No Compelling Evidence for Clathrate Hydrate Formation Under Interstellar Medium Conditions LETTER Over Laboratory Time Scales Mathieu Choukrouna,1, Tuan H

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LETTER

No compelling evidence for clathrate hydrate formation under interstellar medium conditions LETTER over laboratory time scales Mathieu Choukrouna,1, Tuan H. Vua, and Edith C. Fayollea

− In PNAS, Ghosh et al. (1) report their experimental shows a shift of −7cm1), whereas the computed −1 observations of methane and CO2 clathrate formation shift is −86.0 cm . −1 at conditions similar to the interstellar medium (ISM), The emergence of a 2,346 cm CO2 peak is inter- − namely 10 to 30 K and 10 10 mbar. The authors con- preted as due to clathrate formation by analogy to an ducted time-dependent reflection–absorption infrared earlier study (4). However, in that work, the chemical

spectroscopy (RAIRS) of vapor-deposited H2O–CH4 system also contained CH3OH, whose presence ap- and H2O–CO2 mixtures and interpreted new blue pears necessary for CO2 clathrates to form after slow and red shifted peaks from those of trapped CH4 and heating to 120 K. In addition, CO2 clathrates grown CO2 in amorphous ice, respectively, as indicative of epitaxially on other clathrates (5) or formed under clathrate formation. In this letter, we point out poten- higher pressure/temperature conditions (6) have a char- tial pitfalls and caution against the implications drawn acteristic IR signature with double peaks consistently 12 13 for the ISM. observed in the ν3 regions of both CO2 and CO2, In the case of the H2O–CO2 system, Ghosh et al. unlike the signatures observed in figure 2 and figure (1) attribute the observed red shift in the infrared S5 of ref. 1. Furthermore, RAIRS studies of layered

(IR) spectra to hydrogen bonding between CO2 and mixed H2O–CO2 ices (7, 8) reported a peak around − and the water cage. This statement not only 2,346 cm 1, which was interpreted in these studies as

goes against the common knowledge of CO2’s CO2 interacting with or being trapped within amor- lack of affinity for hydrogen bonding, but also phous H2O [albeit not as forming a clathrate as contradicts the calculated structure in figure 2 of inferred by Ghosh et al. (1)]

ref. 1, which clearly shows that there is no available For the H2O–CH4 system, the calculated blue shift 12 12 hydrogen atom in the 5 cage for the CO2 guest for methane in the 5 cage (1) is inconsistent with to form hydrogen bonds with. Any hydrogen bond- previous experimental studies showing that enclathrated ing with the skeleton would greatly distort the methane exhibits a red shift (9, 10). Predicted red

cage structure, as has been suggested for NH3- shifts for other cage types and for CO2 presented bearing clathrates (2). Furthermore, the O–H stretch- in the present study (1) are also consistent with this ingbandinfigureS4ofref.1ischaracteristicof general behavior. amorphous ice (3), while clathrate hydrates are In summary, we find that the experimental data crystalline compounds. and theoretical computations in Ghosh et al. (1) do not Ghosh et al. (1) argue that the close match be- support the interpretation of clathrate hydrate forma- tween experimentally observed shifts and computed tion. The community should be cautioned against shifts indicates formation of a clathrate 512 cage. How- interpreting this article as definitive evidence that − ever, table S1 of ref. 1 claims a shift of −36.0 cm 1 for clathrates can be formed in the ISM within short (ap-

the CO2 antisymmetric stretch (although figure 2 ref. 1 proximately days) laboratory time scales.

1 J. Ghosh et al., Clathrate hydrates in interstellar environment. Proc. Natl. Acad. Sci. U.S.A. 116, 1526–1531 (2019). 2 K. Shin, R. Kumar, K. A. Udachin, S. Alavi, J. A. Ripmeester, Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems. Proc. Natl. Acad. Sci. U.S.A. 109, 14785–14790 (2012). 3 W. Hagen, A. Tielens, J. Greenberg, The infrared spectra of amorphous solid water and ice Ic between 10 and 140 K. Chem. Phys. 56, 367–379 (1981).

aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Author contributions: M.C., T.H.V., and E.C.F. wrote the paper. The authors declare no conflict of interest. Published under the PNAS license. 1To whom correspondence may be addressed. Email: [email protected]. Published online July 3, 2019.

www.pnas.org/cgi/doi/10.1073/pnas.1902381116 PNAS | July 16, 2019 | vol. 116 | no. 29 | 14407–14408 Downloaded by guest on September 23, 2021 4 D. Blake, L. Allamandola, S. Sandford, D. Hudgins, F. Freund, Clathrate hydrate formation in amorphous cometary ice analogs in vacuo. Science 254,548–551 (1991). 5 F. Fleyfel, J. P. Devlin, Carbon dioxide clathrate hydrate epitaxial growth: Spectroscopic evidence for formation of the simple type-II carbon dioxide hydrate. J. Phys. Chem. 95, 3811–3815 (1991). 6 E. Dartois, B. Schmitt, Carbon dioxide clathrate hydrate FTIR spectrum: Near infrared combination modes for astrophysical remote detection. Astron. Astrophys. 504, 869–873 (2009). 7 B. Mate ´ et al., Ices of CO2/H2O mixtures. Reflection-absorption IR spectroscopy and theoretical calculations. J. Phys. Chem. A 112, 457–465 (2008). 8 J. L. Edridge et al., Surface science investigations of the role of CO2 in astrophysical ices. Phil. Trans. R. Soc. A 2013, 20110578 (1994). 9 E. Dartois, D. Deboffle, Methane clathrate hydrate FTIR spectrum: Implications for its cometary and planetary detection. Astron. Astrophys. 490, L19–L22 (2008). 10 S. Subramanian, E. Sloan, Jr, Molecular measurements of methane hydrate formation. Fluid Phase Equilib. 158, 813–820 (1999).

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