Ionization of doped helium nanodroplets: residual helium attached to diatomic cations and their clusters

Electron impact ionization of helium nanodroplets containing a dopant, M, can lead to the detection of both M(+) and helium-solvated cations of the type M(+)·He(n) in the gas phase. The observation of helium-doped ions, He(n)M(+), has the potential to provide information on the aftermath of the charge transfer process that leads to ion production from the helium droplet. Here we report on helium attachment to the ions from four common diatomic dopants, M = N(2), O(2), CO, and NO. For experiments carried out with droplets with an average size of 7500 helium atoms, the monomer cations show little tendency to attach and retain helium atoms on their journey out of the droplet. By way of contrast, the corresponding cluster cations, M(n)(+), where n ≥ 2, all show a clear affinity for helium and form He(m)M(n)(+) cluster ions. The stark difference between the monomer and cluster ions is attributed to more effective cooling of the latter in the aftermath of the ionization event.     

Communication: the formation of helium cluster cations following the ionization of helium nanodroplets: influence of droplet size and dopant

The He(n)(+)/He(2)(+) (n ≥ 3) signal ratios in the mass spectra derived from electron impact ionization of pure helium nanodroplets are shown to increase with droplet size, reaching an asymptotic limit at an average droplet size of approximately 50,000 helium atoms. This is explained in terms of a charge hopping model, where on average the positive charge is able to penetrate more deeply into the liquid helium as the droplet size increases. The deeper the point where the charge localizes to form He(2)(+), the greater the likelihood of collisions with the surrounding helium as the ion begins to leave the droplet, thus increasing the probability that helium will be ejected in the form of He(n)(+) (n ≥ 3) cluster ions rather than He(2)(+). The addition of a dopant alters the He(n)(+)/He(2)(+) ratio for small helium droplets, an observation attributed to the potential energy gradient created by the cation-dopant interaction and its effect in drawing the positive charge towards the dopant in the interior of the droplet.     

Electron impact ionization of water-doped superfluid helium nanodroplets: observation of He(H(2)O)(n)(+) clusters

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.     

Path integral Monte Carlo study of 4He clusters doped with alkali and alkali-earth ions

Path integral Monte Carlo calculations of (4)He nanodroplets doped with alkali (Na(+), K(+) and Cs(+)) and alkali-earth (Be(+) and Mg(+)) ions are presented. We study the system at T = 1 K and between 14 and 128 (4)He atoms. For all studied systems, we find that the ion is well localized at the center of the droplet with the formation of a "snowball" of well-defined shells of localized (4)He atoms forming solid-like order in at least the first surrounding shell. The number of surrounding helium shells (two or three) and the number of atoms per shell and the degree of localization of the helium atoms are sensitive to the type of ion. The number of (4)He atoms in the first shell varies from 12 for Na(+) to 18 for Mg(+) and depends weakly on the size of the droplet. The study of the density profile and of the angular correlations shows that the local solid-like order is more pronounced for the alkali ions with Na(+) giving a very stable icosahedral order extending up to three shells.     

Hydration energies in the gas phase of select (MX)mM+ ions, where M+ = Na+, K+, Rb+, Cs+, NH4+ and X- = F-, Cl-, Br-, I-, NO2-, NO3-. Observed magic numbers of (MX)mM+ ions and their possible significance

