Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm(3) molecule(-1) s(-1)) = -13.25 +/- 0.16 and E-a = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.

An investigation of the germylene addition reaction, GeH2+C2H2: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Time resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with acetylene, C2H2. The reaction was studied in the gas-phase over the pressure range 1-100 Tort, with SF6 as bath gas, at 5 temperatures in the range 297-553 K. The reaction showed a very slight pressure dependence at higher temperatures. The high pressure rate constants (obtained by extrapolation at the three higher temperatures) gave the Arrhenius equation: log(k(infinity)/cm(3) molecule(-1) s(-1)) (-10.94 +/- 0.05) + (6.10 +/- 0.36 kJ mol(-1))/RTln10. These Arrhenius parameters are consistent with a fast reaction occurring at approximately 30% of the collision rate at 298 K. Quantum chemical calculations (both DFT and ab initio G2//B3LYP and G2//QCISD) of the GeC2H4 potential energy surface (PES), show that GeH2 + C2H2 react initially to form germirene which can isomerise to vinylgermylene with a relatively low barrier. RRKM modelling, based on a loose association transition state, but assuming vinylgermylene is the end product (used in combination with a weak collisional deactivation model) predicts a strong pressure dependence using the calculated energies, in conflict with the experimental evidence. The detailed GeC2H4 PES shows considerable complexity with ten other accessible stable minima (B3LYP level), the three most stable of which are all germylenes. Routes through this complex surface were examined in detail. The only product combination which appears capable of satisfying the (P-3) + C2H4.C2H4 was confirmed as a product by GC observed lack of a strong pressure dependence is Ge(P-3) + C2H4. C2H4 was confirmed as a product by GC analysis. Although the formation of these products are shown to be possible by singlet-triplet curve crossing during dissociation of 1-germiranylidene (1-germacyclopropylidene), it seems more likely (on thermochernical grounds) that the triplet biradical, (GeCH2CH2.)-Ge-., is the immediate product precursor. Comparisons are made with the reaction of SiH2 with C2H2.

The addition reaction between silylene and ethyne: further isotope studies, pressure dependence studies, and quantum chemical calculations

Time-resolved kinetic studies of the reaction of dideutero-silylene, SiD2, generated by laser flash photolysis of phenylsilane-d(3), have been carried out to obtain rate constants for its bimolecular reaction with C2H2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 297-600 K. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equation log(k(infinity)/cm(3) molecule(-1) s(-1)) = (-10.05 +/- 0.05) + (3.43 +/- 0.36 kJ mol(-1))/RT ln 10. The rate constants were used to obtain a comprehensive set of isotope effects by comparison with earlier obtained rate constants for the reactions of SiH2 with C2H2 and C2D2. Additionally, pressure-dependent rate constants for the reaction of SiH2 with C2H2 in the presence of He (1-100 Tort) were obtained at 300, 399, and 613 K. Quantum chemical (ab initio) calculations of the SiC2H4 reaction system at the G3 level support the initial formation of silirene, which rapidly isomerizes to ethynylsilane as the major pathway. Reversible formation of vinylsilylene is also an important process. The calculations also indicate the involvement of several other intermediates, not previously suggested in the mechanism. RRKM calculations are in semiquantitative agreement with the pressure dependences and isotope effects suggested by the ab initio calculations, but residual discrepancies suggest the possible involvement of the minor reaction channel, SiH2 + C2H2 - SWPO + C2H4. The results are compared and contrasted with previous studies of this reaction system.

Investigation of the prototype silylene reaction, SiH2+H2O(and D2O): time-resolved gas-phase kinetic studies, isotope effects, RRKM calculations, and quantum chemical calculations of the reaction energy surface

Time-resolved kinetic studies of the reaction of silylene, SiH2, with H2O and with D2O have been carried out in the gas phase at 296 and at 339 K, using laser flash photolysis to generate and monitor SiH2. The reaction was studied over the pressure range 10-200 Torr with SF6 as bath gas. The second-order rate constants obtained were pressure dependent, indicating that the reaction is a third-body assisted association process. Rate constants at 339 K were about half those at 296 K. Isotope effects, k(H)/k(D), were small averaging 1.076 0.080, suggesting no involvement of H- (or D-) atom transfer in the rate determining step. RRKM modeling was undertaken based on a transition state appropriate to formation of the expected zwitterionic donoracceptor complex, H2Si...OH2. Because the reaction is close to the low pressure (third order) region, it is difficult to be definitive about the activated complex structure. Various structures were tried, both with and without the incorporation of rotational modes, leading to values for the high-pressure limiting (i.e., true secondorder) rate constant in the range 9.5 x 10(-11) to 5 x 10(-10) cm(3) molecule' s(-1). The RRKM modeling and mechanistic interpretation is supported by ab initio quantum calculations carried out at the G2 and G3 levels. The results are compared and contrasted with the previous studies.

