D. T. Halfen

Formation of Peptide Bonds in Space: A Comprehensive Study of Formamide and Acetamide in Sgr B2(N)

Extensive observations of acetamide (CH3CONH2) and formamide (NH2CHO) have been conducted toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope (SMT) and the 12 m antenna of the Arizona Radio Observatory. Over the frequency range 65–280 GHz, 132 transitions of acetamide have been observed as individual, distinguishable features, although in some cases they are partially blended. The unblended transitions in acetamide indicate VLSR = 63.2 ± 2.8 km s −1 and ΔV1/2 = 12.5 ± 2.9 km s −1 , line parameters that are very similar to that of formamide (NH2CHO) and other organic species in Sgr B2(N). For formamide, 79 individual transitions were identified over the same frequency region. Rotational diagram analyses indicate the presence of two components for both species in Sgr B2(N). For acetamide, the colder component (Eu < 40 K) exhibits a rotational temperature of Trot = 17 ± 4 K and a column density of Ntot = 5.2 ± 3.5 × 10 13 cm −2 ; the higher energy component has Trot = 171 ± 4 K and Ntot = 6.4 ± 4.7 × 10 14 cm −2 . In the case of formamide, Trot = 26 ± 4 K and Ntot = 1.6 ± 0.7 × 10 14 cm −2 for the colder component with Trot = 134 ± 17 K and Ntot = 4.0 ± 1.2 × 10 14 cm −2 for the warmer region. The fractional abundances of acetamide are f (H2) = 1.7 × 10 −11 and 2.1 × 10 −10 for the cold and warm components, and in formamide, f (H2) = 5.3 × 10 −11 and 1.3 × 10 −10 . The similarity between the abundances and distributions of CH3CONH2 and NH2CHO suggests a synthetic connection. The abundance of acetamide, moreover, is only a factor of three lower than that of formaldehyde, and very similar to acetaldehyde and ketene. CH3CONH2 is therefore one of the most abundant complex organic species in Sgr B2(N), and could be a possible source of larger peptide molecules, as opposed to amino acids.

The rotational spectrum of CuCCH(X̃ Σ1+): A Fourier transform microwave discharge assisted laser ablation spectroscopy and millimeter/submillimeter study

The pure rotational spectrum of CuCCH in its ground electronic state (X̃  (1)Σ(+)) has been measured in the frequency range of 7-305 GHz using Fourier transform microwave (FTMW) and direct absorption millimeter/submillimeter methods. This work is the first spectroscopic study of CuCCH, a model system for copper acetylides. The molecule was synthesized using a new technique, discharge assisted laser ablation spectroscopy (DALAS). Four to five rotational transitions were measured for this species in six isotopologues ((63)CuCCH, (65)CuCCH, (63)Cu(13)CCH, (63)CuC(13)CH, (63)Cu(13)C(13)CH, and (63)CuCCD); hyperfine interactions arising from the copper nucleus were resolved, as well as smaller splittings in CuCCD due to deuterium quadrupole coupling. Five rotational transitions were also recorded in the millimeter region for (63)CuCCH and (65)CuCCH, using a Broida oven source. The combined FTMW and millimeter spectra were analyzed with an effective Hamiltonian, and rotational, electric quadrupole (Cu and D) and copper nuclear spin-rotation constants were determined. From the rotational constants, an r(m)(2) structure for CuCCH was established, with r(Cu-C) = 1.8177(6) Å, r(C-C) = 1.2174(6) Å, and r(C-H) = 1.046(2) Å. The geometry suggests that CuCCH is primarily a covalent species with the copper atom singly bonded to the C≡C-H moiety. The copper quadrupole constant indicates that the bonding orbital of this atom may be sp hybridized. The DALAS technique promises to be fruitful in the study of other small, metal-containing molecules of chemical interest.

