General Chemistry II

General Chemistry IIFundamentals of SpectrophotometryModule 6Electromagnetic RadiationElectromagnetic radiation is convenientlyrepresented as electric and magnetic fieldsthat undergo in-phase, sinusoidal oscillationsat right angles to each other in the direction ofpropagationRequires no supporting medium for its transmission and thus passes readily through a vacuumPlane polarized light-All oscillations of either theelectric or magnetic field lie ina single planeElectromagnetic RadiationCCharacterized by:WavelengthFrequencyVelocityAmplitudeElectromagnetic RadiationAmplitude (A) of a sinusoidal wave is the length of the electric field vector at a maximum in the waveFrequency () is the number of oscillations of the field that occur per second observed at a fixed pointDetermined by the source and independent of the mediumWavelength () is the linear distance between any two equivalent points on successive wavesLike velocity is a function of the mediumWavenumber (1/) as is directly proportional to the frequency (and energy of the radiation)Wave-Particle DualityWave model fails to account for phenomena associated with the absorption and emission of radiant energyParticle model views radiation as a stream of discrete particles, or wave packets, of energy called photonsThe energy of a photon is proportional to the frequency of the radiationDual views of radiation as particles or waves are complementary for elementary particlesWave Parameters

In any medium containing matter, propagation of radiation is slowed down by its interaction with the bound e-s of the matter. The refractive index is a measure of the interaction of radiation with the medium n = 1 for vacuum1.3 1.8 for liquids1.3 2.5 or higher for solidsThe speed of light in any medium is c/n where n is the refractive index of that medium7Electromagnetic Spectrum

Absorption of Light

absorption a process in which e/m energy is transferred to the atoms, ions, or molecules composing a sample. In this process, particles are excited to higher energy excited states.The lowest energy state of an atom, ion or molecule is called the ground state.Absorption occurs at discrete frequencies (E=hv) as a consequence of the quantum nature of matter.The frequencies of absorbed radiation are characteristic of the material and can be used to identify species in the matter and its environment.Planks constantAtomic Absorption

Absorption spectra are simple with absorption at only a few well-defined frequencies because there are only a small number of possible energy states for the absorbing species.

UV and visible radiation can only excite transitions of valence or bonding e-s. X-ray radiation must be used to excite core e-s.Absorption broadens in solvent or solids.Absorption Processes

In absorption spectroscopy the amount of light absorbed as a function of wavelengthProvides both qualitative (identification) and quantitative information about the sampleEmission Processes

Molecular Absorption Absorption spectra are more complex because more energy states are availableelectronic, vibrational and rotationalMicrowave radiation (and longer wavelength IR) stimulates rotational motion of molecule when it is absorbed.Infrared radiation stimulates vibrations of molecules.Visible and ultraviolet radiation causes electrons to be promoted to higher energy orbitals, electronic excitation.X-rays and short-wave length UV break chemical bonds and ionize molecules.What is the wavelength in nanometers of yellow light which had a frequency of 5.09 x 1014 1/s?

13c = The red spectral line of lithium occurs at 671 nm. Calculate the energy of one photon of this light. E=hv and c = therefore E = hc/ =c/

14Spectrophotometry

A monochromator is a device that separates the various wavelengths of light present in a beam of light (prism, grating, or even a filter) thus allowing the selection of a single wavelength.Light of a single wavelength is monochromatic (one color).Transmittance, T, is defined as the fraction of the incident light that passes through the sampleT = P/Po (0 T 1)The percent transmittance is simply 100% x T (0 T 100%)Absorbance Measurement

17Spectrophotometry

Absorbance is defined asA = log10(Po/P) = - log TT = 10-ABeers Law (Beer-Lambert Law)A = bcb = path length (cm), c = concentration (M)molar absorptivity () characteristic of the substance, a measure of how much light is absorbed at a particular wavelengthOther way around18Example: Find the absorbance, transmittance, and percent transmittance of a 10.00 mM solution of a molecule with a molar absorptivity of 65,346 M-1cm-1 in a 1.00 cm cell.A = ebc = (65,346 M-1cm-1)(1.00 cm)(10.0 mM)(1 M/106 mM) = 0.653T = 10-A = 10-0.653 = 0.222 %T = T x 100% = 0.222(100%) = 22.2 %19Spectrophotometry

absorption spectrum graph of the variation in A with The portion of a molecule that is responsible for the absorption of light is referred to as the chromophore Substances that absorb light in the visible region appear colored, the color(s) not absorbed by the substance. 20

Visible Absorption Spectrum of (ferrozine)3Fe(II) in Analysis of Fe

21Calibration CurveIn a chemical analysis by spectrophotometry, it is also important to prepare a series of standard solutions to establish a calibration curve to be used to determine the concentration in the unknown samples. Analysis is valid over the linear region of the calibration curve.A least-squares analysis of the calibration curve is used to determine the unknown concentrations.