Calculations of rovibrational energies and dipole transition intensities for polyatomic molecules using MULTIMODE

We report rigorous calculations of rovibrational energies and dipole transition intensities for three molecules using a new version of the code MULTIMODE. The key features of this code which permit, for the first time, such calculations for moderately sized but otherwise general polyatomic molecules are briefly described. Calculations for the triatomic molecule BF(2) are done to validate the code. New calculations for H(2)CO and H(2)CS are reported; these make use of semiempirical potentials but ab initio dipole moment surfaces. The new dipole surface for H(2)CO is a full-dimensional fit to the dipole moment obtained with the coupled-cluster with single and double excitations and a perturbative treatment of triple excitations method with the augmented correlation consistent triple zeta basis set. Detailed comparisons are made with experimental results from a fit to relative data for H(2)CS and absolute intensities from the HITRAN database for H(2)CO.

Antimalarial drugs based on artemisinin: DFT calculations on the principal reactions

Theoretical calculations have been carried out on the interactions of several endoperoxides which are potential antimalarials, including the clinically useful artemisinin, with two possible sources of iron in the parasite, namely the hexa-aquo ferrous ion [Fe(H2O)(6)](2+) and haeme. DFT calculations show that the reactions of all endoperoxides considered, with both sources of iron, initially generate a Fe-O bond followed by cleavage of the O-O bond to oxygen radical species. Subsequently, they can be transformed into carbon-centred radicals of greater stability. However, with [Fe(H2O)(6)](2+) as the iron source, the oxygen-centred radical species are more likely to react further akin to Fenton's reagent, whereby iron salts encourage hydrogen peroxide to act as an oxidizing agent, and that solvent plays a major role. In contrast, when reacting with haeme, the oxygen-centred radicals interconvert to more stable carbon-centred radicals, which can then alkylate haeme. Subsequent cleavage of the Fe-O bond leads to stable and inactive antimalarial products. These results indicate that the reactivity of the endoperoxides as antimalarials is greater with iron hexahydrates for radical-mediated damage as opposed to haeme, which leads to unreactive species. Since only nanomolar quantities of hydrated metal ions could catalyse the reactions leading to damage to the parasites, this could be an alternative or competitive reaction responsible for the antimalarial activity. (c) 2005 Elsevier B.V. All rights reserved.

Large vibrational variational calculations using 'multimode' and an iterative diagonalization technique

We have favoured the variational (secular equation) method for the determination of the (ro-) vibrational energy levels of polyatomic molecules. We use predominantly the Watson Hamiltonian in normal coordinates and an associated given potential in the variational code 'Multimode'. The dominant cost is the construction and diagonalization of matrices of ever-increasing size. Here we address this problem, using pertubation theory to select dominant expansion terms within the Davidson-Liu iterative diagonalization method. Our chosen example is the twelve-mode molecule methanol, for which we have an ab initio representation of the potential which includes the internal rotational motion of the OH group relative to CH3. Our new algorithm allows us to obtain converged energy levels for matrices of dimensions in excess of 100 000.

The chemistry of intrinsically chiral surfaces

Intrinsically chiral metal and mineral surfaces show enantioselective behaviour without modifiers. Examples are artificial high-Miller-index surfaces of metal single crystals with cubic bulk lattice symmetry, which have no mirror planes and are therefore chiral, or surfaces of naturally occurring crystallites of some common minerals, such as alpha-quartz or calcite. Recent findings with regards to the surface geometry, reactivity and thermal stability of intrinsically chiral surfaces are discussed. A number of enantioselective effects have been reported in connection with the adsorption of small chiral molecules (e.g. alanine, cysteine) on intrinsically chiral surfaces under well-defined conditions. From a combination of experimental surface science techniques and theoretical ab initio model calculations it emerges that these effects are due to a combination of attractive and repulsive adsorbate-substrate and inter-adsorbate interactions.

Full-dimensional vibrational calculations for H5O2+ using an ab initio potential energy surface

We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H5O2+ using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.

