Porous silica gel has been prepared by acdification of water glass. To study aggregation, gelatio... more Porous silica gel has been prepared by acdification of water glass. To study aggregation, gelation andaging use has been made of 29Si-NMR to investigate silica transformations on molecular scale. Qn ratios were used to define distribution of silica in particles and gels. On colloidal scale both 1H relaxation of water and small angle scattering (SAXS, SANS) was very informative, especially because changes in fractal dimension could be used to describe silica transformations during aggregation and aging. Interpretation of fractal dimension in terms of aging mechanisms is performed by computersimulations of both aggregation/aging processes and calculation of the corresponding scattering spectra. Comparison of simulated spectra of aged silica, based on hypothetical aging mechanisms, with experimental spectra emphasized the important role of formation of rings on local scale. In freeze-dried silicas pore distributions were investigated with both neutron scattering (SANS) and physisorptio...
tion of reactive groups on the surface of the particles is During the preparation of silica by ac... more tion of reactive groups on the surface of the particles is During the preparation of silica by acidi®cation of water glass, stochastic, directions of particle–particle bonds are rather primary silica particles form extended and rami®ed aggregates. arbitrary and therefore particle–particle interactions result The growing aggregates form a gel, a tenuous network of intercon- in tenuous aggregates with an amorphous structure. Interac-nected aggregates. After aging and drying of the wet gel, porous tions are not restricted to particle–particle or particle–aggre-silica is obtained. To study the extremely vulnerable aggregates gate bonds, and interactions between aggregates or clustersonly noninvasive methods are allowed. Moreover, because of the of particles are also possible, leading to highly ramified andcolloidal scale many methods based on (atomic or molecular scale) extended aggregates.spectroscopy are not informative. Scanning transmission x-ray mi-Polymerization or particle format...
The resolution available in the King's College London scanning transmission x-ray microscope ... more The resolution available in the King's College London scanning transmission x-ray microscope (STXM) can be exploited to study aggregate structures over a length scale from 100 nm to 10 micrometers that overlaps with and complements that available from small-angle x-ray scattering (SAXS) data. It is then possible to use these combined sets of data to test between different growth models
ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) t... more ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.
ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) t... more ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.
International Journal of Mass Spectrometry and Ion Processes, 1992
The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G**... more The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G** level of theory and by mass spectrometric experiments. The calculations confirm earlier experimental observations that the diazapropylium ions CH3---NH=NH+, 1+, CH3---N=NH+2, 2+ and CH2---NH---NH+2, 3+ and the hydrazonium ion CH2=N---NH+3, 4+, are stable species. Theory predicts 1+ and 2+ to be higher in energy than 3+, by 7-8 kcal mol-1, causing a serious discrepancy with existing experimental values, which indicate that 1+ and 2+ are considerably more stable than 3+. The theoretical values are insensitive to inclusion of electron correlation in the geometry determinations. From a critical evaluation of existing energetic data for N2H+3, CH5N+2 and C2H7N+ ions, and collision experiments on deuterium labelled species, it is concluded that theory is correct and that several reported appearance energy (AE) measurements on hydrazines are probably in error owing to interferences from traces of amines. From AE measurements not affected by these interferences, [Delta]Hf(3+) is proposed to be 204 ± 5 kcal mol-1 from which theory leads us to recommend [Delta]Hf values of 211 ± 5 kcal mol for 1+ and 2+. Ab initio calculated proton affinities for HN=NH, CH3---N=NH and CH3-N---N=CH3 lead to proposed enthalpies for 1+ and 2+ which are consistent with these values. Theory further predicts the ring-closed form of 3+ to be a remarkably stable species (16.7 kcal mol-1 above 3+) but the hydrogen bridged entity CH2 = N 2H2+ previously proposed to be responsible for the facile interconversion between 3+ and 4+, is not a minimum on the potential energy surface. In fact, large energy barriers (42-63 kcal mol-1) prohibit interconversion among ions 1+, 2+, 3+ and 4+, via 1,2-H shifts. Metastable CH5 N+2 ions dissociate to HC[triple bond; length as m-dash]N + NH+4 and to HCNH+ + NH3 and in agreement with experiment, the reacting configuration for HC[triple bond; length as m-dash]N formation is the ion 4+. Formation of HC[triple bond; length as m-dash]N from 4+ is exothermic but the reverse barrier is large (84 kcal mol-1) thus accounting for the persistence of 4+ in the gas phase and in neutral solvents. The small kinetic energy release (KER) accompanying this reaction is rationalized in terms of ion/dipole attraction in the dissociating [HC[triple bond; length as m-dash]N...NH4]+ complex.
