Unimolecular and hydrolysis channels for the detachment of water from microsolvated alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+) clusters

We examine theoretically the three channels that are associated with the detachment of a single water molecule from the aqueous clusters of the alkaline earth dications, [M(H₂O) _n ]²⁺, M = Mg, Ca, Sr, Ba, n ≤ 6. These are the unimolecular water loss (M²⁺(H₂O) _n?1 + H₂O) and the two hydrolysis channels resulting the loss of hydronium ([MOH(H₂O) _n?2]⁺ + H₃O⁺) and Zundel ([MOH(H₂O) _n?3]⁺ + H₃O⁺(H₂O)) cations. Minimum energy paths (MEPs) corresponding to those three channels were constructed at the Møller–Plesset second order perturbation (MP2) level of theory with basis sets of double- and triple-ζ quality. We furthermore investigated the water and hydronium loss channels from the mono-hydroxide water clusters with up to four water molecules, [MOH(H₂O) _n ]⁺, 1 ≤ n ≤ 4. Our results indicate the preference of the hydronium loss and possibly the Zundel-cation loss channels for the smallest size clusters, whereas the unimolecular water loss channel is preferred for the larger ones as well as the mono-hydroxide clusters. Although the charge separation (hydronium and Zundel-cation loss) channels produce more stable products when compared to the ones for the unimolecular water loss, they also require the surmounting of high-energy barriers, a fact that makes the experimental observation of fragments related to these hydrolysis channels difficult.     

Repeller-Induced dissociation of alkyl alcohols in thermospray mass spectrometry

Six alkyl alcohols were studied using thermospray mass Spectrometry. Whereas the dominant ion in the spectrum up to a repeller potential of 120 V was [M + NH₄]⁺, above that potential [M + H]⁺ and fragment ions appeared. The fragments observed were largely due to hydrogen release from alkyl ions ([C_nH_2n+1]⁺ – H2 → [C_nH_2n-1]⁺) and loss of water or some other stable molecule from the same species. The results are compared with those from ionization of the same alcohols under electron impact and photoionization conditions and with results obtained for methanol under thermospray conditions.     

Water exchange rates and mechanisms in tetrahedral [Be(H2O)4]2+ and [Li(H2O)4]+ complexes using DFT methods and cluster‐continuum models

The water exchange reactions in aquated Li⁺ and Be²⁺ ions were investigated with density functional theory calculations performed using the [Li(H₂O)₄]⁺·14H₂O and [Be(H₂O)₄]²⁺·8H₂O systems and a cluster‐continuum approach. A range of commonly used functionals predict water exchange rates several orders of magnitude lower than the experimental ones. This effect is attributed to the overstabilization of coordination number four by these functionals with respect to the five‐coordinated transition states responsible for the associative ( A ) or associative interchange ( I_a ) water exchange mechanisms. However, the M06 and M062X functionals provide results in good agreement with the experimental data: M062X/TZVP calculations yield a concerted I_a mechanism for the water exchange in [Be(H₂O)₄]²⁺·8H₂O that gives an average residence time of water molecules in the first coordination sphere of 260 μs. For [Li(H₂O)₄]⁺·14H₂O the water exchange reaction is predicted to follow an A mechanism with a residence time of inner‐sphere water molecules of 25 ps.     

Ammonia in the ion source. II. Clustering with aromatic nitro compounds

Under positive ion chemical ionization conditions with ammonla at relatively low pressure, aromatic nitro compounds do not form [M + H]⁺ ions but often form ionic clusters [M + NH₄]⁺ and [M + N₂H₇]⁺. Nitrobenzene forms a cluster [2M + NH₄]⁺ and aniline, formed by nucleophilic substitution, leads to a cluster [anilinium ion + nitrobenzene]⁺. The dinitrobenzenes form [M + NH₄]⁺ clusters and show evidence of nitroaniline formation and clustering. 1,3,5-Trinitrobenzene gives little indication of clustering or of substitution. The six isomers of trinitrotoluene appear to be stabilized by the methyl group and form clusters up to [M + N₃H₁₀]⁺. Nucleophilic substitution leads to dinitrotoluidines, which also form clusters with ammonium ions.     

