Uncompensated Resistance Research Articles

Correlation of obesity and the novel adipocytokines, leptin and adiponectin with insulin resistance in type 2 diabetes

Back ground: Type 2 Diabetes mellitus is state of uncompensated insulin resistance. But what actually predisposes to it, is still a controversy. Obesity is often marked as a precondition for such insulin resistance. But the degree of association and mode of pathogenesis are grossly debated.Methods: Here we explored the correlation of physical parameters for obesity like BMI (Body Mass Index), abdominal circumference, waist- hip ratio and the adipocytokines like leptin and adiponectin with the degree of insulin resistance (HOMA-IR score) in 167 patients of type 2 Diabetes mellitus.Result: We found significant positive correlation between BMI, abdominal circumference, waist- hip ratio and insulin resistance. Leptin showed significant positive correlation with all the said physical parameters. But, adiponectin showed a negative correlation with abdominal circumference, waist- hip ratio and fasting insulin. Leptin- adiponectin ratio showed a significant positive correlation with insulin resistance.Conclusion: Leptin shoots up with increased adiposity where as adiponectin gets reduced in central obesity which may contribute to insulin resistance. Though neither of leptin and adiponectin showed any significant correlation with insulin resistance individually, a significant positive correlation was found between leptin and adiponectin ratio and HOMA –IR score. Therefore, the ratio can be an authentic predictor of insulin resistance as it takes into account the interplay of both of the adipocytokines for its causation.DOI: http://dx.doi.org/10.3329/bjpp.v30i1.20791 Bangladesh J Physiol Pharmacol 2014; 30(1):29-32

High surface area diamond-like carbon electrodes grown on vertically aligned carbon nanotubes

Electrochemically active diamond-like carbon (DLC) electrodes featuring high specific surface area have been prepared by plasma-enhanced chemical vapour deposition (CVD) onto densely packed forests of vertically aligned multiwall carbon nanotubes (VACNTs). The DLC:VACNT composite film exhibits a complex topography with web like features and ridges generated by partial coalescence of the DLC over the CNT arrays. DLC:VACNT electrodes exhibit low background responses over a large potential window, low uncompensated resistance, as well as low charge-transfer impedance in the presence of ferrocyanide as a redox probe. The interfacial capacitance associated with the DLC:VACNT electrode is in the range of 0.6mFcm−2, a value two orders of magnitude larger than in conventional flat carbon electrodes. DLC films grown onto single-crystal Si(100) under identical conditions resulted in essentially insulating layers. Conducting-atomic force microscopy studies reveal that the film electro-activity does not arise from specific topographic features in the highly corrugated film. The ensemble of experimental results suggests that the enhanced electrochemical responses are not connected to areas in which the CNT support is exposed to the electrolyte solution. This is remarkable behaviour considering that no dopants have been included during the DLC film growth.

In situ freezing point determination of cryolite baths utilising resistometer measurements

Now close to 30 years ago, Stephen Fletcher developed an instrument that measures uncompensated resistance simultaneously with electrochemical measurements and referred to that instrument as a “resistometer”. In this current work, the resistometer is modified to simultaneously measure the resistance (between the cathode and anode) and the capacitance (which is a combination of the interfacial capacitance of the cathode in series with the interfacial capacitance of the anode) in a Hall–Heroult simulator cell during cooling of a molten cryolite bath. A range of experiments with differing amounts of excess AlF3 are carried out to investigate the effect on the freezing points of the bath. As the bath cools through different freezing events (of the multiple components of the bath), the resistance increases and the capacitance decreases giving valuable information about the freezing behaviour of different compositions of cryolite melts, which has implications for low temperature melts and the investigation of the frozen ledge behaviour in Hall–Heroult cells.

Inappropriate use of the quasi-reversible electrode kinetic model in simulation-experiment comparisons of voltammetric processes that approach the reversible limit.

