Article pubs.acs.org/JPCA
Thermal Processing of Formamide Ices on Silicate Grain Analogue M. Michele Dawley, Claire Pirim, and Thomas M. Orlando* School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, United States ABSTRACT: Fourier transform-infrared spectroscopy (FTIR) and temperature programmed desorption (TPD) have been used to examine the thermal processing of three isotopes of pure formamide ice (HCONH2, DCONH2, and HCOND2) adsorbed on a SiO2 interstellar grain analogue. Pure formamide ice on SiO2 nanoparticles displays at least three different phases that we interpret as a porous phase from ∼70−145 K, a compacted polycrystalline phase from ∼145− 210 K, and a third slow diffusion and sublimation phase from ∼210−380 K. Possible dimerization is also discussed. Formamide desorption from the SiO2 grain surface is characterized by TPD of pure HCONH2 and mixed H2O:HCONH2 ices. Water desorbs at 160 K, and formamide has a TPD peak maximum at ∼228 K. A mean Eact of ∼14.7 kcal/mol (0.64 eV) was obtained using Redhead analysis, indicating strong intermolecular forces within formamide ice. The mixed H2O:HCONH2 ice TPD data suggests possible formamide accumulation if the grains are exposed to temperature cycles 100 K) that originates from solid-state chemistry in high-mass young stellar objects (YSOs).13 Though pure FA ices are unlikely to be present in astrophysical environments, understanding the optical signatures and how they vary with phase transitions are of fundamental interest. This information is necessary because it provides the basis for determining physical and chemical characteristics such as the temperatures and phases FA exits and evolves, as well as how FA is transformed within pure and mixed ices. The latter are more typical and are likely to be present on interstellar grains. Laboratory ice spectra of molecules on grain simulants are helpful when assigning features in the vast observational infrared (IR) databases of the Infrared Space Observatory and the Spitzer Space Telescope. These have sampled regions including high-mass and low-mass YSOs that include interstellar icy grains.6,10,14−16 Also, as the James Webb Space Telescope is expected to launch in 2018, it will begin to explore the distribution of organic molecules and water in star-forming regions and in our own solar system. The main instruments include a mid-IR instrument, a near-IR imager/spectrograph, a near-IR camera, and a near-IR multiobject spectrograph.17 In this article, we examine the thermal processing of FA ice on SiO2 nanoparticles under ultrahigh vacuum (UHV) conditions as a model system to aid observational data and to characterize FA ice morphology. Mid-IR spectra of FA (HCONH2, DCONH2, and HCOND2) ices were measured upon warm-up from 70 to 465 K. Desorption activation energies are determined via coverage-dependent temperature
1. INTRODUCTION Formamide (HCONH2, hereafter denoted FA) is a simple H, N, C, and O containing molecule that is known to form via hydrolysis of HCN1 or via radiation and high temperature processing of ices containing CO and NH3. HCN and NH3, with trace amounts of CO, have been identified in the atmosphere of Titan, Saturn’s largest moon.2,3 In view of the known radiation fluxes and the availability of trace water or other oxygen atom sources, FA may also form on or within nitrogen-containing hydrocarbon (i.e., tholin) aerosols.4 If formed on organic aerosols, the low vapor pressure of FA may prevent a detectable gas-phase signature. Because the large aerosols are eventually deposited on the hydrocarbon and water−ice covered surface of Titan, reactions on Titan can eventually lead to the formation of more complex molecules that may have biological relevance. These reactions may also be enhanced by catalytic mineral particulate surfaces resulting from meteorite impact on Titan’s surface. The persistence or survival of FA and the reaction products depends upon the temperature, structure, and thermal stability of the lowtemperature ices. FA has been tentatively identified in interstellar ices (NGC 7538 IRS9 and W33A)5,6 and has been detected toward the galactic center sources of Sgr A and Sgr B.7 In 2006, Hollis et al. reported two gas-phase transitions of FA toward Sagittarius B2(N),8 a giant molecular cloud with temperatures as low as 40 K in the surrounding envelope and up to 300 K in the dense star-forming regions.9 Warming of icy grains in these regions, due to energy released from star formation, results in higher mobility of radicals and small molecules within the ices (e.g., CO, NH3, H2O, and HCN).6,10 This can induce the formation of larger molecules including FA within the ices11,12 that can then sublime and account for its gas-phase detection.8 FA has © 2014 American Chemical Society
Received: April 23, 2013 Revised: September 30, 2013 Published: January 24, 2014 1220
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was sealed, pumped down, and baked for 24−48 h at 100 °C, as was the sample holder, to remove any contaminants. FA (HCONH2, ≥99.5%) was purchased from Sigma-Aldrich, and isotopic labeled FA liquids (DCONH2 and HCOND2) were purchased from C/D/N Isotopes, Inc. with a purity of 99.5% D and 99.9% D, respectively. The dosing manifold is equipped with a turbo-molecular pump and dry backing (scroll) pump to effectively evacuate any contaminants before each experiment. FA liquid attached to the manifold was freeze− pump−thawed prior to dosing, and FA vapor was dosed into the chamber and onto the cold SiO2 substrate via a precision leak valve. An ion gauge located near the sample monitored the chamber pressure during dosing to maintain a controlled pressure (∼1 × 10−6 Torr) for a specific time. Doses of 25 Langmuir (L) up to ∼1100 L were employed. Typical experiments were carried out as follows. The SiO2 sample was heated to ∼500 K prior to each experiment, and once the pressure had dropped (∼10−8 Torr), the sample was then cooled to 70 K. A background FT-IR spectrum was obtained at 70 K of the bare SiO2 substrate prior to dosing FA. Once dosed, an FT-IR spectrum was again obtained at 70 K. For the subsequent FT-IR measurements as a function of increasing temperature, a constant temperature (± 1 K) was maintained for approximately 5−10 min for each spectral acquisition. Separate TPD measurements were also obtained as a function of increasing temperature at a rate of 0.5 K/s.