The sequential hydration energies and entropies with up to four water molecules were obtained for MXM(+) = NaFNa(+), NaClNa(+), NaBrNa(+), NaINa(+), NaNO(2)Na(+), NaNO(3)Na(+), KFK(+), KBrK(+), KIK(+), RbIRb(+), CsICs(+), NH(4)BrNH(4)(+), and NH(4)INH(4)(+) from the hydration equilibria in the gas phase with a reaction chamber attached to a mass spectrometer. The MXM(+) ions as well as (MX)(m)M(+) and higher charged ions such as (MX)(m)M(2)(2+) were obtained with electrospray. The observed trends of the hydration energies of MXM(+) with changing positive ion M(+) or the negative ion X(-) could be rationalized on the basis of simple electrostatics. The most important contribution to the (MXM-OH(2))(+) bond is the interaction of the permanent and induced dipole of water with the positive charge of the nearest-neighbor M(+) ion. The repulsion due to the water dipole and the more distant X(-) has a much smaller effect. Therefore, the bonding in (MXM-OH(2))(+) for constant M and different X ions changes very little. Similarly, for constant X and different M, the bonding follows the hydration energy trends observed for the naked M(+) ions. The sequential hydration bond energies for MXM(H(2)O)(n)(+) decrease with n in pairs, where for n = 1 and n = 2 the values are almost equal, followed by a drop in the values for n = 3 and n = 4, that again are almost equal. The hydration energies of (MX)(m)M(+) decrease with m. The mass spectra with NaCl, obtained with electrospray and observed in the absence of water vapor, show peaks of unusually high intensities (magic numbers) at m = 4, 13, and 22. Experiments with variable electrical potentials in the mass spectrometer interface showed that some but not all of the ion intensity differentiation leading to magic numbers is due to collision-induced decomposition of higher mass M(MX)(m)(+) and M(2)(MX)(m)(2+) ions in the interface. However, considerable magic character is retained in the absence of excitation. This result indicates that the magic ions are present also in the saturated solution of the droplets produced by electrospray and are thus representative of particularly stable nanocrystals in the saturated solution. Hydration equilibrium determinations in the gas phase demonstrated weaker hydration of the magic ion (NaCl)(4)Na(+).     

M+(12-crown-4) supramolecular cations (M+ = Na+, K+, Rb+, and NH4+) within Ni(2-thioxo-1,3-dithiole-4,5-dithiolate)2 molecular conductor

Monovalent cations (M+ = Na+, K+, Rb+, and NH4+) and 12-crown-4 were assembled to new supramolecular cation (SC+) structures of the M+(12-crown-4)n (n = 1 and 2), which were incorporated into the electrically conducting Ni(dmit)2 salts (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate). The Na+, K+, and Rb+ salts are isostructural with a stoichiometry of the M+(12-crown-4)2[Ni(dmit)2]4, while the NH4+ salt has a stoichiometry of NH4+(12-crown-4)[Ni(dmit)2]3(CH3CN)2. The electrical conductivities of the Na+, K+, Rb+, and NH4+ salts at room temperature are 7.87, 4.46, 0.78, and 0.14 S cm-1, respectively, with a semiconducting temperature dependence. The SC+ structures of the Na+, K+, and Rb+ salts have an ion-capturing sandwich-type cavity of M+(12-crown-4)2, in which the M+ ion is coordinated by eight oxygen atoms of the two 12-crown-4 molecules. On the other hand, the NH4+ ion is coordinated by four oxygen atoms of the 12-crown-4 molecule. Judging from the M(+)-O distances, thermal parameters of oxygen atoms, and vibration spectra, the thermal fluctuation of the Na+(12-crown-4)2 structure is larger than those of K+(12-crown-4)2 and Rb+(12-crown-4)2. The SC+ unit with the larger alkali metal cation gave a stress to the Ni(dmit)2 column, and the SC+ structure changed the pi-pi overlap mode and electrically conducting behavior.     

