Showing papers in "The Journal of Physical Chemistry in 2005"

Tailoring pore properties of MCM-48 silica for selective adsorption of CO2

Abstract: Four different types of amine-attached MCM-48 silicas were prepared and investigated for CO(2) separation from N(2). Monomeric and polymeric hindered and unhindered amines were attached to the pore surface of the MCM-48 silica and characterized with respect to their CO(2) sorption properties. The pore structures and amino group content in these modified silicas were investigated by XRD, FT-IR, TGA, N(2) adsorption/desorption at 77 K and CHN/Si analysis, which confirmed that in all cases the amino groups were attached to the pore surface of MCM-48 at 1.5-5.2 mmol/g. The N(2) adsorption/desorption analysis showed a considerable decrease of the pore volume and surface area for the MCM-48 silica containing a polymeric amine (e.g., polyethyleneimine). The CO(2) adsorption rates and capacities of the amine-attached MCM-48 samples were studied employing a sorption microbalance. The results obtained indicated that in addition to the concentration of surface-attached amino groups, specific interactions between CO(2) and the surface amino groups, and the resultant pore structure after amine group attachment have a significant impact on CO(2) adsorption properties of these promising adsorbent materials.

Comparison of Model Chemistry and Density Functional Theory Thermochemical Predictions with Experiment for Formation of Ionic Clusters of the Ammonium Cation Complexed with Water and Ammonia; Atmospheric Implications

Abstract: The G2, G3, CBS-QB3, and CBS-APNO model chemistry methods and the B3LYP, B3P86, mPW1PW, and PBE1PBE density functional theory (DFT) methods have been used to calculate deltaH(o) and deltaG(o) values for ionic clusters of the ammonium ion complexed with water and ammonia. Results for the clusters NH4(+) (NH3)n and NH4(+) (H2O)n, where n = 1-4, are reported in this paper and compared against experimental values. Agreement with the experimental values for deltaH(o) and deltaG(o) for formation of NH4(+) (NH3)n clusters is excellent. Comparison between experiment and theory for formation of the NH4(+) (H2O)n clusters is quite good considering the uncertainty in the experimental values. The four DFT methods yield excellent agreement with experiment and the model chemistry methods when the aug-cc-pVTZ basis set is used for energetic calculations and the 6-31G* basis set is used for geometries and frequencies. On the basis of these results, we predict that all ions in the lower troposphere will be saturated with at least one complete first hydration shell of water molecules.

Gas diffusion and microstructural properties of ordered mesoporous silica fibers

Abstract: Pore and surface diffusion of carbon dioxide (CO2) and ethylene (C2H4) in the nanopores of ordered mesoporous silica fibers about 200 μm in length was measured by the transient gravimetric method. The experimentally determined pore diffusivity data, coupled with the porosity, pore size, and fiber length, are used to obtain the actual length of the nanopores in silica fibers. These measurements reveal a structure of the ordered nanopores whirling helically around the fiber axis with a spiral diameter of about 15 μm and a pitch value of 1.6 μm. At room temperature the surface diffusion contributes about 10% to the total diffusional flux for these two gases in the nanopores of the ordered mesoporous silica fibers. The surface diffusion coefficients for the ordered mesoporous silica fibers are about 1 order of magnitude larger than the nonordered mesoporous alumina or silica with similar pore size.

Theoretical Investigation of the Dihydrogen Bond Linking MH2 with HCCRgF (M=Zn,Cd; Rg=Ar,Kr)

Abstract: An ab initio computational study of the properties of four linear dihydrogen-bonded complexes pairing MH2 (M = Zn, Cd) with HCCRgF (Rg = Ar, Kr) was undertaken at the MP2/DGDZVP level of theory. The calculated complexation energies of the linear complexes vary between 6.5 kJ/mol for M = Zn to 8.5 kJ/mol for M = Cd. Equilibrium interatomic H...H distances are roughly 2.07 A for all four complexes. The red shifts of the H-C stretching frequency of HCCRgF correlate nicely with the interaction energies.

