Experimental and modeling study of the structure of laminar premixed flames of Tetrahydrofuran/Oxygen/Argon

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1 Experimental and modeling study of the structure of laminar premixed flames of Tetrahydrofuran/xygen/Argon Luc-Sy Tran, Marco Verdicchio, Baptiste Sirjean, Pierre-Alexandre Glaude * Frédérique Battin-Leclerc Laboratoire Réactions et Génie des Procédés (LRGP), CNRS, Université de Lorraine, Nancy, France Abstract To better understand the combustion chemistry of tetrahydrofuran (TF), which is well suited as a model fuel to study the combustion chemistry of its derivatives considered as promising bio-fuels, the structure of laminar premixed low-pressure (. kpa) argon-diluted (%) flames of TF were studied at three equivalence ratios (φ=.,. and.) using gas chromatography analyses. The results consist of mole fraction profiles of about species identified and quantified as a function of the height above the burner. A mechanism for the oxidation of TF have been developed as a first attempt to simulate the TF combustion under flame conditions. verall, the agreement is encouraging for main species and C -C intermediates. owever, further studies are needed to improve the simulation of a number of larger intermediate species. Introduction Today an increasing interest is noted to shift from fossil fuels to bio-fuels. The use of bio-fuels allows a reduction of the dependence on petroleum-based fuels. Moreover burning bio-fuel should not lead to an increase of the total amount of greenhouse gases in the atmosphere. Cyclic ethers of tetrahydrofuran (TF) family, e.g. -methyltetrahydrofuran (MTF) and,-dimethyltetrahydrofuran (DMTF), have been considered as promising bio-fuel compounds for adding to gasoline. They have a high lower heating value (~.-9. MJ/L), which is close to that of furan family fuels (~.-. MJ/L) and of gasoline (~.MJ/L), and higher than that of ethanol (~. MJ/L) [-]. MTF has good antiknock characteristics, and satisfactory performance when mixed in a % blend with gasoline in conventional internal combustion engine []. The TF family fuels can be produced from non-edible biomass [,,]. Some tests for MTF as fuel on engine have been reported. Rudolph and Thomas [] have comparatively analyzed pollutant emissions from a spark-ignition engine performed on mixtures of gasoline with % potential liquid fuels, including ethanol, methanol, methyl t-butyl ether (MTBE), and MTF. The result has shown that the fuel blend containing % MTF has power outputs and carbon monoxide, nitrogen oxides, and non-methane hydrocarbons emissions that most closely resemble unleaded gasoline. MTF has been approved by the USDE as a component of P-series fuels for spark-ignition engine, which were firstly developed by Dr. Stephen Paul of Princeton University and awarded Patent number 99 by the United States Patent and Trademark ffice on December, 99. The P-series fuels are blends of ethanol (- % by vol.), MTF (- %), and C + alkanes (- %), with butane (- %) added to blends that would be used in severe cold-weather conditions to meet engine cold start requirements [-]. In addition, TF, MTF, DMTF and other saturated cyclic ethers have been also identified among the emissions produced during the combustion and autoignition of alkanes and alkenes by isomerization of alkylhydroperoxy radicals [-]. The subsequent reactions of these cyclic ethers can then influence the overall chemical kinetics mechanisms of alkanes and alkenes combustion. Therefore, the fundamental understanding of their combustion chemistry is really wishful, and need to be done before using MTF and DMTF as a fuel in practice. TF is well suited as a model fuel to study the combustion chemistry of saturated cyclic ethers, especially of its derivatives. Very early, the pyrolysis of TF has been studied by Klute and Walters [], Mcdonald et al. [], and Lifshitz et al. []. The later suggested two main possible decomposition pathways of TF which give: (i) ethylene and (C ) bi-radical, and (ii) propene and formaldehyde with a rate constant for the second channel four times lower compared to the first one. The low-temperature (9 K) oxidation of TF has been investigated by Molera et al. [], in a static reactor. A motored engine study on auto-ignition chemistry of acyclic and cyclic ethers, including MTBE, ethyl t-butyl ether-etbe, methyl t-amyl ether-tame, TF, MTF, and tetrahydropyran, has been performed by Leppard [9]. The chemical mechanisms responsible for auto-ignition of both ether classes are detailed, compared, and used to explain the differences in antiknock characteristics. The author explained that, due to a dominance of very reactive alkoxy-carbonyl radicals produced by low-temperature oxygen-addition chemistry in the cyclic ethers auto-ignition, octane number of the cyclic ethers is significantly lower than that of acyclic ethers. Later, the ignition delay times and oxidation of TF have been studied by Dagaut et al. [], in a single-pulse shock tube and in a jet-stirred * Corresponding author: pierre-alexandre.glaude@univ-lorraine.fr Proceedings of the European Combustion Meeting

