Zwitterion Formation in Titan Ice Analogs: Reaction ... - ACS Publications

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Zwitterion Formation in Titan Ice Analogs: Reaction Between HC3N and NH3 Isabelle Couturier-Tamburelli,*,† Bintou Sessouma,‡ Thierry Chiavassa,† and Nathalie Piétri*,† †

UMR CNRS 7345, Physique des Interactions Ioniques et Moléculaires, Equipe de Spectrométries et Dynamique Moléculaires, Aix Marseille Université, Case 252, Centre de St-Jérôme, 13397 Marseille cedex 20, France ‡ Université de Ouagadougou UFR/SEA, 01 B.P. 5509, Ouagadougou 01, Burkina Faso S Supporting Information *

ABSTRACT: A zwitterion is formed in the laboratory at low temperatures in the solid phase from the thermal reaction of HC3N and NH3. We report for the first time its infrared spectrum. We study its reaction using Fourier transform infrared spectroscopy. Its reaction rate is estimated to be k(T) = 2.9 × 105 exp(−2.3 ± 0.1 (kJ mol−1)/RT). Calculations using density functional theory (B3LYP/6-31g**) are used to characterize all the species (complexes, zwitterions, and transition states) and are in good agreement with the infrared spectra. The structure of the zwitterion is determined planar and it is characterized by a N−C bond around 1.5 Å.

1. INTRODUCTION The complex chemistry of the interstellar medium presents a significant challenge for astrophysicists. The interstellar medium is a rich reservoir of molecules. Two of them HC3N and NH3 coexist in astrophysical environments like comets,1 molecular clouds,2−5 and Titan atmosphere.6−8 Indeed, the Cassini space mission to Saturn and the release of Huygens probe onto its largest moon, Titan, has led to a wealth of data on the atmospheric and surface composition. The most interesting compounds detected are hydrocarbons and nitriles (HCN, HC3N, HC5N).9 HC3N was proposed as a key compound because it easily reacts with many nucleophiles species and can induce a rich chemistry. Among these nucleophiles the ammonia is a good candidate. Bernard et al.8 have detected this molecule in the resulting effluents of a gas mixture containing N2 and CH4 subjected to a low pressure electric discharge. In addition, Coustenis et al.10 show from a spectrum of Titan that ammonia could exist in a condensed phase, in the atmosphere and it could be responsible of the complex molecules found in Tholins. So we supposed that more complex products can be formed by thermal or photochemical processes from simpler mixture of HC3N and NH3. In previous work11 it has been shown that at room temperature in the gas phase or in solution a 1:1 Z:E aminoacrylonitrile is readily formed simply by mixing cyanoacetylene and ammonia. Moreover, Eschenmoser et al.12 have pointed out that aminoacrylonitrile should play a role in the prebiotic chemistry more particularly in the formation of carbohydrates and amino acids. Several thermal reactions with ammonia are expected to occur in interstellar medium, have © 2012 American Chemical Society

been already studied. The main example of nucleophilic addition reaction is the reactivity between H2 CO or CH3CHO and NH3, which induce respectively the aminomethanol NH2CH2OH13 or the formation of a chiral molecule, the α aminoethanol NH2CH(CH3)OH.14 In the present work, we study for the first time the thermal reactivity of HC3N:NH3 ices in the 20 K to room temperature range and we try to answer to the question: Could the aminoacrylonitrile be formed on the Titan surface where the temperature can reach 95 K? To identify the products formed, we use FTIR spectroscopy and mass spectroscopy. Isotopic substitutions with 15NC3H, 15NH3, and ND3 and DFT calculations are also performed.

