Hydrogen Isocyanide, HNC: A Key Species in the Chemistry of Titan's Ionosphere?

Icarus 151, 196–203 (2001) doi:10.1006/icar.2000.6569, available online at http://www.idealibrary.com on

Hydrogen Isocyanide, HNC: A Key Species in the Chemistry of Titan’s Ionosphere? Simon Petrie Chemistry Department, the Faculties, Australian National University, Canberra, Australian Capital Territory, 0200, Australia; and School of Chemistry, University College, University of New South Wales, A. D. F. A., Canberra, Australian Capital Territory, 2600, Australia E-mail: [email protected] Received January 18, 2000; Revised November 1, 2000; Posted online April 23, 2001

HNC is a major product of the dissociative recombination reaction of the important ionospheric ion HCNH+ , although to date this neutral has not been identified within Titan’s atmosphere, nor considered as a component in ionospheric or photochemical models. We have studied a simple pseudo-steady-state model for the formation and removal of HNC, in which the loss processes considered are HNC protonation by reaction with H-bearing ions; reaction with H atoms, yielding HCN; reaction with CH3 radicals, to form CH3 CN; and reaction with a population of unidentified X radicals to yield further hypothesized products. Using the ion abundances of C. N. Keller et al. (1998, Planet. Space Sci. 46, 1157–1174) and of M. Banaszkiewicz et al. (2000, Icarus 147, 386–404), we find that the most important loss processes are the reactions with CH3 and with other unidentified radicals. According to our calculations, the HNC concentration reaches a peak of 104 –105 molecules per cubic centimeter at an altitude of 1000–1100 km (i.e., close to the ionospheric peak), but is very much reduced at lower altitudes. We find also that the HNC/HCN ratio in Titan’s atmosphere may approach unity at the ionization peak altitude and above. We discuss prospects for the detection of HNC, or its reaction products, during the Cassini mission. °c 2001 Academic Press Key Words: Titan; planetary ionospheres; molecular processes.

1. INTRODUCTION

Among bodies within the Solar System, Titan is unique in several respects. It is the only moon to possess a dense atmosphere; it is also the smallest body with an atmosphere more massive than Earth’s. The lower atmosphere is dominated by nitrogen (>97%), methane, and argon; methane is the principal constituent above around 1700 km, while hydrogen molecules and atoms become dominant at progressively higher altitudes (see, for example, Nagy and Cravens 1998). Altitudinal profiles for some of the major neutral species at intermediate altitudes, taken from the model of Toublanc et al. (1995), are shown in Fig. 1. The relatively high methane abundance, particularly within the upper atmosphere, results in a rich and complex chemistry (driven by solar photons and by electrons associated

with the saturnian magnetosphere); several larger hydrocarbons and other organic molecules have been identified. Many studies have now addressed the task of modeling Titan’s atmospheric composition (see, for example, Yung et al. 1984, Toublanc et al. 1995, Lara et al. 1996, Banaszkiewicz et al. 2000). Nitriles account for a significant portion of the trace constituents that have been detected within Titan’s atmosphere. Data obtained during the Voyager 1 encounter, and subsequent groundbased observations, have identified HCN (Hanel et al. 1981), NCCN, HCCCN (Kunde et al. 1981), CH3 CN (B´ezard et al. 1993), and NCCCCN (Samuelson et al. 1997) to date, while chemical models (see, for example, Yung et al. 1984, Yung 1987) or laboratory simulations of Titan photochemical processes predict also the presence of CH2 CHCN and CH3 CH2 CN (Thompson et al. 1991) at molefractions greater than 10−10 , lower abundances of species as large as C7 H15 CN (Fujii and Arai 1999), and the existence of nitrile-derived radicals such as CH2 CN (Nesbitt et al. 1990) and CCCN (Huang et al. 1998). Recently, Khilifi et al. (1996) have suggested also that the isonitrile CH3 NC might be produced within the titanian stratosphere; current observational data place an upper limit of 1.3 × 10−9 on the mean stratospheric abundance of this species. An interesting aspect of the Voyager 1 observational data concerning nitriles is that the species HCCCN and NCCN exhibit very large latitudinal variations in abundance (Coustenis et al. 1991, Coustenis and B´ezard 1995), with their highest abundances being evident at the north polar regions (experiencing winter at the time of the Voyager encounters). In addition to their involvement in stratospheric processes, nitriles are believed also to play an important role in the chemistry of the titanian ionosphere, due to their relatively high proton affinities. The first detailed models of the ionosphere (Ip 1990, Keller et al. 1992) identified protonated hydrogen cyanide, HCNH+ , as the dominant positive ion at the ionospheric peak. Subsequent ionospheric models (Fox and Yelle 1997, Keller et al. 1998, Galand et al. 1999, Banaszkiewicz et al. 2000) have placed a greater emphasis on hydrocarbon ion chemistry, but continue to identify HCNH+ as an important ionospheric species. In the present work we consider the hypothesis that

196 0019-1035/01$35.00 c 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.

197

HNC IN TITAN’S IONOSPHERE

which are exothermic and efficient for many hydrocarbon ions; hydrogen-abstraction reactions of HCN+ with neutral hydrogencontaining molecules HCN+ + X H → HCNH+ + X ;

(2)

and C–N bond forming reactions such as N+ + CH4 → HCNH+ + H2

(3)

+ CH+ 3 + N → HCNH + H.

(4)

and

FIG. 1. Profiles for some important neutral constituents of Titan’s atmosphere, in the altitude range from 700 to 1700 km, according to the atmospheric model of Toublanc et al. (1995).