2 reactor, respectively, under a range of conditions of - kpa,. φ., - K. n the basis of these experimental data, a detailed reaction mechanism with species and reactions was proposed and used to describe the ignition and the oxidation of TF. This study has shown that a large amount of aldehydes (formaldehyde, acetaldehyde, and propanal) were produced during the TF oxidation. Recently, Kasper et al. [] investigated the structure of laminar premixed low-pressure TF flames using photoionization (PI) and electron-ionization (EI) molecular-beam mass spectrometry (MBMS). About intermediates were measured and analyzed, but several assumptions have been made for species identification. Large uncertainties in mole fraction values were considered, especially for minor species, and species assignment at some mass (m/z) remained ambiguous and uncertain. Very recently, the kinetics and thermochemistry of TF, MTF and DMTF have been theoretically investigated by Simmie []. From bond dissociation energies (BDE) calculated in that study and their previous paper [], we present in Fig. the structure of TF, MTF, and DMTF, compared to that of furan, MF, and DMF. 9. (TF) 9... (Furan) C C (MTF) (MF).. C C C 99.. (DMTF) (DMF).. C Fig.. Structure of TF, MTF, and DMTF, compared with furan, MF, and DMF. Italic number near the atom: atom label; bold numbers: bond energy (in kcal mol - ) [,]. This figure shows that ring C- and C-C bonds of TF family fuels are much weaker than those of furan family fuels, while C- bonds (in methyl group) have an inverse trend. In MTF and DMTF molecules, C-C bond has the lowest BDE. In the saturated cyclic ethers, the remoteness from the heterocyclic atom strengthens the C- bond and the presence of methyl group slightly weakens the C- bond, while it is not significantly affected in the unsaturated cyclic ethers. The above bibliography shows that the information about the combustion properties of TF is very little in flame conditions. In the present work, as part of a continuing effort to enrich experimental data and to improve the knowledge on the combustion chemistry of this cyclic ether, we report experimental data of lowpressure premixed TF flames with Ar dilution of %, pressure of Torr (. kpa), using on-line gas chromatography (GC) analyses. A new mechanism for the oxidation of TF is proposed as a first attempt to simulate these flames results. Experimental methodology The experimental setup has been developed in the LRGP to study stable species profiles in a laminar premixed flat flame at low-pressure and has been described previously in [,]. Briefly, all flames were stabilized on a McKenna burner (diameter mm) housed in a vacuum chamber which is maintained at Torr (. kpa). The burner is cooled with water at a constant temperature of K. TF liquid fuel (>99. % pure) was supplied by VWR. Liquid fuel was mixed with argon and then evaporated by passing through a CEM (Controlled Evaporator and Mixer) set at K. Liquid and gas flow rates were measured by using mass flow controllers provided by BronKhorst, with a mass flow accuracy of ±.%. Analyses were made using a gas chromatograph (GC) with a heated on-line connection to the probe, three types of columns (Carbosphere, P-Plot Q, and P-Molsieve) and two types of detectors (a flame ionization detector - FID coupled with methanizer and a thermal conductivity detector - TCD). Stable species were identified by the determination of their individual retention times and by mass spectrometry (GC/MS). Flame temperature was measured using a PtRh (%)- PtRh (%) type B thermocouple diameter µm. The initial operating conditions of TF flames are summarized in Table. Table. Flame conditions Gas flow (NL/min) φ TF (gas) Ar Dilution (%) Flow velocity (cm s - ) Modeling A detailed chemical kinetic model for the hightemperature combustion of TF was generated using the automatic generator software EXGAS [,]. Several key kinetic parameters were updated to take into account the specificity of cyclic ethers combustion chemistry. The rate constants of -abstractions by Ḣ, Ȯ, Ȯ, Ċ and Ȯ radicals from TF have been estimated using the Evans-Polanyi correlation of Dean and Bozzelli []. The unimolecular decomposition routes of the resulting TF radicals, i.e, ring-opening reactions and subsequent isomerizations/s, were studied using CBS-QB [] electronic structure calculations and canonical transition state theory. The thermochemical properties of the species involved in the above-mentioned potential energy surface were included in the reaction mechanism to ensure consistency with the kinetic data. The final mechanism involves 9 species and reactions. Simulations were performed using PREMIX software from CEMKIN [9]. To compensate for the perturbations induced by sampling quartz probe and thermocouple, the temperature profile used in simulation is an average between the experimental profiles measured with and without the quartz probe, shifted. mm away from the burner surface.

