Scattering of neutral NH3 clusters off LiF(100): angular distributions of NH3 and small clusters

The scattering behavior of neutral ammonia clusters off a LiF(100) surface is studied. Ammonia clusters are produced by a coexpansion of NH₃ and Kr with an average kinetic energy of 48 meV per monomer molecule. Using single photon VUV laser ionization at λ = 118 nm (hv = 10.49 eV) the mass distribution of scattered particles is obtained in a reflecting time-of-flight mass spectrometer. Compared with the incoming cluster beam the average cluster size of the scattered particles is drastically decreased. The angular distribution of NH ₃ ⁺ and NH ₄ ⁺ after scattering reveals a strong inelastic interaction between the clusters and the LiF(100) surface which is described in the context of a thermokinetic model and a phonon excitation along the (001) azimuth of the LiF(100) surface.     

Reference materials at emission levels of NH3 and HCl

Due to instability in static mixtures, in the past only dynamic preparation techniques for mixtures of NH3 and HCl were maintained; however, recent developments of new passivation techniques for cylinder treatment and commercially available nitrogen with a very low content of impurities have opened up the possibility of preparing stable mixtures in cylinders by gravimetry. Mixtures of NH3 (300 microL/L and 30 microL/L) and HCI (300 microL/L) were prepared in nitrogen in cylinders with different passivation treatments. The mixtures were compared with dynamically prepared mixtures using Photo Acoustic detection (PAS) and FT-IR. The uncertainty is about 3.5% with PAS analysis for NH3 at the 300 microL/L level. For the analysis of HCI with FT-IR the initial uncertainty in the 300 microL/L mixtures is about 7%. Two of the HCI mixtures show significant instability; for the other three mixtures the uncertainty in the measurement is too large to discriminate between stability and instability. The NH3/N2 mixtures are stable within 3.5% for a one-year period. The stability of the 30 microL/L mixtures of NH3 cannot be underpinned by measurement results, but a systematic effect in the measurement procedure seems more likely than instability. Improvement in the measurement procedure should clarify this point.     

Electron attachment to HCl clusters

Negatively charged cluster ions of hydrogen chloride are formed by electron attachment to HCl clusters, which are produced in a seeded supersonic beam traversing a sustained gas discharge. Cluster ions of (HCl) _n ^? , withn=2, and tentatively withn=3 and 4 are observed. Cluster ions like Cl _n ^? , Cl _n ^? (HCl) _m , and withAr attached to them are also seen. The relevance to radiation chemistry of HCl if briefly discussed. Atoms evaporating from the hot, thoriated tungsten filament of the glow discharge lead to clusters such as Th _n ^? and its oxides.     

Microsolvation effects on the electron capturing ability of thymine: thymine-water clusters

The effects of solvation on the stability of thymine and its negative ion have been investigated by explicitly considering the structures of complexes of thymine with up to five water molecules and the respective anions at the B3LYP/DZP++ level of theory. The vertical detachment energy of thymine was predicted to increase gradually with the hydration number, consistent with experimental observations from a photodetachment-photoelectron spectroscopy study J. Schiedt et al., [Chem. Phys. 239, 511 (1998)]. The adiabatic electron affinity of thymine was also found to increase with the hydration number, which implies that while the conventional valence anion of thymine is only marginally bound in the gas phase, it may form a stable anion in aqueous solution.     

Electron solvation dynamics in I-(NH3)n clusters

Femtosecond photoelectron spectroscopy (FPES) is used to monitor the dynamics associated with the excitation of the charge-transfer-to-solvent (CTTS) precursor states in I-(NH3)n = 4-15 clusters. The FPE spectra imply that the weakly bound excess electron in the excited state undergoes partial solvation via solvent rearrangement on a time scale of 0.5-2 ps, and this partially solvated state decays by electron emission on a 10-50 ps time scale. Both the extent of solvation and the lifetimes increase gradually with cluster size, in contrast to the more abrupt size-dependent effects previously observed in I-(H2O)n clusters.     