A theoretical investigation on the structures of (NH3)·(H2SO4)·(H2O)0-14 clusters

Ternary clusters (NH₃)·(H₂SO₄)·(H₂O)_n have been widely studied. However, the structures and binding energies of relatively larger cluster (n > 6) remain unclear, which hinders the study of other interesting properties. Ternary clusters of (NH₃)·(H₂SO₄)·(H₂O)_n, n = 0-14, were investigated using MD simulations and quantum chemical calculations. For n = 1, a proton was transferred from H₂SO₄ to NH₃. For n = 10, both protons of H₂SO₄ were transferred to NH₃ and H₂O, respectively. The NH₄⁺ and HSO₄⁻ formed a contact ion-pair [NH₄⁺-HSO₄⁻] for n = 1-6 and a solvent separated ion-pair [NH₄⁺-H₂O-HSO₄⁻] for n = 7-9. Therefore, we observed two obvious transitions from neutral to single protonation (from H₂SO₄ to NH₃) to double protonation (from H₂SO₄ to NH₃ and H₂O) with increasing n. In general, the structures with single protonation and solvated ion-pair were higher in entropy than those with double protonation and contact ion-pair of single protonation and were thus preferred at higher temperature. As a result, the inversion between single and double protonated clusters was postponed until n = 12 according to the average binding Gibbs free energy at the normal condition. These results can serve as a good start point for studies of the other properties of these clusters and as a model for the solvation of the [H₂SO₄-NH₃] complex in bulk water.     

Gas phase structure of micro-hydrated [Mn(ClO<Subscript>4</Subscript>)]<Superscript>+</Superscript> and [Mn<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>3</Subscript>]<Superscript>+</Superscript> ions probed by infrared spectroscopy

Gas-phase infrared photodissociation spectroscopy is reported for the microsolvated [Mn(ClO₄)(H₂O) _n ]⁺ and [Mn₂(ClO₄)₃(H₂O) _n ]⁺ complexes from n = 2 to 5. Electrosprayed ions are isolated in an ion-trap where they are photodissociated. The 2600–3800 cm⁻¹ spectral region associated with the OH stretching mode is scanned with a relatively low-power infrared table-top laser, which is used in combination with a CO₂ laser to enhance the photofragmentation yield of these strongly bound ions. Hydrogen bonding is evidenced by a relatively broad band red-shifted from the free OH region. Band assignment based on quantum chemical calculations suggest that there is formation of water—perchlorate hydrogen bond within the first coordination shell of high-spin Mn(II). Although the observed spectral features are also compatible with the formation of structures with double-acceptor water in the second shell, these structures are found relatively high in energy compared with structures with all water directly bound to manganese. Using the highly intense IR beam of the free electron laser CLIO in the 800–1700 cm⁻¹, we were also able to characterize the coordination mode (η²) of perchlorate for two clusters. The comparison of experimental and calculated spectra suggests that the perchlorate Cl—O stretches are unexpectedly underestimated at the B3LYP level, while they are correctly described at the MP2 level allowing for spectral assignment.     

Dynamics and structural changes of small water clusters on ionization

Despite utmost importance in understanding water ionization process, reliable theoretical results of structural changes and molecular dynamics (MD) of water clusters on ionization have hardly been reported yet. Here, we investigate the water cations [(H₂O)_{n = 2–6}⁺] with density functional theory (DFT), Möller–Plesset second‐order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The complete basis set limits of interaction energies at the CCSD(T) level are reported, and the geometrical structures, electronic properties, and infrared spectra are investigated. The characteristics of structures and spectra of the water cluster cations reflect the formation of the hydronium cation moiety (H₃O⁺) and the hydroxyl radical. Although most density functionals fail to predict reasonable energetics of the water cations, some functionals are found to be reliable, in reasonable agreement with high‐level ab initio results. To understand the ionization process of water clusters, DFT‐ and MP2‐based Born‐Oppenheimer MD (BOMD) simulations are performed on ionization. On ionization, the water clusters tend to have an Eigen‐like form with the hydronium cation instead of a Zundel‐like form, based on reliable BOMD simulations. For the vertically ionized water hexamer, the relatively stable (H₂O)₅⁺ (5sL4A) cluster tends to form with a detached water molecule (H₂O). © 2013 Wiley Periodicals, Inc.     

