Ground State Spectroscopic Methods

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Vibrational Spectroscopy

Preliminary lecture notes, copyright Pierre Kennepohl, all rights reserved

There are two distinct approaches to vibrational spectroscopy: infrared absorption spectroscopy (a single photon technique) and Raman spectroscopy (a two photon inelastic scattering technique). Each carries its own set of selection rules, thus each can give complementary information regarding the vibrational modes of a molecule.

Infrared spectroscopy

Raman spectroscopy

Literature examples of the use of vibrational spectroscopy in inorganic chemistry

"Imaging of single GaN nanowires by tip-enhanced Raman spectroscopy", N. Marquestaut, D. Talaga, L. Servant, P. Yang, P. Pauzauskie, F. Lagugné-Labarthet, Journal of Raman Spectroscopy, 2009, 40(10), 1441-1445, doi:10.1002/jrs.2404.

This article demonstrates the use of an AFM microscopy used in tandem with a Raman spectroscopy to determine the physical dimensions of a GaN nanowire. Typically well defined objects of sizes on the order of (lambda/2) can be observed by raman, but by using tip-enhanced Raman spectroscopy (TERS) this spatial resolution can be enhanced. The GaN nanowire analyzed in this paper was on the order of ~200 nm. EricPrice

"Effect of high external pressure on the vibrational spectra of crystalline dichloro(1.5-cycloctadiene)platinum(II)", JA Baldwin, IS Butler and DFR Gilson, "Inorganica Chimica Acta", 2006, 359, 3079-3083, doi:10.1016/j.ica.2006.02.003.

This article used high pressure IR and Raman spectroscopy to investigate the bonding in a dichloro-COD platinum complex. They assign the IR and Raman bands based on previous work and use the pressure dependencies of certain bands in the spectra to make conclusions about structural deformations at high pressure and how they can influence the strength of pi-backbonding in this molecule. Kimosten

"A two-dimensional IR correlation spectroscopic study of the conformational changes in syndiotactic polypropylene during crystallization", K. Zheng, R. Liu, Y. Huang, "Polymer Journal", "2010", 42, 81-85 doi:10.1038/pj.2009.304.

This article examines the melting/crystallization of syndiotactic polypropylene by using FTIR and 2D correlation analysis. The changes are seen through the intensities of peaks known to be associated with the crystalline and amorphous forms of the polymer. The 2D correlation analysis gives insight to the order of change within the polymer during these two processes (melting, crystallization). AmberJuilfs

"Isolation of Dysprosium and Yttrium Complexes of a Three-Electron Reduction Product in the Activation of Dinitrogen, the (N₂)^3- Radical", William J. Evans, Ming Fang, Gael Zucchi, Filipp Furche, Joseph W. Ziller, Ryan M. Hoekstra, Jeffrey I. Zink, J. Am. Chem. Soc., 2009, 131, 11195-11202, doi:10.1021/ja9036753.

This article concentrates on synthesis of N₂ complexes involving yttrium and dysprosium, and analysis of the N-N bond order using Raman spectroscopy. Data is compared to computational values, and ¹⁵N isotopic substitution experiments were conducted. Lwence

'Spectroscopic Properties and Quantum Chemistry-Based Normal Coordinate Analysis (QCB-NCA) of a Dinuclear Tantalum complex Exhibiting the Novel Side-On End-On Bridging Geometry of N₂: Correlation to Electronic Structure and Reactivity", Felix Studt, Bruce A. MacKay, Michael D. Fryzuk, Felix Tuczek, J. Am. Chem. Soc., 2003, 126, 280-290, doi:10.1021/ja036997y. TrumanWambach

IR, Raman, isotopic substitution and computations are used in concert to throughly describe the bonding in a dimeric tantalum N₂ compound. IR and Raman spectra are calculated, and assigned to a molecular vibration. Computations are then compared to experimental values. Force constants are assigned to the bonds of interest.

"Probing thin-film morphology of conjugated polymers by Raman spectroscopy", Jessica M. Winfield, Carrie L. Donley, Richard H. Friend, Ji-Seon Kim, J. Appl. Phys., 2010, 107, 024902, doi:10.1063/1.3276257.

This article demonstrates how Raman spectroscopy can be used to compare the thin-film morphology of conjugated polymers at the polymer-substrate interface versus that in the bulk polymer. It is shown that near the substrate interface[poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT)] adopts a more planar conformation (has a lower torsion angle between the alternating units). The morphology of conjugated polymers at interfaces is very important in light-emitting diode and field-effect transistor applications where charge transport occurs near the polymer-substrate interface. (AshleeHowarth)

"Resonance Raman Scattering from Solutions of C₆₀", S.H. Gallagher, R.S. Armstrong, W.A. Clucas, P.A. Lay, C.A. Reed, J. Phys. Chem. A., 1997, 101(16), 2960-2968. doi:10.1021/jp970232t.

The resonance raman excitation spectrum of fullerene (C₆₀) is studied at eight excitation wavelengths. Raman excitation calculations were performed on the five most intense resonance bands to correlate the bands with the vibrational modes of fullerene. Rare (D-type) scattering is observed. It is determined that the symmetry of C₆₀ is distorted by solvent, as there are Raman-silent vibrational modes present. The symmetry of the distorted excited state of fullerene is D_5h. AlexandraAnderson

"Photoinduced Orientation of Azobenzene Chromophores in Amorphous Polymer As Studied by Real-Time Visible and FTIR Spectroscpies", T. Buffeteau, F. Lagugne-Labarthet, M. Pezolet, and C. Sourisseau, Macromolecules, 1998, 31(21), 7312-7320, doi:10.1021/ma980843z.

