SPECTROSCOPY AND ANALYTICAL CHEMISTRY

A variety of spectroscopic techniques can be used to study/elucidate ground and excited state atomic and molecular structures. With high resolution gas phase spectroscopy a rich pattern of energy levels can be revealed and modeled with appropriate Hamiltonians.

SPECTROSCOPY AND ANALYTICAL CHEMISTRY

With appropriate analyses of molecular spectra we can determine values for bond, distances, molecular electric dipole moments, chemical shifts and so on. Molecular symmetry is also readily established by spectroscopic methods.

SPECTROSCOPIC METHODS Spectroscopic and spectrophotometric

techniques (measure both light frequencies and intensities) are useful in analytical chemistry. Spectra provide a “fingerprint” for identifying atoms and molecules. Often we wish to know both the identity of atoms/ions/molecules present as well as their amounts/concentrations.

SPECTROSCOPIC METHODS In spectroscopic experiments light with a

wide range of frequencies/wavelengths can be passed through a sample. The fraction of light absorbed at a particular resonant frequency increases as both the sample concentration and the path length (sample cell size) increase.

SIMPLEST SPECTROPHOTOMETRIC EXPT

Sample

BEER’S LAW

BEER’S LAW

BEERS’S LAW – OTHER QUANTITIES

BEER’S LAW CALCULATION A Cu(NO3)2 (aq) solution at 0.105 M has a

transmittance value of 0.304 for light of 610 nm wavelength in a 5.00 mm cell. Find ε(610nm) for Cu2+(aq). Calculate T for a cell length of 0.50 mm.

Solution: log (I(λ)/Io(λ)) = -ε(λ) [M] l ε = -log(0.304)/(0.105 M x 0.500cm) = 9.85 M-1 cm-1

BEER’S LAW CALCULATION

ANALYTICAL CHEMISTRY AND KINETICS The Beer-Lambert law enables various

spectroscopic techniques to be used in analytical chemistry. It also means that spectroscopy can be used to study rates of reaction. Historically manometric methods (pressure vs time data) were used in gas phase kinetic studies.

CHEMICAL KINETICS

Manometric methods are blunt tools in cases where there are competing or simultaneous reactions (or “subsequent” reactions). In such cases FTIR spectroscopy (for example) provides a convenient method for following changes in concentrations for all reactants/products as a function of time.

KINETICS OF RXN OF Cl AND H2C=O Reactants: Cl2, H2C=O and hν Reactions: Cl2(g) + hν → 2 Cl(g) Cl(g) + H2C=O → CHO(g) + Cl(g) (k2 = 7.8 x 10-11 cm3 molecule-1 s-1 at 298K) CHO(g) + Cl2(g) → HCOCl(g) + Cl(g) Slow subsequent reaction HCOCl(g) → HCl(g) + CO(g)

KINETICS OF RXN OF Cl AND H2C=O Ref: H. Niki et al, Chem. Phys. Lett. 57,

596(1978). For the reaction chosen it could be noted

that. 1. Cl and Cl2 have no IR spectrum. 2. CHO and HCOCl are asymmetric tops with complex rotational structure seen for all IR bands. 3. CO and HCl are linear molecules with simple rotational structure on IR bands.

OTHER SPECTROSCOPIC METHODS Unstable molecules can be studied by other

techniques. For example, formyl chloride using microwave spectroscopy.

HCOOH(g) +PCl5(s) → HCOCl(g) + OPCl3(g) +HCl(g) (at -78 Celsius)

The spectra of HCOCl, OPCl3 and HCl are very complex, moderately complex and simple. Why?

TEMPERATURE EFFECTS Temperatures below or well above room

temperature can be useful in spectroscopic work. Cooling can be accomplished using cryogens or by passing small pulses of gas through a nozzle into a vacuum. AS the gas molecules expand (in opposition to intermolecular forces) their kinetic energy and T drop rapidly – often to 1K or below!

CLASS PROBLEM

We will look briefly at how to tackle the last problem on the 3rd assignment.