Phosphorus-bearing molecules PO and PN at the edge of the Galaxy.

Despite its importance in planet formation and biology1, phosphorus has been identified only in the inner 12 kpc of the Galaxy2-19. The study of this element has been hindered in part by unfavourable atomic transitions2,4,20. Phosphorus is thought to be created by neutron capture on 29Si and 30Si in massive stars20,21, and released into the interstellar medium by Type II supernova explosions2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production23 to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre24. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System25. Supernovae are not present in the Outer Galaxy26, suggesting another source of phosphorus, such as 'Galactic Fountains', where supernova material is redistributed through the halo and circumgalactic medium27. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich28. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars29 which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon21, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.

The millimeter-wave spectrum of the SiP radical (X2Πi): Rotational perturbations and hyperfine structure.

The millimeter/submillimeter-wave spectrum of the SiP radical (X2Πi) has been recorded using direct absorption spectroscopy in the frequency range of 151-532 GHz. SiP was synthesized in an AC discharge from the reaction of SiH4 and gas-phase phosphorus, in argon carrier gas. Both spin-orbit ladders were observed. Fifteen rotational transitions were measured originating in the Ω = 3/2 ladder, and twelve in the Ω = 1/2 substate, each exhibiting lambda doubling and, at lower frequencies, hyperfine interactions from the phosphorus nuclear spin of I = 1/2. The lambda-doublets in the Ω = 1/2 levels appeared to be perturbed at higher J, with the f component deviating from the predicted pattern, likely due to interactions with the nearby excited A2Σ+ electronic state, where ΔEΠ-Σ âˆ¼ 430 cm-1. The data were analyzed using a Hund's case aß Hamiltonian and rotational, spin-orbit, lambda-doubling, and hyperfine parameters were determined. A 2Π/2Σ deperturbation analysis was also performed, considering spin-orbit, spin-electronic, and L-uncoupling interactions. Although SiP is clearly not a hydride, the deperturbed parameters derived suggest that the pure precession hypothesis may be useful in assessing the 2Π/2Σ interaction. Interpretation of the Fermi contact term, bF, the spin-dipolar constant, c, and the nuclear spin-orbital parameter, a, indicates that the orbital of the unpaired electron is chiefly pπ in character. The bond length in the v = 0 level was found to be r0 = 2.076 Å, suggestive of a double bond between the silicon and phosphorus atoms.

A nanometric window on fullerene formation in the interstellar medium: Insights from molecular dynamics studies.

Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules, such as C60 from graphite, which is known to occur in the ISM, in particular, circumstellar environments. Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures, such as pristine and metastable C60 molecules. The pathways leading to molecular C60 formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone-Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie C60 formation. Moreover, they clearly demonstrate that top-down synthesis of C60 from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter.

The structure of ScC2 (X̃2A1): A combined Fourier transform microwave/millimeter-wave spectroscopic and computational study.

Pure rotational spectra of Sc13C2 (X̃2A1) and Sc12C13C (X̃2A') have been measured using Fourier transform microwave/millimeter-wave methods. These molecules were synthesized in a DC discharge from the reaction of scandium vapor, produced via laser ablation, with 13CH4 or 13CH4/12CH4, diluted in argon. The NKa,Kc = 10,1 → 00,0, 20,2 → 10,1, 30,3 → 20,2, and 40,4 → 30,3 transitions in the frequency range of 14 GHz-61 GHz were observed for both species, each exhibiting hyperfine splittings due to the nuclear spins of 13C (I = 1/2) and/or Sc (I = 7/2). These data have been analyzed with an asymmetric top Hamiltonian, and rotational, spin-rotation, and hyperfine parameters have been determined for Sc13C2 and Sc12C13C. In addition, a quartic force field was calculated for ScC2 and its isotopologues using a highly accurate coupled cluster-based composite method, incorporating complete basis set extrapolation, scalar relativistic corrections, outer core and inner core electron correlation, and higher-order valence correlation effects. The agreement between experimental and computed rotational constants, including the effective constant (B + C), is ∼0.5% for all three isotopologues. This remarkable agreement suggests promise in predicting rotational spectra of new transition metal-carbon bearing molecules. In combination with previous work on Sc12C2, an accurate structure for ScC2 has been established using combined experimental (B, C) and theoretical (A) rotational constants. The radical is cyclic (or T-shaped) with r(Sc-C) = 2.048(2) Å, r(C-C) = 1.272(2) Å, and ∠(C-Sc-C) = 36.2(1)°. The experimental and theoretical results also suggest that ScC2 contains a C2 - moiety and is largely ionic.

