Inorganic Spectroscopy.ppt

Inorganic Spectroscopy Diffraction Methods Single-crystal and powder X-ray Neutron X-ray Absorption Spectroscopy (XAS) Magnetism and Magnetic Resonance Methods Magnetic Measurements Nuclear Magnetic Resonance (NMR) Electron Paramagnetic Resonance (EPR) Mössbauer Spectroscopy Electronic and Vibrational Spectroscopy Optical (UV/vis) IR and Resonance Raman Goals of Spectroscopy Define geometric and electronic structure of metal site Ground and excited states Knowledge of these provides insight into function Probe interactions of metal site with substrate and other small molecules to investigate reaction mechanism Difficulties: Some ions (Cu(I), Fe(III)) easier to study than others (Zn(II), Fe(II)) Often need to use multiple methods to get full picture X-ray Diffraction Provides spatial coordinates of atoms Most direct way to find geometric structure Can assess oxidation state of metal center via M-L bond lengths Requires crystal or crystalline powder Crystals can be difficult to obtain, particularly for biological samples or transiently stable species Knowledge of atom connectivity can help with solving structure Larger atoms increase signal intensity Heavy atoms (e.g. Kr, Xe, lanthanide tags) can make the structure easier to solve X-ray Diffraction Crystal has repeating structure -essential to getting a coherent signal Bragg?s Law: n? = 2d sin ? Electrons of atoms in lattice will scatter photons Observed pattern is sum of waves from every atom in the lattice Larger atoms have more electrons and are better at scattering, thereby easier to spot Due to short timescale associate with X-ray diffraction, movement of atoms within lattice will be shown as disorder Sample often cooled to reduce thermal motion, disorder Powder X-ray Diffraction Powder contains large number of small crystallites Orientation is random, unlike single-crystal Light diffracted from atoms in a cone around sample Number and ? positions of maxima a ?fingerprint? for a material Information regarding spatial coordinates very limited Very rapid data collection- amenable for following solid-state reactions with multiple phases or compounds present Glasses and amorphous solids will not generate distinct peaks- technique can be used to determine extent of crystallinity h? Diffraction Pattern Sample Spectrum From Bray, T. H. et al. Inorg. Chem. 45, 8251 (2006) Na2[UO2(IO3)4(H2O)] Single-Crystal X-ray Diffraction Requires large crystal and/or intense source of X-rays Use of single-crystal orients diffraction patterns of individual molecules in sample Instead of cone, a finite number of spots in diffraction pattern Position of spots depends on orientation of atoms within unit cell and orientation of unit cells relative to each other H atoms difficult, if not impossible, to locate due to their low electron densities Nice illustration of powder and single-crystal X-ray diffraction at http://www.msm.cam.ac.uk/doitpoms/tlplib/xray-diffraction/powder.php h? Diffraction Pattern Sample Crystal Structure From Bray, T. H. et al. Inorg. Chem. 45, 8251 (2006) Na2[UO2(IO3)4(H2O)] Atoms represented by thermal ellipsoids Bigger ellipsoids correspond to more thermal motion X-ray Diffraction Limits ?Taking a photograph of a horse does not necessarily tell you how fast it can run?- Jeremy Knowles Conditions used to grow crystal may not be relevant May alter structure May remove or add metal ions Lighter atoms can be misidentified or overlooked entirely With biological samples, structures often refined over years Figure from Lieberman, R. L.; Rosenzweig, A. C. Nature 434, 177 (2005) Neutron Diffraction Diffraction occurs with all particles, not just photons de Broglie wavelength must be comparable to separations of atoms within crystal Neutron beams, like X-rays, can be used to analyze structures of inorganic compounds Requires nuclear reactor or spallation source Requires larger sample, generally, since neutron flux lower than laboratory X-ray sources Neutrons scatter off nuclei, not electrons Lighter atoms can therefore be visualized Neighboring atoms can be distinguished more easily (e.g. N and C) Neutron Diffraction Applied towards study of metal hydride Dorogov, K. Y. et al. Inorg. Chem. 46, 147 (2007) Authors interested in interaction between hydride and silyl ligands X-Ray Absorption Spectroscopy (XAS) X-rays can excite 1s (K edge) and 2s, 2p electrons (L edge) to empty higher energy