modern methods in techniques of analytical chemistry spectroscopic techniques[在分析化學(xué)光譜技術(shù)現(xiàn)代技術(shù)方法](-85)

1、Modern methods in techniques of analytical chemistry: Spectroscopic techniques. Describe the absorption of radiation by molecules and its relationship to molecular structure. Make quantitative calculations, relating the amount of radiation absorbed to the concentration of an absorbing analyte.Descri

2、be the instrumentation required for making measurements.Scope:The Electromagnetic Radiation Light is a form of electromagnetic radiation. Electromagnetic radiation can be considered as a form of radiant energy that is propagated as a transverse wave.It vibrates perpendicular to the direction of prop

3、agation, imparts a wave motion to the radiationWavelength, = c/ vWavenumber, = v/c =1/Wavelength , is the distance between the neighboring peaks of two wave.Frequency, v (nu) the number of cycles passing a fixed point per unit time.Frequency is measure in Hertz, 1 Hz= 1 s-1.Wavenumber, (nu tilde) th

4、e number of complete wavelengths in a given length.e.g.: A wave number of 5 cm-1 indicates there are 5 complete wavelength in 1 cm. Where c = velocity of light (3 x 1010 cm/s)Electromagnetic radiation possesses certain amount of energyE = h = hc/E = energy of a photonh = Plancks Constant (6.62 x 10-

5、34 joule second (J-s)It is apparent that the shorter the wavelength or the greater frequency, the greater energy.In spectrometric methods, the sample solution absorbs electromagnetic radiation from appropriate source, and the amount absorbed is related to the concentration of analyte in the solution

6、.109 107 105 103 101 10-1 10-3 10-5 10-7 10-9 10-11gammaX-raysUltra VioletInfra redmicrowaveRadio waves500600700Violet, indigo, blueGreen, yellowOrange, redColor Wave length (nm)violet400-435indigo435-480blue480-500green500-560yellow560-595orange595-610red610-750Electromagnetic SpectrumHow does matt

7、er absorb radiation?There are 3 basic process:Rotational transitionmolecules rotate, absorb radiation and raised to higher rotational energy level2) Vibrational transitionAtoms or group of atoms within molecule vibrate relative to each other, absorb radiation and raised to higher vibrational energy

8、level. 3) Electronic transition-Electrons of molecule raised to higher electron energy.Wavelenghts and ColorWavelength of Maximum absorption (nm)Color absorptionColor Observed380-420VioletGreen-yellow420-440Violet-blueYellow440-470BlueOrange470-500Blue-greenRed500-520GreenPurple520-550Yellow-greenVi

9、olet550-580YellowViolet-blue580-620OrangeBlue620-680RedBlue-green680-780PurpleGreenIrradiance or radiant power, P (Wm-2), is the energy per second per unit area of the beam of light.Schematic of a simple spectrometer.Derivation of Beers LawIrradiance In, P0Irradiance Out, PPathlenghtReadoutTransmitt

10、ance is T=Po/P =10-kbTransmittance, T, is the fraction of original light not absorbed by the sample.Po = power of incident lightP= power of transmitted light Putting transmittance in logarithmic form:log T = log P/Po = -kb Similar law holds for the dependence of T on concentration, cT=Po/P =10-kc an

11、d log T = log P/Po = -kcCombining these two laws, T = P/P0 =10-abc (Beers Law) Where is the analytes molar absorptivity. = a x molecular weightlog T = log P/P0 = -abcA = -log T = log 1/T = log P0/P = abc = log 100 log %T Continue.Where A = absorbance, a = analytes absoptivity b = pathlenght through

12、the material; c= concentration, ORA = bcSpectroscopy Nomenclature Recommended NameUnitAbsorbance (A)-Absorptivity (a)cm-1 g-1 LPathlength (b)cmTransmittance (T)-Wavelength ()nmConcentration (c)moles per liter (moles L-1)Example 1:A sample in a 1.0-cm cell is determine with a spectrometer to transmit

13、 80% light at certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, What is the concentration of the substance?Solution:The percent of the transmittance is 80%, and so T = 0.80. log 1/T =abc log 1/0.80 = 2.0 cm-1g-1L x 1.0 cm x c log 1.25 = 2.0 g-1L x c c = 0.10/2.0 =

14、0.050 gL-1Example 2:A solution containing 1.00 mg ion (as thiocyanate complex) in 100 mL was observed to transmit 70.0% of the incident light compared to an appropriate blank. What is the absorbance of the solution at this wavelength? What fraction of light would be transmitted by a solution of iron

