Gas-phase Clustering Reactions Research Articles

Mechanisms of SiO oxidation: Implications for dust formation

Reactions of SiO molecules have been postulated to initiate efficient formation of silicate dust particles in outflows around dying (AGB) stars. Both OH radicals and H2O molecules can be present in these environments and their reactions with SiO and the smallest SiO cluster, Si2O2, affect the efficiency of eventual dust formation. Rate coefficients of gas-phase oxidation and clustering reactions of SiO, Si2O2 and Si2O3 have been calculated using master equation calculations based on density functional theory calculations. The calculations show that the reactions involving OH are fast. Reactions involving H2O are not efficient routes to oxidation but may under the right conditions lead to hydroxylated species. The reaction of Si2O2 with H2O, which has been suggested as efficient producing Si2O3, is therefore not as efficient as previously thought. If H2O molecules dissociate to form OH radicals, oxidation of SiO and dust formation could be accelerated. Kinetics simulations of oxygen-rich circumstellar environments using our proposed reaction scheme suggest that under typical conditions only small amounts of SiO2 and Si2O2 are formed and that most of the silicon remains as molecular SiO.

Ambient vapor sampling and selective cluster formation for the trace detection of tributyl phosphate via atmospheric flow tube mass spectrometry

In addition to serving as an f-element ligand and rare-earth method complexing agent, tributyl phosphate is a compound containing core functional groups that mimic those routinely found in degradation products from industrial processes. Because detection of trace quantities of tributyl phosphate can provide insight into the routes of contamination and degradation in the environment, there is a need to develop methods capable of detecting trace quantities of tributyl phosphate. Vapor detection at atmospheric pressure is one approach that is both sensitive and rapid. We present here the use of atmospheric flow tube mass spectrometry for the ambient vapor sampling of tributyl phosphate from headspace of ppb-level solutions in methanol. Gas phase clustering reactions were to enhance detection levels via the addition of small quantities of the dopants diethylamine, triethylamine, and pinacolyl methylphosphonate in the vapor stream. Detection of the tributyl phosphate vapor emanating from these solutions demonstrated a linear range for the protonated tributyl phosphate species of 1–1000 ppb from solution. The clusters of tributyl phosphate with diethylamine, triethylamine, and pinacolyl phosphonate each yielded linear ranges of 1–250 ppb for tributyl phosphate in solution. Despite smaller linear ranges, the addition of these dopant species added a layer of analytical selectivity and reduced variability in signals from quality control samples. These data were obtained using an atmospheric flow tube source coupled to a linear ion trap mass spectrometer, which demonstrates the applicability of trapping systems to the atmospheric flow tube ionization technique while monitoring positive ions.

Studying Chemistry in Micro-compartments by Separating Droplet Generation from Ionization.

Recent studies show that reactions inside micron-sized compartments (e.g., droplets, emulsions) can proceed at significantly accelerated rates and with different mechanisms compared to the same reactions in a macroscopic container. Many of these studies use electrospray ionization (ESI) to both generate droplets and to quantify, via mass spectrometry (MS), droplet reaction kinetics. The highly charged and rapidly evaporating droplets produced in ESI make it difficult to examine precisely the underlying cause for droplet-induced rate enhancements. Additionally, interpretation of the spectra from ESI-MS can be complicated by gas-phase ion-molecule and clustering reactions. Here, we use an approach where droplet generation is separated from ionization, in order to decouple the multiple possible sources of acceleration and to examine more closely the potential role of gas-phase chemistry. The production of sugar phosphates from the reaction of phosphoric acid with simple sugars (a reaction that does not occur in bulk solution but has recently been reported to occur in droplets) is measured using this approach to compare reactivity in droplets (i.e., with compartments) with that in the gas phase (i.e., without compartments). The same product ions that have been previously assigned to in droplet reactions are observed with and without compartmentalization. These results suggest that in some cases, gas-phase processes in the ionization region can potentially complicate the quantification and interpretation of accelerated reactions in droplets using ESI-MS (or one of its variants). In such cases, contributions from in-droplet chemistry cannot be ruled out, but we demonstrate that gas-phase processes can be a significant (and possibly dominant) reaction pathway. We suggest that future studies of rate acceleration in droplets be modified to better assess the potential for non-droplet-related processes. Graphical Abstract ᅟ.

Gas-phase clustering of protonated amino acids with acetonitrile

The gas-phase clustering reactions of protonated amino acids, AAH + (AA = Ala, Val, Ser, Phe, Pro, Gln, Lys and Arg) with acetonitrile were studied with a pulsed ion-beam high-pressure mass spectrometer. The bond energies of the AAH +·(CH 3CN) complexes range from 20 to 11 kcal/mol. While for AlaH +, ValH +, SerH +, PheH +, ProH + and GlnH +, the solvation free energies decrease linearly with increasing gas-phase basicity of AA, deviations from the correlation are observed for LysH + and ArgH +. The bonding free energies for AAH +·(CH 3CN) are compared with those for AAH +·(H 2O) and AAH +·(NH 3).