local orbitals or, with higher energy, the continuum XAS can provide information about oxidation state, coordination geometry, distance and identity of neighboring atoms Conventional sources (e.g. diffractometers) produce low-intensity X-rays not sufficient for absorption spectra Synchrotrons produce high-intensity, tunable X-rays Eight operating synchrotrons in North America XAS K edge data for four Fe(II) complexes at 10 K. The inset shows the pre-edge region, corresponding to 1s?3d transitions. Data are from Jackson Rudd, D. et al. Inorg. Chem. 44, 1221 (2005) XAS Theory- Interpreting the Edge Energy of edge most dependent on identity of metal- negligible overlap between absorption bands of neighboring metal atoms Expulsion of 1s electron more difficult when metal atom is more positively charged (Coulombic attraction) Energy of edge higher for metals in higher oxidation states Ni(II) Zn(II) Cu(II) and Cu(III) complexes with the same ligand set: DuBois, J. L. et al. J. Am. Chem. Soc. 119, 8578 (1997) XAS Theory- Interpreting the Pre-Edge Before the excitation to the continuum, potential transitions from 1s to higher energy atomic orbitals 1s? 3d transitions often observed, provide information on coordination geometry and spin-state These transitions are forbidden by parity, usually weak Introducing p-character into d orbitals (through covalent bonding with ligands) makes these transitions more favorable with more intense bands XAS of an Fe(II) complex from 300 K (black) to 10 K (purple). Increase in pre-edge corresponds to formation of low-spin Fe(II): Jackson Rudd, D. et al. Inorg. Chem. 44, 1221 (2005) Extended X-ray Absorption Fine Structure Abbreviated EXAFS Expelled electron back-scattered from nearby atoms (ligands) Larger neighbors (sulfur, other metals) scatter more efficiently Closer atoms scatter more efficiently- won?t see much beyond inner sphere Amplitude increases with number of scattering atoms Back-scattering interference seen at energies above edge Sinusoidal variation of amplitude with energy Deconvoluting interference can provide information about how close atoms are to electron source (metal) EXAFS- Advantages and Limits EXAFS data can be fit to model to get M-L bonds at 0.01 angstrom accuracy as checked by crystallography Well-defined model compounds can help to analyze data, particularly with smaller donor atoms Cannot tell similarly sized atoms apart (e.g. C, N, and O) Cannot get angular information (L-M-L?), just distances (M-L and M-L?) For accurate XAS data, the sample needs to be pure, but it doesn?t need to be crystalline Sample needs to survive exposure to high-intensity X-rays- photoreduction can occur EXAFS data at RT (blue) and 10 K (black) Sample Study- Rubredoxin Iron-containing protein involved in electron transfer X-ray diffraction had indicated that one Fe-S bond was significantly (0.25 angstroms) shorter than the other three XAS suggested that the four bond lengths were virtually identical (Shulman, R. G. et al. Proc. Nat. Acad. Sci. USA 72, 4003 (1975)) Follow-up X-ray diffraction corroborated the XAS results FESSC is an Fe(III) model compound Sample Questions Why is the energy of the XAS edge much higher than the metal ion?s ionization energy? What structural information can single-crystal X-ray diffraction provide that XAS cannot? What are the advantages to using XAS over single-crystal X-ray diffraction? Magnetic Measurements Measuring magnetic susceptibility can give information about the electronic structure of the metal site Very few organic species are paramagnetic, can easily home in on metal centers in a sample with organic components (diamagnetic metal ions excepted, of course) Probes the ground state at low resolution Multiple magnetic species in a sample will be averaged Not as detailed as electron paramagnetic resonance (EPR) BUT, as we?ll see, you can?t always use EPR Can get bulk susceptibility in three ways: Gouy Balance Superconducting Quantum Interference Device (SQUID) Evan?s Method solution measurement (NMR) Magnetism- Macroscopic Level For an object in an external magnetic field H: B = H + 4p I B is the magnetic induction and I is the intensity of magnetization (magnetic moment/ unit volume of sample) I = kH k is the volume susceptibility (unitless, independent of H) Fz = kV(Hx(dHx/dz)) Hx(dHx/dz) known Fz measured by monitoring weight change of sample N S V Hx Fz Classical Magnetism m = magnetic