Oxygen-containing gas-phase diatomic trications and tetracations: ReO(z+), NbO(z+) and HfO(z+) (z=3, 4)

Three oxygen-containing gas-phase diatomic trications ReO(3+), NbO(3+) and HfO(3+) as well as the diatomic tetracation NbO(4+) have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO(2+), NbO(2+) and HfO(2+), were produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO(z+) (z≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb(x)O(y) clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO(4+), NbO(3+), NbO(2+), HfO(3+) and HfO(2+) with Atom Probe mass spectrometry, whereas ReO(3+) and ReO(2+) apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO(3+), NbO(3+) and HfO(3+) are long-lived metastable gas-phase species, with bond lengths of 1.61 ?, 1.62 ? and 1.86 ?, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO(4+) to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO(4+) is a weakly bound (dissociation barrier ～0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.     

Dissociation of multiply ionized isocyanic acid through electron impact

The dissociation of singly to triply ionized isocyanic acid (HNCO) has been investigated by two- and three-dimensional covariance mapping techniques through electron impact ionization at an electron energy of 200 eV. The absolute cross sections for the various dissociation channels of up to triply ionized HNCO have been measured. The HNCO dications dissociate mostly into ion pairs, while the HNCO trications dissociate mostly into ion triples, both through all the possible bond cleavages and charge allocations. Some major ion-pair dissociation channels of HNCO2+ are supposed to be sequential dissociation through initial charge separation. The metastable decay traces caused by HNCO(2+)-->H(+)+NCO+ and HNCO(+)-->HCO(+)+N have been observed on the covariance map.     

DMSO complexes of trivalent metal ions: first microsolvated trications outside of group 3

The advent of electrospray ionization source opened the door to generation of multiply charged metal ions complexed with organic molecules. A significant amount of work on ligated dications has appeared over the past decade. In contrast, only several microsolvated tripositive ions have been reported, involving solely the few rare earths with the lowest third ionization energies (IEs) of all elements (<23 eV). Here trications of numerous trivalent metals outside of group 3 are shown to coordinate dimethyl sulfoxide (DMSO), an eminent aprotic solvent. These include both main group elements (Al, Ga, In, Bi) and transition metals (V, Fe, Cr) with the third IE up to 31 eV, which is 22 eV above the IE of DMSO. Fragmentation of M(3+)(DMSO)(n) for these metals (plus La, Yb, and Sc) has been characterized in detail using collision-induced dissociation (CID). A rich, highly element specific dissociation chemistry is observed, including the homolytic C-S cleavage in (+3) charge state and various charge-reduction processes, such as dissociative electron and proton transfer and heterolytic S=O cleavage with and without a concomitant proton transfer. Characteristic sizes for the charge reduction in M(3+)(DMSO)(n) and M(2+)(DMSO)(n) have been measured as a function of the relevant elemental IE. These reveal no intrinsic gap between the stabilities of dication and trication complexes, once the IE is adjusted for. This, in particular, suggests that even microsolvated tetracations may exist.     

Gas-phase diatomic trications of Se2(3+), Te2(3+), and LaF3+

Three gas-phase diatomic trications Se(2) (3+), Te(2) (3+), and LaF(3+) have been produced by Ar(+) ion beam sputtering of Se, Te, and LaF(3) surfaces, respectively. These exotic molecular ions were detected at noninteger m/z values in a magnetic sector mass spectrometer for ion flight times of >/=13 micros that correspond to lower limits of their respective lifetimes. Se(2) (3+) and Te(2) (3+) were unambiguously identified by their characteristic isotopic abundances. Ab initio calculations of the electronic structures of Se(2) (3+), Te(2) (3+), and LaF(3+) show that these molecular trications are metastable with respect to dissociation into fragment ions of Se(2+)+Se(+), Te(2+)+Te(+), and La(2+)+F(+), respectively. Their barrier heights are about 0.49, 0.29, and 0.53 eV, and the equilibrium internuclear distances (bond lengths) are about 0.23, 0.27, and 0.26 nm, respectively. The gas-phase diatomic dications Se(2) (2+) and Te(2) (2+) were also observed and unambiguously identified. They were found to be long-lived metastable molecules as well, whereas LaF(2+) is thermochemically stable.     

Gitonic and distonic alkanonium dications (diprotonated alkane dications C(n)H(2n+4(2+)), n = 1-4)(1)

The structures and stabilities of gitonic and distonic alkanonium dications, i.e., diprotonated alkane dications C(n)H(2n+4)(2+) (n = 1-4), were investigated at the MP4(SDTQ)/6-311G**//MP2/6-31G** level. The global minimum energy structures (2, 4, 7, and 10) of the C(n)H(2n+4)(2+) dications are double C--H protonated alkanes to give structures with two two electron three-center (2e-3c) bonds. Two different dissociation pathways for the dications, viz deprotonation and demethylation, were also computed. Demethylation was found to be the favorable mode of dissociation.     

Synthesis, stabilities, and redox behavior of mono-, di-, and tetracations composed of di(1-azulenyl)methylium units connected to a benzene ring by phenyl- and 2-thienylacetylene spacers. A concept of a cyanine-cyanine hybrid as a stabilized electrochromic system

This paper describes the preparation of two tetracations 4a(4+) and 4b(4+) composed of di(1-azulenyl)methylium units based on a new structural principle of a cyanine-cyanine hybrid for the design of electrochromic materials with two color changes. Di- and monocations 5a(2+), 5b(2+) and 6a+, 6b+ composed of di(1-azulenyl)methylium units were also prepared for the purpose of comparison. The pKR+ values of the tetracations are rather high despite their tetracationic structure, although the stability of these cations decreases with the increase of the number of the existing cation units. The cyclic voltammetry (CV) of these cations revealed the presumed multielectron redox properties. However, the tetracations did not exhibit the idealized electrochemical behavior, in which subsequent two-electron reduction was presumed as the cyanine-cyanine hybrid, probably due to the less effective electrochemical interaction among the positive charges. The scope of the creation of the novel polyelectrochromic materials taking the new structural principle is demonstrated by these examples.     

Ab initio/GIAO-CCSD(T) study of propenoyl (H2C=CH-CO+) and isopentenoyl ((CH3)2C=CH-CO+) cations and their superelectrophilic protonated dications

Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)2C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (Calpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. 13C NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert-butyl dication ((CH3)2CCH4(2+)) were also computed at the same level to compare and explore the effect of the additional charge in dications.     

Novel examples of three-dimensional aromaticity: 1,3-dehydro-silaadamantane dications. A theoretical (DFT,GIAO NMR,NICS) study

A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2(2+)-5(2+) has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the (29)Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10(+), suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2(2+) to 5(2+). Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.     

Fragmentation pathways of [Mg(NH3)n]2+ complexes: electron capture versus charge separation

New experimental results are presented from a detailed study of gas-phase [Mg(NH(3))(n)](2+) complexes and their fragmentation pathways. The reactions examined range from those observed as metastable (unimolecular) decompositions through to collision-induced processes, which have been accessed using a variety of collision gases. Measurements of ion intensity distributions coupled with unimolecular decay studies show that [Mg(NH(3))(4)](2+) not only is the most intense species detected but also sits at a critical boundary between complexes that are unstable with respect to charge separation and those that are sufficiently solvated to be deemed stable on the time scale of the experiment. Metastable fragmentation patterns have been used to provide information on the evolution of solvent structure around the central dication. In addition to highlighting the particular significance of [Mg(NH(3))(4)](2+), these measurements show some evidence to suggest the buildup of structures via a hydrogen-bonded network to give conformers of the form (4+1) and (4+2), respectively. Collision-induced dissociation studies show the ions to exhibit several fragmentation pathways, including the loss of NH(3) and NH(3) + H, which are promoted primarily through electron capture dissociation (ECD). This picture contrasts with the conclusion from a number of earlier studies that collisional activation mainly promotes charge separation. From the results presented it is suggested that electron capture may play a more dominant role in the charge reduction of multiply charged metal-ligand species than had previously been appreciated.     

Novel annulene dications from methylated [2.2]metacyclophane monoenes and [e]-ring benzannelated dimethyldihydropyrene

Tetramethyl- and hexamethyl-substituted [2.2]metacyclophane monoenes (10 and 11) are transformed into their corresponding trans-dimethyldihydroethanophenanthrenium dications (14(2+) and 15(2+)) in FSO(3)H x SbF(5) (4:1) and FSO(3)H x SbF(5) (1:1) with SO(2)ClF or SO(2) as the solvent; these 10 pi-dications are equivalent to the C-4/C-5 diprotonated dications of the 2,7-dimethyl derivative of trans-DMDHP, 3a. The trans-12c,12d-dimethyl-12c,12d-dihydrobenzo[e]pyrene (6) reacts with FSO(3)H/SO(2)ClF under surprisingly mild conditions to give initially a persistent diprotonated dication (6H(2)(2+)) and, subsequently, the oxidation dication (6(2+)); the 6(2+):6H(2)(2+) ratio reaches 4:1 after 1 week at low temperature. Protonation of the anti-metacyclophane (13) was also examined. Charge delocalization mode and tropicity in the resulting dications are gauged via detailed NMR studies at 500 MHz.     

