Band Shape Analysis Research Articles

Molecular modelling studies of side-chain rotation in substituted triazine rings

Molecular modelling computation using the gaussian 94 package of programs was carried out to estimate four different barriers to internal rotation about amine nitrogen to sp 2 carbon bonds for two compounds with triazine rings. Whereas two of the processes appear to retain a planar side-chain nitrogen environment throughout the internal rotation, the other two involve pyramidalisation. In the latter two cases, after the barrier is passed, the process may or may not include a transient inversion at nitrogen. The internal rotation for cyclotriazine systems is discussed in relation to those for aniline and formamide. The four barrier magnitudes calculated for the triazines are in the same order as (though somewhat larger than) those measured by variable-temperature NMR bandshape analysis for a relevant compound.

1H and 19F NMR Investigation of the Reaction of B(C6F5)3 with Water in Toluene Solution

Titrations of B(C6F5)3 (1) with water, in toluene-d8 solution, monitored by 19F and 1H NMR at 196 K, showed first the formation of the adduct [(C6F5)3B(OH2)] (2) and then its stepwise transformation into the two aqua species [(C6F5)3B(OH2)]·H2O (3) and [(C6F5)3B(OH2)]·2H2O (4) containing, respectively, one or two water molecules hydrogen-bonded to the protons of the B-bound water molecule. The NMR data show that in each titration step only two species were present in significant concentration: 1 and 2 up to 1 equiv, 2 and 3 between 1 and 2 equiv, 3 and 4 between 2 and 3 equiv. Above 3 equiv the solutions rapidly attained saturation and phase separation occurred (although there was evidence of interaction of 4 with more water molecules). Titrations at room temperature indicated an analogous stepwise course. Variable-temperature experiments demonstrated water exchange between the different aqua species and between the different water sites in the adducts 3 and 4 (“internal” or B-bound and “external” or H-bound). The rate of these processes increased with the amount of water bonded to B(C6F5)3. The exchange of B-bound water among the different B(C6F5)3 molecules (resulting in the 1 ⇔ 2 interconversion) caused the averaging of the 19F resonances of 1 and 2, above 273 K. Band shape analysis in the temperature range 235−312 K provided the kinetic constants, whose dependence on the concentration revealed a dissociative mechanism (ΔH⧧ 67(2) kJ mol-1, ΔS⧧ 58(7) J mol-1 K-1). For the adduct [(C6F5)3B(OH2)]·H2O (3), four different dynamic processes have been recognized: (i) the exchange of H-bound water among different [(C6F5)3B(OH2)] adducts (the 2 ⇔ 3 exchange) or (ii) among different [(C6F5)3B(OH2)].H2O adducts (the 3 ⇔ 4 exchange), (iii) the exchange between H-bound and B-bound water, (iv) the hopping of H-bound water between the two protons of B-bound water. This process was so fast that an averaged signal for the protons of internal water was observed even at 187 K. The rate of the process (i) increased with the concentration of 2, so that separate 19F and 1H signals for 2 and 3 were observed only in very dilute solutions at the lowest temperatures. Linear plots of the kinetic constants (estimated from 1H NMR spectra in the near fast exchange region, temperature range 188−214 K) vs the concentration of 2 allowed the estimation of the constant for the dissociative pathway (4 orders of magnitude faster than for the exchange of B-bound water) and for the bimolecular pathway [ΔH⧧ 30(2) kJ mol-1, ΔS⧧ 3(10) J mol-1 K-1]. Process (ii) was too fast on the NMR time scale to allow any kinetic investigation. Process (iii) caused the parallel broadening of both the 1H signals of 3 at T > 225 K, with a rate quite close to that of the dissociative exchange of water among different B(C6F5)3 molecules. The activation parameters (ΔH⧧ 55(2) kJ mol-1, ΔS⧧ 7(3) J mol-1 K-1, temperature range 233−273 K) allowed no discrimination between the exchange of an entire water molecule and the mere exchange of protons. Even small amounts of 4 accelerated process (iii), due to the occurrence of two much faster processes: the 3 ⇔ 4 exchange and the exchange between the protons of internal and external water in 4. The study of any kind of water mobility concerning the trihydrate 4 was prevented by the occurrence of proton exchange processes (so fast as to broaden the signals of internal and external water even at 188 K), possibly favored by the acidic dissociation of the protons of the B-bonded water molecule of 4.

