An Infrared Spectroscopic Study Toward the Formation of Alkylphosphonic Acids and Their Precursors in Extraterrestrial Environments

The experiments were conducted in a contamination-free ultra-high vacuum (UHV) chamber at pressures of typically 1 × 10⁻¹⁰ Torr achieved by magnetically suspended turbomolecular pumps (Osaka) backed by oil-free scroll pumps (Anest Iwata; Bennett et al. 2004, 2005; Zheng et al. 2006b; Jamieson & Kaiser 2007; Mottl et al. 2007; Jones et al. 2011). Briefly, a highly polished silver wafer is attached to an oxygen-free high conductivity (OFHC) copper target, which in turn is connected to a two-stage closed-cycle helium refrigerator and programmable temperature controller capable of regulating temperatures between 10 and 330 K. High purity phosphine (PH₃), carbon dioxide (CO₂), methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀) premixed gas mixtures were prepared by mixing the individual components in a gas mixing chamber, while oxygen (O₂) and the vapors of ultra-high purity water were introduced to the UHV chamber separately due to their reactivity with phosphine (2.2. ICE COMPOSITION). The preparation of the ices was controlled by a leak valve and introducing the gas mixture via a glass capillary array at 10 K at pressures of about 5 × 10⁻⁸ Torr in the main chamber. The ice thickness ranges from 910 ± 330 nm to 1500 ± 370 nm (Table 1) as determined by the integrated infrared absorption coefficients (Table 2), which we found to be constant among the deposited ice thicknesses. Pure alkane samples were deposited to a thickness of 500 ± 20 nm using laser interferometry (Heavens 1965; Zhou et al. 2014; Turner et al. 2015) with a helium–neon (HeNe) laser operating at 632.8 nm in order to calculate their absorption coefficients (Turner et al. 2015) and precisely locate their absorption bands, which often lie near or overlap bands from other alkanes (Figure 2, Table 3).

The ice mixtures were then irradiated for 60 minutes with 5 KeV electrons at a current of 0 nA (blank), 100 nA, 1000 nA, and 5000 nA from a Specs EQ 22–35 electron gun. The electron beam exposed an area of 3.2 ± 0.3 cm² at an angle of 15° relative to the surface normal with an actual extraction efficiency of 78.8% of the electrons by scanning the beam over the ice surface. The electron trajectories and energy losses inside the ices were simulated by the CASINO code (Hovington et al. 1997). These simulations yielded average penetration depths from 460 ± 70 nm to 640 ± 100 nm translating to average doses between 2.1 ± 0.3 eV and 3.5 ± 0.7 eV absorbed per molecule at 100 nA irradiation current in the deposited ices (Table 1), which scales to between 21 ± 3 eV and 35 ± 7 eV at 1000 nA and between 100 ± 20 eV and 180 ± 40 eV at 5000 nA. For an interstellar ice grain, these doses correspond to a range of approximately 10⁶ to 5 × 10⁷ years, which approaches the lifetime for an interstellar molecular cloud (Strazzulla & Johnson 1991). It is important to note that the penetration depths of the electrons are less than the thickness of the ices ensuring that the electrons interact only with the ices, but not with the silver target. After the irradiation, the irradiated ices remained at 10 K for an additional 60 minutes before being heated to 300 K at a rate of 1 K minute⁻¹. In situ infrared data were collected by a Nicolet 6700 Fourier Transform Infrared Spectrometer at 4 cm⁻¹ resolution throughout the irradiation and temperature programmed desorption (TPD). These data are available as a tar.gz file.

