagnetic (+c)

What kind of spins is a a) Curie and b) Curie-Weiss law applicable to?

a) isolated/ dilute spins with no orbital contribution to the magnetic moment

b) interactions between spins

The four types of magnetism arising from different electron spin alignments

What does the trend look like for magnetism of a) para b) ferro and c) antiferromagnetic compounds with increasing temperature (vs. X_M)?

magnetic orbital

orbital containing an unpaired e^–; SOMO

exchange interactions: a) definition and b) types (2)

a) interaction between different spin (metal) centres with unpaired e^—

b) direct interaction: overlap of magnetic orbitals

superexchange: magnetic orbitals overlap interact through the filled orbitals of bridging ligands

What does it mean if a compound is colourless or white?

No absorbtions/transmissions are possible between the energy levels corresponding to visible light

What are the rules governing the UV excitation of electrons? (2)

• spin selection: cannot change spin multiplicity of the system

• Laporte allowed: no transitions between energy levels of the same symmetry

Why are UV peaks broad?

The beam of light can hit any atom that possesses any energy state that can absorb a given photon; molecules are not the exact same

What is ε for a) symmetry allowed/ charge transfer, b) Laporte allowed, c) Laporte forbidden and d) spin forbidden transitions?

a) >1000

b) ~250

c) 20-100

d) <1

Ligand to Metal Charge Transfer transition

promoting an electron from an orbital that is mainly ligand in character to one of the d orbital on the metal

What are the features of a) metals and b) ligands that support LMCT?

a) higher oxidation states

b) anionic, with non-bonding lone pairs

*raise the ligand orbitals and lower the metal orbitals so they can be closer

What trend is observed for LMCT?

decreases moving left to a period and down a group

LMCT bands can be identified experimentally by…

demonstrating that in different solvents different wavelengths are observed, because the transition involves a change in dipole moment

MLCT ligands (3)

conjugated aromatic systems

CO

CN

TRUE OR FALSE: excited states involve moving an electron to a higher energy level

TRUE, BUT flipping the spin of an electron and moving it down is also an excited state!

What selection rule(s) does a LMCT obey?

both Laporte and spin selection rules

Where do LMCT bands appear?

in visible or near UV range (400-800 nm)

What is the relationship between wavenumber and wavelength?

Inversely proportional?

What effect on the wavelength of absorption (inverse to energy) does a) increasing electronegativity of metal (→↑), b) increasing electronegativity of ligand and c) increasing oxidation state have?

a) increase

b) decrease

c) increase

When does MLCT happen; what ligands is it common for?

• low metal oxidation states

• ligands with low lying pi orbitals eg. aromatic systems, CN, CO

Intervalence charge transfer (IVCT) can occur for…

polynuclear compounds with metals in different O.S.

Moving to lower symmetry generally results in observation of (__) spectral features

more (callback to x-ray crystallography)

TRUE OR FALSE: Jahn-Teller distortion results in more peaks

TRUE