Understanding the Optical Band Shape: Coumarin-153 Steady-State Spectroscopy

We have developed a band-shape analysis of optical transitions in polarizable chromophores characterized by large magnitudes of the transition dipole (intense transitions). The model is tested on steady-state spectra of the coumarin-153 optical dye, employing an explicit solvent description accounting for dipole moment, quadrupole moment, and polarizability of the solvent molecules. The calculations are performed for solvents ranging from nondipolar to strongly dipolar. The solvent dependence of both the experimental Stokes shift and the spectral width is satisfactorily reproduced over the whole polarity range. The optical width is shown to demonstrate a qualitatively different solvent dependence for absorption and emission. The solvent-induced absorption width increases with solvent polarity, whereas the solvent-induced emission width passes through a maximum. This is a result of non-Gaussian statistics of the energy gap fluctuations in polarizable/electronically delocalized chromophores. The total (i.e., solvent and vibrational) emission width tends to pass through a broad maximum at low solvent polarities, decreasing with solvent polarity for highly polar solvents. This results from the combined influence of a solvent-induced mixing of the vacuum adiabatic states and a decrease of the vibrational reorganization energy with increasing solvent polarity. The latter effect arises as a result of a coupling of the vibrational and solvent nuclear modes due to the electronic state occupation number difference, making the vibrational reorganization energy solvent-dependent. The study revels a breakdown of the linear relation between the solvent-induced width and Stokes shift. The model suggests that the Franck−Condon factor of intense optical lines should significantly depend on the magnitude of the transition dipole.

Stereochemically non-rigid transition metal complexes of 2,6-bis(phenyliminomethyl)pyridine and derivatives. Crystal structure of fac-[ReBr(CO) 3(PMTFA)] (PMTFA=2,6-pyridylene-bis-(methylene-2-trifluoromethylaniline)

The Schiff bases 2,6-bis(phenyliminomethyl)pyridine and its derivatives (L) form bidentate chelate complexes with transition metals Re I, Pt IV and Pd II of general formulation fac-[ReBr(CO) 3L], fac-[PtXMe 3L] (X=Cl, Br or I), and cis-[Pd( p-CF 3C 6F 4) 2L]. In solution these complexes are stereochemically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotations of the substituted aromatic rings attached to the metal-coordinated imino nitrogen. Rates and activation energies of these internal motions were measured by one-dimensional NMR bandshape analysis and two-dimensional EXSY NMR experiments over a range of solution temperatures. Free energy values, Δ G ‡ (298.15 K), for the fluxional shifts were in the range 76–97 kJ mol −1 with the Pd II complexes having the lowest energies. An X-ray crystal structure of fac-[ReBr(CO) 3PMTFA] (PMTFA=2,6-pyridylene-bis(methylene-2-trifluoroaniline)) showed the pendant imine function to be in an E-conformation with its nitrogen trans to the pyridyl nitrogen.

Solution fluxionality of some pyridine-2,6-dialdehydes, -diketones and -diesters when acting as bidentate ligands in rhenium(I) and platinum(IV) complexes. Crystal structure of [ReBr(CO) 3L] (L=methylethyldipicolinate)

2,6-Disubstituted pyridines, where the substituents are aldehyde, ketone or ester functions, form bidentate chelate complexes with the transition metal moieties fac-Re IX(CO) 3 (X=halogen). 2-Substituted pyridines, where the substituents are aldehyde or ester functions, form similar types of complexes with the isoelectronic transition metal moieties fac-Re IX(CO) 3 and Pt IVXMe 3. The fac-Re IX(CO) 3 complexes of the 2,6-disubstituted pyridine ligands were shown by 1H NMR spectra to undergo metallotropic shifts whereby the Re coordination switches between adjacent ON pairs of the ONO ligand donor set. Rates and activation energies of these fluxional shifts were measured by dynamic NMR bandshape analysis. Magnitudes of Δ G ‡ (298.15 K) were in the range 59–64 kJ mol −1 for the diketone and diester ligands. The dialdehyde ligand, 2,6-pyridinedicarboxaldehyde, formed an appreciably less-stable Re I complex that was highly fluxional and showed a tendency to dissociation at ambient solution temperatures. The unsymmetrical diester ligand, methylethyldipicolinate, formed two distinct Re I complex species in solution, in the approximate abundance ratio of 2:1, the more abundant structure involving coordination to the carbonyl of the ethyl ester function. This particular complex forms exclusively in the solid state and an X-ray crystal structure of [ReBr(CO) 3L] (L=methylethyldipicolinate) is reported.

