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Infrared Spectra of a Species of Potential Prebiotic and Astrochemical Interest: Cyanoethenethiol (NC-CHdCH-SH) Abdessamad Be´nidar,*,† Robert Georges,† Jean-Claude Guillemin,*,‡ Otilia Mo´,§ and Manuel Ya´n˜ez*,§ Institut de Physique de Rennes, CNRS UMR 6251, UniVersite´ de Rennes 1, 35042 Rennes France, E´cole Nationale Supe´rieure de Chimie de Rennes, CNRS, UMR 6226, AVenue du Ge´ne´ral Leclerc, CS 50837, 35708 Rennes Cedex 7, France, Departamento de Quı´mica, Mo´dulo 13, UniVersidad Auto´noma de Madrid, Campus de Excelencia UAM-CSIC, Cantoblanco, 28049-Madrid, Spain ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: July 15, 2010
Cyanoethenethiol (NC-CHdCH-SH) was obtained in a 8:1 Z:E ratio by flash vacuum thermolysis of the t-butylsulfide derivative. Density functional theory (DFT) and G3 ab initio calculations predict the existence of Z-and E-isomers, each of which exhibits two rotamers as a function of the relative position of the SH group. All these rotameric forms are planar (Cs symmetry) and correspond to synperiplanar and antiperiplanar conformations. Calculations indicate that the synperiplanar Z-isomer is the more stable. In pure form, the cyanoethenethiol rapidly decomposes at room temperature, even at low pressure and partially condenses on the wall of the cell. To record its spectrum, a long optical path of 136 m was necessary, and several successive fillings of the cell were required. On the basis of the calculated harmonic and anharmonic vibrational frequencies, a complete and unambiguous assignment of the experimental spectrum has been carried out. SCHEME 1
Introduction It has been suggested that the addition of simple nucleophiles on cyanoacetylene (H-CtC-CtN) could have formed precursors of life in prebiotic chemistry and that these adducts could be present in the interstellar medium or in planetary atmospheres.1–8 In our group, we have paid particular attention to the addition involving ammonia (NH3) to cyanoacetylene to form 3-amino2-propenenitrile (H2NCHdCHCtN).9,10 Z- and E-isomers were obtained, the Z-isomer being more stable than the E-isomer. With the help of theoretical calculations, the infrared spectra of the E- and Z-isomers were assigned.10 For the Z-isomer, the microwave (MW) spectra of the ground vibrational state and of first three vibrationally excited states belonging to the two lowest normal modes were assigned for the parent species, whereas the ground states were ascribed for five isotopologues.9 Also, a huge increase of the gas phase basicity of this compound compared to the nitrile-free derivative was observed. The presence of the nitrile induces a strong push-pull effect since the basic site is localized on this nitrile group and not on the amino group as observed for R-aminonitriles.11 The extraterrestrial existence of the corresponding sulfur derivative, cyanoethenethiol (3-mercapto-2-propenenitrile), cannot be discarded since several compounds containing sulfur have already been identified in the interstellar medium, in planets, and in comets.12 Previous quantum chemical calculations of the structural and conformational properties predict13 that the Zand the E-forms each have two “stable” planar rotameric forms with the H-S-CdC link of atoms in either a synperiplanar or an antiperiplanar conformation, with the synperiplanar form of the Z-isomer as the global minimum. The MW spectrum of this compound has been recorded, and the spectra of the parent * To whom correspondence should be addressed. † Universite´ de Rennes 1. ‡ ´ Ecole Nationale Supe´rieure de Chimie de Rennes. § Universidad Auto´noma de Madrid.