HCl solvation at the surface and within methanol clusters/nanoparticles II: Evidence for molecular wires

Condensed-phase solvation of HCl on and within methanol nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy, on-the-fly molecular dynamics as implemented in the density functional code Quickstep (which is part of the CP2K package), and ab initio calculations. Adsorption and solvation stages are identified and assigned with the help of calculated infrared spectra obtained from the simulations. The results have been further checked with MP2-level ab initio calculations. The range of acid solvation states extends from the single-coordinated slightly stretched HCl to proton-sharing with Zundel-like methanol O...H+...X- states, and finally to MeOH2+...Cl- units with full proton transfer. Furthermore, once the proton moves to methanol, it is mobilized along methanol molecular chains. Since the proton dynamics reflects the evolving local structures, the "proton" spectra display broad bands usually with underlying continua.     

Characterization of (NH3)(n=1-6)NH+ clusters produced by 252Cf fragments impact onto a NH3 condensed target

This paper reports the first characterization of the (NH(3))(n)NH+ cluster series produced by a 252Cf fission fragments (FF) impact onto a NH(3) ice target. The (NH(3))(n=1-6)NH+ members of this series have been analyzed theoretically and experimentally. Their ion desorption yields show an exponential dependence of the cluster population on its mass, presenting a relative higher abundance at n = 5. The results of DFT/B3LYP calculations show that two main series of ammonium clusters may be formed. Both series follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of the respective {NH(3)NH}+ and {NH(2)NH(2)}+ cores. The energy analysis (i.e., D-plot and stability analysis) shows that the calculated members of the (NH(3))(n-1){NH(2)NH(2)}+ series are more stable than those of the (NH(3))(n-1){NH(3)NH}+ series. The trend on the relative stability of the members of more stable series, (NH(3))(n-1){NH(2)NH(2)}+, shows excellent agreement with the experimental distribution of cluster abundances. In particular, the (NH(3))4{NH(2)NH(2)}+ structure is the most stable one, in agreement with the experiments.     

The solvation of two electrons in the gaseous clusters of Na- (NH3)n and Li- (NH3)n

Alkali metal ammonia clusters, in their cationic, neutral, and anionic form, are molecular models for the alkali-ammonia solutions, which have rich variation of phases with the solvated electrons playing an important role. With two s electrons, the Na(-)(NH(3))(n) and Li(-)(NH(3))(n) clusters are unique in that they capture the important aspect of the coupling between two solvated electrons. By first principles calculations, we demonstrate that the two electrons are detached from the metal by n = 10, which produces a cluster with a solvated electron pair in the vicinity of a solvated alkali cation. The coupling of the two electrons leads to either the singlet or triplet state, both of which are stable. They are also quite distinct from the hydrated anionic clusters Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n), in that the solvated electrons are delocalized and widely distributed among the solvent ammonia molecules. The Na(-)(NH(3))(n) and Li(-)(NH(3))(n) series, therefore, provide another interesting type of molecular model for the investigation of solvated electron pairs.     

生物分子的微溶剂化过程*

研究分子的微溶剂化动力学过程是一个热点课题。应用各种光谱、质谱等实验技术并与从头计算和密度泛函等计算方法相结合,通过对生物分子和溶剂分子在气相中形成的分子团簇的研究,可以使我们了解溶剂分子对生物分子的结构和构型的影响。本文首先介绍了一些先进的实验技术及其应用于溶剂化团簇的研究,综述了近年来发展的几种主要理论计算和溶剂化模型方法。文中介绍了氨基酸分子与水、甲醇等发生微溶剂化过程的最新研究进展,然后分别综述了核酸碱基和碱基对、糖类、神经传递分子的溶剂化团簇的最新研究进展。最后,对该领域的研究前景进行了展望。     

NH4Cl和HCl溶液浸取电石渣制备多晶型CaCO3

以电石渣为原料,分别以NH4Cl溶液和HCl溶液为浸取剂提取电石渣中的钙,以NH4HCO3为碳化剂制备CaCO3。研究了两种浸取工艺对电石渣中Ca^2+离子提取率的影响,并对两种体系浸取得到的Ca^2+离子提取率、二次残渣量和滤液中的杂质情况,以及制得的CaCO3的产率和性状进行了比较。结果表明,在浸取时间为30 min,浸取剂浓度为2.5 mol·L^-1,pH为8的条件下,以NH4Cl溶液浸取电石渣,加入量按Cl^-/Ca^2+物质的量比为2.0,得到Ca^2+离子提取率为92.87%;以HCl溶液浸取电石渣,加入量按Cl-/Ca^2+物质的量比为2.2,得到Ca^2+离子提取率为98.65%。Ca^2+离子滤液中的主要杂质通过调节pH均可以得到有效控制。NH4Cl溶液浸取电石渣Ca^2+离子提取率略低,二次残渣量高(22.39%),制得CaCO3的产率为1.065 g·g^-1电石渣;HCl溶液浸取电石渣Ca^2+离子提取率高,二次残渣量小(8.87%),制得CaCO3的产率为1.203 g·g^-1电石渣。两种浸取液碳化后制得的CaCO3产品纯度都在99%以上,白度在92.2-97.1%之间,通过控制碳化条件均可制得方解石型、球霰石型和文石型三种晶型的CaCO3。     