Many electrode processes that approach the "reversible" (infinitely fast) limit under voltammetric conditions have been inappropriately analyzed by comparison of experimental data and theory derived from the "quasi-reversible" model. Simulations based on "reversible" and "quasi-reversible" models have been fitted to an extensive series of a.c. voltammetric experiments undertaken at macrodisk glassy carbon (GC) electrodes for oxidation of ferrocene (Fc(0/+)) in CH3CN (0.10 M (n-Bu)4NPF6) and reduction of [Ru(NH3)6](3+) and [Fe(CN)6](3-) in 1 M KCl aqueous electrolyte. The confidence with which parameters such as standard formal potential (E(0)), heterogeneous electron transfer rate constant at E(0) (k(0)), charge transfer coefficient (α), uncompensated resistance (Ru), and double layer capacitance (CDL) can be reported using the "quasi-reversible" model has been assessed using bootstrapping and parameter sweep (contour plot) techniques. Underparameterization, such as that which occurs when modeling CDL with a potential independent value, results in a less than optimal level of experiment-theory agreement. Overparameterization may improve the agreement but easily results in generation of physically meaningful but incorrect values of the recovered parameters, as is the case with the very fast Fc(0/+) and [Ru(NH3)6](3+/2+) processes. In summary, for fast electrode kinetics approaching the "reversible" limit, it is recommended that the "reversible" model be used for theory-experiment comparisons with only E(0), Ru, and CDL being quantified and a lower limit of k(0) being reported; e.g., k(0) ≥ 9 cm s(-1) for the Fc(0/+) process.

Theory of Double Potential Step Chronoamperometry at Rough Electrodes: Reversible Redox Reaction and Ohmic Effects

Theory is developed for the DPSC (double potential step chronoamperometry) response for finite fractal and nonfractal electrode roughness with and without uncompensated solution resistance. Mathematical equation for the statistically averaged current transient and its relation to surface structure factor of random roughness is highlighted for reversible charge transfer. Result shows the enhancement of current transient upto outer crossover time for the rough electrode. Current response after application of each potential steps can show upto three regimes, i.e. short, intermediate and long time regimes. In short time, when diffusion layer thickness is small, current response is controlled by the roughness factor. Crossover between short and intermediate time regimes is controlled by diffusion layer thickness scaled with mean square surface curvature. Current behavior in long time (after outer crossover time), when diffusion layer thickness is large compared to width of roughness, becomes identical to smooth electrode. Crossover between intermediate and long time regimes is controlled by mean square width scaled with diffusion layer thickness. For fractal roughness, current has anomalous power law behavior (in intermediate time regime) while for nonfractal roughness current shows crossover between two regimes. Increase in the ratio of reverse and forward current is usually understood as kinetic complications but here we show that this can also emerge out of electrode roughness. Uncompensated solution resistance correction is also included in our theory to account for realistic experimental conditions. Finally, results are compared with the experimental data of a reversible system in viscous medium.

Correction to Surprising Effect of Uncompensated Resistance on the Cyclic Voltammetric Responses for a Reversible Surface-Confined and Uniformly Accessible Redox Couple

Correction to Surprising Effect of Uncompensated Resistance on the Cyclic Voltammetric Responses for a Reversible Surface-Confined and Uniformly Accessible Redox Couple

A Comparison of Fully Automated Methods of Data Analysis and Computer Assisted Heuristic Methods in an Electrode Kinetic Study of the Pathologically Variable [Fe(CN)6]3–/4– Process by AC Voltammetry

Fully automated and computer assisted heuristic data analysis approaches have been applied to a series of AC voltammetric experiments undertaken on the [Fe(CN)6](3-/4-) process at a glassy carbon electrode in 3 M KCl aqueous electrolyte. The recovered parameters in all forms of data analysis encompass E(0) (reversible potential), k(0) (heterogeneous charge transfer rate constant at E(0)), α (charge transfer coefficient), Ru (uncompensated resistance), and Cdl (double layer capacitance). The automated method of analysis employed time domain optimization and Bayesian statistics. This and all other methods assumed the Butler-Volmer model applies for electron transfer kinetics, planar diffusion for mass transport, Ohm's Law for Ru, and a potential-independent Cdl model. Heuristic approaches utilize combinations of Fourier Transform filtering, sensitivity analysis, and simplex-based forms of optimization applied to resolved AC harmonics and rely on experimenter experience to assist in experiment-theory comparisons. Remarkable consistency of parameter evaluation was achieved, although the fully automated time domain method provided consistently higher α values than those based on frequency domain data analysis. The origin of this difference is that the implemented fully automated method requires a perfect model for the double layer capacitance. In contrast, the importance of imperfections in the double layer model is minimized when analysis is performed in the frequency domain. Substantial variation in k(0) values was found by analysis of the 10 data sets for this highly surface-sensitive pathologically variable [Fe(CN)6](3-/4-) process, but remarkably, all fit the quasi-reversible model satisfactorily.

