Rare Isotopic Species Research Articles

Rotational detection of the silicon-carbon chains H2C3Si and HSiCCH

The rotational spectra of two new silicon-bearing carbon chains, H2C3Si and HSiCCH have been detected by means of Fourier-transform microwave spectroscopy in a supersonic jet source equipped with an electrical discharge. On the basis of measurements between 10 and 42GHz, precise spectroscopic constants have been determined for both molecules. Along with H29SiCCH and H30SiCCH, which were detected in natural abundance, several other rare isotopic species of HSiCCH were observed using samples enriched in carbon-13 and D. From this isotopic data, a precise semi-experimental equilibrium structure was derived and compared with a re-evaluated semi-experimental equilibrium structure of the parent silylene SiH2; the deuterium isotopic measurements also provide important clues as to the formation pathway of HSiCCH in our discharge. Because other small Si-bearing chains have been detected in the circumstellar envelope of the evolved carbon star IRC+10216, both chains may be of astronomical interest.

A continued investigation of carbon subsulfide, SCCCS, at 5 [formula omitted]m: New hot bands and isotopic species

The ν3 anti-symmetric stretching mode of linear carbon subsulfide, SCCCS, located at 2100 cm−1 has been reinvestigated at high-spectral resolution in the gas phase using a combination of laser ablation production, free jet expansion and quantum cascade laser detection. Following-up on pioneering studies from the literature (Holland et al., 1988; Holland and Winnewisser, 1991) that reported on the strong ν3 vibrational fundamental, related hot bands orignating from the energetically lowest bending mode ν7 and selected isotopic species, in the present investigation hot bands from the first excited bending modes ν5 and ν6 have been observed for the first time. In addition, using sample targets enriched in 13C permitted the detection of three new isotopic species near 2085 cm−1: S13CCCS, 34S13CCCS, and 34SCC13CS. Selected hot bands of rare isotopic species were also observed. The experimental work has been complemented by high-level CCSD(T) calculations supporting spectroscopic analysis.

Building blocks of dust: A coordinated laboratory and astronomical study of the archtype AGB carbon star IRC+10216

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon-rich Asymptotic Giant Branch stars, as illustrated by the archtype star IRC+10216. The increased sensitivity and angular resolution of high altitude, ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and thus to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra — both in the ground and vibrationally excited states of the normal and rare isotopic species as well — of the principal molecules in the inner region which appear to contain only two or three heavy atoms.Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeter-wave rotational lines that are observed in the inner envelope of IRC+10216. To assess whether vibrationally excited SiC2 and HCN are the carriers of some of the unassigned features observed in this source, millimeter-wave laboratory spectra are compared with Cycle 0 observations from the Atacama Large Millimeter-wave Array (ALMA) under experimental conditions that produce both species. Also highlighted are ongoing laboratory studies of the silicon carbides SiC2 and SiCSi in their ground and excited vibrational states, and c-SiC3 in its ground vibrational state. Following the initial detection of c-SiC3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J⩽48,Ka⩽3 and upper level energies Eu⩽484 K in the range from 178 to 391 GHz, and 35 new transitions of c-SiC3 with J⩽38,Ka⩽20 and Eu⩽875 K between 315 and 440 GHz were measured in the laboratory. In addition, five or six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi and the two lowest rotational transitions in the previously unexplored C–C stretching mode (ν1) of SiC2 were measured in the normal and doubly substituted 13C isotopic species. Areas of continued emphasis and the importance of complementary experimental methods and production techniques to detect and assign vibrational transitions are also highlighted.

Molecular polymorphism: microwave spectra, equilibrium structures, and an astronomical investigation of the HNCS isomeric family.

