J. Phys. Chem. C 2008, 112, 15419–15422
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Catalysis of Addition Reactions by a Negatively Charged Silica Surface Site on a Dust Grain T. P. M. Goumans, C. Richard A. Catlow, and Wendy A. Brown* Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: August 11, 2008
Two common astrochemical surface reactions, hydrogenation and oxidation of unsaturated compounds, increase the molecular complexity in the interstellar medium. The catalytic effect of a negatively charged silica surface site on addition reactions to adsorbed unsaturated organic molecules is investigated computationally. Adsorbates bind relatively strongly to the silanolate group (SiO-) with a pronounced weakening of their multiple bonds, which results in a significant reduction in the activation barrier of the H atom addition to COads, H2CdOads, H2CdCH2,ads and HCtCHads with respect to the corresponding gas phase barriers. More strikingly, the sizable gas phase barrier (21 kJ/mol) for the 3O + CO reaction is almost entirely removed. This strong catalytic effect allows for effective CO2 formation in dark molecular clouds where other routes are inefficient. Furthermore, negatively charged siliceous dust particles could also play an important role in the hydrogenation of CO and hydrocarbons in the interstellar medium. 1. Introduction The chemistry of the interstellar medium (ISM), particularly in star-forming regions in molecular clouds, is an active area of research in both contemporary chemistry and physics.1-3 In particular, the influence of the surfaces of dust grain particles on astrochemistry is being intensively investigated.4 Experiments aim to establish desorption characteristics3,5-8 and gas phase9,10 as well as surface reactivity11-14 at the extremely low temperatures(10-20K)foundintheISM,withtheoreticalcalculations15-19 supporting available or estimated data. These experimental and theoretical data are subsequently used in astrochemical models to simulate the chemical evolution of different regions of the ISM.20-22 Apart from the technical difficulties involved in performing the experiments and calculations, there is an uncertainty factor in the surface chemistry data because the exact composition and structure of the dust grain surfaces remains unknown. Dust grains can be bare or covered in ice and are thought to be made of carbonaceous or silicate material.23 Therefore, experimental model surfaces include graphite,3,6,11 forsterite14 and ground comet material.24 For computational studies, model systems have included polycyclic aromatic hydrocarbons (PAHs),25,26 graphene,16,17 water27and siliceous edingtonite.28 The latter has been used previously to model the formation of methanol (CH3OH) on a silica surface with an embedded cluster approach.28,29 With this approach, it is also possible to study the chemistry on isolated defect sites. Hence, because dust grains are negatively charged on average,30 we have investigated the negatively charged silanol surface site (SiO-) as a potential catalytic site on a model amorphous silica surface.28 The SiO- surface site has already been shown to catalyze the two activated steps in the sequential hydrogenation of CO to CH3OH, the addition of H to CO and to H2CdO.28 While the activation barriers for these reactions are still sizable with respect to the extremely low temperatures of the ISM, the rate of formation of CH3OH is greatly enhanced over the uncatalyzed reaction.28 This * To whom correspondence should be addressed. E-mail: w.a.brown@ ucl.ac.uk.