3 Results and Discussion The carbon (C), hydrogen (), and oxygen () atom balances were checked in all flames. The difference between inlet and outlet is ~- % for C, ~-9 % for and. The chemical structure of the TF flames were investigated at three equivalences ratios (φ=.,. and.), and about species were identified and quantified. owever, only some selected important species of the stoichiometric flame (φ=.) will be presented and discussed in the following paragraphs. The results for the lean and fuel-rich flames are not presented, but it can be noted that the species identifications were similar for all three flames, but with different mole fraction of some intermediates reflecting the differences in equivalence ratios. x - TF Ar (/) As discussed above, a first version of mechanism for the combustion of TF has been developed using, for the estimation of the rate constants, a combination of EXGAS software data, Evans-Polanyi correlation, and CBS-QB theoretical method. This development has focused on the TF reactions and this first version contains a quite complete reaction base for C -C species, however an incomplete set for intermediates C. Therefore, only simulated profiles of reactants, main products and C -C intermediates are presented in the paper. Figures and - present temperature profiles and the mole fraction profiles of chemical species (major and intermediate, at φ=.) as a function of distance above the burner h. The reaction zone peaks at ~- mm above the burner. Figure displays the major species profiles, including reactants (TF and ), diluent (Ar), and main products (C, C,, and ). This figure shows that TF is completely consumed at a height of. mm. A significant mole fraction of (~ - ) remains in the post flame region. The main final products are, to a large extent, C and. The profile of C displays a marked maximum at a height of. mm (~9. - ). It can be seen that there is also a remaining mole fraction of C (~ - ) and (~. - ) in the post flame region. verall, the model satisfactorily reproduces the consumption of reactants (TF and ), the formation of main products (C, C,, and ), and consequently the diluent profiles (Ar). This applies to the profile shapes as well as the mole fraction values. Figure presents the simulated main consumption pathways of TF at h~. mm (T~ K and ~9 % conversion of TF). This figure shows that under these conditions, TF is mainly x Temperature (K) Temperature: With probe Without probe (~%) consumed by -abstractions from C or C positions of TF (the carbon atoms bound to the oxygen atom, see Fig. ) by the flame propagating radicals Ṙ, such as Ḣ, Ȯ, and Ȯ, to produce tetrahydrofur--yl radical (TF-yl-). -abstractions from C or C positions of TF giving the tetrahydrofur--yl radical (TF-yl-), have a lower contribution in the fuel consumption (~%). Figure displays the mole fraction profiles of C -C hydrocarbon intermediates, including methane (C ), acetylene (C ), ethylene (C ), ethane (C ), propyne (pc ), allene (ac ), propene (C ), and propane (C ). Ethylene (Fig. ) is the most abundant intermediate, with a maximum mole fraction of ~ -. Note that x - for unsaturated cyclic ethers such as furan family fuels [], acetylene is the most abundant intermediate. Under the same flame conditions and using the same analytical technique (GC), the ethylene and acetylene mole fractions reach higher levels in the TF flame than those in the ethanol flame reported by Tran et al. [] (a factor of ~.). In the TF flame published by Kasper et al. [] (φ=., Torr, and dilution ~ %), the mole fractions of ethylene, acetylene, and methane are ~- times higher than those in the present TF flame. As shown in Fig., ethylene can be directly formed from fuel through -abstraction from C or C positions, followed by some steps. It is in agreement with the discussions of Kasper et al. [] and Simmie []. The mole fraction profiles of C and C are well predicted by the model (Fig. ). C C α-scission C C + C (C ) -abstractions by R -abstractions by R (~%) (TF) (~%) (TF-yl-) C C x - C C C C + C (C ) (C C) C C C C C C (C -Y) (TF-yl-) + C Fig.. Main consumption pathways of TF at h=. mm (T~ K and ~9 % conversion of TF). The size of the arrows is proportional to the relative flow rates of consumption of a given species. Fig.. Major species and temperature profiles. Symbols: experiment; lines: simulation. C