2. EXPERIMENTAL SECTION 2.1. General Information. Pure cyanoacetylene was synthesized using the method described by Moureu and Bongrand.15 The 15N-cyanoacetylene was prepared by the introduction of 15N enriched ammonia (Cambridge isotopic laboratories 98%) at the appropriate stage of the previous synthesis. Ammonia (14N, 15N, and D) is commercially available as 99.99% pure gas from Air Liquid. Cyanoacetylene and ammonia gases are mixed in different ratios in a pyrex bulb using standard manometric techniques. They are then sprayed onto a gold copper-plated metal surface cooled down at 20 K within a high vacuum chamber (ca. 10−7 Received: June 5, 2012 Revised: October 12, 2012 Published: October 18, 2012 10721

dx.doi.org/10.1021/jp305517k | J. Phys. Chem. A 2012, 116, 10721−10727

The Journal of Physical Chemistry A

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frequencies of 212.3, 9.4, and 18.8 cm−1, respectively. Due to the comparison between the theoretical and experimental shifts, we concluded that the complex formed presents interaction between the Hydrogen of the cyanoacetylene and the Nitrogen of the ammonia. Then, we introduced at 20 K a HC3N:NH3 gas mixture in different proportions (1:1, 1:5, 1:10, 0.5:10.5, 8.5:1.5) without argon. If we want to obtain on the IR spectrum, other bands than those of monomers, the amount of ammonia should be in excess. Thus, we present in Figure 1 the IR spectra obtained by deposition of HC3N/NH3 (0.5:10.5), proportion for which we get the greatest amount of complexes.

mbar). The infrared spectra are recorded between 4000 and 650 cm−1 using a Nicolet serie II Magma system 750 with a MCT detector. A typical spectrum has a 1 cm−1 resolution and is averaged over 100 interferograms. The sample is warmed to 300 K using a heating resistance, and the temperature is controlled using a Lakeshore model temperature controller. The IR spectrum is monitored during the temperature ramp. We usually start the ramp with temperature slope β between 0.5 and 2 K min−1. A typical kinetic experiment is set at a fixed value between 10 and 300 K using a heating resistance and the temperature controller. Infrared spectra are recorded at this fixed temperature at fixed time interval. Mass spectra are recorded using a Hiden HAL VII RGA quadrupole mass spectrometer (QMS), which allows us to follow the products being desorbed during the temperature ramp (temperature-programmed desorption experiments). The ionization source is an 70 eV impact electronic source, and the mass spectra are recorded between 1 and 80 atomic mass units. Mass spectroscopy is used to verify the nature of the desorption products. 2.2. Details of Calculations. Quantum chemical calculations are carried out with the Gaussian 0916 program. The equilibrium structure of compounds and corresponding harmonic frequencies of molecular vibration were predicted using the density functional theory (DFT)17 with the B3LYP hybrid exchange−correlation functional with the 6-31G** basis set. The density functional theory has been widely utilized in the study of reaction mechanism.18 All the calculated structures are optimized at the B3LYP/6-31G** level. Compared to other levels of theory, the B3LYP method has been recognized to be sufficiently accurate for predicting reliable geometries and vibrational frequencies of the stationary points.19 These frequencies are performed on the basis of the optimized geometries at the same level to confirm that all the compounds (complex and zwitterions) have no imaginary frequency and only one transition state. We perform normal coordinated analyses on these transition states structures by intrinsic reaction coordinate (IRC) route20 in both directions (complex and zwitterions). Furthermore, to be as accurate as in our previous paper, all the vibrational frequencies of the different complexes are calculated at a higher level of B3LYP/aug-ccpVTZ.21 We compute the harmonic vibrational frequencies for the 14N, 15N, and D isotopomers. We take into account the environment effect in the calculations to simulate the experiments. The solvation effect of the zwitterion by ammonia was modeled by including two and more NH3 molecules, which are hydrogen bonded like previously developed. The effect of a polar medium (dielectric constant ϵ = 20) were examined.

Figure 1. Solid infrared spectra of HC3N, NH3, and HC3N/NH3 mixture at 20 K in the 4000−650 cm−1 region: (a) NH3; (b) HC3N/ NH3; (c) HC3N. The complex bands are denoted by an asterisk.

