Sulfidosilanes: Complex Molecule Systems via Phase Transfer

Chapter 5

Sulfidosilanes: Complex Molecule Systems via Phase Transfer Catalysis Downloaded by STANFORD UNIV GREEN LIBR on August 2, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch005

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Michael W. Backer , John M. Gohndrone , Simon D. Cook , Simeon J. Bones , and Raquel Vaquer-Perez 1,†

1,†

1

3

Surface and Interface Solutions Center and Service Enterprise Unit, Analytical Sciences, Dow Corning Limited, Cardiff Road, Barry, South Glamorgan, CF63 2YL, United Kingdom Specialty Intermediates Technology Center, Dow Corning Corporation, 2200 West Salzburg Road, Midland, MI 48686

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The complete series of ethoxy and methyl substituted tetrasulfidosilanes (EtO) Me Si(CH ) -S -(CH ) -SiMe (OEt) , with x = 0 - 3, has been prepared via economically favourable and environmentally friendly phase transfer catalysis (PTC) process. UV-absorption spectra and H, C, and Si Nuclear Magnetic Resonance spectra of these systems, actually mixtures of silanes of various sulfur chain length ranging from disulfane S2 up to dodecylsulfane S12 with an average sulfur value (rank) of about 3.75, are discussed in dependence of the varying substitution pattern. Based on the results, response factors in High Pressure Liquid Chromatography analysis are proposed for each sulfane species per silane reflecting the different weight percentage rates of the sulfur chromophore. 3-x

x

2

3

4

1

2

13

3

x

3-x

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Introduction During the last three decades sulfur containing organosilicon compounds such as bis(triethoxysilylpropyl)disulfane (TESPD) and bis(triethoxysilylpropyl)tetrasulfane (TESPT) 1 have become very important coupling agents (/) in a variety of industrial applications. In particular, they have become essential components in the production of automotive tires based on filler reinforced © 2007 American Chemical Society

In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Downloaded by STANFORD UNIV GREEN LIBR on August 2, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch005

50 rubber vulcanizates. (2) The combination of e.g. precipitated silica and sulfidosilanes is improving the physical properties of the tires such as rolling resistance and wet skidding performance, while maintaining abrasion resistance, due to strong coupling between the inorganic filler and the rubber matrix. (3) Numerous methods have been described in the art of preparation of sulfur containing organosilicon compounds. While in earlier times reactions of alkali metal sulfides with elemental sulfur and halohydrocarbyl alkoxysilanes had to be carried out mainly in alcohols under strictly anhydrous conditions, nowadays modern phase transfer catalysis (PTC) techniques (4,5) have been developed for the production of sulfur containing organosilicon compounds in the presence of an aqueous phase (6,7,8,9) and steadily improved to an efficient, and environmentally friendly process that can be safely performed on an industrial scale. (10,11,12,13) During the development of the PTC process, pH control of the aqueous phase and thefine-tuningof the correct oligosulfane distribution by adjustment of the accurate amount of elemental sulfur added to an ionic sulfide species have been identified as critical and very sensitive parameters. (14) Throughout the years, it has been demonstrated that TESPT actually is a mixture of sulfanes with a range from disulfane (S2) to decasulfane (S10) having an average sulfur chain length of around 3.75 - 3.8, and that TESPD normally contains polysulfanes from S2 to S5 with an average sulfur chain length of about 2.1 - 2.15. (75) For quality control purposes and determination of oligosulfane distribution mostly High Pressure Liquid Chromatography (HPLC) techniques are routinely used. (16) However, Nuclear Magnetic Resonance (NMR) and Mass Spectrometry (MS) have also been successfully applied. (17,18) In this paper, the complete series of ethoxy and methyl substituted tetrasulfidosilanes (EtO) . Me Si(CH2)3-S4-(CH2)3-SiMe (OEt) . , with χ = 0 (TESPT 1), 1 (DEMSPT 2), 2 (DMESPT 3), and 3 (TMSPT 4) has been prepared via PTC process. The products have been analysed via HPLC. The UV-absorption spectra of each individual sulfane species per silane series have been recorded and compared. The proton, carbon, and silicon NMR spectra of the silanes have also been taken and discussed in context with the continuously reduced polarity at the silicon center when moving from TESPT 1 to bis(trimethylsilylpropyl)tetrasulfane TMSPT 4. The spectra have been used to conclude on obvious similarities in the sulfidosilane systems in order to calculate reliable response factors for HPLC analysis. 3 x

x

x

3 x

Experimental Compound preparation In a typical experiment under PTC conditions, flaked sodium hydrogensulfide hydrate has been heated with an equimolar amount of alkali hydroxide solution to elevated temperatures (between 60 and 85 °C) in enough water to