programmed desorption (TPD) measurements of pure HCONH2 and mixed H2O:HCONH2 ices adsorbed on SiO2 nanoparticles. The SiO2 nanoparticles are used as a model for interstellar silicate dust grains10,18 and to simulate interactions with catalytic mineral particulate surfaces resulting from meteorite impact on Titan’s surface. The temperature range was chosen to mimic the temperature range of Titan19 and the warming in interstellar sources where FA has been detected.9 In addition, the temperature range overlaps what is expected to be found on early Earth.20 The companion article21 reports on radiation processing of FA ice by Lyman-α photons and 1 keV electrons to simulate reactions that may occur in star forming regions as well as on Titan aerosols.
2. EXPERIMENTAL METHODS The stainless steel UHV chamber (base pressure ≈ 1 × 10−10 Torr) has been described previously;22 however, details relevant to this study will be briefly mentioned. The UHV chamber is coupled to a Bruker Equinox 55 FT-IR spectrometer with an external HgCdTe LN2-cooled detector for transmission FT-IR experiments of the sample under vacuum. For all experiments described here, FT-IR spectra were recorded in the range of 4000−400 cm−1 with 2 cm−1 resolution averaged over 250 scans for both the background and sample scans using OPUS Bruker software. All absorption spectra shown in this study were produced via subtracting out the spectra of the SiO2 at 70 K via the Beer−Lambert law (A = −log (I/Io)), where I is the single channel spectrum taken after FA adsorption and Io is the single channel spectrum taken of the bare SiO2 at 70 K. This subtraction results in the removal of SiO2 bands from the spectra except for those that change during the experiment. Smoothing of the spectra was performed with the OPUS Bruker software; however, care was taken to ensure no loss of spectral features. The UHV chamber is also equipped with a quadrupole mass spectrometer (QMS) to perform TPD measurements using custom LabView software that controls the QMS and maintains a linear temperature ramp (0.5 K/s). The sample holder has been described previously.22 For this study, a 1 × 1 tungsten wire mesh was used to mount the silicon oxide (SiO2) nanoparticle sample in the UHV chamber. The high-surface-area SiO2 nanoparticles are fumed silica, not quartz, and similar powders have been used in other studies as an interstellar analogue.18 The tungsten mesh has a 74.8% open area, with 0.0058″ size openings between each weave. The porous silicon oxide (SiO2, 99.5%) nanoparticles (BET surface area = 640 m2/g) were purchased from US Research Nanomaterials, Inc., with a reported particle size of 15−20 nm. It has been spectroscopically determined that these nanoparticles are composed of SiO2 and Si−OH groups, with a hydroxyl content of >45%. To produce the SiO2 coated substrate, the tungsten mesh was dipped in a mixture of ∼0.48 g of SiO2 nanoparticles in 2 mL of nanopure H2O. The dipcoated mesh was baked in an oven at 110 °C overnight to remove the H2O from the SiO2. The resulting weight of the SiO2 on the mesh was 0.0165 g, after baking. The silicon oxidecoated tungsten mesh sample was then spot-welded to two tantalum horizontal strips and attached to the sample holder. Effective cooling to 70 K and heating to 900 K is possible with this sample, as measured by attaching a K-type thermocouple directly to the SiO2-covered tungsten grid. After affixing the silicon oxide-coated mesh to the sample holder, the chamber
3. RESULTS AND DISCUSSION 3.1. IR Spectra of Formamide Ices from 70 to 465 K. IR peaks have been assigned based on prior HCONH2, DCONH2, and HCOND2 gas-phase, liquid, and condensed-phase reports.23−30 To confirm the assignments and the structural changes that occur, experiments with the FA isotopologues (DCONH2 and HCOND2) were performed and are discussed below. Because the spectra do not change reversibly at different temperatures, most of the IR frequency changes are due to structural changes in the ice. 3.1.1. HCONH2. As shown in Figure 1, the main visible IR vibrations for HCONH2 at 75 K on SiO2 nanoparticles include