4 x - x -.x - x - C C C C.. pc ac C C. Fig.. C -C hydrocarbon intermediates. Symbols: experiment; lines: simulation..x -..,-C,-C -C.x C -C i-c.x C : -methyl--butene -pentene -pentene x - Benzene. Among C hydrocarbon intermediates (Fig. ), C is the most abundant one, with a maximum mole fraction of - (at h=. mm). Dagaut et al. [] and Kasper et al. [] also found this trend in their studies. owever, in the stoichiometric TF flame of Kasper et al., the mole fraction of all C intermediates were measured with higher absolute values than those in the present TF flame (by a factor of -. for C and C, and of 9- for ac and pc ). This too large difference for ac and pc could be caused by the low signal-to-noise ratios of the PI-MBMS or by the difference of dilution ratio. Figure presents the C -C hydrocarbon intermediates, including three isomers of C -,-butadiene (,-C ),,-butadiene (,-C ), and -butyne (-C ), three isomers of C - -butene (- C ), -butene (-C ) and i-butene (i-c ), three isomers of C - -pentene (-C ), -pentene (- C ), and -methyl--butene, and benzene (C ). Among C isomers (Fig. ), the mole fraction of,-butadiene is the largest one with a maximum value of ~. -. For C isomers (Fig. ), -butene is the most abundant one (contribution of ~9% to the three C isomers), with a maximum mole fraction of ~ -. The mole fraction of n-butane (n-c, not shown in Fig. ) reaches a maximum of. -. ther C (not shown in Fig. ), e.g. diacetylene (C ) and iso-butane (ic ) were also detected, but with mole fractions lower than ppm. Among all C intermediates, -butene is the most abundant one followed by n-butane. Kasper et al. [] have detected n-butane, -butene, -butene, and,-butadiene as well in their TF flames using photoionization MBMS. -Butene,,-butadiene and diacetylene were also identified in the TF thermal decomposition studied by Lifshitz et al. []. nly.. Fig.. C -C hydrocarbon intermediates -butene has been identified and quantified in the TF oxidation study of Dagaut et al. [], using GC analysis. For the three C isomers (Fig. ), the contributions of -methyl--butene, -pentene, and -pentene are ~%, ~%, and ~%, respectively. The most abundant one is -methyl--butene, with a maximum mole fraction of ~. -. In the TF flames of Kasper et al. [], -pentene was also detected, but -pentene has been omitted and -methyl--butene was not seen. These C species have not been presented in the TF oxidation study of Dagaut et al. []. The cyclic hydrocarbon intermediates were detected with small mole fractions, ~. - for benzene (C ; Fig. ) and at trace level for,-cyclopentadiene (,-C ). Several oxygenated intermediates were detected in the TF flames. nly some important selected species are presented in the present paper as seen in Fig., including formaldehyde (C), acetaldehyde (C C), propanal (C C), acrolein (C C), acetone (C CC ),,-dihydrofuran (,-DF),,- dihydrofuran (,-DF), furan (C ), -butenal (C C), cyclopropane-carboxaldehyde (cy-c ), and MTF (C ). Formaldehyde, a toxic and cancerogenic species, is measured with the largest mole fraction (maximum of ~.9 - ). The model predicts reasonably the maximum mole fraction of this species. About - % of the formation of formaldehyde comes directly from the TF decomposition via -abstractions from C or C positions, followed by few reactions as seen in Fig.. owever, formaldehyde formation is also dominated by other reactions in the flame, such as Ȯ+Ċ =C+Ḣ, +Ċ C=C+Ȯ+C, C +C =C+C, +C =C +C,

