Prebiotic Astrochemistry from Astronomical Observations and Laboratory Spectroscopy.

The discovery of more than 200 gas-phase chemical compounds in interstellar space has led to the speculation that this nonterrestrial synthesis may play a role in the origin of life. These identifications were possible because of laboratory spectroscopy, which provides the molecular fingerprints for astronomical observations. Interstellar chemistry produces a wide range of small, organic molecules in dense clouds, such as NH2COCH3, CH3OCH3, CH3COOCH3, and CH2(OH)CHO. Carbon (C) is also carried in the fullerenes C60 and C70, which can preserve C-C bonds from circumstellar environments for future synthesis. Elusive phosphorus has now been found in molecular clouds, the sites of star formation, in the molecules PO and PN. Such clouds can collapse into solar systems, although the chemical/physical processing of the emerging planetary disk is uncertain. The presence of molecule-rich interstellar starting material, as well as the link to planetary bodies such as meteorites and comets, suggests that astrochemical processes set a prebiotic foundation.

Destructive Processing of Silicon Carbide Grains: Experimental Insights into the Formation of Interstellar Fullerenes and Carbon Nanotubes.

The detection of the fullerenes C60 and C70 in the interstellar medium (ISM) has transformed our understanding of chemical complexity in space. These discoveries also raise the possibility for the presence of even larger molecules in astrophysical environments. Here we report in situ heating of analog silicon carbide (SiC) presolar grains using transmission electron microscopy (TEM). These heating experiments are designed to simulate the temperature conditions occurring in post-AGB stellar envelopes. Our experimental findings reveal that heating the analog SiC grains to the point of decomposition initially yields hemispherical C60-sized nanostructures, with five- and six-membered rings, which transform into multiwalled carbon nanotubes (MWCNTs) if held isothermally >2 min. These MWCNTs are certainly larger than any of the currently observed interstellar fullerene species, both in overall size and number of C atoms. These experimental simulations suggest that such MWCNTs are likely to form in post-AGB circumstellar material, where the structures, along with the smaller fullerenes, are subsequently injected into the ISM.

The ground state of KO revisited: the millimeter and submillimeter spectrum of potassium oxide.

The millimeter/sub-millimeter spectrum of the KO radical has been recorded in the frequency range 90-534 GHz using direct absorption methods. The radical was synthesized by reacting potassium vapor, produced with a Broida-type oven, with either N2O or O2 mixed in argon carrier gas. Twenty-seven rotational transitions of KO were measured, each exhibiting a doublet structure with a relatively small splitting (∼100-200 MHz) that increased noticeably with frequency. A perturbation was apparent in the rotational lines at energies above ∼120 cm-1, which was more prominent in one doublet component. The data were successfully fit with a Hund's case (c) Hamiltonian, assuming that spectra arise from a 2Πi state, and rotational and effective lambda-doubling constants were determined. Higher order centrifugal distortion terms were needed to account for the perturbation. The spectra could also be fit as a 2Σ+ ground state, but less successfully, and the resulting rotational constant of B = 8235.4 MHz disagreed significantly with that predicted by theory. On the basis of the experimental data, the ground electronic state of KO has been assigned as 2Πi, although the 2Σ+ assignment cannot be entirely ruled out.

Structural determination and gas-phase synthesis of monomeric, unsolvated IZnCH3 (X̃(1)A(1)): a model organozinc halide.

The first experimental structure of a monomeric organozinc halide, IZnCH3, has been measured using millimeter-wave direct absorption spectroscopy in the frequency range 256-293 GHz. IZnCH3 is a model compound for organozinc halides, widely used in cross-coupling reactions. The species was produced in the gas phase by reaction of zinc vapor with iodomethane in the presence of a dc discharge. IZnCH3 was identified on the basis of its pure rotational spectrum as well as those of the isotopically substituted species I(66)ZnCH3, I(64)Zn(13)CH3, and I(64)ZnCD3. IZnCH3 is unmistakably a symmetric top molecule (X̃(1)A1) belonging to the C3v point group, in agreement with DFT calculations, with the following experimentally determined structural parameters: rIZn = 2.4076(2) Å, rZnC = 1.9201(2) Å, rCH = 1.105(9) Å, and ∠H-C-H = 108.7(5)°. The basic methyl group geometry is not significantly altered in this molecule. Experimental observations suggest that IZnCH3 is synthesized in the gas phase by direct insertion of activated atomic zinc into the carbon-iodine bond of iodomethane.