Blueshift and intramolecular tunneling of NH3 umbrella mode in 4He n clusters

We present diffusion Monte Carlo calculations of the ground and first excited vibrational states of NH(3) (4)He(n) for n< or =40. We use the potential energy surface developed by one of us [M. P. Hodges and R. J. Wheatley, J. Chem. Phys. 114, 8836 (2001)], which includes the umbrella mode coordinate of NH(3). Using quantum Monte Carlo calculations of excited states, we show that this potential is able to reproduce qualitatively the experimentally observed effects of the helium environment, namely, a blueshift of the umbrella mode frequency and a reduction of the tunneling splittings in ground and first excited vibrational states of the molecule. These basic features are found to result regardless of whether dynamical approximations or exact calculations are employed.     

Microsolvation of LiH+ in helium clusters: many-body effects and additivity models for the interaction forces

The ab initio calculation of the interaction forces between the LiH+ molecular ion, at its equilibrium geometry, and several He atoms is carried out in order to isolate and assess the importance of many-body contributions in the search for realistic energy and geometry data. The full potential energy surface (PES) with a single helium partner is obtained first by using an aug-cc-pVQZ basis set for He and higher quality ones for Li and H. The calculations were performed at the CAS-SCF plus MRCI level for the lowest potential energy surface over a total of 480 grid points of the two intermolecular Jacobi coordinates, whereas the excited state surface has also been examined in order to exclude the presence of any significant nonadiabatic interaction between the two PESs. A numerical fit of the lower surface is presented and the general physical changes of the ionic interaction when going from the lower to the upper of the two potentials are described and discussed. The fairly limited importance of many-body effects for such systems is seen from further ab initio calculations including several He atoms: our results suggest that, at least in the present case, no strong charge migration occurs after He attachment, and therefore, one could realistically model larger clusters by implementing a sum-of-potentials approach via the presently computed PES.     

HCl/NH_3敏感的卟啉-二氧化硅反蛋白石光子晶体荧光传感器

将反蛋白石结构的光子晶体引入到腐蚀性气体检测体系,制备得到卟啉-二氧化硅反蛋白石光子晶体(TPP-SiO_2IOPCs)荧光传感器。相对于空白样,TPP-SiO_2IOPCs传感器实现了氯化氢(HCl)气体检测信号200倍的增强,这主要归因于反蛋白石型光子晶体的大孔结构和慢光子效应。同时,TPP-SiO_2IOPCs传感器对于HCl气体的猝灭效率可达75%,比空白样的提高了25%,而且经HCl气体处理后的传感器通入氨气(NH_3)后,初始的荧光强度几乎完全恢复。在HCl和NH_3条件下进行5个循环后,TPP-SiO_2IOPCs传感器表现出良好的可重复使用性。该研究对于发展高效的荧光传感器提供了新的思路。     

Photodissociation of HCl and small (HCl)m complexes in and on large Ar n clusters

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Ar n clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes n=140-1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are m=3 and 6 and n=8-248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for.     

Microsolvation of F- in water

Microsolvation of F- in water is studied by ionization and double ionization spectra of (H2O)1-3F- calculated by ab initio methods. It is shown that the presence of the fluorine electrons introduces many-body properties in the spectra which cannot be reproduced by the presence of a negative point charge. The increase of the solvation shell increases the complexity in particular of the double ionization spectra. Ionization and double ionization energies slowly increase with continued solvation, and many-body effects in the inner valence spectra become more prominent.     