compound as well as three isotopomers have been assigned.14 For this compound also, theoretical calculations predict that the protonation takes place at the cyano group. The loss of the proton from the substituent was found to be much more favorable than the deprotonation at the HCdCH group.15 The aim of this paper is to investigate the relative stability of cyanoethenethiol conformers and the barriers for interconversion among them using high-level ab initio calculations as well as their IR spectra, because such spectra can be very useful for identifying molecules in the interstellar medium, in the gas jets of comets, and in the atmospheres of planets. Experimental Section Cyanoethenethiol (HS-CHdCH-CN) is formally an adduct of dihydrogen sulfide (H2S) to cyanoacetylene. It was prepared by flash vacuum pyrolysis at 800 °C of 3-(tert-butylthio)-2propenenitrile. An 8:1 Z:E ratio was obtained. Each stereoisomer was easily identified in 1H NMR spectroscopy by its 3JHH coupling constant of the alkene group (in bold). The integration of the signals gives the Z:E ratio, which allows the easy attribution of the 13C NMR signals. (Z-isomer) 1H NMR (CDCl3): δ 4.17 (d, 1H, 3JHH ) 12.8 Hz, SH); 5.44 (d, 1H, 3 JHH ) 10.4 Hz, CHCN); 7.06 (dd, 1H, 3JHH ) 12.8 Hz, 3JHH ) 10.4 Hz, CHS). 13C NMR (CDCl3): δ 95.3 (CHCN); 115.3 (CN); 143.2 (CHS). (E-isomer) 1H NMR (CDCl3): δ 3.67 (d, 1H, 3JHH ) 11.3 Hz, SH); 5.48 (d, 1H, 3JHH ) 15.9 Hz, CHCN); 7.28 (dd, 1H, 3JHH ) 15.9 Hz, 3JHH ) 11.3 Hz, CHS). 13C NMR (CDCl3): δ 96.5 (CHCN); 116.4 (CN); 144.3 (CHS). The synthesis protocol is summarized in Scheme 1, and details of the experimental procedure can be found in ref 16.
10.1021/jp105650e 2010 American Chemical Society Published on Web 08/16/2010
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Figure 1. Infrared signature of cyanoethenethiol extracted from the experimental recorded spectrum.
Figure 2. The different isomers of cyanoethenethiol. The numbers within parentheses indicate their estimated relative abundance in the gas phase at room temperature.
The sample was evaporated in the cell gas at room temperature. The measured vapor pressure of the prepared cyanoethenethiol was roughly less than 0.1 mbar. The use of the optical White system17 was necessary to record the spectrum. An adjustment of this optical system to achieve a long optical path of 136 m was required to compensate the very low vapor pressure of the studied compound. The gas phase IR spectrum was recorded on 120HR Bruker Fourier transform interferometer. A combination of a Globar source, a KBr beam splitter and a liquid-nitrogen-cooled MCT detector was used to obtain the spectrum in the range (500-4000 cm-1). The final spectrum results from an average of 100 scans at a resolution of 0.5 cm-1. The spectrum calculated from the first scans showed that a large number of observed bands were not attributable to the studied cyanoethenethiol. It was therefore assumed that a part of the vapor in the cell belongs to gaseous impurities. Also, by controlling the measured spectrum, we noted that the intensities of some characteristic bands of the studied compound diminished rather rapidly during the recording of the spectrum. Indeed, at room temperature, this compound began to decompose immediately after evaporation and also probably condense on the walls of the cell. Several successive fillings were required to perform the recording of the final spectrum. Since we could not directly record the infrared signatures of the pure cyanoethenethiol, we sought to extract them from the experimentally obtained spectrum. Indeed, we noted that in the spectrum recorded a few hours after the evaporation of the sample, the strong bands of the cyanoethenethiol disappear completely and only remain visible infrared signatures of gaseous impurities. By adjusting the intensities of the bands of the residual impurities between the two spectra, we extracted the spectrum of the desired cyanoethenethiol (see Figure 1).
were carried out by using a 6-311+G(d,p) basis set. In order to obtain reliable relative stabilities of the different conformers of this system, their total energies were obtained in the framework of the G3B3 theory,21 in which B3LYP/6-31G* optimized geometries and harmonic vibrational frequencies are used instead of the MP2 and HF respectively used in the G3 standard method.22 All calculations were carried out using the Gaussian09 series of programs.23 In order to gain some insight into the corresponding electronic structure and into its possible effects on the relative stability of these systems, we have used the atoms in molecules (AIM) theory.24 In the framework of this approach, we have obtained the corresponding molecular graphs, defined as the ensemble of bond paths and bond critical points (BCPs), the former being the lines connecting two maxima of the electron density and containing the BCP between them, and the latter being critical points in which the density is a minimum along the bond path and maximum in the other two directions. Also the natural charges of the species investigated were obtained by means of the natural bond orbital (NBO) approach.25 In all cases, the B3LYP/6-311+G(d,p) density was used.