Figure 1. Infrared spectra during warm-up after deposition of 1100 L of HCONH2 at 70 K on SiO2 nanoparticle surfaces. Changes in peaks with * are due to thermal effects only and are not due to ice structural modifications. 1221
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the asymmetric and symmetric NH2 stretches (3317 and 3163 cm−1), a C−H stretch (2885 cm−1), the strong CO stretch (1693 cm−1), the weak and broad NH2 bending mode (1633 cm−1), the C−H bending mode (1385 cm−1), and the C−N stretch (∼1327 cm−1). Two weak, broad peaks at 682 and 630 cm−1 represent NH2 wagging and NH2 torsion modes,28 respectively. Upon warm-up to 165 K (Figure 1), several IR bands sharpen including the NH2 and C−H stretches, NH2 (1643 cm−1) and C−H (1390 cm−1) bending modes, C−N stretch (1329 cm−1), and the NH2 wagging and torsion modes at 652 and 631 cm−1, respectively. The sharpening of the bands at 652 and 631 cm−1 is in contrast to a prior study in which the bands were sharper (and split) at lower temperature (108 K compared to 208 K).25 Another main change seen is the splitting and shifting of the CO stretch and the appearance of a shoulder at 1750 cm−1 upon warm-up. To distinguish between temperature-induced absorption band alteration (shape and position) and a true structural change in the ice, several experiments were carried out. For example, annealing a FA ice to 165 K, taking a spectrum, and then cooling to 70 K and subsequently taking another IR spectrum was performed (see Figure 2). After cool down to 70
Figure 3. Infrared spectra during warm-up after deposition of 1100 L of DCONH2 at 70 K on SiO2 nanoparticle surfaces. Changes in peaks with * are due to thermal effects only and are not due to ice structural modifications.
1385 cm−1. The IR absorption intensity of the C−H bending mode for HCONH2 at 75 K was 0.082, whereas for DCONH2 the IR absorption intensity of this vibration band has decreased to 0.023. These changes in the IR vibrations of DCONH2 compared with HCONH2 confirm isotopic substitution at the C−H position. Upon warm-up to 165 K (Figure 3), the main IR vibrational changes are similar to HCONH2 mentioned above, further confirming the original assignments. For this isotope, the C−D stretch at 2178 cm−1 becomes two sharp peaks at 2181 and 2160 cm−1. In addition, the splitting of the CO stretch is confirmed by the appearance of now one sharp peak at 1738 cm−1 representing the CO stretch (with D attached) and a much weaker peak at 1759 cm−1 representing the CO stretch (with H attached). The weaker peak is likely due to H−D isotopic exchange upon warm-up. This small amount of exchange is further confirmed by the small presence of C−H stretches at 2894, 2889, and 2842 cm−1. A similar blue shift (frequency increase) is seen for the main CO stretch, from 1687 cm−1 at 70 K to 1692 cm−1 at 165 K. However, for DCONH2, two weak peaks arise that were not seen in HCONH2. These include a peak at 1172 cm−1 tentatively identified as a C−N stretch (+NHD rock, −NCO bending mode)23 and a peak at 992 cm−1 (C−D wagging mode) that is on the shoulder of the SiO2 vibration at 1011 cm−1. 3.1.3. HCOND2. IR features of HCOND2 taken at 70 K on the SiO2 nanoparticles are shown in Figure 4. Isotopic replacement is confirmed by the significant decrease in the NH2 stretching modes, which had an IR absorption intensity of ∼0.096 for HCONH2 and now only ∼0.039 for HCOND2. In addition, strong peaks representing the ND2 stretches are found at 2507 and 2352 cm−1. Finally, the broad weak bands previously found at 682 and 630 cm−1 representing the NH2 wagging and torsion modes in HCONH2 are now almost nonexistent due to the isotopic substitution in HCOND2. Not only do the changes confirm isotopic replacement in the N−H position, but they also confirm the assignments given to our spectra of HCONH2 ice. Upon warm-up to 165 K (Figure 4), similar changes occur compared to the two previous isotopes discussed. For this ice,
Figure 2. Comparison of infrared spectra of 1100 L of HCONH2 at 70 K on SiO2 nanoparticle surfaces after dosing (bottom), upon warm-up to 165 K (middle), and upon cooling back to 70 K (top). The middle and top spectra both show that the sharpened features are irreversible.