High Temporal Resolution Bio-Sensing Using Nitrogen-Incorporated Nanodiamond Ultra-Microelectrode Array by Fast Scan Cyclic Voltammetry

Microelectrodes have several benefits over macroelectrodes by virtue of their size. Their advantages include lower capacitance and uncompensated resistance loss, enhanced temporal and spatial resolution, usability in highly resistive media, increased signal to noise ratio. An array of microelectrodes can deliver these benefits along with signal amplification. Neurotransmitters, such as dopamine, undergo rapid fluctuations in concentrations occurring at a sub-second time scale. Real-time monitoring and measurement of these concentration changes, in-vivo or in-vitro, requires the use of ultra-microelectrode arrays (UMEAs). A nitrogen incorporated nanodiamond UMEA was fabricated using silicon microfabrication technology and microwave PECVD process. The array of 2500 UMEs, projects above the SiO2 insulating matrix, thus providing a 3-dimensional surface. Fast scan cyclic voltammetry at 300V/s was used to examine the temporal response and sensitivity for dopamine detection. A large negative holding potential was employed to promote DA pre-concentration in between the voltage scans.

Effect of uncompensated solution resistance on quasireversible charge transfer at rough and finite fractal electrode

Theory for the influence of uncompensated solution resistance on quasi-reversible charge transfer at an arbitrary rough electrode is developed. Detailed model analysis is performed for a finite fractal power spectrum; characterized with fractal dimension, lowest, highest cutoff-length scales of roughness and topothesy length (width) of the interface. The composite effect of the real and apparent kinetics is contained in LHΩ which is summation of diffusion-kinetics (LH) and diffusion-ohmic (LΩ) coupling lengths. The current time response is therefore an interplay of LHΩ, diffusion length, three finite fractal lengths and fractal dimension (manifesting themselves at different times). At short time, diffusion length is smaller than LHΩ, current is proportionate to the ratio of real microscopic area and LHΩ. These phenomenological coupling delays and curtails the onset of the anomalous response. As diffusion length becomes greater than LHΩ, there is an emergence of intermediate anomalous region. However, limiting case of large LHΩ approximately constant current is seen, without dynamic roughness effects.

Elucidating Capacitance and Resistance Terms in Confined Electroactive Molecular Layers

Electrochemical analyses on confined electroactive molecular layers, herein exemplified with electroactive self-assembled monolayers, sample current contributions that are significantly influenced by additional nonfaradaic and uncompensated resistance effects that, though unresolved, can strongly distort redox analysis. Prior work has shown that impedance-derived capacitance spectroscopy approaches can cleanly resolve all contributions generated at such films, including those which are related to the layer dipolar/electrostatic relaxation characteristics. We show herein that, in isolating the faradaic and nonfaradaic contributions present within an improved equivalent circuit description of such interfaces, it is possible to accurately simulate subsequently observed cyclic voltammograms (that is, generated current versus potential patterns map accurately onto frequency domain measurements). Not only does this enable a frequency-resolved quantification of all components present, and in so doing, a full validation of the equivalent circuit model utilized, but also facilitates the generation of background subtracted cyclic voltammograms remarkably free from all but faradaic contributions.

Estimation of electrode kinetic and uncompensated resistance parameters and insights into their significance using Fourier transformed ac voltammetry and e-science software tools