Millimeter/sub-mm spectroscopy of the CrBr radical in the high spin X6Σ+ state.

The millimeter/submillimeter spectrum of the CrBr radical has been recorded in the frequency range of 220-300 GHz using direct absorption techniques, utilizing a new instrumental design. This study is the first spectroscopic investigation of this radical species by any method. CrBr was synthesized in a DC discharge by the reaction of chromium vapor, produced in a Broida-type oven, with Br2CH2 in argon. Six to nine rotational transitions were measured for four isotopologues of this molecule in their natural abundances, 52Cr79Br, 52Cr81Br, 53Cr79Br, and 53Cr81Br. Each transition was found to consist of six distinct fine structure components, indicating a 6Σ+ ground electronic state, as observed for CrF and CrCl. Lines originating in the v = 1 and 2 vibrational states were recorded for 52Cr79Br and 52Cr81Br as well. The spectra were analyzed using a Hund's case (b) Hamiltonian, and rotational, spin-spin, and spin-rotation parameters were determined. The third-order spin-rotation constant γs and the fourth order spin-spin term θ were necessary for the analysis; these parameters are thought to play a role in states with high multiplicities. Equilibrium parameters were also derived for the CrBr; a bond length of re = 2.337 282 (30) Å and a vibrational constant of ωe ≅ 300 cm-1 were determined. The sign and magnitude of the spin-spin and spin-rotation constants suggest the presence of nearby 4Π and 6Π excited states in CrBr, lying ∼9000 cm-1 above the ground state. The new instrument design, employing more compact, free-space optics utilizing an offset ellipsoidal mirror, facilitated these measurements.

The pure rotational spectrum of the ZnBr radical (X2Σ+): Trends in the zinc halide series.

The pure rotational spectrum of ZnBr (X2Σ+) has been recorded in the frequency range 259-310 GHz using millimeter-wave direct absorption techniques. This study is the first quantitative spectroscopic investigation of this free radical. ZnBr was synthesized in a DC discharge by the reaction of zinc vapor in argon with one of three reagents: BrCH3, Br2CH2, or Br2. Eight rotational transitions were measured for six isotopologues (64Zn79Br, 64Zn81Br, 66Zn79Br, 66Zn81Br, 68Zn79Br, and 68Zn81Br), all of which exhibited spin-rotation interactions. Furthermore, transitions originating in the v = 1 through 3 excited vibrational states were obtained for certain isotopologues. Five rotational transitions were also recorded for 67Zn79Br, in which hyperfine splittings were observed arising from the 67Zn nucleus (I = 5/2). The spectra were analyzed using a Hund's case (bßJ) Hamiltonian, and rotational, spin-rotation, and 67Zn magnetic hyperfine constants were determined. Equilibrium parameters were also derived for the 64Zn79Br, 64Zn81Br, 66Zn79Br, and 66Zn81Br isotopologues, including the vibrational constant, ωe = 286 cm-1. The equilibrium bond length was derived to be re = 2.268 48(90) Å. Analysis of the 67Zn hyperfine parameters suggest a decrease in ionic character in ZnBr from the other known zinc halides, ZnF and ZnCl.

Extreme 13C,15N and 17O isotopic enrichment in the young planetary nebula K4-47.

