Chem 1140; Spectroscopy• UV-VIS• IR• NMRThe Absorption LawsI1 I0 II2Detectortransmitted lightintensity of the incident beamI2I1Overall transmittance: T =(what is actually measured)II0Internal transmittance: Ti = (of interest to the spectroscopist)Usually T0 = Ti Such differences as might exist can be minimized by using matched cells and setting T for the reference at 100%.UV-VIS SpectroscopyP. Wipf 1 Chem 1140The quantity I/I0 is independent of the intensity (I) of the source andproportional to the number of absorbing moleculesLambert-Beer Law:logI0I= ! . l . c= Al = path length of the absorbing solution in [cm]c = concentration in moles/literLog I0/I = absorbance or optical density = A! = molar extinction coefficient [1000 cm2 mol-1]! i s a characteristic of a g iven compound, or more accurately, the lightabsorbing system of the compound, the so called chromophore. ! i scorrelated to the size (Å) of the chromophore and of course wave-lengthdependent.! = f (")! ! f (c) (Approximation! An accuratedetermination of ! r equires determiningA at various concentrations)!: 10 - 105(scales with extended #-systems)(dyes)P. Wipf 2 Chem 1140UV-Vis spectra are usually plotted as A vs. ! plots:A!21bathochromichypsochromichyperchromichypochromic[nm]!maxshifts"#"$n#"$Though there are discrete levels of electronic excited states in amolecule, we do not observe absorption lines but broad peaks; thechange in vibrational and rotational energy levels during absorption oflight leads to peaks containing vibrational and rotational finestructure. Due to additional interaction with solvent molecules, thisfine structure is blurred out, and a smooth curve is observed. (vapourphase: one can observe vibrational fine structure).P. Wipf 3 Chem 1140Selection RulesThe irradiation of organic compounds may or may not give rise to excitation of electrons from one orbital to another orbital. There are transitions between orbitals that are quantum mechanically forbidden.Two selection rules:-!Spin-rule:!! The total spin S may not change during transition (S ! S; T ! T)-!Symmetry rule:! e—transitions between orbitals of identical symmetries!!!! are not allowed.!!!! (for ex. Even/uneven with regard to inversion).n !* even even! In reality, these quantum mechanical rules are not rigidly observed, however, due to molecular vibrations, the intensities (") of "forbidden" transitions are significantly reduced (and are usually of diagnostic importance).n # $* band near 300 nm of ketones; " = 10-100 benzene 260 nm band with " = 100-1000.P. Wipf 4 Chem 1140ChromophoresDefinition:! Chromophore – light absorbing electron system of a compoundRules:! - ! or n- orbitals that do not interact lead to a spectrum that is the ! sum of the individual absorptions of the isolated chromophores. - the longer the conjugated system, the longer the wavelength of the absorption maximum and the higher its intensity. - a bathochromic and hyperchromic effect is observed, when atoms with n-orbitals are directly attached to a chromophore (-OH, -OR, NH2, SH, SR, Hal…) = auxochromic groups. ! Isolated ChromophoresOf the once listed in table, only few are of practical significance (Vacuum-UV)C C!"max = 190nm~!#$%h&!!#P. Wipf 5 Chem 1140P. Wipf 6 Chem 1140! Conjugated ChromophoresC C!"max = 190nm~!#$%h&!!#!!"AHHSnASS=Auxochrome!3"!2!1P. Wipf 7 Chem 1140! Benzene and aromatic compoundsThe UV spectra of benzenes are characterized by three major bands which have been given a variety of names. P. Wipf 8 Chem 1140Only ! is allowed. B band is forbidden (loss of symmetry due to molecular vibrations; shows vibrational fine structure).MO Diagram of FerroceneFe FeCp22 Cp-4 pa2u, e1u4 sa1g4 pa1g, e1g, e2ge1u!a1g!e2g!a2u!e2ue2g, e2ue1ua1ge2ge1g, e1ue1ue1ga1ga2ua1g, a2u-50050100150200250300200 300 400 500 600 700 800UV Spectrum of Ferroceneextinction [cm-1/M]lambda [nm]Ferrocene has a molar extinction coefficient of 96 M-1cm-1 at 442 nmP. Wipf 9 Chem 1140Nomenclature of Electronic Transitions; Symbols of Symmetry ClassesSymbols of symmetry classes:A: sym. (according to a Cn operation)B: antisym. (according to a Cn operation)E: 2-fold degenerate stateT: 3-fold degnerate stateIndices:g: sym. (according to an inversion operation)u: antisym. (according to an inversion operation)1: sym. (according to a C2 axis that is orthogonal to a Cn axis)2: antisym. (according to a C2 axis that is orthogonal to a Cn axis)‘: sym. (according to a plane of symmetry "n that is orthogonal to a Cn axis)‘: antisym. (according to a plane of symmetry "n that is orthogonal to a Cn axis)Examples:1A2 1A11B1u 1A1g1B2u 1A1g1E1u 1A1gCalculation of spectra:! Bonus Problem: Calculate UV and IRSpectra of Ferrocene and Acetylferrocene,and Compare to Experimental Data; can youdesign a Ferrocene derivative that is green-colored?P. Wipf 10 Chem 1140Infrared Spectroscopy After considering ultraviolet and visible radiation (200-800 nm) which is energetic enough to affect the electronic levels in a molecule, we shall now consider radiation which has a longer wavelength: infrared radiation which extends beyond the visible into the microwave region and is capable of affecting both the vibrational and the rotational energy levels in molecules.Range of commercial instruments:!! 2500 nm to 16’000 nm!! physical chemists:!! 25000 – 160’000 Å!! analytical chemists:!! 2.5 – 16 microns (µ)!! organic chemists:!! 4000 – 625 cm-1!wavenumber = 1/" = !/c"normalized frequency"Use: simple, rapid, reliable means for functional group identification.Vibrational modes For a molecule comprised of N atoms, there are 3N-6 normal modes of vibration (3N-5 for linear molecules). To a good approximation, however, some of these molecular vibrations are associated with the vibrations of individual bonds or functional groups (localized vibrations) while others must be considered as vibrations of the whole molecule.Localized vibrations are:Stretching modes:C Hsimplecoupled:CH HsymmetricasymmetricCH HP. Wipf 11 Chem 1140Bending modes:CHHscissoring (sym)CHHrocking (asym)in plane