This chapter reviews different experimental strategies that determine the geometrical arrangement of atoms in space which make up a particular molecule. It explains the choice of techniques to use when dealing with crystalline, liquid, or gas phase samples. It also outlines the structural relationship between functional groups in an organic molecule and the arrangements of ligands around a metal centre in a transition metal complex, including the way in which a cardiovascular drug molecule might bind within the protein cavity in haemoglobin. The chapter looks at the methods that rely on the interaction of photons or electrons with the molecule of interest. It illustrates the use photons that exploit whichever regions of the electromagnetic spectrum are appropriate to.

This chapter presents a brief introduction to rotational and vibrational spectroscopy. It explains how pure rotational spectroscopy can be used in the precise determination of molecular geometry in small molecules and provide a direct means to deduce the shape of larger molecules. It also looks at how vibrational spectroscopy can be used in the measurement of geometrical parameters of gas phase molecules and in providing information about the structure and shape of molecules in the condensed phase. The chapter discusses the important role vibrational spectroscopy plays in characterization and analysis in organic chemistry where it is used typically in tandem with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. It analyses concepts on a molecular scale by exploring how light interacts with the energy levels associated with rotational and vibrational degrees of freedom.

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Cross-linking of ethylene propylene diene monomer (EPDM) rubbers containing different amounts of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) dienes was examined by EPR spectroscopy. The cross-linking was initiated by thermal decomposition of dicumyl peroxide at 440K. The concentration of free radicals increased towards the end of the cross-linking process before reaching a maximum and decaying to zero. This is explained by the spatial confinement of the radicals in the cross-linked rubber, which leads to increased life time and, hence, higher radical concentration at a time when most peroxide has decomposed. The EPR spectra showed the presence of two components: a well-resolved spectrum overlapping a broad line. Both components are assigned to allyl radicals possessing different mobility. The more mobile component is assigned to allyl radicals along the EPDM chains, whereas the immobilised allyl radicals are formed in the cross-links. The stability of the allyl radicals decreases in the order DCPD>ENB>VNB. EPDMs containing two dienes show more persistent radicals than their single-diene counterparts. The most persistent radicals are observed for highly cross-linked (e.g., 28% ENB) or mixed diene EPDMs (e.g., 2.2% DCPD–4.4% ENB); the EPR spectra of free radicals in these systems can be observed for several hours.

We report on the use of EPR spectroscopy and spin trapping technique to detect free radical intermediates formed in the presence of gold nanoparticles. Phosphine- and amine-protected gold nanoparticles were found to initiate air oxidation of organic substrates containing active hydrogen atoms, such as amines and phosphine oxides. Nanoparticles protected by stronger bound ligands (e.g., thiols) were inactive in these reactions. We also found that gold nanoparticles are able to abstract a halogen atom from the halogenated compounds, presumably due to the high affinity of gold metal for halogens. Reaction of Au nanoparticles with chloroform showed an unusual inverse isotope effect. The trichloromethyl spin adduct was observed when Au nanoparticles were mixed with CDCl(3) but not with CHCl(3). This unexpected behaviour suggests that C-H bond breaking is not the rate-determining step in Au-initiated hydrogen abstraction.

A spin-trapping EPR technique has been employed to explore the generation of hydroxyl radicals from reactions between a series of first row transition metal ions and aqueous hydrogen peroxide at pH 10, and with a range of chelating agents (EDTA, DTPMP and the readily biodegradable ligands S,S-EDDS and IDS). In the absence of these chelating agents only Cu(II) generates a significant level of hydroxyl radicals; in their presence with Cu(II) EDTA and IDS give similar behaviour whereas EDDS and DTPMP inhibit hydroxyl radical generation. For Fe(II), EDTA, DTPMP and IDS significantly enhance ( radical)OH production under these conditions whereas EDDS does not. Results from model cellulose damage experiments broadly confirm the findings for copper, though experiments with Fe(II) lead to somewhat contrasting results. Our findings are discussed in terms of binding constants and implications for alkaline peroxygen bleaching systems.