International Journal of Mass Spectrometry and Ion Processes, 1992
The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G**... more The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G** level of theory and by mass spectrometric experiments. The calculations confirm earlier experimental observations that the diazapropylium ions CH3NHNH+, 1+, CH3NNH+2, 2+ and CH2NHNH+2, 3+ and the hydrazonium ion CH2NNH+3, 4+, are stable species. Theory predicts 1+ and 2+ to be higher in energy than 3+, by 7–8 kcal mol−1, causing a serious discrepancy with existing experimental values, which indicate that 1+ and 2+ are considerably more stable than 3+. The theoretical values are insensitive to inclusion of electron correlation in the geometry determinations. From a critical evaluation of existing energetic data for N2H+3, CH5N+2 and C2H7N+ ions, and collision experiments on deuterium labelled species, it is concluded that theory is correct and that several reported appearance energy (AE) measurements on hydrazines are probably in error owing to interferences from traces of amines.From AE measurements not affected by these interferences, ΔHf(3+) is proposed to be 204 ± 5 kcal mol−1 from which theory leads us to recommend ΔHf values of 211 ± 5 kcal mol for 1+ and 2+. Ab initio calculated proton affinities for HNNH, CH3NNH and CH3-NNCH3 lead to proposed enthalpies for 1+ and 2+ which are consistent with these values.Theory further predicts the ring-closed form of 3+ to be a remarkably stable species (16.7 kcal mol−1 above 3+) but the hydrogen bridged entity CH2 = N 2H2+ previously proposed to be responsible for the facile interconversion between 3+ and 4+, is not a minimum on the potential energy surface. In fact, large energy barriers (42–63 kcal mol−1) prohibit interconversion among ions 1+, 2+, 3+ and 4+, via 1,2-H shifts.Metastable CH5 N+2 ions dissociate to HCN + NH+4 and to HCNH+ + NH3 and in agreement with experiment, the reacting configuration for HCN formation is the ion 4+. Formation of HCN from 4+ is exothermic but the reverse barrier is large (84 kcal mol−1) thus accounting for the persistence of 4+ in the gas phase and in neutral solvents. The small kinetic energy release (KER) accompanying this reaction is rationalized in terms of ion/dipole attraction in the dissociating [HCN⋯NH4]+ complex.
Porous silica gel has been prepared by acdification of water glass. To study aggregation, gelatio... more Porous silica gel has been prepared by acdification of water glass. To study aggregation, gelation andaging use has been made of 29Si-NMR to investigate silica transformations on molecular scale. Qn ratios were used to define distribution of silica in particles and gels. On colloidal scale both 1H relaxation of water and small angle scattering (SAXS, SANS) was very informative, especially because changes in fractal dimension could be used to describe silica transformations during aggregation and aging. Interpretation of fractal dimension in terms of aging mechanisms is performed by computersimulations of both aggregation/aging processes and calculation of the corresponding scattering spectra. Comparison of simulated spectra of aged silica, based on hypothetical aging mechanisms, with experimental spectra emphasized the important role of formation of rings on local scale. In freeze-dried silicas pore distributions were investigated with both neutron scattering (SANS) and physisorptio...
tion of reactive groups on the surface of the particles is During the preparation of silica by ac... more tion of reactive groups on the surface of the particles is During the preparation of silica by acidi®cation of water glass, stochastic, directions of particle–particle bonds are rather primary silica particles form extended and rami®ed aggregates. arbitrary and therefore particle–particle interactions result The growing aggregates form a gel, a tenuous network of intercon- in tenuous aggregates with an amorphous structure. Interac-nected aggregates. After aging and drying of the wet gel, porous tions are not restricted to particle–particle or particle–aggre-silica is obtained. To study the extremely vulnerable aggregates gate bonds, and interactions between aggregates or clustersonly noninvasive methods are allowed. Moreover, because of the of particles are also possible, leading to highly ramified andcolloidal scale many methods based on (atomic or molecular scale) extended aggregates.spectroscopy are not informative. Scanning transmission x-ray mi-Polymerization or particle format...
The resolution available in the King's College London scanning transmission x-ray microscope ... more The resolution available in the King's College London scanning transmission x-ray microscope (STXM) can be exploited to study aggregate structures over a length scale from 100 nm to 10 micrometers that overlaps with and complements that available from small-angle x-ray scattering (SAXS) data. It is then possible to use these combined sets of data to test between different growth models
ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) t... more ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.
ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) t... more ABSTRACTCombination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.
