PRODUCTION OF HEXAMETHYLENETETRAMINE IN PHOTOLYZED AND IRRADIATED INTERSTELLAR COMETARY ICE ANALOGS

H. COTTIN,<fnref n="1" /> C. SZOPA,<fnref n="2" /> AND M. H. MOORE<fnref n="1" />

Received 2001 August 30; accepted 2001 September 26; published 2001 October 15

ABSTRACT

We report hexamethylenetetramine (HMT) detection in organic residues resulting from H₂O : CO : CH₃OH : NH₃
ices submitted to UV photolysis or proton irradiation. This is the first time HMT is detected after proton irradiation
of an interstellar or cometary ice analog, whereas this molecule was suspected to be a characteristic signature of
UV versus ion processing. This result strengthens the probability of HMT presence in the interstellar medium and
in comets, where it may eventually be detected with the COSAC (COmetary Sampling And Composition) experiment
on board the upcoming Rosetta mission.

*Subject headings:* astrochemistry — comets: general — ISM: molecules — methods: laboratory — space vehicles: instruments

1. INTRODUCTION

Unraveling the nature of the organic components of inter-
stellar dust and comets is critical to understanding the chemical
evolution of materials from the interstellar medium to the solar
system. A classic laboratory approach to this problem is to start
with various ice mixtures at ~10 K and photolyze with vacuum
ultraviolet, irradiate with charged particles, or simply warm the
ices, simulating the processing of ice-mantled dust in molecular
clouds or the solar system. When warmed to room temperature,
these processed icy mixtures leave a residue, which is generally
presumed to be a close analog of the organic mantles of dust
in molecular clouds (see, for instance, Greenberg 1982, Alla-
mandola, Sandford, & Valero 1988, or Cottin, Gazeau, & Rau-
lin 1999 for reviews on that subject).

Three kinds of energetic processing occur on icy coated dust
grains in interstellar clouds or in the outer layers of comet ices
in the solar system. In interstellar clouds, icy coated dust par-
ticles are subjected to processing by

1. Charged particles: Galactic cosmic rays.

2. UV photons: Lyα photons from neighboring stars in the diffuse outer regions or UV photons induced by Galactic cosmic rays in the inner regions of dense clouds.

3. Thermal processes: cycling between the cold dense cloud center and warmer diffuse regions.

In the solar system, the outer layers of comets undergo the
same processes:

1. Charged particles: Galactic cosmic rays, mainly in the Kuiper belt and the Oort clouds. This process has the largest effect on the outer few meters of the nucleus.

2. UV photons: solar UV, mainly in the inner solar system when the comet is close to the Sun. This process would affect the outer few micrometers of the nucleus.

3. Thermal processes: during the formation of the solar system (depending on the region in which the comet accretes) and in the inner Solar system (when the comet approaches perihelion).

As a result of the diversity of environments involved, con-
straining the degrees to which different processes affect cosmic
ices is a highly convoluted problem. Differences between the
products synthesized during processing, according to the en-
ergy sources, could give information on the history of comets.
So far the laboratory experiments have emphasized the ana-
lytical method (infrared spectroscopy, liquid and gas chro-
matography, mass spectroscopy, etc.) to detect the widest range
of synthesized molecules rather than the form of the deposited
energy (Cottin et al. 1999).

The facilities at NASA's Goddard Cosmic Ice Laboratory are equipped with a Van de Graaff accelerator and a UV lamp system so that ices can be processed using both protons and UV photons. Results can be compared using the same analytical method. Thus, a direct comparison of synthesized compounds is possible, with the aim of finding if the production of a molecule can be an indicator of the processing history of ices. The first set of data comparing UV photolysis and ion irradiation of ices at Goddard showed that the yield of major products was similar for a simple ice containing H₂O and CO₂ (Gerakines, Moore, & Hudson 2000) but different for pure CO (Gerakines & Moore 2001).

