Oscar Martinez

DETECTION OF E-CYANOMETHANIMINE TOWARD SAGITTARIUS B2(N) IN THE GREEN BANK TELESCOPE PRIMOS SURVEY

The detection of E-cyanomethanimine (E-HNCHCN) toward Sagittarius B2(N) is made by comparing the publicly available Green Bank Telescope (GBT) PRIMOS survey spectra to laboratory rotational spectra from a reaction product screening experiment. The experiment uses broadband molecular rotational spectroscopy to monitor the reaction products produced in an electric discharge source using a gas mixture of NH3 and CH3CN. Several transition frequency coincidences between the reaction product screening spectra and previously unassigned interstellar rotational transitions in the PRIMOS survey have been assigned to E-cyanomethanimine. A total of eight molecular rotational transitions of this molecule between 9 and 50?GHz are observed with the GBT. E-cyanomethanimine, often called the HCN dimer, is an important molecule in prebiotic chemistry because it is a chemical intermediate in proposed synthetic routes of adenine, one of the two purine nucleobases found in DNA and RNA. New analyses of the rotational spectra of both E-cyanomethanimine and Z-cyanomethanimine that incorporate previous millimeter-wave measurements are also reported.

On the molecular structure of HOOO.

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H(2)O and O(2) heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H(2) and O(2) as precursor gases. Using equal mixtures of normal and (18)O(2), it has been possible to detect three new isotopic species, H(18)OOO, HO(18)O(18)O, and H(18)O(18)O(18)O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O(2) → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r(0)) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 Å), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 Å), and a more acute [angle]HOO angle (92.4°) when compared to equilibrium (r(e)) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (r(e) (emp)) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.

EXPERIMENTAL AND THEORETICAL STUDIES OF REACTIONS BETWEEN H ATOMS AND NITROGEN-CONTAINING CARBANIONS

Nitrogen-containing organic compounds are important in the interstellar medium (ISM). The detection of some corresponding anions, C n N– (n = 1, 3, 5), highlights the importance of laboratory studies of their chemistry, especially with the most abundant atomic species in the ISM, hydrogen atom. This work is a combined experimental and computational study of a series of nitrogen-containing carbanions reacting with H atoms. The anions include C n N– (n = 1-6), C n N2 – (n = 1, 3-5), and C n N3 – (n = 2, 4), and reactions mainly proceed through associative detachment (A– + H → AH + e –) or fragmentation pathways. Experimental measurements of the reaction rate constants were made with the flowing afterglow-selected ion flow tube technique. Ab initio theoretical calculations were carried out to explore the reaction mechanisms and investigate the factors determining reactivities. Reaction is mainly determined by the characteristics of the potential energy surfaces along the approach of the reactants. In addition, angular momentum conservation of the anion-H atom collision may have the universal effect of decreasing the reaction efficiencies. Our results indicate that C n N– (n = 1-6) and C n N2 – (n = 1, 3-5) can be destroyed in reactions with H atoms by either forming the corresponding neutral monohydride compound or fragmenting into smaller anionic and neutral species. However, C n N3 – (n = 2, 4) anions are unreactive, such that they may exist in H atom rich regions in the ISM.

Further Insight into the Reaction FeO+ + H2 → Fe+ + H2O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling

The reactions of FeO(+) with H2, D2, and HD were studied in detail from 170 to 670 K by employing a variable temperature selected ion flow tube apparatus. High level electronic structure calculations were performed and compared to previous theoretical treatments. Statistical modeling of the temperature and isotope dependent rate constants was found to reproduce all data, suggesting the reaction could be well explained by efficient crossing from the sextet to quartet surface, with a rigid near thermoneutral barrier accounting for both the inefficiency and strong negative temperature dependence of the reactions over the measured range of thermal energies. The modeling equally well reproduced earlier guided ion beam results up to translational temperatures of about 4000 K.

Microwave detection of sulfoxylic acid (HOSOH).

Sulfoxylic acid (HOSOH), a chemical intermediate roughly midway along the path between highly reduced (H2S) and highly oxidized sulfur (H2SO4), has been detected using Fourier transform microwave spectroscopy and double resonance techniques, guided by new high-level CCSD(T) quantum-chemical calculations of its molecular structure. Rotational spectra of the two most stable isomers of HOSOH, the putative ground state with C2 symmetry and the low-lying C(s) rotamer, have been measured to high precision up to 71 GHz, allowing accurate spectroscopic parameters to be derived for both isomers. HOSOH may play a role in atmospheric and interstellar chemistry, and the present work provides the essential data to enable remote sensing and/or radioastronomical searches for these species. Spectroscopic characterization of HOSOH suggests that other transient intermediates in the oxidation of SO2 to H2SO4 may be amenable to laboratory detection as well.

The influence of spin effects on the gas phase reactions of carbanions with N and O atoms.

