SCIENCE HIGHLIGHTS Code SS Weekly Report – April 18, 2020

Title: Paper on the production of hexamethylenetetramine-methanol (HMT-methanol) in astrophysical ices to appear in the journal Astrobiology and to be highlighted on the journal issue’s cover.

Short Story:

Ultraviolet photolysis of ice mixtures relevant to dense molecular clouds frequently leads to the production of more complex compounds, many of which are of astrobiological interest. Through a series of elegant isotopic studies, we have determined that the photolysis of ices containing H2O, CH3OH, CO, and NH3 yields detectable quantities of hexamethylenetetramine-methanol (hereafter HMT-methanol, see structure below) in room-temperature refractory residues. This molecule differs from HMT, which is known to be abundant in similar ice photolysis residues, by the replacement of a peripheral H atom with a CH2OH group. Like HMT, HMT-methanol is likely to be an amino acid precursor. This research will appear in a forthcoming paper entitled “The production and potential detection of hexamethylenetetramine-methanol in space” in the journal Astrobiology (May 2020, vol. 20, #5, DOI: 10.1089/ast.2019.2147). The authors of the paper are Drs. Christopher Materese (NASA GSFC / NASA ARC), Michel Nuevo (BAERI, NASA ARC), Scott Sandford (NASA ARC), Partha Bera (BAERI, NASA ARC), and Timothy Lee (NASA ARC). This paper will also be highlighted on the cover of the issue (see figure below).

Although it was not possible to isolate HMT-methanol or purchase a reference standard to obtain an infrared spectrum of the pure material, we computed infrared spectra for both HMT and HMT-methanol using ab initio quantum chemistry methods. We note that while we cannot directly validate the computed spectrum of HMT-methanol, the computed infrared spectrum for regular HMT shows good agreement with laboratory-derived infrared spectra from our earlier work that showed HMT is a major product of ice photolysis.

HMT has tetrahedral (Td) symmetry, whereas HMT-methanol has C1 symmetry. Since HMT-methanol lacks the high symmetry of HMT, it also produces rotational transitions that could potentially also be observed at longer wavelengths by radio telescopes, although establishing the exact positions of these transitions may be challenging. It is likely that HMT-methanol represents an abundant member of a larger family of functionalized HMT molecules that may be present in icy environments in space.

As a follow-on to this work, we computed the infrared spectra of a number of HMT variants and showed that, as a family, these molecules may be detectable in space via their combined infrared spectra since the HMT skeletal vibrational frequencies are not significantly affected by the substitution of a H with another functional group. This additional work has appeared in the paper “The Calculated Infrared Spectra of Functionalized Hexamethylenetetramine (HMT) Molecules” by Partha Bera, Scott Sandford, Timothy Lee, and Michel Nuevo that was recently published in The Astrophysical Journal (vol. 884:64, 16 pp; DOI: https://doi.org/10.3847/1538-4357/ab3c4f

The structural parameters computed for hexamethylenetetramine (HMT; left) and HMT-methanol (right) using the wB97X-D/cc-pVTZ method. Bond lengths are in Å, and bond angles are in degrees

Macintosh HD:Users:scottsandford:Desktop:Sandford Work-USE THIS:Documents:Papers in Progress:2018 HMT-methanol Paper:Highlight Material:ASTv20n5 cover.pdf

The cover of the issue in which the paper will appear, showing both the structure and computed infrared spectrum of HMT-methanol.

POC: Scott Sandford (SSA)

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