DETECTION OF A NEW INTERSTELLAR MOLECULE: THIOCYANIC ACID HSCN

A new interstellar molecule, HSCN (thiocyanic acid), an energetic isomer of the well-known species HNCS, has been detected toward Sgr B2(N) with the Arizona Radio Observatory 12 m telescope. Eight rotational transitions in the Ka = 0 ladder were observed in the 2 mm and 3 mm bands. Five consecutive transitions in the 3 mm band are unblended, but three in the 2 mm band are partially masked by lines of other molecules. The peak intensity of all eight transitions are well described by a rotational temperature that is in very good agreement with that of many other molecules in this source. The line width and radial velocity of HSCN match closely with those of the ground state isomer HNCS (isothiocyanic acid), HNCO (isocyanic acid), and HOCN (cyanic acid); preliminary maps indicate that all four molecules are similarly distributed in Sgr B2. Although HSCN is calculated to lie over 3000 K higher in energy than HNCS, its column density of 1.3 × 1013 cm–2 in Sgr B2(N) is only three times lower than that of HNCS. The fractional abundances of HSCN and HNCS relative to H2 are 4.5 × 10–12 and 1.1 × 10–11. By analogy with the isomeric pair HCN and HNC, these two sulfur-bearing isomers are plausibly formed from a common cation precursor.

INTERSTELLAR DETECTION of METHYL ISOCYANATE CH3NCO in Sgr B2(N): A LINK from MOLECULAR CLOUDS to COMETS

A new interstellar molecule, CH3NCO (methyl isocyanate), has been detected using the 12 m telescope of the Arizona Radio Observatory (ARO). CH3NCO was identified in spectra covering 48 GHz (68–116 GHz) in the 3 mm segment of a broadband survey of Sgr B2(N). Thirty very favorable rotational lines (Ka = 0 and Ka = 1 only; Eu < 60 K) originating in five consecutive transitions (J = 8 7, 9 8, 10 9, 11 10, and 12 11) in both the A and E internal rotation species are present in this frequency range. Emission was observed at all of the predicted frequencies, with 17 lines appearing as distinct, uncontaminated spectral features, clearly showing the classic a-type, asymmetric top pattern, with ≈ 20–70 mK. The CH3NCO spectra also appear to exhibit two velocity components near VLSR ≈ 62 and 73 km s−1, both with ΔV1/2 ≈ 10 km s−1—typical of molecules such as CH2CHCN, HNCO, and HCOOCH3 in Sgr B2(N). The column density of CH3NCO in Sgr B2(N) was determined to be Ntot ≈ 2.3 × 1013 and 1.5 × 1013 cm−2 for the 62 and 73 km s−1 components, corresponding to fractional abundances, relative to H2, of f ≈ 7.6 × 10−12 and 5.0 × 10−12, respectively. CH3NCO was recently detected in volatized material from comet 67P/Churyumov–Gerasimenko by Rosettas Philae lander, with an abundance ~1.3% of water; in Sgr B2(N), CH3NCO is roughly ~0.04% of the H2O abundance.

The pure rotational spectra of SrSH (X̃ 2A′) and SrS (X 1Σ+): Further studies in alkaline-earth bonding

The pure rotational spectrum of the SrSH radical in its ground electronic (X  2A′) and vibrational states has been measured using millimeter/submillimeter-wave direct absorption techniques. This work is the first observation of SrSH with rotational resolution. The spectrum of its deuterium isotopomer SrSD and SrS (X  1Σ+) has been recorded as well. These species were created by the reaction of strontium vapor and H2S, in the presence of a dc discharge. SrS was also made with CS2. For SrSH and SrSD, eight rotational transitions were recorded, respectively, for which asymmetry components up to Ka=8 were measured; fine structure was also resolved in each component. Thirteen transitions of SrS in each of its v=0, 1, and 2 states have additionally been observed. These data have been analyzed and spectroscopic parameters determined for all three species, including spin-rotation terms for the strontium hydrosulfides. From an r0 structure calculation, the bond angle in SrSH was determined to be 91.48(3)°, very c...