HNC, formed by the dissociative recombination of HCNH+ , may be detectable within Titan’s upper atmosphere and may also play a part in the formation of the more complex nitriles found on Titan. 2. THEORETICAL METHODS

We have employed calculations at the Gaussian-2 (G2) (Curtiss et al. 1991), CBS-Q (Ochterski et al. 1996), and CBSAPNO (Montgomery et al. 1994) levels of ab initio theory, to investigate the thermochemistry of various processes. These theoretical methods are all well-characterized, widely used, and highly accurate composite quantum chemical techniques generally considered reliable to within ±8 kJ mol−1 or better (Petersson et al. 1998). 3. RESULTS AND DISCUSSION

3.1. Ionospheric HCNH+ Ionospheric models of Titan (Ip 1990; Gan et al. 1992; Keller et al. 1992, 1994, 1998; Keller and Cravens 1994, Roboz and Nagy 1994; Fox and Yelle 1997; Nagy and Cravens 1998, Galand et al. 1999, Banaszkiewicz et al. 2000) all place the altitude of peak electron concentration—the “ionospheric peak”— at between 1000 and 1200 km altitude, which is consistent also with results obtained from analysis of the Voyager 1 results (McNutt and Richardson 1988, Bird et al. 1997). The models and the observational data also indicate that the peak electron concentration is ∼1000–8000 cm3 . There are several pathways to HCNH+ within Titan’s ionosphere. For example, Ip’s model (1990) includes protonation reactions of HCN X H+ + HCN → HCNH+ + X,

(1)

The most significant of these processes, near the ionization peak, is (1) which, for X H+ = C2 H+ 5 , is expected to account for about 70% of the HCNH+ produced (Keller et al. 1998). Another process leading to HCNH+ , which has not been previously considered (and for which no experimental verification yet exists), is the associative ionization reaction CH + NH → HCNH+ + e;

(5)

reaction (5) is exothermic and, by analogy with other exothermic associative ionization processes (Dalgarno et al. 1973, Oppenheimer and Dalgarno 1977, Petrie and Bohme 1994, Petrie 1999), most probably lacks any activation barrier and might therefore be expected to be efficient. Nevertheless, several other product channels (HCN + H, CH2 + N, C + NH2 , CN + H2 ) are also exothermic for the above reactants. Furthermore, while NH may be a comparatively abundant radical, CH is expected to have a very low abundance within the titanian atmosphere: CH is identified as a trace species in the photochemical models of Yung et al. (1984) and Lara et al. (1996), but does not feature in the model of Toublanc et al. (1995). (In this regard, it is interesting to note that an additional source of CH has recently been identified, in the reaction of N(2 D) with HCN (Kurosaki and Takayanagi 1999); however, since CH is consumed reasonably efficiently by reaction with abundant methane (Yung et al. 1984, Lara et al. 1996), this new CH source is probably not of great significance.) It is likely, therefore, that reaction (5)—while noteworthy as a novel source of ionization within the atmosphere—will not dominate over other routes to HCNH+ , except perhaps at altitudes substantially above the ionospheric peak where photolysis of methane and its other fragments might give rise to comparatively high CH abundances. 3.2. HNC Formation The reaction of HCNH+ with a free electron HCNH+ + e → HCN + H

(6a)

→ HNC + H

(6b)

198

SIMON PETRIE

is one of a handful of dissociative recombination reactions for which the branching ratio to products has received serious scrutiny. Experimental difficulties have thus far prevented a laboratory study of the product distribution for this reaction, but a number of theoretical studies of reaction (6), including high-level ab initio quantum chemical investigations, have been performed (Herbst 1978, Talbi and Ellinger 1998, Shiba et al. 1998, Jursic 1999): these studies have concurred that the reaction produces HCN and HNC in approximately equal yields. This can be compared with the results of observational studies of cold, dark interstellar clouds (Harju 1989, Hirota et al. 1998), in which the abundance of HNC is typically found to exceed that of HCN: a mean value of HNC : HCN ∼ 2.1 ± 1.2 has been obtained in a recent detailed radioastronomical study of 19 dark cloud cores (Hirota et al. 1998). Reaction (6) is believed to be one of the principal sources of both HCN and HNC: other interstellar pathways exist to HCN, such as the hydrogen-abstraction reactions of the abundant CN radical with hydrogenated molecules CN + HX → HCN + X,

geometries of HNC and of the corresponding subunit within HCNH+ do not differ greatly, and thus there is no reason to suspect a substantial degree of vibrational or rotational excitation; the HNC product might feasibly be electronically excited, but radiative emission to the ground state is likely to occur on a much shorter timescale than collision with another molecule at the altitudes of peak HCNH+ abundance; and translational excitation of the products will be channeled almost exclusively (>96%) into the light fragment (H) rather than into the heavy fragment (HNC). The most probable consequence of collision with any of the most abundant species—N2 , Ar, or CH4 —is quenching of any internal excitation of the HNC, so that it is reasonable to consider the reactions of thermalized HNC with trace components of the upper atmosphere. We will explore this notion in greater detail in the following section. HNC will be subject to photochemical processing, both in terms of isomerization HNC + hν → HCN

(10a)

HNC + hν → CN + H.

(10b)

(7) and photolysis

while the only other interstellar pathway to HNC that has been seriously considered involves the reaction of C+ with ammonia C+ + NH3 → CNH+ 2 +H CNH+ 2 + e → HNC + H → CN + [H2 or H + H].