Carbon, nitrogen and oxygen are the three most abundant elements in the Galaxy after hydrogen and helium. Whereas hydrogen and helium were created in the Big Bang, carbon, nitrogen and oxygen arise from nucleosynthesis in stars. Of particular interest1,2 are the isotopic ratios 12C/13C, 14N/15N and 16O/17O because they are effective tracers of nucleosynthesis and help to benchmark the chemical processes that occurred in primitive interstellar material as it evolved into our Solar System3. However, the origins of the rare isotopes 15N and 17O remain uncertain, although novae and very massive stars that explode as supernovae are postulated4-6 to be the main sources of 15N. Here we report millimetre-wavelength observations of the young bipolar planetary nebula K4-47 that indicate another possible source for these isotopes. We identify various carbon-bearing molecules in K4-47 that show that this object is carbon-rich, and find unusually high enrichment in rare carbon (13C), oxygen (17O) and nitrogen (15N) isotopes: 12C/13C = 2.2 ± 0.8, 16O/17O = 21.4 ± 10.3 and 14N/15N = 13.6 ± 6.5 (uncertainties are three standard deviations); for comparison, the corresponding solar ratios7 are 89.4 ± 0.2, 2,632 ± 7 and 435 ± 57. One possible interpretation of these results is that K4-47 arose from a J-type asymptotic giant branch star that underwent a helium-shell flash (an explosive nucleosynthetic event that converts large quantities of helium to carbon and other elements), enriching the resulting planetary nebula in 15N and 17O and creating its bipolar geometry. Other possible explanations are that K4-47 is a binary system or that it resulted from a white dwarf merger, as has been suggested for object CK Vul8. These results suggest that nucleosynthesis of carbon, nitrogen and oxygen is not well understood and that the classification of certain stardust grains must be reconsidered.

The pure rotational spectrum of the T-shaped AlC2 radical (X[combining tilde]2A1).

The pure rotational spectrum of the AlC2 radical (X[combining tilde]2A1) has been measured using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the frequency range 21-65 GHz. This study is the first high-resolution spectroscopic investigation of this molecule. AlC2 was created in a supersonic jet from the reaction of aluminum, generated by laser ablation, with a mixture of CH4 or HCCH, diluted in argon, in the presence of a DC discharge. Three transitions (NKa,Kc = 101 → 000, 202 → 101, and 303 → 202) were measured, each consisting of multiple fine/hyperfine components, resulting from the unpaired electron in the species and the aluminum-27 nuclear spin (I = 5/2). The data were analyzed using an asymmetric top Hamiltonian and rotational, fine structure, and hyperfine constants determined. These parameters agree well with those derived from previous theoretical calculations and optical spectra. An r0 structure of AlC2 was determined with r(Al-C) = 1.924 Å, r(C-C) = 1.260 Å, and θ(C-Al-C) = 38.2°. The Al-C bond was found to be significantly shorter than in other small, Al-bearing species. The Fermi contact term established in this work indicates that the unpaired electron in the valence orbital has considerable 3pza1 character, suggesting polarization towards the C2 moiety. A high degree of ionic character in the molecule is also evident from the quadrupole coupling constant. These results are consistent with a T-shaped geometry and an Al+C2- bonding scheme. AlC2 is a possible interstellar molecule that may be present in the circumstellar envelopes of carbon-rich AGB stars.

Examining transition metal hydrosulfides: The pure rotational spectrum of ZnSH (X̃2A').

The pure rotational spectrum of the ZnSH (X̃2A') radical has been measured using millimeter-wave direct absorption and Fourier transform microwave (FTMW) methods across the frequency range 18-468 GHz. This work is the first gas-phase detection of ZnSH by any spectroscopic technique. Spectra of the 66ZnSH, 68ZnSH, and 64ZnSD isotopologues were also recorded. In the mm-wave study, ZnSH was synthesized in a DC discharge by the reaction of zinc vapor, generated by a Broida-type oven, with H2S; for FTMW measurements, the radical was made in a supersonic jet expansion by the same reactants but utilizing a discharge-assisted laser ablation source. Between 7 and 9 rotational transitions were recorded for each isotopologue. Asymmetry components with Ka = 0 through 6 were typically measured in the mm-wave region, each split into spin-rotation doublets. In the FTMW spectra, hyperfine interactions were also resolved, arising from the hydrogen or deuterium nuclear spins of I = 1/2 or I = 1, respectively. The data were analyzed using an asymmetric top Hamiltonian, and rotational, spin-rotation, and magnetic hyperfine parameters were determined for ZnSH, as well as the quadrupole coupling constant for ZnSD. The observed spectra clearly indicate that ZnSH has a bent geometry. The rm(1) structure was determined to be rZn-S = 2.213(5) Å, rS-H = 1.351(3) Å, and θZn-S-H = 90.6(1)°, suggesting that the bonding occurs primarily through sulfur p orbitals, analogous to H2S. The hyperfine constants indicate that the unpaired electron in ZnSH primarily resides on the zinc nucleus.