The pure rotational spectrum of HPS (X̃1A′): Chemical bonding in second-row elements

The pure rotational spectrum of HPS, as well as its (34)S and D isotopologues, has been recorded at microwave, millimeter, and submillimeter wavelengths, the first observation of this molecule in the gas phase. The data were obtained using a combination of millimeter direct absorption, Fourier transform microwave (FTMW), and microwave-microwave double-resonance techniques, which cover the total frequency range from 15 to 419 GHz. Quantum chemical calculations at the B3LYP and CCSD(T) levels were also performed to aid in spectral identification. HPS was created in the direct absorption experiment from a mixture of elemental phosphorus, H(2)S, and Ar carrier gas; DPS was produced by adding D(2). In the FTMW study, these species were generated in a pulsed discharge nozzle from PH(3) and H(2)S or D(2)S, diluted in neon. The spectra recorded for HPS and its isotopologues exhibit clear asymmetric top patterns indicating bent structures; phosphorus hyperfine splittings were also observed in HPS, but not DPS. Analysis of the data yielded rotation, centrifugal distortion, and phosphorus nuclear spin-rotation parameters for the individual species. The r(m) ((1)) structure for HPS, calculated from the rotational constants, is r(H-P) = 1.438(1) Å, r(P-S) = 1.9320(1) Å, and θ(H-P-S) = 101.85(9)°. Empirically correcting for zero-point vibrational effects yields the geometry r(e)(H-P) = 1.4321(2) Å, r(e)(P-S) = 1.9287(1) Å, and θ(e)(H-P-S) = 101.78(1)°, in close agreement with the r(m) ((1)) structure. A small inertial defect was found for HPS indicating a relatively rigid molecule. Based on these data, the bonding in this species is best represented as H-P=S, similar to the first-row analog HNO, as well as HNS and HPO. Therefore, substitution of phosphorus and sulfur for nitrogen and oxygen does not result in a dramatic structural change.

Investigating the limits of chemical complexity in sagittarius B2(N): A rigorous attempt to confirm 1,3-dihydroxyacetone

A search for confirming transitions of 1,3-dihydroxyacetone, (CH2OH)2CO (hereafter DHA), has been made toward Sgr B2(N) using the Arizona Radio Observatory Submillimeter Telescope (SMT) and 12 m telescope at millimeter wavelengths. In addition, data for this species have been obtained at the Green Bank Telescope. In all, 63 new transitions of DHA were sought; no plausible emission was detected at 97% of these frequencies. Typical peak-to-peak limits of 10-30 mK were achieved, indicating that DHA is not present in Sgr B2(N) at the level reported by Widicus Weaver & Blake in 2005, by at least a factor of 300. The upper limit to the DHA column density is Ntot < 5 × 1013 cm-2, suggesting that this species, a three-carbon keto sugar, is less abundant than the two-carbon aldehyde sugar, glycolaldehyde (CH2OHCHO), by a factor of at least 4. If sugars of higher complexity are formed in interstellar clouds, their synthesis may proceed one carbon atom at a time, possibly through aldehyde intermediates.

The rotational spectrum of the CCP (XΠr2) radical and its C13 isotopologues at microwave, millimeter, and submillimeter wavelengths

