Internal rotation analysis of the microwave and millimeter wave spectra of fluoral (CF3CHO)

The rotational spectrum (4-40 GHz and 50-330 GHz) has been measured and analyzed for trifluoroacetaldehyde, also known as fluoral (CF₃CHO), which is one of the degradation products of the fluorinated contaminants emitted into the atmosphere. The complexity of the spectroscopic analysis of this molecule arises from the strong coupling between the internal rotation motion of CF₃ group and the overall rotation of the molecule. The value obtained for its coupling constant (ρ = 0.91723481(49)) is comparable to the corresponding value of methanol (CH₃OH, ρ = 0.81), which is known for its complex spectrum. A total of 12,322 transitions of the ground, the first and second excited torsional states (ΔE_1υt = 62.0183(13)cm^-1; ΔE_2υt = 120.3315(13)cm^-1) with J ≤ 50 were included in the analysis that was performed employing the rho-axis-method (RAM), and the RAM36 code. A fit within experimental error (root mean square deviation equals to 35 kHz) has been achieved for this dataset using 47 parameters of the RAM torsion-rotation Hamiltonian. In the course of the analysis, it became evident that for such high ρ value, as it is determined for fluoral, a larger than usual torsional basis set at the first diagonalization step of the two-step diagonalization procedure is required for achieving a fit within experimental error.

Rotational spectroscopic study and astronomical search for propiolamide in Sgr B2(N)

CONTEXT

For all the amides detected in the interstellar medium (ISM), the corresponding nitriles or isonitriles have also been detected in the ISM, some of which have relatively high abundances. Among the abundant nitriles for which the corresponding amide has not yet been detected is cyanoacetylene (HCCCN), whose amide counterpart is propiolamide (HCCC(O)NH2).

AIMS

With the aim of supporting searches for this amide in the ISM, we provide a complete rotational study of propiolamide from 6 GHz to 440 GHz.

METHODS

Time-domain Fourier transform microwave (FTMW) spectroscopy under supersonic expansion conditions between 6 GHz and 18 GHz was used to accurately measure and analyze ground-state rotational transitions with resolved hyperfine structure arising from nuclear quadrupole coupling interactions of the 14N nucleus. We combined this technique with the frequency-domain room-temperature millimeter wave and submillimeter wave spectroscopies from 75 GHz to 440 GHz in order to record and assign the rotational spectra in the ground state and in the low-lying excited vibrational states. We used the ReMoCA spectral line survey performed with the Atacama Large Millimeter/submillimeter Array toward the star-forming region Sgr B2(N) to search for propiolamide.

RESULTS

We identified and measured more than 5500 distinct frequency lines of propiolamide in the laboratory. These lines were fitted using an effective semi-rigid rotor Hamiltonian with nuclear quadrupole coupling interactions taken into consideration. We obtained accurate sets of spectroscopic parameters for the ground state and the three low-lying excited vibrational states. We report the nondetection of propiolamide toward the hot cores Sgr B2(N1S) and Sgr B2(N2). We find that propiolamide is at least 50 and 13 times less abundant than acetamide in Sgr B2(N1S) and Sgr B2(N2), respectively, indicating that the abundance difference between both amides is more pronounced by at least a factor of 8 and 2, respectively, than for their corresponding nitriles.

CONCLUSIONS

Although propiolamide has yet to be included in astrochemical modeling networks, the observed upper limit to the ratio of propiolamide to acetamide seems consistent with the ratios of related species as determined from past simulations. The comprehensive spectroscopic data presented in this paper will aid future astronomical searches.

Rotational spectroscopic study of S-methyl thioformate: A global laboratory analysis of ground and excited torsional states up to 660 GHz

CONTEXT

S-methyl thioformate CH3SC(O)H is a monosulfur derivative of methyl formate, a relatively abundant component of the interstellar medium (ISM). S-methyl thioformate being, thermodynamically, the most stable isomer, it can be reasonably proposed for detection in the ISM.

AIMS

This work aims to experimentally study and theoretically analyze the ground and first torsional excited states for CH3SC(O)H in a large spectral range for astrophysical use.

METHODS

S-methyl thioformate was synthesized as a result of a reaction of methyl mercaptan with acetic-formic anhydride. The millimeter-wave spectrum was then recorded for the first time from 150 to 660 GHz with the solid-state spectrometer located at Lille.