(8) (9a) (9b)

Reaction (9) is likely to favor HNC formation over other products (Jursic 1999), but isomerization of the singlet CNH+ 2 product of reaction (8) to the more stable form HCNH+ is essentially quantitative, and consequently CNH+ 2 is not considered a significant source of HNC (Talbi and Herbst 1998). The large HNC : HCN ratio observed in the cold clouds, the lack of other significant routes to HNC, and the expectation that HNC should be depleted at least as rapidly through a variety of chemical reactions as is the lower-energy isomer HCN have led radioastronomers to suggest (Hirota et al. 1998) that the product distribution of reaction (6) should be assigned as HNC + H (60%) and HCN + H (40%). This is the branching ratio that we adopt in the remainder of this discussion, and we assume also a total recombination coefficient of 6.3 × 10−7 cm3 molecule−1 s−1 (Ip 1990). Dissociative recombination is a highly exothermic process— for channel (6b), the exothermicity is 5.60 eV (Hunter and Lias 1998, Hansel et al. 1998)—and, in consequence, the HNC initially produced by dissociative recombination will be both translationally and internally excited. Will this excitation promote unimolecular isomerization to HCN, or enhance the bimolecular reactivity of the HNC under the conditions of Titan’s upper atmosphere? We strongly suspect not, since the equilibrium

The thresholds for these processes are, respectively, 141 and ˚ (Bowman et al. 1993, Hansel 465 kJ mol−1 (8510 and 2570 A) et al. 1998, Chase 1998). In the model presented here, we shall neglect any inclusion of photoprocessing of HNC, because it is very probable that neither reaction (10a) nor (10b) is a significant loss process for a species likely to be consumed reasonably rapidly by chemical processing as explored in the subsequent sections. The lifetime against photolysis will be at least 107 s, according to the upper-atmosphere photolysis rates for analogous processes with similar energetic requirements (for example, C2 H2 → C2 H + H) (Yung et al. 1984). While the energy required for isomerization might appear rather low— corresponding to a near-IR photon—this nevertheless suggests a lifetime of at least 105 s by analogy with other processes possessing a similar threshold (Yung et al. 1984). Furthermore, it can be argued that dynamical obstacles to the interconversion of one linear isomer to the other isomer, necessitating the channeling of sufficient energy into the two degenerate bending modes of HNC rather than dissipation into the stretching modes, electronic excitation, or rotational excitation of this species, will drastically limit the quantum yield for photoisomerization at photon energies not greatly above the isomerization threshold. We would suggest that the timescale against HNC photoisomerization (for a solar zenith angle of 60◦ , which is standard for the titanian photochemical models) probably exceeds 106 s, which is also significantly greater than the lifetime of this species against chemical reaction at all but the highest altitudes in our calculations.

199

HNC IN TITAN’S IONOSPHERE

TABLE I Thermochemistry of the Reaction X + HNC → Products, for Various Radicals X

3.3. Likely HNC Loss Processes, and Implications of HNC Reactivity Mechanisms by which HNC is likely to be removed include protonation X H+ + HNC → X + HCNH+ ,

1H0◦ (kJ mol−1 )a X CN + H

X NC + H

HX + CN

−53(−61, −58, −61)

0

29(30, 32, 35)

C

−131(−9, 3, 22)

(−16, −22, −16)

124(131, 135, 135)

CH

(−94, −94, −73)

(14, 13, 14)

39(46, 54, 60)

CH2

−112(−109, −110, −102)

(−9, −12, −6)

6(2, 6, 9)

CH3

−42(−47, −44, −44)

57(54, 55, 54)

26(27, 33, 33)

X

(11)

H

the H-catalysis isomerization process H + HNC → HCN + H,

(12)

and a more general reaction with radicals

N

X + HNC → X CN + H

(13a)

NH

→ X H + CN.

(13b)

NH2

HNC has a proton affinity (PA) of 772 kJ mol−1 (Hunter and Lias 1998), so all ions having PA values less than this will readily undergo reaction (11): this includes the ion HCNH+ itself (yielding HCN as the neutral product), as well as virtually all the hydrogen-bearing ions likely to be encountered in the titanian + + + + ionosphere (N2 H+ , CH+ 5 , C2 H3 , C2 H5 , C4 H3 , HCCCNH , and + + others, but excluding CH3 and NH4 ). It is conceivable that some of these reactions may occur also as addition processes: for ex+ ample, addition of C2 H+ 3 or C2 H5 to HNC would yield proto+ nated vinyl cyanide (C2 H3 CNH ) or protonated ethyl cyanide (C2 H5 CNH+ ) respectively, but at low pressures the occurrence of the substantially exothermic proton transfer channel (11) is likely to dominate. We adopt a generic value of 3.5 × 10−9 cm3 molecule−1 s−1 for reactions of type (11): this value is similar to the rate coefficients suggested by Ip (1990) for protonation of HCN under titanian ionospheric conditions. We assume also, for simplicity, that all ions (present with a total concentration equal to the electron concentration) are capable of protonating HNC. Reaction (12) has received some scrutiny via high-level ab initio calculations (Talbi et al. 1996), in the context of its influence upon interstellar chemistry. The potential energy surface for this reaction exhibits a 17.6 kJ mol−1 (∼2000 K) activation barrier, ensuring that the reaction is prohibitively slow at cold IS cloud temperatures. Because of the barrier, this process will also be inefficient within Titan’s upper atmosphere: the rate coefficient at 200 K is calculated to be approximately 2 × 10−14 cm3 molecule−1 s−1 (Talbi et al. 1996), although the comparatively high abundance of H atoms at some altitudes indicates that reaction (12) will still be of some significance. The generalized reaction of HNC with radicals, reaction (13), is very difficult to assess in the absence of any experimental studies to date: aside from the “trivial” case where X = H, discussed above, this process appears to have been examined only for the instance of X = C2 H, in another study pertinent to IS cloud chemistry (Fukuzawa and Osamura 1997). It is interesting to note that, for X = C2 H, no barrier to reaction was found,