Following the Interstellar History of Carbon: From the Interiors of Stars to the Surfaces of Planets.

The chemical history of carbon is traced from its origin in stellar nucleosynthesis to its delivery to planet surfaces. The molecular carriers of this element are examined at each stage in the cycling of interstellar organic material and their eventual incorporation into solar system bodies. The connection between the various interstellar carbon reservoirs is also examined. Carbon has two stellar sources: supernova explosions and mass loss from evolved stars. In the latter case, the carbon is dredged up from the interior and then ejected into a circumstellar envelope, where a rich and unusual C-based chemistry occurs. This molecular material is eventually released into the general interstellar medium through planetary nebulae. It is first incorporated into diffuse clouds, where carbon is found in polyatomic molecules such as H2CO, HCN, HNC, c-C3H2, and even C60+. These objects then collapse into dense clouds, the sites of star and planet formation. Such clouds foster an active organic chemistry, producing compounds with a wide range of functional groups with both gas-phase and surface mechanisms. As stars and planets form, the chemical composition is altered by increasing stellar radiation, as well as possibly by reactions in the presolar nebula. Some molecular, carbon-rich material remains pristine, however, encapsulated in comets, meteorites, and interplanetary dust particles, and is delivered to planet surfaces. Key Words: Carbon isotopes-Prebiotic evolution-Interstellar molecules-Comets-Meteorites. Astrobiology 16, 997-1012.

Millimeter-wave spectroscopy of CrC (X(3)Σ(-)) and CrCCH (X̃ (6)Σ(+)): Examining the chromium-carbon bond.

Pure rotational spectroscopy of the CrC (X(3)Σ(-)) and CrCCH (X̃ (6)Σ(+)) radicals has been conducted using millimeter/sub-millimeter direct absorption methods in the frequency range 225-585 GHz. These species were created in an AC discharge of Cr(CO)6 and either methane or acetylene, diluted in argon. Spectra of the CrCCD were also recorded for the first time using deuterated acetylene as the carbon precursor. Seven rotational transitions of CrC were measured, each consisting of three widely spaced, fine structure components, arising from spin-spin and spin-rotation interactions. Eleven rotational transitions were recorded for CrCCH and five for CrCCD; each transition in these cases was composed of a distinct fine structure sextet. These measurements confirm the respective (3)Σ(-) and (6)Σ(+) ground electronic states of these radicals, as indicated from optical studies. The data were analyzed using a Hund's case (b) Hamiltonian, and rotational, spin-spin, and spin-rotation constants have been accurately determined for all three species. The spectroscopic parameters for CrC were significantly revised from previous optical work, while those for CrCCH are in excellent agreement; completely new constants were established for CrCCD. The chromium-carbon bond length for CrC was calculated to be 1.631 Å, while that in CrCCH was found to be rCr-C = 1.993 Å - significantly longer. This result suggests that a single Cr-C bond is present in CrCCH, preserving the acetylenic structure of the ligand, while a triple bond exists in CrC. Analysis of the spin constants suggests that CrC has a nearby excited (1)Σ(+) state lying ∼16 900 cm(-1) higher in energy, and CrCCH has a (6)Π excited state with E ∼ 4800 cm(-1).

Prebiotic chemical evolution in the astrophysical context.

An ever increasing amount of molecular material is being discovered in the interstellar medium, associated with the birth and death of stars and planetary systems. Radio and millimeter-wave astronomical observations, made possible by high-resolution laboratory spectroscopy, uniquely trace the history of gas-phase molecules with biogenic elements. Using a combination of both disciplines, the full extent of the cycling of molecular matter, from circumstellar ejecta of dying stars - objects which expel large amounts of carbon - to nascent solar systems, has been investigated. Such stellar ejecta have been found to exhibit a rich and varied chemical content. Observations demonstrate that this molecular material is passed onto planetary nebulae, the final phase of stellar evolution. Here the star sheds almost its entire original mass, becoming an ultraviolet-emitting white dwarf. Molecules such as H2CO, HCN, HCO(+), and CCH are present in significant concentrations across the entire age span of such nebulae. These data suggest that gas-phase polyatomic, carbon-containing molecules survive the planetary nebula phase and subsequently are transported into the interstellar medium, seeding the chemistry of diffuse and then dense clouds. The extent of the chemical complexity in dense clouds is unknown, hindered by the high spectral line density. Organic species such as acetamide and methyl amine are present in such objects, and NH2CHO has a wide Galactic distribution. However, organophosphorus compounds have not yet been detected in dense clouds. Based on carbon and nitrogen isotope ratios, molecular material from the ISM appears to become incorporated into solar system planetesimals. It is therefore likely that interstellar synthesis influences prebiotic chemistry on planet surfaces.