Computational Details Density functional theory (DFT) calculations have been carried out to obtain the structure and the harmonic and anharmonic vibrational frequencies of the cyanoethenethiol. Among the different functionals available nowadays, the B3LYP hybrid functional, which includes Becke’s three-parameter nonlocal hybrid exchange potential18 and the nonlocal correlation functional of Lee, Yang, and Parr,19 was chosen because its good performance in obtaining reliable geometries and vibrational frequencies is well documented.20 These geometry optimizations
Results and Discussion Relative Stabilities and Isomerization Barriers. In agreement with previous findings reported in the literature, our B3LYP/6-311+G(d,p) DFT calculations on the structural and conformational properties of cyanoethenethiol showed that this compound exists in the form of two Z- and E-isomers, each of them presenting two rotamers depending on the relative position (synperiplanar and antiperiplanar) of the SH group. The four rotamers are planar Cs-symmetry structures. The same predictions are obtained when the G3B3 method is used. These two rotamers are identified by adding the number 1 or 2, respectively, to the letter (Z or E) that identifies each conformer (see Figure 2). Table 1 summarizes the total and relative energies of all stable conformers obtained at the G3B3 level of theory. It can be seen that the Z2-structure is the global minimum favored by 5.6, 5.9, and 7.0 kJ/mol relative to the Z1, E1, and E2 conformers, respectively. In this respect, it is worth noting that, although the vibrational frequencies of the different forms are not identical, the differences between their zero point vibrational frequencies are negligibly small and always lower than 1.5 kJ mol-1.
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TABLE 1: G3B3 Calculated Gibbs Free Energies (G, hartrees)a and Relative Gibbs Free Energies (∆G, kJ mol-1) of the Different Isomers of Cyanoethenethiol and the Transition States Connecting them structure
G
∆G
mole fraction (%)
Z2 Z1 E2 E1 TSZ2E2 TSZ1E1 TSZ2Z1 TSE2E1
-568.808450 -568.806305 -568.805781 -568.806215 -568.686983b -568.688481b -568.799626 -568.800250
0 5.6 7.0 5.9 303.9b 300.0b 23.2 21.5
79.6 8.3 4.7 7.4
a Values calculated at 298.2 K. b Values obtained at the G3 level, because of convergency problems in the B3LYP/6-31G* optimization of this transition state.
Figure 3. Molecular graph corresponding to the Z2 isomer of cyanoethenethiol. Red dots denote BCPs. Note that no BCP is found between the SH group and the CN group. Electron densities are in atomic units (a.u.)
The enhanced stability of the Z conformers with respect to the E ones was explained in terms of the conjugation between the lone pairs of the substituent and the CdC double bond, through a charge transfer from the heteroatom lone pair into the πcc* antibonding orbital. The slightly enhanced stability of rotamer Z2 with respect to Z1 could be attributed to the formation in the former of an intramolecular hydrogen bond. However, an AIM analysis of this rotamer (see Figure 3) does not show the existence of any BCP between the H(S) hydrogen and the N of the cyano group. However, a certain electrostatic stabilizing interaction between the positive charge of the former (natural charge +0.15) and the negative charge of the latter (natural charge -0.35) should be expected. This is actually reflected in a slight elongation of the S-H bond in rotamer Z2 with respect to rotamer Z1. Although these energy gaps between the four possible structures are small (see Table 1), they are large enough to predict that the synperiplanar Z-isomer would be the dominant form present in the gas phase at 298 K. Also, importantly, the activation barriers calculated at the G3B3 level associated with the Z2-E2 and the Z1-E1 isomerizations are very high (304 and 300 kJ mol-1, respectively). It is worth noting that the G3 calculated barrier is significantly higher than that previously reported at the B3LYP/6-311G(d,p) level of theory.13 Also, the Z2-Z1 internal rotation barrier, although much lower than the previous one, is high enough so we can safely assume that both rotamers should be observable at room temperature. The same applies to the E1-E2 internal rotation barrier (see Table 1). Assuming a Boltzmann distribution and using the relative free energies reported in Table 1, the estimated molar fractions (%) for each isomer are summarized in the same table. It is worth