C2 H

(3, −3, 16)

(129, 128, 152)

151(138, 144, 146)

(20, −20, −12)

(152, 205, 198)

61(78, 83, 85)

−27(−25, −25, −23)

(161, 161, 163)

11(17, 23, 24)

−184(−173, −177, −170) (−65, −65, −61) −91(−92, −84, −89)

C2 H 3 b

−85(−85, −82, −85)

(5, 6, 4)

0(3, 11, 7)

C2 H 5 b

−38(−41, −42, −41)

52(50, 50, 49)

44(42, 46, 47)

−99(−108, −110, −109)

(−4, −9, −8)

−53(−61, −58, −61)

−85(−99, −106, −99)

(161, 158, 163)

37(40, 44, 47)

(−27, −28, −28)

(234, 224, 236)

−34(−29, −25, −23)

CNc O OH

a Reaction enthalpy. A negative value indicates an exothermic product channel: such values are underlined (as are those whose endothermicity is exceeded by the total experimental uncertainty) to highlight viable product channels. Experimental values, in Roman script, are taken from the tabulations of Lias et al. (1988) or Chase (1998) unless otherwise indicated. Theoretical values obtained in this study are parenthesized and in italic script: in the order given, they are the CBS-APNO, CBS-Q, and G2 values. All enthalpies are 298 K values. b Experimental enthalpies for C H and C H are from Tsang(1996). 2 3 2 5 c Theoretical values for NCCN and NCNC have been previously reported by Petrie(1998).

implying that C2 H + HNC is a viable route to HCCCN even at IS cloud temperatures: this contrasts to the barrier evident for X = H, and indicates that reactions of the type (13) will need to be closely examined in a systematic fashion in order to determine for which radicals the reaction possesses activation barriers. In Table I, we provide an analysis of the thermochemistry of possible product channels for reaction (13). Since it is very uncertain as to which radicals will react rapidly with HNC, we have chosen not to examine a highly detailed set of reactions of type (13). Instead, we shall consider only the abundant methyl radical (CH3 ) as a specific reactant, with an additional population of unspecified X radicals that react rapidly with HNC. We investigate two possibilities: in model 1, the reaction of the abundant methyl radical (CH3 ) with HNC is efficient CH3 + HNC → CH3 CN + H

(14)

with a rate coefficient of 5 × 10−11 cm3 molecule−1 s−1 ; while in model 2 this reaction is very inefficient, k14 = 1 × 10−14 cm3

200

SIMON PETRIE

FIG. 2. Calculated profiles for HNC, in the altitude range from 800 to 1700 km, for the six separate cases described within the text: namely, CH3 + HNC fast (Model 1) or slow (Model 2), with atmospheric concentrations of other reactants derived from the Keller/Yung, the Keller/Toublanc, or the Banaszkiewicz ionosphere (note that the latter model extends only to 1400 km altitude). Differences between the results obtained from the Keller and the Banaszkiewicz ion abundances reflect, in part, the different zenith angles (60◦ and 30◦ , respectively) that were used in the studies of Keller et al. (1998) and Banaszkiewicz et al. (2000). Also shown in the figure, as the thinner curve, is the assigned abundance profile of the generic radical X (here divided by 100 for the convenience of the axis scale) employed in our calculations.

molecule−1 s−1 . In both models, we include the generic species X with a relevant rate coefficient of 5 × 10−11 cm3 molecule−1 s−1 . The altitudinal profile for X is as shown in Fig. 2. 3.4. Prospects for HNC Detection We can incorporate reactions (6), and (11)–(14) into a simple pseudo-steady-state model detailing the principal routes to HNC formation and removal. Assumption of a steady state dictates that 0.6 × α6 [HCNH+ ][e] = [HNC]{k11 [e] + k12 [H] + k13 [X ] + k14 [CH3 ]}. (15) To determine the concentration of HNC according to this model, we have used the rate coefficients given in the preceding sections, with the ion/electron concentration profiles determined by Keller et al. (1998) or by Banaszkiewicz et al. (2000). The ion concentrations of Keller et al. were in turn determined using the atmospheric neutral abundances of Yung et al. (1984) or of Toublanc et al. (1995): the main relevant differences between the abundances of the Yung and Toublanc photospheric models are that Yung et al. predict higher HCN and CH3 abundances above ∼1100 km, and a greater degree of altitude dependence in the CH3 abundance between 700 and 1000 km, than do Toublanc et al. In contrast to the Keller et al. ion chemistry model, the Banaszkiewicz et al. (2000) study featured a coupled photochemical and ion chemistry model, and might therefore be expected to provide greater insight into the overall chemical pro-