K, the IR spectrum still displayed the same new and sharpened vibrational features seen at 165 K. Similar experiments were done for 210, 290, and 380 K annealing temperatures (not shown). Thus, the IR band changes with temperature have been found to be irreversible in this study. IR features seen upon warm-up were still observed upon cooling to 70 K for all the temperatures tested confirming that a true structural modification of FA ice occurs upon warm-up with FA locked into new configurations when cooled back to 70 K on the SiO2. 3.1.2. DCONH2. IR features of DCONH2 taken at 70 K on SiO2 nanoparticles are shown in Figure 3, and most of the vibrations occur at about the same frequencies as for HCONH2. Two exceptions that confirm isotope replacement include the C−D stretching mode, now visible at 2178 cm−1, that replaces the C−H stretch previously found at 2885 cm−1, and a decrease in intensity seen for the C−H bending mode at 1222
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Figure 4. Infrared spectra during warm-up after deposition of 1100 L of HCOND2 at 70 K on SiO2 nanoparticle surfaces. Changes in peaks with * are due to thermal effects only and are not due to ice structural modifications.
Figure 5. Comparison of infrared spectra at 165 K during warm-up after deposition at 70 K of 1100 L of (A) HCONH2, (B) DCONH2, and (C) HCOND2 on the SiO2 nanoparticle surfaces. Changes in peaks with * are due to thermal effects only and are not due to ice structural modifications.
sharpening of the ND2 stretches occurs giving bands at 2485, 2437, 2390, 2357, and 2340 cm−1. Compared to the sharpening seen in the NH2 stretches for the previous two ices, here we see several additional bands. The bands at 2390, 2357, and 2340 cm−1 overlap with background CO2; thus, CO2 may contribute. However, the sharpness of the band at 2340 cm−1 is difficult to explain as simply CO2; thus, it must at least be partially due to the ND2 stretching modes. A weak peak at 2281 cm−1 is tentatively assigned23 to the ND2 bending mode because there is no additional or shifted ND2 bending vibration seen near the weak NH2 bending mode at 1646 cm−1. Note this is only weakly visible due to isotopic exchange. In addition, weak peaks at 3277 and 3229 cm−1 represent the NH2 stretching modes. Another weak band at 2689 cm−1 is visible, and we tentatively assign this as a C−H stretch. In this ice, we again see the new CO stretch (with H attached to carbon, 1733 cm−1), but we do not see the deuterated companion peak (CO with D attached) as seen in the DCONH2 ice. Again, the main CO stretch blue shifts from 1665 cm−1 at 70 K to 1674 cm−1 at 165 K. Because this peak shifts comparably in all three experiments, its frequency appears very sensitive to differences that occur in the FA ice geometry (discussed further in Section 3.3). A weak peak at 942 cm−1 is assigned to the ND2 rocking mode (−CN stretch), and it overlaps with the SiO2 vibration at 958 cm−1. Finally, both the NH2 wagging and torsion modes are much weaker and are now shifted to 634 and 569 cm−1, respectively, compared to the other isotopes at 165 K. The shifting is evidence for assignment to ND2 wagging and torsion modes at these frequencies. Therefore, this further confirms the assignments of these features in all the FA isotopes. 3.2. SiO2 IR Vibrations. Several IR bands (denoted by * in Figures 1−5) are seen upon warm-up and are attributed to the molecular rearrangement of the SiO2 substrate and/or thermal effects on the SiO2 vibrations. The negative IR peak seen in all the isotopic experiments near 1253 cm−1 that becomes more pronounced with increasing temperature is reversible and is thus a thermal effect on the vibration. It corresponds to the LO (longitudinal) partner of the TO (transverse) phonon mode of the asymmetric motion of Si−O−Si bridges in which adjacent oxygen atoms execute the asymmetric motion in phase with
each other.31 In addition, the peak at 1007 cm−1 is due to reversible thermal effects on the SiO2 bands (Si−O−Si stretches at ∼1050−1230 cm−1).32 This is confirmed by the disappearance of these bands when the SiO2 was annealed to 505 K and cooled back to 70 K. However, the peak at 960 cm−1 (Si−O−H stretch32) likely involves the surface hydroxyls of the buried silicate surface. The nonreversible change of this feature with increasing temperature leads to a structural change that may or may not influence FA ice. An optical screening effect was mentioned previously by Yates et al.,33 in which the SiO2 underlying substrate vibrations may be inhibited by an overlying ice until a sufficient temperature is reached. It has been ruled out here by verifying the warm-up behavior in control SiO2 IR warm-up experiments performed without FA, as well as with H2O dosed on the SiO2 as a control to monitor the warm-up behavior with a different ice (not shown). All the same SiO2 features were visible in both experiments upon warm-up. 3.3. Molecular Rearrangement upon