In order to synthesize phosphorus-bearing molecules—including alkylphosphonic acids—in interstellar analog ices, appropriate precursor molecules that have been detected or predicted to exist on low temperature grains must be utilized; these precursor molecules must also contain carbon (C), hydrogen (H), oxygen (O), and phosphorus (P) as present in alkylphosphonic acids. First, in astrophysically relevant ices, water (H₂O) presents the dominating component and can provide the required oxygen and hydrogen (Boogert et al. 2015). Second, oxygen can also be provided by carbon dioxide (CO₂), which has been detected on interstellar grains at levels of 19%–28% the abundance of water ice (Boogert et al. 2015). Goldsmith et al. (2011) detected molecular oxygen toward Orion and at levels suggesting O₂ comprises at most 1% of interstellar oxygen, and laboratory experiments have verified that molecular oxygen most likely represents a radiolysis product of water ice (Zheng et al. 2006a, 2006b). The reaction of phosphine with these free oxygen atoms is expected to form, e.g., the phosphorus oxoacids H₃PO₂, H₃PO₃, and H₃PO₄ (Figure 1), whose infrared spectra have been investigated by Chapman & Thirlwell (1964), Brun (1970), and Ahmadi et al. (2005), respectively. Third, considering the molecular structure of the alkylphosphonic acids and the existence of an alkyl moiety (Figure 1), astrophysically relevant alkanes shall also be a component of the analog ices. Here, methane (CH₄) has been detected in interstellar ices at levels of up to 5% the abundance of water (Boogert et al. 2015). Ethane (C₂H₆) has never been detected in interstellar ices, but, based on laboratory experiments, it presents the principal alkane, along with propane (C₃H₈) and butane (C₄H₁₀), formed upon radiolysis of methane ices at 10 K (Bennett et al. 2006; Kim et al. 2010). Finally, a carrier of phosphorus is needed. Astrochemical models of interstellar clouds suggest that phosphorus should be depleted onto interstellar grains as phosphine (PH₃) at levels below the current detection limit via infrared spectroscopy (Turner et al. 1990; Charnley & Millar 1994; Wakelam et al. 2013). Note that phosphine was identified in the circumstellar envelope of IRC+10216 (Agundez et al. 2014, 2008) as well as in the atmospheres of Jupiter (Ridgway et al. 1976) and Saturn (Gillett & Forrest 1974). Since the initial atmospheric makeup is generally related to the chemistry of the local solar nebula during planetary formation and molecular clouds provide the material for planetary formation, phosphine is expected to be present in molecular clouds and hence also on interstellar grains. Based on these considerations, we select eight sets of phosphine-rich model ices with rising complexity. Here, ice I helps to unravel the basic processes on the decomposition of phosphine in irradiated ices, whereas ice II probes potential carbon–phosphorus bond couplings. Ices III to V elucidate the reactivity of oxygen atoms released from molecular oxygen, carbon dioxide, and water with phosphine and the inherent formation of oxygen–phosphorus bonds as present in (alkyl) phosphonic acids. Eventually ices VI to VIII explore the formation of all functional groups of alkylphosphonic acids by adding the C1 to C4 alkanes to the phosphine–water and phosphine–carbon dioxide ices.

Pure phosphine (PH₃) ice (Figures 3–5, Table 4) displayed the distinct peaks for ν₂ (985 cm⁻¹) and ν₄ (1097 cm⁻¹) as well as an intense peak centered at 2305 cm⁻¹ resulting from the unresolved bands of ν₁ and ν₃. The irradiated ice produced mostly diphosphine (P₂H₄), which was easily identified at 100 nA irradiation and consistent with the results of Turner et al. (2015). For all irradiations, the ν₁ (2280 cm⁻¹), ν₂ (2225 cm⁻¹), and ν₃ (1056 cm⁻¹) normal modes were observed, while additional vibrations of ν₅ (648 cm⁻¹), ν₈ (2289 cm⁻¹), ν₁₁ (870 cm⁻¹), ν₁₂ (632 cm⁻¹), 2ν₂ (4467 cm⁻¹), and 2ν₁ (4520 cm⁻¹) were observed at higher doses. In addition, triphosphane (P₃H₅) was observed at higher doses, with the δ(HPP) stretches observed at 721 and 778 cm⁻¹. The phosphorus–hydrogen stretching vibration for triphosphane and higher order phosphanes is likely present but obscured by the more intense stretches of phosphine and diphosphine. Not surprisingly, the residues that remained after the ice sublimed contained phosphorus–hydrogen stretching and, to a lesser extent, δ(PH₂), which suggests the residue consists of large molecular weight phosphanes that do not sublime at room temperature even under ultra-high vacuum. This experiment shows that phosphine molecules readily react to produce more complex phosphanes (such as those seen in Turner et al. 2015) and provides the first piece of the puzzle in unraveling the capabilities of phosphine to react with neighboring constituents in ices.