The solid NH3 infrared spectrum (Figure 1a) can be characterized by four fundamental modes noted ν1, ν2, ν3, and ν4 whose frequencies are 3212, 1060, 3373, and 1626 cm−1 respectively23 (Table 1). The pure solid HC3N bands (Figure Table 1. Experimental Frequencies of NH3 and HC3N in Pure Solids (ν = Stretching, δ = Bending) NH3 modes

Jetzki νNH νNH δNH δNH

ν3 ν1 ν4 ν2

23

3378 3216 1644 1066

modes ν1 ν2 ν3 2ν5 ν4 ν5

3. RESULTS AND DISCUSSION 3.1. Experiments. 3.1.1. Infrared Absorption Spectra of HC3N/NH3. The CA/NH3 different gas mixtures are prepared at room temperature into primary vacuum pumped line adjusted by standard manometric techniques. In previous work22 we have shown that when the gas mixture HC3N/NH3/Ar (1/10/ 500) is trapped on the surface cooled at 20 K, the IR spectrum shows the response of the 1:1 complex HC3N:NH3 and higher order complexes 1:n (n ≥ 1) trapped in argon matrix. The interaction between these two molecules is illustrated in the IR spectrum by a shift of the monomer absorption bands. For the ν1, ν2, and ν3 modes of HC3N complexed with one NH3 molecule, we observed in argon matrix a shift toward lower

νCH νCN νCC νCC δCCH

this work 3373 3212 1626 1060

15

NH3

this work 3365 3298 1626 1060 HC3N

ND3 Jetzki23 2513 2335 1196 822

this work 2500 2325 1183 822 HC3N15

this work

this work

3203 2272 2067 1494 883 759

3203 2247 2056 1480 871 742

1c) display five fundamental vibrationals modes listed in Table 1. The spectrum recorded after codeposition of HC3N/NH3 at 20 K shows the absorption bands of NH3, which is in excess, and new absorption bands where the most intense are situated at 2890, 2257, and 2044 cm−1 (Figure 1b). These bands are respectively shifted toward lower frequencies by 324, 12, and 25 cm−1 to the pure HC3N solid, showing that HC3N is totally associated with NH3 in the mixture. In Table 2, we compared the experimental shifts with theoretical ones (Supporting 10722



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Table 2. Experimental and Calculated Frequencies Shifts Scaled with a 0.96 Factor for the HC3N Subunit in HC3N/ NH3, HC3N/2NH3, and HC3N/4NH3 Experiments (Δν = νmonomer − νcomplex) HC3N:nNH3 Δνcalc modes (HC3N) ν1 ν2 ν3

νCH νCN νCC

Δνexp

1:1

1:2

1:4

324 12 25

216 13 24

226 10 23

312 13 33

Information) for the HC3N/nNH3 (n = 1, 2, and 4) with a structure similar to those obtained in cryogenic matrix.22 These results show that the largest frequency shift is obtained for the νCH mode of HC3N, in agreement with the experimental results. This mode (ν1) is much more sensitive to the environment than the CN (ν2) or CC (ν3) modes, which keep almost the same frequency shifts. Although these calculations better model the gas phase, we can consider the structure of the complex obtained in solid phase. The results obtained in cryogenic matrix are coherent with the 1:1 complex formation because we observe in the matrix a shift (Δνexp) of 212 cm−1 of the ν1 mode. Nevertheless, the Table 2 analysis shows us that the solid phase results are more consistent with a higher order molecular complex (n = 4 for exemple). But there are probably complexes of varying stoichiometries as indicated by the broadness of the band due to the CH stretching vibration (which is the most sensitive to the envirennement). The best results are obtained with HC3N:nNH3 (n = 4) with a calculated shift of 312 cm−1 with respect to the 324 cm−1 found in our experiment. We can conclude without ambiguity that these bands are due to the association between HC3N and n molecules of NH3 with n > 1. This conclusion is confirmed by isotopic experiments results obtained for the HC3N/4NH3 complexes (Table 3). Once the matrix is warmed, these bands decrease and new strong absorption bands appear. Some of them are present in the area of NH3 polymers and are difficult to attribute without ambiguity. Among the bands obtained, those observed at 2769, 2204, 1980, and 1504 cm−1 (Figure 2 and Table 4) correspond to the formation of a new reactional product noted A. From around 60 K another set of absorption bands appears in the same area at 2763, 2199, 1964, and 1498 cm−1 (Figure 3). It seems to be the same product with IR bands shifted to lower frequencies, resulting in a change of the A local environment. We can note that this shift takes place at the beginning of the crystallization of ammonia, which occurs around 65 K as shown by the change on the infrared spectral features of NH3. From 80 K the absorption bands of A decreases and the bands of the complex start to grow again. NH3 begins to sublimate at 95 K and disappears at 125 K. At this temperature the complex has