This article shows the use of visible (birefringent studies) and infrared (polarization modulated FTIR) spectroscopy to study the dynamics of photoinduced orientation of doped and co-polymers of azobenzenes. Polarization modulated infrared spectroscopy (PM-FTIR)is a form of IR which allows the observation of the amount of order in a given anisotropic sample. The authors used this technique to observe the orientation (by photoinduction) and relaxation of DR1 in a matrix of PMMA. JackyYim

"Charging effects on bonding and catalyzed oxidation of CO on Au-8 clusters on MgO", Yoon B, Hakkinen H, Landman U, Worz AS, Antonietti JM, Abbet S, Judai K, Heiz U, SCIENCE, 2005, 307(5708), 403-407, doi:10.1126/science.1104168.

This article investigates on how gold octamers bound to oxygen-vacancy F-center defects on MgO(001) activate adsorbed CO and O2 and catalyze the oxidation of CO. The authors use infrared spectroscopy to measure the stretch vibration of CO and it shows a red shift by 25 to 50 cm-1. This shift is caused by enhanced backdonation from the gold nanocluster into the antibonding 2pi* orbital of the CO. All the experimental results agree with quantum calculations. CuilingXu

"Real-time monitoring of microwave-promoted organometallic ligand-substitution reactions using in situ Raman spectroscopy", T. M. Barnard and N. E. Leadbeater, Chem. Commun., 2006, 3615. doi:10.1039/b608793k.

The article demonstrates an application of Raman spectroscopy in monitoring progress in organometallic ligand-substitution reactions in situ, while using microwave irradiation to facilitate the reaction. This allows reaction conditions to be easily optimized and does not require much trial and error, but requires the help of some literature data. The theory is tested with the substitution of a CO ligand with some ligands in Mo(CO)₆, where the course of substitution is monitored with Raman spectroscopy without aliquots removed and conserves time. ReneeMan

"Synthesis, Crystal Structure, and Vibrational Spectroscopy of K₂Ca₄Si₈O₂₁—An Unusual Single-Layer Silicate Containing Q2 and Q3 Units", E. Arroyabe, R. Kaindl, D.M. Tbbens and V. Kahlenberg, Inorg. Chem., 2009, 48, 11929–11934., doi:10.1021/ic901762u.

The author synthesized a novel silicate, and determined its crystal structure. Confocal Raman spectrum of a single crystal of this silicate was also studied in this article. The author assigned the fequencies to specific vibrational modes, and concluded that K₂Ca₄Si₈O₂₁ is a silicate based on loop-branched chains. JiazhangWang

"Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of n-Alkanethiols on the Coinage Metal Surfaces, Cu, Ag, Au.", P. Laibinis, G. Whitesides, D. Allara, Y. Tao, A. Parikh, R. Nuzzo, Journal of the American Chemical Society, 1991, 113, 7152-7167, doi:10.1021/ja00019a011.

Reflection-absorption infrared spectroscopy (RAIRS) is used to determine the crystallinity and cant angle of straight-chain alkanethiols adsorbed on Cu and Ag. These measurements are then compared to the well-studied system of alkanethiols adsorbed on Au. RAIRS uses p-polarized incident light to take advantage of the "surface selection rule," which states that only vibrations with a transition-dipole moment component perpendicular to the plane of the metal substrate can be observed. The intensity of a particular vibration is compared to that of a bulk spectrum and the orientation of the vibration relative to the substrate can be determined. BrianSahli

"Water’s Role in Reshaping a Macrocycle’s Binding Pocket: Infrared and Ultraviolet Spectroscopy of Benzo-15-crown-5-(H2O)n and 4′-aminobenzo-15-crown-5-(H2O)n, n ) 1, 2.", V. Alvin Shubert, Christian W. Muller, and Timothy S. Zwier J. Phys. Chem. A, 2009, 113, 8067–8079, [1].

In order to investigate the water's role in reshaping a macrocycle's bingdding pocket of two kinds of chemicals, the author used the IR and UV spectroscopy to both of them. The author wanted to investigate the binding site and the binding number of water molecules to the two kinds of macrocyle. Both Experimental and Computational Summary of the frequencies are shown in the article. Zhengyu Chen

"Synthesis and characterization of germanium sulfide aerogels.", Kennedy K. Kalebaila, Daniel G. Georgiev, Stephanie L. Brock J. Non-Cryst. Solids, 2006, 352, 232-240. doi:10.1016/j.jnoncrysol.2005.11.035.

During the synthesis of germanium sulfide aerogels, samples that had been transferred to vials sealed with a rubber septum seemed to form GeS gels rather than GeS_x, meaning a reduction of germanium. Powder-XRD and EDS supported this argument; however XPS indicated the germanium atom had actually become slightly more highly charged, rather than becoming reduced. Raman spectroscopy showed a complete absence of GeS peaks, as well as the presence of peaks assigned to GeO₂. It was thus concluded that either during transfer or while aging in the vial the samples had become exposed to air and/or moisture, and the germanium had oxidized. Since O is more electronegative than S, this accounts for the higher binding energy in the XPS. GeO₂ crystallizes similarly to GeS, and oxygen would not be observed in the EDS data due to limitations in the setup used. Andrew Priegert

"Phenol Nitration Induced by an {Fe(NO)₂}₁₀ Dinitrosyl Iron Complex", Nhut Giuc Tran, Harris Kalyvas, Kelsey M. Skodje, Takahiro Hayashi, Pierre Monne-Loccoz, Paige E. Callan, Jason Shearer, Louis J. Kirschenbaum, and Eunsuk Kim, J. Am. Chem. Soc., 2011, 133, 1184-1187. doi:10.1021/ja108313u.