dipole moment of N molecules I = macroscopic magnetic moment E = -m ? H (lowest energy when dipole aligned with field) Energy of system lowered by external magnetic field Never perfect alignment due to thermal energy I = -(dE/dH) Quantum mechanics dictates that only certain energies are allowed (Van Vleck treatment) H No Field Experimentally Measured Susceptibility c = cdia + cpara + cTIP Diamagnetic component: Associated with all electrons Induced magnetic moment opposes applied magnetic field From Pascal?s constants or measurements on diamagnetic analog Small magnitude No T dependence [Fe(II)L6] [Zn(II)L6] Paramagnetic species of interest Diamagnetic reference Experimentally Measured Susceptibility c = cdia + cpara + cTIP Paramagnetic component: Associated with angular momentum of unpaired electrons Aligned with applied magnetic field Large magnitude, major component for paramagnetic complexes T dependent Temperature Independent Paramagnetic (TIP) component: From 2nd order Zeeman Becomes important at higher temperatures (< 5% of c at 300 K) Magnetic Resonance Spectroscopies When a particle has a magnetic moment, its orientation in an applied magnetic field will impact its energy Transitions between these states can be triggered by low-energy photons (magnetic resonance) Nuclei: nuclear magnetic resonance (NMR), radiofrequency light Electrons: electron paramagnetic resonance (EPR), microwave light Lower energy Higher energy NMR Spectroscopy Used for the study of molecular structure in solids and solutions Paramagnetic metal ions with slower electron-spin relaxation times may have shifted 1H and 13C NMR signals relative to those from the free ligand Paramagnetic metal species usually either EPR-active or NMR-active Resonances may be spread out far beyond typical organic values Other paramagnetic metal ions will broaden peaks to flatness Often useful to compare paramagnetic molecule with diamagnetic analog (e.g substitute Mg(II) for Mn(II)) Figure from Tei, L. et al. J. Chem. Soc., Dalton Trans. (2000) NMR Spectroscopy- Benefits and Limits Can provide information on connectivity- which atoms are near each other Nearby atoms will impact chemical shift and splitting Can be used to monitor kinetic processes Appearance of new resonances/disappearance of old resonances Broadening of resonance peaks Cannot be used to measure bond distances or bond angles Requires visible nuclei in high abundance 1H (99.98%) easy to see 13C (1.1%) more difficult without isotopic enrichment Relatively slow technique, particularly compared to optical and vibrational spectroscopies NMR Theory Nucleus has spin I In applied magnetic field: 2I + 1 orientations, with different energies Energy gaps of (h/2p)?B, where ? is the magnetogyric ratio of the nucleus and B is the magnitude of the magnetic field Small energy gap Poor sensitivity with weak magnetic field Increasing field strength increases population differences between I energy levels, allows for greater absorption I = 1/2 mI = -(1/2)(h/2p)?B mI = +(1/2)(h/2p)?B B Resonance Features Energy of peak depends on Particle (electron, sort of nucleus) Chemical environment Magnetic field strength Width of feature connected to life-time of excited state ?E ?t ~ h/2p Relaxation processes will shorten life-time, broaden observed resonance peak (sometimes beyond recognition) Spin-spin relaxation (T2), from interaction of nuclei Spin-lattice relaxation (T1), from loss of energy to vibrational and rotational modes Relaxation Processes T1 and T2 are life-times, their inverses are rates With a normal sample T1 ~ 10-2 ? 100 s With paramagnetic samples, T1 values much smaller Can be as low as 10-4 s More efficient relaxation associated with more intense magnetic fields associated with these ions Can broaden NMR signals past limit of detection Chemical Shifts Frequency of NMR transition depends on local magnetic field Neighboring atoms will perturb magnetic field around each nucleus Chemical shift (?) of resonance quantifies the impact of these perturbations and provides information on nearby atoms Lower ? values correspond to better shielding, generally higher electron densities Predict order of 1H chemical shifts Chemical Shifts and Paramagnetism Nearby paramagnetic metal ion spreads chemical shifts over wider frequency range Shift depends largely on distance of target nucleus from metal ion Can deconvolute complicated portions of NMR spectrum