CH2203 – Spectroscopy of Inorganic Compounds

Electronic/UV-Vis/Absorption Spectroscopy

This is based on the movement of an electron from a ground state to an excited state, each

transition will relate to a discrete energy value characteristic to the system being analysed, allowing

structures to be calculated. The absorbance is dependent on the Beer-Lambert law;

A = εcl

The probability of the absorption occurring is given by the value of ε, the extinction coefficient.

For inorganic compounds this method is mainly used to analyse d-d transitions, these are generally

weak transitions performed in the visible region of light, and charge transfer processes in transition

metal complexes.

The spectra can give the crystal field splitting parameter (∆ or 10Dq), tell whether the species is high

or low spin and give the geometry (e.g. octahedral or tetrahedral, metal oxidation states, ligand

properties). Since d-d transitions are normally in the visible region, most transition metal complexes

are coloured (if they absorb through the visible they are black, if partially they display the

complimentary colour to that absorbed, if not at all they are white) and give broad absorption peaks,

this is since electronic transitions are rapid (around 10-15s) and so molecules are in many vibrational

states at the time of the transition leading to absorption of a range of energies.

The most common geometry in transition metal chemistry is octahedral, where the 6 ligands lie on

the x,y and z axes, in this arrangement the greatest interaction occurs with the dx2 and dx2-y2 (the

t2g) orbitals as they lie along the axes, the remaining orbitals (the eg, dxy, dxz and dyz orbitals) lie

between the axes reducing the interaction.

In a tetrahedral geometry the ligands approach the dxy, the dxz and the dyz orbitals, the t2 orbitals,

while the dx2 and dx

2-y

2 remain at a lower energy, the e orbitals.

The probability of a transition occurring, effectively the value of ε, is given by three selection rules.

1. The spin rule

Only one electron is involved in any transition, transitions are allowed when there is no

change in the spin multiplicity (i.e. ∆S=0) of the ground and excited states. In a d5 high spin

complex transition will be forbidden as the electron would be forced to change spin, in a d5

low spin complex transition would be allowed as it would be possible for electron spin to

remain the same.

2. ∆l=±1

Only changes that involve a change in the orbital angular momentum quantum number (l) of

1 are allowed, transitions within the same sub-level are forbidden (i.e. s→p allowed, p→d

allowed, d→d forbidden)

3. The Laporte Selection Rule

In a centrosymmetric system a transition is only allowed if it is between orbitals of different

symmetries (i.e. u→g or g→u, ungerade and gerade being the German for symmetrical and

asymmetrical, gerade for centrosymmetric orbitals such as s or d orbitals, ungerade for non-

centrosymmetric orbitals such as p orbitals).

This would suggest that all d-d transitions are forbidden in octahedral complexes and helps

to explain why f-f transitions are so weak in lanthanide compounds however some are

witnessed due to vibronic coupling which causes a level of asymmetry in otherwise

centrosymmetric systems.

The Spectrochemical Series

Early UV-vis studies showed that the position of the bands varied in a consistent manner for a set of

metal ions and ligands, leading to the spectrochemical series.

The value of ∆o on coordination to a metal increases along the series;

I-<Br-<Cl-<F-<O2-<H2O<NH3<CN-<PR3<CO

Tetrahedral Complexes

Since in a tetrahedral complex the crystal field splitting energy is 4/9 of that in an octahedral

complex d-d transitions occur at a lower energy (and so a higher wavelength) than those in an

octahedral complex. Also of importance is that tetrahedral complexes do not have a centre of

symmetry, so d-d transitions do not break the Laporte selection rule. If the spin selection rule is

obeyed then d-d absorptions are more intense than for octahedral complexes.

Charge Transfer Bands

These occur in addition to d-d transitions and can be either Ligand to Metal Charge Transfer (LMCT)

or Metal to Ligand Charge Transfer (MLCT). These involve the temporary movement of an electron

from ligand to metal or vice versa, the transitions are generally spin allowed and Laporte allowed

and so are very intense with an ε of 1000-10000M-1cm-1. LMCT requires an oxidisable ligand (e.g. O2-)

and a high oxidation state metal (easily reduced), quite often these are n→d transitions, i.e. S2-

→Cd2+ (5s).

MLCT requires a low oxidation state metal and a reducible ligand e.g. [Ru(bipy)3]2+ which shows

MLCT absorption at 450nm.

Nuclear Magnetic Resonance Spectroscopy

Nuclear Spin (Angular Momentum)

Nuclear spin arises from unpaired proton/neutron spin, thus isotopes with an odd mass will have a

nuclear spin, though not all odd atomic mass isotopes are useful. Energy levels in NMR arise from

the interaction of the nuclear spin with the spectrometers magnetic field (if the nuclei align with the

field they are low energy, if against the field, high energy), however they will also be affected by

other nuclear spins and magnetic fields established by electrons in the sample.

The nuclear spin is quantised and given the quantum number I. I can be an integral or half-integral

and nuclei with spin larger than ½ are quadrupolar and typically give broad lines.