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre.

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of 37(+16)(-10) microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

The electronic structure of vanadium monochloride cation (VCl(+)): tackling the complexities of transition metal species.

Six electronic states (X (4)Σ(-), A (4)Π, B (4)Δ, (2)Φ, (2)Δ, (2)Σ(+)) of the vanadium monochloride cation (VCl(+)) are described using large basis set coupled cluster theory. For the two lowest quartet states (X (4)Σ(-) and A (4)Π), a focal point analysis (FPA) approach was used that conjoined a correlation-consistent family of basis sets up to aug-cc-pwCV5Z-DK with high-order coupled cluster theory through pentuple (CCSDTQP) excitations. FPA adiabatic excitation energies (T0) and spectroscopic constants (re, r0, Be, B0, D¯e, He, ωe, v0, αe, ωexe) were extrapolated to the valence complete basis set Douglas-Kroll (DK) aug-cc-pV∞Z-DK CCSDT level of theory, and additional treatments accounted for higher-order valence electron correlation, core correlation, and spin-orbit coupling. Due to the delicate interplay between dynamical and static electronic correlation, single reference coupled cluster theory is able to provide the correct ground electronic state (X (4)Σ(-)), while multireference configuration interaction theory cannot. Perturbations from the first- and second-order spin orbit coupling of low-lying states with quartet spin multiplicity reveal an immensely complex rotational spectrum relative to the isovalent species VO, VS, and TiCl. Computational data on the doublet manifold suggest that the lowest-lying doublet state ((2)Γ) has a Te of ∼11 200 cm(-1). Overall, this study shows that laboratory and theoretical rotational spectroscopists must work more closely in tandem to better understand the bonding and structure of molecules containing transition metals.

Terahertz spectroscopy of 25MgH (X2Σ+) and (67)ZnH (X2Σ+): bonding in simple metal hydrides.

Pure rotational spectra of (25)MgH (X(2)Σ(+)) and (67)ZnH (X(2)Σ(+)) were recorded using direct absorption techniques. These free radicals were synthesized by the reaction of metal vapor, generated in a Broida-type oven, with H2 in a dc discharge. The N = 0 → 1 and N = 1 → 2 transitions were recorded for both species in the frequency range 342-789 GHz. Spin-rotation and metal and proton hyperfine interactions were resolved in the spectra. From these data, rotational, fine structure, and hyperfine constants were determined, including the Fermi contact, dipolar, and electric quadrupole parameters of the (25)Mg and (67)Zn nuclei. Comparison of the metal and proton hyperfine terms suggests that the unpaired electron resides in a σ molecular orbital that has significant s contributions from both the metal and the hydrogen atoms. The dipolar terms for both metals are relatively large, in contrast to those of the proton, and indicate spσ and possibly sdσ (zinc only) orbital hybridization. The quadrupole constants arise principally from the p/d orbital character of the unpaired electron, although there is a non-negligible polarization contribution. These results suggest significant covalent character in both MgH and ZnH, in contrast to their fluoride analogs.

Gas-phase synthesis and structure of monomeric ZnOH: a model species for metalloenzymes and catalytic surfaces.

Monomeric ZnOH has been studied for the first time using millimeter and microwave gas-phase spectroscopy. ZnOH is important in surface processes and at the active site of the enzyme carbonic anhydrase. In the millimeter-wave direct-absorption experiments, ZnOH was synthesized by reacting zinc vapor, produced in a Broida-type oven, with water. In the Fourier-transform microwave measurements, ZnOH was produced in a supersonic jet expansion of CH(3)OH and zinc vapor, created by laser ablation. Multiple rotational transitions of six ZnOH isotopologues in their X(2)A' ground states were measured over the frequency range of 22-482 GHz, and splittings due to fine and hyperfine structure were resolved. An asymmetric top pattern was observed in the spectra, showing that ZnOH is bent, indicative of covalent bonding. From these data, spectroscopic constants and an accurate structure were determined. The Zn-O bond length was found to be similar to that in carbonic anhydrase and other model enzyme systems.

