Chapter 13-15 Molecular Absorption and Luminscence PDF

Ultraviolet/Visible Molecular Absorption Spectrometry Chapter 13-15 1 Molecular absorption spectrometry involves measurement of transmittance or abs...

Ultraviolet/Visible Molecular Absorption Spectrometry Chapter 13-15 1 Molecular absorption spectrometry involves measurement of transmittance or absorbance of a solution contained in a transparent cell having path length b in cm. The concentration of an analyte is linearly related to absorbance as per Beer’s law. Application of Beer’s law to a mixture with no interaction among them its components. 2 1  13-A. MEASUREMENT OF TRANSMITTANCE AND ABSORBANCE Figure 13-1, reflection occurs at the air/wall interfaces and at the wall/solution interfaces. In addition, attenuation of a beam may occur due to scattering by large molecules. To compensate for these effects, transmittance of analyte solution is usually compared with that of an identical cell containing only solvent. Experimental transmittance and absorbance that closely approximate the true transmittance and absorbance are then obtained with the equations 3 13B-2 Limitations to Beer’s Law. Some of these deviations are. Fundamental or real limitations Chemical deviations Instrumental deviations A) Real Deviation At high concentrations (> 0.01 M), deviation from linear relationship is caused by 1. Interaction between molecules increases causing changes in charge distribution, molecular association, light scattering, reflective and refractive incises and in turn the ability to absorb a given wavelength. 2. A similar effect is sometimes observed at low concentrations when high electrolyte concentrations are used 4 2  B) Apparent Chemical Deviations Arise when an analyte dissociates, associates, or reacts with a solvent to produce a product having a different absorption spectrum from the analyte. For example, the color change associated with a typical indicator HIn arises from shifts in the equilibrium 5 6 3  7 Figure 13-3. plot of data in Table 13-2.  Departures from Beer’s law arise when the absorbing system is capable of undergoing dissociation or association.  Note that the direction of curvature is opposite at the two wavelengths. 8 4  C) Instrumental Deviations due by use of Polychromatic light The laws assumes the use of monochromatic radiation. Unfortunately, a single wavelength is seldom practical because monochromator produces a symmetric band of wavelengths around the desired one. Figure 13-4, greater departures from linearity is expected with increasing differences between €‘ and €“. 9 Deviations from Beer’s law by the use of a polychromatic light when the is no changes in absorption as a function of wavelength. Figure 13-5. D) Instrumental Deviations in Presence of Stray Radiation When measurements are made in presence of stray radiation, the curve is deviating either positive or negative (Figure 13-6). Instrument Components (All UV-VIS-NIR instruments are composed of five components) (1) Light sources, (2) Wavelength Selectors, (3) Sample containers, (4) Radiation (input) Transducers. (5) Signal Processor and Readout (Output) Transducers. 10 5  13D. Instrumentation D2 + Ee D2* D + D + h Sensitivity? 100 – 400 (150-375 nm), 400 – 2500 nm Tungsten-Halogen Lamps Selectivity? 11 Sensitivity? Selectivity? 12 6  Double-beam instruments offer the advantages of :- 1. They provide continuous recording of absorbance or transmittance spectra. 2. They compensate for fluctuations in the radiant source and drift in the transducer and amplifier. 3. They compensate for variations in source intensity with wavelength. Diode array/multichannel instrument is a powerful tool for 1. Studying transient intermediates in moderately fast reactions. 2. Kinetic studies. 3. Qualitative and quantitative determination of components exiting from liquid chromatographic or capillary electrophoresis columns. 4. Time dependence measurements at multi wavelengths. 5. Several array-detector systems with fiber-optic probes are present Disadvantages its moderately high cost, Resolution, usually 1 to 2 nm. 13 14 7  Some Typical instruments 15 16 8  17 18 9  Applications 19 UV-VIS spectrophotometry has widespread application for identification and determination of thousands inorganic and organic species. It is the most widely used quantitative analysis techniques in chemical and clinical laboratories throughout the world. 14B-ABSORBING SPECIES M* excited state The absorption of UV-VIS with lifetime of radiation is a two-step process, 10-8 to 10 s, excitation followed by relaxation M The lifetime of M* is usually very short that its concentration at M + h M* any instant is ordinarily negligible. The amount of thermal energy Relaxation involves conversion of the excitation energy to heat; decomposition evolved by relaxation is usually not to new species or emission of detectable. fluorescence or phosphorescence. 