Tuning of redox potential and visible absorption band of ruthenium(II) complexes of (benzimidazolyl) derivatives: synthesis, characterization, spectroscopic and redox properties, X-ray structures and DFT calculations

Six ruthenium(II) complexes have been prepared using the tridentate ligands 2,6-bis(benzimidazolyl) pyridine and bis(2-benzimidazolyl methyl) amine and having 2,2'-bipyridine, 2,2':6',2 ''-terpyridine, PPh3, MeCN and chloride as coligands. The crystal structures of three of the complexes trans-[Ru(bbpH(2))(PPh3)(2)(CH3CN)I(ClO4)(2) center dot 2H(2)O (2), [Ru(bbpH(2))(bpy)Cl]ClO4 (3) and [Ru(bbpH(2))(terpy)](ClO4)(2) (4) are also reported. The complexes show visible region absorption at 402-517 nm, indicating that it is possible to tune the visible region absorption by varying the ancillary ligand. Luminescence behavior of the complexes has been studied both at RT and at liquid nitrogen temperature (LNT). Luminescence of the complexes is found to be insensitive to the presence of dioxygen. Two of the complexes [Ru(bbpH(2))(bpy)Cl]ClO4 (3) and [Ru(bbpH(2))(terpy]ClO4)(2) (4) show RT emission in the NIR region, having lifetime, quantum yield and radiative constant values suitable for their application as NIR emitter in the solid state devices. The DFT calculations on these two complexes indicate that the metal t(2g) electrons are appreciably delocalized over the ligand backbone. (C) 2006 Elsevier B.V. All rights reserved.

Evaluation of data for atmospheric models: master equation/RRKM calculations on the combination reaction, BrO+NO2 -> BrONO2, a conundrum

Experimental data for the title reaction were modeled using master equation (ME)/RRKM methods based on the Multiwell suite of programs. The starting point for the exercise was the empirical fitting provided by the NASA (Sander, S. P.; Finlayson-Pitts, B. J.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Orkin, V. L.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15; Jet Propulsion Laboratory: Pasadena, California, 2006)(1) and IUPAC (Atkinson, R.; Baulch, D. L.; Cox, R. A.: R. F. Hampson, J.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data. 2000, 29, 167) 2 data evaluation panels, which represents the data in the experimental pressure ranges rather well. Despite the availability of quite reliable parameters for these calculations (molecular vibrational frequencies (Parthiban, S.; Lee, T. J. J. Chem. Phys. 2000, 113, 145)3 and a. value (Orlando, J. J.; Tyndall, G. S. J. Phys. Chem. 1996, 100,. 19398)4 of the bond dissociation energy, D-298(BrO-NO2) = 118 kJ mol(-1), corresponding to Delta H-0(circle) = 114.3 kJ mol(-1) at 0 K) and the use of RRKM/ME methods, fitting calculations to the reported data or the empirical equations was anything but straightforward. Using these molecular parameters resulted in a discrepancy between the calculations and the database of rate constants of a factor of ca. 4 at, or close to, the low-pressure limit. Agreement between calculation and experiment could be achieved in two ways, either by increasing Delta H-0(circle) to an unrealistically high value (149.3 kJ mol(-1)) or by increasing , the average energy transferred in a downward collision, to an unusually large value (> 5000 cm(-1)). The discrepancy could also be reduced by making all overall rotations fully active. The system was relatively insensitive to changing the moments of inertia in the transition state to increase the centrifugal effect. The possibility of involvement of BrOONO was tested and cannot account for the difficulties of fitting the data.

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcal/mol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively. (C) 2008 American Institute of Physics.

MULTIMODE quantum calculations of intramolecular vibrational energies of the water dimer and trimer using ab initio-based potential energy surfaces

We report vibrational configuration interaction calculations of the monomer fundamentals of (H2O)(2), (D2O)(2), (H2O)(3), and (D2O)(3) using the code MULTIMODE and full dimensional ab initio-based global potential energies surfaces (PESs). For the dimer the HBB PES [Huang , J. Chem. Phys 128, 034312 (2008)] is used and for the trimer a new PES, reported here, is used. The salient properties of the new trimer PES are presented and compared to previous single-point calculations and the vibrational energies are compared with experiments. (C) 2008 American Institute of Physics.

Tests of MULTIMODE calculations of rovibrational energies of CH4

We report variational calculations of rovibrational energies of CH4 using the code MULTIMODE and an ab initio force field of Schwenke and Partridge. The systematic convergence of the energies with respect to the level of mode coupling is presented. Converged vibrational energies calculated using the five-mode representation of the potential for zero total angular momentum are compared with previous, benchmark calculations based on Radau coordinates using this force field for zero total angular momentum and for J = 1. Very good agreement with the previous benchmark calculations is found. (c) 2006 Elsevier B.V. All rights reserved.