Theory of surface micelles of semifluorinated alkanes

Surface structures of semifluorinated alkanes F(CF(2))(n)(CH(2))(m)H (referred to as FnHm) spread on the air/water interface are investigated theoretically. The study is focused on the disklike surface micelles that were recently identified by AFM and scattering techniques at sufficiently high surface concentrations. We show that (1) the micelles emerge as a result of liquid/liquid (rather than liquid/gas) phase separation in the Langmuir layer; (2) the micelles are islands of the higher-density phase with roughly vertical orientation of FnHm molecules (F-parts extend toward air, H-parts toward water) and the matrix is the lower density-phase where the FnHm diblocks are nearly parallel to the water surface; (3) the micelles and the hexagonal structure they form are stabilized by the electrostatic interactions which are mainly due to the vertical dipole moments of the CF(2)- CH(2) bonds in the vertical phase; and (4) the electrostatic repulsive interactions can serve to suppress the micelle size polydispersity.     

Synthesis of a uranyl persulfide complex and quantum chemical studies of formation and topologies of hypothetical uranyl persulfide cage clusters

The compound Na(4)[(UO(2))(S(2))(3)](CH(3)OH)(8) was synthesized at room temperature in an oxygen-free environment. It contains a rare example of the [(UO(2))(S(2))(3)](4-) complex in which a uranyl ion is coordinated by three bidentate persulfide groups. We examined the possible linkage of these units to form nanoscale cage clusters analogous to those formed from uranyl peroxide polyhedra. Quantum chemical calculations at the density functional and multiconfigurational wave function levels show that the uranyl-persulfide-uranyl, U-(S(2))-U, dihedral angles of model clusters are bent due to partial covalent interactions. We propose that this bent interaction will favor assembly of uranyl ions through persulfide bridges into curved structures, potentially similar to the family of nanoscale cage clusters built from uranyl peroxide polyhedra. However, the U-(S(2))-U dihedral angles predicted for several model structures may be too tight for them to self-assemble into cage clusters with fullerene topologies in the absence of other uranyl-ion bridges that adopt a flatter configuration. Assembly of species such as [(UO(2))(S(2))(SH)(4)](4-) or [(UO(2))(S(2))(C(2)O(4))(4)](4-) into fullerene topologies with ~60 vertices may be favored by use of large counterions.     

Role of the uranyl oxo group as a hydrogen bond acceptor

Density functional theory calculations have been used to evaluate the geometries and energetics of interactions between a number of uranyl complexes and hydrogen bond donor groups. The results reveal that although traditional hydrogen bond donors are repelled by the oxo group in the [UO(2)(OH(2))(5)](2+) species, they are attracted to the oxo groups in [UO(2)(OH(2))(2)(NO(3))(2)](0), [UO(2)(NO(3))(3)](-), and [UO(2)Cl(4)](2-) species. Hydrogen bond strength depends on the equatorial ligation and can exceed 15 kcal mol(-1). The results also reveal the existence of directionality at the uranyl oxo acceptor, with a weak preference for linear U═O---H angles.     

Electrostatic-gated transport in chemically modified glass nanopore electrodes

Electrostatic-gated transport in chemically modified glass nanopore electrodes with orifice radii as small as 15 nm is reported. A single conical-shaped nanopore in glass, with a approximately 1 microm radius Pt disk located at the pore base, is prepared by etching the exposed surface of a glass-sealed Pt nanodisk. The electrochemical response of the nanopore electrode corresponds to diffusion of redox-active species through the nanopore orifice to the Pt microdisk. Silanization of the exterior glass surface with Cl(Me)(2)Si(CH(2))(3)CN and the interior pore surface with EtO(Me)(2)Si(CH(2))(3)NH(2) introduces pH-dependent ion selectivity at the pore orifice, a consequence of the electrostatic interactions between the redox ions and protonated surface amines. Nanopore electrodes with very small pore orifice radii (< approximately 50 nm) display anion permselectively at pH < 4, as demonstrated by electrochemical measurement of transport through the pore orifice. Ion selective transport vanishes at pH > 6 or when the pore radius is significantly larger than the Debye screening length, consistent with the observed ion selectivity resulting from electrostatic interactions. The ability to introduce different surface functionalities to the interior and exterior surfaces of glass nanopores is demonstrated using fluorescence microscopy to monitor the localized covalent attachment of 5- (and 6)-carboxytetramethylrhodamine succinimidyl ester to interior pore surfaces previously silanized with EtO(Me)(2)Si(CH(2))(3)NH(2).     