Molecular anions have been recently detected in the denser regions of the interstellar medium. However, the chemical reactions of molecular anions with atomic species that are abundant in the ISM remain largely unexplored. This work is an experimental and computational study of CH(2)CN(-), CH(3)CHCN(-), (CH(3))(2)CCN(-), and CH(2)CHO(-) reacting with N and O atoms. In all cases the reactions of anions with O atoms exhibit larger reaction rate constants compared to the corresponding reactions with N atoms. Our study indicates that spin-forbidden reactions are the probable pathways in the reactions with N atoms, whereas spin-allowed reactions are the dominant processes in the reactions with O atoms. The major factor influencing the reaction rate constants of anions with N and O atoms is whether a spin-allowed barrierless pathway exists. The rich chemistry observed in this work provides a greater understanding of the ion-atom reaction processes, as well as some new avenues for further spin chemistry research.

Formation of Gas-Phase Glycine and Cyanoacetylene via Associative Detachment Reactions

The recent discovery of molecular negative ions in the interstellar medium suggests that these species may be reactive intermediates in astrochemical processes. Our recent studies indicate that these anions, despite their high electron binding energies, are reactive with atomic reagents. In this work, we report that two species of interstellar interest, glycine (NH(2)CH(2)COOH) and cyanoacetylene (HC(3)N), can be readily formed by associative detachment of their corresponding deprotonated anions with hydrogen atoms. The reaction rate constants for glycine anion (NH(2)CH(2)CO(2)(-)) and cyanoacetylene anion (C(3)N(-)) with H atom have been measured to be 2.0 +/- 0.5 x10(-10) cm(3) s(-1) and 5.4 +/- 0.2 x 10(-10) cm(3) s(-1), respectively, where the error bars represent one standard deviation of the mean; the estimated total error is +/- 50%. A possible reaction mechanism for chemical conversion of species observed in the interstellar medium is also described.

GAS-PHASE REACTIONS OF POLYCYCLIC AROMATIC HYDROCARBON ANIONS WITH MOLECULES OF INTERSTELLAR RELEVANCE

We have studied reactions of small dehydrogenated polycyclic aromatic hydrocarbon anions with neutral species of interstellar relevance. Reaction rate constants are measured at 300?K for the reactions of phenide (C6H? 5), naphthalenide (C10H? 7), and anthracenide (C14H? 9) with atomic H, H2, and D2 using a flowing afterglow-selected ion flow tube instrument. Reaction rate constants of phenide with neutral molecules (CO, O2, CO2, N2O, C2H2, CH3OH, CH3CN, (CH3)2CO, CH3CHO, CH3Cl, and (CH3CH2)2O) are also measured under the same conditions. Experimental measurements are accompanied by ab initio calculations to provide insight into reaction pathways and enthalpies. Our measured reaction rate constants should prove useful in the modeling of astrophysical environments, particularly when applied to dense regions of the interstellar and circumstellar medium.

Detection and structure of HOON: microwave spectroscopy reveals an O-O bond exceeding 1.9 Å.

A Glimpse of HOON Bonds between two oxygen atoms are relatively weak, as manifested in the sometimes explosive reactivity of O2 and various peroxides. Thus, although nitrous acid (HONO) can be rearranged on paper to an isomer with an O-O rather than N-O bond, nitrosyl-O-hydroxide (HOON) has been considered too unstable to be observed. Crabtree et al. (p. 1354) used microwave spectroscopy to detect HOON formation in a dilute gaseous mixture of NO and OH in neon. Isotopic substitutions enabled determination of its structure, which included an unusually long O-O bond. Spectroscopy reveals an isomer of nitrous acid previously considered too unstable to observe. Nitric oxide (NO) reacts with hydroxyl radicals (OH) in the gas phase to produce nitrous acid, HONO, but essentially nothing is known about the isomeric nitrosyl-O-hydroxide (HOON), owing to its perceived instability. We report the detection of gas-phase HOON in a supersonic molecular beam by Fourier transform microwave spectroscopy and a precise determination of its molecular structure by further spectroscopic analysis of its 2H, 15N, and 18O isotopologs. HOON contains the longest O–O bond in any known molecule (1.9149 ± 0.0005 Å) and appears surprisingly stable, with an abundance roughly 3% that of HONO in our experiments.

Detection of Two Highly Stable Silicon Nitrides: HSiNSi and H3SiNSi

The formation mechanisms of silicon nitride and silicon nitrogen hydrogen films, both produced by chemical vapor deposition (CVD) techniques and widely used in electronic device fabrication, are poorly understood. Identification of gas-phase intermediates formed from starting materials, typically silane, ammonia, and/or nitrogen, is a critical step in assessing the interplay between gas and surface processes in film formation. Two potential intermediates in this process, HSiNSi and H3SiNSi, have now been detected in a molecular beam by means of rotational spectroscopy. Both molecules were produced in electrical discharges of CVD-like gas mixtures and are the most readily observed silicon-nitrogen-containing molecules in the 6-20 GHz frequency range, though neither has been the subject of prior experimental or theoretical studies. HSiNSi and H3SiNSi are likely formed from reactions involving the silanitrile radical (SiN, isoelectronic to CN), implying that similar gas-phase reactions may be involved in film growth.