Cation [M = H+, Li+, Na+, K+, Ca2+, Mg2+, NH4+, and NMe4+] interactions with the aromatic motifs of naturally occurring amino acids: a theoretical study

Ab initio (HF, MP2, and CCSD(T)) and DFT (B3LYP) calculations were done in modeling the cation (H(+), Li(+), Na(+), K(+), Ca(2+), Mg(2+), NH(4)(+), and NMe(4)(+)) interaction with aromatic side chain motifs of four amino acids (viz., phenylalanine, tyrosine, tryptophan and histidine). As the metal ion approaches the pi-framework of the model systems, they form strongly bound cation-pi complexes, where the metal ion is symmetrically disposed with respect to all ring atoms. In contrast, proton prefers to bind covalently to one of the ring carbons. The NH(4)(+) and NMe(4)(+) ions have shown N-H...pi interaction and C-H...pi interaction with the aromatic motifs. The interaction energies of N-H...pi and C-H...pi complexes are higher than hydrogen bonding interactions; thus, the orientation of aromatic side chains in protein is effected in the presence of ammonium ions. However, the regioselectivity of metal ion complexation is controlled by the affinity of the site of attack. In the imidazole unit of histidine the ring nitrogen has much higher metal ion (as well as proton) affinity as compared to the pi-face, facilitating the in-plane complexation of the metal ions. The interaction energies increase in the order of 1-M < 2-M < 3-M < 4-M < 5-M for all the metal ion considered. Similarly, the complexation energies with the model systems decrease in the following order: Mg(2+) > Ca(2+) > Li(+) > Na(+) > K(+) congruent with NH(4)(+) > NMe(4)(+). The variation of the bond lengths and the extent of charge transfer upon complexation correlate well with the computed interaction energies.     

Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.     

Structure and photoabsorption properties of cationic alkali dimers solvated in neon clusters

We present a theoretical investigation of the structure and optical absorption of M(2)(+) alkali dimers (M=Li,Na,K) solvated in Ne(n) clusters for n=1 to a few tens Ne atoms. For all these alkali, the lowest-energy isomers are obtained by aggregation of the first Ne atoms at the extremity of the alkali molecule. This particular geometry, common to other M(2)(+)-rare gas clusters, is intimately related to the shape of the electronic density of the X (2)Σ(g)(+) ground state of the bare M(2)(+) molecules. The structure of the first solvation shell presents equilateral Ne(3) and capped pentagonal Ne(6) motifs, which are characteristic of pure rare gas clusters. The size and geometry of the complete solvation shell depend on the alkali and were obtained at n=22 with a D(4h) symmetry for Li and at n=27 with a D(5h) symmetry for Na. For K, our study suggests that the closure of the first solvation shell occurs well beyond n=36. We show that the atomic arrangement of these clusters has a profound influence on their optical absorption spectrum. In particular, the XΣ transition from the X (2)Σ(g)(+) ground state to the first excited (2)Σ(u)(+) state is strongly blueshifted in the Frank-Condon area.     

Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He(N)) in their 6(2)P(1/2) ((2)Π(1/2)) state upon photo-excitation as well as the immersion of Cs(+) into the He(N) upon photo-ionization via the 6(2)P(1/2) ((2)Π(1/2)) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6(2)P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He(N) is excited with photon energies close to the atomic D(1)-line, which implies an attractive character of the excited state system (Cs?-He(N)) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He(N) in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6(2)P(1/2) ((2)Π(1/2)) to the 6(2)S(1/2) ((2)Σ(1/2)) state help to explain the experimental data. The stability of the Cs?-He(N) system allows to form Cs(+) snowballs in the He(N), where we use the non-desorbing 6(2)P(1/2) ((2)Π(1/2)) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6(2)P(1/2) ((2)Π(1/2)) state gives rise to a very high yield of immersed Cs(+) in contrast to an ionization scheme via the 6(2)P(3/2) ((2)Π(3/2)) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs(2) (+)-He(N) aggregates are observed in time-of-flight mass spectra.     

Communication: Barium ions and helium nanodroplets: Solvation and desolvation

The solvation of Ba(+) ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p (2)P(1∕2) ← 6s (2)S(1∕2) and 6p (2)P(3∕2) ← 6s (2)S(1∕2) transitions of Ba(+) are found to be identical to those recorded in bulk He II [H. J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafer, and A. Winnacker, Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of-flight mass spectra suggest that following the excitation of the solvated Ba(+) ions, these are being ejected from the helium droplets either as bare Ba(+) ions or as small Ba(+)He(n) (n < 20) complexes.     