The conformational diastereomers of 5-substituted-5H-6-chlorodibenzo[a,c]cycloheptene

Conformational diastereomers of 5-substituted-5H-6-chlorodibenzo[a,c]cycloheptene 1 are observed at room temperature in solution. Empirical force field and the AM1 semi-empirical quantum mechanics calculations indicate that the conformation of the cycloheptatriene ring in 1 is a boat (B) form. In the B form, the C-5 substituent can be oriented in equatorial (e) and axial (a) directions. The e–a interchange involves inversion of the biphenyl moiety in the range of 14.1–17.7 kcal mol−1, determined by 1H-NMR band shape analysis. The free energy difference between the e–a forms is not reproduced by the AM1 calculations. The e–a equilibrium in derivatives of 1 depend on the solvent polarity: for 1i the e–a ratio in CDCl3 (28 ∶ 72) changes to (55 ∶ 45) in DMSO-d6.The conformational space of the flexible side chains in all mono-substituted compounds except 1k is searched by molecular mechanics and the AM1 semi-empirical method and the relative heats of formation of all rotamers are considered in estimation of the e–a ratio. Many of the rotamers show an internal hydrogen bond calculated by the AM1 method. The diethylamino derivative of 1 (1l) was found just in the e form. The preferred conformation of 1h in the solid state was found by X-ray crystallography of a single crystal to be e with a strong OH⋯N internal hydrogen bond in the side chain. Dissolution of the 1h single crystals at low temperature and recording the 1H-NMR spectrum at −60 °C show that assignment of the e form as the dominant one at room temperature for 1h is justified.

Intramolecular excited charge-transfer states in donor–acceptor derivatives of naphthalene and azanaphthalenes

Electronic structure and molecular conformation of selected donor–acceptor isomeric derivatives of naphthalene, quinoline and quinazoline as well as a family of isomeric triazanaphthalenes containing N,N-dimethylaniline as an electron donor in the excited charge transfer singlet (1CT) states have been investigated. The mechanism of the 1CT←S0 absorption and 1CT → S0 emission is discussed in terms of the Mulliken–Murrell model of the CT complexes and the Marcus formalism of radiative charge transfer. The determination of the electronic transition dipole moments corresponding to the CT absorption and fluorescence and the band-shape analysis of the corresponding spectra lead to the quantities relevant for the 1CT → S0 charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple LCAO MO description from the properties of individual chromophores.

Thermally Averaged Spectroscopic Parameters of the Weakly Bound Dimers

Spectroscopic parameters in an ensemble of the weakly bound dimers are derived making use of the quantum and classical statistical average over thermally excited rovibrational states. Argon dimers and oxygen dimers are taken as examples. It is shown that thermally averaged characteristics of dimers rapidly change as temperature of a gas rises. In an ensemble of quasi-diatomic tightly bound dimers, the high-temperature limit of the effective dissociation energy amounts to roughly D0/3, where D0 is the dissociation energy from the ground state. It is demonstrated that the effective rotational Beff(T) constant for the oxygen dimers statistically averaged at near room temperature is merely one half of the ground state B0 value. This conclusion is in agreement with recent spectroscopic observations. Rotationally resolved spectroscopic probe of (O2)2 in the slit-nozzle expansion resulted in B0 = 0.095 cm−1, while bandshape analysis of the room-temperature collision-induced absorption in the oxygen fundamental allowed an estimate of Beff ≈ 0.038 cm−1. The role of anisotropy of intermolecular interaction and the formation of various types of metastable dimeric states are discussed.

Synthesis, characterization, and photophysical properties of a series of supramolecular mixed-valence compounds.

The synthesis and characterization of 10 cyano-bridged trinuclear mixed-valence compounds of the form [(NH3)5M-NC-FeII(CN)4-CN-M'(NH3)5]n+ (M = RuIII, OsIII, CrIII, or PtIV; n = 2, 3, or 4) is reported. The electronic spectra of these supramolecular compounds exhibit a single intervalent (IT) absorption band for each nondegenerate Fe-->M/M' transition. The redox potential of the Fe(II) center is shifted more positive with the addition of each coordinated metal complex, while the redox potentials of the pendant metals vary only slightly from their dinuclear counterparts. As a result, the Fe-->M IT bands are blue-shifted from those in the corresponding dinuclear mixed-valence compounds. The energies of these IT bands show a linear correlation with the ground-state thermodynamic driving force, as predicted by classical electron transfer theory. Estimates of the degree of electronic coupling (Hab) between the metal centers using a theoretical analysis of the IT band shapes indicate that most of these values are similar to those for the corresponding dinuclear species. Notable exceptions occur for the Fe-->M IT transitions in Os-Fe-M (M = Cr or Pt). The enhanced electronic coupling in these two species can be explained as a result of excited state mixing between electron transfer and/or ligand-based charge transfer states and an intensity-borrowing mechanism. Additionally, the possibility of electronic coupling between the remote metal centers in the Ru-Fe-Ru species is discussed in order to explain the observation of two closely spaced redox waves for the degenerate Ru(III) acceptors.