Warm-up. It is immediately apparent from Figures 1, 3, and 4 that all three isotopes have major changes occurring in their IR features starting at ∼145 K, with the changes most evident near 165− 190 K in all three isotopes. Figure 5 presents a direct comparison of the mid-IR spectra taken of the three FA ices upon warm-up to 165 K, where the sharpest features are apparent. Table 1 also gives a corresponding list of the band positions for the IR spectra taken at 165 K. These changes include sharpening of the respective FA vibrations already present and the appearance of a few distinct features that occur only above 145 K. The sharpened IR features seen at 165 K begin to have a notable reduction in intensity at ∼210 K; i.e., the temperature at which FA begins to desorb (seen also in TPD). The IR features above 210 K, although broad, are not identical to those seen at 70 K for any of the three ices. Because of its intense spectral feature, the CO bond of FA is a good probe to highlight new molecular positions or local disturbances. See Figure 6A for the maximum peak position and Figure 6B for the IR absorption intensity of the main C O stretch as a function of increasing temperature for the three 1223
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Table 1. Infrared Vibrational Frequencies (cm−1) for Formamide Isotopes (1100 L Dosed at 70 K) after Warm-up to 165 K on the SiO2 Nanoparticles;a Corresponding Spectra Are Shown in Figure 5
a
assignment @ 165 K
HCONH2
DCONH2
HCOND2
NH2 asym stretch NH2 sym stretches CH stretches ND2 asym stretches ND2 sym stretches ND2 bend CD stretches CO stretch CO stretch NH2 bend CH bend (−CO stretch) CN stretch (+NH2 [or ND2] rock, −NCO bend) CN stretch (+HD rock, −NCO bend) Si−O−Si phonon modes34 CD wagging Si−OH stretching ND2 rock (−CN stretch) NH2 wagging (+NH2 torsion) NH2 torsion (−NH2 wagging)
3292 3199, 3150 2890, 2808
3290 3185, 3153 2894 (w), 2889, 2842
3277 3229 2890,2689 (vw) 2485, 2437 2390, 2357, 2340 2281
960
2181, 2160 1759 (w), 1738 1692 1645 1392 (w) 1311 1172 (?) 1011 (w) 992 957
652 631
646 625
1750 1703 1643 1390 1329 1007
1733 1674 1646 (vw) 1401, 1390 (sh) 1347, 1350 (sh) 1086 (w), 1010 958 942 634 (for ND2) 569 (for ND2)
w = weak; vw = very weak; sh = shoulder on main peak.
Figure 6. CO stretch (A) infrared maximum peak position and (B) peak IR absorption intensity as a function of increasing temperature for three formamide isotopes. The changes indicate at least three distinct molecular rearrangements at (1) 70−145 K, (2) 145−210 K, and (3) 210−380 K. Lines are not fit but are drawn to guide the eyes.
environments overlap. It is important to note here that the IR absorption intensity changes seen were also seen for annealed ices. We attribute differences in the maximum peak position and IR absorption intensity (Figure 6) of the main CO stretch upon warm-up to represent three new CO stretches in three different geometrical ice environments, depending on the temperature: (1) a first phase that is likely porous between 70 and 145 K, (2) a second phase previously referred in the literature as crystalline26,29 between 145 and 210 K, and (3) a third phase between 210 and 380 K where the disappearance of the bands of the condensed layer indicates that FA diffuses from the free volume (pores) between the SiO2 grains as desorption begins. At 70 K, pure FA ice is likely amorphous and consists of monomers, stable dimers, and aggregates. The presence of a single CO stretch between 70 and 145 K suggests that FA molecules exhibit a local symmetry, possibly condensing as dimeric units and forming puckered sheets.35 However, during
FA isotopes. As shown in Figure 6A, the peak position generally changes to higher frequencies from 145 to 165 K and then red shifts back to lower frequencies starting at about 210 K. The only exception is the HCOND2 ice where the peak position continues to slightly blue shift from 240 K up to 380 K. The influence of the nitrogen-deuterium bond (ND) on the CO stretch of FA may account for this frequency increase at the higher temperatures. As shown in Figure 6B, the IR absorption intensity of the band starts to increase at ∼150 K, also evidenced by the sharpened bands in the IR spectra. Also, the full width at half-maximum (fwhm) of the main CO stretching band is 58.7 cm−1 at 70 K and 29.4 cm−1 at 165 K, confirming the band’s sharpening/narrowing upon warm-up. Figure 6B shows another change in the intensity of the band starting at 210 K for all three ices as desorption begins until the intensities fall to near zero at ∼460 K, indicating complete desorption. Also, the main CO band appears to contain more than one feature (depending upon temperature) and might be a combination of bands at certain temperatures when ice 1224