Figure 2. Infrared spectra evolution of solid HC3N/NH3 between 25 and 55 K in the 4000−650 cm−1 region. (a) HC3N/NH3 at 25 K. (b) HC3N/NH3 at 55 K. (c) Subtraction spectrum between 55 and 25 K. The complex band is denoted by *. The zwitterion A bands are denoted by ●.

totally disappear and we observe the absorption bands of solid HC3N alone (Figure 3). To identify the product formed, we performed isotopic substitution experiments using a NH3:HC315N, 15NH3:HC315N, and ND3:HC315N mixtures in a 0.5:10.5 concentration ratio. Concerning the NH3:HC315N experiment, after deposition we observe the bands of the complexed molecules at 2920, 2234, and 2033 cm−1, respectively, shifted by 283, 13, and 23 cm−1 compared to the case of the HC315N monomer (Table 3). During the annealing the same effect as in the nonisotopic experiment is observed. Until 65 K we observe the growing of the A(15N) new reactionnal compound bands at 2186, 1971, and 1503 cm−1 (Table 4), which is followed by the growing of the same product A(15N) bands (2179, 1956, and 1504 cm−1) slightly shifted when NH3 begins to crystallize . The two other isotopic experiments (Tables 3 and 4) lead respectively to the different isotopes of A during the warming up of the sample. From all these experiments, we can say that during the annealing, HC3N reacts with NH3 molecules (in excess) to yield a new product, A. The crystallization of NH3 around 65 K induces a shift on the frequencies of this compound of few cm−1. This phenomenon is observed on all the frequencies and could be explained by a structural modification of A induced by a change of the environment. Then, we observe a regular decrease of NH3 absorption bands until its total desorption at about 125 K. Above this temperature, the absorption bands of the compound formed disappear and those of the HC3N:nNH3 complex increase. We observe the product A only between 55 and 120 K (Figure 3). Above this temperature the amount of NH3 is not sufficient in the medium to observe it.

Table 3. Experimental and Calculated Frequencies Shifts Scaled with a 0.96 Factor for the HC3N Subunit in HC3N/4NH3, HC315N/4NH3, HC315N/415NH3, and HC315N/4ND3 Experiments (Δν = νmonomer − νcomplex) HC3N:4NH3 modes (HC3N) ν1 ν2 ν3

νCH νCN νCC

HC315N:4NH3

HC315N:415NH3

HC315N:415ND3

Δνexp

Δνcalc

Δνexp

Δνcalc

Δνexp

Δνcalc

Δνexp

Δνcalc

324 12 25

312 13 33

283 13 23

313 13 31

306 12 22

313 13 30

268 12 19

312 13 31

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Table 4. Experimental Infrared Absorption Bands (cm−1) Relative to the New Species, A, Yielded between HC3N and NH3a modes ν1 ν2 combination ν5 combination ν6

νNH νNH νCN νCC

HC315N:15NH3

HC315N/NH3

HC3N/NH3

15

15

15

15

HC315N:ND3

A1

A2

A( N)1

A( N)2

A( N N)1

A( N N)2

A( ND)1

A(15ND)2

3384 3337 2769 2204 1980 1504

3382

3382 3346 2760 2186 1971 1503

3377 3345 2773 2179 1956 1504

3370 3286 2736 2187 1973 1495

3370 3333 2740 2180 1955 1493

2464

2382

2180 1972 1482

2177 1955 1483

2763 2199 1964 1498

15

15

15

a

A1 and A2 are relative at two different forms of A depending of the local environment and temperature. A1 is the low temperature compound and A2 is the high one.