Cs*He(n) exciplexes in solid 4He

We present a theoretical and experimental study of the laser-induced formation process and of the emission spectra of Cs*He(n) exciplexes in the hcp and bcc phases of solid helium. Two different exciplex molecules are detected: a linear triatomic Cs*He2, which can exist in two electronic states: APi(1/2) and BPi(3/2), and a larger complex, where six or seven He atoms form a ring around a single cesium atom in the 6P(1/2) state. A theoretical model is presented, which allows the interpretation of the experimentally observed spectra.     

Fragmentation of ionized doped helium nanodroplets: theoretical evidence for a dopant ejection mechanism

We report a theoretical study of the effect induced by a helium nanodroplet environment on the fragmentation dynamics of a dopant. The dopant is an ionized neon cluster Ne(n) (+) (n=4-6) surrounded by a helium nanodroplet composed of 100 atoms. A newly designed mixed quantum/classical approach is used to take into account both the large helium cluster zero-point energy due to the light mass of the helium atoms and all the nonadiabatic couplings between the Ne(n) (+) potential-energy surfaces. The results reveal that the intermediate ionic dopant can be ejected from the droplet, possibly with some helium atoms still attached, thereby reducing the cooling power of the droplet. Energy relaxation by helium atom evaporation and dissociation, the other mechanism which has been used in most interpretations of doped helium cluster dynamics, also exhibits new features. The kinetic energy distribution of the neutral monomer fragments can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experiments. The results also reveal the predominance of Ne(2) (+) and He(q)Ne(2) (+) fragments and the absence of bare Ne(+) fragments, in agreement with available experimental data (obtained for larger helium nanodroplets). Moreover, the abundance in fragments with a trimeric neon core is found to increase with the increase in dopant size. Most of the fragmentation is achieved within 10 ps and the only subsequent dynamical process is the relaxation of hot intermediate He(q)Ne(2) (+) species to Ne(2) (+) by helium atom evaporation. The dependence of the ionic fragment distribution on the parent ion electronic state reached by ionization is also investigated. It reveals that He(q)Ne(+) fragments are produced only from the highest electronic state, whereas He(q)Ne(2) (+) fragments originate from all the electronic states. Surprisingly, the highest electronic states also lead to fragments that still contain the original ionic dopant species. A mechanism is conjectured to explain this fragmentation inhibition.     

A quantum dynamical treatment of symmetry-induced kinetic isotope effects in the formation of He2+

Kinetic isotope effects for He(2)(+) formation are calculated quantum dynamically using high-quality Born-Oppenheimer (BO) potentials for two electronic states of He(2)(+) and an accurate treatment of all nonadiabatic BO corrections. The two potentials are coupled only when the helium isotopes are different, and the calculations reveal that this coupling is sufficient to allow the two sets of distinguishable reactants, (4)He(+) + (3)He or (3)He(+) + (4)He, to yield He(2)(+) with comparable efficiency over a wide temperature range. Consequently, the potential coupling provides a significant formation rate enhancement for the low isotopic symmetry reactants, as compared to the symmetrical cases (e.g., (4)He(+) + (4)He or (3)He(+) + (3)He). The computed symmetry-induced kinetic isotope effects (SIKIEs) are in substantial agreement with the available experimental results and represent the first theoretical demonstration of this unusual kinetic phenomenon. Possible application of SIKIE to ozone formation and other chemical systems is discussed.     

Sizes of large He droplets

Helium droplets spanning a wide size range, N(He) = 10(3)-10(10), were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range N(He) = 10(5)-10(7). Moreover, the measurements give the average sizes in the previously uncharacterized range of very large droplets of 10(7)-10(10) atoms. The droplet sizes and beam flux increase rapidly at nozzle temperatures below 6 K, which is ascribed to the formation of droplets within the nozzle interior. The mass spectra of the droplet beam upon electron impact ionization have also been obtained. The spectra show a large increase in the intensity of the He(4) (+) signal upon increase of the droplet size, an effect which can be used as a secondary size standard in the droplet size range N(He) = 10(4)-10(9) atoms.     