Distinctive unimolecular gas-phase reactivity of [M(en)2]2+ (M=Ni, Cu) dications and their inclusion complexes with the macrocyclic cavitand Cucurbit[8]uril

Electrospray ionization mass spectrometry makes it possible to generate gas-phase bis-ethylenediamine nickel and copper dications, [M(en)(2)](2+) (M = Ni, 1; M = Cu, 2), as well as their {[M(en)(2)]@cuc[8]}(2+) inclusion complexes with the macrocyclic cavitand cucurbit[8]uril (cuc[8]). The unimolecular gas-phase reactivity of these species has been investigated by electrospray ionization tandem mass spectrometry with a quadrupole-time-of-flight configuration. Distinctive fragmentation pathways have been observed for the free and encapsulated [M(en)(2)](2+) (M = Ni, Cu) dications under collision-induced dissociation (CID) conditions. The dications [M(en)(2)](2+) (M = Ni, Cu) dissociate according to several competitive pathways that involve intra-complex hydrogen or electron-transfer processes. Most of these channels are suppressed after encapsulation inside the cucurbit[8]uril macrocycle and, as a consequence, a simplification of the {[M(en)(2)]@cuc[8]}(2+) fragmentation pattern is observed. The results obtained demonstrate that the encapsulation of a coordination complex inside a host molecule can be used to alter the nature of the product ions generated under CID conditions.     

Exchange interaction of 5,5'-(m- and p-phenylene)bis(10-phenyl-5,10-dihydrophenazine) dications and related analogues

[structures: see text] 5,5'-(m-Phenylene)bis[(10-aryl-5,10-dihydrophenazine) dications, 3a2+ and 3b2+, and their p-analogues 4a2+ and 4b2+, were prepared, and their exchange interaction was investigated. The EPR spectra of these dications at 123 K in a butyronitrile matrix showed the population of a triplet state. The temperature dependence of the EPR signal intensity (absolute value(delta m(s)) = 2) showed that these dications had singlet ground states with deltaE(ST)/k(B) = -27 to -21 K for the m-isomer 3(2+) and with deltaE(ST)/k(B) = -10 to -8 K for the p-isomer 4(2+). Theoretical calculation of the exchange interaction J for these dications at the orthogonal torsion angle geometries was carried out for 3a2+ and 4a2+ and for (m- and p-phenylene)bisphenothiazine dications 1(2+) and 2(2+) using the broken-symmetry approach for the singlet states. A good correlation was observed between the calculated J and a MO-energy term in the triplet state, deltaE(TMO) = absolute value(HOMO(alpha) - (HOMO - 1)(alpha)). The calculated J values were negative in the order of 10 K for the m-dications (J/k(B) = -14.7 K for 1(2+), -11.5 K for 3a(2+)), but much smaller negative values were found for the p-isomers (J/k(B) = -0.9 K for 2(2+), -0.8 K for 4a2+). The smaller absolute value(J) values for the p-dications are qualitatively consistent with the experimental deltaE(ST) (2J) values.     

Dications of fluorenylidenes. The relationship between redox potentials and antiaromaticity for meta- and para-substituted diphenylmethylidenefluorenes

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.     

Dications of 3-phenyl-indenylidene dibenzo[a.d]cycloheptene: the role of charge in the antiaromaticity of cationic systems

Dications of 9-(3-phenyl-1H-inden-1-ylidene)-5H-dibenzo[a,d]cycloheptene, 5(2+), were prepared by oxidation with SbF(5) in SO(2)ClF, and their magnetic behavior was compared to dications of 9-(3-phenyl-1H-inden-1-ylidene)-9H-fluorene, 2(2+). The good correlation between the experimental (1)H NMR shifts for the dications that were oxidized cleanly and the chemical shifts calculated by the GAIO method supported the use of the nucleus independent chemical shifts, NICS, to evaluate the antiaromaticity of the indenyl systems of 2(2+)/5(2+) and their unsubstituted parent compounds, 6(2+) and 7(2+), as well as the antiaromaticity of the fluorenyl system of 2(2+)/7(2+) and the aromaticity of the dibenzotropylium system of 5(2+)/6(2+). Antiaromaticity was shown to be directly related to the amount of charge in the antiaromatic systems, with the antiaromatic systems more responsive to changes in the calculated NBO charge than the aromatic systems. The antiaromaticity was also shown to be directly related to the amount of delocalization in the ring system. The aromaticity of the dibenzotropylium system was much less responsive to changes in the amount of charge in the tropylium system, because the aromatic system was much more completely delocalized. Thus, antiaromatic species are more sensitive probes of delocalization than aromatic ones.     