International Journal of Mass Spectrometry and Ion Processes, 1992
The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G**... more The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G** level of theory and by mass spectrometric experiments. The calculations confirm earlier experimental observations that the diazapropylium ions CH3---NH=NH+, 1+, CH3---N=NH+2, 2+ and CH2---NH---NH+2, 3+ and the hydrazonium ion CH2=N---NH+3, 4+, are stable species. Theory predicts 1+ and 2+ to be higher in energy than 3+, by 7-8 kcal mol-1, causing a serious discrepancy with existing experimental values, which indicate that 1+ and 2+ are considerably more stable than 3+. The theoretical values are insensitive to inclusion of electron correlation in the geometry determinations. From a critical evaluation of existing energetic data for N2H+3, CH5N+2 and C2H7N+ ions, and collision experiments on deuterium labelled species, it is concluded that theory is correct and that several reported appearance energy (AE) measurements on hydrazines are probably in error owing to interferences from traces of amines. From AE measurements not affected by these interferences, [Delta]Hf(3+) is proposed to be 204 ± 5 kcal mol-1 from which theory leads us to recommend [Delta]Hf values of 211 ± 5 kcal mol for 1+ and 2+. Ab initio calculated proton affinities for HN=NH, CH3---N=NH and CH3-N---N=CH3 lead to proposed enthalpies for 1+ and 2+ which are consistent with these values. Theory further predicts the ring-closed form of 3+ to be a remarkably stable species (16.7 kcal mol-1 above 3+) but the hydrogen bridged entity CH2 = N 2H2+ previously proposed to be responsible for the facile interconversion between 3+ and 4+, is not a minimum on the potential energy surface. In fact, large energy barriers (42-63 kcal mol-1) prohibit interconversion among ions 1+, 2+, 3+ and 4+, via 1,2-H shifts. Metastable CH5 N+2 ions dissociate to HC[triple bond; length as m-dash]N + NH+4 and to HCNH+ + NH3 and in agreement with experiment, the reacting configuration for HC[triple bond; length as m-dash]N formation is the ion 4+. Formation of HC[triple bond; length as m-dash]N from 4+ is exothermic but the reverse barrier is large (84 kcal mol-1) thus accounting for the persistence of 4+ in the gas phase and in neutral solvents. The small kinetic energy release (KER) accompanying this reaction is rationalized in terms of ion/dipole attraction in the dissociating [HC[triple bond; length as m-dash]N...NH4]+ complex.
International Journal of Mass Spectrometry and Ion Processes, 1992
The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G**... more The CH5N+2 system has been investigated by ab initio MO calculations at the SDCl/6-31G**//6-31G** level of theory and by mass spectrometric experiments. The calculations confirm earlier experimental observations that the diazapropylium ions CH3NHNH+, 1+, CH3NNH+2, 2+ and CH2NHNH+2, 3+ and the hydrazonium ion CH2NNH+3, 4+, are stable species. Theory predicts 1+ and 2+ to be higher in energy than 3+, by 7–8 kcal mol−1, causing a serious discrepancy with existing experimental values, which indicate that 1+ and 2+ are considerably more stable than 3+. The theoretical values are insensitive to inclusion of electron correlation in the geometry determinations. From a critical evaluation of existing energetic data for N2H+3, CH5N+2 and C2H7N+ ions, and collision experiments on deuterium labelled species, it is concluded that theory is correct and that several reported appearance energy (AE) measurements on hydrazines are probably in error owing to interferences from traces of amines.From AE measurements not affected by these interferences, ΔHf(3+) is proposed to be 204 ± 5 kcal mol−1 from which theory leads us to recommend ΔHf values of 211 ± 5 kcal mol for 1+ and 2+. Ab initio calculated proton affinities for HNNH, CH3NNH and CH3-NNCH3 lead to proposed enthalpies for 1+ and 2+ which are consistent with these values.Theory further predicts the ring-closed form of 3+ to be a remarkably stable species (16.7 kcal mol−1 above 3+) but the hydrogen bridged entity CH2 = N 2H2+ previously proposed to be responsible for the facile interconversion between 3+ and 4+, is not a minimum on the potential energy surface. In fact, large energy barriers (42–63 kcal mol−1) prohibit interconversion among ions 1+, 2+, 3+ and 4+, via 1,2-H shifts.Metastable CH5 N+2 ions dissociate to HCN + NH+4 and to HCNH+ + NH3 and in agreement with experiment, the reacting configuration for HCN formation is the ion 4+. Formation of HCN from 4+ is exothermic but the reverse barrier is large (84 kcal mol−1) thus accounting for the persistence of 4+ in the gas phase and in neutral solvents. The small kinetic energy release (KER) accompanying this reaction is rationalized in terms of ion/dipole attraction in the dissociating [HCN⋯NH4]+ complex.
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Papers by Harold van Garderen