20 20 10  The absorption of UV-VIS radiation results in excitation of bonding electrons. Thus, max of absorption can be correlated with types of bonds. Three types of electronic transitions involved:- (1) , , and n electrons, (2) d and f electrons (3) charge transfer electrons 14B-1 Absorbing species containing , , and n electrons These include:-  All organic molecules and ions and number of inorganic anions.  Organic compounds absorb electromagnetic radiation because they contain valence electrons that can be excited to higher energy levels at wavelength greater than 185 nm.  Examples: C-C, C-H, C=C and n electrons on O, S, N and halogens. 21 21 Types of Electronic Transition  Bond  Bond S + S + Head to head Overlap Lateral Overlap 22 22 11  In  Bonds, charge density is distributed symmetric around the bond axis. In  bonds, charge density distribution is characterized by a nodel plane (region of low charge density) along the bond axis and a maximum density in regions above and below the plane. Each double bond contains 1 and 1. Definition : Chromophores: Chromophores are functional groups responsible for absorption in UV-VIS region. It contains valence electrons with relatively low excitation energy. 23 23 Figure 14-3, Four types of transitions are possible: 24 24 12  -  * transition Occurs by radiation absorption. Excitation requires high energy corresponding to UV or vacuum UV region. Examples, CH4 contains 4 C-H bonds & exhibits absorption maximum at 125 nm. C2H6 has an absorption peak at 135 nm due to C-H and C-C bonds. C—C is less strong than C—H bond, less energy is required for excitation and absorption occurs at longer wavelength.. n-  * transition Occurs in saturated compounds with unshared electrons. Less energetic than -* , occurs in the rang 150-250 nm depending on the kind of bond and the molecular structure. Has low  value (100-3000 L cm-1 mol-1). Shifted to shorter wavelength in polar solvents. 25 25 n -  and  - * transitions Occurs in most organic compounds in the range 200-700 nm. Occurs in unsaturated compounds having  bonds (orbitals).  values for n-* is in the range of 10-100 and for -* is 1000-10,000 Lcm-1mol-1. Effect of solvent polarity : Shifted to shorter wavelength with increasing solvent polarity arises from the increased solvation of the unbounded electron pair Definitions : a) Hypsochromic Effect = Blue Shift An effect leads to shift the absorption maximum of a band to shorter wavelength. b) Bathochromic Effect = Red Shift An effect leads to shift the absorption maximum of a band to longer wavelength. 26 26 13  Organic Chromophores Table 14-2. common organic chromophores and their approximate max. These data can serve as rough guide for identifying functional groups. max. is affected by type of solvent and structural details of molecule. 27 27 Effect of Conjugation of Chromophores Delocalization of  bonds lower the energy level of the * orbital and give it less anti-bonding character. Thus max is shifted to longer wavelength (Table 14-3). Examples: CH2=CH−CH=CH2 has max 20 nm longer than its unconjugated butadiene CH2=CH=CH−CH2. Aldehydes, ketones, carboxylic acids, , unsaturated compounds..etc give similar behavior 28 28 14  Absorption by Aromatic Systems (Benzene)  Aromatic hydrocarbons are characterized by three sets of bands: E1 band at 184 nm (=60,000); a weaker E2 band at 204 nm (€=7900); and a more weaker B band, at 256 nm (=200).  All three bands for benzene are strongly affected by ring substitution; and solvents (Table 14-4 & Fig 14-5) 29 29 30 30 15  Auxochrome A) Shift max to longer wavelength Is a functional group that does not itself absorb in UV region, But when (bathochromic effect or red shift). it is added to a chromophore it causes B) Increase the coloring intensity () Example: 1. OH and NH2 have an auxochromic effect on the benzene chromophore, particularly band B (Table 14-4) 2. Auxchromes have at least one pair of n electron interact with the  electron of the ring, enriching and stabilize it, lower energy shift to longer wavelength. 3. Phenolate ion is more effective than phenol because it has extra pair of electrons. 4. Anilinium ion has no effect as it has no unshared electrons compared to aniline.. Absorption by Inorganic Anions  A number of inorganic anions exhibit UV absorption peaks corresponding to n - * transitions.  