Trends in alkali metal hydrosulfides: a combined Fourier transform microwave/millimeter-wave spectroscopic study of KSH (X1A').

The pure rotational spectrum of KSH (X(1)A') has been measured using millimeter-wave direct absorption and Fourier transform microwave (FTMW) techniques. This work is the first gas-phase experimental study of this molecule and includes spectroscopy of KSD as well. In the millimeter-wave system, KSH was synthesized in a DC discharge from a mixture of potassium vapor, H2S, and argon; a discharge-assisted laser ablation source, coupled with a supersonic jet expansion, was used to create the species in the FTMW instrument. Five and three rotational transitions in the range 3-57 GHz were recorded with the FTMW experiment for KSH and KSD, respectively, in the K(a) = 0 component; in these data, potassium quadrupole hyperfine structure was observed. Five to six transitions with K(a) = 0-5 were measured in the mm-wave region (260-300 GHz) for the two species. The presence of multiple asymmetry components in the mm-wave spectra indicates that KSH has a bent geometry, in analogy to other alkali hydrosulfides. The data were analyzed with an S-reduced asymmetric top Hamiltonian, and rotational, centrifugal distortion, and potassium electric quadrupole coupling constants were determined for both isotopolgues. The r0 geometry for KSH was calculated to be r(S-H) = 1.357(1) Å, r(K-S) = 2.806(1) Å, and θ(M-S-H) (°) = 95.0 (1). FTMW measurements were also carried out on LiSH and NaSH; metal electric quadrupole coupling constants were determined for comparison with KSH. In addition, ab initio computations of the structures and vibrational frequencies at the CCSD(T)/6-311++G(3df,2pd) and CCSD(T)/aug-cc-pVTZ levels of theory were performed for LiSH, NaSH, and KSH. Overall, experimental and computational data suggest that the metal-ligand bonding in KSH is a combination of electrostatic and covalent forces.

The impact of recent advances in laboratory astrophysics on our understanding of the cosmos.

An emerging theme in modern astrophysics is the connection between astronomical observations and the underlying physical phenomena that drive our cosmos. Both the mechanisms responsible for the observed astrophysical phenomena and the tools used to probe such phenomena-the radiation and particle spectra we observe-have their roots in atomic, molecular, condensed matter, plasma, nuclear and particle physics. Chemistry is implicitly included in both molecular and condensed matter physics. This connection is the theme of the present report, which provides a broad, though non-exhaustive, overview of progress in our understanding of the cosmos resulting from recent theoretical and experimental advances in what is commonly called laboratory astrophysics. This work, carried out by a diverse community of laboratory astrophysicists, is increasingly important as astrophysics transitions into an era of precise measurement and high fidelity modeling.

The microwave and millimeter spectrum of ZnCCH (X̃2Σ+): a new zinc-containing free radical.

The pure rotational spectrum of the ZnCCH (X̃(2)Σ(+)) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7-260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05% acetylene in argon, using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene in a dc discharge. Thirteen rotational transitions were recorded for the main species, (64)ZnCCH, and between 4 and 10 for the (66)ZnCCH, (68)ZnCCH, (64)ZnCCD, and (64)Zn(13)C(13)CH isotopologues. The fine structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium, and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a (2)Σ Hamiltonian, and rotational, spin-rotation, and H, D, and (13)C hyperfine parameters have been established for this radical. From the rotational constants, an r(m) ((1)) structure was determined with r(Zn-C) = 1.9083 Å, r(C-C) = 1.2313 Å, and r(C-H) = 1.0508 Å. The geometry suggests that ZnCCH is primarily a covalent species with the zinc atom singly bonded to the C≡C-H moiety. This result is consistent with the hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The spin-rotation constant indicates that an excited (2)Π state may exist ∼19,000 cm(-1) in energy above the ground state.

The microwave and millimeter rotational spectra of the PCN radical (X3Σ-).