Controlled Aggregation of Multiple Guanidinium Ions through a Hydrogen‐Bonded Network Assembly with Deprotonated Forms of Kemp’s Triacid

A series of five complexes that incorporate the guanidinium ion and various deprotonated forms of Kemp’s triacid (H₃KTA) have been synthesized and characterized by single‐crystal X‐ray analysis. The complex [C(NH₂)₃⁺] ? [H₂KTA^?] ( 1 ) exhibits a sinusoidal layer structure with a centrosymmetric pseudo‐rosette motif composed of two ion pairs. The fully deprotonated Kemp’s triacid moiety in 3 [C(NH₂)₃⁺] ? [KTA^3?] ( 2 ) forms a record number of eighteen acceptor hydrogen bonds, thus leading to a closely knit three‐dimensional network. The KTA^3? anion adopts an uncommon twist conformation in [(CH₃)₄N⁺] ? 2 [C(NH₂)₃⁺] ? [KTA^3?] ? 2 H₂O ( 3 ). The crystal structure of [(nC₃H₇)₄N⁺] ? 2 [C(NH₂)₃⁺] ? [KTA^3?] ( 4 ) features a tetrahedral aggregate of four guanidinium ions stabilized by an outer shell that comprises six equatorial carboxylate groups that belong to separate [KTA^3?] anions. In 3 [(C₂H₅)₄N⁺] ? 20 [C(NH₂)₃⁺] ? 11 [HKTA^2?] ? [H₂KTA^?] ? 17 H₂O ( 5 ), an even larger centrosymmetric inner core composed of eight guanidinium ions and six bridging water molecules is enclosed by a crust composed of eighteen axial carboxyl/carboxylate groups from six HKTA^2? anions.     

DFT quantum-chemical study of the hydrolysis products of Fe(II) and Fe(III) aqua-complexes

The quantum-chemical DFT calculations of thermodynamic characteristics of the reactions of formation of binuclear dihydroxobridging [Fe(H₂O)₄(μ-OH)₂Fe(H₂O)₄] ⁿ⁺ and oxobridging [Fe(H₂O)₅(μ-O)Fe·(H₂O)₅] ⁿ⁺ (n = 2, 4) cations, the hydrolysis products of cations [Fe(H₂O)₆] ^m+ (m = 2, 3). It is shown that effects of solvation lead to higher energetic stability of the dihydroxobridging binuclear compounds in aqueous solutions.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant [M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these [M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 ? C_nH_2n]⁺ and [M + 73 ? 2CnH_2n]⁺. Small amounts of [M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the [M + 73 ? 2CnH_2n]⁺ or [(CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no [M + 73] ⁺ ions, but do give [M + H]⁺ ions, which allow the determination of the relative molecular mass. These [M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only [M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives [M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.     

Theoretical study of beryllium (II) complexes using CATIVIC: New parametric method

Calculations of several beryllium complexes {[Be(H₂O)_n]²⁺ (n = 1–4), [BeOH(H₂O)_n]⁺ (n = 1–3), and [Be(OH)₂(H₂O)_n] (n = 1, 2)} were carried out to compare different ab initio (density functional theory, MP2) and parametric (PM3(tm), CATIVIC) methods. Results show that the parametric method CATIVIC gives geometries and energies closer to the ab initio geometries than the PM3(tm) method due to the inclusion of the atomic excitation energies of the neutral atoms as well as the ions and to the dependence of the molecular parameters on the system charge. The molecular electronic density analysis of the Be? O bonds shows that the Be–water interaction in the [Be(H₂O)_n]²⁺ complexes can be considered as a closed‐shell interaction with a σ character in the bond while in the [Be(OH)₂(H₂O)_n] complexes the Be? water bond have π character. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Hexanuclear Niobium Cluster Compounds with Protonated N-Base Cations

Octahedral clusters of the [M₆X₁₂] type offer numerous possibilities to form structural arrangements through different choices of bonding situations. In this paper a series of new cluster compounds of the transition metal niobium is described, which consist of the [Nb₆Cl₁₈]^2–, and in one case [Nb₆Cl₁₈]^3–, anion and protonated N-base cations ([MIm-H]⁺, [nPr₃N-H]⁺, [TMGu-H]⁺, and [Tzn-H]⁺). They all are prepared using water scavenger compounds [SOCl₂ or (Ac)₂O] under oxidising conditions, resulting in two-electron (or one-electron, respectively) oxidized cluster units with respect to the starting material [Nb₆Cl₁₄(H₂O)₄] · 4H₂O. Of five members of this group single-crystal X-ray structures were determined. The cluster anions exist in all structures as discrete units. The acidic H atoms of all N-bases are hydrogen bonded to H acceptors, in 4 cases to outer, exo bonded Cl atoms of the cluster unit and in one case to the O atom of a co-crystallized THF molecule. In [TMGu-H]₂[Nb₆Cl₁₈] chains of cluster anions exist hydrogen-bonded through bridging [TMGu-H]⁺ cations. ESI mass spectra of [MIm-H]₂[Nb₆Cl₁₈] · 2SOCl₂ and [TMGu-H]₂[Nb₆Cl₁₈] show the expected isotopic distribution patterns for the anions together with other peaks associated to chloride mass losses and/or reduction processes.     