Nuclear spin, I, has 2I+1 mI states (with values +1 to -1) which in the absence of a magnetic field are

degenerate.

When placed in a magnetic field (strength equal to B0) their degeneracy is removed giving 2I+1

energy levels with mI states, with the lower levels only slightly more populated (~1 in 105 at 25oC)

according to the Boltzmann distribution, making the NMR signals weak.

The selection rule for NMR is that transitions must have a ∆mI=±1, the energy level of each state is

given by;

E = B0γhmI

Where γ is the gyromagnetic ratio

Therefore to increase the strength of the NMR signal, the strength of the applied magnetic field may

be increased. Since ∆mI is limited to ±1 the energy differences are equal and proportional to γ, this is

different for all nuclei providing characteristic values of E. As different nuclei have different E values,

they will resonate at different frequencies.

I Resonant Frequency (MHz)

Abundance Receptivity Quadrupolar Moment

1H 1/2 200.1 100% 1000 0 7Li 3/2 77.76 92.6% 272 -0.04 10B 3 21.50 19.7% 4 0.08 11B 3/2 64.20 80.3% 133 0.04 19F ½ 188.33 100% 83.4 0 29Si 1/2 39.70 4.7% 0.4 0 31P 1/2 81.00 100% 66 0

55Mn 5/2 49.35 100% 175 0.4 195Pt 1/2 43.01 33.8% 3.4 0

Scalar Coupling

When there are different NMR active nuclei present in a molecule, scalar coupling will occur

between them. This arises as the nuclear magnetic moment of other NMR active nuclei surrounding

the nuclei of interest produce additional, albeit small, magnetic fields. If it occurs between the same

nuclei in different environments it is called homonuclear coupling, and if it is between different

elements it is called heteronuclear coupling. The observed nuclei are affected by these resultant

magnetic fields, the number and nature will depend on the nature of the surrounding active nuclei.

This effect is independent of the strength of the applied field and so the coupling constants will not

vary between NMR machines.

Scalar coupling is actually a polarisation of the electrons within the bonds between the nuclei, if one

nuclei is to spin upwards then an electron, in an orbital between this nuclei and the coupling nuclei,

can minimise its energy by aligning to spin in the same direction. As the electron on the coupled

nuclei is in the same orbital it must have the opposite spin, and how this interacts with the nuclear

spin of B will affect the energy of the spin state, perturbing the energy levels of the spin system.

In essence the nuclear spin of the first nuclei can sense that of the second due to polarisation of

electrons in bonds. The spin state of the second nuclei can affect the resonance energy and lead to

various multiplicities.

As the number of bonds between the nuclei increases the polarisation effect will decrease, as such it

is uncommon to observe coupling over more than 4 bonds.

In a situation where one of the nuclei has a spin of 3/2 there are 4 mI states (2I+1). The other nuclei

can sense the 4 different spin states which are all equally probably and so the first nuclei is split into

4 lines of equal intensity. In general, when a nuclei is coupled to a nuclei of spin I, the observed

multiplicity is;

2nI + 1

NMR Spectra of Nuclei with I=1/2

These are similar to 1H NMR spectra, though usually with different chemical shift ranges. Both

homonuclear and heteronuclear coupling can occur.

Two important nuclei, both of which are 100% abundant, are 19F and 31P.

The spectra of 19F or 1H can be simplified using a technique known as decoupling, to produce

singlets. Conventionally this is written as 19F{1H} NMR if it were to be a 1H decoupled 19F spectrum.

To achieve this the sample is irradiated at the desired decoupled nuclei’s resonant frequency while

obtaining the NMR, this causes the decoupled nuclei to rapidly resonate between spin states so that

the species undergoing NMR can only ‘see’ an average of the multiple states, removing coupling.

Decoupling can be used for any combination of nuclei.

PF5 has a trigonal bypyramidal structure, single crystal x-ray studies indicate there are two distinct P-

F bonds (axial and equatorial) with P-Faxial=158.0pm and P-Fequatorial=152.2pm. For this species to be

spectroscopically detected it would have to exist in that energy state for a finite time. The

approximate timescale for NMR is 10-7 seconds (varying depending on the applied field and species

being studied as well as the species relaxation rate). The species PF5, at room temperature, is

fluxional, with fluorine atoms exchanging amongst each other. Since this occurs at a rate faster than

the NMR timescale only an average environment is observed, in which all fluorine atoms are

equivalent and not coupled to each other (just to the phosphorus). The process, by which the

fluorine atoms interchange, is known as Berry pseudo-rotation. To prevent this mechanism

interfering with the NMR spectra the species can be cooled down, the temperature at which this

mechanism is halted is known as the coalescent temperature.

NMR Spectra of Nuclei with I>1/2

Fundamentally these are the same as for nuclei with I=1/2 but more complicated. A commonly

analysed species is 11B, a quadrupolar nucleus with nuclear spin 3/2.