Among the complex organic refractory molecules that have
been synthesized during cometary and interstellar ice analog
experiments, polyoxymethylene [POM; (CH₂-O)ₙ] and hexa-
methylenetetramine (HMT; C₆H₁₂N₄) seem to be of prime in-
terest. POM and polymers of the same family have been
detected when ice mixtures containing formaldehyde and am-
monia are slowly warmed to room temperature, without any
photolysis or ion irradiation processing (Schutte, Allamandola,
& Sandford 1993a, 1993b). After UV photolysis of ices such
as H₂O : CH₃OH : CO : NH₃ (10 : 5 : 1 : 1), POM-like poly-
mers have also been detected, but HMT represents ~60% of
the organic residue at 300 K (Bernstein et al. 1995). Thus, it
looks like POM is a good indicator for thermal processing of
ices, while HMT plays the same role concerning UV photolysis.
HMT is of particular interest since it forms amino acids when
acid-hydrolyzed (Wolman et al. 1971).

Bernstein et al. (1995) suggested that ion irradiation of a
mixture of H₂O : CH₃OH : CO : NH₃ would not lead to the pro-
duction of HMT. Rather than a conversion of CH₃OH into
H₂CO, which is the first step toward HMT formation during
photolysis, acetone would form, leading to the production
of linear molecules such as polyamino-dimethyl-methylene

<footnote n="1">¹ Astrochemistry Branch, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771; hcottin@lepvax.gsfc.nasa.gov, ummmhm@lepvax.gsfc.nasa.gov.</footnote>

<footnote n="2">² Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR 7583 CNRS, Universités Paris VII-Paris XII, 94010 Créteil Cedex, France; szopa@lisa.univ-paris12.fr.</footnote>
<figure><img src="image_1.png" /><figcaption>FIG. 1.—Chromatogram of HMT injected in an MXT-1 column (see text for details). (a) Total chromatogram. It shows the sample is released 19 minutes after injection. The same retention time for mass 42 is shown in (b) and for mass 140 in (c). (d) Mass spectrum of HMT (from the National Institute of Standards and Technology Mass Spectral Database). Characteristic peaks for HMT are 42 and 140 amu.</figcaption></figure>

(PADMM; $[-C(CH_3)_2-NH-]_n$). HMT would then be a signature of UV processing, and PADMM a signature of proton processing. This hypothesis originated with a tentative infrared identification of acetone in irradiated water ice containing methanol by Baratta et al. (1994), which was repeated by Moore, Ferrante, & Nuth (1996) and Palumbo, Castorina, & Strazzulla (1999). However, Hudson & Moore (2000), recently showed that there is no evidence of acetone formation in proton-irradiated methanol ices. Instead, H₂CO has now been identified in both H₂O : CH₃OH and H₂O : CO ices. Therefore, HMT formation in proton-irradiated ices was expected in irradiated H₂O : CH₃OH : CO : NH₃ ices and has been confirmed by the results reported in this Letter.

For the first time, we present the analysis of two organic residues originating from the same ice composition. One residue was from UV-photolyzed H₂O : CH₃OH : CO : NH₃ ice and the second residue from the same ice after ion irradiation. This Letter focuses specifically on HMT formation, in order to check the hypothesis that HMT is a specific signature of UV-processed ice. Analysis of other organic compounds will require new analytical developments and is in progress.

## 2. EXPERIMENT

The experimental setup has been described in detail by Hudson & Moore (1995) and Moore & Hudson (1998, 2000). A gas mixture is condensed onto an aluminum cold mirror ($T < 20$ K) in a vacuum chamber ($P \sim 10^{-8}$ torr). Gases are mixed in a glass bulb, and the concentration of each is determined by its partial pressure. The resulting ice is assumed to have the same composition as the gas phase. The ice thickness is determined by measuring laser interference fringes during deposition. The typical rate is 3–5 µm hr⁻¹. Ice can be photolyzed

with a UV lamp (average energy: 7.4 eV; flux: ~3 × 10¹⁴ photons cm⁻² s⁻¹) or irradiated with 0.8 MeV protons from a Van de Graaff accelerator. Source and purity of used compounds are the following: triply distilled H₂O with a resistance greater than 10⁷ ohms cm; CO, Matheson research grade, 99.99%; CH₄, Matheson research grade, 99.999%; NH₃, Matheson anhydrous, 99.99%; CH₃OH, Sigma-Aldrich HPLC grade, 99.9%; HMT, Fluka, greater than 99.5%.