Characterization of the Electronic Excited-State Energetics and Solution Structure of Lanthanide(III) Complexes with the Polypyridine Ligand 6,6'-Bis[bis(2-pyridylmethyl)aminomethyl]-2,2'-bipyridine

Abstract: Absorption, emission, and excitation spectra for solid-state and solution of Tb(III), Dy(III), and Gd(III) complexes with the polypyridine ligand 6,6'-bis[bis(2-pyridylmethyl)-aminomethyl]-2,2'-bipyridine (C36H34N8) are presented. Measurements of excited-state lifetimes and quantum yields in various solvents at room temperature and 77 K are also reported and used to characterize the excited-state energetics of this system. Special attention is given to the characterization of metal-to-ligand energy transfer efficiency and mechanisms. The measurement of circularly polarized luminescence (CPL) from the solution of the Dy(III) complex following circularly polarized excitation confirms the chiral structure of the complexes under study. No CPL is present in the luminescence from the Eu(III) or Tb(III) complex because of efficient racemization. The variation of the magnitude of the CPL as a function of temperature from an aqueous solution of DyL is used for the first time to characterize the solution equilibria between different chiral species.

Theoretical Investigation of the Weakly Dihydrogen Bonded Complexes FArCCH..HBeX (X=H, F, Cl, Br)

Abstract: An ab initio computational study of the properties of four linear dihydrogen-bonded complexes formed between the first compound with an Ar-C chemical bond (FArCCH) and HBeX (X = H, F, Cl, and Br) molecules was undertaken at the MP2/6-311++G(2d,2p) level of theory. The calculated complexation energy at MP2 and G2(MP2) levels decreases in the order HBeH...HCCArF > BrBeH...HCCArF > ClBeH...HCCArF > FBeH...HCCArF. The intermolecular stretching frequency, and shifts within the monomers, are compared with the energetic strength of complexation.

State-resolved Dynamics of the CH(A2Delta) Channels from Singleand Multiple Photon Dissociation of Bromoform in the 10-20 eV EnergyRange

Abstract: Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A2Delta) and CH(B2Sigma) radicals. Vibrational and rotational state distributions of the CH(A2Delta) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A2Delta) radicals is observed, the most likely process leading to CH(A) formation is CHBr3 --> CH + 3Br rather than CHBr3 --> CH + Br + Br2. The rotational Boltzmann temperatures in the CH(A --> X) emission spectra for v' = 0 and v' = 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(Ipar - Iper)/(Ipar + 2Iper) = -0.024 +/- 0.005] is constant across the action spectrum; a small net absorption dipole of CHBr3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr3 molecule. The state distributions of the CH(A2Delta) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T(v'=0)(rot)= 4250 +/- 300 K; T(v'=1)(rot)= 3100 +/- 550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T(v'=0)(rot)= 3650 +/- 150 K; T(v'=1)(rot)= 2400 +/- 200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.

The effects of surface temperature on the gas-liquid interfacial reaction dynamics of O(3P) + squalane

Abstract: OH/OD product state distributions arising from the reaction of gas-phase O(3P) atoms at the surface of the liquid hydrocarbon squalane C30H62/C30D62 have been measured. The O(3P) atoms were generated by 355 nm laser photolysis of NO2 at a low pressure above the continually refreshed liquid. It has been shown unambiguously that the hydroxyl radicals detected by laser-induced fluorescence originate from the squalane surface. The gas-phase OH/OD rotational populations are found to be partially sensitive to the liquid temperature, but do not adapt to it completely. In addition, rotational temperatures for OH/OD(v'=1) are consistently colder (by 34+/-5 K) than those for OH/OD(v'=0). This is reminiscent of, but less pronounced than, a similar effect in the well-studied homogeneous gas-phase reaction of O(3P) with smaller hydrocarbons. We conclude that the rotational distributions are composed of two different components. One originates from a direct abstraction mechanism with product characteristics similar to those in the gas phase. The other is a trapping-desorption process yielding a thermal, Boltzmann-like distribution close to the surface temperature. This conclusion is consistent with that reached previously from independent measurements of OH product velocity distributions in complementary molecular-beam scattering experiments. It is further supported by the temporal profiles of OH/OD laser-induced fluorescence signals as a function of distance from the surface observed in the current experiments. The vibrational branching ratios for (v'=1)/(v'=0) for OH and OD have been found to be (0.07+/-0.02) and (0.30+/-0.10), respectively. The detection of vibrationally excited hydroxyl radicals suggests that secondary and/or tertiary hydrogen atoms may be accessible to the attacking oxygen atoms.