A detailed IR study of the order–disorder phase transition of NaNO 2

The infrared spectra of sodium nitrite NaNO 2 have been investigated between 400 and 4000 cm −1 over a wide range of temperature (27–250°C). The results are reported during the order–disorder phase transition (II→I) at temperature about 161°C. The study includes spectral analysis and interpretation of band shape, peak height, band width and band intensities of the internal modes of vibrations as functions of temperature. The present results strongly confirm the phase transformation process from the ordered state II to the disordered state I of NaNO 2 by clear variations obtained in the spectral parameters for the bending modes ν 2 (≈830 cm −1) asymmetric stretching mode ν 3 (≈l270 cm −1) and combination mode ( ν 1+ ν 3) (≈2552 cm −1). The rotational energy barrier for the nitrite group NO 2 is determined for both ordered and disordered phases using ν 2 and ν 3 modes. Also, the phase transition process is found to be activated by an energy of 0.16 eV in the regions of these two modes.

Dynamics of solitary water in benzene and hexafluorobenzene: An infrared and Raman study

Solutions of water very diluted (xw∼10−3 mf) in benzene and hexafluorobenzene have been investigated using vibrational spectroscopy. In these two solvents, it was found that water is essentially in its monomeric form. The band-shape analysis of the infrared and Raman profiles associated with the symmetric v1 and antisymmetric v3 stretching vibrations of the water molecule in benzenic compounds has been performed and compared with the spectral results obtained for water diluted in liquid CCl4, considered here as the standard “inert” solvent. It is found that the reorientational motions of water around its different axes are always more hindered in benzene and hexafluorobenzene than in liquid CCl4. Moreover, the reorientational motion of the main symmetry axis (z) of water is more hindered than that of the y-axis (in the molecular plane). Although the rotational behavior of water appears similar in both aromatic solvents, the shifts in the vibrational frequencies and the change in the vibrational intensities suggest that the solute–solvent interactions in these two solutions are in fact quite different. In benzene, a weak H-bonding type of interaction with water is likely to exist in the liquid state in agreement with previous works. In contrast, in liquid perfluorinated benzene, a Lewis acid–base interaction is more likely to be involved. This conclusion is consistent with our previous ab initio calculations [Y. Danten et al., J. Phys. Chem. 103, 3530 (1999)].

Size analysis of nanocrystals in semiconductor doped silicate glasses with anomalous small-angle x ray and Raman scattering

Semiconductor doped glasses containing CdSxSe1−x nanocrystallites embedded in a silicate glass matrix were investigated. The dimensions of the nanocrystallites are in the range of a few nanometers and vary as a function of a secondary heat treatment. The confinement of such quantum dots for elementary excitations depends strongly on their size. In order to obtain a mean particle size and the size distribution, anomalous small angle x-ray scattering (ASAXS) and low-frequency inelastic Raman scattering measurements were performed. The sizes and the size distributions were evaluated for samples of different mean crystallite radius and composition x. The results of Raman measurements agree well with those of ASAXS, if both the acoustic mode damping across the nanocrystallite–matrix interface and the particle size distribution are taken into account in the Raman band shape analysis. The concentration of nanocrystallites in the glass matrix was determined by using the technique of contrast variation. Scattering curves were recorded at three energies below but close to the K-absorption edge of selenium (12.66 keV) and at 9.64 keV for comparison, which is significantly below the absorption edge.