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warm-up to 145−210 K, long-range diffusion and rearrangements take place and likely generate a reorganization/phase transition. Itoh and Shimanouchi have accounted for low frequency band splitting in term of hydrogen bonding and intermolecular interactions within FA ices at 208 and 108 K. This could contribute to some of the features seen in our spectra, although their study was performed under ambient conditions.25 The reorganization or phase transition is supported by the irreversible changes in the bands upon annealing. While the ice is reported to be crystalline at ∼145− 160 K,29 the observed increase in CO stretching vibrations suggests that individual FA molecules exhibit lower local symmetry; i.e., CO vibrations take place in different environments.35 Indeed, the warm-up likely induces a 3dimensional rearrangement of FA molecules with molecular compaction. We note here that a prior study36 of photonstimulated desorption (PSD) of FA on Si(100) at 96 K showed using near edge X-ray absorption fine structure spectroscopy (NEXAFS) that the CNO plane is titled at an angle of ∼42° away from the surface. They suggested that the FA at 96 K was not amorphous on a single crystal substrate. However, very recent VUV spectroscopy37 and FTIR studies38 report that FA undergoes an amorphous to crystalline phase change at ∼160 K.37 Our results using grain analogues are consistent with these results. During warm-up to 210−380 K, a slow sublimation phase is also observed where lateral diffusion and grain-to-grain transport likely dominates. Figure 7 shows the TPD spectra (100−600 K) of H2O, HCONH2, and a H2O:HCONH2 (2:1) binary mixture dosed at 70 K onto SiO2 nanoparticles. Figure 7A shows that the maximum desorption temperature peak observed for H2O from SiO2 nanoparticles (160 K) is consistent with the desorption temperature of water multilayer (bulk) ice from a porous Si surface.39 Water molecules, a higher vapor pressure component, serve as calibration for the TPD experiment. Figure 7B indicates that FA desorption from SiO2 begins near 200 K, with a peak maximum at ∼228 K. Desorption is nearly complete by ∼450 K, which is consistent with the IR spectra of HCONH2 shown upon warm-up (Figure 1) in which no FA peaks are observed at this temperature. The lower desorption temperature of H2O compared to FA from SiO2 nanoparticles indicates weaker intermolecular forces within H2O ice than within FA ice, which is consistent with FA ice persisting as dimeric units in the 200−300 K temperature range.40 Sivaraman et al. proposed the formation of FA dimers and polymers to explain the crystalline form of the FA ice.38 While neither new higher mass fragments nor a difference in the intensity of the FA parent ion (m/z = 45) or fragment ion HNCOH+ (m/z = 43) after annealing to the crystallization temperature were observed in our TPD spectra (not shown), the actual configuration of our experiment does not permit us to rule out desorption of higher ordered structures or polymers. The inset in Figure 7B shows TPD spectra of m/z 45 (FA) as a function of increasing multilayer coverage dosed at 70 K onto the SiO2 substrate. The Redhead peak maximum method,41,42 which assumes activation parameters that are independent of coverage, was used to extract the activation energy of desorption from the TPDs of FA on SiO2. The Redhead equation is
Figure 7. (A) TPD spectrum of H2O molecules (m/z = 18) from pure H2O adsorbed on SiO2 nanoparticles. Note maximum desorption at 160 K. (B) TPD spectra of pure HCONH2 molecules (m/z = 45) adsorbed on SiO2 nanoparticles. Note maximum peak desorption occurs at ∼228−233 K. The inset shows no coverage dependence of the maximum desorption temperature peak. (C) TPD spectrum of H2O (m/z = 18, open circles, blue) from a mixed H2O:HCONH2 ice (2:1) compared to a TPD spectrum of the NH4+ fragment (closed triangles, black) from pure HCONH2 ice on SiO2 nanoparticles. The latter NH4+ TPD is due to formamide fragmentation in the electron impact source. Ramp rate is 0.5 K/s for all 3 panels.
where R is the Boltzmann constant, v is the frequency factor, Tm is the desorption peak maximum, and b is the heating rate (0.5 K/s). The mean values were calculated from several TPD experiments at different coverages (Figure 7B inset). The Redhead equation yields a mean Eact of approximately 14.7 kcal/mol (0.64 eV) for Tm = 228 K for all experiments at doses of 25−1100 L at a sample temperature of 70 K. An error of 100 K) in high-mass YSOs and is in agreement with the observations of Bisschop et al. in which FA was labeled as a “hot molecule” that originates from solid-state chemistry in the ices.13 Bisschop et al. also suggested that FA desorbs from the ice at higher temperatures and is likely present in a different, more tightly bound ice layer.13 Our work suggests possible phases of an FA ice layer and concur with their conclusions regarding its desorption. The companion article21 on the radiation processing of pure and mixed FA ices explores the effects of the molecular rearrangement seen at ∼165 K on the Lyman-α and electron irradiation product yields.