Examples: nitrate at 313 nm), carbonate (217 nm), nitrite (360 and 280 nm), 31 azide (230 mu), and trithiocarbonate (500 nm) ions. 31 14B-2. Absorption Involving d and f Electrons Most transition-metal ions absorb in the UV-VIS region. Lanthanide & actinide absorption result from transitions of 4f & 5f electrons In the 1st & 2nd transition-metal series, 3d and 4d electrons are responsible. A) Absorption by Lanthanide and Actinide Ions  Lanthanide and Actinide ions absorb in the UV-VIS regions forming narrow, well-defined, and characteristic absorption peaks,  Bands are little affected by the type of ligand associated with the metal ion or solvent species because absorption caused by 4f and 5f inner orbitals which are screened from external influences by electrons occupying orbitals with higher principal quantum numbers. Figure 14-6 26Fe 2+ 1s2, 2s2, 2p6, 3s2, 3p6, 4s2 , 3d4 3+ 60Nd , 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, , 4d10, , 5p6, , 6s2, 4f4 3 32 16  33 33 34 34 17  B) Absorption by Elements of the First and Second Transition-Metal Series  Tend to absorb visible radiation. The ions and complexes of  Absorption bands are often broad (Figure 14-7) and the 18 elements strongly influenced by chemical environmental in the first two factors. e.g. Cu2+ form pale blue aquo copper (II) transition series complex and a dark blue Cu(NH3)4 complex.  The spectral characteristics of transition metals involve electronic transitions among the various energy levels of these d orbitals. Two theories The Crystal Field Theory (previously studied in explain colors inorganic chemistry). of transition- metal ions and Molecular Orbital Theory ligand 35 effects. 35 Both theories are based on splitting of the d-orbital energies upon complexation with ligand or solvent molecules due to electrostatic repulsion between the electron pair of the donor and the electrons in the various d orbitals of the central metal ion. 36 36 18  Examples: M + 6L ML6 Octahedral structure M + 4L ML4 Tetrahedral or Square Planar Z Z Z X X X Y Y Y 37 37  Charge on the metal ion and its position in the periodic table. The magnitude  The ligand field strength, which is a measure of the extent to which a complexing group will split the energies of the d of E electrons depends upon  Ligand field strengths increases in the order I- < Br- < Cl- < F- < OH- < C2O4-- < H2O < SCN- < NH3 < ethylenediamine < o- phenanthroline < NO2 < CN-  As the field strength increases, the wavelength of the absorption maxima decreases. Table 14-5. 38 38 19  14B-3 Charge-Transfer Absorption (C.T. band) Species exhibit C.T. have max>10,000 providing highly sensitive means for detecting and determining such absorbing species. Many inorganic complexes exhibit C.T. absorption and called C.T. complexes. h In any C.T. Electron Donor Electron Acceptor e-transferee SCN- Fe3+ / Fe(SCN)2+ Fe2+ o-phenanthroline Examples Quinone Hydroquinone Amines, Aromatics, I2 39 Sulfides,..etc 39 40 40 20  14C-1 Methods of Plotting Spectra 41 41 14C. Application of absorption measurement to qualitative analysis UV-VIS have limited application for qualitative analysis because the number of absorption maxima and minima are relatively few. 14C-2 Solvents: should be 1) Transparent 2) Has no possible effects. Polar solvents (water, alcohols, esters, and ketones) obliterate spectral fine structure arising from vibrational effects. Nonpolar solvents give spectra like in gas phase. Solvent 42 Cutoff 42 21  14D. Quantitative analysis by absorption measurements Important characteristics include: 1. Wide applicability to both organic and inorganic systems, 2. Sensitivities of 10-4 to 10-5 M and can often be extended to 10-6–10-7 M. 3. Moderate to high selectivity, 4. Good accuracy (1–3% & can be reduced to few tenths of a percent), 5. Ease and convenience of data acquisition. 14D-1 Scope Very broad A 95% of all quantitative determinations in the field of health are performed by UV-VIS spectrophotometry. Over 3,000,000 daily tests carried out in United States. Applications to Absorbing Species  Tables 14-2, 14-3, and 14-4 list many common organic chromophors. Compounds containing one or more of these groups can be measured.  Inorganic species absorb light such as nitrite, nitrate, and chromate ions; 43 osmium and ruthenium tetroxides; molecular iodine; and ozone. 43 Applications to Non-absorbing Species Based on color reaction between the analyte and a reagent to yield products that absorb strongly in the UV-VIS regions. Examples 1. The complexing agents SCN-, for iron, cobalt, and molybdenum, H2O2- for titanium, vanadium, and chromium; I- for bismuth, palladium, and tellurium. O-phenanthroline for iron, DMG for nickel, Diethyldithiocarbamate for copper, and Diphenyldithiocarbazone for lead. Greatest change in A/unit concentration 14D-2 Procedural Details Maximum Sensitivity a) Selection of Good adherence to Beer's Law wavelength  max for an absorption The Curve is often flat in this region, less sensitive to uncertainties in wavelength peak 44 represents setting of the instrument. 44 22  b) Variables Influence Absorbance 1. Nature of Solvent. 2. pH of Solution. 3. Temperature. 4. Electrolyte Concentration. 5. Presence of Interfering substances. All should be controlled to avoid changes in absorbance. c) Cleaning and handling of Cells 1. Calibrate against one another to detect differences arise from scratches, etching, …..etc. 2. Use recommended Erikson and Surles cleaning and drying technique in which:- A. A lens paper is soaked in spectroscopic pure methanol. Then the paper is held with a hemostat. B. Methanol is allowed to evaporate leaving cell surface free from containments. 45 C. Avoid using dry lens papers that may leave lints and films on surface. 45 d) Relationship between absorbance & concentration A Calibration Curve Method XX Calibration curve is established using standard X solutions in the concentration range of the method. X It is not safe to assume adherence to Beer’s law X and use a single standard to determine . The results of an analysis should never be based X X on a literature value for molar absorptivity. Interfering ions from matrix should be considered. e.g. the effect of SO4 and PO4 ions in complex Concentration formation leads to color fading. Standard addition method is often used in case of high matrix effect, 46 46 23  e) Standard Addition Method 1. Adding increments of a standard solution to sample aliquots of the same size. 2. Each solution is then diluted to a fixed volume before measuring its absorbance. 3. For a single addition, unknown concentration can be calculated using: Cx, Vx and A1 are the conc., volume and absorbance of unknown sample. Cs, Vs and A2 are the conc., volume and absorbance of standard sample. 47 47 48 48 24  f) Analysis of Mixtures of Absorbing Substances  For a Mixture, the total absorbance is At= A1+ A2+ A3 + …An  For two substances, two equation produced and CM and CN are obtained by solving the two equations.  For mixture with n substances, n equations are formulated, but with higher uncertainties. 49 49 14E PHOTOMETRIC TITRATIONS  Used to locate the equivalence point of a titration, provided the analyte, the reagent, or the titration product absorbs radiation. 14E-1 Titration Curves  A photometric titration curve is a plot of absorbance, corrected for volume changes, as a function of the volume of titrant.  The curve consists of two lines with different slopes, The end point is taken as the intersection of the extrapolated linear portions of the curve. Figure 14-18. Absorbing system must obey Beer’s law; otherwise, the titration curve will lack the linear regions needed for extrapolation to the end point. Absorbance has to be corrected for volume changes Adil 50 =A measured [(V+v)/V] 50 25  Photometers or spectrophotometers 14E-2 Instrumentation modified to permit insertion of titration vessel in light path Photometric Titrations are performed using-: Probe type Photometer 14E-3 Applications of Photometric Titration- Gives more accurate results for determining end point. It has been applied to all types of reactions. Redox Reactions with characteristic absorption spectra. Acid-Base reactions using A/B indicators. EDTA and complexmetric titrations. Example: Bi EDTA Cu EDTA EDTA titration for51Cu2+ & Bi3+ =0 0 51 Continuous Variation Method 52 26  Molar Ratio Method 53 54 54 27  Luminescence Spectroscopy An emission of light by excited molecules Luminescence Photoluminescence Chemiluminescence Other examples Fluorescence Phosphorescence Bioluminescence Sonoluminescence Electroluminescence 1. In photoluminescence, energy is provided by absorption of IR, visible or ultraviolet light. 2. In chemiluminescence, energy is provided by chemical reactions. 3. In bioluminescence, energy is provided by biological reactions. It can be considered as chemiluminescence occur in55 living cells. 55 In molecular luminescence, analyte molecules are excited to give emission spectra carry qualitative and quantitative information about the analyte. Common types of luminescence Fluorescence Phosphorescence Chemiluminescence referred to by the Fluorescence & general term. phosphorescence are alike in Photoluminescence that excitation is brought about by absorption of photons. Fluorescence Phosphorescence max Fl & Ph - Does not involve a change -Involves a change in are always in electron spin during electron spin. longer than max relaxation. -Long lived, easily - Short-lived, 56 (t