catalytic effect could be further enhanced by quantum mechanical tunnelling through the activation barrier. The barriers for the two activated H atom addition steps needed for CH3OH formation were found to be reduced because the negatively charged silica surface site destabilizes both the CO and the H2CdO multiple bonds.28 In this study we extend our investigations to the reactivity of other unsaturated species adsorbed on this negatively charged defect. We study the surface-catalyzed activation barriers for H atom addition to a C-C double bond (H2CdCH2) and a C-C triple bond (HCtCH) and the formation of CO2 from 3O + CO:
Hg + HCtCHads f H2CdCHads
(1)
Hg + H2CdCH2,ads f H3CsCH2,ads
(2)
Og + COads f 3CO2,ads
(3)
3
In all cases, we study the Eley-Rideal mechanism where the unsaturated species is adsorbed on a negatively charged silicate surface site and reacts with an H or O atom coming from the gas phase. Experimentally, it has been established that both H2CdCH2 and HCtCH ices are fully hydrogenated at low temperatures (10-25 K) to yield H3CsCH3.31 The rate of hydrogenation increases with decreasing temperature and the rate of the H + H2CdCH2 reaction is higher than that of the H + HCtCH reaction. Therefore, no H2CdCH2 is observed during the H-bombardment of HCtCH ice.31 CO2 ices have long been observed in the ISM.32 The relatively high solid phase abundance of CO233,34 indicates that it is formed predominantly on grain surfaces rather than forming in the gas phase before freezing out. Because solid CO2 has been observed toward quiescent dark clouds,35 there must exist an efficient CO2 formation route on surfaces without the need for energizing or ionizing events. We have previously studied the reaction of 3O + CO, OH + CO and O + HCO on a carbonaceous PAH surface.26 This surface was found to lower the barrier for the 3O + CO reaction, especially for the hot atom mechanism, whereby an unthermalized adsorbed O atom reacts with an adsorbed CO molecule. However, with the rates calculated from the activation barriers, CO2 formation in the ISM via this surface
10.1021/jp8042297 CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
15420 J. Phys. Chem. C, Vol. 112, No. 39, 2008 route is only viable when adsorbed CO is capped by water ice to increase the desorption temperature of the CO.7 2. Computational Methods An embedded cluster approach is employed in which a small cluster is treated with quantum mechanics (QM) and then embedded in a larger cluster, treated at the computationally cheaper molecular mechanics (MM) level. The details of the setup of the calculation have been described previously,29 but the essentials are discussed below. We use the modular ChemShell code36 with GAMESS-UK37 for the QM and DL-POLY for the MM calculations. The QM/ MM boundaries (SiQM-OMM) are treated with a link-atom plus charge-shifting approach, as has been developed for zeolites.36 The model grain surface is the siliceous edingtonite (100) surface, which is a good computational model for the surface of amorphous silica.38 A hemispherical cluster with a radius of 25 Å is cut from the fully optimized 2D-surface (MM) and surrounded by charges to restore the Madelung potential. The QM setup is chosen to reproduce the gas phase activation barriers for the reactions under consideration (see validation, below). The QM-cluster is treated with the MPWB1K functional, which has been designed to give good activation energies as well as model nonbonding interactions.39 We use a 6-31+G** (5D) basis for the O- atom of the reactive surface site, a 6-31G* (5D) basis for the remainder of the cluster, and a 6-311G** basis for the adsorbates and incoming atoms. For the MM-atoms we use modified Hill and Sauer40 potentials.29 Scaled zero-point energy corrections (0.9573)41 are included and no counterpoise corrections are applied. Activation energies for the surface addition reactions are given with respect to the adsorbed unsaturated molecule and the gas phase atoms. Validation. To benchmark our computational approach, we initially calculated the activation barriers for the gas phase reactions. The B97-1 functional, which performs well for several astrochemical reactions42 and was accordingly used to study the H + CO and H + H2CdO reactions on surfaces,28,29 performs poorly for the 3O + CO reaction.26 However, the MPWB1K functional performs much better,26 with the MPWB1K/ 6-311G** activation barrier for 3O + CO activation (21.1 kJ/ mol) being slightly lower than the high-level CCSD(T)/augcc-pV5Z result of 24.7 kJ/mol.43 Likewise, the MPWB1K/ 6-311G** calculated activation barriers for the H + H2CdCH2 (10.7 kJ/mol) and the H + HCtCH (17.3 kJ/mol) reactions are in excellent agreement with the high accuracy values of 11.8 and 17.9 kJ/mol, respectively, at the QCISD(T) level extrapolated to the basis set limit.33 Even though the gas phase barriers at the MPWB1K/6-311G** level are slightly lower than the high-accuracy ab initio values, this approach is sufficiently accurate to predict trends in surface-catalyzed reactions from the lowering of these gas phase barriers. 3. Results The physisorption interaction of all of the unsaturated species with the negatively charged silanolate group is stronger than with the protonated silanol group (SiOH), as was observed previously for CO and H2CO.28 This stronger interaction, in turn, stretches and weakens the multiple bond.28 In Table 1, the physisorption energy of H2CdCH2, HCtCH and CO on the negative silanolate group are summarized, as well as the changes in bond length and in infrared frequency (unscaled) of the multiple bond upon adsorption. HCtCH has a very strong physisorption interaction with the silanolate group and also displays the largest red-shift of the C-C stretching frequency.