51 keep the mixture soluble. The specifically calculated molar amounts of elemental sulfur have then been added under vigorous stirring, and a short period for equilibration of the generated oligosulfide species has been allowed. After addition of the PTC catalyst, tetrabutylammonium bromide (TBAB), two molar equivalents (to HS") of the according chloropropylsilane have been added over a period that permitted control of the exotherm. The nucleophilic substitution reaction was followed by gas chromatography analysis until chloropropylsilane had disappeared or reached a stable level. The mixture was cooled down and water added to dissolve formed alkali chloride/bromide. The aqueous phase was drained off, and the organic phase was filtered and dried according to various known methods. A typical equipment setup is displayed in Figure 1.

Figure 1. Experimental setup of Phase Transfer Catalysis process.

HPLC analysis The distribution of the various sulfur containing compounds was analyzed by High Pressure Liquid Chromatography (HPLC). Typical run conditions were as follows: the reaction samples were diluted in cyclohexane and then filtered through a 0.45 μχι PTFE membrane (e.g. PURADISC™ 25TF of Whatman®) into a vial. 10 μ\ sample of filtrate was injected via an autosampler into a HPLC system (e.g. Waters 2690 system with Waters 996 Diode Array detector). The sample was fractionated on a RP18 column (e.g. Phenomenex Gemini) using a water/alcohol Radient mixture as mobile phase. Thefractionswere investigated via UV-absorption using λ = 254 nm as the appropriate excitation wavelength. Different UV-sensitivities of every single sulfane species were averaged by


52 division of the respective peak area through specific, empirically evaluated, response factors. The response factors (or inverse values to be multiplied) of TESPT 1 are known from literature (16) and listed in Table I. They reflect the hyperchromy with every additional sulfur atom in the chain and elemental sulfur. Table I. H P L C Response Factors of 1


RF 1

S2 1.0

S3 3.52

S4 6.39

S9 S10 57 S8 S6 31.30 37.26 26.08 17.39 20.87 13.04

S5 9.78

NMR Spectroscopy Liquid state NMR spectra were obtained using a JEOL Lambda 400 MHz spectrometer with a 5 mm autotunable probe operating at the frequencies ω = 400.18 (*H), 100.62 ( C), and 79.50 ( Si) MHz. The deuterated solvent CDC1 (99.8% deuterated) was dried over molsieve A4 prior to use. The isotropic shifts were calibrated to the signals of CHC1 at δ = 7.26 (*H) and δ = 77.00 ppm ( C) correlated to Me Si (TMS, δ = 0.0 ppm). For S i chemical shift measurements, one drop of Me Si can be directly added as an internal standard with the signal to be set at δ = 0.00 ppm. The NMR acquisition parameters for the three nuclei were set in the following manner: (i) *H 90° pulse length 5.1 sweep width 10 kHz, number of scans 32, relaxation delay 3 s; (ii) ^C-^H} decoupling 90° pulse length 4.50 sweep width 24 kHz, number of scans due to the signal-tonoise ratio, relaxation delay 60 s, (iii) S i inverse gated { H} decoupling 90° pulse length 5.45 \ls, sweep width 20 kHz, number of scans due to the signal-tonoise ratio, relaxation delay 60 s. In order to achieve high spectral resolutions for both the Si and C spectra, such as 0.07 Hz for optimal separation of peaks, the addition of a relaxation aid as chromium (tris)acetylacetonate [Cr(acac) ] has been relinquished, and a high data point density of 128 k has been applied. The probe was heated to a constant temperature t = 25 °C during acquisition to remove any heating effect the decoupler may have had on the sample. ,3

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3

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4

4

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Results and Discussion Phase Transfer Catalysis A simplified scheme of the formation of bis(silylpropyl)polysulfanes is shown in Figure 2 by the reaction of chloropropyltriethoxysilane (CPTES) with a caustic solution of sodium hydrogensulfide and elemental sulfur. -

SH* + OH + χ S + 2 Cl(CH ) Si(OEt) — (EtO)3Si(CH )3Si (CH )3Si(OEt)3 + 2 CY + H 0 2

3

3

2

+x

2

Figure 2. Simplified reaction equation for experiment 1.