Picture from website of Dr. Michael K. Denk (University of Guelph) Lanthanide Tags Installing a paramagnetic metal ion onto a peptide can give information about how the protein folds and how water accessible the labeled site is Lanthanide(III) ions can alter the chemical shifts of organic compounds up to 40 angstroms away Can aid with solving crystal structures, may sometimes fluoresce Carboxylate-heavy tag usually installed at either N- or C-terminus Ln(III) can bind to Ca(II)-binding sites, if present Image from Martin, L. J. et al. J. Am. Chem. Soc. 129, 7106 (2007) Spin-Spin Coupling Nearby nuclei will couple with each other Extent of coupling decreases dramatically with distance When nuclei are equivalent (interchangeable through symmetry operations), they will not couple Neighbor splits signal(s) by 2I +1 Multiple nuclei give more complicated patterns Homonuclear coupling between nuclei of same element Already encountered in 1H NMR spectra of organic chemistry Heteronuclear coupling between spins of different elements Weakening bonds tends to reduce coupling I = 9/2 Sample Questions Predict what the 1H and 2H NMR spectra of HD will look like (note: I = 1 for D) Predict the relative chemical shifts (1H) of M(H2) species relative to the dihydride MH2 How will the 1H chemical shift of a metal hydride change with increasing metal oxidation number Electron Paramagnetic Resonance (EPR) Also known as ESR (Electron Spin Resonance) Metal ion must have unpaired electrons: Cu(II), Co(II), Fe(III), Mn(II), Mo(V) For reasons which will be elucidated later, ions with odd numbers of electrons most readily analyzed by EPR Can detect ions in mM range Apply magnetic field, make electron spin states non-degenerate, and excite electron into higher energy level H?Zee = bH×(L + 2S) {Zeeman Effect} EMJ = gbHMJ E = +1/2 gbH (Ms = ½) E = -1/2 gbH (Ms = -½) + H?Zee Experimental Details Magnetic field varied with microwave frequency kept constant S band, 0.1 cm-1 X band, 0.3 cm-1 (most commonly used) Q band, 1.16 cm-1 (offers higher resolution) When h? = gbH, resonance- electron is promoted to higher MJ state Selection rules: For transverse configuration, ?MJ = ±1 (most commonly used) For longitudinal configuration, ?MJ = 0 Sample Preparation Sample normally cooled to 77 K or 4 K to reduce speed of relaxation processes which would broaden line-widths Samples diluted in solvent or diamagnetic solid as well Solid or solution samples can be analyzed Kramers? Rule ?If an ion has an odd number of electrons, the degeneracy of every level must remain a least twofold in the absence of a magnetic field? (Drago, R. S. Physical Methods for Chemists, p. 559) MJ = ±1/2,? ±J Lowest energy level will be a doublet, which can be made non-degenerate by applying a magnetic field (Kramers Doublet) EPR operational in these cases Case on right for Co(II), d7 Spin-orbit coupling Magnetic Field Kramers? Rule With even numbers of electrons, degeneracy can be removed by crystal field, leaving singlet levels far apart in energy MJ = 0,? ±J Energy gap outside the microwave spectrum; EPR cannot be used Case on right for V(III), d2 Paramagnetic, but not observable by EPR Interpreting EPR Spectra Three parameters of interest: g value: determines energy gap in magnetic field Hyperfine splitting, AM: resonance split by spin-coupling between metal nucleus and electron Superhyperfine splitting, AL: resonances further split by spin-coupling between ligand nuclei and electron The first derivative of the signal is normally presented- makes parameters easier to measure g Values Anisotropic- value of g dependent on orientation relative to magnetic field Potentially up to three g values: gx, gy, gz Axial, two g values Rhombic, three g values In frozen solution or powder sample, the resultant signal will contain a mixture of all orientations EPR of mononuclear Cu(II) complex from Pratt, R. C. et al. Inorg. Chem. 43, 8030 (2004) Rhombic EPR spectra with g1 >> g2 > g3 g Values Spin-only g value = 2.0023 Organic free radicals will have g values around this value In metal complexes, g values will deviate from 2.0023 due to the coupling of the electron spin with orbital angular momentum g = [J(J+1) ? L(L+1) + S(S+1)] + 1 2J(J+1) L = orbital angular momentum, J = total angular momentum Different Lx,y,z values for each d orbital Covalency of d orbitals will counter impact of angular momentum and reduce the g values g values