5 x - C C C x - Propanal Acrolein Acetone x -,-DF,-DF Furane -Butenal x - Cyclopropanecarboxaldehyde MTF +Ċ =C+Ċ, and +C =C+. In comparison with formaldehyde, acetaldehyde is found in lower amount with a maximum mole fraction ~. -. The formation of this species is slightly over predicted by the model. An important part of acetaldehyde is produced from the combination of hydroperoxy radical (Ȯ ) and formylmethyl radical (Ċ C) which comes mostly from the decomposition of fuel (Fig. ). Under similar conditions, the ethanol flame reported by Tran et al. [] produced a much larger mole fraction of acetaldehyde (. - at φ=.), while the mole fraction of formaldehyde in the ethanol flame (. - at φ=.) is quite comparable to that in the TF flame. Note that acetaldehyde and formaldehyde mole fractions reach higher levels in the stoichiometric TF flame reported by Kasper et al. [] than those in the present TF flame (by a factor of ). owever, the ratio of formaldehyde to acetaldehyde ( ) is similar for the two studies at φ=. For other aldehydes, the mole fraction of propanal (~ - ) and acrolein (~ - ) reach higher levels than those of -butenal (~. - ) and cyclopropanecarboxaldehyde (. - ). In comparison with the studies of Dagaut et al. [] and Kasper et al. [], the acetaldehyde/propanal ratio - approximately ~-. - is quite comparable at φ=, despite the individual propanal and acetaldehyde mole fractions being different between the three studies. -Butenal was also identified in the TF flame reported by Kasper et al. []. The presence of cyclopropanecarboxaldehyde, as mentioned by Kasper et al [] can neither be confirmed nor excluded based on the flame-sampled PIE curves because of a complicated convolution of signals at m/z =. owever, this species is well identified and quantified in the present TF flames (Fig. ). As discussed by the team of Lifshitz [,], the isomerization of,-dihydrofuran can produce -butenal and cyclopropanecarboxaldehyde which subsequently isomerizes to -butenal as seen in Fig.. Figure shows that the,-dihydrofuran is ~. times more abundant than,-dihydrofuran, and its mole fraction reaches maximum ~. - at h=. mm. The peak mole fraction of furan is ~. - at h=. mm. s of MTF (~. - ) are comparable to that of,-dihydrofuran (~. - ). Kasper et al. [] have also detected,-dihydrofuran,,-dihydrofuran, and MTF in their TF flames. In the study of Dagaut et al. [], only three oxygenated intermediates (formaldehyde, acetaldehyde, and propanal) were measured; others such as dihydrofurans were detected at trace levels. (,-dihydrofuran) C Fig.. Selected oxygenated intermediates. Symbols: experiment; lines: simulation. (-butenal) (cyclopropane- carboxaldehyde) Fig.. Formation of cyclopropanecarboxaldehyde and -butenal from,-dihydrofuran [,]. ther C oxygenated intermediates (not shown in Fig. ), such as ethylene oxide, ketene, and dimethyl ether were also detected in the present TF flame, but with quite low amounts (less than ppm). Since ethylene oxide and ketene are very reactive, there is possibly a larger uncertainty in their mole fractions. Note that ethylene oxide and dimethyl ether were not found in both earlier studies of Dagaut et al. [] and Kasper et al. []. Conclusions This paper presents new experimental results about the chemical structure of low-pressure laminar premixed flame of TF using on-line GC and GC-MS. About stable species were identified and quantified as a function of distance to the burner. A number of selected important species have been presented, discussed and compared to those in previous studies reported in the literature. Among all intermediate species measured for the TF flames, the production of ethylene is the largest. Note that for unsaturated cyclic ethers such as furan family fuels acetylene is the most abundant one as reported in the literature. The comparison of the present results and that obtained for the combustion of ethanol studied previously under similar conditions shows that the combustion of tetrahydrofuranic fuels produces a much lower maximum mole fraction of acetaldehyde, compared to the ethanol combustion, while formaldehyde formation is quite comparable in both fuels classes. Despite the structure of laminar premixed low-pressure TF flames investigated using MBMS has been reported in the literature, it should be noted that