noting that these theoretical estimates are in excellent agreement with the experimental evidence obtained through our NMR spectroscopy measurements. Experimentally, it is possible to distinguish isomers E and Z but not to distinguish between their respective rotamers. Hence the experimental information only indicates, as mentioned above, that the proportion of Z and E conformers in cyanoethenethiol is 8:1, which is in excellent agreement with the theoretical estimates (88% Z and 12% E). Vibrational Analysis. Calculations of the harmonic and anharmonic vibrational frequencies were performed at the same level of theory indicated above. The infrared intensities were calculated within the harmonic approximation. The corresponding values together with the assignment of each normal mode for the four forms are reported in Table 2. As expected, harmonic wavenumbers are systematically higher than the experimental values, the root-mean-square (rms) deviation being 90 cm-1. The agreement between anharmonic and experimental values is much better, the rms deviation being 19 cm-1. Although experimentally there is a mixture of conformers, the amount of the forms Z1, E1, and E2 in the sample is so small that the corrections in the spectra, if their presence is taken into account, are too small to be easily distinguished from noise. Therefore, the assignment of the different absorption bands was made by comparing the experimental vibrational frequencies with those calculated for the Z2 rotamer (see Table 3), as if it was the only species present in the sample. One can observe a very good agreement between the experimental values and the calculated harmonic frequencies scaled by a factor 0.9615 proposed by Scott and Radom26 and the nonscaled calculated anharmonic ones. This good agreement is also illustrated by comparison of the simulated and the observed spectra of this compound (see Figure 4). The simulated spectrum was obtained by convoluting the nonscaled anharmonic frequencies with Lorentz lineshapes with a half-width of 5 cm-1, and the intensities of each conformer are adjusted with respect to its estimated mole fraction. Spectroscopic Analysis. In the cyanoethenethiol compound, the stretching of the CH bond of the two groups CH (CN) and CH (SH) are coupled. The two hydrogen atoms vibrate either in phase or out of phase. A similar coupling is observed for the CH (CN) and CH (SH) in-plane deformation and out-of-plane deformation for the two Z1 and Z2 conformers. Conversely, for the E1 and E2 rotamers, these vibrations are localized in each fragment. CH stretching bands of all isomers exhibit, as expected, weak or very weak intensities. Calculated frequencies of the E1 and E2 forms are [3026, 3016 cm-1] and [3057, 3050 cm-1] for the CH (CN) and CH (SH) groups respectively. In the other Z1 and Z2 conformers, the absorption bands of the out-of-phase CH stretching are predicted at [3054, 3057 cm-1] and the CH in-phase stretching at [3046, 3011 cm-1]. Consistent with these predictions, we assign the two weak bands observed at 3079 and 3063 cm-1 to ν1 (CH out-of-phase stretching) and ν2 (CH in-phase stretching). It is worth noting that the anharmonicity corrections are larger for the in-phase than for the out-of-phase stretchings, so while harmonic values predict the in-phase combination to have a slightly higher frequency than the out-of-phase combination, for the anharmonic values is the other way around. Differently from the CH stretching, the CH in-plane bending modes are very sensitive to conformational effects. For the inphase vibration, there is a decrease of almost 100 cm-1 on going from the Z-type isomers to the E-type ones. Interestingly, for the out-of-phase in-plane CH bending mode, this frequency shifting goes in the opposite direction, and the band appears at
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TABLE 2: Calculated Harmonic, νharm, and Anharmonic, νanharm, Vibrational Frequencies (cm-1) and Infrared Intensities, IIR (kJ/mol), for the Different Isomers of Cyanoethenethiol isomer Z1 assignment
νharm
C-C-S bending S-H bending out of plane C-CdC bending out of plane C-CtN bending (in plane) C-CtN bending (out of plane) C-CdC bending (in plane) C-S stretch C-H bending out of plane (in phase) C-H bending out of plane (out of phase) C-C stretch S-H bending C-H bending in plane (in phase) C-H bending in plane (out of phase) CdC stretch CtN stretch S-H stretch C-H stretch out of phase C-H stretch in phase
119 190 316 357 547 650 716 740 945 971 1010 1224 1373 1621 2319 2681 3178 3198
νanharm 119 25 302 350 536 646 709 729 922 951 978 1205 1344 1571 2283 2551 3055 3046
isomer E1a
isomer Z2 IIR 2.5 12.5 3.5 2.0 6.5 10.9 55.2 18.4 0.4 1.4 29.4 4.3 15.0 81.8 32.6 0.2 4.6 0.2
νharm 127 264 371 374 546 665 717 721 949 957 1023 1228 1384 1610 2314 2646 3177 3193
νanharm 124 242 367 341 537 655 702 711 931 932 995 1210 1363 1541 2270 2528 3057 3011
IIR
νharm b
6.6 10.7 0.2 6.7 2.9 1.2 35.0 56.2 1.50 1.4 23.9 14.4 5.0 73.7 40.0 0.6 3.7 0.1
117 161c 206 330d 497 517 808e 843f 943g 951h 1049i 1267 1323 1632 2320 2686 3165j 3192k
isomer E2a IIR
νanharm b
52 162c 149 333d 489 520 800e 828f 925g 928h 1052i 1243 1304 1593 2292 2561 3026j 3057k
19.4 7.0 4.8 0.2 6.4 2.8 0.9 36.3 25.2 54.4 3.9 6.1 1.2 126.5 65.5 0.8 2.9 1.8
νharm b
154 162c 327 343d 498 514 806e 806f 956g 959h 1046i 1290 1318 1629 2321 2657 3164j 3181k
νanharm b
158 166c 317 346d 492 517 803e 797f 932g 936h 1046i 1265 1301 1584 2285 2544 3016j 3050k
IIR 0.7 7.8 10.0 2.2 3.4 4.5 37.3 2.9 30.3 54.3 5.7 6.5 3.5 102.6 64.1 2.2 3.9 2.2
a For the E1 and E2 conformers, the assignments do not always correspond to that indicated in the first column. The correct assignment is given in a footnote. b SH bending out of plane + CCC bending. c CCC bending (in plane). d CCS bending. e C-H bending out of plane and out of phase. f C-S stretch. g S-H bending. h C-H bending out of plane (in phase). i C-C stretch. j C-H(CN) stretch. k C-H(SH) stretch.