Activation of methane by zinc: gas-phase synthesis, structure, and bonding of HZnCH3.

The methylzinc hydride molecule, HZnCH3, has been observed in the gas phase for the first time in the monomeric form using high-resolution spectroscopic techniques. The molecule was synthesized by two methods: the reaction of dimethylzinc with hydrogen gas and methane in an AC discharge and the reaction of zinc vapor produced in a Broida-type oven with methane in a DC discharge. HZnCH3 was identified on the basis of its pure rotational spectrum, which was recorded using millimeter/submillimeter direct-absorption and Fourier transform microwave techniques over the frequency ranges 332-516 GHz and 18-41 GHz, respectively. Multiple rotational transitions were measured for this molecule in seven isotopic variants. K-ladder structure was clearly present in all of the spectra, indicating a molecule with C3v symmetry and a (1)A1 ground electronic state. Extensive quadrupole hyperfine structure arising from the (67)Zn nucleus was observed for the H(67)ZnCH3 species, suggesting covalent bonding to the zinc atom. From the multiple isotopic substitutions, a precise structure for HZnCH3 has been determined. The influence of the axial hydrogen atom slightly distorts the methyl group but stabilizes the Zn-C bond. This study suggests that HZnCH3 can be formed through the oxidative addition of zinc to methane in the gas phase under certain conditions. HZnCH3 is the first metal-methane insertion complex to be structurally characterized.

Laboratory Studies of Methane and Its Relationship to Prebiotic Chemistry.

To examine how prebiotic chemical evolution took place on Earth prior to the emergence of life, laboratory experiments have been conducted since the 1950s. Methane has been one of the key molecules in these investigations. In earlier studies, strongly reducing gas mixtures containing methane and ammonia were used to simulate possible reactions in the primitive atmosphere of Earth, producing amino acids and other organic compounds. Since Earth's early atmosphere is now considered to be less reducing, the contribution of extraterrestrial organics to chemical evolution has taken on an important role. Such organic molecules may have come from molecular clouds and regions of star formation that created protoplanetary disks, planets, asteroids, and comets. The interstellar origin of organics has been examined both experimentally and theoretically, including laboratory investigations that simulate interstellar molecular reactions. Endogenous and exogenous organics could also have been supplied to the primitive ocean, making submarine hydrothermal systems plausible sites of the generation of life. Experiments that simulate such hydrothermal systems where methane played an important role have consequently been conducted. Processes that occur in other Solar System bodies offer clues to the prebiotic chemistry of Earth. Titan and other icy bodies, where methane plays significant roles, are especially good targets. In the case of Titan, methane is both in the atmosphere and in liquidospheres that are composed of methane and other hydrocarbons, and these have been studied in simulation experiments. Here, we review the wide range of experimental work in which these various terrestrial and extraterrestrial environments have been modeled, and we examine the possible role of methane in chemical evolution. Key Words: Methane-Interstellar environments-Submarine hydrothermal systems-Titan-Origin of life. Astrobiology 17, 786-812.

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A.

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.

Structures of Solvent-Free, Monomeric LiCCH, NaCCH, and KCCH.

The simplest alkali metal acetylides MCCH were made in the gas phase for the first time (see drawing), and their bond lengths were determined by millimeter/submillimeter spectroscopy of their isotopomers. The M-C bond lengths rCM are the shortest known for organoalkali metal compounds. In the case of LiCCH, the experimentally determined Li-C distance of 1.888 Å has an estimated accuracy of ±0.0005 Å, which should allow a rigorous test of theoretical methods.

Observations of interstellar formamide: availability of a prebiotic precursor in the galactic habitable zone.