6 the information from another independent quantitative measurement such as GC was very valuable to confirm species identifications and to unambiguously identify the respective isomers. The obtained experimental results provide database for the analysis of intermediates in flames, which is being used to develop a new mechanism for TF combustion. A first version of the model is used to simulate the chemical structure of the TF flame. A comparison between simulations and experiments shows that, overall, the agreement is encouraging for main species and C -C intermediates. owever, the precision on the rate coefficients for some of the important reactions along the main TF combustion pathways need further studies, and a reactions base need to be set more completely. It is our ongoing works and future scope of this project. Acknowledgements This work was funded by the European Commission through the Clean ICE Advanced Research Grant of the European Research Council. References [] M. Thewes, M. Muether, S. Pischinger, M. Budde, A. Brunn, A. Sehr, P. Adomeit, J. Klankermayer, Energy Fuels () 9-. [] L.-S. Tran, B. Sirjean, P.-A. Glaude, R. Fournet, et F. Battin-Leclerc, Energy () -. [] J.M. Simmie, J. Phys. Chem. A () -. [] T.J. Wallington, W.. Siegl, R. Liu, Z. Zhang, R.E. uie, M.J. Kurylo, Environ. Sci. Technol. (99) [] W. Yang, A. Sen, ChemSusChem () 9-. [] J.-P. Lange, E. van der eide, J. van Buijtenen, R. Price, ChemSusChem (). [] T.W. Rudolph, J.J. Thomas, Biomass (9) -9. [] S.F. Paul, Alternative fuel, U.S. Patent 99. [9] S.F. Paul, Prepr. Symp. Am. Chem. Soc., Div. Fuel Chem. (99) -. [] DE, P-series Fuels Part II, Federal Register (999) -9. [] W.R. Leppard, SAE Transactions 9 (9) 9-9. [] W.R. Leppard, SAE Transactions 9 (99) 9-9. []. erbinet, F. Battin-Leclerc, S. Bax,. Le Gall, P.-A. Glaude, R. Fournet, Z. Zhou, L. Deng,. Guo, M. Xie, F. Qi, Phys. Chem. Chem. Phys. () 9-. []. erbinet, S. Bax, P.-A. Glaude, V. Carre, F. Battin-Leclerc, Fuel 9 () -. [] C.. Klute, W.D. Walters, J. Am. Chem. Soc. (9) -. [] G. Mcdonald, N.M. Lodge, W.D. Walters, J. Am. Chem. Soc. (9) -. [] A. Lifshitz, M. Bidani, S. Bidani, J. Phys. Chem. 9 (9) -9. [] M.J. Molera, A. Couto, J.A. Garcia-Dominguez, Int. J. Chem. Kinet. (9) -. [9] W.R. Leppard, SAE Transactions (99) 9-. [] P. Dagaut, M. McGuinness, J.M. Simmie, M. Cathonnet, Combust. Sci. Technol. (99) - 9. [] T. Kasper, A. Lucassen, A.W. Jasper, W. Li, P.R. Westmoreland, K. Kohse-öinghaus, B. Yang, J. Wang, T.A. Cool, N. ansen, Z. Phys. Chem. () -. [] J.M. Simmie,.J. Curran, J. Phys. Chem. A (9) -. [] E. Pousse, P.-A. Glaude, R. Fournet, F. Battin- Leclerc, Combust. Flame (9) 9-9. [] L.-S. Tran, P.-A. Glaude, R. Fournet, F. Battin- Leclerc, Energy Fuels () DI:./efx [] V. Warth, N. Stef, P.-A. Glaude, F. Battin- Leclerc, G. Scacchi, G.-M. Côme, Combustion and Flame (99) -. [] F. Buda, R. Bounaceur, V. Warth, P.-A Glaude, R. Fournet, F. Battin-Leclerc, Combust. Flame () -. [] A.M. Dean, J.W. Bozzelli, Gas-Phase Combustion Chemistry. NewYork: W.C. Gardiner, Springer-Verlag,. [] J.A. Montgomery, M.J. Frisch, J.W. chterski, G.A. Petersson, J. Chem. Phys. (999) -. [9] R.J. Kee, F.M. Rupley, J.A. Miller, Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics, Report No. SAND9-9, Sandia National Laboratories, 99. [] Z. Tian, T. Yuan, R. Fournet, P.-A. Glaude, B. Sirjean, F. Battin-Leclerc, K. Zhang, F. Qi, Combust. Flame () -. [] A. Lifshitz, M. Bidani, S. Bidani, J. Phys. Chem. 9 (99) 9-. [] F. Dubnikova, A. Lifshitz, J. Phys. Chem. A ()-.