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The addition of methane (CH₄) to phosphine ices (Figures 6–8, Table 5) was observed through the ν₃ (3002 cm⁻¹) and ν₄ (1302 cm⁻¹) fundamentals, as well as a small peak for the infrared inactive ν₂ (1525 cm⁻¹) vibration. Upon irradiation, phosphine produced diphosphine and triphosphane in a similar manner to ice I, while the only hydrocarbon seen at all irradiations was ethane (C₂H₆), which displayed peaks for ν₅ (2880 cm⁻¹), ν₆ (1370 cm⁻¹), ν₁₀ (2968 cm⁻¹), ν₁₁ (1461 cm⁻¹), ν₁₂ (821 cm⁻¹), and ν₈ + ν₁₁ (2936 cm⁻¹). At higher doses, two bands were seen for propane (C₃H₈), the ν₁ (2956 cm⁻¹) and ν₂₁ (912 cm⁻¹) vibrations, while at 5000 nA the only unsaturated compound produced, ethylene (C₂H₄), was observed at ν₇ (947 cm⁻¹) and ν₉ (3087 cm⁻¹). The reaction of phosphine and methane was limited compared to reactions with themselves, and the only product identified in the irradiated ices was methylphosphine (CH₃PH₂), for which the associated vibrations of ω(CH₃) (984 cm⁻¹), δ_as(CH₃) (1412 cm⁻¹), ν(PH) (2300 cm⁻¹), and ν(CH) (912 cm⁻¹) were observed. Methylphosphine is significant because it establishes that ices of phosphine and methane can lead to phosphorus–carbon bonding and with further oxidation, methylphosphine could be a precursor in the formation of methylphosphonic acid. Finally, the peaks in the residue fall into three general categories and mostly appear only after 5000 nA irradiation, which suggests that products of phosphine and methane have a sufficiently high vapor pressure to sublime below room temperature except at high irradiation doses. The first category is phosphorus–hydrogen stretching (2260 cm⁻¹), which is typical of a phosphine-containing ice and is the only infrared band that was seen at less than 5000 nA. Second is a group of four peaks associated with carbon–hydrogen stretching: ν_s(CH) on CH₂ (2861 cm⁻¹), ν_s(CH) on CH₃ (2889 cm⁻¹), ν_as(CH) on CH₂ (2912 cm⁻¹), and ν_as(CH) on CH₃ (2949 cm⁻¹). The third group of vibrations belongs to the P–CH₃ moiety: ρ(CH₃) (867 and 883 cm⁻¹), δ_s(CH₃) (1275 cm⁻¹), and δ_as(CH₃) (1409 cm⁻¹). Thus, these bands indicate that the molecules in the residue contain phosphorus–hydrogen bonds, carbon–hydrogen bonds, and most significantly, carbon–phosphorus bonds. Here, a substantial step in the systematic untangling of the production of alkylphosphonic acids is supplied as phosphorus–carbon bonds were observed from a simple binary ice mixture. These results show that the bottom-up approach for synthesis of the phosphorus–carbon bond is in fact feasible within interstellar ices (Turner et al. 2016).