Gas-phase ion–molecule reactions in dilute mixtures of dimethyl ether in krypton. Bimolecular and clustering reactions

The results of high-pressure variable-temperature and variable ionizing electron energy studies of gas-phase ion-molecule reactions of dimethyl ether in krypton are presented. Near the ionization threshold a series of peaks corresponding to (CH₃OCH₃)_nH⁺ (n = 1-4) clusters are observed. At higher ionizing electron energies, two new series of peaks appear, corresponding to [CH₃OCH₂]⁺(CH₃OCH₃)_n and [(CH₃)₃O]⁺ (CH₃OCH₃)_n clusters. The onium ion, [(CH₃)₃O]⁺, has been previously reported at elevated temperatures under methane chemical ionization conditions. It was suggested that the onium ion is formed by reaction of (CH₃)₂OH⁺ with CH₃OCH₃ with subsequent elimination of methanel, i.e. by fragmentation of an adduct ion. The present results strongly suggest that, under our conditions, [CH₃OCH₂]⁺ rather than thermal (CH₃)₃OH⁺, is the precursor to [(CH₃)₃O]⁺.     

The Structural Features of the Hydrated Ferrous Ion Clusters: [Fe(H2O) n ]2+ (n?=?1–19)

The low-lying structures of the hydrated ferrous ion clusters [Fe(H₂O) _n ]²⁺ (n?=?1?C19) were extensively searched at the level of the density functional theory. The results show that the first hydration shell consists of six water molecules, and the second hydration shell contains seven water molecules. Furthermore, it is found that all the lowest-energy states of [Fe(H₂O) _n ]²⁺ (n?=?1?C19) clusters are spin quintet states. These lowest-energy states keep well even at finite temperatures. The analyses of the successive water binding energy and natural charges population on ferrous ion clearly show that the influence of ferrous ion on the surrounding water molecules goes beyond the second hydration shell.     

Alkali metal (Na and K)-mediated supramolecular assembly of oxovanadium(V) complexes: Synthesis and structural characterization

The syntheses and characterization of alkali metal complexes [{VO₂L}M(H₂O}_n] (1 and 2) [M = Na⁺ (1), K⁺ (2)] of anionic cis-dioxovanadium(V) species (LVO₂⁻) of the Schiff base 2-hydroxybenzoylhydrazone of 2-hydroxybenzaldehyde have been reported. The number of coordinated water molecules in [{VO₂L}M(H₂O}_n] decreases as the charge density of the alkali metal ion decreases (n = 5 for Na⁺ and 1 for K⁺). These compounds represented M⁺-mediated supramolecular assembly [{VO₂L}M(H₂O}_n]_∝ with an infinite polymeric structure containing an alternating array of cis-dioxo vanadium(V), [VO₂L]⁻, units and aquated metal ion centres, as confirmed by X-ray crystallographic investigation of both. All the compounds are characterized by elemental analysis, IR, UV–Vis and NMR spectroscopy.     

Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx]+ (x=1–3): On the Influence of Oxidation States

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuO_x]⁺ (x=1–3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]⁺/CH₄ couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO₂]⁺ brings about the generations of [Ru,O,C,H₂]⁺/H₂O, [Ru,O,C]⁺/H₂/H₂O, and [Ru,O,H₂]⁺/CH₂O, whereas [RuO₃]⁺ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H₂). Regarding the reactions with H₂, as compared to CH₄, both [RuO]⁺ and [RuO₂]⁺ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO₃]⁺ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H−H bond activation. Furthermore, the reactivity patterns of [RuO_x]⁺ (x=1–3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues [OsO_x]⁺/CH₄/H₂ (x=1–3) and the [FeO]⁺/H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.     