REFERENCES
(1) Gerakines, P. A.; Moore, M. H.; Hudson, R. L. Ultraviolet Photolysis and Proton Irradiation of Astrophysical Ice Analogs Containing Hydrogen Cyanide. Icarus 2004, 170, 202−213. (2) Lundell, J.; Krajewska, M.; Rasanen, M. Matrix Isolation Fourier Transform Infrared and ab Initio Studies of the 193 nm Induced Photodecomposition of Formamide. J. Phys. Chem. A 1998, 102, 6643−6650. (3) Lavvas, P.; Galand, M.; Yelle, R. V.; Heays, A. N.; Lewis, B. R.; Lewis, G. R.; Coates, A. J. Energy Deposition and Primary Chemical Products in Titan’s Upper Atmosphere. Icarus 2011, 213, 233−251. (4) Clark, R. N.; Curchin, J. M.; Barnes, J. W.; Jaumann, R.; Soderblom, L.; Cruikshank, D. P.; Brown, R. H.; Rodriguez, S.; Lunine, J.; Stephan, K.; et al. Detection and Mapping of Hydrocarbon Deposits on Titan. J. Geophys. Res.: Planets 2010, 115, E100051− E1000528. (5) Raunier, S.; Chiavassa, T.; Duvernay, F.; Borget, F.; Aycard, J. P.; Dartois, E.; d’Hendecourt, L. Tentative Identification of Urea and Formamide in ISO-SWS Infrared Spectra of Interstellar Ices. Astron. Astrophys. 2004, 416, 165−169. (6) Gibb, E. L.; Whittet, D. C. B.; Boogert, A. C. A.; Tielens, A. Interstellar Ice: The Infrared Space Observatory Legacy. Astrophys. J., Suppl. Ser. 2004, 151, 35−73. (7) Gottlieb, C. A.; Palmer, P.; Rickard, L. J.; Zuckerma., B. Studies of Interstellar Formamide. Astrophys. J. 1973, 182, 699−710. (8) Hollis, J. M.; Lovas, F. J.; Remijan, A. J.; Jewell, P. R.; Ilyushin, V. V.; Kleiner, I. Detection of Acetamide (CH3CONH2): The Largest Interstellar Molecule with a Peptide Bond. Astrophys. J. 2006, 643, L25−L28. (9) de Vicente, P.; Martin Pintado, J.; Wilson, T. L. A Hot Ring in the Sgr B2 Molecular Cloud. The 4th International Joint meeting of the European Southern Observatory (ESO) and Cerro Tololo Inter-American Observatory (CTIO), La Serena, Chile, March 10−15, 1996; The Galactic Center, Astronomical Society of the Pacific Conference Series; Gredel, R., Ed.; Astronomical Society of the Pacific (ASP): San Francisco, CA, 1996; Vol. 102, pp 64−67. (10) Oberg, K. I.; Boogert, A. C. A.; Pontoppidan, K. M.; van den Broek, S.; van Dishoeck, E. F.; Bottinelli, S.; Blake, G. A.; Evans, N. J. The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars. Astrophys. J. 2011, 740, 109−125. (11) Brucato, J. R.; Strazzulla, G.; Baratta, G. A.; Rotundi, A.; Colangeli, L. Cryogenic Synthesis of Molecules of Astrobiological Interest: Catalytic Role of Cosmic Dust Analogues. Origins Life Evol. Biospheres 2006, 36, 451−457. (12) Brucato, J. R.; Baratta, G. A.; Strazzulla, G. An Infrared Study of Pure and Ion Irradiated Frozen Formamide. Astron. Astrophys. 2006, 455, 395−399. (13) Bisschop, S. E.; Jorgensen, J. K.; van Dishoeck, E. F.; de Wachter, E. B. M. Testing Grain-Surface Chemistry in Massive HotCore Regions. Astron. Astrophys. 2007, 465, 913−U123. (14) Boogert, A. C. A.; Pontoppidan, K. M.; Knez, C.; Lahuis, F.; Kessler-Silacci, J.; van Dishoeck, E. F.; Blake, G. A.; Augereau, J. C.; 1226
dx.doi.org/10.1021/jp404026s | J. Phys. Chem. A 2014, 118, 1220−1227
The Journal of Physical Chemistry A
Article
Bisschop, S. E.; Bottinelli, S.; et al. The C2D Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. I. H2O and the 5−8 μm Bands. Astrophys. J. 2008, 678, 985−1004. (15) Pontoppidan, K. M.; Boogert, A. C. A.; Fraser, H. J.; van Dishoeck, E. F.; Blake, G. A.; Lahuis, F.; Oberg, K. I.; Evans, N. J., II; Salyk, C. The C2D Spitzer spectroscopic survey of ices around lowmass young stellar objects. II. CO2. Astrophys. J. 2008, 678, 1005− 1031. (16) Oberg, K. I.; Boogert, A. C. A.; Pontoppidan, K. M.; Blake, G. A.; Evans, N. J.; Lahuis, F.; van Dishoeck, E. F. The C2D Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. III. CH4. Astrophys. J. 2008, 678, 1032−1041. (17) The James Webb Space Telescope, NASA, 2012. http://www. jwst.nasa.gov/. (18) Rajappan, M.; Yuan, C.; Yates, J. T. Lyman-Alpha Driven Molecule Formation on SiO2 Surfaces: Connection to