RESULTS

A set of 3545 lines is determined and combined with 54 previously measured lines in the microwave region, belonging to ground state ν t = 0 as well as 1391 transitions in the first excited state of torsion ν 18 = 1. Some 164 lines were also assigned to ν 18 = 2 for the A-species. A global fit was performed using the BELGI-Cs code taking into account the large splitting of A and E lines due to methyl internal rotation motion with a relatively low barrier, V3 = 127.4846(15) cm-1.

CONCLUSIONS

Using our spectroscopy work, a deep search of S-methyl thioformate was carried out in the IRAM 30m and ALMA data of different high-mass star-forming regions (Orion KL and Sgr B2). We derived an upper limit to the CH3SC(O)H column density in these regions.

The millimeter-wave spectrum of methyl ketene and the astronomical search for it

CONTEXT

The analysis of isomeric species of a compound observed in the interstellar medium (ISM) is a useful tool to understand the chemistry of complex organic molecules. It could, likewise, assist in the detection of new species.

AIMS

Our goal consists in analyzing one of the two most stable species of the C3H4O family, methyl ketene, whose actual rotational parameters are not precise enough to allow its detection in the ISM. The obtained parameters will be used to search for it in the high-mass star-forming regions Orion KL and Sagittarius B2, as well as in the cold dark clouds TMC-1 in the Taurus Molecular Cloud and Barnard 1 (B1-b).

METHODS

A millimeter-wave room-temperature rotational spectrum of methyl ketene was recorded from 50 to 330 GHz. The internal rotation analysis of its ground state and first torsional excited state was performed with the rho-axis method employing the RAM36 program.

RESULTS

More than 3000 transitions of the rotational spectrum of the ground state (Kamax = 18) and first torsional excited state (Kamax = 13) of methyl ketene were fitted using a Hamiltonian that contains 41 parameters with an RMS (root mean square) of 41 kHz. Column density limits were calculated but no lines were detected in the ISM belonging to methyl ketene.

Rotational spectrum of methoxyamine up to 480 GHz: a laboratory study and astronomical search

AIMS

Methoxyamine is a potential interstellar amine that has been predicted by gas-grain chemical models for the formation of complex molecules. The aim of this work is to provide direct experimental frequencies of its ground-vibrational state in the millimeter- and submillimeter-wave regions to achieve its detection in the interstellar medium.

METHODS

Methoxyamine was chemically liberated from its hydrochloride salt, and its rotational spectrum was recorded at room temperature from 75 to 480 GHz using the millimeter-wave spectrometer in Valladolid. Many observed transitions revealed A-E splitting caused by the internal rotation of the methyl group, which had to be treated with specific internal rotation codes.

RESULTS

Over 400 lines were newly assigned for the most stable conformer of methoxyamine, and a precise set of spectroscopic constants was obtained. Spectral features of methoxyamine were then searched for in the Orion KL, Sgr B2, B1-b, and TMC-1 molecular clouds. Upper limits to the column density of methoxyamine were derived.

Laboratory study of methyl isocyanate ices under astrophysical conditions

Methyl isocyanate has been recently detected in comet 67P/ Churyumov-Gerasimenko (67P/CG) and in the interstellar medium. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CH3NCO have been obtained at temperatures of relevance for astronomical environments. The spectra are dominated by a strong, characteristic multiplet feature at 2350-2250 cm-1, which can be attributed to the antisymmetric stretching of the NCO group. A phase transition from amorphous to crystalline methyl isocyanate is observed at ~ 90 K. The band strengths for the absorptions of CH3NCO in ice at 20 K have been measured. Deuterated methyl isocyanate is used to help with the spectral assignment. No X-ray structure has been reported for crystalline CH3NCO. Here we advance a tentative theoretical structure, based on Density Functional Theory (DFT) calculations, derived taking as a starting point the crystal of isocyanic acid. A harmonic theoretical spectrum is calculated then for the proposed structure, and compared with the experimental data. A mixed ice of H2O and CH3NCO was formed by simultaneous deposition of water and methyl isocyanate at 20 K. The absence of new spectral features indicates that methyl isocyanate and water do not react appreciably at 20 K, but form a stable mixture. The high CH3NCO/H2O ratio reported for comet 67P/CG, and the characteristic structure of the 2350-2250 cm-1 band, make of it a very good candidate for future astronomical searches.