sensitive to chemical environment Metal Hyperfine Depends on spin of metal?s nucleus As with g values, metal hyperfine is anisotropic Three components, the first of which is isotropic: Fermi Contact: d electron spin polarizes the s electrons at the metal?s nucleus, producing negative spin density at nucleus Spin Dipolar: dipoles of electron spin and nuclear spin couple Orbital Dipolar: dipole of electron angular momentum couples to the dipole of the nuclear spin As with g values, covalency will reduce AM constants (impact Fermi contact and spin dipolar terms) With nuclear spin of I, will see (2I + 1) lines e.g. 63Cu, I = 3/2, each g value split into 4 lines AM g Metal Nuclei Different isotopes may have different I values Will often see multiple isotopes overlaid in spectrum Isotope Natural Abundance Spin I 50V 0.24% 6 51V 99.76% 7/2 52Cr 90.46% 0 53Cr 9.54% 3/2 55Mn 100% 5/2 56Fe 91.72% 0 57Fe 2.25% ½ 59Co 100% 7/2 63Cu 69.09% 3/2 65Cu 30.91% 3/2 Superhyperfine Same as physical basis as hyperfine, but with nuclei of ligands Much weaker interaction than hyperfine, often not fully seen Each signal further split into (2IL +1) peaks Increased covalency in M-L bond increases density of the d electron near the ligand?s nucleus Better covalency therefore increases AL Below example with Cr(V) from Gez, S. et al. Inorg. Chem. 44, 2934 (2005) 53Cr: natural abundance ~ 10%, I = 3/2 52Cr: major isotope, I = 0 Superhyperfine from five O atoms seen in 53Cr satellites Mössbauer Spectroscopy Photons are emitted by nuclei undergoing radioactive decay through excited nuclear states 57Co ? 57Fe* ? 57Fe + h? The gamma rays emitted by the source do not lose energy due to recoil, allowing them to be absorbed by the same sort of nuclei The emitted photons are absorbed by nuclei in the sample 57Fe + h? ? 57Fe* Energy of absorbance and quadrupole splitting provides insight into chemical environment of target element Oxidation state Spin-state (high-spin or low-spin) Coordination sphere Mössbauer Spectroscopy Due to the different chemical environments of the atom in the source and sample, the radiation emitted from the source will not be at exactly the energies required for absorbance by the sample The gamma ray energy is modulated by moving the sample at a set velocity relative to the source The Doppler effect brings the sample into resonance with the source The spectrum plots the absorbance vs. the velocity of the sample Au-albumin data from Canumalla, A. et al. Inorg. Chem. 38, 3268 (1999) Velocity (mm/s) Mössbauer Spectroscopy- Radiation Source Radioactive source must decay with a reasonable lifetime Radioactive element in source must decay through an isotope with a reasonable natural abundance Gamma ray must be at relatively low energy for Mössbauer effect to significantly occur Mössbauer limited to the study of 40K, 57Fe, 61Ni, 67Zn, 129I, 119Sn, 121Sb, and 197Au due to these requirements 57Co has a half-life of 270 days, long enough to run experiments, short enough to provide an adequate number of photons Energy of gamma emission from 57Co = 14.4 keV Natural abundance of 57Fe is 2.25% Samples often must be highly concentrated or enriched with 57Fe for a decent signal Interpreting a Spectrum Two parameters to consider: The isomer shift, ? The quadrupole splitting, EQ The isomer shift is at the center of the signal The peaks of the signal are split by EQ ? EQ Interpreting a Spectrum- Isomer Shift The isomer shift depends on the electron density at the nucleus of the absorber The s orbitals contribute to this electron density Only orbitals with finite probability of finding electron at nucleus The electrons in the valence d orbitals shield the nucleus from the s electrons and thereby reduce the electron density Compounds with a large positive ? have lower electron density at the nucleus- higher oxidation states High-spin Fe(II): +0.6 --- +1.4 High-spin Fe(III): +0.25 --- +0.85 High-spin Fe(IV): 0 --- +0.3 Low-spin Fe(II): -0.3 --- +0.5 Low-spin Fe(III): -0.2 --- +0.2 Low-spin Fe(IV): 0 --- +0.1 Interpreting a Spectrum- Quadrupole Splitting The quadrupole splitting depends on the interaction of the nuclear quadrupole moment with the electric field gradient near the nucleus This splits excited state into two energy levels An asymmetric electric field leads to a large EQ The occupation of the d-orbitals the primary determinant Largest values for high-spin Fe(II)- (t2g)4 (eg)2, low-spin