2


53 As displayed in Figure 3, the first step of the phase transfer catalysis cycle for sulfurization is the ion exchange in the aqueous phase between disodium polysulfide and the quarternary ammonium bromide. The more hydrophobic ammonium polysulfide complex, in concentrated form visible as a thin brown interlayer ("omega phase", see Figure 1), is then transferred into the organic phase consisting of chloropropylsilane and later also of sulfidosilane product. The high nucleophilicity of the sulfide ion leads to the substitution of the chlorine in the bimolecular reaction. The leaving hydrophilic chloride is finally brought back with the ammonium cation into the aqueous layer. Here the catalyst cycle is starting again with the ion exchange step under generation of the catalyst-polysulfide complex and sodium chloride. The exergonic formation of sodium halogenide by-product can be seen as the actual driving force of the whole reaction, and the progress of the reaction can be followed by the increasing amount of sodium chloride precipitating out of the warm saturated aqueous solution. Organic Phase Intrinsic Reaction (S 2) N

+

2

+

2 R-Cl + Q S - Q · x

+

+

2

+

2 Na Cl'+ Q S "Q " X

'

+

R-S -R + 2 Q C1" x

+

2

+

+

» Na S "Na + 2 Q C1" x

Ion Exchange

Aqueous Phase

Figure 3. Phase Transfer Catalysis cycle for sulfurization of chloropropylsilanes. 2

2

The equilibrium distribution of (poly)sulfide ions (S ", S ', HS") in aqueous solutions under variable pH conditions has been investigated for a long time either by fast protonation to form the analogous, not highly stable sulfanes (19,20) for analysis or by optical spectrometric analysis (21,22), even at higher temperatures. (23) Nevertheless, all results remain effected by method specific uncertainties that do not help to predict accurate sulfidosilane product distributions to support the design of PTC experiments. The estimation of acidity concentration constants (pK ) in the sequence of S " > 14.00, S ~ = 9.7, S ' = 7.5, S * = 6.3, to S " 5.7 (18) confirms the observation that pH values of aqueous sulfide solutions gradually drop and the average sulfur chain length S of the sulfidosilane, not unexpectedly of course, grows with the amount of elemental sulfur added. The operational range of pH has to be finally adjusted between around pH = 12 as the limitation of the stability of alkoxysilanes against x

2

a

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4

2

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avg


54 condensation reaction and neutral conditions of around pH = 7 when formation and emission of hydrogen sulfide can be observed (pK (HS") = 6.9). During the PTC reaction a constant drop of the pH value towards neutral conditions can be measured reflecting the ongoing consumption of the sulfide species. Thefine-tuningto the desired organic sulfane distribution is carried out by the adjustment of elemental sulfur amounts that are added to the aqueous sulfide solution. Figure 4 exhibits the strong dependence of the distribution on slightest changes in the starting ratio of sulfur to (hydrogen) sulfide. As shown, the weight percentage of bis(silylpropyl)disulfane S2 is rapidly and of trisulfane S3 slowly decreasing when the sulfur/sulfide ratio is increased from 2.6 to 3.0. At the same time, the tetrasulfane S4 content remains nearly constant while higher sulfanes are increasingly formed. It is also noted that as the sulfur / sulfide ratio increases unincorporated sulfur begins to be observed in the final product distribution. It is obvious that average sulfiir chain lengths higher than S = 4.5 - 5.0 will hardly be achieved within aqueous phase chemistry.


a

avg

Figure 4. Sulfane distribution in 1 in dependence of sulfur /sulfide ratio.