An NMR and computational study of a lithium amide and ethereal ligand exchange

The activation parameters for the exchange of an ethereal ligand in a chiral lithium amide was determined from full bandshape analysis of the dynamic NMR spectra. For diethyl ether, the activation parameters were deltaH++=11.0 kcalmol(-1) and deltaS++=12.0 cal K(-1)mol(-1). The exchange of the tetrahydrofuran ligand proceeds with a similar activation enthalpy deltaH++=11.2 kcalmol(-1); however, the entropy is close to zero. deltaS++=1.6 cal K(-1)mol(-1). The dissociation of the ethereal ligand was also modeled by means of semiempirical (PM3) and density functional (DFT) methods. The experimental 13C NMR chemical shifts for the carbons of uncoordinated and coordinated ethers were calculated with the GIAO-DFT (B3PW91/6-31G(d)) computational method.

Ion Association and Ion Solvation Effects at the Crystalline−Amorphous Phase Transition in PEO−LiTFSI

From band shape analyses of the strong 740 cm-1 Raman mode of the TFSI anion, we deduce that the crystalline state of the P(EO)nLiTFSI system contains a very small amount of ion pairs (≈7%), most probably belonging to the minor amorphous phase, for the n ≥ 6 range. For all-amorphous samples, we found a small amount of ion pairs for n ≥ 8, but a significant amount (≈24%) for n = 6. This increase in ion pair formation coincides with a decrease in the relative solvation of lithium cations by polymeric ether oxygens, as detected from a careful study of the 863 cm-1 polymer-cation “breathing mode”. We therefore propose that the local ionic structure is preserved during the melting of n ≥ 8 compositions, but that there is a changeover from a predominately ether oxygen lithium coordination to a combined ether oxygen and anionic coordination of the lithium cations for more concentrated samples upon melting.

Intramolecular electron transfer on the vibrational timescale and rate constants estimated by IR absorption band shape analysis

Intramolecular electron transfer on the vibrational timescale and rate constants estimated by IR absorption band shape analysis

Reorganization Parameters of Electronic Transitions in Electronically Delocalized Systems. 2. Optical Spectra

This is a modification of the condensed-phase band shape analysis of optical spectra to treat electronically delocalized systems. It incorporates the dependence of the optical observables on the optical transition dipole. Absorption and emission band shapes are calculated on the basis of adiabatic free energy surfaces defined as the functions of the reorganization parameters invariant to electronic delocalization and the parameter of electronic delocalization. The latter is given through the ratio of the transition and differential chromophore dipoles. The linear response relations commonly used to connect optical observables to reorganization parameters of electron transfer reactions break down for electronically delocalized systems. The optical Stokes shift depends strongly on the delocalization parameter deviating downward from twice the adiabatic reorganization energy with electronic delocalization. The Stokes shift is a rising function of solvent polarity for localized systems. An inverted solvent dependence develops for delocalized complexes: the Stokes shift decreases with solvent polarity. The spectrum of permissible solvent fluctuations is limited from the low-energy side by the adiabatic splitting of the free energy terms inducing narrowing of the emission optical bands compared to the absorption bands.

Ft-Raman Study of Three Synthetic Solid Solutions Formed by Orthorhombic Sulfates: Celestite-Barytes, Barytes-Anglesite and Celestite-Anglesite

In the present work, three different solid solutions whose end-members are the orthorhombic sulfates celestite (SrSO4), barytes (BaSO4) and anglesite (PbSO4) are studied using FT-Raman spectroscopy. Sulfate anion symmetric internal modes have been examined in detail by means of band-shape analysis and component fitting procedures. the symmetric stretching mode v1(A1) changes its wavenumber position linearly with the cationic composition of the samples which further confirms the ideal character of the solid solutions studied. the corresponding full-width at half-height is strongly increased in the central components of the different solid solutions which can be understood as an effect of the positional disorder induced by random cationic substitution. Similar results are observed in the symmetric bending mode, v2 (E). the study of the low frequency spectral region permits one to differentiate translational modes of the sulfate anion, which changes its wavenumber position when the alkali-earth metal ion changes from rotational modes. This permits the tentative band assignment of the anglesite rotational Raman bands at 134 and 152 cm−1, which were previously not assigned.