process of this new product, we have measured the experimental activation barrier. 3.1.2. Rate of Zwitterion Formation. Then, we measure the zwitterion formation rate k(T) on an NH3:HC3N ice mixture, when NH3 and HC3N are mixed. The rate of the reaction can be written as v = k[NH3]α[HC3N]β, where α and β are the partial orders of the reaction related to NH3 and HC3N, and [NH3] and [HC3N] are the molar fractions of NH3 and HC3N complexed, respectively. To study this reaction, we use a NH3:HC3N mixture in a 10.5:0.5 ratio and we measure the reaction rate as a function of temperature. The NH3:HC3N mixture is as quickly as possible brought to a fixed temperature, and the evolution with time of the species IR bands is recorded at this fixed temperature (Figure 4). From both the Figure 3. Infrared spectra of the thermal evolution of a HC3N/NH3 binary mixture in a 0.5/10.5 ratio at 70, 90, 100, 120, and 140 K. The complex band is denoted by *. The zwitterion A bands are denoted by ●.

To help to the attribution of the A compound, we performed the same experiments using mass spectroscopic analysis. During the desorption experiment made between 20 K and room temperature, only the bands corresponding to the ammonia and HC3N are detected. In the IR spectra, we have seen that the product bands disappear with the sublimation of ammonia. The A product could be the aminoacrylonitrile, which is a stable product at room temperature24 obtained by addition of NH3 on HC3N. However, A is not the aminoacrylonitrile like that confirmed by mass spectrometry. No relevant signal is observed during the desorption process at m/z 68, corresponding to the parent ion of aminoacrylonitrile. Finally, four relevant peaks are observed at 145 K, when the HC3N product is desorbing, m/z 51, 50, 25, and 24, corresponding to the parent ion of HC3N, the loss of one H atom, and the loss of one CN or HCN, respectively.25 In addition, Askeland et al.26 have performed calculations (MP2/aug-cc-pVTZ) to model the neutral−neutral reaction between HC3N and NH3. The complex obtained evolves toward a transition state whose enthalpy is 129 kJ/mol higher than the sum of the enthalpies of the ammonia and cyanoacetylene to form aminoacrylonitrile, which is more stable than the complex by 133 kJ/mol. With regard to the IR and mass spectroscopic results, we can conclude that the aminoacrylonitrile is not the product formed. In view of these results, and the work obtained on the thermal reactivity of NH3 on C3O2,27 we considered the formation of a zwitterion between NH3 and HC3N. To better understand the formation

Figure 4. Decay of HC3N:NH3 absorption and increase of zwitterion over 110 min at 55 K, monitored by infrared spectroscopy at 2272 (HC3N/NH3 complex) and 2204 cm−1 (zwitterion).

disappearance of the complex and the formation of A product along with time, we derive a reaction rate k′, where k′ = k[NH3]α, considering that [NH3] is constant because NH3 is in excess in the mixture with respect to HC3N or complex. The single exponential decay of HC3N:NH3 (Figure 4) indicates a first partial order for HC3N complexed (β = 1). As for NH3, the partial order α is derived to be 3 (Figure 5), from reaction rate measurements considering the same temperature (T = 60.0 K) and different NH3 molar fractions, as seen in Table 5. Finally, the reaction is found to have a rate law v = k[NH3]3[HC3N] and Table 5 gives the reaction rates measured for different temperatures. This set of reaction rates as a function of temperature is fitted to an Arrhenius law k(T) = A exp(−Ea/ RT), where Ea is the activation energy and A is the preexponential factor, as shown in Figures 5 and 6. We find Ea = 10724