Solvation effects on angular distributions in H- (NH3)n and NH2(-) (NH3)n photodetachment: role of solute electronic structure

We report 355 and 532 nm photoelectron imaging results for H(-)(NH(3))(n) and NH(2)(-)(NH(3))(n), n = 0-5. The photoelectron spectra are consistent with the electrostatic picture of a charged solute (H(-) or NH(2)(-)) solvated by n ammonia molecules. For a given number of solvent molecules, the NH(2)(-) core anion is stabilized more strongly than H(-), yet the photoelectron angular distributions for solvated H(-) deviate more strongly from the unsolvated limit than those for solvated NH(2)(-). Hence, we conclude that solvation effects on photoelectron angular distributions are dependent on the electronic structure of the anion, i.e., the type of the initial orbital of the photodetached electron, rather than merely the strength of solvation interactions. We also find evidence of photofragmentation and autodetachment of NH(2)(-)(NH(3))(2-5), as well as autodetachment of H(-)(NH(3))(5), upon 532 nm excitation of these species.     

Hydrogen bonding and multiphonon structure in copper pyrazine coordination polymers

We report a systematic investigation of the temperature-dependent infrared vibrational spectra of a family of chemically related coordination polymer magnets based upon bridging bifluoride (HF(2)-) and terminal fluoride (F-) ligands in copper pyrazine complexes including Cu(HF(2))(pyz)(2)BF(4), Cu(HF(2))(pyz)(2)ClO(4), and CuF(2)(H(2)O)(2)(pyz). We compare our results with several one- and two-dimensional prototype materials including Cu(pyz)(NO(3))(2) and Cu(pyz)(2)(ClO(4))(2). Unusual low-temperature hydrogen bonding, local structural transitions associated with stronger low-temperature hydrogen bonding, and striking multiphonon effects that derive from coupling of an infrared-active fundamental with strong Raman-active modes of the pyrazine building-block molecule are observed. On the basis of the spectroscopic evidence, these interactions are ubiquitous to this family of coordination polymers and may work to stabilize long-range magnetic ordering at low temperature. Similar interactions are likely to be present in other molecule-based magnets.     

Slow relaxation in a one-dimensional rational assembly of antiferromagnetically coupled [Mn4] single-molecule magnets

Four discrete Mn(III)/Mn(II) tetranuclear complexes with a double-cuboidal core, [Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)](ClO(4))(4).2CH(3)CN (1), [Mn(4)(hmp)(6)(H(2)O)(4)](ClO(4))(4).2H(2)O (2), [Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)](ClO(4))(2).4H(2)O (3), and [Mn(4)(hmp)(6)(Hhmp)(2)](ClO(4))(4).2CH(3)CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO(4))(2).6H(2)O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO(3) (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn(2+)-Mn(3+) and Mn(3+)-Mn(3+) magnetic interactions are ferromagnetic in 1-3 leading to an S(T) = 9 ground state for the Mn(4) unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn(4)] core geometry result in an S(T) = 1 ground state in 4. A one-dimensional assembly of [Mn(4)] units, catena-{[Mn(4)(hmp)(6)(N(3))(2)](ClO(4))(2)} (5), was obtained in the same synthetic conditions with the subsequent addition of NaN(3). Double chairlike N(3)(-) bridges connect identical [Mn(4)] units into a chain arrangement. This material behaves as an Ising assembly of S(T) = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Delta(xi) and Delta(tau) were estimated and successfully compared to the experimental values.     