Infrared spectroscopy of divalent zinc and cadmium crown ether systems

The gas-phase structures of transition-metal dication (Zn(2+) and Cd(2+)) complexes with varying sized crown ethers, 12-crown-4 (12c4), 15-crown-5 (15c5), and 18-crown-6 (18c6), are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and quantum mechanical calculations. The measured spectra span the 750-1600 cm(-1) infrared range, utilizing light generated by a free electron laser, and are compared to predicted spectra calculated at the B3LYP/6-311+G(d,p) or B3LYP/Def2TZVP levels of theory. Spectra with the largest and most flexible crown ether, 18c6, indicate that the crown is highly distorted, wrapping in a tight cage-like structure around both dications studied. The 15c5 adopts a folded orientation for the Zn(2+) complex yet is almost planar when complexed with the larger Cd(2+) ion. The Zn(2+)(12c4) spectrum has bands appearing at lower frequencies than the other systems, consistent with an open conformation such that the metal is exposed, lying above the center of mass of the crown ether ring. The open structures of the Zn(2+)(12c4) and Cd(2+)(15c5) complexes have implications for solvent interactions in the condensed phase. The conformation of each metal-crown complex is highly dependent on metal size, charge, and crown ether flexibility, such that a delicate balance of minimizing the metal-oxygen bond lengths but maximizing the oxygen-oxygen distances arises. These competing influences are reflected in both the spectra and lowest-energy conformations.     

Bond-formation versus electron transfer: C-C-coupling reactions of hydrocarbon dications with benzene

The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.     

Nitro and nitroso transformations in superacids

Novel transformations involving the nitro and nitroso groups in superacid media are discussed and summarized. NO₂-diprotonation is a key reaction for nitro PAHs, forming N,N-dihydroxyiminium–arenium dications whereas nitrosoarenes are N,O-diprotonated to form hydroxyiminium–arenium dications. In nitropyrenes a facile cyclization involving the nitro group leads to five- and/or six-membered ring heterocyclic cations. Nitro cyclization also occurs in nitroalkylbenzenes but at higher temperatures. Multinuclear NMR data for the resulting persistent NO₂-diprotonated dications and their cyclized analogs are gathered and the structural requirements to bring about nitro cyclization are assessed. Conjugated nitroalkenes such as nitrostyrene and nitroethene are NO₂-diprotonated to form hydroxyiminium–carbenium dications. β-Heteroatom-substituted nitroethylenes are C,O-diprotonated and subsequently form hydroxynitrilium ions. Nitronate salts and unsubstituted nitroalkanes are activated in superacids via their protonated nitronic acid. β-ethoxycarbonyl-substituted nitroalkanes are activated via the O,O-diprotonated aci-nitro species with further O-protonation to give dioxonium–carbenium trications or via hydroxynitrilium ions. The NMR characteristics for the dihydroxyiminium–carbenium dications and hydroxynitrilium ions from various 2-nitroalkenes and their nucleophile quenching-derived cations are also gathered. The mechanistic aspects emphasizing dicationic (and tricationic) intermediates, their interplay and the potential synthetic benefits of these transformations which greatly extend the chemistry of the nitro group are highlighted.     

Dissociation of multiply ionized alkanes from methane to n-butane due to electron impact

The dissociation of up to triply ionized alkanes from methane to n-butane has been studied through electron impact ionization at an electron energy of 200 eV. The ionic fragments are recorded by a covariance mapping technique using a focusing time-of-flight mass spectrometer. The absolute cross-sections for the numerous dissociation channels of all the alkane monocations and dications, as well as propane and n-butane trications have been obtained. All the alkane dications are unstable and dissociate mostly into ion pairs, in which the production of proton pairs is found to be a major dissociation channel. The ionic products from the dissociation of ethane, propane and n-butane dications have a similar distribution, in which the first few abundant ion species are H⁺, CH₃⁺, C₂H₃⁺ and C₂H₂⁺. The total single ionization cross-sections agree well with the results of the binary-encounter-Bethe method. The distortion of the ion coincidence islands has been explained as due to the deviation from the space focusing condition of the mass spectrometer. Metastable decays, such as C₃H₅²⁺→CH₃⁺+C₂H₂⁺ in propane and C₄H₁₀⁺→C₃H₇⁺+CH₃ in n-butane have been observed and identified using the initial and final angles of the decay traces.     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.     

Acetonitrile complexes of triply charged metal ions: are ligated trications intrinsically more prone to charge reduction than dications?

The development of electrospray has enabled the generation of gas-phase solvated multiply charged metal ions. Complexes involving both protic ligands (water, alcohols) and aprotic ones (e.g., ketones, DMSO, acetonitrile) were found for dications, but trications were compatible with aprotic ligands only. Here, to probe this difference in stability, acetonitrile complexes of metal trications were formed by ESI and collisionally fragmented. Proton transfer, electron transfer, and heterolytic cleavage channels previously found for dications were observed. Characteristic sizes for these processes suggest an intrinsic gap in the stability of dication and trication complexes against proton transfer, but not against electron transfer.