Thermochemical stabilities of the gas-phase cluster ions of halide ions with rare gas atoms

Gas-phase clustering reactions of halide ions (X −) with rare gas atoms (Rg) have been investigated using a pulsed-electron beam high-pressure mass spectrometer. The thermochemical stability of the cluster ions X −(Rg) n has been determined. To evaluate bond energies that are difficult to obtain experimentally, ab initio calculations were also carried out. Among the cluster ions investigated, the interaction between the hardest F − ion and the softest Xe atom was found to be strongest (6.3 kcal/mol).

Thermochemical Stabilities and Structures of the Cluster Ions OCS +, S 2+, H +(OCS), and C 2H 5+ with OCS Molecules in the Gas Phase

The gas-phase clustering reactions of OCS+, S2+, H+(OCS), and C2H5+ ions with carbonyl sulfide (OCS) molecules were studied using a pulsed electron-beam high-pressure mass spectrometer and applying density functional theory (DFT) calculations. In the cluster ions OCS+(OCS)(n) and H+(OCS)(OCS)(n), a moderately strong, here referred to as "semi-covalent", bond was formed with n = 1. However, the nature of bonding changed from semi-covalent to electrostatic with n = 1 --> 2. The bond energy of S2(+)(OCS) was determined experimentally to be 12.9 +/- 1 kcal/mol, which is significantly smaller than that of the isovalent S2(+)(CS2) complex (30.9 +/- 1.5 kcal/mol). DFT based calculations predicted the presence of several isomeric structures for H+(OCS)(OCS)(n) complexes. The bond energies in the C2H5+(OCS)(n) clusters showed an irregular decrease for n = 1 --> 2 and 7 --> 8. The nonclassical bridge structure for the free C2H5+ isomerized to form a semi-covalent bond with one OCS ligand, [H3CCH2...SCO]+, i.e., reverted to classical structure. However, the nonclassical bridge structure of C2H5+ was preserved in the cluster ions C2H5+(OCS)(n) below 140 K attributable to the lack of thermal energy for the isomerization. DFT calculations revealed that stability orders of the geometric isomers of H+(OCS)(OCS)(n) and C2H5+(OCS)(n) changed with increasing n values.

Thermochemical stabilities and structures of the gas-phase cluster ions of halide ions with OCS

Gas-phase clustering reactions of halide ions with carbonyl sulfide were studied using a pulsed electron-beam mass spectrometer. For F −, formation of a stable FCSO − species was found with the F −–CSO binding energy of >30 kcal/mol (the calculated value=33.41 kcal/mol). DFT calculations showed that the three-fold in-plane coordination to the n=1 species is stable. For Cl −, Br − and I −, the bond energies of cluster ions are in the range of 5–10 kcal/mol with symmetric geometries. All cluster geometries except for F − with n=1 are controlled by the σ ∗ vacant orbital of OCS with participation of the sulfur 3d z 2−( x 2+ y 2) atomic orbital.

Clustering and activation in reactions of CoCp + with hydrogen and methane

Gas-phase clustering reactions of CoCp + with H 2 and with CH 4 were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp + ion was found to form strong interactions with two ligands. The first and second H 2 groups cluster to CoCp + with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH 4 groups cluster to CoCp + with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H 2 and 2.2 kcal/mol for the third CH 4 clustering to CoCp + suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H 2-elimination from CoCp(CH 4) 2 + was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H 2-elimination product is ( c-C 5H 6)Co +C 2H 5. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH 3 groups to the C 5-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C 5-ring ligand.

Gas-Phase Solvation of O2+, O2-, O4-, O3-, and CO3- with CO

Equilibria for the gas-phase clustering reactions of O2+, O2-, O4-, O3-, and CO3- with CO were measured with a pulsed electron-beam mass spectrometer. Van't Hoff plots of the equilibrium constants ...

Gas-phase solvation of Cl −, NO 2−, CH 2NO 2−, CH 3NO 2− and CH 3NO 4− by CH 3NO 2

The gas-phase clustering reactions in the systems containing nitromethane were investigated with a pulsed electron-beam high-pressure mass spectrometer. The thermochemical stabilities, Δ H° n−1, n and Δ S° n−1, n , of cluster ions X −(CH 3NO 2) n ( n≤4), X=Cl, NO 2, CH 2NO 2, CH 3NO 2 and CH 3NO 4, were determined. For CH 3NO 4 −(CH 3NO 2) n , the binding energies are found to be significantly lower than those in corresponding cluster ions having Cl −, NO 2 −, CH 2NO 2 − and CH 3NO 2 − as the core ions. This is ascribed to the dispersion of negative charge in the CH 3NO 4 − core ion for which the methyl-peroxynitrate structure is postulated.