In this communication, the authors report that exposure of Fe(TMEDA)(NO)₂ to O₂ forms a complex whose IR spectrum displays one upshifted and one downshifted NO band relative to the starting material. The product of this reaction is postulated to be Fe(TMEDA)(NO)(ONOO), which is corroborated by the data generated from IR, EPR, and EXAFS studies. Fe(TMEDA)(NO)(ONOO) is photosensitive, and its decomposition under white light is characterized by the replacement of the two NO bands by one single band, consistent with photodecomposition of the peroxynitrite ligand. CatherineChow

"A Combined Experimental and Theoretical Study of Iron Dinitrogen Complexes: Fe(N₂), Fe(NN)_x (x=1-5) and Fe(NN)₃^-",Zhang-Hui Lu, Ling Jiang, and Qiang Xu, J. Phys. Chem. A, 2010, 114, 2157-2163. doi:10.1021/jp9106298.

In this paper the authors use laser ablation to produce iron atoms which they then expose to dinitrogen. Six iron nitrogen complexes were synthesized and characterized with a variety of techniques. Comparison of DFT predicted vibrational stretches and experimental stretches help to assign the observed stretches in the IR spectrum. The observed vibrational frequencies were then used to make conclusions about the nature of the NN bond, and therefore about the degree of back donation from the iron. FraserPick

"Crystal structure and infrared spectroscopy of cobalt(II) complex with 4-imidazoleacetate anions and methanol molecules",Piotr drozdzewski, Barbara Pawlaka and Tadeusz Glowiak, J. Mol. Struct., 2003, 654, 111-118. [2]

The authors report a Cobalt(II) novel complex and its crystal structure. They use IR spectroscopy to characterize the complex and attempt to assign absorption bands to corresponding vibrations. Their main focus is to give further evidence that the IR spectrum supports the different coordinating species of methanol which they propose in their paper.DineshAluthge

"Pt,In and Pd,In catalysts for the hydrogenation of nitrates and nitrites in water. FTIR characterization and reaction studies", F.A. Marchesinia, L.B. Gutierreza, C.A. Querinia and E.E. Miró, "Chemical Engineering Journal.,2010,159, 203-211. [3]

The authors in this paper attempted to determine the mechanism and ability of Pt and Pd to act as a catalyst for the hydrogenation of nitrates and nitrites found in water. They use FTIR spectroscopy to determine the active sites on the catalyst be allowing it to react with CO and determining if there are any CO-Pd or CO-Pt bonds that have formed. They also use IR spectroscopy to determine the presence of any carbonate ions which would be the result of the oxidation of CO to CO2 in the presence of oxygen, and thus show the reactivity of the metal sites. PhillipTaylor

"O₂ Chemistry of Dicopper Complexes with Alkyltriamine Ligands. Comparing Synergistic Effects on O₂ Binding", Company, A.; Lamata, D. ; Poater, A.; Sola, M.; Rybak-Akimova, E.V.; Que, L., Jr.; Fontrodona, X.; Parella, T.; Llobet, A.; Costas, M. Inorg. Chem., 2006, 45, 5239-5241. [4]

In this communication, synthesis of two dicopper complexes with hexaaza ligands is reported. Also, their reactivity towards O₂ is compared with a similar monocopper system. Comparability of these systems is confirmed by FT-IR analysis, and resonance Raman spectroscopy is employed to identify the unique Cu₂^III-(μ-O)₂ feature of the oxygenated complex. Yang Cao

"Application of photoacoustic infrared spectroscopy in the forensic analysis of artists’ inorganic pigments", Eleanor L. von Aderkas, Mirela M. Barsan, Denis F.R. Gilson, and Ian S. Butler Spectrochimica Acta Part A, 2010, 77, 954–959. [5]

In this article, the authors use Fourier-transform photoacoustic infrared (PAIR) and Raman spectroscopy to analyse 12 inorganic pigments commonly used by artists today. Joanna De Witt

"Resonance Raman Spectra of Rubredoxin, Desulforedoxin, and the Synthetic Analogue Fe(S2-o-xyl)2-: Conformational Effects", Vittal K. Yachandra, Jeffrey Hare, I. Moura, and Thomas G. Spira J. Am. Chem. Soc., 1983, 105, 6455-6461. [6]

The resonance raman (rR) spectra of rubredoxin and an analogue were investigated. Assignments of the bands were made and used to probe the correct geometry/conformation of the compounds. The rR spectrum of oxidized rubredoxin was also taken and compared to that of the parent rubredoxin, giving insight into the geometric changes of the protein upon oxidation. Caterina Ramogida

"Single Crystal X-Ray Structure of BeF2: α-Quartz", Pallavi Ghalsasi and Prasanna S. Ghalsasi Inorg. Chem., 2011, 50, 86-89. [7]

BeF2 is highly ionic and covalent crystal bearing SiO2 tetrahedral framework. Infrared spectra shows two peaks associated with TO-LO and TO modes, respectively. Jiazhang Wang

"Forbidden Vibrational Modes in Iron(II), Ruthenium(II), and Osmium(I1) Hexacyanides: A Tunneling, IR, and Raman Spectroscopy Study" Hipps, K. W.; Williams, S. D.; Mazur, U. J. Am. Chem. Soc. 1984, 23(22), 3500-3505. [See Journal]

Forbidden vibrational modes are explored in various octahedral complexes. Peter Christensen

Nuclear Magnetic Resonance Spectroscopy

File:2009W2-C529-S021a.pdf Nuclear magnetic resonance spectroscopy is probably the most widely used spectroscopic technique in modern chemistry. It allows for rapid and accurate characterization of organic molecules as well as more in-depth studies of electronic and geometric structure of a wide range of molecules.The technique relies on the quantum mechanical behaviour of atomic nuclei, most particularly their ground state angular momentum ( $I_{g}$ ). NMR active nuclei must have $I_{g}>0$ such that they have more than one magnetic state (# of states = $2I+1$ ). NMR uses the Zeeman effect, the splitting of angular momentum states in a magnetic field, to create a situation where one can probe the nuclear states with electromagnetic radiation.

