MP2 and B3LYP calculations were performed on complexes of nitric acid with water using the 6-311++G(2d,p) basis set to determine optimized geometries and binding energies for HNO(3)(...)nH(2)O systems (n = 1-4). The structures for the global minima for n = 1-4 have homodromic rings formed by successive hydrogen bonds. The potential energy surface for the HNO(3)(...)nH(2)O clusters is quite shallow. The first stable ion-pair configuration is obtained for a HNO(3)(...)4H(2)O Complex. The ion pair, H3O+-NO3, is separated by the three H2O molecules forming an Eigen-ion (H9O4+) type structure. The transition states and activation barriers for n = 1-4 were also determined. The zero-point corrected transition-state barrier for the ion pair is only 0.5 kcal/mol. Larger HNO(3)(...)nH(2)O clusters (n up to 32) were also determined to be dominated by the ion-pair motif.

Water was reacted with gas-phase oxyanions having the general composition SixOyHz- that were formed and isolated in an ion trap-secondary ion mass spectrometer (IT-SIMS). The radical SiO2.- reacted slowly with H2O to abstract HO., forming SiO3H-, at a rate of 8 x 10(-13) cm(3) molecule(-1) s(-1), corresponding to an efficiency of about 0.03% compared with the theoretical collision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3- at a rate that was approximately 0.4-0.7% efficient. SiO4H3- did not undergo further reaction with water. The multiple reaction pathways by which radical SiO3.- reacted with H2O were kinetically modeled using a stochastic approach. SiO3.- reacted with water by three parallel reaction pathways: (1) abstraction of a radical H-. to form SiO3H-, which then reacted with a second H2O to form SiO4H3-; (2) abstraction of a radical OH. to form SiO4H-, which further reacted by consecutive H-. abstractions to form SiO4H2.- and then SiO4H3-; and (3) condensation with H2O to form SiO4H2.-, which subsequently abstracted a radical H-. from a second H2O to form SiO4H3-. In all of these reactions, the rate constants were determined to be very slow, as determined by both direct measurement and stochastic modeling. For comparison, the even electron ion Si2O5H- was also investigated: it underwent condensation with H2O to form Si2O6H3-, with a rate constant corresponding to 50% efficiency. The reactions were also modeled using ab initio calculations at the UB3LYP/6-311 + G(2d,p) level. Addition of H2O to SiO3.-, SiO3H-, and Si2O5H- was calculated to be approximately 42, 45, and 55 kcal mol(-1) exothermic, respectively, and encountered low activation barriers. Modeling Of SiO2.- and SiO3.- reactions with H2O failed to produce radical abstraction reaction pathways observed in the IT-SIMS, possibly indicating that alternative reaction mechanisms are operative.

Gas-phase Si and Al oxyanions were formed by particle bombardment, isolated by mass, and then reacted with H2S in an ion trap secondary ion mass spectrometer (IT-SIMS). The reactions proceeded by different reaction pathways depending on whether the oxyanions were even- or odd-electron species. The radical anion SiO2bullet- reacted with H2S by abstracting a(bullet)SH radical to form the even-electron Sio(2)SH(-). Once formed, the even electron Sio(2)SH(-) reacted with a second H2S molecule by O-for-S exchange to form SiOS2H-. The radical anion SiO3bullet- abstracted an H-bullet radical from H2S to form even-electron SiO3H-, which then underwent two consecutive O-for-S exchange reactions with H2S to form SiO2SH- and SiOS2H-. For the reactions of the even-electron anion AlO2-, the products of two consecutive O-for-S exchange reactions with H2S were AlOS- and AlS2-. The radical abstraction reactions and the O-for-S exchange reactions of SiO3H-, AlO2 and AlOS- were efficient in the 30-50% range. The efficiency of the O-for-S exchange reaction of SiO2SH- (producing SiOS2H-) was substantially less efficient at 8%.

Hydration of aluminum oxide anion clusters was studied in the gas phase using an ion trap secondary ion mass spectrometer. Hydration of both AlO2- and Al2O4H- occurred by the consecutive addition of two H2O molecules. For hydration of AlO2-, the rate constants for addition of the first and second water molecules are 4 x 10(-11) and 4 x 10(-10) cm(3) molecule(-1) s(-1), respectively. The first and second hydration rate constants for Al2O4H- are 2 x 10(-9) and 8 x 10(-10) cm(3) molecule(-1) s(-1), respectively. A comparison of the experimental rate constants to the theoretical rate constants reveals that addition of the first H2O to AlO2- is only 2% efficient, whereas addition of the first H2O to Al2O4H- is 100% efficient. Ab initio calculations were performed to assist in the interpretation of the kinetic results. Reaction mechanisms and energetics for the hydration of the AlO2- system were calculated using the HF/6-311+G(d(Al),p), B3LYP/6-31+G(d), B3LYP/6-311+G-(2d,p), B3LYP/6-311+G(3d2f,2p), and MP2/6-311+G(2d,p) levels of theory. Calculations on the hydration of the Al2O4H- system were performed using the B3LYP/6-311+G(2d,p) level of theory. Ab initio results revealed that the addition of the first and second waters, for both the AlO2- and Al2O4H- systems, results in the formation of four-membered transition states, with simultaneous Al-O bond formation and proton transfer. However, a significant later transition state is observed, with respect to the Al-O and H-O bond lengths, for the addition of the second water molecule in the Al2O4H- system. A comparison of the reaction mechanisms and energetics was not sufficient to account for the 2 orders of magnitude difference in rate constants; however, the reactivity differences do correlate with the dipole moment of the aluminum oxide anions, which may serve to preorient the incoming water molecule, thus enhancing the reaction rate.

The direct detection of the nerve agent VX (methylphosphonothioic acid, S-[2-[bis(1-methylethyl)amino]ethyl] O-ethyl ester) on milligram quantities of soil particles has been achieved using ion trap secondary ion mass spectrometry (IT-SIMS). VX is highly adsorptive toward a wide variety of surfaces; this attribute makes detection using gas-phase approaches difficult but renders the compound very amenable to surface detection. An ion trap mass spectrometer, modified to perform SIMS, was employed in the present study. A primary ion beam (ReO4-) was fired on axis through the ion trap, where it impacted the soil particle samples. [VX + H]+, [VX + H]+ fragment ions, and ions from the chemical background were sputtered into the gas-phase environment of the ion trap, where they were either scanned out or isolated and fragmented (MS2). At a surface concentration of 0.4 monolayer, intact [VX + H]+, and its fragment ions, were readily observable above background. However, at lower concentrations, the secondary ion signal from VX became obscured by ions derived from the chemical background on the surface of the soil particles. MS2 analysis using the ion trap was employed to improve detection of lower concentrations of VX: detection of the 34S isotopic ion of [VX + H]+, present at a surface concentration of approximately 0.002 monolayer, was accomplished. The study afforded the opportunity to investigate the fragmentation chemistry of VX. Semiempirical calculations suggest strongly that the molecule is protonated at the N atom. Deuterium labeling showed that formation of the base peak ion (C2H4)N(i-C3H7)2+ involves transfer of the amino proton to the phosphonothioate moiety prior to, or concurrent with, C-S bond cleavage. To manage the risk associated with working with the compound, the vacuum unit of the IT-SIMS was located in a hood, connected by cables to the externally located electronics and computer.