cessing within the ionosphere; comparison between the Keller et al. (1998) and the Banaszkiewicz et al. (2000) ion abundances is complicated, however, by the differences in zenith angle (60◦ and 30◦ , respectively, in the two studies), and consequently the Banaszkiewicz et al. ionospheric peak is at a somewhat lower altitude. In each of the six simple HNC chemical models studied here (i.e., Model 1 and Model 2, for each of the Keller/Yung, Keller/ Toublanc, and Banaszkiewicz ionospheres), we have solved Eq. (15) using the relevant HCNH+ , e, H, and CH3 abundances at 100–km intervals (over the altitude range 800–1700 km for Keller/Yung and Keller/Toublanc, and 800–1400 km for Banaszkiewicz), combined with our “fabricated” X abundance profile for all other reactive radicals. The resultant HNC profiles are shown in Fig. 2. All six models predict an HNC peak concentration of between 1 and 7 times 104 cm−3 at the approximate altitude of the ionization peak in the respective models (this altitudinal dependence is unsurprising since ion/electron recombination is the sole source of HNC considered in our model), with a dramatic reduction in HNC concentration at lower altitudes. The calculations based on the Keller/Yung ionosphere display a greater dependence on CH3 reactivity than do the values for the Keller/Toublanc and Banaszkiewicz ionospheres (CH3 is more abundant in the model of Yung et al.). For all of the ionospheric models, CH3 is found to be the principal sink for HNC at all altitudes provided that reaction (14) is rapid, while we anticipate that loss due to reaction of HNC with the “unidentified” radicals grouped together as X is likely to be the other major sink. We would urge experimental and/or theoretical investigations of the reactions of HNC with various radicals, such as have already been reported for H (Talbi et al. 1996) and for C2 H (Fukuzawa and Osamura 1997) to help resolve this issue. We should caution that the HNC concentrations determined here are necessarily rather approximate, not only because of the uncertainties in the relevant chemistry but also because the pseudo-steady-state treatment employed neglects any influence of the “new” reactions (6) and (11)–(14) upon the abundances of related species such as HCN and HCNH+ . (In this context, the partitioning of reaction (6) between HCN and HNC production can act as a net “sink” for HCN, and this high-altitude process may well act in concert with the principally low-altitude removal of HCN by polymerization: Banaszkiewicz et al. (2000) have commented that consideration of HCN polymerization is needed in order to satisfactorily model the observed thermospheric mixing ratio profile of HCN.) We would argue that a more detailed analysis of the interrelationship between HNC and other species within the titanian ionosphere is, at present, inappropriate given the large gaps in our understanding of HNC’s reaction chemistry in this environment. Nevertheless, it seems evident from our study that HNC is an important, if minor, constituent near the ionospheric peak: its formation accounts for between 20 and 40% of the ion/electron neutralization events in the altitude range 1000–1200 km.

201

HNC IN TITAN’S IONOSPHERE

Prospects for the direct detection of HNC by the Cassini mission appear rather poor, if this molecule is as reactive as we have supposed: while the IR spectrum of HNC is fairly well established (Maki and Sams 1981, Winter and Jones 1982, Burkholder et al. 1987, Northrup et al. 1997, Nezu et al. 1998), HNC is not expected to be present in sufficient concentration at any altitude to permit its spectroscopic detection, and detection of HNC by mass spectrometry will be very severely hampered by the nearimpossibility of distinguishing HNC+ from HCN+ (Petrie et al. 1990). Groundbased observation by radioastronomy is feasible in principle, but seems beyond the capabilities of existing equipment: the least abundant species detected to date by this method, using the IRAM 30-m radiotelescope, is HCCCN (B´ezard et al. 1992) with an overall column density 10 times greater than our most optimistic value for HNC. Indeed, the failure to detect HNC in previous radioastronomical surveys of Titan gives an indication that this species cannot be significantly more abundant than we have estimated. The principal difficulty in detecting HNC is that its highest concentration is estimated to occur within the ionosphere, where total number densities are rather low: all molecules detected within Titan’s atmosphere to date are concentrated principally within the stratosphere or elsewhere within the lower atmosphere, in regions of much higher number density. A consequence of our conjecture, for a rapid reaction between CH3 and HNC, is that we would expect a localized maximum in CH3 CN concentration near the ionospheric peak, since an efficient reaction (14) would substantially outweigh, at high altitudes, the contributions made by the processes CN + CH4 → CH3 CN + H

(16)

CN + C2 H6 → CH3 CN + CH3 ,

(17)

and

of magnitude at this altitude. Of course, the reaction of CH3 with HNC has not yet been studied, either experimentally or by quantum chemical calculations, and it may well be that this reaction is much less efficient than our more optimistic estimate: in this case, it is highly likely that the best indicator of HNC chemistry will be the product of some unidentified radical (X ) with HNC. According to our calculations, HNC concentrations are negligible at lower altitudes—less than 10 molecule cm−3 at 700 km and below—because we have considered only the dissociative recombination reaction (6) as a route to HNC. This may be too “pessimistic” a view: other authors have determined (Copeland et al. 1992) that HNC is a minor product of the reactions of CN with various hydrocarbons, and such reactions may therefore constitute a low-altitude pathway to HNC on Titan. Nevertheless, such H-atom abstraction reactions are generally rather slow, result in production of much more HCN than HNC (Copeland et al. 1992), and involve the reaction of a radical, CN, which is present in comparatively low concentration in Titan’s atmosphere according to model calculations (Yung et al. 1984, Yung 1987). Finally, we wish to stress that the dissociative recombination reaction (6) is not restricted to Titan’s ionosphere. The ionospheres of Neptune and Triton, for example, are other outerSolar-System environments in which HNC production might be expected to occur: HCN has been detected within Neptune’s stratosphere (Rosenqvist et al. 1992, Marten et al. 1993, Monks et al. 1993), while some similarities have been noted between the hypothesized chemistries of Titan’s and Triton’s atmospheres (see, for example, Thompson et al. 1989, Strobel et al. 1991). It is to be hoped that a consideration of HNC formation and destruction may lead to a more complete picture of the atmospheric processes within these and other extraterrestrial environments. 4. CONCLUSION