Goumans et al. TABLE 1: Physisorption Energies (Eads) in kJ/mol, Change in Bond Length (∆r) in pm and Change in IR Frequency (∆hν) in cm-1 for HCtCH, H2CdCH2 and CO Adsorbed on a Silanolate Group on a Negatively Charged Siliceous Edingtonite Surface adsorbate
Eads (kJ/mol)
∆r (pm)
∆hν (cm-1)
HCtCH H2CdCH2 CO
52.9 22.2 12.1
0.64 0.28 0.68
-81 -9 -60
TABLE 2: Activation Energies for the Gas Phase Reactions 1-3 and for the Analogous Reaction Where the Unsaturated Species Is Adsorbed on a Negatively Charged Silanol Site on an Edingtonite Surface activation energy (kJ/mol) reaction
gas phase
adsorbed
(1) Hg + HCtCH f H2CdCH (2) Hg + H2CdCH2 f H3CsCH2 (3) 3Og + CO f 3CO2
17.3 10.7 21.1
6.4 2.5 0.6
We note that at this level of theory, MPWB1K/6-311G**, the adsorption energy of CO on a silanolate group (12.1 kJ/mol) is lower than previously calculated at the B97-1/6-31+G* level (18.1 kJ/mol).26 Consequently, the CO stretch is also less redshifted (-60 cm-1) at the MPWB1K/6-311G** level compared to the B97-1/6-31+G* level (-78 cm-1). The gas phase and surface reaction activation barriers for reactions 1-3 are listed in Table 2, and the calculated transition states, including bond lengths and angles, are depicted in Figure 1. The activation energies in Table 2 clearly show that the weakening of the multiple bond has a pronounced effect on the activated addition of H and/or O gas phase atoms to the multiple bond. The addition of H atoms to H2CdCH2 and HCtCH to yield H3C-CH3 is thought to proceed efficiently through tunnelling at temperatures as low as 10 K on the surface of pure ices,31 despite the activation barrier being considerable. However, when the hydrocarbons are adsorbed on a negatively charged surface site, the activation barriers are strongly reduced from 17.3 to 6.4 kJ/mol for Hg + HCtCHads and from 10.5 to 2.5 kJ/mol for Hg + H2CdCH2,ads. Because of this lower barrier, the H atom addition reactions will proceed much more rapidly, especially as tunneling will also be more efficient for the surface reactions than for the gas phase reactions. The most dramatic surface catalytic effect is, however, the elimination of the barrier for the 3O + CO reaction, which is reduced from 21.1 kJ/mol in the gas phase to 0.6 kJ/mol on the surface. Further analysis of the gas phase approach of 3O to COads shows that there is a long-range complex, with an O-CO distance of 2.28 Å and an energy of -0.5 kJ/mol below the separated reactants (including zero-point energy correction). Because of the shallow energy depth of the long-range complex, an impinging O atom is not likely to be stabilized in the longrange complex, but will either recoil or continue to overcome the low-energy barrier to CO2-formation. The very low barrier makes reaction 3 a feasible pathway to CO2 formation in dark molecular clouds. Our previous calculations showed that CO2 formation from 3O + CO is also catalyzed by a carbonaceous surface.26 However, in that case, the activation barrier is still substantial (13.2 kJ/mol) and the reaction rate for CO2 formation only becomes substantial at elevated temperatures of 60 K. With the negligible barrier for 3Og + COads on a negatively charged silica grain, CO2 formation will already occur efficiently at temperatures as low as 10-20 K.
Catalysis of Addition Reactions
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15421
Figure 1. Transition state geometries of the QM-clusters for reactions 1 (left), 2 (middle) and 3 (right). White H, light grey (green): Si, gray: C, dark gray (red): O.