Ab initio and DFT calculations of three-body interactions in chiral mixtures

In order to elucidate the enthalpic stabilization of a 2-methyl-1,4-butanediol system (2M14BD) and a 3-chloro-1,2-propanediol (3C12PDO) system by mixing of each (R)- and (S)-enantiomers, three-body interaction energies are obtained by PW91/6-311G** and MP2/6-311G** level calculations. The differences between homochiral interactions and heterochiral interactions in a 3C12PDO system are found. On the other hand, in 2M14BD systems, very slight differences can be observed between the three-body interaction energies of the three ternary systems. Further, the relationship between excess enthalpies and chiral interactions is discussed.     

Infrared predissociation spectroscopy of M+ (C6H6)(1-4)(H2O)(1-2)Ar(0-1) cluster ions, M = Li, Na

Infrared predissociation (IRPD) spectra of Li(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar(0-1) and Na(+)(C(6)H(6))(2-4)(H(2)O)(1-2)Ar(1) are presented along with ab initio calculations. The results indicate that the global minimum energy structure for Li(+)(C(6)H(6))(2)(H(2)O)(2) has each water forming a π-hydrogen bond with the same benzene molecule. This bonding motif is preserved in Li(+)(C(6)H(6))(3-4)(H(2)O)(2)Ar(0-1) with the additional benzene ligands binding to the available free OH groups. Argon tagging allows high-energy Li(+)(C(6)H(6))(2-4)(H(2)O)(2)Ar isomers containing water-water hydrogen bonds to be trapped and detected. The monohydrated, Li(+) containing clusters contain benzene-water interactions with varying strength as indicated by shifts in OH stretching frequencies. The IRPD spectra of M(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar are very different for lithium-bearing versus sodium-bearing cluster ions emphasizing the important role of ion size in determining the most favorable balance of competing noncovalent interactions.     

Multinuclear NMR spectroscopic and theoretical study on the interactions between diperoxovanadate complex and picoline-like ligands

To understand the substituting effects of organic ligands on the reaction equilibrium, the interactions between diperoxovanadate complex [OV(O(2))(2)(H(2)O)](-) and a series of picoline-like ligands in solution were explored using 1D multinuclear ((1)H, (13)C, and (51)V) magnetic resonance, 2D diffusion ordered spectroscopy (DOSY), and variable temperature NMR in 0.15 mol/l NaCl ionic medium for mimicking the physiological conditions. The order of reactive capability of the picoline-like ligands with [OV(O(2))(2)(H(2)O)](-) is found to be picolinamide>N-methylpicolinamide>methyl picolinate>ethyl picolinate approximately propyl picoliniate>isopropyl picolinate. The substituting group influences the reactivity by either steric effect or electron-donating effect. Competitive coordination interactions result in a series of new seven-coordinated peroxovanadate species [OV(O(2))(2)L](-) (L=picoline-like ligands). Their coordination ways were confirmed by density functional calculations.     

Hydrogen bonded arrays: the power of multiple hydrogen bonds

Hydrogen bond interactions in small covalent model compounds (i.e., deprotonated polyhydroxy alcohols) were measured by negative ion photoelectron spectroscopy. The experimentally determined vertical and adiabatic electron detachment energies for (HOCH(2)CH(2))(2)CHO(-)(2a), (HOCH(2)CH(2))(3)CO(-) (3a), and (HOCH(2)CH(2)CH(OH)CH(2))(3)CO(-) (4a)reveal that hydrogen-bonded networks can provide enormous stabilizations and that a single charge center not only can be stabilized by up to three hydrogen bonds but also can increase the interaction energy between noncharged OH groups by 5.8 kcal mol(-1) or more per hydrogen bond. This can lead to pK(a) values that are very different from those in water and can provide some of the impetus for catalytic processes.     