which have been previously identified as routes to CH3 CN on Titan (B´ezard et al. 1993, Lara et al. 1996). The reactions of CN with methane and ethane have overall rate coefficients at 195 K of 1.75 × 10−14 and 2.24 × 10−11 cm3 molecule−1 s−1 , respectively (Sims et al. 1993); note that CH3 CN production is expected to be only a minor channel, accounting for 5%, or less, of the reactive collisions (Lara et al. 1996). The detection of a high-altitude “hump” in the CH3 CN concentration profile may well provide a more reliable signature of the HNC chemistry in Titan’s atmosphere than any attempt to detect HNC directly, since CH3 CN is much less reactive than we consider HNC to be in the present model (and will therefore persist at substantially higher concentrations in the altitude range considered here). The photochemical model of Toublanc et al. (1995; see also Anicich and McEwan 1997), for example, estimates a mixing ratio of 8 × 10−9 for CH3 CN at 1000 km, corresponding to ∼100 molecules cm−3 : if reaction (14) is fast, then we would estimate the CH3 CN molefraction to exceed the Toublanc value by two or more orders

HNC is a previously unconsidered but important trace species within Titan’s upper atmosphere. In contrast to other closedshell species for which both ion/molecule and radical/neutral formation pathways are generally viable, HNC is likely to be formed almost entirely by an ion/molecule mechanism; as a result, the concentration profile for HNC as a function of altitude is expected to follow the typical profile for a polyatomic ion rather than a neutral radical or molecule. A peak concentration of between 104 and 105 molecules cm−3 is calculated for an altitude of 1000 to 1100 km. High-altitude HNC is unlikely to be directly detectable by the Cassini mission (or by groundbased radioastronomy), but we anticipate that the “signature” of HNC reactivity may be evident in the altitudinal profiles of daughter species such as CH3 CN. Nevertheless, much additional study is necessary before the reaction chemistry of HNC, within Titan’s atmosphere or within other extraterrestrial environments, is properly characterized.

202

SIMON PETRIE

ACKNOWLEDGMENTS The author thanks Vincent G. Anicich (JPL, Caltech) and Bruno B´ezard (Observatoire de Paris, Meudon) for helpful advice concerning the detectability of HNC. Constructive criticism by the referees is also gratefully acknowledged.

REFERENCES

Hansel, A., CH. Scheiring, M. Glantschnig, W. Lindinger, and E. E. Ferguson 1998. Thermochemistry of HNC, HNC+ , and CF+ 3 . J. Chem. Phys. 109, 1748–1750. Harju, J. 1989. HCN and HNC observations towards dark clouds. Astron. Astrophys. 219, 293–302. Herbst, E. 1978. What are the products of polyatomic ion-electron dissociative recombination reactions? Astrophys. J. 222, 508–516.

Banaszkiewicz, M., L. M. Lara, R. Rodrigo, J. J. L´opez-Moreno, and G. J. Molina-Cuberos 2000. A coupled model of Titan’s atmosphere and ionosphere. Icarus 147, 386–404.

Hirota, T., S. Yamamoto, H. Mikami, and M. Ohishi 1998. Abundances of HCN and HNC in dark cloud cores. Astrophys. J. 503, 717–728. Huang, Y., K. H. Patrick, and J. B. Halperns 1998. Nitrile photochemistry in Titan’s atmosphere: Why NCCN is an important source of CN and why C3 N is an important free radical. In Laboratory Space Science Workshop Harvard–Smithsonian Center for Astrophysics, 1998, p. 153.

B´ezard, B., A. Marten, and G. Paubert 1992. First ground-based detection of cyanoacetylene on Titan. Bull. Am. Astron. Soc. 24, 953.

Hunter, E. P. L., and S. G. Lias 1998. Evaluated gas phase basicities and proton affinities of molecules: an update. J. Phys. Chem. Ref. Data 27, 413–656.

B´ezard, B., A. Marten, and G. Paubert 1993. Detection of acetonitrile on Titan. Bull. Am. Astron. Soc. 25, 1100.

Ip, W. H. 1990. Titan’s upper ionosphere. Astrophys. J. 362, 354–363. Jursic, B. S. 1999. Complete basis set ab initio study of potential energy surfaces of the dissociative recombination reaction HCNH+ + e. J. Molec. Structure Theochem. 487, 211–220.

Anicich, V. G., and M. J. Mcewan 1997. Ion-molecule chemistry in Titan’s ionosphere. Planet. Space Sci. 45, 897–921.