4. Discussion Using embedded cluster calculations, we have previously shown that a negatively charged defect on a siliceous surface (SiO-) catalyzes the addition of H atoms to COads and H2CdOads,28 reducing the activation barriers with respect to the gas phase by 6.4 and 3.4 kJ/mol, respectively. In this study, we have shown that this catalytic surface site also lowers the hydrogenation barrier for HCtCHads and H2CdCH2,ads, with a reduction of the barriers by 10.9 and 8.2 kJ/mol respectively. Even more dramatic is the effect on the 3Og + COads reaction, where the gas phase barrier almost disappears, being reduced from 21.1 kJ/mol in the gas phase to 0.6 kJ/mol when CO is adsorbed on a SiO- site. The negatively charged defect lowers the barrier for atom addition reactions to multiple bonds as these bonds are stretched and weakened (Table 1) and they are also electrostatically polarized (see below) by the negative charge. However, there is no straightforward correlation between the calculated infrared shift, the adsorption energy and the catalytic effect on the addition reactions. For instance, the red-shift (-9 cm-1) and bond extension (+0.28 pm) for H2CdCH2 is only small, but the adsorption energy is larger than that of CO (Table 2). Furthermore, the relative reduction of the gas phase H-addition barrier is larger (-77%) than that of HCtCH (-63%), despite HCtCH being more strongly bound and having a much stronger red-shift (-81 cm-1). The C-end hydrogenation barrier of HCdN has also been calculated (not shown). For this reaction (Hg + HCtNads f H2CdNads) the barrier is only moderately reduced from 25.1 kJ/mol in the gas phase to 19.5 kJ/mol on the surface, despite HCtN being the strongest bound (106.7 kJ/mol) to the surface and the most red-shifted species (-257 cm-1). This moderate catalytic effect could be due to the regioselectivity of the catalyst: HCtNads binds to the silanolate group via the H-end, bringing the C atom closest to the surface, which leads to an unfavorable approach in the transition state. Likewise, calculations of the activation energies for H-addition to the C atom closest to the surface in HCtCHads and H2CdCH2,ads give values of 12.8 and 5.4 kJ/mol, respectively, about twice as large as the corresponding activation energies for addition to the distant C atom (Figure 1, Table 2). This regioselectivity for the catalyzed hydrogenation reactions could originate either from a frontier orbital effect or an electrostatic effect. In the first case, the π*-orbital of the most distant atom of the double bond will have the largest amplitude, while in the latter it will have the largest partial negative charge. In both cases the approach of the incoming H atom is favored toward the distant atom of the double bond in the transition state. Orbital
and charge analysis of the reactant adsorbate species indicate that the regioselectivity is most likely to be caused by the electrostatic effect, as the polarization of the multiple bonds is such that the partial negative charge is on the distant atom. This study clearly shows that a straightforward prediction of the catalysis of addition reactions to unsaturated species is not possible from adsorption energy, bond stretching and red-shift of the adsorbate alone. Therefore, the transition states for all of these reactions need to be calculated in order to evaluate the catalytic effect. Nevertheless, our calculations show that all addition reactions to the unsaturated species considered in this study are catalyzed by a negatively charged defect on the siliceous surface. The activation of a multiple bond by a negatively charged surface defect could also be applied to other catalytic addition reactions to unsaturated molecules. However, the simultaneous dosing of H atoms and unsaturated molecules would likely deactivate the catalytic site because the H atoms are very reactive toward the SiO- group (see astrophysical implications section below). In addition, under laboratory conditions, it will be difficult to create a large concentration of active sites because of the build-up of charge which would hamper turnover rates. However, in the ISM turnover rates are very low in terms of laboratory timescales and just one very reactive site per hundreds of dust grains would still greatly enhance the transformation rates of interstellar molecules overall. Astrophysical Implications. We have studied the reactivity of the SiO- surface as a potential catalyst for interstellar addition reactions. This reactive surface defect could be created on the surface of amorphous silica dust grains when the grain is impacted by electrons, cosmic rays, protons, photons or other dust grains. Nearby adsorbates could migrate to adsorb on this favorable surface site and are thus activated toward addition reactions. Alternatively, when no adsorbate migrates to this favorable surface site, it is most likely to be eventually impacted by an H atom, which is a relatively abundant interstellar species. The reaction with an H atom is exothermic (-174 kJ/mol) and yields a silanol group (SiOH) and an electron, deactivating the catalytic site. However, since interstellar dust grains are thought to be negatively charged,30 negative charges must be generated relatively frequently and SiO- surface defects are therefore plausible sites for the excess electron to reside on an amorphous silica surface. In order to obtain a more quantitative insight into the catalytic properties of the SiO- surface defects, the potential dynamical processes upon defect creation must be studied in greater detail. Nevertheless, this study provides a qualitatively possible route to the formation of complex molecules in the ISM via surface catalyzed addition reactions.