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structures and stabilities of the HC3N/NH3 complex and zwitterion, density functional theory (DFT)18 calculations were carried out using the DFT B3LYP method19 with the 6-31g** basis set. The effects of a polar medium were examined by the self-consistent reaction field (SCRF) solvation method28 by introducing a dielectric constant ϵ = 20 close to the ammonia value. The optimized geometry of the zwitterion (H3N+, − CHC()CN) is given in Scheme 1. Scheme 1. Optimized Geometries (B3LYP/6-31g**) of the HC3N/NH3 Zwitterion (Lengths in Å and Angles in deg)

Figure 5. Partial order α derivated by measuring the ln[k′] at 60 K as a function of ln [NH3].

Table 5. Reaction Rates k(T) Measured for a HC3N:NH3 Ice Mixture at Fixed Temperatures temp (K) 55 60 60 60 60 63 66

k′ (s−1) 8.51 8.14 7.98 5.84 6.16 4.97 3.79

× × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4

[NH3]

k (s−1) = k′/[NH3]0.3

0.955 0.91 0.86 0.82 0.955 0.955 0.955

4.47 × 10−4

Harmonic vibrational frequencies were determined at the stationary points and compared to experimental data. These calculations do not model really the solid phase, but if we compare the frequency theoretical and experimental shifts obtained for the zwitterion, we can provide some conclusions. Table 6 displays these experimental shifts between the Table 6. Experimental and Calculated Frequency Shifts Scaled with a 0.96 Factor in HC3N/NH3, HC315N/NH3, HC315N/15NH3, and HC315N/ND3 Zwitterions (Δν = νzwitterion HC3N/NH3 − νisotopes)a

3.23 × 10−4 2.61 × 10−4 1.98 × 10−4

2.3 ± 0.1 kJ mol−1 and A = 2.9 × 105 s−1. Introducing a temperature dependence of the pre-exponential factor does not improve the fit. Then, we can conclude without ambiguity that the aminoacrylonitrile is not formed during these experiments. To confirm the presence of zwitterion, we performed DFT calculations. 3.2. Theoretical Calculations. 3.2.1. Geometries and Vibrational Analysis of Zwitterion. To establish the molecular a

Frequencies reported for the zwitterion are those measured at 60 K (notes A2 in Table 4).

zwitterion issue from (HC3N/NH3) and zwitterion issue from the different isotopic mixtures (HC315N/ND3, HC315N/ NH3, and HC315N/15NH3; Supporting Information). It is worthwhile to note the good agreement between the experimental and calculated frequency shifts for the νCN (ν5 mode) of each isotope. Indeed, the theoretical frequency shift predicted for the HC315N/ND3 zwitterion is 27 cm−1 for the νCN stretching, which is the same that the experimental one (Table 6). A very good agreement is also observed for νND (920 cm−1 (experimental) versus 881/865 cm−1 (calculated)). 3.2.2. Reaction Profile Starting from the Complex (HC3N:nNH3). During the experiment we demonstrate that the zwitterion formation occurs when the NH3 is present in excess compared to HC3N. So, although a zwitterionic type structure exists on the potential energy surface, a more stable equilibrium structure can be obtained by solvatation. The solvation effect of the zwitterion by ammonia was modeled by

Figure 6. Arrhenius plot of ln[k] against 1/T for the formation of the zwitterions and the best-fit straight line. 10725



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The introduction of a second NH3 molecule in the complex improves the activation energy. In fact, the way between the HC3N:2NH3 complex to the zwitterion in interaction with NH3 asks only 38.6 kJ/mol in a polar medium (ϵ = 20). With the introduction of two other NH3 molecules the reactional pathway is considerably modified. The zwitterion is obtained with an activation energy of 8.3 kJ/mol (Scheme 3). This last activation energy is consistent with our experimental conditions where the zwitterion is formed above 55 K according to a nucleophilic addition reaction between cyanoacetylene and ammonia. Compared to the complex, the zwitterion is stabilized by 39.7 kJ/mol, indicating an exothermic process, which demonstrates the stability of the zwitterion when it is solvated by more NH3 molecules. In fact, the zwitterion is stable until the NH3 desorption, which induces its destruction and the reappearance of HC3N. According to the Scheme 2, the energy barrier to produce HC3N from zwitterion is about 21 kJ/mol.