Microsolvation of the sodium and iodide ions and their ion pair in acetonitrile clusters: a theoretical study

The structural and thermodynamic properties of Na+(CH3CN)n, I-(CH3CN)n, and NaI(CH3CN)n clusters have been investigated by means of room-temperature Monte Carlo simulations with model potentials developed to reproduce the properties of small clusters predicted by quantum chemistry. Ions are found to adopt an interior solvation shell structure, with a first solvation shell containing approximately 6 and approximately 8 acetonitrile molecules for large Na+(CH3CN)n and I-(CH3CN)n clusters, respectively. Structural features of Na+(CH3CN)n are found to be similar to those of Na+(H2O)n clusters, but those of I-(CH3CN)n contrast with those of I-(H2O)n, for which "surface" solvation structures were observed. The potential of mean force calculations demonstrates that the NaI ion pair is thermodynamically stable with respect to ground-state ionic dissociation in acetonitrile clusters. The properties of NaI(CH3CN)n clusters exhibit some similarities with NaI(H2O)n clusters, with the existence of contact ion pair and solvent-separated ion pair structures, but, in contrast to water clusters, both types of ion pairs adopt a well-defined interior ionic solvation shell structure in acetonitrile clusters. Whereas contact ion pair species are thermodynamically favored in small clusters, solvent-separated ion pairs tend to become thermodynamically more stable above a cluster size of approximately 26. Hence, ground-state charge separation appears to occur at larger cluster sizes for acetonitrile clusters than for water clusters. We propose that the lack of a large Na+(CH3CN)n product signal in NaI(CH3CN)n multiphoton ionization experiments could arise from extensive stabilization of the ground ionic state by the solvent and possible inhibition of the photoexcitation mechanism, which may be less pronounced for NaI(H2O)n clusters because of surface solvation structures. Alternatively, increased solvent evaporation resulting from larger excess energies upon photoexcitation or major solvent reorganization on the ionized state could account for the observed solvent-selectivity in NaI cluster multiphoton ionization.     

Ion exchange behaviour of 18 elements on amphoteric resin Retardion 11A8 in ammonium chloride,NH4Cl+NH3 and NH4Cl+HCl media

Distribution coefficients of 18 elements in the systems: amphoteric resin Retardion 11A8-NH₄Cl₂ (NH₄Cl+NH₃) and (NH₄Cl+HCl) were measured by batch equilibration technique with the use of radioactive tracers. On the basis of these data ageneral qualitative interpretation of ion exchange behaviour of inorganic ions on Retardion 11A8 in dependence on pH and composition of aqueous phase is given. The possibility of achieving useful radiochemical separations using Retardion 11A8 columns and exploiting both cation and anion exchange functions of the resin is demonstrated.     

Liquefaction of solid-state BH3NH3 by gaseous NH3

This paper reports for the first time that under ammonia atmosphere, ammonia borane (AB) reversibly absorbs up to at least 6 equiv of NH(3), forming liquid AB(NH(3))(n) (n = 1-6) complexes at 0 °C. Reasonable structures for AB(NH(3))(n) were identified via density functional theory calculations, which indicate that the strong classical hydrogen bond formed between the lone pair of NH(3) and the -NH(3) of AB is the driving force for the absorption of ammonia by AB. By use of the van't Hoff equation, the enthalpy change (ΔH) for AB to absorb one NH(3) was determined to be -2.24 kcal/mol, which is in good agreement with the theoretical calculations. Other organic amines were screened to further confirm the role of the N lone pair; only 1,4-diazabicyclo[2.2.2]octane (DABCO) formed a stable adduct, which X-ray structural analysis showed was the DABCO-BH(3) species. Finally, Raman spectra of AB(NH(3))(n) were collected, and its unique spectral features are also discussed.     

Infrared spectroscopy of Li(NH3)n clusters for n=4-7

Infrared spectra of Li(NH3)(n) clusters as a function of size are reported for the first time. Spectra have been recorded in the N-H stretching region for n=4-->7 using a mass-selective photodissociation technique. For the n=4 cluster, three distinct IR absorption bands are seen over a relatively narrow region, whereas the larger clusters yield additional features at higher frequencies. Ab initio calculations have been carried out in support of these experiments for the specific cases of n=4 and 5 for various isomers of these clusters. The bands observed in the spectrum for Li(NH3)(4) can all be attributed to N-H stretching vibrations from solvent molecules in the first solvation shell. The appearance of higher frequency N-H stretching bands for n > or =5 is assigned to the presence of ammonia molecules located in a second solvent shell. These data provide strong support for previous suggestions, based on gas phase photoionization measurements, that the first solvation shell for Li(NH3)(n) is complete at n=4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH3)(4) is found to be the basic structural motif.