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Three sources of oxygen were chosen for this study: water (H₂O), carbon dioxide (CO₂), and dioxygen (O₂). While small amounts of O₂ have been detected in the interstellar medium, it largely serves as a good foundation for the study of reactions of atomic oxygen with phosphine and as a carbon-free reference for comparison with the carbon dioxide ices. Although previous studies (Bennett & Kaiser 2005; Ennis et al. 2011) observed the O₂ fundamental stretch in pure oxygen ices at 1549 cm⁻¹, these mixed ices with phosphine show no features related to O₂ (Figures 9–11, Table 6). However, the phosphine bands are less intense, and upon irradiation with even 100 nA, their intensities decline to approximately 10%, which indicates that irradiated dioxygen is much more efficient at reacting with phosphine than methane. After irradiation, several oxidized phosphorus peaks were observed at the expense of phosphanes, as only two peaks for diphosphine (ν₁ (2284 cm⁻¹) and ν₃ (1060 cm⁻¹)) were seen. Also, the irradiated oxygen produced ozone (O₃), observed at ν₃ (1040 cm⁻¹) and its overtone 2ν₃ (2108 cm⁻¹). In general, the broad absorption features assigned to phosphorus with oxygen are identified as function groups that could belong to several molecules and numerous combinations of phosphorus, oxygen, and hydrogen. At lower frequencies exists a complex of band bounded by ν(P–O) with peak intensity between 930 and 1000 cm⁻¹ and by ν(P=O) with peak intensity ranging from 1160 to 1270 cm⁻¹. Intermediate between these bounds exists δ(PH₂) bands near 1050 cm⁻¹, which includes the ν₃ fundamental seen in this and previous ices. The P–OH moiety is evident by the location of oxygen–hydrogen stretches around 2170 and 2700 cm⁻¹, and the band at 1680 cm⁻¹ indicates that O=P–OH is also present. That the band at 2700 cm⁻¹ is greater intensity than 2170 cm⁻¹ suggests a high concentration of phosphoric acid for the 1000 nA residues, while the roughly equal intensities at 5000 nA indicate greater amounts of phosphonic acid (Socrates 2004). However, this method is limited in the irradiated ices because the ν₄(H₂O) band contributes to the 1680 cm⁻¹ absorption, thus complicating the comparison of peak intensities. Other water bands that formed from irradiation include ν_L (800 cm⁻¹) and the broad oxygen–hydrogen stretches of ν₁ and ν₃ between 3000 and 3600 cm⁻¹. While water is not unexpected from this experimental mixture, the ease at which phosphine is oxidized to form compounds similar to phosphonic and phosphoric acid with P=O, P–OH, and combined O=P–OH is promising for phosphine's potential as a precursor to alkylphosphonic acids.

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Carbon dioxide (CO₂) adds a level of complexity in that four elements are present in this ice (Figures 12–14, Table 7). Although carbon is again present in this system like in ice II, the purpose of adding carbon dioxide is as a source of oxygen from the decomposition of carbon dioxide to carbon monoxide (CO) and atomic oxygen, and thus the expected results are more similar to ice III. A downside to using carbon dioxide is that its most intense peak, ν₃ (2335 cm⁻¹) not only overlaps with ν₁/ν₃ of phosphine but dwarfs and obscures phosphine and some products with phosphorus–hydrogen bonding. Besides the ν₃ vibration and its carbon-13 isotopic peak (2274 cm⁻¹), the ν₂ band appears prominently at 665 cm⁻¹. The only carbon-containing compound detected in the irradiated was carbon monoxide at 2137 cm⁻¹ and for ¹³CO, 2092 cm⁻¹. The carbonyl functional group, ν(C=O), was also observed around 1765 cm⁻¹, but given its broad low intensity and overlapping neighboring peaks the specific nature of this carbonyl stretch cannot be ascertained. Other than these bands, the products were similar to the system of phosphine and oxygen. Water was observed at all irradiations through the ν₁, ν₂, ν₃, and ν_L bands. Similarly, the oxidized phosphorus functional groups of ν(P–O) (950 cm⁻¹), ν(P=O) (1150–1300 cm⁻¹), δ(O=P–OH) (1712 cm⁻¹), and P–OH ν(OH) (2170 and 2680 cm⁻¹) were observed at after irradiation and in the residues for the 1000 nA and 5000 nA experiments. Diphosphine was also detected after irradiation at 1000 nA, while the functional groups ν(PH) and δ(PH₂) were detected both after irradiation and in the residues. A comparison of the P–OH bands at 1712, 2170, and 2680 cm⁻¹ indicates that for the 1000 and 5000 nA experiments, phosphonic acid is the dominant source of the P–OH moiety in the residue. The 5000 nA residue also has a band assigned to phosphorus–carbon stretching (745 cm⁻¹), although no other carbon stretches are available to help elucidate the nature of the compounds that might contain this bond. These results readily supply the next step in understanding the possible formation mechanism toward alkylphosphonic acids, as phosphonic acid vibrations are easily detected.