We conducted a study on interstellar formamide, NH2CHO, toward star-forming regions of dense molecular clouds, using the telescopes of the Arizona Radio Observatory (ARO). The Kitt Peak 12 m antenna and the Submillimeter Telescope (SMT) were used to measure multiple rotational transitions of this molecule between 100 and 250 GHz. Four new sources of formamide were found [W51M, M17 SW, G34.3, and DR21(OH)], and complementary data were obtained toward Orion-KL, W3(OH), and NGC 7538. From these observations, column densities for formamide were determined to be in the range of 1.1×10(12) to 9.1×10(13) cm(-2), with rotational temperatures of 70-177 K. The molecule is thus present in warm gas, with abundances relative to H2 of 1×10(-11) to 1×10(-10). It appears to be a common constituent of star-forming regions that foster planetary systems within the galactic habitable zone, with abundances comparable to that found in comet Hale-Bopp. Formamide's presence in comets and molecular clouds suggests that the compound could have been brought to Earth by exogenous delivery, perhaps with an infall flux as high as ~0.1 mol/km(2)/yr or 0.18 mmol/m(2) in a single impact. Formamide has recently been proposed as a single-carbon, prebiotic source of nucleobases and nucleic acids. This study suggests that a sufficient amount of NH2CHO could have been available for such chemistry.

Jet-launching structure resolved near the supermassive black hole in M87.

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

The chemistry in circumstellar envelopes of evolved stars: following the origin of the elements to the origin of life.

Mass loss from evolved stars results in the formation of unusual chemical laboratories: circumstellar envelopes. Such envelopes are found around carbon- and oxygen-rich asymptotic giant branch stars and red supergiants. As the gaseous material of the envelope flows from the star, the resulting temperature and density gradients create a complex chemical environment involving hot, thermodynamically controlled synthesis, molecule "freeze-out," shock-initiated reactions, and photochemistry governed by radical mechanisms. In the circumstellar envelope of the carbon-rich star IRC+10216, >50 different chemical compounds have been identified, including such exotic species as C(8)H, C(3)S, SiC(3), and AlNC. The chemistry here is dominated by molecules containing long carbon chains, silicon, and metals such as magnesium, sodium, and aluminum, which makes it quite distinct from that found in molecular clouds. The molecular composition of the oxygen-rich counterparts is not nearly as well explored, although recent studies of VY Canis Majoris have resulted in the identification of HCO(+), SO(2), and even NaCl in this object, suggesting chemical complexity here as well. As these envelopes evolve into planetary nebulae with a hot, exposed central star, synthesis of molecular ions becomes important, as indicated by studies of NGC 7027. Numerous species such as HCO(+), HCN, and CCH are found in old planetary nebulae such as the Helix. This "survivor" molecular material may be linked to the variety of compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon-rich fragments originally formed in circumstellar shells.

Competition between metal-amido and metal-imido chemistries in the alkaline earth series: an experimental and theoretical study of BaNH.

The pure rotational spectrum of BaNH in its X(1)Sigma(+) ground electronic state has been recorded using millimeter/submillimeter direct absorption methods; data for the deuterium and barium 137 isotopomers have been measured as well. The molecules were produced by the reaction of ammonia or ND(3) and barium vapor in the presence of a dc discharge. Transitions arising from the ground vibrational state and the excited vibrational bending (01(1)0) and heavy atom stretching (100) modes were measured. The rotational spectrum indicates a linear structure, with B(0)(BaNH) = 7984.549 MHz and B(0)(BaND) = 7060.446 MHz. An r(m)((1)) structure has been determined, yielding r(BaN) = 2.077 +/- 0.002 Angstroms and r(NH) = 1.0116 +/- 0.0006 Angstroms. Density functional calculations using an extensive Slater-type basis set with inclusion of scalar relativistic effects gives geometrical parameters and vibrational frequencies for BaNH in excellent agreement with those determined by experiment. The molecular orbital and natural bond order analyses of the BaNH wave function show Ba-N pi bonds formed by electron donation from the formally filled N 2p orbitals of the imido group to the empty Ba 5d orbitals. The multiple bonding between Ba and N stabilizes the linear geometry and, along with the relative ease of oxidation of the Ba atom, favors formation of the metal-imido species over that of the metal-amido species that have been found from similar studies with Mg, Ca, and Sr atoms in this group.

The first precise molecular structure of a monomeric transition metal cyanide, copper(I) cyanide.