The pure rotational spectrum of CCP (X (2)Pi(r)) has been measured at microwave, millimeter, and submillimeter wavelengths (17-545 GHz), along with its (13)C isotopologues ((13)C(13)CP, C(13)CP, and (13)CCP). The spectra of these species were recorded using a combination of millimeter/submillimeter direct absorption methods and Fourier transform microwave (FTMW) techniques. The phosphorus dicarbides were created in the gas phase from the reaction of red phosphorus and acetylene or methane in argon in an ac discharge for the direct absorption experiments, and using PCl(3) as the phosphorus source in a pulsed dc nozzle discharge for the FTMW measurements. A total of 35 rotational transitions were recorded for the main isotopologue, and between 2 and 8 for the (13)C-substituted species. Both spin-orbit components were identified for CCP, while only the Omega = 12 ladder was observed for (13)C(13)CP, C(13)CP, and (13)CCP. Hyperfine splittings due to phosphorus were observed for each species, as well as carbon-13 hyperfine structure for each of the (13)C-substituted isotopologues. The data were fitted with a Hunds case (a) Hamiltonian, and rotational, fine structure, and hyperfine parameters were determined for each species. The r(m)(1) bond lengths established for CCP, r(C-C) = 1.289(1) A and r(C-P) = 1.621(1) A, imply that there are double bonds between both the two carbon atoms and the carbon and phosphorus atoms. The hyperfine constants suggest that the unpaired electron in this radical is primarily located on the phosphorus nucleus, but with some electron density also on the terminal carbon atom. There appears to be a minor resonance structure where the unpaired electron is on the nucleus of the end carbon. The multiple double bond structure forces the molecule to be linear, as opposed to other main group dicarbides, such as SiC(2), which have cyclic geometries.

Hydroxyacetone (CH3COCH2OH): A Combined Microwave and Millimeter-Wave Laboratory Study and Associated Astronomical Search

A combined laboratory and astronomical investigation has been conducted on the methyl sugar hydroxyacetone (CH3COCH2OH). Rotational transitions of this species in the ground torsional state (vt = 0) were recorded using both millimeter-wave direct absorption techniques and Fourier transform microwave spectroscopy. A total of 1145 lines of CH3COCH2OH were analyzed in the frequency range 4 to 180 GHz, including transitions arising from both A- and E-symmetry species. A modified rho-axis method Hamiltonian was needed for the analysis because of the presence of perturbations resulting from the torsional motion of the methyl group in this molecule. Assignment of the E-species was particularly problematic as a consequence of significant mixing between the ground and torsionally excited levels. The complete data set was fitted using 21 spectroscopic parameters and had a global rms of 90 kHz; the barrier to internal rotation was established to be 65.3560(22) cm-1. An astronomical search was subsequently conducted for hydroxyacetone at 2 and 3 mm using the 12 m telescope of the Arizona Radio Observatory. Twenty-eight favorable transitions arising from both A- and E-species, each consisting of collapsed quartets, were searched for toward Sgr B2(N). Although credible features were detected at several frequencies of hydroxyacetone, there were a sufficient number of missing lines to rule out an interstellar detection. An upper limit to the column density of Ntot < 5 × 1012 cm-2 was derived for CH3COCH2OH in Sgr B2(N), indicating that this species is an order of magnitude less abundant than glycolaldehyde (CH2OHCHO).

Precise rest frequencies for AlH and AlD (X 1Σ+): A reassessment of the aluminum hyperfine structure

The J = 0 → 1 transition of AlH at 377 GHz and the J = 1 → 2 line of AlD at 393 GHz have been recorded using submillimeter direct absorption methods. The two species were created by the reaction of aluminum vapor and H2 or D2 gas in a DC discharge. The 27Al quadrupole hyperfine splittings, which are critical for the astronomical identification of these molecules, were measured for both AlH and AlD. In the case of AlH, the observed hyperfine pattern was not consistent with previous measurements. Values of the hyperfine parameters have thus been revised to be eQq = -48.59 MHz and CI = 0.306 MHz, which agree very closely with recent ab initio predictions. For AlD, eQq was established to be -48.48 MHz with CI = 0.156 MHz—the first determination of the hyperfine parameters for the molecule. These measurements should enable viable astronomical searches to be conducted for AlH and AlD, presumably with space-borne platforms. Diatomic hydrides containing a cosmically abundant element such as aluminum are fundamental for interstellar and circumstellar chemistry.