The pure rotational spectrum of the PCN radical (X(3)Σ(-)) has been measured for the first time using a combination of millimeter/submillimeter direct absorption and Fourier transform microwave (FTMW) spectroscopy. In the millimeter instrument, PCN was created by the reaction of phosphorus vapor and cyanogen in the presence of an ac discharge. A pulsed dc discharge of a dilute mixture of PCl(3) vapor and cyanogen in argon was the synthetic method employed in the FTMW machine. Twenty-seven rotational transitions of PCN and six of P(13)CN in the ground vibrational state were recorded from 19 to 415 GHz, all which exhibited fine structure arising from the two unpaired electrons in this radical. Phosphorus and nitrogen hyperfine splittings were also resolved in the FTMW data. Rotational satellite lines from excited vibrational states with v(2) = 1-3 and v(1) = 1 were additionally measured in the submillimeter range. The data were analyzed with a Hund's case (b) effective Hamiltonian and rotational, fine structure, and hyperfine constants were determined. From the rotational parameters of both carbon isotopologues, the geometry of PCN was established to be linear, with a P-C single bond and a C-N triple bond, structurally comparable to other non-metal main group heteroatom cyanides. Analysis of the hyperfine constants suggests that the two unpaired electrons reside almost exclusively on the phosphorus atom in a π(2) configuration, with little interaction with the nitrogen nucleus. The fine structure splittings in the vibrational satellite lines differ significantly from the pattern of the ground state, with the effect most noticeable with increasing v(2) quantum number. These deviations likely result from spin-orbit vibronic perturbations from a nearby (1)Σ(+) state, suggested by the data to lie ~12,000 cm(-1) above the ground state.

Millimeter-wave rotational spectroscopy of FeCN (X 4Δi) and FeNC (X 6Δi): determining the lowest energy isomer.

The pure rotational spectrum of FeCN has been recorded in the frequency range 140-500 GHz using millimeter/sub-millimeter direct absorption techniques. The species was created in an ac discharge of Fe(CO)(5) and cyanogen. Spectra of the (13)C, (54)Fe, and (57)Fe isotopologues were also measured, confirming the linear cyanide structure of this free radical. Lines originating from several Renner-Teller components in the ν(2) bending mode were also observed. Based on the observed spin-orbit pattern, the ground state of FeCN is (4)Δ(i), with small lambda-doubling splittings apparent in the Ω = 5/2, 3/2, and 1/2 components. In addition, a much weaker spectrum of the lowest spin-orbit component of FeNC, Ω = 9/2, was recorded; these data are consistent with the rotational parameters of previous optical studies. The data for FeCN were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling parameters were determined. Rotational constants were also established from a case (c) analysis for the other isotopologues, excited vibronic states, and for FeNC. The r(0) bond lengths of FeCN were determined to be r(Fe-C) = 1.924 Å and r(C-N) = 1.157 Å, in agreement with theoretical predictions for the (4)Δ(i) state. These measurements indicate that FeCN is the lower energy isomer and is more stable than FeNC by ~1.9 kcal/mol.

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.

The rotational spectrum of CuCCH(X̃ 1Σ+): 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.

The pure rotational spectrum of the CrS radical in its X 5Π(r) state.

The pure rotational spectrum of the CrS radical has been measured in its ground X (5)Π(r) state using gas-phase millimeter/submillimeter direct absorption methods. The molecule was created by the reaction of chromium vapor, sublimed in a Broida-type oven, with hydrogen sulfide. Eleven rotational transitions were recorded for this free radical in the frequency range of 280-405 GHz; in most transitions, all five spin components were observed, and lambda-doubling was resolved in the Ω=0, 1, and 2 ladders. The data were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling constants were established. Higher order spin and spin-orbit terms were essential in the analysis. The lambda-doubling constants indicate a nearby (5)Σ(+) state at an energy of ∼1500-2000 cm(-1). A bond length of 2.0781 Å was derived for CrS from the data, which is larger than the value of 2.0682 Å found for MnS by ∼0.01 Å. In contrast, the bond distance for MnO is greater than that of CrO by 0.03 Å, an illustration of the subtle differences between 3d oxide and sulfides. CrS is the second molecule in a (5)Π state that has been studied by rotational spectroscopy.