Bird, M. K., R. Dutta-Roy, S. W. Asmar, and T. A. Rebold 1997. Detection of Titan’s ionosphere from Voyager 1 radio occultation observations. Icarus 130, 426–436. Bowman, J. M., B. Gazdy, J. A. Bentley, T. J. Lee, and C. E. Dateo 1993. Ab initio calculation of a global potential, vibrational energies, and wave functions for ˜ X ˜ emission spectrum. J. Chem. Phys. HCN/HNC, and a simulation of the A99, 308–323. Burkholder, J. B., A. Sinha, P. D. Hammer, and C. Howard 1987. High-resolution Fourier transform infrared spectroscopy of the fundamental bands of HNC. J. Mol. Spectrosc. 126, 72–77. Chase, M. W., Jr. 1998. NIST–JANAF Thermochemical Tables, fourth ed. J. Phys. Chem. Ref. Data 9, 1–1951. Copeland, L. R., F. Mohammad, M. Zahedi, D. H. Volman, and W. M. Jackson 1992. Rate constants for CN reactions with hydrocarbons and the product HCN vibrational populations: Examples of heavy-light-heavy abstraction reactions. J. Chem. Phys. 96, 5817–5826. Coustenis, A., and B. B´ezard 1995. Titan’s atmosphere from Voyager infrared observations. IV. Latitudinal variations of temperature and composition. Icarus 115, 126–140. Coustenis, A., B. B´ezard, D. Gautier, A. Martin, and R. Samuelson 1991. Titan’s atmosphere from Voyager infrared observations. III. Vertical distributions of hydrocarbons and nitriles near Titan’s north pole. Icarus 89, 152–167. Curtiss, L. A., K. Raghavachari, G. W. Trucks, and J. A. Pople 1991. Gaussian-2 theory for molecular energies of first- and second-row compounds. J. Chem. Phys. 94, 7221–7230. Dalgarno, A., M. Oppenheimer, and R. S. Berry 1973. Chemiionization in interstellar clouds. Astrophys. J. 183, L21–L24. Fox, J. L., and R. V. Yelle 1997. Hydrocarbon ions in the ionosphere of Titan. Geophys. Res. Lett. 24, 2179–2182. Fujii, T., and N. Arai 1999. Analysis of N-containing hydrocarbon species produced by a CH4 /N2 microwave discharge: Simulation of Titan’s atmosphere. Astrophys. J. 519, 858–863. Fukuzawa, K., and Y. Osamura 1997. Molecular orbital study of neutral–neutral reactions concerning HC3 N formation in interstellar space. Astrophys. J. 489, 113–121. Galand, M., J. Lilensten, D. Toublanc, and S. Maurice 1999. The ionosphere of Titan: Ideal diurnal and nocturnal cases. Icarus, 140, 92–105. Gan, L., C. N. Keller, and T. E. Cravens 1992. Electrons in the ionosphere of Titan. J. Geophys. Res. A 97, 12137–12151. Hanel, R., B. Conrath, F. M. Flasar, V. Kunde, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, L. Herath, M. Allison, D. Cruickshank, D. Gautier, P. Gierasch, L. Horn, R. Koppany, and C. Ponnamperuna 1981. Infrared observations of the saturnian system from Voyager 1. Science 212, 192–200.

Keller, C. N., and T. E. Cravens 1994. One-dimensional multispecies magnetohydrodynamic models of the wakeside ionosphere of Titan. J. Geophys. Res. A 99, 6527–6536. Keller, C. N., V. G. Anicich, and T. E. Cravens 1998. Model of Titan’s ionosphere with detailed hydrocarbon ion chemistry. Planet. Space Sci. 46, 1157– 1174. Keller, C. N., T. E. Cravens and L. Gan 1992. A model of the ionosphere of Titan. J. Geophys. Res. A 97, 12,117–12,135. Keller, C. N., T. E. Cravens, and L. Gan 1994. One-dimensional multispecies magnetohydrodynamic models of the ramside ionosphere of Titan. J. Geophys. Res. A 99, 6511–6525. Khilifi, M., P. Paillous, P. Bruston, F. Raulin, and J. C. Guillemin 1996. Absolute IR band intensities of CH2 N2 , CH3 N3 , and CH3 NC in the 250–4300 cm−1 region and upper limits of abundance in Titan’s stratosphere. Icarus 124, 318–328. Kunde, V. G., A. C. Aikin, R. A. Hanel, D. E. Jennings, W. C. Maguire, and R. E. Samuelson 1981. C4 H2 , HC3 N and C2 N2 in Titan’s atmosphere. Nature 292, 686–688. Kurosaki, Y., and T. Takayanagi 1999. Ab Initio Molecular Orbital Study of the N(2 D) + HCN(1 6) Reaction. J. Phys. Chem. A 103, 9323–9329. Lara, L. M., F. Lellouch, J. J. Lopez-Moreno, and R. Rodrigo 1996. Vertical distribution of Titan’s atmospheric neutral constituents. J. Geophys. Res. E 101, 23,261–26,038. Lias, S. G., J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin, and W. G. Mallard 1988. Gas-phase ion and neutral thermochemistry. J. Phys. Chem. Ref. Data 17, Suppl. No. 1. Maki, A. G., and R. L. Sams 1981. High temperature, high resolution infrared spectral measurements on the HNC–HCN equilibrium system. J. Chem. Phys. 75, 4178–4182. Marten, A., D. Gautier, T. Owen, D. B. Sanders, H. E. Matthews, S. K. Atreya, R. P. J. Tilanus, and J. R. Deane 1993. First observations of CO and HCN on Neptune and Uranus at millimeter wavelengths and the implications for atmospheric chemistry. Astrophys. J. 406, 285–297. McNutt, R. L., Jr., and J. D. Richardson 1988. Constraints on Titan’s ionosphere. Geophys. Res. Lett. 15, 709–712. Monks, P. S., P. N. Romani, F. L. Nesbitt, M. Scanlon, and L. J. Stief 1993. The kinetics of the formation of nitrile compounds in the atmospheres of Titan and Neptune. J. Geophys. Res. 98, 17,115–17,122. Montgomery, J. A., Jr., J. W. Ochterski, and G. A. Petersson 1994. A complete basis set model chemistry. IV. An improved atomic pair natural orbital method. J. Chem. Phys. 101, 5900–5909.