Common uses of NMR in inorganic chemistry

identification of compounds
help with mechanism elucidation
kinetics studies
isotopic labelling experiments
solid state identification
paramagnetic/diamagnetic verification

Literature examples of NMR in inorganic chemistry

"Neutral-Ligand Complexes of Bis(imino)pyridine Iron: Synthesis, Strucutre, and Spectrososcopy", S.C. Bart, E. Lobkovsky, E. Bill, K. Wieghardt, P.J. Chirik, "Inorg. Chem.", 2007, 46, 7055-7063, http://dx.doi.org/10.1021/ic700869h

A series of bis(imino)pyridine (PDI) iron complexes are synthesized and characterized using I.R. NMR, and Mossbauer spectroscopy. It is found that temperature independent paramagnetism occurs (TIP). TIP if found to be a function of donor strength. Upon two electron reduction of the system the ligand displays redox activity. Weak field ligands stabilize the S=0 ground state, where the redox active ligand (S=1) is antiferromagnetically coupled to the Fe(II) high spin iron center (S=1). Stronger field ligands stabilize the excited state (S=1).TrumanWambachTruman

"Synthesis, Structures, Bonding, and Redox Chemistry of Ditungsten Butadiyne Complexes with WCCW Backbones", J. Sun, E. Shaner, M.K. Jones, D.C. O'Hanlon, J.S. Mugridge, and M.D. Hopkins, Inorganic Chem., 2010, 49, 1687-1698, doi:10.1021/ic902088x.

This article reports the synthesis of a new Tungsten-Carbon triply bonded species of the form XL₄WCCWL₄X, where L = 1/2 dmpe, 1/2 depe, P(OMe)₃ and X = Cl, OTf. They used both X-ray crystallography and NMR to observe the molecule. They also performed DFT calculations to observe the bonding of the species. Through ³¹P NMR they were able to observe the degradation of the molecule by seeing the formation of free ligands. AmberJuilfs

"Na-Y Zeolite as a Highly Active Catalyst for the Hydroamination of α,β-Unsaturated Compounds with Aromatic Amines", K. Komura, R. Hongo, J. Tsutsui, and Y. Sugi, Catal. Lett., 2009, 128, 203-209, doi:10.1007/s10562-008-9738-4.

This article reports the use of Zeolite as a reusable, green solid catalyst for hydroamination to produce fine chemicals. The authors use the hydroamination of aniline with methyl acrylate as a their test reaction and found Na-Y shows high catalytic activity and mono-product selectivity. To study the mechanism by which this transformation occurs, ¹³C MAS NMR and solution ¹³C NMR of methyl acrylate adsorpted Na-Y zeolite were compared. It was found that Na-Y activates methyl acrylate through a Lewis-acid interaction between the carbonyl and sodium ion. JackyYim

"Insertion Reactions of trans-Mo(dmpe)₂(H)(NO) with Imines", F. Liang, H. W. Schmalle, and H. Berke, Inorg. Chem., 2004, 43, 993-999, doi:10.1021/ic0301391.

The insertion of a disubstituted aromatic imine into the metal-hydrogen bond of a molybdenum hydride complex is studied using variable temperature ¹H NMR. By comparing the relative peak intensities of product and reactant at different temperatures the standard enthalpy and entropy of reaction are determined. This is done using a van't Hoff plot, which plots ln K vs. 1/T. The calculated change in enthalpy and entropy were -48.8 kJ/mol and -33 J/(K mol), respectively. BrianSahli

"Structural and thermodynamical properties of CuII amyloid-beta16/28 complexes associated with Alzheimer's disease"". Guilloreau, L.; Damian, L.; Coppel, Y.; Mazarguil, H.; Winterhalter, M.; Faller, P. Journal Of Biological Inorganic Chemistry: JBIC: A Publication Of The Society Of Biological Inorganic Chemistry 2006, 11, 1024-1038, doi:10.1007.

Amyloid-beta peptide interaction with the paramagnetic metal Cu(II) is investigated by 1D proton NMR. The paramagnetic shift/broading that occurs in the NMR signals of amino acid residues that are bound with Cu(II) are used to identify them as ligands. The signals from other residues are also broadened and shifted due to the influence of the paramagnetic metal, and so substoichiometric amounts of Cu(II) are used in order to see the most dramatic broadening occur at the coordinated amino acids.EricPrice

"Diphenylphosphino- or Dicyclohexylphosphino-Tethered Boryl Pincer Ligands: Synthesis of PBP Iridium(III) Complexes and Their Conversion to Iridium-Ethylene Complexes", Y. Segawa, M. Yamashita, and K. Nozaki, Organometallics, 2009, 28, 6234-6242, doi:10.1021.

This article focuses on synthesis and characterization of various Ir PBP pincer complex derivatives. NMR studies are of interest as up to 4 different NMR active nuclei (³¹P, ¹¹B, ¹H, and ¹³C) may be bound to the metal centre at once.Lwence

"Characterization of wurtzite indium nitride synthesized from indium oxide by In-115 MAS NMR spectroscopy", W.S. Jung, O.H. Han and S.A. Chae, Materials Letters, 2007, 61, 3413-3415, doi:10.1016/j.matlet.2006.11.083.