Copper(I) cyanide is an important reagent in organic, organometallic, and supramolecular chemistry because of both the copper center and the versatile cyanide ligand. Solid-phase CuCN and many of its derivatives show oligomeric or polymeric structures, a trait shared by other metal cyanides. Often, it is difficult to specify the orientation of the cyano ligand in an X-ray structure. Here the first preparation and precise structure of a monomeric transition metal cyanide is reported. Gas-phase reaction between copper vapor and cyanogen (NCCN) clearly gives CuCN (not CuNC). The precise structure of CuCN so produced is determined by millimeter/submillimeter-wave spectroscopy. Because of the highly efficient synthesis and the presence of significant amounts of two copper isotopes, such strong signals were seen that natural-abundance materials allowed observation of transitions for the four isotopomers (63)Cu(12)C(14)N, (65)Cu(12)C(14)N, (63)Cu(13)C(14)N, and (63)Cu(12)C(15)N and the determination of r(o), r(s), and r(m)((2)) structures. All data unequivocally show a linear geometry and that the carbon of cyanide is bound to copper with a Cu-C distance of 1.82962(4) A in the r(m)((2)) structure, which is likely to be closest to the equilibrium geometry.

Gas-phase synthesis, submillimeter spectra, and precise structure of monomeric, solvent-free CuCH3.

The most-studied organocopper, methylcopper (CH(3)Cu), has been synthesized for the first time as a monomer, free of other ligands or coordinated solvent, and detected using millimeter-wave rotational spectroscopy. The molecule was created in the gas phase by the reaction of copper vapor and several different precursors, including CH(4) and tetramethylsilane. An obvious symmetric top pattern was observed, indicative of C(3)(v) symmetry. For confirmation, rotational spectra of four isotopically substituted species, (65)Cu(12)CH(3), (63)Cu(12)CD(3), (63)Cu(13)CH(3), and (65)Cu(13)CH(3), were recorded as well, following large-scale syntheses of specifically labeled precursors. From this data set, accurate spectroscopic constants and a structure for CH(3)Cu were derived. The r(m)((2)) geometry shows the shortest alkyl carbon-copper(I) bond length known (1.8809 A) and a rather large H-C-H angle in the methyl group (109.88 degrees). The measured rotational and distortion constants are well reproduced by QCISD calculations. This study should allow further refinement of theoretical calculations on methylcopper and other organocopper derivatives, which are valuable synthetic intermediates in organic chemistry.

The rotational spectrum and dynamical structure of LiOH and LiOD: a combined laboratory and ab initio study.

Millimeter wave rotational spectroscopy and ab initio calculations are used to explore the potential energy surface of LiOH and LiOD with particular emphasis on the bending states and bending potential. New measurements extend the observed rotational lines to J=7<--6 for LiOH and J=8<--7 for LiOD for all bending vibrational states up to (03(3)0). Rotation-vibration energy levels, geometric expectation values, and dipole moments are calculated using extensive high-level ab initio three-dimensional potential energy and dipole moment surfaces. Agreement between calculation and experiment is superb, with predicted Bv values typically within 0.3%, D values within 0.2%, ql values within 0.7%, and dipole moments within 0.9% of experiment. Shifts in Bv values with vibration and isotopic substitution are also well predicted. A combined theoretical and experimental structural analysis establishes the linear equilibrium structure with re(Li-O)=1.5776(4) A and re(O-H)=0.949(2) A. Predicted fundamental vibrational frequencies are v1=923.2, v2=318.3, and v3=3829.8 cm(-1) for LiOH and v1=912.9, v2=245.8, and v3=2824.2 cm(-1) for LiOD. The molecule is extremely nonrigid with respect to angular deformation; the calculated deviation from linearity for the vibrationally averaged structure is 19.0 degrees in the (000) state and 41.9 degrees in the (03(3)0) state. The calculation not only predicts, in agreement with previous work [P. R. Bunker, P. Jensen, A. Karpfen, and H. Lischka, J. Mol. Spectrosc. 135, 89 (1989)], a change from a linear to a bent minimum energy configuration at elongated Li-O distances, but also a similar change from linear to bent at elongated O-H distances.