Selective binding of monovalent cations to the stacking G-quartet structure formed by guanosine 5'-monophosphate: a solid-state NMR study

We report a solid-state multinuclear ((23)Na, (15)N, (13)C, and (31)P) NMR study on the relative affinity of monovalent cations for a stacking G-quartet structure formed by guanosine 5'-monophosphate (5'-GMP) self-association at pH 8. Two major types of cations are bound to the 5'-GMP structure: one at the surface and the other within the channel cavity between two G-quartets. The channel cation is coordinated to eight carbonyl oxygen atoms from the guanine bases, whereas the surface cation is close to the phosphate group and likely to be only partially hydrated. On the basis of solid-state (23)Na NMR results from a series of ion titration experiments, we have obtained quantitative thermodynamic parameters concerning the relative cation binding affinity for each of the two major binding sites. For the channel cavity site, the values of the free energy difference (Delta G degrees at 25 degrees C) for ion competition between M(+) and Na(+) ions are K(+) (-1.9 kcal mol(-1)), NH(4)(+) (-1.8 kcal mol(-1)), Rb(+) (-0.3 kcal mol(-1)), and Cs(+) (1.8 kcal mol(-1)). For the surface site, the values Delta G degrees are K(+) (2.5 kcal mol(-1)), NH(4)(+) (-1.3 kcal mol(-1)), Rb(+) (1.1 kcal mol(-1)), and Cs(+) (0.9 kcal mol(-1)). Solid-state NMR data suggest that the affinity of monovalent cations for the 5'-GMP structure follows the order NH(4)(+) > Na(+) > Cs(+) > Rb(+) > K(+) at the surface site and K(+) > NH(4)(+) > Rb(+) > Na(+) > Cs(+) > Li(+) at the channel cavity site. We have found that the cation-induced stability of a 5'-GMP structure is determined only by the affinity of monovalent cations for the channel site and that the binding of monovalent cations to phosphate groups plays no role in 5'-GMP self-ordered structure. We have demonstrated that solid-state (23)Na and (15)N NMR can be used simultaneously to provide mutually complementary information about competitive binding between Na(+) and NH(4)(+) ions.     

Competition between pi and non-pi cation-binding sites in aromatic amino acids: a theoretical study of alkali metal cation (Li+, Na+, K+)-phenylalanine complexes

To understand the cation-pi interaction in aromatic amino acids and peptides, the binding of M(+) (where M(+) = Li(+), Na(+), and K(+)) to phenylalanine (Phe) is studied at the best level of density functional theory reported so far. The different modes of M(+) binding show the same order of binding affinity (Li(+)>Na(+)>K(+)), in the approximate ratio of 2.2:1.5:1.0. The most stable binding mode is one in which the M(+) is stabilized by a tridentate interaction between the cation and the carbonyl oxygen (O[double bond]C), amino nitrogen (--NH(2)), and aromatic pi ring; the absolute Li(+), Na(+), and K(+) affinities are estimated theoretically to be 275, 201, and 141 kJ mol(-1), respectively. Factors affecting the relative stabilities of various M(+)-Phe binding modes and conformers have been identified, with ion-dipole interaction playing an important role. We found that the trend of pi and non-pi cation bonding distances (Na(+)-pi>Na(+)-N>Na(+)-O and K(+)-pi>K(+)-N>K(+)-O) in our theoretical Na(+)/K(+)-Phe structures are in agreement with the reported X-ray crystal structures of model synthetic receptors (sodium and potassium bound lariat ether complexes), even though the average alkali metal cation-pi distance found in the crystal structures is longer. This difference between the solid and the gas-phase structures can be reconciled by taking the higher coordination number of the cations in the lariat ether complexes into account.     