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While the irradiation of molecular oxygen and carbon dioxide produces a free reactive oxygen atom, reactions with water (Figures 15–17, Table 8) are expected to proceed differently as the hydroxyl radical (OH) is formed instead during irradiation. The unirradiated ice shows consistent features to the water produced in ices III and IV, including the broad ν_L band centered near 750 cm⁻¹ and its small overtone at 1465 cm⁻¹, the asymmetric ν₂ band, the combination band of ν_L and ν₂ that falls in the ν(PH) region at 2335 cm⁻¹, and the complex of ν(OH) absorptions between 3000 and 3600 cm⁻¹. These broad absorptions include ν₁, which is split between vibrations that are in-phase and out-of-phase with its neighbors (3050 and 3500 cm⁻¹), and ν₃, which is also split between the transversal (3200 cm⁻¹) and longitudinal (3350 cm⁻¹) vibrations. The thickness of this ice is around 1 μm, which is sufficient for the transversal mode to be observed and not neutralized by the metal surface selection rule (MSSR; Horn et al. 1995; Zondlo et al. 1997; Maté et al. 2003). The transversal mode, which arises from a transition dipole moment parallel to the surface and absorbs the s-polarized component of the infrared radiation, can be canceled out in much thinner films by the MSSR. Also, the notch seen in this peak can be attributed to coupling of the real refractive index (n) and the absorption index (k), resulting in a change of reflectivity that becomes more pronounced with increasing film thickness (Horn et al. 1995). Furthermore, this complex includes a prominent absorption at 3570 cm⁻¹ that was not seen in previous non-phosphorus containing ices. A similar spectral feature was seen by de Barros et al. (2015), which they assigned to dangling OH bonds. The irradiated ice is comparatively simple, with the only non-oxygen-containing bands belonging to P₂H₄ (1060 and 2286 cm⁻¹), ν(PH) (2220–2300 cm⁻¹), and δ(PH₂) (1035 cm⁻¹). The latter two features are also observed in the residue. Compared to carbon dioxide and molecular oxygen, water reacted with phosphine less readily and produced fewer oxygenated phosphorus compounds. At 100 nA irradiation, the spectrum was nearly unchanged, and even at 1000 nA the only new phosphorus/oxygen bands were small signals for ν(P=O) (1135 cm⁻¹) and P–OH ν(OH) (2780 cm⁻¹). However, small signals for ν(P–O) and δ(O=P–OH) could be obscured by the ν_L and ν₂ bands of water, and ν(P–O) was visible in the residue. All of these vibrations were visible in the 5000 nA experiment, although still muted compared to the carbon dioxide ices. Missing from all irradiated ices was the P–OH ν(OH) at 2170 cm⁻¹, although a small peak appeared in the residue at 5000 nA. Unlike the carbon dioxide and molecular oxygen residues at 5000 nA, these peaks from the water experiments indicate a low concentration of phosphonic acid and instead greater amounts of phosphoric and phosphinic acid (H₃PO₂). In summary, ices of phosphine with oxygen, carbon dioxide, and water each can produce, given sufficient irradiation, compounds that include a phosphorus-oxygen double bond (P=O), single bond (P–O), including to a hydroxyl group (P–OH), and the combination of these functional groups (O=P–OH). However, these signals were visible in the oxygen ices after just 100 nA irradiation, while carbon dioxide required 1000 nA and water needed the highest dose of 5000 nA for detection. These ice mixtures also contained phosphorus–hydrogen bonding that appeared comparatively more intense in the residues than they did in the irradiated ices, which suggests a large number of phosphorus–oxygen compounds were able to sublime from the ice while more phosphorus–hydrogen compounds remained in the residue. These results help to show possible barriers in the synthesis of the phosphonic acid group within water ice, which is the major component of interstellar ices.