Creation of coating surfaces possessing superhydrophobic and superoleophobic characteristics with fluoroalkyl end-capped vinyltrimethoxysilane oligomeric nanocomposites having biphenylene segments

Fluoroalkyl end-capped vinyltrimethoxysilane oligomeric nanocomposites having biphenylene units [R(F)-(VM-SiO(2))(n)-R(F)/Ar-SiO(2)] were prepared by the sol-gel reaction of the corresponding oligomer [R(F)-(VM)(n)-R(F)] with 4,4'-bis(triethoxysilyl)-1,1'-biphenyl [Ar-Si(OEt)(3)] under alkaline conditions. R(F)-(VM-SiO(2))(n)-R(F)/Ar-SiO(2) nanocomposites were applied to the surface modification of PMMA to exhibit not only a good oleophobicity imparted by fluorine but also a fluorescent emission ability on the surface. Methanol sol solutions of R(F)-(VM-SiO(2))(n)-R(F)/Ar-SiO(2) nanocomposites were effective for the surface modification of glass through the dipping technique to exhibit good oleophobicity with superhydrophobicity on the modified glass surface. On the other hand, 1,2-dichloroethane sol solutions enabled R(F)-(VM-SiO(2))(n)-R(F)/Ar-SiO(2) nanocomposites to exhibit both superhydrophobic and superoleophobic characteristics on the modified surface through dipping the glass in these sol solutions.     

Effects of Dissipation on Contact Angle Measurements Using a Dynamic Method

We report studies of the effect of hydrothermal treatment on physical properties such as crystalline phase, size, and morphology of nanosized cadmium sulfide (CdS) particles. CdS precipitates have been synthesized by the reaction of Cd(NO(3))(2) with Na(2)S at room temperature. These CdS precipitates have been hydrothermally treated in the range 120-240 degrees C with variation of the treatment time. The effects of acid catalysts and other additives were also investigated. The particles prepared were characterized by XRD, TEM, and BET methods. With increased hydrothermal treatment temperature and time, crystallization from amorphous to crystalline form, cubic or hexagonal, and an increase of particle size occurred. CdS particles of well-developed hexagonal form were obtained at a hydrothermal treatment temperature of 240 degrees C; the primary hexagonal grain size was on the order of 20-30 nm. The addition of an acid catalyst, HCl, or of Cd(NO(3))(2) into the precipitate sol promoted crystal growth and phase transformation during the hydrothermal treatment, but another additive, Na(2)S, showed the opposite trend. It appears that hydrothermal treatment combined with proper additives could be an effective method for preparation of nanosize crystalline CdS particles. Copyright 2001 Academic Press.     

Studies of finite molecular chains: synthesis, structural, magnetic and inelastic neutron scattering studies of hexa- and heptanuclear chromium horseshoes

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et(2)NH}(2){[Et(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)][HO(2)CCMe(3)](2)}(2)] (4), [{[Me(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]2.5 H(2)O}(4)] (5) and [{[iPr(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr...Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetra-horseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A)=-1.1 meV and J(B)=-1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and J(B) is the exchange within the body of the horseshoe (1 meV=8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18 meV. The single-ion anisotropy parameters for Cr(III) were also derived for the two compounds as: for 1, D(Cr)=-0.028 meV and |E(Cr)|=0.005 meV; for 6, D(Cr)=-0.031 meV. Magnetic-field-dependent inelastic neutron scattering experiments on 1 allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12 meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.     