Infrared absorption and Raman scattering ofOH−andOD−stretching-mode vibrations in alkali fluorides: Isolated defects

The stretching-mode (SM) of ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ defects in LiF, NaF, and KF hosts has been studied with Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy, at temperatures from 4 K to 500 K. In all three hosts the $〈111〉$ orientation of the ${\mathrm{OH}}^{\ensuremath{-}}$ molecular axis is confirmed both by polarized Raman spectra and by stress-induced FTIR dichroism. The latter technique also yields the values of the disc-shaped elastic dipole tensor, aligning the ${\mathrm{OH}}^{\ensuremath{-}}$ parallel to an applied $〈111〉$ uniaxial stress with a Curie law. Accurate band-shape analysis of low-temperature FTIR absorption and ${T}_{2g}$ Raman spectra yields for low concentrations of ${\mathrm{OH}}^{\ensuremath{-}}$ in LiF and KF consistent values of their nearest-neighbor reorientational tunneling parameters $(\ensuremath{\Delta}=1.2$ and $0.6{\mathrm{cm}}^{\ensuremath{-}1},$ respectively). For ${\mathrm{OH}}^{\ensuremath{-}}$ in NaF and for ${\mathrm{OD}}^{\ensuremath{-}}$ in all three hosts spectral substructure from tunneling is undetectable in the narrow Lorentzian-shaped SM lines. The strong temperature broadening that occurs for the SM absorption and ${T}_{2g}$ Raman bands and the smaller broadening of the ${A}_{1g}$ line can be fitted with models for the dephasing of the transition by phonons, considering the presence of isotropic and anisotropic dephasing components. In spite of the very different amplitudes of the ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ SM vibrations, their relative infrared absorption and Raman intensities are found to be very similar; the only exception is an extremely weak oscillator strength of the ${\mathrm{OD}}^{\ensuremath{-}}$ absorption in KF.

Band-Shape Analysis of Delayed Slow-Passage ODMR in the Photoexcited Triplet State of Tryptophan

Delayed slow passage optically detected magnetic resonance (ODMR) band shapes of the triplet state of tryptophan, lysyl-tryptophanyl-lysine, N-acetyl tryptophanamide, and ribonuclease T1 from A. oryzae (RNaseT1) in low-temperature aqueous glass are analyzed using data obtained from independent microwave-induced delayed phosphorescence (MIDP) measurements as fixed input parameters. This method which uses only the band center frequency, Gaussian width, overall amplitude, and baseline position as variables is compared with a previously employed empirical analysis (Wu, J. Q.; Ozarowski, A.; Davis, S. K.; Maki, A. H. J. Phys. Chem. 1996, 100, 11496) that requires many more variables. Comparable quality of fit is found for the new, more rigorous method for each band except for the Tx ↔ Tz (D−E) ODMR transition which (with the exception of RNaseT1) deviates systematically when using the fixed kinetic parameters. The quality of fit of this band becomes comparable to that of the multiparameter empirical method upon introduction of dispersion in kx, which assumes values between −1 and −2 s-1/GHz. The sign and magnitude of this dispersion are consistent with previous kinetic data. Band centers and widths are altered only slightly from those obtained by the earlier empirical analysis.

X-ray diffraction and phosphorus-31 NMR studies of the dynamically disordered 3:2 phenol–triphenylphosphine oxide complex

The 3:2 phenol–triphenylphosphine oxide (phenol–TPPO) adduct was studied by means of X-ray diffraction together with high-resolution solid-state 31P NMR spectroscopy. The X-ray results showed that the crystalline structure, which belongs to the triclinic P[1 with combining macron] space group, involves disorder. There are two molecules of TPPO and three phenol molecules per unit cell. One of the latter is disordered across an apparent inversion centre. Analysis of NMR data in conjunction with the crystal structure allowed the origin of such disorder to be established, showing that it is dynamic in nature. Variable-temperature NMR experiments were performed and a coalescence temperature was found at 247 K. Spectra recorded below this temperature showed two phosphorus signals. The kinetics for the phenol residue exchanging between two different TPPO moieties (two-site exchange with equal populations) were determined. Thermodynamic parameters for the motion were calculated from Eyring plots. For temperatures ranging from 262.9 to 221.5 K, the bandshape analysis technique was used to derive the required data. For lower temperatures, the selective polarisation inversion experiment (SPI) was performed, whilst high temperature values were derived from variable-temperature T1ρ studies. The activation enthalpy (ΔH‡), calculated using the results obtained by bandshape analysis, T1ρ and SPI, was determined as 38 kJ mol−1, while the activation entropy (ΔS‡) was found to be −23 J mol−1 K−1 (assuming the transmission coefficient is ½). Phosphorus-31 shielding tensor anisotropies have been derived for this system by spinning sideband analysis at both fast and slow-exchange limits and it has been shown that the tensor is axially symmetric. Single-crystal experiments show that the symmetry axis of the tensor is along the P[double bond, length half m-dash]O bond (within experimental error).