15、 four times as concentrated.Solution:T = 0.700 A = log 1/0.700 = log 1.43 = 0. 155(b) 0.155 = ab (0.0100 g/L) ab = 15.5 L/g From A= abc A = 15.5 L/g x (4x0.1000 g/L) = 0.620 log 1/T = 0.620 T = 0.240The absorbance of the new solution could have been calculated more directly: Beers Law Assumption/Lim

16、itation The light being shined on the sample must be monochromatic (one color or wavelenght) The analyte must not be participate in a concentration dependent equilibrium This isnt good technique for many weak acid systems, as dilution increases dissociation and HA and A- probably dont have the same

17、absorbance.Beers Law Assumption/LimitationProblems: Calibration curves are found to be nonlinear because occuring of deviation. Deviation from linearity are divided into three categories:Fundamental : Law is valid for low concentration analyte. At higher concentration, there will be interaction betw

18、een particle of analyte that may change the value of .2) Chemical: when the absorbing species is involved in an equilibrium reaction. HA H+ + A- HA will absorbs the wavelength and contribute to and C value. However, if the equilibrium shifts to right, less HA available for absorption and will result

19、 in non linearity of the curve.3) Instrumentation: 2 principal limitation: i) Stricly valid for purely monochromatic instrumentation. ii) The effect of leakage light from imperfections within wavelength selector. This phenomena is called stray radiation. Deviation from beers law:continueFrom the bee

20、rs law, the absorbance against the conc. A straight line passing through origin is obtained (linear graph)However, deviation might occurs. Deviation is due to the following factors:- A foreign substance having colour particle may affect the absorption & extinction coefficient.Deviation also occur if

21、 colored solute ionized or dissociates in the solution; e.g.- benzyl alcohol in chloroformDue to the presence of impurities that fluoresce or absorb at the absorption wave length.If monochromatic light is not used deviation may occurs.If width of the slit is not proper.If the solution species underg

22、oes polymerization Block of Diagram - SpectrometerSourceMonochromatorSampleDetectorReadout (meter or recorder)Sources : Tungsten lamp (visible), hydrogen or deuterium discharge tube (ultraviolet), hot wires, light bulb or glowing seramic (IR), laser Monochromator: prism, difraction grating, optical

23、filters.Sampel cell: cuvets, KBr, UV/IR quartz.Detector: phototube, photomultiplier tube, spectrophotometer (UV) thermocouples, bolometers (IR)Two types of MonochromatorGrating: A fundamental property of gratings is that the angle of deviation of all but one of the diffracted beams depends on the wa

24、velength of the incident light. Therefore, a grating separates an incident polychromatic beam into its constituent wavelength components, i.e., it is dispersive. Because of their dispersive properties, gratings are commonly used in monochromators and spectrometers.Two types of MonochromatorPrism: El

25、ectromagnetic radiation is refracted because index of refraction of prism material is different from air.Shorter wavelengths are refracted more than longer wavelengthThe effects of refraction is to spread the wavelength apart into different wavelengthBy rotating the prism, different wavelength can b

26、e made to pass through an exit slid and to the sampleTo do a Spectroscopic AnalysisYou need:A continuous light sourceA wavelength selectorA sample cellA detectorThe sample cell is called cuvet and can be made of many substancesGlass (good for visible)Quartz (UV-vis)NaCl/KBr (IR)Spectroscopic Procedu

27、reYou may have a single-beam or double beamSingle-beam instrument has one sample holder, you must swap blank and sampleDouble-beam instrument splits light output between two holders so you can measure blank and sampleA baseline spectrum is a spectrum of a reference solution (solvent or reagent blank

28、)We try to do an analysis at the max if we canSensitivity is greatest at maximum absorbanceCurve is relatively flat in case the monochromator drifts and is off by a little in wavelengthThe Single-Beam SpectrometerHow Do UV spectrometers work?Two photomultiplier inputs, differential voltage drives am

29、plifier.Matched quartz cuvettesSample in solution at ca. 10-5 M.System protects PM tube from stray lightD2 lamp-UVTungsten lamp-VisDouble Beam makes it a difference techniqueRotates, to achieve scanThe polychromatic light is separated into monochromatic wavelengths by diffraction. n = d(sin + sin )O

30、ptics of the Grating MonochromatorIn the equation n = d(sin + sin ) n is the order of the diffraction n = 1, 2, 3 etc, d is the number of lines etched on the grating, is the angle of the incident beam and is the angle of the emerging beam.Optics of the Grating MonochromatorThe photodiode array detec

31、torThe photodiode array detector Key components:Light SourceMonochromatorSample/reference holderRadiation detectionReadout device UV InstrumentationSpectroscopic ProcedureYou should always try to keep the absorbance reading of your sample below 1.Because % transmittance is related logarithmically wi

32、th concentration, it means that from 1-99% transmittance you can detect 2 orders of magnitude in analyte concentration.Any orders of magnitude greater than that will be detected in the range of 0-1% T.In order to maximize accuracy, you should dilute the solution if you have to so that the transmitta

33、nce reading is not maxed out in that region.An Electronic SpectrumAbsorbanceWavelength, , generally in nanometers (nm)0.04008001.0200UVVisiblemaxwith certain extinction Make solution of concentration low enough that A 1(Ensures Linear Beers law behavior)Even though a dual beam goes through a solvent

34、 blank, choose solvents that are UV transparent.Can extract the value if conc. (M) and b (cm) are knownUV bands are much broader than the photonic transition event. This is because vibration levels are superimposed on UV.Ultraviolet Spectroscopy200-400 nm photons excite electrons from a bonding orbi

35、tal to a * antibonding orbital.Conjugated dienes have MOs that are closer in energy.A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. =ChromophoreExampleSolventlmax (nm)emaxType of transitionAlkenen-Heptane17713,000pp*Alkynen-Heptan0002,

36、000160pp*_Carbonyln-Hexanen-Hexane1862801802931,00016Large12ns*np*ns*np*CarboxylEthanol20441np*AmidoWater21460np*AzoEthanol3395np*NitroCH3NO2Isooctane28022np*NitrosoC4H9NOEthyl ether30066510020_np*NitrateC2H5ONO2Dioxane27012np*Absorption Characteristics of Some Common ChromophoresSolvents for UV (sh

37、owing high energy cut-offs)Water205CH3CN210C6H12210Ether210EtOH210Hexane210MeOH210Dioxane220THF220CH2Cl2235CHCl3245CCl4265benzene280Acetone300Various buffers for HPLC, check before using. Deviation from Beers Law Beers law is only valid for low concentration, up to 10 mM; The intermolecular distance

38、s in a given solution will decrease, eventually reach a point at which neighboring molecules mutually affect the charge distribution of the other affect Chemical processes such as the reversible association-dissociation of analyte molecules, or the ionization of a weak acid in an unbuffered solvent.

39、Instrumentation limitation-incident beam may be polychromatic . Processes other than analyte absorption result in significant decrease in the power of the incident beam; Reference cell is used to correct these processes; Reference cell is often prepared by adding distilled water to an absorption cel

40、l; The reference cell is then placed in the path of the light beam, and the power of the radiation exiting the reference cell is measured and taken as P0 for the sample cell.Background Correction Linear calibration curve; Nonlinear calibrationCalibration CurvesAn Example-PulegoneFrequently plotted a

41、s log of molar extinction So at 240 nm, pulegone has a molar extinction of 7.24 x 103Antilog of 3.86 Infrared SpectroscopyIntroductionSpectroscopy is an analytical technique which helps determine structureIt destroys little or no sampleThe amount of light absorbed by the sample is measured as wavele

42、ngth is variedInfrared spectroscopy is very useful for obtaining qualitative information about the molecules. But molecule must possess certain properties in order to undergo absorption.IR SpectroscopyThe presence and also the environment of functional groups in organic molecule can be identified by

43、 infrared (IR) spectroscopy. Infrared spectroscopy is nondestructive. Moreover, the small quantity of sample needed, the speed with which spectrum can be obtained, the relatively low cost of the spectrometer, and I wide applicability of the method combine to make infrared spectroscopy one the most u

44、seful tools available to the organic chemist lowhighFrequency (n)EnergyX-RAYULTRAVIOLETINFRAREDMICRO- WAVERADIOFREQUENCYUltravioletVisibleVibrationalinfraredNuclear magneticresonance200 nm400 nm800 nm2.5 mm15 mm1 m5 mshortlongWavelength (l)highlowBLUEREDTHE ELECTROMAGNETIC SPECTRUMX-rayUV/VisibleInf

45、raredMicrowaveRadio Frequency Bond-breaking Electronic Vibrational Rotational Nuclear and Electronic SpinREGIONENERGY TRANSITIONSTypes of Energy Transitions in Each Region of the Electromagnetic Spectrum(NMR) Principles IR SpectroscopyEnergy: E=hwhere: is the frequency in hertzIn IR, frequency is co