TABLE 3: Comparison of the Observed and Calculated Frequencies of the Z2 Isomer of Cyanoethenethiol calculated
a
observed
no.
assignment
scaleda νharm
νanharm
ν
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18
C-H stretch in-phase C-H stretch out of phase S-H stretch CtN stretch CdC stretch C-H bending in plane (out of phase) C-H bending in plane (in phase) S-H bending C-C stretch C-H bending out of plane (out of phase) C-H bending out of plane (in phase) C-S stretch C-CdC bending (in plane) C-CtN bending (out of plane) C-CtN bending (in plane) C-CdC bending out of plane S-H bending out of plane C-C-S bending
3070 3054 2544 2225 1548 1331 1180 984 920 912 693 689 639 525 360 357 254 122
3011 3057 2528 2270 1541 1363 1210 995 933 931 711 702 655 537 341 367 242 124
3079.3 3063.0 2566.7 2223.7 1571.7 1352.6 1193.6 1004.1 932.4 926.8 717.5 703.4 674.0
A scaling factor of 0.9615 was used.
higher frequencies (about 100 cm-1) for the E-isomers than for the Z-isomers (see Table 2). Hence, the two bands observed in the experimental spectra at 1193 and at 1352 cm-1 are assigned respectively to ν6 (CH in-plane bending out of phase) and to ν7 (CH in-plane bending in phase), which agree very well with the anharmonic values calculated for the Z2-isomer (1210 and 1363 cm-1, respectively). Concerning the CH out-of-plane bending, in the case of the Z isomers, the oscillator strength for this mode is 50 times greater for the in-phase vibration than for the out-of-phase mode. The order of these two modes appears reversed in the case of the E isomers, with the in-phase mode being of lower frequency than the out-of-phase mode. Still, however, the intensity of the former is about 50 times that of the latter. In the observed spectrum, we assign frequencies at 927 and 703 cm-1 to ν10 (CH out-of-plane bending out of phase) and ν11 (CH out-ofplane bending in phase), respectively, again in good agreement
with the anharmonic values (931 and 711 cm-1, respectively) calculated for the Z2 isomer. In the vinylic compounds, infrared spectra are characterized by strong bands absorbing around 1650 cm-1 which belong to the νCdC double bond stretching. As expected, our calculations predict very strong intensity bands for all conformers in this region. Frequencies are separated by about 10 cm-1 between the two forms Z and E. Consistently with these calculations, the experimental spectrum is characterized by a very intense band absorbing at 1517.7 cm-1, which is in good agreement with the CC double bond anharmonic stretching ν5 of the Z2 compound. This mode is also strong for the other isomers, but their relative molar fraction is not high enough to make them be easily detected in our spectrum. The CN stretching vibration in these compounds gives rise to a medium-strong intensity absorption band. In the Z2measured spectrum, this mode ν7 is assigned at 2223 cm-1. As
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Figure 4. Comparison of the observed and simulated spectra of cyanoethenethiol. The latter was obtained by convoluting the nonscaled anharmonic frequencies with Lorentz line shapes with a half-width of 5 cm-1, and the intensities of each conformer were adjusted with respect to its estimated mole fraction.