In transient forms of voltammetry, quantitative analysis of electrode kinetics and parameters such as uncompensated resistance (Ru) and double layer capacitance (Cdl) are usually undertaken by comparing experimental and simulated data. Commonly, the skill of the experimentalist is heavily relied upon to decide when a good fit of simulated to experimental data has been achieved. As an alternative approach, it is now shown how data analysis can be based on implementation of e-science software tools. Previously, a standard heuristic data analysis approach applied to the oxidation of ferrocene in acetonitrile (0.1M Bu4NPF6) at a glassy carbon electrode using higher order harmonics available in Fourier transformed ac voltammetry implied that the heterogeneous charge transfer rate constant k0 is ⩾0.25cms−1 with the charge transfer coefficient (α) lying in the range of 0.25–0.75. Application of e-science software tools to the same data set allows a more meaningful understanding of electrode kinetic data to be provided and also offers greater insights into the sensitivity of the IRu (Ohmic drop) on these parameters. For example, computation of contour maps based on a sweep of two sets of parameters such as k0 and Ru or α and k0 imply that α is 0.50±0.05 and that k0 lays in the range 0.2–0.4cms−1 with Ru around 130Ohm. Quantitative evaluation of k0, α and Ru for the quasi-reversible [Fe(CN)6]3-+e⇌[Fe(CN)6]4- process at a glassy carbon electrode in aqueous media is also facilitated by use of e-science software tools. In this case, when used in combination with large amplitude Fourier transformed ac voltammetry, it is found for each harmonic that k0 for the electrode process lies close to 0.010cms−1, α is 0.50±0.05 and Ru is ⩽10Ohm.

Current loss-of-function mutations in the thyrotropin receptor gene: when to investigate, clinical effects, and treatment

Thyroid-stimulating hormone receptor (TSHR) loss-of-function (LOF) mutations lead to a wide spectrum of phenotypes, ranging from severe congenital hypothyroidism (CH) to mild euthyroid hyperthyrotropinemia. The degree of TSH resistance depends on the severity of the impairment of the receptor function caused by the mutation and on the number of mutated alleles In this review data about genotype-phenotype correlation and criteria for clinical work-up will be presented and discussed. Complete TSH resistance due to biallelic LOF TSHR mutations must be suspected in all patients with severe not syndromic CH and severe thyroid hypoplasia diagnosed at birth by neonatal screening. Partial forms of TSH resistance show a more heterogeneous hormonal and clinical pattern . In these cases TSH serum levels are above the upper limit of normal range for the age but with a very variable pattern, free thyroxine (T4) concentrations are within the normal range and thyroid size can be normal or hypoplastic at ultrasound scan. An early substitutive treatment with L-T4 must be mandatory in all patients with severe CH due to complete uncompensated TSH resistance diagnosed at birth by neonatal screening. The usefulness of substitutive treatment appears much more controversial in patients with subclinical hypothyroidism due to partial TSH resistance in whom the increased TSH concentration should be able to compensate the mild functional impairment of the mutant receptor. Together with standard criteria we recommend also an accurate clinical work-up to select patients who are candidates for a LOF TSHR mutation. Conflict of interest:None declared.

A Dielectric Model of Self-Assembled Monolayer Interfaces by Capacitive Spectroscopy

The presence of self-assembled monolayers at an electrode introduces capacitance and resistance contributions that can profoundly affect subsequently observed electronic characteristics. Despite the impact of this on any voltammetry, these contributions are not directly resolvable with any clarity by standard electrochemical means. A capacitive analysis of such interfaces (by capacitance spectroscopy), introduced here, enables a clean mapping of these features and additionally presents a means of studying layer polarizability and Cole-Cole relaxation effects. The resolved resistive term contributes directly to an intrinsic monolayer uncompensated resistance that has a linear dependence on the layer thickness. The dielectric model proposed is fully aligned with the classic Helmholtz plate capacitor model and additionally explains the inherently associated resistive features of molecular films.

Electron Transfer Dynamics of Iridium Oxide Nanoparticles Attached to Electrodes by Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of carboxylated alkanethiolates (-S(CH(2))(n-1)CO(2)(-)) on flat gold electrode surfaces are used to tether small (ca. 2 nm d.) iridium(IV) oxide nanoparticles (Ir(IV)O(X) NPs) to the electrode. Peak potential separations in cyclic voltammetry (CV) of the nanoparticle Ir(IV/III) wave, in pH 13 aqueous base, increase with n, showing that the Ir(IV/III) apparent electron transfer kinetics of metal oxide sites in the nanoparticles respond to the imposed SAM electron transfer tunneling barrier. Estimated apparent electron transfer rate constants (k(app)(0)) for n = 12 and 16 are 9.8 and 0.12 s(-1). Owing to uncompensated solution resistance, k(app)(0) for n = 8 was too large to measure in the potential sweep experiment. For the cathodic scans, coulometric charges under the Ir(IV/III) voltammetric waves were independent of potential scan rate, suggesting participation of all of the iridium oxide redox sites (ca. 130 per NP) in the NPs. These experiments show that it is possible to control and study electron transfer dynamics of electroactive nanoparticles including, as shown by preliminary experiments, that of the electrocatalysis of water oxidation by iridium oxide nanoparticles.