46、mmonly expressed as wave numbers ( , in Reciprocal cm, or cm-1) Where Absorption of radiation in this region by a typical organic molecule results in the excitation of vibrational, rotational, and bending modes, while the molecule itself remains in its electronic ground state. Molecular asymmetry is

47、 a requirement for excitation by infrared radiation and fully symmetric molecules do not display absorbance in this region unless asymmetric stretching or bending transitions are possible. Symmetric stretchAssymmetric stretchSymmetric bendingPrinciples IR Spectroscopy Principles IR Spectroscopy For

48、the purpose of routine organic structure determination, the most important absorptions in the infrared region are the simple stretching vibrations. For simple systems, these can be approximated by considering the atoms as point masses, linked by a “spring” having a spring constant k and following Ho

49、okes Law. Principles IR SpectroscopyUsing this simple approximation, the equation shown in below can be utilized to approximate the characteristic stretching frequency (in cm-1) of two atoms of mass m1 and m2, linked by a bond with a spring constant k: Where =m1m2/(m1+m2) , also called “reduced mass

50、”Absorption of Infrared RadiationOnly bonds which have significant dipole moments will absorb infrared radiation.Dipole is the polar covalent bond in which a pair of electron is shared unequally.For absorption occur, there must be a charge in the dipole moment (polarity) of the molecule. A diatomic

51、molecule must have a permanent dipole in order to absorb, but larger molecule do not.DIPOLE MOMENTSBonds which do not absorb infrared include: Symmetrically substituted alkenes and alkynes Many types of C-C Bonds Symmetric diatomic moleculesH-H Cl-Cl Molecular VibrationsLight is absorbed when radiat

52、ion frequency = frequency of vibration in moleculeCovalent bonds vibrate at only certain allowable frequenciesAssociated with types of bonds and movement of atomsVibrations include stretching and bending Light source: Nichrome wire that glows when an electrical current is passed through;Interferomet

53、er: no monochrometerDetector: thermocouple detector, whose output voltage varies with changes caused by varying levels of radiation striking the detector.IR Instrumentation IR InstrumentationNo two molecules of different structure will have exactly the same natural frequency of vibration, each will

54、have a unique infrared absorption pattern or spectrum.Two Uses:IR can be used to distinguish one compound from another.Absorption of IR energy by organic compounds will occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound; thus, infrared s

55、pectrum gives structural information about a molecule.The absorptions of each type of bond (NH, CH, OH, CX, C=O, CO, CC, C=C, CC, CN, etc.) are regularly found only in certain small portions of the vibrational infrared region, greatly enhancing analysis possibilities.Infrared Spectroscopy (IR)The In

56、frared SpectrumA plot of absorption intensity (% Transmittance) on the y-axis vs. frequency (wavenumbers) on the x-axis.Infrared Spectroscopy (IR)Principal Frequency Bands (from left to right in spectrum)OH3600 cm-1 (Acids - Very Broad, Alcohols - Broad)NH3300 - 3500 cm-1 (2, 1, 0 peaks 1o, 2o, 3o)C

57、N2250 cm-1 (Nitrile)CC2150 cm-1 (Acetylene)C=O1685 - 1725 cm-1 (1715) (Carbonyl)C=C1650 cm-1 (Alkene); 4 absorptions 1450-1600 (aromatic)CH21450 cm-1 (Methylene Group)CH31375 cm-1 (Methyl Group)CO900 - 1100 cm-1 (Alcohol, Acid, Ester, Ether, Anhydride) -CH(Saturated Alkane absorptions on Right side

58、of 3000 cm-1)=C-H(Unsaturated Alkene absorptions on Left side of 3000 cm-1)=C-H(Aromatic absorptions) Verify at 1667 2000 cm-1 C-H(Unsaturated Alkyne absorptions on Left side of 3000 cm-1)Infrared Spectroscopy (IR)Suggested approach for analyzing IR SpectraStep 1. Check for the presence of the Carbo

59、nyl group (C=O) at 1715 cm-1. If molecule is conjugated, the strong (C=O) absorption will be shifted to the right by 30 cm-1,i.e., 1685 cm-1 If the Carbonyl absorption is present, check for:Carboxylic Acids-Check for OH group (broad absorptionnear 3300-2500 cm-1)Amides-Check for NH group (1 or 2 abs

60、orptions near 3500 cm-1) Esters-Check for 2 C-O group (medium absorptions near 1300-1000 cm-1)Anhydrides-Check for 2 C=O absorptions near 1810 and 1760 cm-1Aldehydes-Check for Aldehyde CH group (2 weak absorptions near 2850 and 2750 cm-1)Ketones-Ketones (The above groups have been eliminated)Infrare