Hydrogen bonds in gas-phase clusters between halide ions and olefins

Gas-phase clustering reactions of halide ions (X − = F −, Cl −, Br −, and I −) with ethylene (C 2H 4) and propylene (C 3H 6) were studied with a pulsed electron beam mass spectrometer. Bonding energies of all cluster ions were found to be less than 10 kcal/mol, i.e., no anion-initiated polymerization of C 2H 4 and C 3H 6 took place. For the cluster F −(C 2H 4) n , a small gap in the binding energy is observed between n = 4 and 5 suggesting that the first shell is completed with n = 4. For larger halide ions, the bond energies for the clusters X −(C 2H 4) n were found to be nearly n independent. For Cl −(C 3H 6) n a steep decrease in binding energies was observed between n = 2 and 3 and n = 3 and 4. The structure of the cluster ions was investigated by ab initio calculations. X −(C 2H 4) n complexes were calculated to have hydrogen-bond geometries regardless of the identity of the halide ions, and bidentate (chelate) type geometries of X −(C 3H 6) 1 were found.

The structures, thermodynamics, and nature of interactions of silanium cations shellvated by molecular hydrogen, SiH 3+(H 2) n ( n=1–11)

Theoretical studies of the structures, thermodynamics, and nature of interactions for gas-phase clustering reactions of SiH 3 + with H 2 are presented with the largest system found, SiH 3 +(H 2) 11. The shell structure of the studied systems has been revealed with shells of capacity of two, three, and six molecules. The properties of shell occupying molecules indicate that the characteristics of species within the same shell are almost exclusively determined by the field of the central SiH 3 + cation with little perturbation coming from neighboring molecules.

How is the Fluoride Ion Bound to O2, N2, and CO Molecules?

Fundamental gas-phase clustering reactions for F-(Sol)n-1 + Sol = F-(Sol)n for Sol = O2, N2, and CO were studied by using a pulsed electron beam mass spectrometer. While the interaction of F- with O2 ligands is mainly electrostatic, those with N2 and CO have some contribution from charge transfer (i.e., the formation of incipient covalent bonds in the complexes F- → N2 and F- → CO). The cluster ion F-(CO)6 was found to have an octahedral structure with equivalent F-···CO bonds. F-(O2)n was found to involve the smallest binding energy among F- cluster ions ever reported.

Gas-phase clustering reactions of NO + and NO 2+ with CH 3NO 2

The gas-phase clustering reactions of NO + and NO 2 + with CH 3NO 2 were studied with a pulsed electron beam high-pressure mass spectrometer. Thermochemical quantities, Δ H n−1, n o and Δ S n−1, n o, of the clusters NO +(CH 3NO 2) n and NO 2 +(CH 3NO 2) n with n=1–4, were determined. The −Δ H n−1, n o values show a gradual decrease with n, suggesting that the interaction involved in the clusters is mainly electrostatic. The measured bond enthalpy of NO +–CH 3NO 2, 25.2±0.9 kcal/mol, is slightly greater than that of NO 2 +–CH 3NO 2, 22.1±0.9 kcal/mol. The thermochemical stabilities of both NO +(CH 3NO 2) n and NO 2 +(CH 3NO 2) n clusters with n=2–4 are found to be about the same. This Letter supports the results of Cacace and co-workers derived from FT–ICR measurements.

Gas-phase equilibria studies of C 2H 2N + and C 3H 4N + with CH 3CN

Thermodynamic quantities, ΔH° n−1, n and ΔS° n−1, n , for the gas-phase clustering reactions of C 2H 2N + and C 3H 4N + with CH 3CN have been studied using a pulsed electron beam high-pressure mass spectrometer. For the C 2H 2N +(CH 3CN) n clusters studied, in the first step the C 2H 2N + ion was seen to disappear rapidly after the electron pulse and the equilibrium of this ion for the reaction forming C 2H 2N + ·CH 3CN was not observed under conditions of the experiments. In the case of C 3H 4N +(CH 3CN) n , the − ΔH° n−1, n values show a large gap with n = 1 → 2, suggesting that the adduct ion, C 3H 4N + ·CH 3CN, is a core in the cluster. Comparison of the bond enthalpies of C 2H 2N +(CH 3CN) n ⩽ 5 and C 3H 4N +(CH 3CN) n ⩽ 4 with those of alkali metal ions solvated by CH 3CN shows that at n ⩾ 2 the ion-ligand interaction in the clusters is mainly electrostatic.