Magic number silicon dioxide-based clusters: laser ablation-mass spectrometric and density functional theory studies

The magic number silica clusters [(SiO(2))(n)O(2)H(3)](-) with n = 4 and 8 have been observed in the XeCl excimer laser (308 nm) ablation of various porous siliceous materials. The structural origin of the magic number clusters has been studied by the density functional theoretical calculation at the B3LYP/6-31G** level, with a genetic algorithm as a supplementary tool for global structure searching. The DFT results of the first magic number cluster are parallel to the corresponding Hartree-Fock results previously reported with only small differences in the structural parameters. Theoretical calculation predicts that the first magic number cluster (SiO(2))(4)O(2)H(4) and its anion [(SiO(2))(4)O(2)H(3)](-) will most probably take pseudotetrahedral cage-like structures. To study the structural properties of the second magic number cluster, geometries of the bare cluster (SiO(2))(8), the neutral complex cluster (SiO(2))(8)O(2)H(4), and the anionic cluster [(SiO(2))(8)O(2)H(3)](-) are fully optimized at the B3LYP/6-31G** level, and the corresponding vibrational frequencies are calculated. The DFT calculations predict that the ground state of the bare silica octamer (SiO(2))(8) has a linear chain structure, whereas the second magic number complex cluster (SiO(2))(8)O(2)H(4) and its anion [(SiO(2))(8)O(2)H(3)](-) are most probably a mixture of cubic cage-like structural isomers with an O atom inside the cage and several quasi-bicage isomers with high intercage interactions. The stabilization of these structures can also be attributed to the active participation of the group of atoms 2O and 4H (3H for the anion) in chemical bonding during cluster formation. Our theoretical calculation gives preliminary structural interpretation of the presence of the first and second magic number clusters and the absence of higher magic numbers.     

Mixed micellar aggregates of nonionic and anionic surfactants with short hydrophobic tails: a microcalorimetric study

Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH(3)-(CH(2))(5)-(OCH(2)-CH(2))(5)OH (C(6)E(5)) in aqueous solution at constant molality of the ionic surfactant CH(3)-(CH(2))(5)-SO(-)(3)Na(+)(C(6)SNa) at 25 degrees C. The experimental data obtained by a stepwise dilution process allowed evaluation of the C(6)E(5) first interaction parameter at several constant molalities of C(6)SNa. The C(6)E(5) critical micelle composition as a function of the C(6)SNa molality was also estimated. The experimental calorimetric data, together with the mixed micelles composition computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the Deltah(Mic) of micellization. The experimental data are compared to those predicted by the ideal solution model and regular solution model of mixed micellization. From a calorimetric study performed on the water-hexanol-C(6)SNa and water-penthaethylene glycol-C(6)SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C(6)E(5)-C(6)SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.     

Dendrimers with a copper(I) bis(phenanthroline) core: synthesis, electronic properties, and kinetics

The copper(I) bis(chelate) complex Cu(L(0))(2) has been prepared from 2,9-diphenethyl-1,10-phenanthroline and Cu(CH(3)CN)(4)BF(4). Derivative Cu(L(0))(2) has been characterized by NMR, UV-vis spectroscopy, and X-ray crystallography. Interestingly, owing to the presence of the ethylene linker, the interligand pi-pi stacking interactions between the phenyl rings and the phenanthroline subunits in Cu(L(0))(2) do not induce significant distortions of the pseudotetrahedral symmetry around the Cu(I) center in the solid state or in solution. Following the synthesis of Cu(L(0))(2), dendrimers Cu(L(1)(-)(4))(2) with a Cu(I) bis(2,9-diphenethyl-1,10-phenanthroline) core surrounded by Fréchet type dendritic branches have been prepared and the kinetics of their cyanide-assisted demetalation studied. Importantly, the surrounding dendritic wedges have no significant influence on the coordination geometry of the Cu(I) center, as deduced from their absorption spectra. Therefore, the variations of the rate constants only reflect changes resulting from the presence of the dendritic branches. The kinetics of the cyanide-mediated demetalation reaction indeed revealed that cyanide diffusion through the dendritic shell is slightly influenced by the size of the branches. Significant effects were observed in the kinetics when going from the third to the fourth generation and have been ascribed to changes in the lipophilicity around the metallic core as a result of dendritic encapsulation.     