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Ice VI (Figures 18–20, Table 9) represents the first of three ice mixtures capable of forming an alkylphosphonic acid. Previous ices have shown that phosphine with methane is capable of forming the phosphorus–carbon bond, while phosphine with carbon dioxide results in functional groups present in a phosphonic acid. The unirradiated ice contained the expected peaks seen in ices II and IV, and the spectrum after 100 nA irradiated was fairly unremarkable with only small peaks for CO (2137 cm⁻¹), ν(PH) (2330 cm⁻¹), δ(O=P–OH) (1700 cm⁻¹), and ν(POC) (800 cm⁻¹), the latter, which also remained in the residue. Increasing the current to 1000 nA provided more detailed results with the only compounds detected being CO, C₂H₆ (ν₁₀, ν₁₁, and ν₁₂), and H₂O (ν₂ and ν₃). The phosphorus–carbon bond was established by bands from ν(PC) (722 cm⁻¹), P–CH₃ ρ(CH₃) (875 cm⁻¹), and P–CH₃ δ_as(CH₃) (1419 cm⁻¹), with only the latter remaining in the residue. Absorptions of phosphorus and oxygen include ν(P–O) (955 cm⁻¹), ν(P=O) (1130–1360 cm⁻¹), δ(O=P–OH) (1720 cm⁻¹), and also in the residue P–OH ν(OH) (2790 cm⁻¹). This residue also contains carbon–hydrogen stretching at 2895 cm⁻¹ (CH₃ ν_s(CH)) and 2977 cm⁻¹ (CH₃ ν_as(CH)) and phosphorus–hydrogen vibrations at 1050 cm⁻¹ (δ(PH₂)) and 2267 cm⁻¹ (ν(PH)). The 5000 nA experiment shows similar and more intense results. Additional peaks for C₂H₆ (ν₅ and ν₆) appeared as did the P–OH ν(OH) stretch at 2130 cm⁻¹, although the broad H₂O ν_L band now masks many low frequency peaks. Notably, this is the first ice mixture in which diphosphine was not identified, which suggests that phosphine is sufficiently diluted by other compounds that it cannot react with itself to form a phosphorus–phosphorus bond. The residue spectrum has two new peaks of note: first is P–CH₃ (δ_s(CH₃)) at 1302 cm⁻¹ and the second is P–O–CH₃ (δ(CH₃)) at 1456 cm⁻¹, which along with the ν(P–O–C) stretching seen at 1000 nA shows the potential for this ice mixture to form methyl ester phosphorus oxoacids. The degree to which these bands exist in the irradiated ice is hidden by CH₄ (ν₄) and C₂H₆ (ν₁₁), respectively. Also, the residue spectrum lacks the P–OH ν(OH) stretch at 2130 cm⁻¹, which indicates the residue has a higher concentration of phosphoric acid.

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The alkylphosphonic acids discovered in the Murchison meteorite not only included methylphosphonic acid but also contained C2 to C4 alkyl side chains. Because of this, the C2 to C4 alkanes—ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀)—where added to the mixture of ice VI to study its potential to form each of the alkylphosphonic acids (Figures 21–23, Table 10). These new alkanes are well-represented in the unirradiated ice with fundamentals for ethane (ν₅ (2878 cm⁻¹), ν₆ (1370 cm⁻¹), ν₁₀ (2970 cm⁻¹), ν₁₁ (1464 cm⁻¹), and ν₁₂ (820 cm⁻¹)), propane (ν₁/ν₂ (2956 cm⁻¹), ν₃ (2871 cm⁻¹), ν₄ (1471 cm⁻¹), ν₆ (1383 cm⁻¹), ν₁₈ (1370 cm⁻¹, with ethane ν₆), and ν₂₅ (748 cm⁻¹)), and butane (ν₁₂ (2956 cm⁻¹, with propane ν₁/ν₂), ν₁₃ (2922 cm⁻¹), ν₁₄ (1460 cm⁻¹), ν₁₇ (728 cm⁻¹), ν₂₉ (2860 cm⁻¹), and ν₃₂ (1377 cm⁻¹)) observed along with numerous overtones and combination bands. Despite the complex mixture, the irradiation products were straightforward. Water (ν_L, ν₁, ν₂, and ν₃) and CO were observed as expected, beginning with 100 nA, and like ice VI the residue of the 100 nA experiment was minimal. The phosphorus-oxygen double bond was already seen at 100 nA (ν(P=O) at 1170 cm⁻¹ and δ(O=P=OH) at 1700 cm⁻¹), and although diphosphine was not detected again at any dose, ν(PH) appears at 100 nA irradiation. After 1000 and 5000 nA irradiation the residue spectra showed additional signs of interactions between phosphorus and carbon. The ν(PC) (770 cm⁻¹), P–CH₃ δ_s(CH₃) (1300 cm⁻¹), and P–CH₂R δ(CH₂) (1413 cm⁻¹, where R=alkyl group or H) vibrations confirm the phosphorus–carbon bond, while P–O–CH₃ δ(CH₃) (1455 cm⁻¹) and P–O–C₂H₅ δ(CH₃) (1373 cm⁻¹) not only demonstrate methoxy linkages to phosphorus but also that larger alkanes, such as in this case the ethyl group, can be incorporated in alkoxy functional groups with phosphorus. Also of note in the 5000 nA irradiated ice is the detection of ethylene, first seen in ice II, at 1434 cm⁻¹ (ν₁₂), 3068 cm⁻¹ (ν₂ + ν₁₂), and 3086 cm⁻¹ (ν₉). Similar to ice VI, ν(P–O) (970 cm⁻¹) and P–OH ν(OH) (2650 cm⁻¹) were detected while only a scarce amount of P–OH ν(OH) (2180 cm⁻¹) was observed—none of which remained in residues. Finally, as might be expected from the alkane mixture, the residues show a complex of carbon–hydrogen stretches from 2850 to 2990 cm⁻¹.