The oxidation of phenols, cinnamic acids and methyl aryl sulfides by hydrogen peroxide, using three catalyst systems, [L2Mn2IV(μ-O)3](PF6)2, L=1,4,7-trimethyl-1,4,7-triazacyclononane; [LMnIV(OMe)3(PF6); and MnII/L/H2O2, have been studied. The results from a combination of spectroscopic and kinetic studies, coupled with Hammett correlations and 18O labelling experiments, suggest that with each system the active oxidant is an electrophilic, mononuclear oxo-manganese (V) species. The influence of additives that can act as co-ligands for the manganese species has been investigated, with a view to controlling the activity/selectivity of the active oxidant. The two-step, sulfide–sulfoxide–sulfone, oxidation shows an unusual switch in the philicity of the active oxidant from electrophilic in the first step to nucleophilic in the second. Mechanisms for the oxidations are proposed.

EPR spectroscopy has been employed to detect directly radicals formed from a variety of polyolefins (PE, PP and EPM) during reaction with peroxide-derived alkoxyl radicals generated by thermolysis. Conditions have been chosen to reflect those employed in polyolefin grafting, degradation and cross-linking. Radical assignment is assisted, in particular, by the recognition of the effects of chirality on the β-proton hyperfine splittings. Quantitative analysis provides information on the selectivity of the initial attack (e.g. methine protons>methylene protons for PP); notable differences in selectivity between alternating EPM and other EPM samples are discussed. The detection of longer-lived allyl radicals detected for PE is explained in terms of alkyl radical disproportionation with subsequent reaction of the product alkene.

Spin-Trapping-Experimente belegen die Entstehung Schwefel-zentrierter Radikale beim Austausch von Triphenylphosphanliganden an Goldnanopartikeln durch Alkanthiole in Gegenwart von Luft (siehe Bild). Als Schlüsselschritt wird die Oxidation des Alkanthiols durch auf den Nanopartikeln adsorbierten Disauerstoff vorgeschlagen. Dass ein solcher Prozess möglich ist, bestätigt die Oxidation von BH4− oder tBuOOH durch Luft mit Au-Nanopartikeln als Katalysator.

UV-Vis, H-1 NMR and EPR (with spin-trapping) spectroscopy, MS and product studies have been used to study the reactions of dihydrazide and monohydrazide esters of oxalic acid in aqueous base (pH 10.5). In the absence of dioxygen, the dihydrazides are stable although the monohydrazide esters are rapidly hydrolysed in a reaction that gives monohydrazide oxalate anions, but not aryl radicals. However, in contrast both types of compound are rapidly degraded by dioxygen to give aryl radicals. The mechanisms involve an initial oxidation of the hydrazide anion by dioxygen followed by hydrolysis to give the corresponding aryl diazene anion. Further oxidation and loss of nitrogen yields aryl radicals. A mechanistic reaction scheme is proposed to account for the role of dioxygen, the fate of the aryl radicals and the products formed. The scope of these reactions as a non-photochemical source of aryl radicals under mild conditions is considered.

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

A series of Au nanoparticles modified with a nitroxide-functionalized ligand was prepared with a range of spin-label coverage. The X-band EPR spectra of frozen solutions of these nanoparticles showed coverage-dependent line-broadening due to dipole-dipole interactions between spin labels. We developed a methodology to analyze such spectra in terms of geometrical features of the nanoparticles (e.g. Au core size and the length of the spin-labeled ligand). Our method is based on the assumption that the spectral line shape is determined by the average distance between nearest-neighboring spin labels adsorbed on the Au particle. Geometrical and statistical analysis then relates this distance to the line shape parameter d(1)/d, which was calibrated using a model system. Application of this methodology to the experimental spectra provided information about the conformation of ligands on the Au surface. We found that, if the spin-labeled ligand is substantially longer than the surrounding protecting layer, it does not adopt a fully stretched conformation but wraps around the particle immediately above the layer of surrounding ligand. Our results also show that the ligands do not adsorb cooperatively on the Au surface.

Reaction of Cu(BF(4))(2).6H(2)O with the N(3)O(2) donor ligand H(2)L (where H(2)L = N-benzyl-N',N''-di-tert-butyl-disalicyl-triaminocyclohexane) results in the formation of a novel Cu(II)L complex, 1. X-Ray crystallography of it shows the Cu(II) centre coordinated by two phenolate oxygens and two imine nitrogens in a distorted square plane with an elongated bond to the amine nitrogen (2.512 A) in the axial position. EPR spectroscopy gives g values of g(1) = 2.277, g(2) = 2.100, g(3) = 2.025, and A(1) = 15.6 mT which are consistent with the distorted square pyramidal coordination environment determined from the X-ray structure. UV/visible and electrochemical analysis of shows that it undergoes two reversible processes assigned to the successive oxidation of the phenolate oxygens to phenoxyl radicals, the first at E((1/2)) = 0.89 V (DeltaE = 81 mV, vs. Ag/AgCl) and the second at E((1/2)) = 1.13V (DeltaE = 84 mV, vs. Ag/AgCl). Chemical oxidation results in the formation of a species, assigned as [1](+)(.) which is EPR silent due to antiferromagnetic coupling between the Cu(II) centre and the bound phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.

