Electronic Spectroscopy 2

Optical Electronic Spectroscopy 2 Lecture Date: January 28th, 2008 Molecular UV-Visible Spectroscopy  Molecular UV-Visible spectroscopy is driven by electronic absorption of UV-Vis radiation.  Molecular UV-Visible spectroscopy can: – Enable structural analysis – Detect molecular chromophore – Analyze light-absorbing properties (e.g. for photochemistry)  Basic UV-Vis spectrophotometers acquire data in the 190-800 nm range and can be designed as “flow” systems. Figures from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm#uv1 1 Molecular UV-Vis Spectroscopy: Terminology  UV-Vis Terminology – Chromophore: a UV-Visible absorbing functional group – Bathochromic shift (red shift): to longer wavelengths – Auxochrome: a substituent on a chromophore that causes a red shift – Hypsochromic shift (blue shift): to shorter wavelengths – Hyperchromic shift: to greater absorbance – Hypochromic shift: to lesser absorbance Molecular UV-Vis Spectroscopy: Transitions  Classes of Electron transitions – HOMO: highest occupied molecular orbital – LUMO: lowest unoccupied molecular orbital – Types of electron transitions: (1) , π and n electrons (mostly organics) (2) d and f electrons (inorganics/organometallics) (3) charge-transfer (CT) electrons 2 Molecular UV-Vis Spectroscopy: Theory  Molecular energy levels and absorbance wavelength:   * and   π* transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis. n  * and π  * transitions: non-bonding electrons (lone pairs), wavelength(max) in the 150-250 nm region. n  π* and π  π* transitions: most commontransitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiplebonds with max = 200-600 nm. Figure from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm Molecular UV-Vis Spectroscopy: Theory  d/f orbitals – transition metal complexes – UV-Visspectra of lanthanides/actinides are particularly sharp, due to screening of the 4f and 5f orbitals by lower shells. – Can measure ligand fieldstrength, and transitions between d-orbitalsmade non-equivalent by the formationof a complex  Charge transfer (CT) – occurs when electron-donorand electron-acceptorproperties are in the same complex – electron transfer occurs as an “excitation step” – MLCT (metal-to-ligandcharge transfer) – LMCT (ligand-to-metalcharge transfer) – Ex: tri(bipyridyl)iron(II),which is red – an electron is exictedfrom the d-orbital of the metal into a π* orbital on the ligand 3 Molecular UV-Vis Spectroscopy: Absorption  max is the wavelength(s) of maximum absorption (i.e. the peak position)  The strength of a UV-Visible absorption is given by the molar absorptivity ():  = 8.7 x 1019 P a where P is the transitionprobability (0 to 1) – governed by selection rules and orbitaloverlap, and a is the chromophorearea in cm2  Again, the Beer-Lambert Law: A= ebc Molecular UV-Vis Spectroscopy: Quantum Theory  UV-Visiblespectra and the states involvedin electronic transitions can be calculated with theories ranging from Huckel to ab initio/DFT.  Example: π  π* transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empiricalexcited-states methods(Gaussian 03W): HOMO πu bonding molecular orbital LUMO πg antibonding molecular orbital 4 Molecular UV-Visible Spectrophotometers  Continuum UV-Vis sources – the 2H lamp: Hamamatsu L2D2 lamps  Tungsten lamps used for longer wavelengths.  The traditional UV-Vis design – double-beam grating systems Figure from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm#uv1 Molecular UV-Visible Spectrophotometers  Diode array detectors can acquire all UV-Visible wavelengths at once.  Advantages: – Sensitivity (multiplex) – Speed  Disadvantages: – Resolution Figure from Skoog, et al., Chapter 13 5 ... - tailieumienphi.vn