Fe(III)- (t2g)5 Lowest values for high-spin Fe(III)- (t2g)3 (eg)2, low-spin Fe(II)- (t2g)6 Covalent interaction with ligands can alter electric field gradient as can distortions from ideal octahedral geometry (e.g. Jahn-Teller, inequivalent ligands, tetragonal distortions) EQ Values From Drago, R. S. ?Physical Methods for Chemists? Compound EQ ? FeSO4?7H2O 3.2 1.19 FeSO4 2.7 1.2 FeCl2?4H2O 3.00 1.35 FeC4H4O6 2.6 1.25 FeF2 2.68 --- FeCl3?6H2O 0.2 0.85 FeCl3 0.2 0.5 Fe(NO3)3?9H2O 0.4 0.4 Fe2(C2O4)3 0.5 0.45 Fe2O3 0.12 0.47 K4[Fe(CN)6] ?3H2O --- -0.13 K3[Fe(CN)6] 0.26 -0.15 Electronic Spectroscopy Very valuable, since most organic compounds are colorless or weakly colored Three sorts of UV/vis bands seen for metal complexes: Ligand to ligand transitions (e.g. porphyrins, pyridine rings) Transitions between metal ion?s d orbitals (d-d) Transitions between ligand and metal orbitals (LMCT, MLCT) Spectra depend on spin-state, oxidation state, and coordination sphere of metal ion Figure from Goldsmith, C. R. et al. J. Am. Chem. Soc. 124, 83 (2002) Experimental Details Sample put in glass or quartz cuvette with set dimensions Optical pathlength (l) Absorbance (A) measured by comparing incident intensity (Io) to intensity measured after the beam goes through the sample (I) A = log (Io/I) Beer-Lambert law relates A to l, concentration (C), and molar extinction coefficient (?) A = C ? l Wavelengths (?) and ? values of peak maxima measured Pathlength (l) h? h? (less intense) Sample, concentration C Selection Rules Like XAS, electronic absorption spectroscopy probes excited states For ground state (?g) and excited state (?e), an electronic transition is allowed if: ? ?g M ?e d? ? 0 M is the transition moment operator M largely the change in the electric dipole during the transition Note that spin must remain unchanged during transition, otherwise the integral = 0 and the transition is spin-forbidden d-d transitions are parity forbidden since the d-orbitals have g symmetry and the M operator has u symmetry Why do We See d-d Bands? Although d-d transitions are formally forbidden, the lower symmetry of most metal centers mixes these transitions with higher energy electric dipole-allowed charge transfer excited states, yielding weak bands For a spin-allowed d-d transition, ? = 10-100 M-1 cm-1 Typically, these are at very low energies (? > 600 nm) Exact energy dependent on ligand field strength and the specific electronic configurations involved (Tanabe-Sugano diagrams) d5 Spectroscopic Terms- Free Ions Many transitions seen for promotion of single electron, as seen in experimentally obtained spectra and Tanabe-Sugano diagrams Different microstates for each electronic configuration Russell-Saunders coupling used for lighter atoms (3d series) Main symbol associated with L of microstate (S, P, D, F, G, etc) L is sum of individual angular momenta of electrons Left superscript (multiplicity) given by 2S +1 Highest multiplicities and L values are lowest in energy 3P 3S 1P 1S Sample Questions Go back to the Tanabe-Sugano diagram for d5. For each microstate on the left-hand side, sketch the electronic configuration How many spin-allowed transitions are there in the free Mn(II) ion? Most UV/vis spectra are acquired with 1 cm pathlengths. How will the data change if a 10 cm pathlength cell is used? Spectroscopic Terms- Complexes Crystal field lowers angular momentum (also seen in magnetism) Multiplicity remains the same (2S + 1) The rest of the label is the symmetry label of the overall electronic orbital state Tanabe-Sugano diagrams show how atomic term symbols convert into molecular term symbols (as function of crystal field strength) Higher energy term symbols tend to have lower multiplicities, electrons in closer proximity to each other E states correspond to two equivalent microstates T states correspond to three equivalent microstates 2D 2T2g Simplest Cases- d1 and d9 Can think either in terms of electrons or electron holes With d1 and d9, a single electron (d1) or electron hole (d9) Only one transition possible The letter of the electronic state gives the degeneracy A non-degenerate E doubly degenerate T triply degenerate The multiplicity will not change for spin-allowed transitions Can get energy splitting directly from absorption peak Something More Difficult- d2 Different relative spatial orientations of electrons give rise to different electronic states for t2g1 eg1 