This article outlines the characterization of wurtzite indium nitride (w-InN) samples made using different conditions using 115-In MAS NMR spectroscopy. The authors used this technique, in addition to XRD, to identify the different phases present in each sample and found that samples made using higher temperatures had a w-InN structure with extra In incorporated, due to thermal decomposition. Kimosten

"29Si NMR Relaxation of Silicated Nanoparticles in Tetraethoxysilane-Tetrapropylammonium Hydroxide-Water System (TEOS-TPAOH-H2O)", M. Haouas, D. Petry, M. Anderson, F. Taulelle, J. Phys. Chem. C, 2009, 113, 10838-10841, doi:10.1021/jp903454f.

In an attempt to understand how zeolites nucleate and grow, silicon-29 NMR relaxation times are measured from a silicalite-1 precursor solution. Nanoparticles and silicate oligomers in the solution all give rise to observable resonances. From these resonances the Qn distribution of nanoparticles in the solution can be measured and therefore the progressive connectivity of the nanoparticles to form zeolites can be followed. (AshleeJHowarth)

"Multiple Functional Groups of Varying Ratios in Metal-Organic Frameworks", Hexiang Deng, Christian J. Doonan, Hiroyasu Furukawa, Ricardo B. Ferreira, John Towne, Carolyn B. Knobler, Bo Wang, Omar M. Yaghi, Science, 2010, 327, 846-850, doi:10.1126/science.1181761.

This article reports a set of synthesized multivariate metal-organic frameworks (MTV-MOF). The frameworks are composed of metal-oxide joints and 1,4-benzenedicarboxylate and its derivatives links and a "complex" secondary structure formed by multivaried arrangements of many functional groups. The structures is ordered, and highly porous. They can achieve better capacity and selectivity in gas storage. ¹³C CP/MAS NMR study indicates the presence of multi functional organic links in the MOF backbone without unbound link in crystals. ¹H NMR spectra indicates the link ratio of their crystals. Jiazhang

"Rhodium(0) Metalloradicals in Binuclear C-H Activation", F. F. Puschmann, H. Grutzmacher, B. de Bruin, J. Am. Chem. Soc. 2010, 132, 73-75, [8].

The paper uses ¹H NMR to monitor the C-H activation reaction of a 17-electron Rhodium(0) species into two 16-electron complexes. As the reaction proceeds, the broad paramagnetic peaks disappear and the signals corresponding to the dimagnetic products grow. The growth of peaks allows monitoring of the reaction, hence kinetic studies can be performed. ReneeMan

"Crystal-Structure Determination of Powdered Paramagnetic Lanthanide Complexes by Proton NMR Spectroscopy, Gwendal Kervern, Anthony D'Aleo, Loic Toupet, Olivier Maury, Lyndon Emsley, and Guido Pintacuda, Angew. Chem. Int. Ed., 2009, 48, 3082-3086, doi: 10.1002/anie.200805302

Solid-state NMR spectroscopy can be used for structure determination of microcrystalline paramagnetic solids at natural isotopic abundance. The protocol makes use of paramagnetic effects, measured on suitably recorded ¹H NMR spectra, to define the conformation of a molecule in the lattice and the intermolecular packing in the solid phase. CuilingXu

"Adsorption-Desorption Induced Structural Changes of Cu-MOF Evidenced by Solid State NMR and EPR Spectroscopy", Yijiao Jiang, Jun Huang, Besnik Kasumaj, Gunnar Jeschke, Michael Hunger, Tamas Mallat, and Alfons Baike, J. Am. Chem. Soc., 2009, 131, 2058-2059, [9]

The authors' work uses ¹¹B MAS NMR and EPR to present direct experimental evidence of a porous copper metal-organic framework's structural flexibility during adsorption-desorption of a few mbar of CH₃OH and CH₃CN. Their observations indicate that these MOFs can undergo structural changes under relatively mild conditions. Further understanding of these dynamic changes is necessary for the rational design of MOFs for technical applications. Joanna De Witt

"Natural-Abundance ⁴³Ca Solid-State NMR Spectroscopy of Bone", Jiadi Xu, Peizhi Zhu, Zhehong Gan, Nadder Sahar, Mary Tecklenburg, Michaed D. Morris, David H. Kohn, and Ayyalusamy Ramamoorthy, J. Am. Chem. Soc., 2010, 132, 11504-11509,[10]

The calcium environment of a cortical bone sample, along with bone model complexes such as carbonated apatite and hydroxyapatite were probed via ⁴³Ca solid-state magic angle spinning (MAS) NMR spectroscopy. Through analysis of the shift frequencies, this technique was used to provide valuable information about the coordination chemistry and ionic charge of calcium ions in bone and bone model complexes. Caterina Ramogida

"⁷³Ge NMR Spectral Investigations of Singly Bonded Oligogermanes"', Monika L. Amadoruge, Claude H. Yoder, Julia Hope Conneywerdy, Katie Heroux, Arnold L. Rheingold, and Charles S. Weinert, Organometallics, 2009,28, 3067-3073,[11]

The synthesis of several germanium oligomers and their characterization with ⁷³Ge NMR is reported. The NMR resonances are correlated with the chemical makeup of the oligomers. Crystallographic data as well as ¹³C and ¹H NMR is also used to validate the assignments made using ⁷³Ge NMR.

Dinesh Aluthge

"Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer Complex", Robert Langer, Gregory Leitus, Yehoshoa Ben-David, David Milstein, Angew. Chem. Int. Ed. 2011, 50, 2120, doi:10.1002/anie.201007406.

The authors synthesized a highly efficient iron pincer catalyst for the hydrogenation of ketones which operates under very mild conditions (4.1 atm H₂, 26 - 28 C). It was extensively characterized through heteronuclear and 2D NMR spectroscopy. NMR spectroscopic studies of the dearomatized benzonitrile and benzophenone adducts led the authors to propose a mechanism for this transformation. CatherineChow

"A Molecular Reel: Shuttling of a Rotor by Tumbling of a Macrocycle", Kazuhiro Yamauchi, Atsuhisa Miyawaki, Yoshinori Takashima, Hiroyasu Yamaguchi, and Akira Harada, J. Org. Chem. 2010, 75, 1040–1046, [12].