15422 J. Phys. Chem. C, Vol. 112, No. 39, 2008 Dust grain surfaces are thought to catalyze a range of interstellar reactions.1,4,15,44-46If two open-shell species encounter each other on the surface, a barrierless addition is likely to occur, with the surface stabilizing the product via energy-transfer. For instance, H + H, H + O, H + HO, H + HCO and O + HCO are all radical-radical additions without an activation barrier. However, there is usually an activation barrier on the reaction pathway of either H or O with any closed-shell species. At the very low temperatures in the ISM (10-20 K), these barriers are sufficient to prohibit hydrogenation and oxidation reactions. Our calculations indicate that negatively charged silica sites on a dust grain can increase reaction rates by reducing effective activation barriers. Furthermore it is envisaged that quantum mechanical tunneling increases reaction rates significantly.46,47 CO2 is a very abundant interstellar ice species and consequently there must exist an efficient route for CO2 formation, presumably on the surface of dust grain particles.34,48,49 While we have previously shown that the formation of CO2 is also catalyzed by a model carbon dust grain,26 the activation barrier of the 3Og + COads f 3CO2,ads reaction is almost completely negated by the negatively charged silanolate surface site studied here. The silanolate surface-catalyzed addition therefore offers an attractive astrochemical route for the formation of CO2. 5. Conclusions The embedded cluster calculations presented here show that a negatively charged defect on a silica surface (silanolate) effectively catalyzes the addition of atoms (H, O) to unsaturated molecules adsorbed on it. The negative charge polarizes and destabilizes the multiple bonds, which in turn lowers the barriers to addition. Therefore, complex interstellar molecules could be formed effectively via surface-catalyzed hydrogenation and/or oxidation routes. The hydrogenations of HCtCHads and H2CdCH2,ads are strongly catalyzed, but the 3Og + COads reaction is most efficiently catalyzed by the silanolate surface site, by reducing the gas phase barrier from 21.1 to 0.6 kJ/mol for the Eley-Rideal reaction. Acknowledgment. The EPSRC is acknowledged for a postdoctoral fellowship for TPMG (EP/D500524) and for computer resources on HPCx used through the EPSRC-funded (EP/D504872) Materials Chemistry consortium. We also thank UCL Research Computing and NSCCS for use of their computer resources. Paul Sherwood, Alexey Sokol and Ben Slater are acknowledged for their help with ChemShell, and Huub van Dam is thanked for implementing the MPWB1K functional in GAMESS-UK. This work forms part of the research currently being undertaken in the UCL Centre for Cosmic Chemistry and Physics. References and Notes (1) Herbst, E. J. Phys. Chem. A 2005, 109, 4017. (2) van Dishoeck, E. F Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12249. (3) Williams, D. A.; Brown, W. A.; Price, S. D.; Rawlings, J. M. C.; Viti, S. Astron. Geophys. 2007, 48, 25. (4) Williams, D. A.; Herbst, E. Surf. Sci. 2002, 500, 823. (5) Amiaud, L.; Fillion, J. H.; Baouche, S.; Dulieu, F.; Momeni, A.; Lemaire, J. L. J. Chem. Phys. 2006, 124. (6) Brown, W. A.; Bolina, A. S. Mon. Not. R. Astron. Soc. 2007, 374, 1006. (7) Collings, M. P.; Dever, J. W.; Fraser, H. J.; McCoustra, M. R. S. Astrophys. Space Sci. 2003, 285, 633. (8) Acharyya, K.; Fuchs, G. W.; Fraser, H. J.; van Dishoeck, E. F.; Linnartz, H. Astron. Astrophys. 2007, 466, 1005. (9) Costes, M.; Daugey, N.; Naulin, C.; Bergeat, A.; Leonori, F.; Segoloni, E.; Petrucci, R.; Balucani, N.; Casavecchia, P. Faraday Discuss. 2006, 133, 157.
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JP8042297