Two sets of experiments have been performed with the same ice composition: H₂O : CH₃OH : CO : NH₃ (10 : 5 : 1 : 1). This composition was chosen to be the same as the one studied in Bernstein et al. (1995) and includes the most abundant interstellar and cometary molecules with the exception of CO₂ and H₂ (Ehrenfreund & Charnley 2000). During the first set of experiments, two successive ~0.5 µm ice layers were photo-lyzed each during 9 hr, in order to repeat Bernstein et al. experiment. Ices where very thin to ensure UV processing throughout the whole ice depth. The second experimental set consists of an ~10 µm ice sample, irradiated with 0.8 MeV protons. This energy has a penetration range of ~20 µm and will process the whole ice depth. Irradiation processing therefore is more efficient than UV processing at producing a larger amount of organic residue. Dose estimates for UV photons are described in Gerakines et al. (2000) and for protons in Moore & Hudson (1998). The proton irradiation dose has been set to be equivalent to the UV energy deposited, i.e., roughly 25 eV molecule⁻¹.

Organic residues were recovered from the aluminum mirror with methanol. Analyses were performed using a Varian Saturn II gas chromatograph–mass spectrometer (GC-MS). GC separations were performed on an MXT-1 (Restek) capillary column (length = 30 m, internal diameter = 0.18 mm, inner coating = 0.6 µm). Analysis settings were: column inlet pressure = 17 psi (relative to the atmospheric pressure), $T_{column}$ = 60°C (0–5 minutes) and then 20°C minute⁻¹ until 160°C, $T_{injection}$ = 300°C, $T_{transfer to MS}$ = 200°C. This column and settings were chosen specifically for the detection of HMT. Detection was performed with a Finnigan-Varian mass spectrometer, with an ion trap (1–650 amu; resolution: 1 amu). Secure GC-MS identification of HMT is achieved by comparing the retention time of standard HMT with the retention time of peaks resulting from the organic residue injected with the same analysis settings and by matching the mass spectrum with the HMT reference mass spectrum (Fig. 1). Blank experiments are performed before each organic sample injection in order to check the contamination level of the column.

## 3. RESULTS AND DISCUSSION

GC-MS analyses of residues from both UV-photolyzed and ion-irradiated H₂O : CH₃OH : CO : NH₃ ices are shown in Figure 2. Concerning the UV sample, there is no clear indication of HMT in the total chromatogram (Fig. 2a). This is due to the very low amount of residue recovered after the photolysis experiment. Nevertheless, its presence is clearly revealed on the specific plots (Figs. 2b and 2c) for masses 42 and 140 (main HMT MS fragments), both of which show a peak at the same retention time as measured for the HMT standard. The peculiar shape of the peaks is not clearly understood. It is probably due to the high polarity of amino groups such as in HMT, which interact badly with stationary phases of GC columns. Identification of HMT is confirmed by the mass spectrum sampled on peaks 42 and 140 attributed to HMT (Fig. 2d). Masses detected with amu > 140 are due to recombination in the ion
<figure><img src="image_2.png" /><figcaption>FIG. 2.—Chromatograms collected during analysis of UV-processed residue (left panels) and proton-processed residue (right panels). Upper panels are the total chromatogram compared to the two characteristic masses of HMT 42 and 140. Lower panels show the mass spectra corresponding to the peaks associated to HMT.</figcaption></figure>

trap of the spectrometer. This detection is consistent with Bernstein et al. (1995).

The proton-processed ice was much thicker than the UV one; therefore, the amount of residue is larger, ensuring a better signal, as shown in the right panels of Figure 2. HMT is clearly detected in the total chromatogram thanks to its retention time, 19 minutes (Fig. 2e). This detection is confirmed by specific plots for masses 42 and 140 (Figs. 2f and 2g) and by the mass spectrum sampled on the peak attributed to HMT (Fig. 2h). No trace of substituted HMT (HMT skeleton with a CH₃, OH, CHO, or COOH group instead of one H, which should result in a parent mass of 154, 156, 168, and 184, respectively) has been so far detected in our samples (spectra not shown).