Astrochemistry on Dust Grains in the Interstellar Medium. J. Chem. Phys. 2011, 134, 0643151−06431510. (19) Lunine, J. I.; Horst, S. M. Organic Chemistry on the Surface of Titan. Atti Accad. Naz. Lincei, Cl. Sci. Fis., Mat. Nat., Rend. 2011, 22, 183−189. (20) Miyakawa, S.; Cleaves, H. J.; Miller, S. L. The Cold Origin of Life: A. Implications Based on the Hydrolytic Stabilities of Hydrogen Cyanide and Formamide. Origins Life Evol. Biospheres 2002, 32, 195− 208. (21) Dawley, M. M.; Pirim, C.; Orlando, T. M. Radiation Processing of Formamide and Formamide:Water Ices on Silicate Grain Analogue. J. Phys. Chem. A 2013, DOI: 10.1021/jp4042815. (22) Dawley, M. M.; Michalkova, A.; Hill, F. C.; Leszczynski, J.; Orlando, T. M. Adsorption of Formamide on Kaolinite Surfaces: A Combined Experimental and Theoretical Infrared Study. J. Phys. Chem. C 2012, 116, 23981−23991. (23) Fogarasi, G.; Balazs, A. A Comparative ab Initio Study of Amides. 1. Force-Fields and Vibrational Assignments for Formamide, Acetamide, and N-Methylformamide. J. Mol. Struct. 1985, 26, 105− 123. (24) Mortensen, A.; Nielsen, O. F.; Yarwood, J.; Shelley, V. Vibrational-Spectra of Mixtures of Isotopomers of Formamide Anomalies in the Carbonyl Stretching Region. J. Phys. Chem. 1994, 98, 5221−5226. (25) Itoh, K.; Shimanouchi, T. Vibrational Spectra of Crystalline Formamide. J. Mol. Spectrosc. 1972, 42, 86−99. (26) Torrie, B. H.; Brown, B. A. Raman and Far-Infrared Spectra of Formamide at Temperatures Down to 20 K. J. Raman Spectrosc. 1994, 25, 183−187. (27) Parmeter, J. E.; Schwalke, U.; Weinberg, W. H. Interaction of Formamide with the Ru(001) Surface. J. Am. Chem. Soc. 1988, 110, 53−62. (28) Hudson, R. L.; Khanna, R. K.; Moore, M. H. Laboratory Evidence for Solid-Phase Protonation of HNCO in Interstellar Ices. Astrophys. J., Suppl. Ser. 2005, 159, 277−281. (29) Khanna, R. K.; Lowenthal, M. S.; Ammon, H. L.; Moore, M. H. Molecular Structure and Infrared Spectrum of Solid Amino Formate (HCO2NH2): Relevance to Interstellar Ices. Astrophys. J., Suppl. Ser. 2002, 140, 457−464. (30) Evans, J. C. Infrared Spectrum and Thermodynamic Functions of Formamide. J. Chem. Phys. 1954, 22, 1228−1234. (31) Ding, Y.; Chu, X.; Hong, X.; Zou, P.; Liu, Y. The Infrared Fingerprint Signals of Silica Nanoparticles and Its Application in Immunoassay. Appl. Phys. Lett. 2012, 100, 0137011−0137013. (32) Niznansky, D.; Rehspringer, J. L. Infrared Study of SiO2 Sol to Gel Evolution and Gel Aging. J. Non-Cryst. Solids 1995, 180, 191−196. (33) Zhuang, J.; Rusu, C. N.; Yates, J. T. Adsorption and Photooxidation of CH3CN on TiO2. J. Phys. Chem. B 1999, 103, 6957−6967. (34) Kirk, C. T. Quantitative-Analysis of the Effect of DisorderInduced Mode-Coupling on Infrared-Absorption in Silica. Phys. Rev. B 1988, 38, 1255−1273.
(35) Mardyukov, A.; Sanchez-Garcia, E.; Rodziewicz, P.; Doltsinis, N. L.; Sander, W. Formamide Dimers: A Computational and Matrix Isolation Study. J. Phys. Chem. A 2007, 111, 10552−10561. (36) Ikeura-Sekiguchi, H.; Sekiguchi, T.; Kitajima, Y.; Baba, Y. Inner Shell Excitation and Dissociation of Condensed Formamide. Appl. Surf. Sci. 2001, 169, 282−286. (37) Sivaraman, B.; Raja Sekhar, B. N.; Jones, N. C.; Hoffmann, S. V.; Mason, N. J. VUV Spectroscopy of Formamide Ices. Chem. Phys. Lett. 2012, 554, 57−59. (38) Sivaraman, B.; Raja Sekhar, B. N.; Nair, B. G.; Hatode, V.; Mason, N. J. Infrared Spectrum of Formamide in the Solid Phase. Spectrochim. Acta, Part A 2013, 105, 238−244. (39) Chen, Y.; Chen, H.; Aleksandrov, A.; Orlando, T. M. Roles of Water, Acidity, and Surface Morphology in Surface-Assisted Laser Desorption/Ionization of Amino Acids. J. Phys. Chem. C 2008, 112, 6953−6960. (40) Ladell, J.; Post, B. The Crystal Structure of Formamide. Acta Crystallogr. 1954, 7, 559−564. (41) Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203−211. (42) King, D. A. Thermal Desorption from Metal-Surfaces. Surf. Sci. 1975, 47, 384−402.
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