Millimeter wave spectra of carbonyl cyanide

CONTEXT

More than 30 cyanide derivatives of simple organic molecules have been detected in the interstellar medium, but only one dicarbonitrile has been found and that very recently. There is still a lack of high-resolution spectroscopic data particularly for dinitriles derivatives. The carbonyl cyanide molecule is a new and interesting candidate for astrophysical detection. It could be formed by the reaction of CO and CN radicals, or by substitution of the hydrogen atom by a cyano group in cyanoformaldehyde, HC(=O)CN, that has already been detected in the interstellar medium.

AIMS

The available data on the rotational spectrum of carbonyl cyanide is limited in terms of quantum number values and frequency range, and does not allow accurate extrapolation of the spectrum into the millimeter-wave range. To provide a firm basis for astrophysical detection of carbonyl cyanide we studied its millimeter-wave spectrum.

METHODS

The rotational spectrum of carbonyl cyanide was measured in the frequency range 152 - 308 GHz and analyzed using Watson's A- and S-reduction Hamiltonians.

RESULTS

The ground and first excited state of v5 vibrational mode were assigned and analyzed. More than 1100 distinct frequency lines of the ground state were fitted to produce an accurate set of rotational and centrifugal distortion constants up to the eighth order. The frequency predictions based on these constants should be accurate enough for astrophysical searches in the frequency range up to 500 GHz and for transition involving energy levels with J ≤ 100 and Ka ≤ 42. Based on the results we searched for interstellar carbonyl cyanide in available observational data without success. Thus, we derived upper limits to its column density in different sources.

The millimeter wave spectrum of methyl cyanate: a laboratory study and astronomical search in space<sup/>

AIMS

The recent discovery of methyl isocyanate (CH3NCO) in Sgr B2(N) and Orion KL makes methyl cyanate (CH3OCN) a potential molecule in the interstellar medium. The aim of this work is to fulfill the first requirement for its unequivocal identification in space, i.e. the availability of transition frequencies with high accuracy.

METHODS

The room-temperature rotational spectrum of methyl cyanate was recorded in the millimeter wave domain from 130 to 350 GHz. All rotational transitions revealed A-E splitting owing to methyl internal rotation and were globally analyzed using the ERHAM program.

RESULTS

The data set for the ground torsional state of methyl cyanate exceeds 700 transitions within J″ = 10 - 35 and [Formula: see text] and newly derived spectroscopic constants reproduce the spectrum close to the experimental uncertainty. Spectral features of methyl cyanate were then searched for in Orion KL, Sgr B2(N), B1-b, and TMC-1 molecular clouds. Upper limits to the column density of methyl cyanate are provided.

A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks

The recent analysis of the composition of the frozen surface of comet 67P/Churyumov-Gerasimenko has revealed a significant number of complex organic molecules. Methyl isocyanate (CH3NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified. We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH3NCO in Orion is only a factor of ten below those of HNCO and CH3CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface. We also compare our abundances with those derived recently for this molecule towards Sgr B2 (Halfen et al. 2015). A more accurate abundance of CH3NCO is provided for this cloud based on our extensive laboratory work.

Searching for trans ethyl methyl ether in Orion KL. Astron Astrophys 2015;582:L1. [PMID: 26869726 PMCID: PMC4747151 DOI: 10.1051/0004-6361/201526255]

We report on the tentative detection of trans ethyl methyl ether (tEME), t-CH₃CH₂OCH₃, through the identification of a large number of rotational lines from each one of the spin states of the molecule towards Orion KL. We also search for gauche-trans-n-propanol, Gt-n-CH₃CH₂CH₂OH, an isomer of tEME in the same source. We have identified lines of both species in the IRAM 30 m line survey and in the ALMA Science Verification data. We have obtained ALMA maps to establish the spatial distribution of these species. Whereas tEME mainly arises from the compact ridge component of Orion, Gt-n-propanol appears at the emission peak of ethanol (south hot core). The derived column densities of these species at the location of their emission peaks are ≤(4.0 ± 0.8) × 10¹⁵ cm^-2 and ≤(1.0 ± 0.2)× 10¹⁵ cm^-2 for tEME and Gt-n-propanol, respectively. The rotational temperature is ~100 K for both molecules. We also provide maps of CH₃OCOH, CH₃CH₂OCOH, CH₃OCH₃, CH₃OH, and CH₃CH₂OH to compare the distribution of these organic saturated O-bearing species containing methyl and ethyl groups in this region. Abundance ratios of related species and upper limits to the abundances of non-detected ethers are provided. We derive an abundance ratio N(CH₃OCH₃)/N(tEME) ≥ 150 in the compact ridge of Orion.

VUV photoionization and dissociative photoionization of the prebiotic molecule acetyl cyanide: theory and experiment

The present combined theoretical and experimental investigation concerns the single photoionization of gas-phase acetyl cyanide and the fragmentation pathways of the resulting cation. Acetyl cyanide (AC) is inspired from both the chemistry of cyanoacetylene and the Strecker reaction which are thought to be at the origin of medium sized prebiotic molecules in the interstellar medium. AC can be formed by reaction from cyanoacetylene and water but also from acetaldehyde and HCN or the corresponding radicals. In view of the interpretation of vacuum ultraviolet (VUV) experimental data obtained using synchrotron radiation, we explored the ground potential energy surface (PES) of acetyl cyanide and of its cation using standard and recently implemented explicitly correlated methodologies. Our PES covers the regions of tautomerism (between keto and enol forms) and of the lowest fragmentation channels. This allowed us to deduce accurate thermochemical data for this astrobiologically relevant molecule. Unimolecular decomposition of the AC cation turns out to be very complex. The implications for the evolution of prebiotic molecules under VUV irradiation are discussed.

Photochemical cycloaddition reactions of cyanoacetylene and dicyanoacetylene

Photolysis of cyanoacetylene with 185- or 206-nm light yields 1,3,5-tricyanobenzene while 254-nm radiation yields a mixture of tetracyanocyclooctatetraenes, 1,2,4- and 1,3,5-tricyanobenzene. A polymer of cyanoacetylene is the major photoproduct. 1,3,5-Tricarbomethoxybenzene was the only photoproduct identified from the irradiation of methyl propiolate at 254 nm. Mono-, di-, and tricyanobenzenes are formed by irradiation of mixtures of acetylene and cyanoacetylene at 185, 206, and 254 nm along with trace amounts of cyclooctatetraenes. No photoadducts were detected on photolysis of mixtures of cyanoacetylene and CO or HCN. The tetracyanocyclooctatetraene structures were established by UV, MS, and NMR analyses. The 1H NMR of the product mixture exhibited a singlet at delta 7.028 consistent with either 1 or 2 and two singlets at delta 6.85 and 6.91 assigned to 3. Photolysis of mixtures of dicyanoacetylene and acetylene with either 185- or 206-nm light yielded 1,2-dicyanobenzene and (E,Z)-1-buten-3-yne-1,4-dicarbonitrile. These products were also obtained using 254-nm light along with a mixture of tetracyanocyclooctatetraenes. The same three singlets were observed in this product mixture as were observed in the tetracyanocyclooctatetraenes obtained from cyanoacetylene. From this observation it was concluded that the delta 7.02 signal is due to 2 and not 1. The photolysis of cyanoacetylene and dicyanoacetylene in the presence of ethylene with 185-nm light yields 1-cyanocylobutene and 1,2-dicyanocyclobutene, respectively. 2-Cyanobutadiene and 2,3-dicyanobutadiene are the photoproducts with 254-nm light. Reaction pathways are proposed to explain these findings.

Structural and conformational properties of 2-propynylphosphine (propargylphosphine) as studied by microwave spectroscopy supplemented by quantum chemical calculations

The microwave spectrum of 2-propynylphosphine (propargylphosphine), H-C triple bond C-CH2-PH2, has been investigated in the 18-26.5 and 32-48 GHz spectral regions at about -50 degrees C. Two conformers with different orientation of the phosphino group, denoted conformer I and conformer II, respectively, were assigned. Conformer I has a symmetry plane (Cs symmetry) with both hydrogen atoms of the phosphino group pointing toward the triple bond (C-C-P-H dihedral angles approximately 47 degrees from syn-periplanar (0 degrees )). The C-C-P-H dihedral angles are 73 and 167 degrees, respectively, from syn-periplanar in conformer II. Only one of the hydrogen atoms of the phosphino group points toward the triple bond in this rotamer. Conformer I is 1.5(20) kJ/mol more stable than II. The dipole moment of II was determined to be (in units of 10(-30) C m) mu(a) = 0 (assumed), mu(b) = 3.05(7), mu(c) = 1.60(9), and mu(tot) = 3.44(9) [mu(tot) = 1.03(3) D]. Two vibrationally excited states were assigned for each of the two rotamers I and II. Their frequencies were determined by relative intensity measurements. Many of the transitions of conformer II were split into two components presumably because of tunneling of the phosphino group. The tunneling frequency was determined to be 0.814(42) MHz for the ground vibrational state and 11.49(18) MHz for the first excited state of the C-P torsional vibration. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311++G(3df,2pd) basis set reproduced experimental rotational constants, quartic centrifugal distortion constants, and dipole moment components within a few percent. The energy difference between the two conformers was calculated using the Gaussian-2 theory, and conformer I was found to be more stable than conformer II by 2.1 kJ/mol.

Acidity trends in alpha,beta-unsaturated alkanes, silanes, germanes, and stannanes

The gas-phase acidity of ethyl-, vinyl-, ethynyl-, and phenyl-substituted silanes, germanes, and stannanes has been measured by means of FT-ICR techniques. The effect of unsaturation on the intrinsic acidity of these compounds and the corresponding hydrocarbons was analyzed through the use of G2 ab initio and DFT calculations. In this way, it was possible to get a general picture of the acidity trends within group 14. As expected, the acid strength increases down the group, although the acidity differences between germanium and tin derivatives are already rather small. As has been found before for amines, phosphines, and arsines, the carbon, silicon, germanium, and tin alpha,beta-unsaturated compounds are stronger acids( )than their saturated analogues. The acidifying effect of unsaturation is much larger for carbon than for Si-, Ge-, and Sn-containing compounds. The allyl anion is better stabilized by resonance than its Si, Ge, and Sn analogues, [CH(2)(-)(delta)--CH(+)(delta)(') --CH(2)(-)(delta)](-) vs [CH(2)(-)(delta)()II = CH(-)(delta)()III - XH(2)(-)(delta)()IV](-) (X = Si, Ge, Sn). The enhanced acid strength of unsaturated compounds is essentially due to a greater stabilization of the anion with respect to the neutral, because the electronegativity of the alpha,beta-unsaturated carbon group increases with its degree of unsaturation. The phenyl derivatives are systematically weaker acids than the corresponding ethynyl derivatives by 15-20 kJ mol(-)(1). Experimentally, toluene acidity is very close to that of propyne, because the deprotonation of propyne takes place preferentially at the =CH group rather than at the -CH(3) group.

Partial pressures and nature of products. Application to the photolysis of PH3 and NH3 in the atmosphere of Jupiter and Saturn

The photolysis of mixtures of gases containing NH3 or PH3 presents important differences mainly due to the strength of the X-H bond. On some examples, these differences are evidenced and the consequences for mixtures of gases containing these two compounds are shown: the photolysis of ammonia and ethylene mainly gives ethyl-, butyl- and hexylamine whereas the photolysis of phosphine and ethylene leads to ethyl- and vinylphosphine. When gaseous mixtures of NH3, PH3 and ethylene are photolyzed together, the presence of phosphine dramatically decreases the formation of nitrogen derivatives. The relevance of such lab studies to the atmospheres of Jupiter and Saturn is discussed.

Rotational Spectrum of Vinylarsine

The rotational spectrum of vinylarsine in the ground state has been studied in the range 7-320 GHz. The spectra of a syn conformer and a gauche conformer have been unambiguously assigned on the basis of the existence of a b-type or a c-type spectrum. Rotational constants, quartic, and some sextic centrifugal distortion constants were derived. For the syn form, measurements of low J aR0,1 transitions in a pulsed-nozzle Fourier transform microwave spectrometer (FTMWS) enabled the determination of the diagonal elements of the quadrupole tensor, as well as two spin-rotation constants. Ab initio calculations performed at the MP2 level using the 6-311++G(3df, 3pd) basis set reproduced experimental rotational constants within 0.2%. Copyright 1998 Academic Press.