in octahedral field Anticipate three spin-allowed transitions for d2 ions in octahedral field The allowed two-electron transition from 3T1g to 3A2g has not been observed experimentally Structural Information from d-d Bands? In theory, information about the coordination geometry can be obtained from the number and energy of the d-d bands Difficult since bands are very weak, may strongly overlap, or may not appear at all Additionally, some geometries may give rise to similar spectra (e.g. distorted tetrahedral and trigonal bipyramidal) With Zn(II)-containing carbonic anhydrase, replacing Zn(II) with Co(II) yielded an optical spectrum consistent with a tetrahedrally coordinated metal ion Tetrahedrally coordinated Zn(II) in carbonic anhydrase since confirmed with X-ray crystallographic analysis Ligand to Metal Charge Transfer Bands LMCT bands much more intense than d-d bands since ligand valence orbitals are primarily p atomic orbitals (u symmetry) Bands usually higher in energy than d-d bands Intensity of band scales with donor strength of ligand Intensity and energy may be altered by switching solvent As ligand becomes easier to oxidize, the energy of the LMCT band decreases (e.g. Cu-Br lower in energy than Cu-F) In order for LMCT to occur, metal ion must be capable of accepting an electron from the ligand (better for higher oxidation states) Low energy, intense charge transfer transitions indicate strongly covalent M-L bonds Case Study- Blue Copper Proteins Named for their distinctive blue color Plastocyanin has electronic transition at ~ 600 nm, with ? ~ 5000 M-1 cm-1 The intensity and energy of the feature are consistent with a strongly covalent Cu-L bond Strongly covalent bond between copper and sulfur from a cysteinate ligand Optical spectrum of poplar plastocyanin (Holm, R. H. et al. Chem. Rev. 96, 2239 (1996) Soft- soft interactions ? covalent, strong spectra in UV/Vis Review Questions How many d-d transitions would you expect for an octahedrally coordinated Mn(III) ion? If a sample does not have a Mössbauer signal, what does it say about its iron content? How would a strongly covalent Cu-S bond impact the electron transfer pathways through a blue copper center? What is needed for a MLCT band? Vibrational Spectroscopy Infrared spectroscopy (IR) known from organic chemistry Theory treats bonds as springs, with force constant k and reduced mass (m) that depends on masses of partners (mA, mB) Vibrational quantum number v m = (mAmB)/(mA + mB) Ev = (v + 0.5)(h/2p)(k/m)1/2 When v = 0, zero-point energy Fundamental transitions associated with Dv = +1 Overtones correspond to Dv = +2 or greater Number and sorts of vibrational bands dictated by symmetry Electric dipole selection rule in effect for IR IR Active Raman Active IR Inactive Raman Active Raman Spectroscopy With IR, radiation passed through sample, spectrum derives from variation of transmission with frequency With regular Raman, sample irradiated with laser, spectrum derives from energy lost (Stokes) or gained (anti-Stokes) by photon Change in energy correlated to vibrational excitation Polarizability of vibration determines visibility IR and Raman often yield complementary vibrational data No common lines if the molecule has a center of symmetry (i) IR Active Raman Active IR Inactive Raman Active Resonance Raman Tune laser to electronic absorption feature Electronic excitation enhances vibrations associated with the feature Intensity can increase by 3-4 orders of magnitude With biomolecules, a good way to look at vibrations associated with the chromophore at the active site Often used to look at O-O stretches, which are not visualized by IR Can substitute 18O for 16O; the shift in the stretching frequency provides insight into the nature of the O-O bond Case Study- Cu Superoxo Model Data from Maiti, D. et al. J. Am. Chem. Soc. 129, 264 (2007) Looking at Cu(I)-O2 adduct Is O2 end-on or side on? Resonance Raman used to look at Cu-O and O-O bonds Excitation at 413 nm (LMCT band) Frequencies, isotope shifts consistent with end-on structure Review Questions If you do not see an enhancement in a vibrational peak going from Raman to resonance Raman, what does it say about that vibration?s relationship to the electronic transition? Estimate the ratio of the vibrational energies associated with O-H and O-D stretches Estimate the ratio of the vibrational energies associated with 16O-18O and 16O-16O stretches