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The final ice mixture replaces carbon dioxide in ice VII with water as the oxygen source (Figures 24–26, Table 11). The unirradiated ice is as reported in previous ice mixtures, and although the phosphorus–hydrogen stretching area is no longer overlapped with bands from carbon dioxide, it is immediately evident that small, low frequency bands are partially obscured by H₂O (ν_L) and that oxygen–hydrogen stretching will complicate the nearby overlapping carbon–hydrogen stretching region. Diphosphine, which was absent in the CO₂ ices VI and VII, returns at all irradiation doses, and this marks the only difference seen in the 100 nA ice other than a small amount of P–OH ν(OH) near 2740 cm⁻¹. The residue at 100 nA is more notable than in ices VI and VII and includes phosphorus–hydrogen, phosphorus–oxygen (perhaps as P–O–C), oxygen–hydrogen, and carbon–hydrogen bonds. At 1000 nA, the irradiated ice contains two additional peaks, demonstrating that the phosphorus–carbon bond is formed: CH₃PH₂ ω(CH₃) at 985 cm⁻¹ and P–CH₂R (where R is an alkyl group or H) δ(CH₂) at 1414 cm⁻¹. The residue includes oxygen–hydrogen stretches, including that for P–OH at 2640 cm⁻¹, as well as ν(PO), ν(P=O), and δ(O=P–OH). Besides P-H and C-H stretches, the residue also shows P–CH₂R δ(CH₂) and P–O–CH₃ δ(CH₃) at 1452 cm⁻¹. A small peak at 2140 cm⁻¹, which is normally associated with the broad P–OH v(OH) vibration, appears in the 5000 nA irradiated ice and likely belongs to CO, and this peak does not remain in the residue, leading again to the conclusion that phosphoric acid contributes mostly to the ν(OH) stretching. The P–CH₃ ρ(CH₃) vibration (880 cm⁻¹) and ν(PH) stretch of CH₃PH₂ (2308 cm⁻¹) can also be seen in this ice, as well as the v₁₂ (1438 cm⁻¹), v₂ + v₁₂ (3066 cm⁻¹), and v₉ (3084 cm⁻¹) bands of ethylene. The residue at 5000 nA shows P–O–C₂H₅ (1374 cm⁻¹), which was not seen at lower doses but again shows the capability for multi-carbon alkyl groups to create compounds with phosphorus and oxygen.

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Irradiated phosphine ices regularly form, with a few exceptions, diphosphine and residues with ν(PH) and δ(PH₂). The reaction of phosphine with methane and higher order alkanes can be seen with the low-frequency phosphorus–carbon bond, the P–CH₃ ρ(CH₃) vibration, the P–CH₂R δ(CH₂) band, where R can be an alkyl group or hydrogen (if hydrogen, this band would be labeled P–CH₃ δ_as(CH₃) and occurs at the same frequency), the P–CH₃ δ_s(CH₃), and the vibrations of the compound methylphosphine (CH₃PH₂). The addition of oxygen to mixture via carbon dioxide, water, or dioxygen often results in residues with ν(P–O), ν(P=O), and δ(O=P–OH), and two bands for P–OH ν(OH). When an alkane is present, P–O–CH₃ δ(CH₃) and P–O–C₂H₅ δ(CH₃) were also observed. This systematic study of the irradiation of interstellar ice analogs, from simple to complex, shows that not only can phosphine readily react to interstellar ices, but it is in fact capable of producing all of the necessary bonding to produce alkylphosphonic acids. In conclusion, all functional groups necessary to form methylphosphonic acid are present (Figure 27), although whether they are present simultaneously in the same molecule cannot be ascertained using only infrared spectroscopy. However, more sensitive experiments are being designed to extract the true identity of these residues and the confirmation of alkylphosphonic acids in phosphine-containing interstellar ices.

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