Characterization of the cation and temperature dependence of DNA quadruplex hydrogen bond properties using high-resolution NMR

Variations in the hydrogen bond network of the Oxy-1.5 DNA guanine quadruplex have been monitored by trans-H-bond scalar couplings, (h2)J(N2N7), for Na(+)-, K(+)-, and NH(4)(+)-bound forms over a temperature range from 5 to 55 degrees C. The variations in (h2)J(N2N7) couplings exhibit an overall trend of Na(+) > K(+) > NH(4)(+) and correlate with the different cation positions and N2-H2...N7 H-bond lengths in the respective structures. A global weakening of the (h2)J(N2N7) couplings with increasing temperature for the three DNA quadruplex species is accompanied by a global increase of the acceptor (15)N7 chemical shifts. Above 35 degrees C, spectral heterogeneity indicates thermal denaturation for the Na(+)-bound form, whereas spectral homogeneity persists up to 55 degrees C for the K(+)- and NH(4)(+)-coordinated forms. The average relative change of the (h2)J(N2N7) couplings amounts to approximately 0.8 x 10(-3)/K and is thus considerably smaller than respective values reported for nucleic acid duplexes. The significantly higher thermal stability of H-bond geometries in the DNA quadruplexes can be rationalized by their cation coordination of the G-quartets and the extensive H-bond network between the four strands. A detailed analysis of individual (h2)J(N2N7) couplings reveals that the 5' strand end, comprising base pairs G1-G9* and G4*-G1, is the most thermolabile region of the DNA quadruplex in all three cation-bound forms.     

Fourier transform infrared spectroscopy of azide and cyanate ion pairs in AOT reverse micelles

Evidence for ion pair formation in aqueous bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles (RMs) was obtained from infrared spectra of azide and cyanate with Li(+), Na(+), K(+), and NH(4)(+) counterions. The anions' antisymmetric stretching bands near 2000 cm(-1) are shifted to higher frequency (blueshifted) in LiAOT and to a lesser extent in NaAOT, but they are very similar to those in bulk water with K(+) and NH(4)(+) as the counterions. The shifts are largest for low values of w(o) = [water]/[AOT] and approach the bulk value with increasing w(o). The blueshifts are attributed to ion pairing between the anions and the counterions. This interpretation is reinforced by the similar trend (Li(+)>Na(+)>K(+)) for producing contact ion pairs with the metal cations in bulk dimethyl sulfoxide (DMSO) solutions. We find no evidence of ion pairs being formed in NH(4)AOT RMs, whereas ammonium does form ion pairs with azide and cyanate in bulk DMSO. Studies are also reported for the anions in formamide-containing AOT RMs, in which blueshifts and ion pair formation are observed more than in the aqueous RMs. Ion pairs are preferentially formed in confined RM systems, consistent with the well established ideas that RMs exhibit reduced polarity and a disrupted hydrogen bonding network compared to bulk water and that ion-specific effects are involved in mediating the structure of species at interfaces.     

Role of alkali metal cation size in the energy and rate of electron transfer to solvent-separated 1:1 [(M+)(acceptor)] (M+ = Li+, Na+, K+) ion pairs.