Anomalous Change of Bond Energies in the Cluster Ion N2H+(H2)n

Gas-phase clustering reactions N2H+(H2)n-1 + H2 = N2H+(H2)n with n = 1−13 were studied using a pulsed-electron beam mass spectrometer. Anomalous changes of binding energies in the cluster ion N2H+(H2)n were observed, i.e., the bond energies of N2H+(H2)n increase with n = 2 → 5. The molecular orbital (MO) calculation revealed that the charge transfer takes place in the cluster ion N2H+(H2)n from the H2 ligands to the N2H+ moiety, with increase of n leading to the self-tightening of the bonds in the cluster ions. The experimental bond energies of N2H+(H2)n (n = 2−7) were reproduced by the ab initio MO calculations. The larger cluster ions N2H+(H2)n (n = 10−13) were found to coexist with smaller ones with n = 6−9 in the temperature region of 65∼40 K. This suggests the presence of the structural isomers.

Thermochemistry and structure of the cluster ions CH 3+(CO) n and SiH 3+(CO) n in the gas phase

The gas-phase clustering reactions of CH 3 + and SiH 3 + with CO were studied using a high-pressure mass spectrometer. For the cluster CH 3 +(CO) n , the acylium ion CH 3CO + is generated with n = 1, and it forms the core in larger clusters. Theoretical calculations predict that CH 3CO + is 58.6 kcal/mol more stable than the isomeric CH 3OC + ion. The interaction between CH 3CO + and CO is mainly electrostatic and the CH 3CO + ion is solvated preferentially by three CO ligands, giving the shell structure, CH 3CO +(CO) 3(CO) n−4 . The bond dissociation energy of SiH 3 + … CO is 34 kcal/mol. The cluster ion SiH 3 +(CO) n was found to have the shell structure SiH 3 +(CO) 2(CO) 6(CO) n−8 . Theoretical binding energies for both CH 3 +(CO) n and SiH 3 +(CO) n differ from the observed ones by less than 0.7 kcal/mol. The n = 3 and 4 geometries have a staggered conformation of CO coordinations relative to CH and SiH bonds.

Thermochemistry of the SiH3+(SiH4)n and SiH3+(H2)n cluster ions

Gas-phase clustering reactions of SiH3+ with SiH4 and H2 were studied using a pulsed-electron beam mass spectrometer. The bond dissociation energy of SiH3+ … SiH4 was estimated to be ⩾35 kcal/mol and those of SiH3+ (H2)n with n = 1 and 2 were measured to be 14.2 and 4.8 kcal/mol in good agreement with the theoretical values. The clusters are found to have the shell structures, SiH3+ (sol)2(sol)n−2 for both sol = SiH4 and H2.

The small binding energies of the negative cluster ions: SF 5−(SF 6) 1, SF 6−(SF 6) 1 and F −(SF 6) n ( n = 1 and 2), in the gas phase

The gas-phase clustering reactions of SF 5 −, SF 6 − and F − ions with SF 6 were measured using a high-pressure mass spectrometer. The bond energies of SF 5 −(SF 6) 1 and SF 6 −(SF 6) 1 were estimated to be ⩽2.3 kcal/mol. The bond energies of F −(SF 6) n with n = 1 and 2 were determined to be 5.4 and ≈4.1 kcal/mol, respectively. This result indicates that the SF 6 molecule is the weakest electrophilic reagent toward F − ever reported. This is mainly due to the exchange repulsion between the negative ions and fluorine atoms surrounding the reactive S atom. The structure of the cluster ion F −(SF 6) 1 whose binding energy was surrrisingly small was investigated by ab initio MO calculations.

Gas-phase solvation of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O

Gas-phase clustering reactions of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O were measured with a pulsed electron-beam high pressure mass spectrometer. The bond in NO+(N2O)n is found to be electrostatic, while those in O+2(N2O)1, N2O+(N2O)1, and N2OH+(N2O)1 have covalent character. The observed n dependence of −ΔH0n−1,n for the clustering reactions suggests that the cluster ions have the structures of the core ion plus ligand molecules, i.e., NO+(N2O)3(N2O)n−3, O+2N2O(N2O)n−1, (N2O)+2(N2O)n−1, H+N2O(N2O)1(N2O)n−1, and H3O+(N2O)3(N2O)n−3. The N2O molecule forms stronger bonds with NO+, O+2, and H3O+ ions than the isoelectronic CO2 molecule, indicating that N2O is a stronger nucleophilic reagent than CO2.