Large amplitude Fourier transformed ac voltammetry at a rotating disc electrode: a versatile technique for covering Levich and flow rate insensitive regimes in a single experiment

The theory for large amplitude Fourier transformed ac voltammetry at a rotating disc electrode is described. Resolution of time domain data into dc and ac harmonic components reveals that the mass transport for the dc component is controlled by convective-diffusion, while the background free higher order harmonic components are flow rate insensitive and mainly governed by linear diffusion. Thus, remarkable versatility is available; Levich behaviour of the dc component limiting current provides diffusion coefficient values and access to higher harmonics allows fast electrode kinetics to be probed. Two series of experiments (dc and ac voltammetry) have been required to extract these parameters; here large amplitude ac voltammetry with RDE methodology is used to demonstrate that kinetics and diffusion coefficient information can be extracted from a single experiment. To demonstrate the power of this approach, theoretical and experimental comparisons of data obtained for the reversible [Ru(NH(3))(6)](3+/2+) and quasi-reversible [Fe(CN)(6)](3-/4-) electron transfer processes are presented over a wide range of electrode rotation rates and with different concentrations and electrode materials. Excellent agreement of experimental and simulated data is achieved, which allows parameters such as electron transfer rate, diffusion coefficient, uncompensated resistance and others to be determined using a strategically applied approach that takes into account the different levels of sensitivity of each parameter to the dc or the ac harmonic.

Electrode Kinetics Associated with Tetracyanoquinodimethane (TCNQ), TCNQ•–, and TCNQ2– Redox Chemistry in Acetonitrile As Determined by Analysis of Higher Harmonic Components Derived from Fourier Transformed Large Amplitude ac Voltammetry

The dc and first to eighth harmonic components derived from Fourier transformed large amplitude ac voltammetry have been used to determine the heterogeneous electron transfer kinetics (k0) of electron transfer processes involving reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ), oxidation and reduction of TCNQ•–, and oxidation of TCNQ2– in acetonitrile. This protocol permits removal of a possible contribution from the cross redox reaction between TCNQ and TCNQ2–. k0 values were determined by comparison of the experimental data with simulations that take into account the effect of uncompensated resistance and double layer capacitance. Values of 0.30 ± 0.05 cm s–1 were obtained at platinum and glassy carbon electrodes for both TCNQ0/•– and TCNQ•–/2– processes when studies were undertaken over a range of concentrations using both reduction and oxidation perspectives. Difficulties associated with measurement of electrode kinetics near the reversible limit and comparisons with literature data are considered.

Chronically Implanted, Nafion-Coated Ag/AgCl Reference Electrodes for Neurochemical Applications

Fast-scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes can be used to measure behaviorally correlated dopamine changes in the extracellular fluid of the brain of freely moving rats. These experiments employ a chronically implanted Ag/AgCl reference electrode. When dopamine measurements are taken 4 days after implantation, there is often a potential shift, typically greater than +0.2 V, in the anodic and cathodic peaks in the cyclic voltammogram for dopamine. In this work, we optimized a method to coat sintered Ag/AgCl reference electrodes with the perfluorinated polymer, Nafion, to prevent this shift. We find that we can stabilize reference electrodes for up to 28 days. Immunohistochemistry of the tissue around the implant site shows extensive glial encapsulation around both bare and Nafion-coated devices. However, the lesion around bare electrodes has a rough texture implying that these cells are strongly adsorbed onto the bare reference electrode, while the lesion around a Nafion-coated electrode shows that cells are more intact implying that they adsorb less strongly. EDS and SEM analysis of the surface of the electrodes confirms this by visualizing a heavy build up of plaques, organic in nature, only on bare electrodes. Impedance spectroscopy indicates no difference between the impedance of bare and Nafion-coated Ag/AgCl electrodes, indicating that glial encapsulation does not lead to an increase in uncompensated resistance between the working and reference electrodes. The electrochemical shift therefore must be due to the unique chemical microenvironment around the reference electrode that alters the chloride equilibrium, a process that the Nafion coating prevents.