The effect of cation size on the rate and energy of electron transfer to [(M(+))(acceptor)] ion pairs is addressed by assigning key physicochemical properties (reactivity, relative energy, structure, and size) to an isoelectronic series of well-defined M(+)-acceptor pairs, M(+) = Li(+), Na(+), K(+). A 1e(-) acceptor anion, alpha-SiV(V)W(11)O(40)(5-) (1, a polyoxometalate of the Keggin structural class), was used in the 2e(-) oxidation of an organic electron donor, 3,3',5,5'-tetra-tert-butylbiphenyl-4,4'-diol (BPH(2)), to 3,3',5,5'-tetra-tert-butyldiphenoquinone (DPQ) in acetate-buffered 2:3 (v/v) H(2)O/t-BuOH at 60 degrees C (2 equiv of 1 are reduced by 1e(-) each to 1(red), alpha-SiV(IV)W(11)O(40)(6-)). Before an attempt was made to address the role of cation size, the mechanism and conditions necessary for kinetically well behaved electron transfer from BPH(2) to 1 were rigorously established by using GC-MS, (1)H, (7)Li, and (51)V NMR, and UV-vis spectroscopy. At constant [Li(+)] and [H(+)], the reaction rate is first order in [BPH(2)] and in [1] and zeroth order in [1(red)] and in [acetate] (base) and is independent of ionic strength, mu. The dependence of the reaction rate on [H(+)] is a function of the constant, K(a)1, for acid dissociation of BPH(2) to BPH(-) and H(+). Temperature dependence data provided activation parameters of DeltaH = 8.5 +/- 1.4 kcal mol(-1) and DeltaS = -39 +/- 5 cal mol(-1) K(-1). No evidence of preassociation between BPH(2) and 1 was observed by combined (1)H and (51)V NMR studies, while pH (pD)-dependent deuterium kinetic isotope data indicated that the O-H bond in BPH(2) remains intact during rate-limiting electron transfer from BPH(2) and 1. The formation of 1:1 ion pairs [(M(+))(SiVW(11)O(40)(5-))](4-) (M(+)1, M(+) = Li(+), Na(+), K(+)) was demonstrated, and the thermodynamic constants, K(M)(1), and rate constants, k(M)(1), associated with the formation and reactivity of each M(+)1 ion pair with BPH(2) were calculated by simultaneous nonlinear fitting of kinetic data (obtained by using all three cations) to an equation describing the rectangular hyperbolic functional dependence of k(obs) values on [M(+)]. Constants, K(M)(1)red, associated with the formation of 1:1 ion pairs between M(+) and 1(red) were obtained by using K(M)(1) values (from k(obs) data) to simultaneously fit reduction potential (E(1/2)) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E(1/2) values on the ratio of thermodynamic constants K(M)(1) and K(M)(1)red. Formation constants, K(M)(1), and K(M)(1)red, and rate constants, k(M)(1), all increase with the size of M(+) in the order K(Li)(1) = 21 < K(Na)(1) = 54 < K(K)(1) = 65 M(-1), K(Li)(1)red = 130 < K(Na)(1)red = 570 < K(K)(1)red = 2000 M(-1), and k(Li)(1) = 0.065 < k(Na)(1) = 0.137 < k(K)(1) = 0.225 M(-1) s(-1). Changes in the chemical shifts of (7)Li NMR signals as functions of [Li(5)1] and [Li(6)1(red)] were used to establish that the complexes M(+)1 and M(+)1(red) exist as solvent-separated ion pairs. Finally, correlation between cation size and the rate and energy of electron transfer was established by consideration of K(M)(1), k(M)(1), and K(M)(1)red values along with the relative sizes of the three M(+)1 pairs (effective hydrodynamic radii, r(eff), obtained by single-potential step chronoamperometry). As M(+) increases in size, association constants, K(M)(1), become larger as smaller, more intimate solvent-separated ion pairs, M(+)1, possessing larger electron affinities (q/r), and associated with larger k(M)(1)() values, are formed. Moreover, as M(+)1 pairs are reduced to M(+)1(red) during electron transfer in the activated complexes, [BPH(2), M(+)1], contributions of ion pairing energy (proportional to -RT ln(K(M)(1)red/K(M)(1)) to the standard free energy change associated with electron transfer, DeltaG degrees (et), increase with cation size: -RT ln(K(M)(1)red/K(M)(1)) (in kcal mol(-1)) = -1.2 for Li(+), -1.5 for Na(+), and -2.3 for K(+).