The author sythesized two molecular shuttles with several macrocycles. Two kinds of molecular ([2]Rotaxane and [3]Rotaxane) shuttles are characterized by 2D NMR and Kinetics. The conformations and direction of the macrocycles in both molecules are elucidated with 2D ROESY NMR measurements, while the mechanism of the tumbling process are studied by Kinetics. They found that the macrocycles in the molecular shuttles may not be the same when they are in different solvents. In DMSO and H2O, the comformation of the molecular shuttle [2]Rotaxane changed with the polarity of solvent. It can be a prototype of molecular machines capable of shuttling a part back and forth in the molecule.. Zhengyu Chen

"Rhodium Catalyzed Acylation with Quinolinyl Ketones: Carbon-Carbon single bond activation as the Turnover-Limiting Step of Catalysis", Colin M. Rathbun and Jeffrey B. Johnson, J. Am. Chem. Soc. 2011,133, 2031, [13].

In this paper the authors investigate the mechanism of a catalytic alkene insertion into a carbon carbon bond. The authors identify the carbon carbon bond activation to be the rate limiting step in the transformation. 1H NMR is used to monitor reactions rates, and to determine the kinetic isotope effect of 12C/13C substitution at different carbon positions in the substrate. FraserPick

"Structure elucidation of the unprecedented asymmetric bis-chelate complex [Pd(1,3-bis(di(o-methoxy-m-methylphenyl)phosphino)propane)₂]²⁺ in the solid state and in solution", Tiddo J. Mooibroek, Martin Lutz, Anthony L. Spek, Elisabeth Bouwman, Dalton Trans. 2010,39, 11027.

When studying dppp-derived ligands and their PdL₂ complexes, one particularly bulky ligand gave a complex ³¹P NMR spectrum, where only a single signal was expected. It was discovered that although the two ligands are both identical and symmetric, they coordinate asymmetrically to give four signals, which show splitting characteristic of four inequivalent phosphorus atoms. AndrewPriegert

"Probing the interface of core shell particles of GaPO4 and AlPO4 by 31P MAS NMR spectroscopy", S.K. Kulshreshthaa, O.D. Jayakumarb, B. Vishwanadhc and V. Sudarsanb, Solid State Sciences 2011,13, 484-487. [14]

In this article, the authors used 31P NMR studies in order to determine the reactive sites on nanoparticles of GaPO4 and AlPO4. They found a multi-component NMR pattern of five peaks that showed that there were varying numbers of Al and Ga ions near the 31P nuclei at the interface, which they believed gave evidence for bond formation between the nanoparticles. PhillipTaylor

"NMR Observations of ¹³C-Enriched Coenzyme B₁₂ Bound to the Ribonucleotide Reductase from Lactobacillus Leichmannii", Brown, K. L.; Li, J.; Zou, X. Inorg. Chem. 2006, 45, 9172-9174. [15]

In this paper, the cobalt-bond carbon in coenzyme B₁₂ is enriched with ¹³C and titrated with RTPR, a B₁₂ –dependent reductase from Lactobacillus leichammii. The author concludes that ground-state Co-C bond distortion is not utilized for Co-C hemolysis by this enzyme, in that neither the ¹³C-NMR chemical shift nor the ¹H-¹³C coupling constants is significantly changed upon the coenzyme-enzyme binding. Yang Cao

"Comparative NMR Properties of H₂ and HD in Toluene-d₈ and in H₂/HD@C₆₀ Chen, J. Y. C.; Marti, A. A.; Turro, N. J.; Komastu, K.; Murata, Y.; Lawler, R. G. J. Phys. Chem. B 2010, 114, 14689-14695. [16]

The spin-lattice relaxation times (T₁) are measured for H₂ and HD incarcerated in C₆₀. The relaxation times observed were then compared to T₁ measured for mixtures of H₂ and HD. It was found that spin-lattice relaxation occurs at a much faster rate when incarcerated in C₆₀. Peter Christensen

Electron Paramagnetic Resonance Spectroscopy

Electron Paramagnetic Resonance (EPR) spectroscopy is a technique that uses the same principles as those in NMR, but applied to the angular momentum states of electrons rather than nuclei. EPR is also sometime called electron spin resonance (ESR) although this terminology is somewhat outdated. As with NMR, the Zeeman effect only applies if there exists more than one angular momentum state of the system, therefore unparied electrons (such that $S>0$ ) are required to observe an EPR signal. Given that the gyromagnetic ratio of an electron is approximately $10^{3}$ times larger than that of a proton, the Zeeman splitting requires much larger photon energies for similar magnetic fields. EPR therefore generally uses microwaves rather than radiowaves. Although the technique is essentially identical to NMR, the information content of the technique is quite different in many respects given that the origin of observed chemical shifts are very different.

Common uses of EPR in inorganic chemistry

determining the spin state of a compound

Literature examples of EPR in inorganic chemistry

" EPR Studies of SiBNC Preceramic Polymers and Ceramic Employing Isotope Labeling", Y.H. Sehlleier, Y. Akdogan, A. Verhoeven, E. Roduner, and M. Jansen, Chem. Mater., 2008, 20, 7563-7569. doi:10.1021/cm801889w.