Non-localized ligand-to-metal charge transfer excited states in (Cp)2Ti(IV)(NCS)2

The bent d(0) titanium metallocene (Cp)(2)Ti(NCS)(2) exhibits an intense phosphorescence from a ligand-to-metal charge transfer triplet excited state at 77 K in an organic glass substrate and a poly(methyl methacrylate) plastic substrate. Quantum chemical calculations and spectroscopic studies show that the orbital parentage of this triplet state arises from the promotion of an electron from an essentially nonbonding symmetry adapted pi molecular orbital located on the NCS(-) ligands to a d(z)2-(y)2 orbital located on the Ti metal. Standard infrared spectroscopy of (Cp)(2)Ti(NCS)(2) in its ground electronic state at 77 K reveals a pair of closely spaced absorptions at (2072 cm(-1), 2038 cm(-1))(glass) and (2055 cm(-1), 2015 cm(-1))(plastic) that are assigned, respectively, to the symmetric and antisymmetric CN stretching modes of the two coordinated NCS(-) ligands. Low-temperature (77 K) time-resolved infrared spectroscopy that accesses the phosphorescing triplet excited state on the ns time scale shows an IR bleach that is coincident with the two ground state CN stretching bands and an associated grow-in of a pair of new IR bands at slightly lower energies (2059 cm(-1), 2013 cm(-1))(glass) and (2049 cm(-1), 1996 cm(-1))(plastic) that are assigned, respectively, to the symmetric and antisymmetric CN stretches in the emitting triplet state. These transient IR bands decay with virtually identical lifetimes to those observed for the phosphorescence decays when measured under identical experimental conditions. Singular value decomposition analysis of the time-resolved infrared data shows that the observed transient IR features arise from the same electronic manifold as measured through luminescence studies. The close similarity between the ground state and excited-state CN stretching bands in (Cp)(2)Ti(NCS)(2) indicates that symmetry breaking does not occur in forming the charge-transfer triplet excited-state manifold; i.e., electron density is withdrawn from a delocalized pi MO spread across both NCS(-) ligands. Calculations at several levels of theory reveal a delocalized ligand-to-metal charge transfer excited triplet manifold. These calculations closely reproduce the relative intensity ratios and frequencies of the symmetric and antisymmetric transient infrared vibrations in the CN region. This study is the first time-resolved infrared investigation of a ligand-to-metal charge-transfer excited state and the first to be performed at cryogenic temperatures in thin-film organic glass and plastic substrates.     

Self-exchange reaction kinetics of metallocenes revisited: insights from the decamethylferricenium-decamethylferrocene reaction at variable pressure

Rate constants k(ex) and volumes of activation deltaV(ex) have been obtained using (1)H NMR for the self-exchange reaction of the [(eta(5)-C(5)(CH(3))(5))(2)Fe](+) hexafluorophosphate and tetrafluoroborate with [(eta(5)-C(5)(CH(3))(5))(2)Fe] in acetone-d(6) (deltaV(ex) = -8.6 +/- 0.3 cm(3) mol(-)(1)), dichloromethane-d(2), and (semiquantitatively) in acetonitrile-d(3). Under the experimental conditions, ion pairing was significant only in CD(2)Cl(2), but even that produced only a minor reduction in k(ex) and so had a negligible effect on deltaV(ex) ( = -6.4 +/- 0.2 cm(3) mol(-)(1) with PF(6)(-)). In all cases, deltaV(ex) is negative and consistent with a simple two-sphere activation model, rather than with that of Weaver et al. (Nielson, R. M.; McManis, G. E.; Safford, L. K.; Weaver, M. J. J. Phys. Chem. 1989, 93, 2152) in which the barrier crossing rate is limited by solvent dynamics. Similarly, the approximately 5-fold increase in k(ex) on going from [(eta(5)-C(5)H(5))(2)Fe](+/0) to [(eta(5)-C(5)(CH(3))(5))(2)Fe](+/0) in acetone can be explained with the two-sphere model on the basis of the effects of reactant size on the solvent reorganization energy, without reference to solvent dynamics.