The synthesis of templated mesoporous silicas with a narrow pore size distribution can be achieved in a water/acetonitrile/n-dodecylamine system. The benefits of using this system compared to aqueous ethanol to prepare silicas are the much enhanced wall thickness, leading to greater structural stability, as well as a lack of co-surfactancy from acetonitrile, which means that the pore size remains constant over a wide range of solvent compositions. The particles obtained from this route are cylindrical, with partially aligned mesopores, in contrast to those obtained from aqueous ethanol, which are spherical and have a wormhole structure. A further major advantage is that the amine-functionalised materials, formed by direct co- condensation, are nucleophilic, in contrast to those prepared from aqueous ethanol. This latter feature allows for the elaboration of these amine functions to attach a range of more complex functionality, examples of which are given.

A series of Au nanoparticles functionalised with nitroxide spin labels has been prepared and studied by EPR spectroscopy. Samples with low coverage of the spin label were used to investigate the dynamics of the surface-attached labels at different distances from the Au surface. The rotational correlation times of spin labels vary from 10(-10) s to more than 3 x 10(-9) s, depending on the chain length of the label and the surrounding ligand. The samples with higher coverage of the spin label show an increasing contribution of the exchange interaction between nitroxides adsorbed in a close proximity to each other on the same nanoparticle. Quantitative analysis of the EPR spectra of these samples suggests the presence of non-equivalent binding sites on the surface of Au nanoparticles. Additionally, EPR signals of isolated radical pairs were observed at intermediate coverage.

l,4,7-Trimethyl-l,4,7-triazacyclononane (TMTACN), MnSO4 and H2O2, in basic aqueous acetonitrile, is an effective system for the epoxidation of cinnamic acid. The influence of a selection of organic additives, potential co-ligands for the manganese species, on the reactions has been studied by UV–vis spectroscopy and ESI-MS. The mechanism of the most efficient system, with added oxalic acid, has been investigated in more detail using cinnamic acid and seven of its 3- or 4-substituted derivatives. A Hammett correlation of rate data shows that the active oxidant is electrophilic (ρ value −0.63). Oxygen (18O) labelling experiments reveal that H2O2 and not H2O is the source of the oxygen in the epoxide. Possible mechanisms for the reactions are discussed.

Using three readily-prepared free-radicals of DPPH type, with different oxidation potential, it is possible to generate, via one electron transfer or hydrogen-atom abstraction, short-lived radicals, characterised by the EPR spin-trapping technique. PBM DMPO and DEPMPO were employed as spin-traps. The results show that DPPH and its congeners can be successfully employed for generation of short-lived oxygen-, sulfur-, carbon-, nitrogen-, and phosphorus-centered radicals.

The oxidation of phenolic substrates with H2O2 catalysed by [MnIV2(mu-O)3(TMTACN)2](PF6)2 1, (TMTACN, 1,4,7-trimethyl-1,4,7-triazacyclononane) has been investigated by use of ESI mass spectrometry. The role of the phenols as one-electron reductants and as co-ligands in the stabilisation and reaction of an intermediate O=MnV species has been analysed and the presence of a variety of manganese species in solution has been explained. Our results lead to a proposed mechanism for the catalytic oxidation of phenols in this system.

The mechanism of a place-exchange reaction of ligand-protected Au nanoparticles was investigated using diradical disulfide spin labels. Analysis of reaction mixtures using a combination of GPC and EPR allowed us to determine concentration profile and propose a kinetic model for the reaction. In this model, only one branch of the disulfide ligand is adsorbed on the Au surface during exchange; the other branch forms mixed disulfide with the outgoing ligand. The two branches of the disulfide ligand therefore do not adsorb in adjacent positions on the surface of Au nanoparticles; this was ultimately proven by the powder EPR spectra of frozen exchange reaction mixtures. Our data also suggest the presence of different binding sites with different reactivity in the exchange reaction. The most-active sites are likely to be nanoparticle surface defects.