22Calibration Curve

23Example: Determine the concentration of a solution of (ferrozine)3Fe(II) that exhibits an absorbance of 0.500 at a wavelength of 562 nm. The molar absorptivity of the (ferrozine)3Fe(II) complex is 27,932 M-1cm-1 at 562 nm.c = A/eb = (0.500)/(27,932 M-1cm-1)(1.00 cm) = 1.79 x 10-5 M

Limitations of Beers Law (1)Beers law is an example of a limiting law and applies to solutions < 0.01M. Solute-solute and solute-matrix interactions effect the absorption properties of the solute due to their proximity (average distance)Apparent deviations arise when an analyte dissociates, associates, or reacts with a solvent to produce a product with a different absorption spectrum from the analyteMost spectrometers produce polychromatic radiation with a Guassian distribution centered around a particular wavelength characterized by a band width. If the molar absorption coefficient varies over this range of wavelengths then deviations from linearity occur (Deviations from Beers law are not significant if the effective bandwidth of the monochromator is less than one-tenth of the width of the absorption band at half-height)Limitations of Beers law (2)Normally the radiation exiting a monochromator is contaminated by stray light with a wavelength outside the nominal wavelength range for the selected band. At high concentrations and longer path lengths stray radiation can be an important source of non-linearityMismatched cells (optical quality) results in an intercept term in the Beers law expression. Can usually be easily handled by linear regression calibration methodsCombined Electronic, Vibrational, and Rotational Transitions

Changes in the vibrational and rotational states of a molecule generally accompany an electronic transition.Electronic absorption bands in molecules are generally very broad because many different vibrational and rotational levels are available at slightly different energies.

What Happens to Absorbed Energy After an electronic absorption transition occurs (So S1)vibrational relaxation (radiationless transition, R1) vibrational energy is transferred to other molecules or the solvent by collisions.part of absorbed photon energy is converted to heat.The molecule could enter a highly excited vibrational state of a lower energy electronic state having the same energy (internal conversion, IC), and relaxation may occur through radiationless transitions, R2 as a result of collisions. In this way all absorbed energy is converted to heat.What Happens to Absorbed Energy Molecule can cross into a different spin state, T1 (intersystem crossing, ISC), undergo further radiationless relaxation R3, could then undergo another intersystem crossing to So and then radiationless relaxation R4.In this way all absorbed energy is converted to heat.What Happens to Absorbed Energy

Molecule could relax by emission of a photonS1 So is fluorescence, FT1 So is phosphorescence, PThese transitions can terminate in any of the vibrational levels of the So state.Less common than other relaxation processes in solution.What Happens to Absorbed Energy

The lifetime of fluorescence is always very short 10-8 to 10-4 sThe lifetime of phosphorescence is much longer 10-4 to 102 s, and therefore less likely.The relative rates of these various processes depend upon the nature of the moleculethe solventthe experimental conditions (T, P)Absorption vs Emission Spectra In general, fluorescent and phosphorescence have lower energy than that of the absorbed radiation as a result of vibrational relaxation occurring before emission. molecules emit at longer s than those of the radiation they absorb.The emission spectrum is roughly the mirror image of the absorption spectrum.Wavelength of maximum absorbance / emission do not coincide because of geometric relaxation that occurs after absorption and before emission (o emission > o absorption). Excitation and Emission Spectra

Excitation and Emission Spectra

An emission spectrum is obtained by fixing the excitation scanning the emission emission intensity vs emission An excitation spectrum is obtained by fixing the emission scanning the excitation emission intensity vs excitation *An excitation spectrum looks very much like an absorption spectrum.Luminescence in Analytical Chemistry If the absorbance of a solution is small at the excitation wavelength, , then the emission intensity is proportional to radiant power of incident radiation and the concentration of emitting species.I = kPocSensitivity can be increased by increasing the incident radiant power.

Luminescence measurements are good for trace analysis when absorbance is too weak to measure.Luminescence in Analytical Chemistry

Applications of SpectrophotometryAnalysis of a MixtureWhen a mixture of species are present in a solution being analyzed by spectrophotometry, The absorbance detected corresponds to the sum of the absorbances of all species. The spectrophotometer is unable to differentiate the absorbance from different species.Applications of SpectrophotometryAnalysis of a MixtureIf the species present in the mixture have different absorptivities at different wavelengths, then the concentrations of the individual species can be mathematically determined.A = Xb[X] +Yb[Y] + Zb[Z} + . . .where is the molar absorptivity of each species at the wavelength in question and b is the cell pathlength.Applications of SpectrophotometryCase 1. Mixture overlaps significantly at all s measurements at many are necessary.

Applications of SpectrophotometryCase 2. Relatively little overlap in some regions, measurements at 2 where one species dominates at each.

Applications of SpectrophotometryCase 2. Relatively little overlap in some regions, measurements at 2 where one species dominates at each.

Mixture of Two CompoundsCompound 1 has a dominant absorption at 1 and a concentration c1Compound 2 has a dominant absorption at 2 and a concentration c2The molar absorption coefficients for both compounds at both wavelengths are knownAT,1 = 1b c1 + 1b c2(1) AT,2 = 2b c1 + 2b c2(2)

Solve as a simultaneous equation with c1 and c2 asunknowns Optical InstrumentsCommon componentsstable source of radiant energytransparent container for holding the sampledevice that isolates a restricted region of the spectrum for measurementdetector (converts radiant energy to an electrical signal)software or electromechanical device to process and display signal Absorption MeasurmentsDouble-Beam in Space InstrumentPhotodiode-Array SpectrometerGeneral Instrument for Fluorescence

Nearly all fluorescence instruments employ double-beam optics to compensate for fluctuations in radiant powerPrism MonochromatorGrating MonochromatorPhotomultiplier TubesSpectrometer