An offline correction method for uncompensated series resistance and capacitance artifacts from whole-cell patch clamp recordings of small cells

Whole-cell patch clamp recordings from small cells, such as in Drosophila, suffer from large series resistance (R_e) and capacitive artifacts [1,3,4]. While the series resistance error is caused by only the electrode, the capacitive artifact is caused by both the electrode and membrane capacitances. These artifacts cannot be compensated for with the standard amplifier circuits because of the feedback ringing and other instabilities caused by small cells. Standard electrophysiological measurement procedures for determining ion channel parameters using the voltage-clamp protocol suffer the most from these artifacts because R_e affects the effective membrane voltage on the cell. For instance, a standard electrophysiological technique is to record multiple voltage protocols from multiple cells, average the traces from the same cells, and leak-subtract the average. This cell average trace is then normalized by the estimated capacitance of the cell and can be averaged across cells. During this procedure, an uncompensated R_e perturbs the expected outcome in two ways: Within the same cell, R_e changes during the collection of multiple trials; usually increasing with time because of the cumulative clogging of the electrode tip. Between cells, both electrode and seal leak parameters vary, which affect R_e and the leak that runs through it, both of which affect the membrane voltage, V_m. We propose a simple and straightforward method and a publicly available Matlab implementation to address both of these problems. We use a standard model of electrode resistance and capacitance [2] together with an isopotential neuron model to simulate passive responses during recordings. For each recording, we use voltage clamp steps that exhibit passive responses from the cell to fit the passive parameters of our passive model. This allows estimating passive parameters for each trial and independent subtraction of the artifacts. The success of this method is dependent on having access to non-leak subtracted recordings as otherwise R_e and its effects cannot be estimated. Using this method, we estimated errors from two different voltage clamp datasets. One surprising result was that variable R_e contributes more significantly than the variable capacitance, such that capacitance normalization fails to regularize responses. In summary, we propose this offline series resistance estimation and removal method to complement voltage-clamp experiments and to reduce errors during data analysis procedures.

Surprising Effect of Uncompensated Resistance on the Cyclic Voltammetric Responses for a Reversible Surface-Confined and Uniformly Accessible Redox Couple

A traditional diagnostic for cyclic voltammetric responses is the plot of the peak current i(peak) vs the scan rate |v| or vs |v|(1/2). For a reversible redox couple with redox moieties freely diffusing in bulk solution, a plot of i(peak) vs |v|(1/2) will be linear; for a surface-confined and uniformly accessible reversible couple, a plot of i(peak) vs |v| will be linear; however, if there is significant uncompensated resistance, it is the plot of i(peak) vs |v|(1/2) that will be linear even if the redox species are surface confined. The present work offers a theoretical basis for this result.

Activation Parameters Derived From a Temperature Dependent Large Amplitude ac Voltammetric Study of the Electrode Kinetics of the Cp2M0/+ Redox Couples (M = Fe, Co) at a Glassy Carbon Electrode

Heterogeneous charge transfer rate constants (k°) for the oxidation of ferrocene, Fc (k° = 0.21 cm s(-1) at 20 °C), and the reduction of the cobaltocenium cation, Cc(+) (k° = 0.12 cm s(-1) at 18 °C), in CH(2)Cl(2) (0.5 M Bu(4)NPF(6)) at a glassy carbon (GC) electrode were determined as a function of temperature using the higher harmonic components available in Fourier transformed large amplitude ac voltammetry. The measured k° values lie at the upper end of the analytically useful range available for the acquisition of meaningful kinetic data for these very fast (i.e., close to reversible) processes. Measurements of the kinetics are facilitated by the ac technique because contributions resulting from uncompensated resistance and slow electrode kinetics, both of which also vary with temperature, can be distinguished via their different contributions to individual harmonics. Values of k° measured over the temperature range of -18 to +20 °C for the Fc(0/+) and Cc(+/0) processes gave linear Arrhenius plots yielding free energies of activation (ΔG(‡)) estimates of 17.0 and 18.0 kJ mol(-1), respectively, in excellent agreement with theoretical values derived from Marcus theory. An analogous temperature dependence is indicated in a study of the Fc(0/+) process in acetonitrile (0.1 M Bu(4)NPF(6)), but the greater k° value evident makes the temperature dependence of these parameters more difficult to quantify as departures from reversibility are minimal, even in the higher harmonics.