HNC IN TITAN’S IONOSPHERE Nagy, A. F., and T. E. Cravens 1998. Titan’s ionosphere: A review. Planet. Space Sci. 46, 1149–1155. Nesbitt, F. L., G. Marston, and L. J. Stief 1990. Role of the methylene amidogen (H2 CN) radical in the atmospheres of Titan and Jupiter. In First International Conference on Laboratory Research for Planetary Atmospheres, pp. 244–248. Nezu, M., T. Amano, and K. Kawaguchi 1998. Transition dipole moments for the vibrational fundamentals of HNC determined from the Herman-Wallis effect. J. Mol. Spectrosc. 192, 41–46. Northrup, F. J., G. A. Bethardy, and R. G. Macdonald 1997. Infrared absorption spectroscopy of HNC in the region 2.6 to 3.1 µm. J. Mol. Spectrosc. 186, 349–362. Ochterski, J. W., G. A. Petersson, and J. A. Montgomery, Jr. 1996. A complete basis set model chemistry. V. Extensions to six or more heavy atoms. J. Chem. Phys. 104, 2598–2619. Oppenheimer, M., and A. Dalgarno 1977. Associative ionization and interstellar TiO+ and TiO. Astrophys. J. 212, 683–684. Petersson, G. A., D. K. Malick, W. G. Wilson, J. W. Ochterski, J. A. Montgomery, Jr., and M. J. Frisch 1998. Calibration and comparison of the Gaussian-2, complete basis set, and density functional methods for computational thermochemistry. J. Chem. Phys. 109, 10570–10579. Petrie, S. 1998. Proton affinities of dicyanogen isomers: Is there a preferred site of protonation for CNCN? J. Phys. Chem. A 102, 7835–7840. Petrie, S. 1999. Group IIIA metal dihalide ions: Identification of a possible new class of associative ionization reactions. Int. J. Mass Spectrom. 184, 191–199. Petrie, S., and D. K. Bohme 1994. Associative ionization processes within interstellar clouds. Astrophys. J. 436, 411–417. Petrie, S., C. G. Freeman, M. Meot-Ner, M. J. McEwan, and E. E. Ferguson 1990. An experimental study of HCN+ and HNC+ ion chemistry. J. Am. Chem. Soc. 112, 7121–7126. Roboz, A., and A. F. Nagy 1994. The energetics of Titan’s ionosphere. J. Geophys. Res. A 99, 2087–2093. Rosenqvist, J. E. Lellouch, P. N. Romani, G. Paubert, and T. Encrenaz 1992. Millimeter-wave observations of Saturn, Uranus, and Neptune: CO and HCN on Neptune. Astrophys. J. Lett. 392, 99–102. Samuelson, R. E., L. A. Mayo, M. A. Knuckles, and R. J. Khanna 1997. C4 N2 ice in Titan’s north polar stratosphere. Planet. Space Sci. 45, 941–948.

203

Shiba, Y., T. Hirano, U. Nagashima, and K. Ishii 1998. Potential energy surfaces and branching ratio of the dissociative recombination reaction HCNH+ + e− : An ab initio molecular orbital study. J. Chem. Phys. 108, 698–705. Sims, I. R., J.-L. Queffelec, D. Travers, B. R. Rowe, L. B. Herbert, J. Karth¨auser, and I. W. M. Smith 1993. Rate constants for the reactions of CN with hydrocarbons at low and ultra-low temperatures. Chem. Phys. Lett. 211, 461-8. Strobel, D. E., R. R. Meier, M. E. Summers, and D. J. Strickland 1991. Nitrogen airglow sources—Comparison of Triton, Titan, and Earth. Geophys. Res. Lett. 18, 689–692. Talbi, D., and Y. Ellinger 1998. Potential energy surface for the electronic dissociative recombination reaction of HCNH+ : Astrophysical implications on the HCN/HNC abundance ratio. Chem. Phys. Lett. 288, 155–164. Talbi, D., and E. Herbst 1998. An extensive ab initio study of the C+ + NH3 reaction and its relation to the HNC/HCN abundance ratio in interstellar clouds. Astron. Astrophys. 333, 1007–1015. Talbi, D., Y. Ellinger, and E. Herbst 1996. On the HNC/HCN abundance ratio: A theoretical study of the H + CNH ↔ HCN + H exchange reactions. Astron. Astrophys. 314, 688–692. Thompson, W. R., T. J. Henry, J. M. Schwartz, B. N. Khare, and C. Sagan 1991. Plasma discharge in N2 + CH4 at low pressures: Experimental results and applications to Titan. Icarus 90, 57–73. Thompson, W. R., S. K. Singh, B. N. Khare, and C. Sagan 1989. Triton— Stratospheric molecules and organic sediments. Geophys. Res. Lett. 16, 981–984. Toublanc, D., J. P. Parisot, J. Brillet, D. Gautier, F. Raulin, and C. P. McKay 1995. Photochemical modeling of Titan’s atmosphere. Icarus 113, 2–26. Tsang, W. 1996. Heats of formation of organic free radicals by kinetic methods. In Energetics of Organic Free Radicals (J. Martinho Simoes, A. Greenberg, and J. F. Liebman, Eds.), pp. 22–58, Blackie, London. Winter, M. J., and W. J. Jones 1982. Vibration-rotati on infrared emission spectrum of hydrogen isocyanide, HNC, at 2.75 µm. J. Chem. Soc. Faraday Trans II 78, 585–594. Yung, Y. L. 1987. An update of nitrile photochemistry on Titan. Icarus 72, 468–472. Yung, Y. L., M. Allen, and J. P. Pinto 1984. Photochemistry of the atmosphere of Titan: Comparison between model and observations. Astrophys. J. Suppl. Ser. 55, 456–506.