Oxidative dehydrogenation of propane over V_2O_5/MoO_3/Al_2O_3 and V_2O_5/Cr_2O_3/Al_2O_3 : Structural Characterization and catalytic function

Abstract: The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VOx and MoOx or VOx and CrOx on Al2O3 were examined using Raman and UV -visible spectroscopies, the dynamics of stoichiometric reduction in H 2, and the oxidative dehydrogenation of propane. VO x domains on Al2O3 modified by an equivalent MoO x monolayer led to dispersed binary structures at all surface densities. MoOx layers led to higher reactivity for VO x domains present at low VO x surface densities by replacing V-O-Al structures with more reactive V -O-Mo species. At higher surface densities, V -O-V structures in prevalent polyvanadates were replaced with less reactive V -O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV -visible data indicate that polyvanadates predominant on Al 2O3 convert to dispersed binary oxide structures when MoO x is deposited before or after VOx deposition; these structures are less reducible and show higher UV -visible absorption energies than polyvanadate structures on Al 2O3. The deposition sequence in binary Mo -V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO x on Al2O3 led to more reactive VOx domains than those deposited on pure Al2O3 at similar VOx surface densities. At all surface densities, the replacement of V -O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO x domains; it also led to higher propene selectivities via the selective inhibition of secondary C 3H6 combustion pathways, prevalent in VOx-Al2O3, and of C3H8 combustion routes that lead to low alkene selectivities on CrO x-Al2O3. VOx and CrOx mix significantly during synthesis or thermal treatment to form CrVO 4 domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO x domains can be modified significantly by the formation of binary dispersed structures. VO x-CrOx structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VOx domains present at similar surface densities on pure Al 2O3 supports.

Quantum State-Resolved Energy Transfer Dynamics at Gas-Liquid Interfaces: IR Laser Studies of CO^d2 Scattering from Perfluorinated Liquids

Abstract: An apparatus for detailed study of quantum state-resolved inelastic energy transfer dynamics at the gas-liquid interface is described. The approach relies on supersonic jet-cooled molecular beams impinging on a continuously renewable liquid surface in a vacuum and exploits sub-Doppler high-resolution laser absorption methods to probe rotational, vibrational, and translational distributions in the scattered flux. First results are presented for skimmed beams of jet-cooled CO(2) (T(beam) approximately 15 K) colliding at normal incidence with a liquid perfluoropolyether (PFPE) surface at E(inc) = 10.6(8) kcal/mol. The experiment uses a tunable Pb-salt diode laser for direct absorption on the CO(2) nu(3) asymmetric stretch. Measured rotational distributions in both 00(0)0 and 01(1)0 vibrational manifolds indicate CO(2) inelastically scatters from the liquid surface into a clearly non-Boltzmann distribution, revealing nonequilibrium dynamics with average rotational energies in excess of the liquid (T(s) = 300 K). Furthermore, high-resolution analysis of the absorption profiles reveals that Doppler widths correspond to temperatures significantly warmer than T(s) and increase systematically with the J rotational state. These rotational and translational distributions are consistent with two distinct gas-liquid collision pathways: (i) a T approximately 300 K component due to trapping-desorption (TD) and (ii) a much hotter distribution (T approximately 750 K) due to \"prompt\" impulsive scattering (IS) from the gas-liquid interface. By way of contrast, vibrational populations in the CO(2) bending mode are inefficiently excited by scattering from the liquid, presumably reflecting much slower T-V collisional energy transfer rates.

Global Search for Minimum Energy (H2O)n Clusters, n = 3−5

Abstract: The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere.