Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association.

The deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H3+ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H3+, especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H3+ only, considering that the protonated ions produced in the destruction of H3+ by its main competitors present in the primitive nebula, i.e., H2O, CO, and N2, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H2O, CO, and N2, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10-16 to 10-19 cm3 s-1, which is reasonable for such reactions and has to be compared to the rates of 10-16 to 10-18 cm3 s-1, obtained with H3+. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe.

Communication: The formation of CHe(2+) by radiative association.

The detection of ArH(+) has revived the interest in the search for noble gas containing species. Despite helium being the second most abundant element in the universe (He/H ∼ 1/10), it has never been observed in any other form than that of a neutral/ionized atom in the interstellar medium. Because He is the "most noble" gas, its non-observation as part of neutral molecular systems is understandable. It is more surprising for charged species, especially HeH(+) whose spectral signatures are well documented in the laboratory. The purpose of this work was to find a simple positive ion containing He, and likely to be observed as an alternative to undetected HeH(+). Among the HeX(2+) diatomics formed with first row atoms, we focused on X = C because of both its relative abundance and the magnitude of its ionization potentials with respect to He. The formation of CHe(2+) by radiative association is the center of this study. The question was addressed by means of numerical simulations using high level ab initio calculations of the CHe(2+) potential surface, followed by a quantum chemical determination of the rate coefficients for the corresponding radiative association in the range of 10 to 1000K. The radiative association path shows a potential well deep enough to accommodate 20 vibrational levels, and no barrier to oppose the reaction. The rate coefficient varies from ∼4.5 × 10(-20) cm(3)s(-1) to ∼2.5 × 10(-22) cm(3)s(-1) for the temperatures considered. The present study suggests that the existence of this species has to be searched for mainly in highly irradiated regions.

Arsenic in prebiotic species: a theoretical approach.

A recent controversy about the presence of arsenic in biological systems prompted us to investigate the possible replacement of phosphorus by arsenic in prebiotic species small enough to be potentially identified in space. Systematic computational experiments were carried out on simple systems able to form a peptide or analogous bond. Density Functional Theory (DFT) within the B3LYP formalism, MP2 and CCSD(T) methods were used to determine the most stable isomers that can possibly form from the [C,H,O,As] and [C,3H,O,As] sets of atoms. It was found that HAsCO, like HPCO and HNCO was the most stable isomer. With three hydrogen atoms, the peptide-like bond (AsH(2)-CH=O) is not the most stable structure, contrary to NH(2)-CH=O. It is ∼9 kcal mol(-1) higher than the most stable structure, CH(2)[double bond, length as m-dash]As-OH. To assess the plausibility of the As to P substitution, a comparative study of the dimethylphosphate (DMP) and dimethylarsenate (DMA) anions was then carried out. It was found that the gauche-gauche arrangement that mimics the helix structure is the most stable one in both model molecules, showing that there is no structural evidence to discard the hypothesis of the possible inclusion of As in place of P in the DNA architecture. The topological analysis of the ELF function showed a weakening by 50% of two As-O covalent bonds in all the DMA conformers. It means that if As replaces P, the structure of the DNA helix could be weakened. Rotational constants and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.

Looking for homochirality in the inter-stellar medium.

In this report we address the question of whether some chiral molecules have a probability of being detected in the interstellar medium (ISM). To this end we rely on the Minimum Energy Principle which states that the most abundant isomer of a given generic formula should be that of lowest energy. The relative stability of the chiral molecules with respect to the other possible species of the same chemical formula are calculated by means of quantum simulations based on density functional theory (DFT). The result is that no chiral isomer in the C(3)H(6)O (acetone), C(2)H(5)ON, C(3)H(7)ON (amide), C(2)H(5)O(2)N, C(3)H(7)O(2)N (amino acid) families is the most stable species. This is also true of the C(2)(H(2)O)(2) and C(3)(H(2)O)(3) species when restricted to the sugar families, but another chiral molecule of the same chemical formula, i.e. lactic acid HOCH(CH(3))COOH is the most stable of all structures. Two other molecules with an NH(2) group, namely, NH(2)CH(CH(3))CN, the precursor of alpha-alanine and NH(2)CH(CH(3))OH, the simplest chiral molecule, are also the most stable species in their respective families. These three molecules satisfy the conditions for being detected according to the Minimum Energy Principle. With dipoles moments of 2.3, 2.7 and 1.6 Debye respectively, they make appealing targets. The present study should encourage laboratory experiments to determine rotational constants of higher precision prior to submission of observation proposals.

Preliminary study of the influence of environment conditions on the successive hydrogenations of CO.

The successive hydrogenation of CO has been investigated by two methods. The first is hydrogenation of a CO surface. The second is co-injection of CO molecules and H atoms. Both methods have been performed at 3 and 10 K. In the first method, the interaction of H atoms with solid CO at 10 K shows that CO is consumed to form H(2)CO and CH(3)OH. No trace of species such as HCO and CH(3)O is detected. No product was observed when the same experiment was performed at 3 K. In the second method, when H and CO are codeposited at 10 K, HCO and CH(3)O are observed. In fact, the yield of these intermediate species depends on the amount of the H radicals interacting with CO molecules. At 3 K, the presence of H(2) in the solid screens the hydrogenation reaction. This causes a termination for the reaction in the stage of the formation of HCO and H(2)CO. At 10 K, H(2) cannot condense, and the reaction between CO and H is total. In this case, species such as HCO, H(2)CO, CH(3)O, and CH(3)OH are observed.

H3(+) as a trap for noble gases-3: multiple trapping of neon, argon, and krypton in X(n)H3(+) (n = 1-3).

Recent studies on the formation of XH(3)(+) noble gas complexes have shown strategic implications for the composition of the atmospheres of the giant planets as well as for the composition of comets. One crucial factor in the astrophysical process is the relative abundances of the noble gases versus H(3)(+). It is the context in which the possibility for clustering with more than one noble gas (X(n)H(3)(+) up to n = 3) has been investigated for noble gases X ranging from neon to krypton. In order to assert our results, a variety of methods have been used including ab initio coupled cluster CCSD and CCSD(T), MP2, and density functional BH&HLYP levels of theory. All complexes with one, two, and three noble gases are found to be stable in the Ne, Ar, and Kr families. These stable structures are planar with the noble gases attached to the apices of the H(3)(+) triangle. The binding energy of the nth atom, defined as the X(n)H(3)(+) --> X(n-1)H(3)(+) + X reaction energy, increases slightly with n varying from 1 to 3 in the neon series, while it decreases in the argon series and shows a minimum for n = 2 in the krypton series. The origin of this phenomenon is to be found in the variations in the respective vibrational energies. A topological analysis of the electron localization function shows the importance of the charge transfer from the noble gases toward H(3)(+) as a driving force in the bonding along the series. It is also consistent with the increase in the atomic polarizabilities from neon to krypton. Rotational constants and harmonic frequencies are reported in order to provide a body of data to be used for the detection in laboratory prior to space observations. This study strongly suggests that the noble gases could be sequestered even in an environment where the H(3)(+) abundance is small.

Electronic structure of simple phosphorus containing molecules [C,xH,O,P] candidate for astrobiology (x=1, 3, 5).

The present report is a prospective study aimed at finding phosphorus containing compounds for astrobiology. Since PN, PC and HCP are the only species detected so far, it was deemed reasonable to enlarge the quest for phosphorus compounds to mixed carbon oxygen containing compounds [C,xH,O,P] analogue to the CHON family. Ab initio Møller-Plesset (MP2), Coupled Cluster (CCSD(T)) and Density Functional Theory (DFT) were used. State of the art level of theory, CCSD(T)/cc-pVQZ, was necessary to show that CH3-PH2=O is the most stable isomer, with CH3-PH-OH close by in the [C,5H,O,P] sub-family. This structure has the same C-P-O connectivity as the most stable compound of the [C,3H,O,P] sub-family, CH3-P=O but differs from the simplest [C,H,O,P] system HP=C=O. Rotational constants B=7.1377 and C=6.0636 GHz associated with a dipole moment of 4.2 Debye together with an IR spectrum with very strong bands at 1214, 2282, 2264 and 1039 cm(-1) have been calculated for CH3-PH2=O. For CH3-P=O, one has B=7.9881 and C=6.4659 GHz, a dipole moment of 2.9 Debye and four IR bands at 1198, 623, 835, 1256 cm(-1) of medium intensity. The simplest HPCO system with B=5.5206 and 5.3952 GHz and a dipole moment of 0.8 Debye has only one very strong IR frequency at 2037 cm(-1). The above values should be precise enough to encourage laboratory experiments on these prototype molecules.

H(3) (+) as a trap for noble gases--2: structure and energetics of XH(3) (+) complexes from X=neon to xenon.

The affinity of H(3) (+) to combine with noble gases X has been investigated from neon to xenon using ab initio coupled cluster [CCSD and CCSD(T)] and density functional BH&HLYP levels of theory. For all noble gases, the stable structures belong to a C(2v) symmetry with an apex of the H(3) (+) triangle pointing to the noble gas. The structure of the complexes changes gradually from a practically pure Ne-H(3) (+) arrangement to a situation close to XeH(+)-H(2). A topological analysis of the electron localization function is used to illustrate the changes in the bonding along the series. The lowest dissociation energies of NeH(3) (+) and ArH(3) (+) ( approximately 1 and approximately 7 kcalmol) correspond to the breaking of the complexes according to X+H(3) (+), while the lowest dissociation energies of KrH(3) (+) and XeH(3) (+) ( approximately 8 and approximately 3 kcalmol) correspond to the breaking according to XH(+)+H(2). Rotational constants and harmonic frequencies are reported. Apart from XeH(3) (+) whose dipole moment (mu=2.6 D) may not be large enough, all the other complexes with dipole moments in the range of 6-8 D should be reasonable targets for detection by microwave spectroscopy. The present calculations are intended to stimulate both laboratory experiments and spatial observations since the possible sequestration of noble gases by H(3) (+) may have strong implications on the composition of astrophysical objects.

Looking for the PC bond in space: HPCO and HPCS as possible tracers.

This paper looks at the possibility of astronomical detection of molecules of prebiotic interest containing phosphorus. Attention has been focused on the most stable species that can be formed with the four [C,H,O,P] atoms. Ab initio coupled cluster molecular orbital methods and density functional theory have been used. It is found that the best candidate for detection is singlet HPCO (corresponding to singlet HNCO), which lies 23.5 kcal/mol lower than the next isomer (HOCP) and has a non-negligible dipole moment of 0.8 D. Theoretical rotational constants and infrared (IR) spectra have been determined for HPCO and the closely related HPCS. By correcting the known inadequacies in the calculations with the average theoretical to experimental ratio from two benchmark molecules (HOCO+ and HNCO), it is possible to obtain rotational constants and IR frequencies of considerably higher accuracy. The corrected values of B = 5.5322 GHz and C = 5.4071 GHz for HPCO, and B = 3.0416 GHz and C = 3.0014 GHz for HPCS should be accurate to within a few tenths of a percent. It can also be predicted that only the 2000 cm(-1) (HPCO) and 1400 cm(-1) (HPCS) IR bands have sufficient intensity to be searched in the low-abundance conditions prevailing in space.

Theoretical infrared spectra of some model polycyclic aromatic hydrocarbons: effect of ionization.

In order to test the hypothesis of ionized polycyclic aromatic hydrocarbons (PAHs) as possible carriers of the UIR bands, we realized a computational exploration on selected PAHs of small dimension in order to identify which changes ionization would induce on their IR spectra. In this study we performed ab initio calculations of the spectra of neutral and positively ionized naphthalene, anthracene, and pyrene. The results are significantly important. The frequencies in the cations are slightly shifted with respect to the neutral species, but no general conclusion can be reached from the three molecules considered. By contrast, the relative intensities of most vibrations are strongly affected by ionization, leading to a much better agreement between the calculated CH/CC vibration intensity ratios and those deduced from observations.

Theoretical IR spectra of ionized naphthalene.

We report the results of a theoretical study of the effect of ionization on the IR spectrum of naphthalene, using ab initio molecular orbital theory. For that purpose we determined the structures, band frequencies, and intensities of neutral and positively ionized naphthalene. The calculated frequencies and intensities allowed an assignment of the most important bands appearing in the newly reported experimental spectrum of the positive ion. Agreement with the experimental spectrum is satisfactory enough to take into consideration the unexpected and important result that ionization significantly affects the intensities of most vibrations. A possible consequence on the interpretation of the IR interstellar emission, generally supposed to originate from polycyclic aromatic hydrocarbons (PAHs), is briefly presented.

Calculations on the rate of the ion-molecule reaction between NH3+ and H2.

The rate coefficient for the ion-molecule reaction NH3(+) + H2 --> NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He --> NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.

Is interstellar detection of higher members of the linear radicals CnCH and CnN feasible?

Rotational constants and dipole moments for linear-chain radicals CnCH and CnN are estimated using a combination of ab initio molecular orbital calculations and observed data on the starting members of the series. CnCH with n = 0-5 have been observed by radioastronomy in carbon-rich interstellar clouds; higher members of the series have 2 pi ground states with large dipole moments and are strong candidates for observation. CN and C3N have also been observed by radioastronomy; higher members of the series, with the possible exception of C5N, have 2 pi ground states with near-zero dipole moments making their interstellar detection hopeless under present observational conditions. C5N can be a strong candidate only if it has a 2 sigma ground state, and our best computations so far indicate that this is not the case.