Thus, HMT is detected in both samples and should henceforth not be considered as a characteristic signature of a specific process. According to the mechanism described in Bernstein et al. (1995), the first stage toward its synthesis is the production of H₂CO by oxidation of CH₃OH. This can be achieved by UV photolysis (Allamandola et al. 1988) or proton irradiation (Hudson & Moore 2000). But CO in water also produces formaldehyde by UV (Allamandola et al. 1988) or proton processes (Moore, Khanna, & Donn 1991). Experiments starting with ¹³C-labeled methanol reported in Bernstein et al. indicated that the carbon in HMT originated from methanol rather than from CO. But this is certainly due to a more efficient conversion rate of methanol into formaldehyde than CO into formaldehyde. Then ices containing only one carbon source (CO or CH₃OH) should yield production of HMT but with a higher efficiency with CH₃OH. Addition of CO₂ in the initial ice mixture should not induce any effect regarding HMT production as no detectable amount of H₂CO is reported after UV (Gerakines et al. 1999) or proton (Moore et al. 1991; Brucato, Palumbo, & Stazzulla 1997) irradiation of H₂O : CO₂ ices.

Our results strengthen the probability for the presence of

HMT in the interstellar medium as being one product of radiation chemistry (UV and charged particles) between the most abundant interstellar molecules detected so far. But its actual IR detection is jeopardized since its strongest infrared bands at ~10 µm overlap the Si-O silicate band (Bernstein et al. 1994, 1995). Its detection on comets from remote sensing has the same difficulties. Bernstein et al. (1995) suggested that the degradation of HMT on cometary grains ejected from the nucleus could possibly be responsible for the extended source of the CN radical observed in comet Halley (Klavetter & A'Hearn 1994). This could be an interesting indirect detection of HMT. But Cottin et al. (2001) have tested this hypothesis regarding the photostability of HMT and its photodegradation products and concluded that the photolysis of HMT is very unlikely to account for the amount of extended CN in comets. Nevertheless, the idea of thermal degradation of HMT on very hot cometary grains remains to be investigated in the laboratory. Indeed, HMT is known to decompose into amines and nitriles at temperatures as low as 500 K (Iwakami, Takazono, & Tsuchiya 1968), a temperature that could easily be reached by small grains in cometary comae (Crifo 1988; Greenberg & Li 1998).

The coming in situ investigation of a cometary nucleus (46P/Wirtanen) with the ESA Rosetta mission will search for complex organics. The COSAC (COmetary Sampling And Composition) experiment (Rosenbauer et al. 2001) on board the Rosetta lander includes a gas chromatography system devoted to the analysis of the nucleus molecular composition (Szopa et al. 2000). One of the columns selected to be used in the experiment is an MXT-1, similar to the one we used for our laboratory analysis of HMT, with some differences: length = 10 m, internal diameter = 0.18 mm, and inner coating = 0.1 µm. If present on P/Wirtanen in sufficient amounts, HMT should then be detected, as shown in Figure 3.
<figure><img src="image_3.png" /><figcaption>FIG. 3.—HMT injected along with other organic compounds expected to be present on a cometary nucleus (including benzene, toluene, pyridine, octane, nonane, decane, pentanol, xylene, naphtalene, indene, and trioxane). The top chromatogram was collected at *T* = 90°C and the bottom one at 60°C, on the same MXT-1 column as the one selected to be part of the COSAC experiment on board the *Rosetta* lander. Length = 10 m, internal diameter = 0.18 mm, and inner coating = 0.1 µm. The analysis has been performed within the spatial requirement of the mission (*T* between 30°C and 90°C).</figcaption></figure>

If not, a compound such as HMT, likely to be synthesized by proton and UV irradiations in the interstellar medium, might have been destroyed during the collapse of the presolar cloud or in the early turbulent nebula. Indeed, temperatures above 800 K are necessary to produce the crystalline phase silicates observed in comet Hale-Bopp (Crovisier et al. 1997). Temperature *T* > 800 K is consistent with estimations of conditions in the inner presolar nebula (Mousis et al. 2000). They are sufficient to destroy HMT (Iwakami et al. 1968). The nondetection of HMT could therefore be an indication that most of the interstellar material has been reprocessed at high temperature before the accretion of the comet and that mainly thermal processes dominated comet P/Wirtanen ices history.

This work was performed while H. C. held an NRC-NASA/GSFC Associateship. This work was supported by NASA through RTOP 344-02-57 and 344-33-01 and the French Space Agency (CNES). Claude Smith and Steve Brown of the NASA/Goddard Radiation Facility are thanked for assistance with the proton irradiation.

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