-Diborane B2H6

In diborane the 2 boron atoms are equivalent, as are the 4 terminal hydrogen atoms (Ht) and the 2

bridging hydrogen atoms (Hb). The signal for Hb is as expected, a septet of quintets (coupling to 2 11B,

I=3/2, and 4Ht, I=1/2). The pattern for Ht is complicated however, although there is a 1JHB coupling to

the nearest boron atom, there is also 3JHB coupling to the other boron atom, since the hydrogen

atoms terminal to each boron are undergoing the same effects from different perspectives they are

considered magnetically inequivalent, despite being chemically equivalent.

Magnetic inequivalence gives rise to second order effects in spectra which make them complicated

and difficult to analyse.

On top of this there is the isotope effect to consider. The 1H NMR shows coupling of 4 equivalent

hydrogen atoms to 11B (I=3/2) to give 4 peaks of equal intensity, but 11B is only 80% abundant whilst 10B (I=3) is 20% abundant. This couples to the hydrogen atoms to give 7 lines of equal intensity, since

there is a mixture of isomers present in the sample the spectra are superimposed. The relative

intensities for each line are calculated by;

Relative Intensity =Abundance of Isomer

No .of Lines

The superimposed patterns would have the same central point but with different couplings.

Chemical Shift

The chemical shift of a peak or set of peaks is effected by the shielding of a nucleus, the less shielded

the nucleus is the more positive the shift value. An electronegative substituent, a positive charge or

a higher oxidation state will affect the ability of the electrons to shield the nucleus, leading to de-

shielding and a downfield shift in δ, the chemical shift.

Infrared/Vibrational Spectroscopy

Molecules undergo a complex series of vibrations which depend on;

-the symmetry of the molecule

-mass of the bonded atoms, as according to Hooke’s law;

Absorbed Frequency =1

2πc

k

μ

μ =m1m2

(m1 + m2)

-bond strengths

Theory shows that these can be resolved into a limited number of normal modes of vibration. For a

molecule with ‘n’ atoms there will be 3n-6 vibrational modes if it is non-linear, and 3n-5 vibrational

modes if it is linear.

The selection rule for infrared spectroscopy is; a vibration will only absorb energy if a dipole change

occurs (the magnitude of the change is proportional to peak intensity).

As with organic compounds, inorganic bonds have characteristic absorption frequency ranges, the

position of a band can give the same sorts of information on bond strength, the mode of attachment

of ligands etc.

Metal Carbonyl Complexes

C=O absorptions are particularly useful when running infrared spectra on inorganic complexes, they

are strong bands in the region above 1500cm-1, a region otherwise unoccupied when using nujol.

The frequency of the C=O absorption can be lowered by a process called back-bonding. While there

is a sigma bond established by the carbon atom to the metal, the d-orbitals of the metal are pushing

unwanted electrons into the empty anti-bonding p-orbital on the carbon. This forces the bond

slightly apart, weakening it and lowering the absorption of the C=O stretching frequency. The

position of the C=O stretch can show a lot about the bonding mode of the carbonyl as a ligand. A

terminal M-CO will have a stretch above 1900cm-1, while a bridging M2-CO will be around 1750-

1900cm-1, a CO ligand bridging 3 metal atoms will be lower again, around 1650-1730cm-1.

As the positive charge on the complex increases the level of back-bonding will decrease, this is due

to the d-orbitals on the metal contracting with an increase in positive charge, reducing the overlap

with the π* orbital and reducing the effect of the back-bonding.

Metal Hydride Complexes

Also important in inorganic infrared spectroscopy are metal hydride complexes, of the formula

LnMHn. As with carbonyl complexes the M-H bond normally has stretching modes which absorb

outside the fingerprint region. A terminal M-H appears around 1700-2200cm-1, a bridging M-H will

absorb around 1100-1400cm-1, beginning to fall into the fingerprint region. If deuteration of the

metal hydride to form M-2D was to be performed, the change in wavelength could be predicted

using Hooke’s law.

For MH;

νM−H =1

2πc

k

1

For MD;

νM−D =1

2πc

k

2

And so;

νM−H

νM−D=

11

12

= 1.41

∴ νM−D = 0.71(νM−H )

Linkage Isomers

Some ligands can coordinate to metal centres through different atoms, this gives rise to linkage

isomerism. Infrared spectroscopy can be used to help determine which type of coordination is

occurring. For example the nitro group, NO2-.

Compare two linkage isomers;

In the first example the absorption will be around 1300-1340cm-1 and 1350-1450cm-1, in the second

it will be around 1050-1200cm-1, indicative of an N-O single bond, and 1380-1480cm-1, indicative of

M=O character.

Another important group of ligands in linkage isomerism is cyanoto-type ligands.

M-S-C≡N Thiocyanato 2140,700cm-1

M-O-C≡N Cyanate 2100,1200cm-1

M-N=C=S Isothiocyanate 2080,820cm-1

M-N=C=O Isocyanate 2100,1375cm-1