This article examines the pyrolysis process of preceramic polymers to ceramic materials. They use isotope labeled EPR, SQUID and NMR to look at the polymers after pyrolysis at a variety of temperatures. It was shown that the unpaired electron spends some time around the ¹³C nuclei as well as the ¹⁵N nuclei from the broadening of the EPR spectra. AmberJuilfs

" EPR Study of the Low-Sping [d³; S=1/2], Jahn-Teller-Active, Dinitrogen Complex of a Molybdenum Trisamido Amine", R.L. McNaughton, J.M. Chin, W.W. Weare, R.R. Schrock, and B.M. Hoffman, J. Am. Chem. Soc., 2007, 129, 3480-3482. doi:1021/ja068546u.

This article explores the Mo-N₂ complex which is capable of catalytic turn over, forming ammonia. The Mo-N₂ complex exists as an extremely rare low spin d³ complex, S=1/2. The electronic and vibronic strucutre of the Mo-N₂ complex are described using EPR. TrumanWambach

"Synthesis and Characterization of V^V(3,6-DBSQ)(3,6-DBCat)₂, a d⁰ Metal Complex with Dioxygenase Catalytic Activity", A.M. Morris, C.G. Pierpont, and R.G. Finke, Inorg. Chem., 2009, 48, 3496-3498, doi:10.1021/ic802122q.

This article reports the synthesis of V^V(3,6-DBSQ)(3,6-DBCat)₂ (3,6-DBSQ = 3,6-di-tert-butylsemiquinone, 3,6-DBCat = 3,6-di-tert-butylcatechol), which is characterized with X-ray crystallography, UV-Vis and EPR spectroscopy. It was also found that this complex had catalytic activity towards dioxygenase. Using EPR, it was shown that the 3,6-DBSQ radical couples to the ⁵¹V center as well as two protons form the SQ ring, which provides supporting evidence of chemical structure. JackyYim

"Hopping of Thiolate Ligands between Au Nanoparticles Revealed by EPR Spectroscopy", M. Zachary and V. Chechik, Angew. Chem. Int. Ed. 2007, 46, 3304-3307, doi.10.1002/anie.200604070 .

The use of EPR spectroscopy in monitoring the exchange of stabilizing ligands between Au nanoparticles is explored in this article. One set of nanoparticle was functionalized by nitroxide spin label, and the particles are left overnight with another set of octanethiol-protected nanoparticles for the ligand exchange to occur. The EPR spectrum of the reaction mixture changes from a broad line to a triplet, as the nitroxide ligands exchange with the octanethiol ligands. Kinetic studies using the EPR spectra have shown that the maximum conversion of the reaction is independent of the initial concentrations, indicating that the rate-determining step of the reaction is the slow desorption of the nitroxide ligand from the surface of the Au nanoparticle. ReneeMan

"Isolation of Dysprosium and Yttrium Complexes of a Three-Electron Reduction Product in the Activation of Dinitrogen, the (N₂)^3- Radical", William J. Evans, Ming Fang, Gael Zucchi, Filipp Furche, Joseph W. Ziller, Ryan M. Hoekstra, Jeffrey I. Zink, J. Am. Chem. Soc., 2009, 131, 11195-11202, doi:10.1021/ja9036753.

EPR spectroscopy is used to show a radical electron existing on a dinitrogen ligand, verifying that the synthesized side-on bridged yttrium complex is indeed the first example of a (N₂)^3- compound. EPR spectroscopy studies were furthered with ¹⁵N₂ coordination, resulting in altered splitting patterns.Lwence

"Spin Trapping of Au-H Intermediate in the Alcohol Oxidation by Supported and Unsupported Gold Catalysts", M. Conte, H. Miyamura, S. Kobayashi, and V. Chechik, J. Am. Chem. Soc., 2009, 131, 7189-7196. doi:10.1021/ja809883c.

The mechanism of alcohol oxidation by supported and unsupported gold catalysts is studied using electron paramagnetic resonance (EPR) spectroscopy and spin trapping. The spin traps DMPO and PBN are nitrones that react with short-lived radicals to produce a more stable spin adduct that is itself a radical. The spin adduct can then be detected using EPR spectroscopy, allowing reaction intermediates to be identified. Isotopic labelling and anaeorbic oxidation experiments are also performed; the authors conclude that the reaction proceeds through a Au-H intermediate, and that the role of O₂ is to regenerate the catalyst as opposed to oxidizing the alcohol. BrianSahli

"In Vitro Monitoring of Poly(ortho ester) Degradation by Electron Paramagnetic Resonance Imaging", S. Capancioni et al., Macromolecules, 2003, 36, 6135-6141. doi:10.1021/ma034365q.

This paper outlines the use of EPR techniques to track the degradation of biodegradable polymers in the body. The polymers were used as drug delivery systems and EPR was used on nitroxide radical labeled polymers to track changes in pH over time during the degradation process. Kimosten

"Two conformations in the human kinesin power stroke defined by X-ray crystallography and EPR spectroscopy", C.V. Sincdelar, M.J. Budny, S. Rice, N. Naber, Nature Structural Biology, 2002, 9, 844. doi:10.1021/jmsb852. AlexandraAnderson

" A Stable and Crystalline Triarylgermyl Radical: Structure and EPR Spectra", Christian Drost, Jan Griebel, Reinhard Kirmse, Peter L�nnecke, and Joachim Reinhold, Angew. Chem. Int. Ed., 2009, 48, 1962-1965. doi:10.1002/anie.200805328.

In this paper, the author showed the synthesis of the first stable triarylgermyl radical: Ge[3,5-tBu₂-2,6-(EtO)₂C₆H]₃. X-ray crystallography and EPR were used to determine the structure of the radical both in solid state and solution. Studies showed that the radical is planar in solid state and pyramidal in solution.

CuilingXu

"Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments", Morita, Y.; Suzuki, S.; Sato, K.; Takui, T.Nature,2011,3,197-204. [17].