expected, this mode is not sensitive to the isomeric effect, as in the case of the cyanovinylamine.10 Our DFT calculations predict this vibration to appear always around 2270 cm-1 for all the studied isomers, the intensities being twice as large for the Eforms than for the Z-isomers. The stretch of the carbon-carbon single bond and the carbon-sulfur bond correspond to the modes noted as ν9 and ν12, having weak and moderate intensities, respectively. Both modes appear to be slightly sensitive to isomerization. The two observed bands located at 935 and 699 cm-1 are in very good agreement with the anharmonic values (932 and 702 cm-1, respectively) calculated for Z2-cyanoethenethiol. The calculated ν9 mode anharmonic frequencies for the Z1, E1, and E2 isomers are 951, 1052, and 1046 cm-1, respectively. In the same order, the calculated anharmonic values for ν12 are 709, 828, and 797 cm-1. The two visible vibration modes in the experimental spectrum involving the SH bond are the SH stretching ν3 and the SH bending ν8 modes. The stretching mode absorbs at 2566.7 cm-1 observed, 2528 cm-1 calculated with very weak intensity. The bending mode absorbs at 1004 cm-1 observed, 995 cm-1 calculated, but is 20 times more intense than the stretching. In the region below 600 cm-1, not covered by our experimental analysis, our calculations predict the existence of six bands corresponding to the C-CN in-plane bending ν13 and ν15, the C-CN out-of-plane bending ν14, the C-CdC bending ν16, the S-H out-of-plane bending ν17, and the C-C-S bending ν18, respectively, of the Z1, Z2, E1 and E2 isomers (see Table 2). All these modes are expected to absorb with moderate to weak intensities. Conclusions G3B3 high-level ab initio calculations indicate, in agreement with previous theoretical studies, that cyanoethenethiol exists in four different conformations, two Z-type rotamers, namely Z1 and Z2, and two E-type rotamers, namely E1 and E2. From the calculated Gibbs free energies, the expected ratio between the Z and the E forms in the gas phase should be 8:1 at room
temperature, in good agreement with the experimental evidence obtained from NMR spectroscopy. Also, importantly, the different isomerization barriers connecting these four structures are high enough as to conclude that the four forms should be experimentally accessible at room temperature, with the equilibrium mixture containing 79.6% of Z2, 8.3% of Z1, 4.3% of E2, and 7.4% of E1. The complete assignment of the gas-phase infrared spectrum of cyanoethenethiol has been carried out, using calculated anharmonic vibrational frequencies and the fact that structure Z2 is clearly dominant, so that the contributions of the other three forms to the observed spectrum are negligibly small. Acknowledgment. This work has been partially supported by the DGI Project No. CTQ2009-13129-C01, by the Project MADRISOLAR2, ref.: S2009PPQ/1533 of the Comunidad Auto´noma de Madrid, by Consolider on Molecular Nanoscience CSD2007-00010, and by the COST Action CM0702. A generous allocation of computing time at the CCC of the UAM is also acknowledged. J.-C.G. and A.B. thank the Centre National d’Etudes Spatiales (CNES) and the program PCMI (INSUCNRS) for financial support. M.Y. acknowledges The Ministry of Science and Innovation of Spain for a postdoctoral contract (Project No. CTQ2009-07197-E). References and Notes (1) Sanchez, R. A.; Ferris, J. P.; Orgel, L. E. Science 1966, 154, 784– 788. (2) Guillemin, J. C.; Breneman, C. M.; Joseph, J. C.; Ferris, J. P. Chem.sEur. J. 1998, 4, 1074–1082. (3) Ferris, J. P.; Sanchez, R. A.; Orgel, L. E. J. Mol. Biol. 1968, 33, 693–698. (4) Ksander, G.; Bold, G.; Lattmann, R.; Lehmann, C.; Fruh, T.; Xiang, Y. B.; Inomata, K.; Buser, H. P.; Schreiber, J.; Zass, E.; Eschenmoser, A. HelV. Chim. Acta 1987, 70, 1115–1172. (5) Drenkard, S.; Ferris, J.; Eschenmoser, A. HelV. Chim. Acta 1990, 73, 1373–1390. (6) Wagner, E.; Xiang, Y. B.; Baumann, K.; Guck, J.; Eschenmoser, A. HelV. Chim. Acta 1990, 73, 1391–1409. (7) Eschenmoser, A. Origins Life EVol. Biospheres 1994, 24, 389–423. (8) Eschenmoser, A.; Loewenthal, E. Chem. Soc. ReV. 1992, 21, 1–16.
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