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Spectroscopy P. Wipf 1 Chem 1140 Chem 1140; Spectroscopy • UV-VIS • IR • NMR UV-VIS Spectroscopy The Absorption Laws I 1 I0 I I2 Detector intensity of the transmitted incident beam light I2 Overall transmittance: T = (what is actually measured) I1 I Internal transmittance: T i = (of interest to the spectroscopist) I0 Usually T0 = Ti Such differences as might exist can be minimized by using matched cells and setting T for the reference at 100%. P. Wipf 2 Chem 1140 The quantity I/I0 is independent of the intensity (I) of the source and proportional to the number of absorbing molecules I0 . Lambert-Beer Law: log = ! l c = A I l = path length of the absorbing solution in [cm] c = concentration in moles/liter Log 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 light absorbing system of the compound, the so called chromophore. ! i s correlated to the size (Å) of the chromophore and of course wave-length dependent. ! = f (") ! ! f (c) (Approximation! An accurate determination of ! r equires determining A at various concentrations) !: 10 - 105 (scales with extended #-systems) (dyes) P. Wipf 3 Chem 1140 UV-Vis spectra are usually plotted as A vs. ! plots: A hyperchromic shifts 2 hypsochromic bathochromic hypochromic 1 "#"$ n#"$ !max [nm] ! Though there are discrete levels of electronic excited states in a molecule, we do not observe absorption lines but broad peaks; the change in vibrational and rotational energy levels during absorption of light leads to peaks containing vibrational and rotational fine structure. Due to additional interaction with solvent molecules, this fine structure is blurred out, and a smooth curve is observed. (vapour phase: one can observe vibrational fine structure). P. Wipf 4 Chem 1140 Selection Rules The 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 5 Chem 1140 Chromophores Definition:! Chromophore – light absorbing electron system of a compound Rules:! - ! 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 Chromophores Of the once listed in table, only few are of practical significance (Vacuum-UV) !# !# h& C C $% ! ! "max =~ 190nm P. Wipf 6 Chem 1140 P. Wipf 7 Chem 1140 ! Conjugated Chromophores !# !# h& C C $% ! ! "max =~ 190nm !3" !" A A !2 ! n !1 H S S H S=Auxochrome P. Wipf 8 Chem 1140 ! Benzene and aromatic compounds The UV spectra of benzenes are characterized by three major bands which have been given a variety of names. P. Wipf 9 Chem 1140 Only ! is allowed. B band is forbidden (loss of symmetry due to molecular vibrations; shows vibrational fine structure). MO Diagram of Ferrocene - Fe FeCp2 2 Cp ! e1u ! a1g ! 4 p e2g ! a2u, e1u a2u e2g, e2u e2u 4 s UV Spectrum of Ferrocene a1g 300 250 e 200 4 p 1u a1g, e1g, e2g a1g 150 e2g 100 e , e extinction [cm-1/M] 1g 1u 50 e1u 0 e1g -50 200 300 400 500 600 700 800 a , a a2u 1g 2u lambda [nm] a Ferrocene has a molar extinction coefficient of 96 M-1cm-1 at 442 nm 1g P. Wipf 10 Chem 1140 Nomenclature of Electronic Transitions; Symbols of Symmetry Classes Symbols of symmetry classes: A: sym. (according to a Cn operation) Examples: B: antisym. (according to a Cn operation) 1 1 A2 A1 1 1 E: 2-fold degenerate state B1u A1g 1 1 T: 3-fold degnerate state B2u A1g 1 1 E1u A1g Indices: 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) Calculation of spectra: ! Bonus Problem: Calculate UV and IR Spectra of Ferrocene and Acetylferrocene, and Compare to Experimental Data; can you design a Ferrocene derivative that is green- colored? P. Wipf 11 Chem 1140 Infrared 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: coupled: simple H H H H C H C C symmetric asymmetric P. Wipf 12 Chem 1140 Bending modes: H H H H C C in plane deformations scissoring (sym) rocking (asym) H H H H C C out of plane deformations wagging (sym) twisting (asym) Transmission O H C C C C other stretching N H C N C O C H X Y Z C N bending and stretching stretching combination bands N O stretching FINGERPRINT N H bending REGION Absorbance 2500 1500 -1 4000 3000 2000 1000 cm wave numbers P. Wipf 13 Chem 1140 Selection rules 1. In order to observe an absorption, the dipole moment of the excited vibrational state must differ from that of the ground state. Reason:!oscillating dipole interacts with oscillating electric vector of h! O O observed H C C H H C C H not observed Analysis of IR - Spectra of Unknown Compounds 1. frequency, shape, intensity of an absorption band have all to be considered in the interpretation 2. all characteristic absorption frequencies of a functional group have to be considered (band can be missing or is caused by another function) 3. first the obvious absorptions should be identified: !!!!!!!!!!!!!!!!X-H, C=O, C=C, out of plane C-H 4. subsequently the strong absorptions in the fingerprint region should be analyzed 5. possible structures can now be proposed and have to be checked with reference spectra (or data from other spectroscopic techniques) P. Wipf 14 Chem 1140 An Introduction to NMR Spectroscopy 1H NMR 13C NMR The types of information accessible via high resolution NMR include: 1. Functional group analysis (chemical shifts) 2. Bonding connectivity and orientation (J coupling), 3. Through space connectivity (Overhauser effect) 4. Molecular Conformations, DNA, peptide and enzyme sequence and structure. 5. Chemical dynamics (lineshapes, relaxation phenomena). http://www.chem.ucla.edu/%7Ewebspectra/ P. Wipf 15 Chem 1140 Nuclear Spin:!The proton is a spinning charged particle and has also a magnetic moment. 1H: - nuclear spin quantum number B m = 1/2 - such a nucleus is is described as having a nuclear spin I of 1/2 m = +1/2 m = -1/2 lower higher energy energy Because nuclear charge is the opposite of electron charge, a nucleus whose magnetic moment is parallel to the magnetic field has the lower energy. The difference in energy is given by:!!" = h# B0/2$ # = magnetogyric ratio ( a constant, typical for a nucleus, which essentially reflects the strength of the nuclear magnet) Bo = strength of the applied magnetic field h = Planck’s constant (3.99 x 10-13 kJ s mol-1) P. Wipf 16 Chem 1140 Note that as the field strength increases, the difference in energy between any two spin states increases proportionally. ! nuclear spin quantum # m =-1/2 Ei = -mh# Bo/2$ "! = -mµ Bo O N degenerate m = +1/2 O Bo [values of = 1/ were picked for m, so that the difference in energy between two neighboring states will always be an integer multiple of Bo (!h/2")]. The number of nuclei in the low energy state (N!) and the number in the high energy state (N") will differ by an amount determined by the Boltzmann distribution: N"/N! = e(-#$/kT) k = 1.381 x 10-23JK-1 When a radio frequency (RF) signal is applied, this distribution is changed if the radio frequency matches #$. #$ = h% = h& B0/2' % = resonance frequency = & B0/2' % i s therefore dependent upon both the applied field strength and the nature of the nucleus. P. Wipf 17 Chem 1140 1H: in a 2.35 T field (earth magnetic field = 0.00006 T) = 0.999984 ~ 1 in 106 (! = 100 MHz) - The difference in population of t he two states i s exceedingly small, in the order of few parts per million.
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