Regioselectivity of the photochemical addition of phosphine to unsaturated hydrocarbons in the atmospheres of Jupiter and Saturn

Phosphine (PH3) has been observed in the atmospheres of Jupiter and Saturn. We have studied the regioselectivity in the gaseous phase of the photochemical addition of PH3 to propene 1, propadiene 2, propyne 3, 1,3-butadiene 4 and 1,3-butadiyne 5. The photolysis were performed at 185 and 254 nm. The volatile products formed in these reactions were characterized by 1H and 31P NMR. The n-propylphosphine 6 and the isopropylphosphine 7 were the major products observed in the photolysis of PH3 with propene. The allylphosphine 8 was obtained when most of the light was absorbed by propene. This allylphosphine was the main product formed in the photolysis of PH3 in the presence of propadiene; the methylvinylphosphine 10 being not detected in these experiments, the reaction presents a very high regioselectivity. When most of the light was absorbed by propadiene, the propargylphosphine 9 was also observed. The photolysis of PH3 in the presence of propyne led to the E- and Z-1-propenylphosphines 12a,b and small amounts of methylvinylphosphine 10. Even when most of the light was absorbed by propyne, the propargylphosphine 9 was not observed. The Z-1-butene-3-ynylphosphine 13a and a mixture of primary phosphines containing the E-and Z-2-butenylphosphines 14a,b were obtained as major products when 1,3-butadiyne and 1,3-butadiene respectively where photolyzed with PH3. A high regioselectivity was thus observed in the photolysis of PH3 with an alkyne or an allene but alkenes led to mixtures of products.

Absolute absorption coefficient of C6H2 in the mid-UV range at low temperature; implications for the interpretation of Titan atmospheric spectra

The interpretation of mid-UV albedo spectra of planetary atmospheres, especially that of Titan, is the main goal of the SIPAT (Spectroscopie uv d'Interet Prebiologique dans l'Atmosphere de Titan) research program. This laboratory experiment has been developed in order to systematically determine the absorption coefficients of molecular compounds which are potential absorbers of scattered sunlight in planetary atmospheres, with high spectral resolution, and at various temperatures below room temperature. From photochemical modelling and experimental simulations, we may expect triacetylene (C6H2) to be present in the atmosphere of Titan, even though it has not yet been detected. We present here the first determination of the absolute absorption coefficient of that compound in the 200-300 nm range and at two temperatures (296 K and 233 K). The temperature dependence of the C6H2 absorption coefficient in that wavelength range is compared to that previously observed in the case of cyanoacetylene (HC3N). We then discuss the implications of the present results for the interpretation of Titan UV spectra, where it appears that large uncertainities can be introduced either by the presence of trace impurities in laboratory samples or by the variations of absorption coefficients with temperature.

Experimental simulation of Titan's organic chemistry at low temperature

A wide range of experiments has already been carried out to simulate the chemical evolution of Titan. Such experiments can provide useful information on the possible nature of minor constituents, mostly organic, likely to be present in Titan's atmosphere. Indeed, all but one of the organic compounds already detected in Titan's atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, mainly CH2N2, and polyynes, namely C6H2, have never been detected in experimental simulation. It turned out that these compounds were thermally unstable, and the temperature conditions used during the simulation experiments (including conditions used for chemical analysis) were not appropriate. We have recently started a new program of simulation experiments using temperature conditions close to those of Titan's environment, more compatible with the build-up and detection of organics only stable at low temperature. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range of 100-150 K. The analysis of the obtained products was performed through FTIR, GC and GC-MS techniques. GC-peak identification was done owing to its mass spectrum and, in most cases, by comparison of the retention time and of the mass spectrum with standards. We report here the first detection in Titan's simulation experiments of C6H2. Its abundance is a few 10(-2) relative to C4H2. We also report a tentative identification of HC5N (to be confirmed by use of standard) with an abundance of a few 10(-2) relative to HC3N. The possible presence of HC5N suggested by our work provides the occurrence of very novel pathways in the formation of Titan's organic aerosols, involving not only C and H but also N atoms.