HPLC Analysis Figure 5 displays a typical HPLC chromatogram of bis(triethoxysilylpropyl)tetrasulfane 1 prepared via PTC route. Using the described parameter the peaks are well separated, and sulfanes up to the dodecasulfane S12 have been identified. There is minimal evidence of condensed siloxanes at retention times of around 30 minutes typically detected in higher amounts in material prepared under far more drastic anhydrous conditions. Elemental sulfur is well separated and elutes between the disulfane S2 and trisulfane S3. Applying the response factors of Table I onto the peak integrals an average sulfane chain length of S . = 3.75 has been calculated. The specific weight percentages of each sulfane species of the analysed mixture of 1 are listed in Table II. Elemental sulfur has been calculated to a level of about 0.6 wt%. avg


55 Table II. Typical sulfane distribution in 1 S2 s wt% 16.9


x

S3 30.1

S4 24.0

S5 15.4

S6 7.7

S7 3.3

S8 1.6

S9 0.8

S 1 0 Syg 0.2 3.75 a

Equal experimental conditions have been used for the reaction of sodium polysulfide with chloropropyldiethoxymethyl-, -dimethylethoxy- and -trimethylsilane to prepare the sulfidosilanes 2, 3,and 4. The HPLC analysis of the ethoxy substituted silanes showed a high similarity to the chromatogram of 1 with very close retention times and nearly identical peak integrals and appearances. As displayed in Figure 6, the peaks of the bis(trimethylsilylpropyl)tetrasulfane 4, however, are shifted to longer retention times due to the reduced interaction of the non polar silane with the polar solvent mixture of the mobile phase. In the same way a peak broadening, especially of the signals of the shorter sulfanes as S2 and S3, is observed that leads to a false impression about the peak integral ratios. In fact, they are still very close to the chromatogram of silane 1. In order to further evaluate the feasibility of using the known response factors of the sulfanes of TESPT 1 as a basic system also for the determination of the sulfane distribution and the average sulfane chain lengths of silanes 2, 3, and 4, the UV absorption spectrum of each individual sulfane species has been recorded via diode array detector. The normalized UV absorption spectra of S2 to S8 of silane 1 are shown in Figure 7. It is necessary to remark that further quantitative investigations of the UV spectra require pure material of each sulfane species. This would allow a correct determination of the molar extinction coefficients for final comparison of the absorption strength of the silanes and ultimately accurate response factors. In Figure 7, S2 and S4 exhibit unique behaviour. S2 has a very strong and sharp absorption maximum in the quartz UV border region at λ , ^ = 202 nm and then only another weaker local maximum at l = 252 nm. S4, however, as the only sulfane surprisingly does not show a maximum near ^ = 200 nm, but the strongest absorption at λ^χ = 210 nm and another well defined local maximum at 299 nm. For all other sulfanes maxima appear at around X = 200 nm and, with a stepwise bathochromic shift, from 214 (S3) to 224 (S8) nm. They also exhibit a broad plateau towards the longer wavelengths side with onset points at λ = 330 - 340 nm for the longer sulfane chains. The bathochromic shift with every additional sulfur unit is correlating with an increasing conjugative effect of the free electrons at the sulfur atoms. This UV absorption behaviour has been also reported for the homologous sulfane series H S (24) and other organically endcapped polysulfanes. (25) Surprisingly, the correspondence of the individual absorption maxima is extremely good for all sulfanes of the same chain lengths. An excellent example is the absorption maximum of di-n-butyldisulfane that has been found at X = 251.5 nm. This is very close to the absorption maxima ofthe disulfanes of 1, 2, 3, and 4 analyzed at λ^χ = 252 nm. This gives rise to the suggestion that the absorption in the lower energy region is rarely influenced by m a x

m a x

max

2

n

max



Figure 5. Typical HPLC chromatogram of TESPT 1.



I•

10.00

11.00

12.00

13.00

•, ••· •• 14.00

16.00

17.00

18.00

19.00

20.00

23.00

24.00

• •,•• 21.00 22.00 Mnutes

25.00

26.00

27.00

Figure 6. HPLC chromatogram of TMSPT 4.

15.00

I

28.00

29.00


30.00

31.00

32.00

• · .··>•!