including two and more NH3 molecules, which are hydrogen bonded like previously developed. The effects of a polar medium (dielectric constant ϵ = 20) were examined. In these calculations, the zwitterion has a planar structure and is characterized by a N−C bond around 1.5 Å. The HC3N subunits in the complex are transformed into CC functional group in the zwitterion. The characteristic frequencies of the latter are significantly different from those of the van der Waals complexes. As indicated by their large dipole moment (between 6.5 and 13.2 D) and charge distributions, the zwitterion is best described as charge transfer species. Hence it is strongly stabilized in the presence of polar medium. A reaction profile is proposed, and the transition state structures are given in Schemes 2 and 3. Experimental Scheme 2. Reaction Profile of Zwitterion Formation from HC3N:NH3 Complex (Dielectric Medium ϵ = 20)

4. CONCLUSION Our detailed kinetic analysis reveals correspondence between the disappearance of complex and the appearance of the HC3N−NH3 zwitterion formed upon nucleophilic addition reaction. The same results have already observed with NH312 The pseudo-first-order kinetics fit the experimental results. Nevertheless, our results do not consider the inhomogeneity of the medium, and therefore they can only give indicative activation energy barriers. The experimental value (around 3 kJ/mol) found is in satisfactory agreement with the calculated barrier (8.6 kJ/mol) for at least four molecules of NH3 in interaction with HC3N. The addition of more NH3 molecules should still lower this energy. In our experimental conditions, the reaction occurs between HC3N and NH3 in excess in the mixture, as soon as 55 K. The zwitterion is stable until 90 K, the temperature at which NH3 starts to sublimate. From an astrochemical point of view, this implies that for the Titan surface with the temperature around 95 K, the zwitterion could be formed and further contributed to the surface reactivity. On the other hand, spontaneous formation of aminoacrylonitrile at this temperature can hardly occur on Titan because of the high activation energy (about 129 kJ/mol). The transformation kinetics of the NH3:HC3N complexes into zwitterions followed pseudo-first-order rate laws, indicating that the complexes do not isomerize directly to the zwitterion in a unimolecular reaction but that exchange with an ammonia molecule from the surrounding medium is involved.

Scheme 3. Reaction Profile of Zwitterion Formation from HC3N:4NH3 Complex (Dielectric Medium ϵ = 20)

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ASSOCIATED CONTENT

S Supporting Information *

vibrational spectra showed that the zwitterion is formed when HC3N is solvated by at least two NH3 molecules. To be in agreement with this experimental observation, four NH3 molecules were involved in the whole process. After the complexation between one NH3 molecule and the hydrogen of HC3N, the zwitterion was obtained with an activation energy of 147.6 kJ/mol. This activation energy is not consistent with the experimental one. The introduction of a dielectric constant ϵ = 20 reduces the transition state energy by around half, which is calculated to be 69.9 kJ/mol (Scheme 2). It is important to note that the complex is 48.5 kJ/mol more stable than the zwitterion and in such a condition its formation is impossible.

Computational results about HC3N and HC3N:nNH3 complexes and all calculated frequencies of the zwitterions and its isotopes. This information is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 10726



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(28) (a) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991, 113, 4776−4782. (b) Wong, M. W.; Wiberg, K. B.; Frisch, M. J. J. Chem. Phys. 1991, 95, 8991−8998.

ACKNOWLEDGMENTS The theoretical part of this work was conducted with the technical means of “Centre de Competence en Modelisation Moléculaire de Marseille” and the EPOV program. The authors express their gratitude to Dr. Y. Ferro (PIIM, SDM, Aix Marseille University, France) for the valuable discussion.

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REFERENCES

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