Organic chemistry in Titan's atmosphere: new data from laboratory simulations at low temperature

Many experiments have already been carried out to simulate organic chemistry on Titan, the largest satellite of Saturn. They can provide fruitful information on the nature of minor organic constituents likely to be present in Titan's atmosphere, both in gas and aerosol phases. Indeed, all the organic compounds but one already detected in Titan's atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, and polyynes, first of all C6H2, have never been detected in experimental simulation thus far. All these compounds are thermally unstable, and the temperature conditions used during the simulation experiments were not appropriate. We have recently started a new program of simulation experiments with temperature conditions close to that of Titan's environment. It also uses dedicated analytical techniques and procedures compatible with the analysis of organics only stable at low temperatures, as well solid products of low stability in the presence of O2 and H2O. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range 100-150 K. Products were analysed by FTIR, GC and GC-MS techniques. GC-peaks were identified by their mass spectrum, and, in most cases, by comparison of the retention time and mass spectrum with standard ones. We report here the first detection in Titan simulation experiments of C6H2 and HC5N. Their abundance is a few percent relative to C4H2 and HC3N, respectively. Preliminary data on the solid products indicate an elemental composition corresponding to (H11C11N)n. These results open new prospects in the modeling of Titan's haze making.

Photolysis of phosphine in the presence of acetylene and propyne, gas mixtures of planetary interest

Phosphine (PH3) 1 has been observed in the atmospheres of Jupiter and Saturn. We have studied the photochemical reactions of this compound with acetylene (C2H2), an alkyne also detected in these atmospheres. The volatile products formed in these reactions were characterized by H, 31P and 13C NMR. The ethenylphosphine 2 is the first product formed in the photolysis of PH3 in the presence Of C2H2. Photolysis of PH3 in the presence of propyne (C3H4) led to the formation of the Z- and E-prop-1-enylphosphines and traces of 1-methylethenylphosphine. A reaction pathway is proposed. The initial step is the dissociation of PH3 to hydrogen and PH2 radicals. Addition of the phosphinyl radical on alkyne occurs as the next step. Vinylphosphines are then formed by radical combination. This proposed reaction pathway takes into account the nature of the products and studies devoted to the photolysis of germane (GeH4) or hydrogen sulfide (H2S) in the presence of alkyne. Attempts to detect the methylidynephosphine HC triple bond P (the isoelectronic compound of HC triple bond N), in the photolysis products of PH3-C2H2 mixtures were unsuccessful. The application of these findings to Jovian and Saturn atmospheric chemistry is discussed.

Germane photochemistry. Photolysis of gas mixtures of planetary interest

Photolysis of germane (GeH4) in the presence of acetylene (C2H2), propyne (C3H4) or phosphine (PH3) with a 185 nm mercury lamp has been studied. The volatile products formed in these reactions are characterized by 1H, 31P and 13C NMR. Vinylgermanes are the first reaction products formed in the photolysis of GeH4 with alkynes. A reaction pathway is proposed. The initial step is the dissociation of germane 1 to hydrogen and GeH3 radicals. Addition of the germyl radical on alkyne is proposed as the next step. Vinyl-germanes are then formed by radical combination. Photolysis of ethenylgermane 2 gives diethenylgermane 3 in the presence of acetylene and digermaethane 4 in the presence of GeH4. The application of these findings to Jovian and Saturn atmospheric chemistry is discussed.

The photolysis of NH3 in the presence of substituted acetylenes: a possible source of oligomers and HCN on Jupiter

Photolysis of NH3 in the presence of propyne yields dimethylketazine (4) as the main product along with dimethylketimine, isopropylamine, and propioazine (7). Dimethylketazine and isopropylamine are the principal reaction products when the photolysis is performed at 198 K. The conversion to dimethylketazine is about 35 times greater at 198 K because it is not volatile and condenses on the wall of the photolysis cell out of the UV flux. Photolysis of dimethylketazine at 185 nm yields acetonitrile and small amounts of N-methyldimethylketimine (8). Photolysis of 8 gives acetonitrile. Photolysis of NH3 in the presence of 2-butyne gives the cis and trans isomers of 2-butene as the principal products along with the corresponding azine (9). Photolysis of azine 9 yields acetonitrile and propionitrile. Photolysis of hydrazine in the presence of propyne yields acetonitrile and isopropylamine but no azines were detected as reaction products. Quantum yields and percentage conversion to products are reported. These studies show that acetylenic hydrocarbons formed by the photolysis of methane in the stratosphere of Jupiter may react with radicals formed by NH3 photolysis to give nonvolatile yellow-brown polymers, dialkylazines, alkylnitriles, and eventually HCN. This scenario accounts for the observation of both HCN and chromophores on Jupiter.