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Figure 7. UV-Absorption spectra of bis(triethoxysilylpropyl)sulfanes S2-S8.

the substituents if they are not carrying inductive or unsaturated groups that can interact themselves with the sulfane electrons. Figure 8 displays an overlay of the absorption spectra of the pentasulfanes S5 of the sulfidosilanes 1 - 4. In the for HPLC analysis relevant UV-region of λ = 240 - 260 nm (standard detector wavelength λ = 254 nm) all silanes demonstrate the same absorption behaviour. In the far-UV region, of course, the influence of the substituents e.g. oxygen, is made visible by the different absorption strength of each silane. As mentioned earlier, it would be possible with pure sulfane species to determine the actual molar extinction coefficients at each individual wavelength, especially at λ = 254 nm. This would finally prove the accuracy of the increasing response factors with respect to the huge hyperchromic effect of every additional sulfur unit in the chain (24). The UV absorption spectrum of elemental sulfur (usually orthorhombic aSulfur S (26)) has also been taken and qualitatively compared to the linear octasulfane S8 of 1 (see Figure 9). Surprisingly, both molecules show the same maxima positions in the far-UV region at λ = 200 and 224. However, the elemental sulfur exhibits a local maximum of λ = 272 nm. This is close to the maximum normally found for the trisulfanes of 1 - 4. The ring strengths of the elemental sulfur corona (H

2

3

Y

2

13


2

Y

2

!

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13

x

x

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spectra displayed in Figure 12. The S i NMR analysis shows the expected iso tropic shift ranges for trialkoxyalkylsilanes (1, δ = -45.88 - -46.33 ppm), dialkoxydialkyl silanes (2, δ = -5.45 - -5.83 ppm), alkoxytrialkylsilanes (3, δ = 16.40 - 16.53 ppm), and tetraalkylsilanes (4, δ = 1.55 - 1.73 ppm) according to the different shielding power of the substituents. While the intensity profiles of the peaks are in sequential order for S2 to S8 from left to right for the signals of silane 1, the signals of S2 and S7 in silane 2 are starting to migrate in direction towards the signals of S3 and S6. In the spectrum of silane 3 these signals are even overlapping. A full set of signals for S2 to S8, however, surprisingly in reverse order, is observed for the signals of silane 4. Together with the general convergence of the signals (Δδ(1) = 0.55 ppm, Δδ(3) = 0.13 ppm), this is an indicator that the positive polarization of the silicon by the alkoxy substituents is recognized by the electronegative sulfur chain via some sort of backbonding through the space. On the other hand, an intermolecular interaction of the silicon atom with neighbouring sulfane chains cannot be completely ruled out either. The comparison of the integral ratios of resolved peaks confirms again the high similarity of the sulfane composition of the four silanes.


63


Conclusions Four phase transfer catalysis experiments have been carried out preparing bis(silylpropyl)oligosulfanes with increasing number of ethoxy substituents being replaced by methyl groups at the silicon atom. NMR analyis of the generated mixtures demonstrated an excellent similarity of the molar distribution of the individual sulfane species and the corresponding average sulfane chain lengths. HPLC analysis also exhibited the same integral ratios of the sulfane species within each silane representing molecules of different sulfur content with respect of the molecular structure. The average formula of the silanes 1-4, the corresponding Molecular Weight and the consequently increasing theoretical sulfur content are listed in Table III. Table III. Formula, Molecular Weights, and wt% Sulfur of 1 -4 Molecule TESPT 1 DEMSPT 2 DMESPT3 TMSPT 4

Formula C18H42O6S12S375

C|6H3 04Si2S ,75 8

3

C14H34O2S12S3.75 C12H30S12S3 75

Mw 530.95 470.89 410.84 350.79

wt%S 22.65 25.54 29.27 34.28

As a first result the following response factors are proposed for the HPLC analysis of the silanes 2 - 4 as listed in Table IV. The response factors are so far based purely on the ratio of the molecular weights of the specific sulfane Sx (2,3,4) to Sx (1) multiplied with the according response factor RF 1 of Table I. Better UV-absorption analysis will certainly lead to more accurate adjustments. Table IV. Proposed H P L C Response Factors of 2,3, and 4 RF/Sx RF2 RF3 RF 4

S2 1.14 1.34 1.61

S3 3.99 4.61 5.46

S4 7.19 8.22 9.60

S9 S10 S8 S7 S5 S6 Set 10.93 14.48 19.21 22.93 28.53 34.10 37.26 12.39 16.28 21.45 25.45 31.49 37.45 37.26 14.29 18.60 24.28 28.59 35.13 41.53 37.26

Acknowledgments The authors would like to thank T. Materne, L. Stelandre, S.K. Mealey, S.C. Winchell, G.R. Haines, and D.J. Bunge for fruitful discussions. The help by H. Yue, T.J. Rivard, and M.R. Reiter (all Dow Corning) in the development of the analytical methods is gratefully acknowledged.