Phenanthrene is a planar molecule in which three benzene rings are fuzed in a triangular fashion. In its neutral ground state phenanthrene has an uneven number of electrons and is therefore a radical. Phenanthrene, and its derivatives belong to a class of molecules known as 'open-shell graphene fragments'. These graphene fragment molecules are of great interest to the study of quantum electronic devices. PeterChristensen

"Isolation of a radical dianion of nitrogen oxide (NO)²⁻", William J. Evans, Ming Fang, Jefferson E. Bates, Filipp Furche, Joseph W. Ziller, Matthew D. Kiesz, Jeffrey I. Zink, Nature Chem. 2010, 2, 644-647, doi:10.1038/nchem.701.

The authors trapped the radical dianion of nitrogen oxide, (NO)^2-, by treating a dinitrogen complex with 1.5 equivalents of NO. EPR analysis indicated that an unpaired electron was present in the product, and its g value (2.0077) is consistent with a product in which this unpaired electron is associated mostly with the ligand, rather than the metal. This formation of the (NO)^2- anion is further supported by the nitrogen coupling constants for the 14N and 15N versions of the product, which are similar to radical anions of aliphatic azo compounds. DFT calculations indicate that the unpaired spin resides mostly on a π* orbital of the NO ligand and that the asymmetry of this orbital likely leads to larger coupling constants than observed for the related (N₂)^3- complex. CatherineChow

"Characterization of the 17-Electron Radical Cation [CpCo( 1,5-COD)]⁺ by ESR Spectroscopy", Teen T. Chin, Laura Inman Sharp, and William E. Geiger, Organometallics. 1995, 14, 1322-1326,[18]

A 17 electron cobalt complex is generated by oxidation of the corresponding 18e complex. An ESR spectrum is obtained at 77 K, ESR parameters are used to determine the metal orbital contributions to the molecular orbital, which contains the single unpaired electron. DineshAluthge

"EPR and DFT studies of the one-electron reduction product of phospholium cations", Prashant Adkine, Thibault Cantat, Eliane Deschamps, Louis Ricard, Nicolas Mezailles, Pascal Le Floch and Michel Geoffroy, Phys. Chem. Chem. Phys 2006, 8, 862-868

Several phospholium cations were synthesized. The one-electron radical products of electrochemical reduction were studied by EPR and theoretical DFT calculations. The radical is observed to be delocalized through the five-membered heterocycle. AndrewPriegert

"Photoexcited Triplet State Properties of Brominated and Nonbrominated Ga(III)-Corroles as Studied by Time-Resolved Electron Paramagnetic Resonance", Linn Wagnert, Roy Rubin, Alexander Berg, Atif Mahammed, Zeev Gross, and Haim Levanon, J. Phys. Chem. B 2010, 114, 14303–14308, [19].

The time-resolved electron paramagnetic resonance was used to test both brominated and non-brominated Ga(III)-corrole complexes, in order to find the change caused by the bromination of the corrole macrocycle. Both measured results and simulated results of two kinds of compounds were shown. And the results were also used to compare with spectra of the other metal complexes. The author believe Ga(III)-corrole shares the same trend trend with recently examined free-base, Al(III), Sb(III), Sb(V), and Sn(IV) complexes. Zhengyu Chen

"Formation, Structure, and EPR Detection of a High Spin Fe^IV-Oxo SpeciesDerived from Either an Fe^III-Oxo or Fe^III-OH Complex", Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188. [20]

A new high spin oxoiron(IV) complex is prepared from a well characterized oxoiron(III) species. This oxoiron(IV) complex gives the first observable EPR features in the parallel-mode EPR spectrum with g-values at 8.19 and 4.06. A simulation for an S =2 species could accurately predict the intensity, line shape, and position of the observed signals, which shows that EPR spectroscopy could serve as a useful tool to investigate high spin oxoiron(IV) complexes. The structure of such complex is also confirmed by X-ray diffraction methods. Yang Cao

"EPR Study of the Photolysis of Methyl- and Adenosylcobinamides in the Presence of Phospine and Pyridine Bases. Evidence for the Need of a Judicious Choice of Irradiation Temperature and Solvent to Assess Ligand Binding", Joao Gomes, Baltazar de Castro, Maria Rangel, Organometallics, 2008, 27, 2536-2543. [21]

Methylcobinamide and adenosylcobinamide, models of coenzyme B₁₂, in the presence of phosphine and pyridine bases in methanol were characterized by EPR. The behaviour of the cobalt(II) fragments formed after irradiation was analyzed, to gain better understaning of the Co-C bond cleavage, and determine the effect of the presence of an axial base on these systems. Caterina Ramogida

"An Iridium(IV) Species, [Cp*Ir(NHC)Cl]⁺, Related to Water-Oxidation Catalyst",Timothy P. Brewster, James D. Blakemore, Nathan D. Schley, Christopher D. Incarvito,Nilay Hazari, Gary W. Brudvig, and Robert H. Crabtree Organometallics, 2011, 30, 965. [22]

The desire to produce carbon neutral fuels has sparked an increase in research focused on finding renewable and sustainable sources of reductive equivalents that can be used to reduce carbon dioxide. To this end the oxidation of water producing oxygen, protons and electrons represents an attractive target. In this paper the authors report an attempt to characterize a highly unstable Cp*Ir(IV) species proposed as an intermediates in the oxidation of water. The authors use EPR as well as cyclic voltammetry to probe the redox behaviour of a series of Ir precatalysts. The authors conclude that a supporting NHC ligand effectively stabilizes the Ir system so that EPR observation of Ir(IV) can be made. This result is consistent with catalytic trials indicating that the NHC stabilized complex is less active presumably due to a longer lived Ir(IV) intermediate. FraserPick

Mössbauer Spectroscopy

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