Adduct ion formation by metal complexes under methane negative chemical ionization conditions

Negative chemical ionization mass spectrometry is used as a probe to examine reactions between hydrocarbon radicals and metal complexes in the gas phase. The methane negative chemical ionization mass spectra of 27 complexes of cobalt(II ), nickel(II ) and copper(II ) in the presence of O₄, O₂N₂ and N₄ donor atom sets are characterized by two dominant series of adduct ions of the form [M + C_nH_2n]^? and [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Insertion of the CH radical into the ligand followed by radical/radical recombination and electron capture is proposed as the major mechanism leading to the formation of [M + C_nH_2n]^? adduct ions. A second pathway involves ligand substitution by C_nH_2n+1 radicals concomitant with H elimination and electron capture. Oxidative addition at the metal followed by ionization is suggested as the principal pathway for the formation of [M + C_nH_2n+1]^? adduct ions.     

Theoretical study of the desorption of neutral and ionic alkali metal atoms from the excited Li+(H2O)n = 1-4 and Na+(H2O)n = 1-4 cluster models: Electronic excitation charge transfer

In this work, the potential energy curves of several low-lying excited states of M⁺(H₂O)_{n = 1-4} (M = Li and Na) clusters with one M─O bond, related to the stretching of their M─O bond, were calculated in the gas phase. The time-dependent density functional theory and direct-symmetry-adapted cluster-configuration interaction were used in this study separately. Theoretical calculations showed that the charge transfer occurred between M⁺ and (H₂O)_n in the excited clusters so that the neutral metal atom was obtained at the dissociation limit of the potential curves. The excited potential curves of clusters were also calculated in the presence of the electrostatic field of water (EFW), and it was found that the charge transfer was blocked in the presence of EFW. The effect of the size of the (H₂O)_n cluster on the shape of the excited potential curves was investigated to observe how the M─O bond was affected in the excited states depending on the (H₂O)_n size. It was found that the increase in the size of the (H₂O)_n cluster increased the number of bonding excited potential curves. The difference between the electron density of the excited and ground electronic states was calculated to see how the charge transfer was affected by the size of the (H₂O)_n cluster.     

Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products

The mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p-XC₆H₄) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO₂NHR', RSO₂SR, RSSR and NH₂R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO₂C₆H₄NH₂, RSOC₆H₄NH₂ and RSC₆H₄NH₂ (in the case of arylsulfinyl arylamides of the type of X = H) The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]⁺, [M ? R]⁺, [M ? R + H]⁺, [M ? SO]⁺, [RS]⁺, [NHR']⁺, [NHR' + H]⁺, [RSO]⁺, [RSO + H]⁺, [R]⁺, [R + H]⁺, [R']⁺ and [M ? OH]⁺, and that the thermal decomposition products give the following ions: [RSO₂SR]⁺, [RSSR]⁺, [M ? O]⁺, [M + O]⁺ and [RSOC₆H₄NH₂]⁺.     

A mass spectral study of 1-(aryldimethylsilyl)-2-propanones and their isomeric 2-(aryldimethylsiloxy)-1-propenes

The mass spectra of a series of β-ketosilanes, p-Y? C₆H₄Me₂SiCH₂C(O)Me and their isomeric silyl enol ethers, p-Y? C₆H₄Me₂SiOC(CH₃)?CH₂, where Y = H, Me, MeO, Cl, F and CF₃, have been recorded. The fragmentation patterns for the β-ketosilanes are very similar to those of their silyl enol ether counterparts. The seven major primary fragment ions are [M? Me·]⁺, [M? C₆H₄Y·]⁺, [M? Me₂SiO]⁺˙, [M? C₃H₄]⁺˙, [M? HC?CCF₃]⁺˙, [Me₂SiOH]⁺˙ and [C₃H₆O]⁺˙ Apparently, upon electron bombardment the β-ketosilanes must undergo rearrangement to an ion structure very similar to that of the ionized silyl enol ethers followed by unimolecular ion decompositions. Substitutions on the benzene ring show a significant effect on the formation of the ions [M? Me₂SiO]⁺˙ and [Me₂SiOH]⁺˙, electron donating groups favoring the former and electron withdrawing groups favoring the latter. The mass spectral fragmentation pathways were identified by observing metastable peaks, metastable ion mass spectra and ion kinetic energy spectra.