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1 8ι



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References

1. Plueddemann, E.P. Silane Coupling Agents, Plenum Press, New York, 1982. 2. Agostini, G,; Bergh, J.; Materne, T.; New Compound Technology, paper presented at the Akron Rubber Group Meeting, October 1994, Akron (OH). 3. R. Rauline, Compagnie Generale des Etablissements Michelin, Michelin & Cie, EP 0501127 1991, US 5227425 1993. 4. Braendstroem, A, Principles of Phase Transfer Catalysis by Quarternary Ammonium Salts in Advances in Physical Organic Chemistry, Gold, V.; Bethell, D., Eds., Academic Press, London 1977, p. 267. 5. Starks, C.M.; Liotta, C.L.; Halpern, M.E., Eds. Phase Transfer Catalysis Fundamentals, Applications, and Industrial Perspectives, Chapman & Hall, New York, 1994. 6. The Goodyear Tire & Rubber Co., US 5,405,985, 1995. 7. The Goodyear Tire & Rubber Co., US 5,468,893, 1995. 8. The Goodyear Tire & Rubber Co., US 5,583,245, 1996. 9. The Goodyear Tire & Rubber Co., US 5,663,396, 1997. 10. Dow Corning Corporation, US 6,384,255, 2002. 11. Dow Corning Corporation, US 6,384,256, 2002. 12. Dow Corning Corporation, US 6,448,426, 2002. 13. Dow Corning Corporation, US 6,534,668, 2003. 14. Backer, M.W.; Buesing, C.A.; Gohndrone, J.M.; Maki, W.C.; Rivard, T.J.; Yue, H. Polymer Preprints 2004, 45, 660. 15. Luginsland, H.-D. Reactivity of the Sulfur Functions of the Disulfane Silane TESPD and Tetrasulfane Silane TESPT, paper presented at the ACS Meeting, April 1999, Chicago (IL). 16. ASTM D6844 Standard Test Methodfor Silanes Used in Rubber Formulations (bi (triethoxysilylpropyl)sulfanes): Characterization by High Performance Liquid Chromatography (HPLC), 2002. 17. Backer M. W.; Buesing, C.A.; Dingman, J.L., Gohndrone, J.M.; Hu, S.; Maki, W.C.; Mealey, S.K.; Thomas, B.; Yue, H. Polymer Preprints 2003, 44(1), 245. 18. Backer M.W.; Buesing, C.A.; Dingman, J.L., Gohndrone, J.M.; Hu, S.; Maki, W.C.; Mealey, S.K.; Thomas, B.; Yue, H. Use of LC, NMR and MS for Sulfidosilane Material Characterization, presented by S.K. Mealey at the Fourth International Symposium on Silanes and other Coupling Agents, June 2003, Orlando (FL). 19. Teder, A. Acta Chem.Scand.1971, 25, 1722. 20. Schwarzenbach, G.; Fisher, A. Helv. Chim. Acta 1960, 49, 1365. 21. Teder, A. Ark for Kemi, 1969, 31, 173. 22. Giggenbach, W. Jnorg Chem. 1972, 11, 1201. 23. Giggenbach, W. Inorg Chem. 1974, 13, 1724. 24. Feher, F.; MUnzner, H. Chem. Per. 1963, 96, 1131. 25. Steudel, R. Chem. Rev. 2002, 102, 3905. 26. Wiberg N., Ed. Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 91.-100. verbesserte Auflage, Walter de Gruyter, Berlin, 1985, pp. 471-525. 27. Papamokos, G.V.; Demetropoulos, I.N. J. Phys. Chem. A 2002, 106, 1661.


Sulfidosilanes: Complex Molecule Systems via Phase Transfer

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