The rotational spectra of thioisocyanic acid (HNCS), and its three energetic isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution by a combination of chirped-pulse and Fabry-Pérot Fourier-transform microwave spectroscopy between 6 and 40 GHz in a pulsed-jet discharge expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid (HSNC), calculated here to be 35-37 kcal mol(-1) less stable than the ground state isomer HNCS, have been detected for the first time. Precise rotational, centrifugal distortion, and nitrogen hyperfine coupling constants have been determined for the normal and rare isotopic species of both molecules; all are in good agreement with theoretical predictions obtained at the coupled cluster level of theory. On the basis of isotopic spectroscopy, precise molecular structures have been derived for all four isomers by correcting experimental rotational constants for the effects of rotation-vibration interaction calculated theoretically. Formation and isomerization pathways have also been investigated; the high abundance of HSCN relative to ground state HNCS, and the detection of strong lines of SH using CH3CN and H2S, suggest that HSCN is preferentially produced by the radical-radical reaction HS + CN. A radio astronomical search for HSCN and its isomers has been undertaken toward the high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m Green Bank Telescope. While we find clear evidence for HSCN, only a tentative detection of HNCS is proposed, and there is no indication of HCNS or HSNC at the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations from a single-excitation temperature model, suggesting weak masing may be occurring in some transitions in this source.

Rotational Spectroscopy of Isotopologues of Silicon Monoxide, SiO, and Spectroscopic Parameters from a Combined Fit of Rotational and Rovibrational Data

Pure rotational transitions of silicon monoxide, involving the main ((28)Si(16)O) as well as several rare isotopic species, were observed in their ground vibrational states by employing long-path absorption spectroscopy between 86 and 825 GHz (1 ≤ J" ≤ 18). Fourier transform microwave spectroscopy was used to study the J" = 0 transition frequencies in the ground and several vibrationally excited states. The vibrational excitation of the newly studied isotopologues extend to between υ = 9 and 29 for (28)Si(17)O and (30)Si(16)O, respectively. Data were extended for some previously investigated species up to υ = 51 for the main isotopologue. The high spectral resolution allowed us to resolve the hyperfine structure in (28)Si(17)O caused by the nuclear electric quadrupole and magnetic dipole moments of (17)O for the first time, and to resolve the much smaller nuclear spin-rotation splitting for isotopic species containing (29)Si. These data were combined with previous rotational and rovibrational (infrared) data to determine an improved set of spectroscopic parameters of SiO in one global fit which takes the breakdown of the Born-Oppenheimer approximation into account. Highly accurate rotational transition frequencies for this important astronomical molecule can now be predicted well into the terahertz region with this parameter set. In addition, a more complete comparison among physical properties of group 14/16 diatomics is possible.

Rotational spectra of rare isotopic species of fluoroiodomethane: Determination of the equilibrium structure from rotational spectroscopy and quantum-chemical calculations

Supported by accurate quantum-chemical calculations, the rotational spectra of the mono- and bi-deuterated species of fluoroiodomethane, CHDFI and CD(2)FI, as well as of the (13)C-containing species, (13)CH(2)FI, were recorded for the first time. Three different spectrometers were employed, a Fourier-transform microwave spectrometer, a millimeter/submillimter-wave spectrometer, and a THz spectrometer, thus allowing to record a huge portion of the rotational spectrum, from 5 GHz up to 1.05 THz, and to accurately determine the ground-state rotational and centrifugal-distortion constants. Sub-Doppler measurements allowed to resolve the hyperfine structure of the rotational spectrum and to determine the complete iodine quadrupole-coupling tensor as well as the diagonal elements of the iodine spin-rotation tensor. The present investigation of rare isotopic species of CH(2)FI together with the results previously obtained for the main isotopologue [C. Puzzarini, G. Cazzoli, J. C. López, J. L. Alonso, A. Baldacci, A. Baldan, S. Stopkowicz, L. Cheng, and J. Gauss, J. Chem. Phys. 134, 174312 (2011); G. Cazzoli, A. Baldacci, A. Baldan, and C. Puzzarini, Mol. Phys. 109, 2245 (2011)] enabled us to derive a semi-experimental equilibrium structure for fluoroiodomethane by means of a least-squares fit procedure using the available experimental ground-state rotational constants together with computed vibrational corrections. Problems related to the missing isotopic substitution of fluorine and iodine were overcome thanks to the availability of an accurate theoretical equilibrium geometry (computed at the coupled-cluster singles and doubles level augmented by a perturbative treatment of triple excitations).

Dense molecular gas in nearby gas-rich active galaxies★

We present 3 mm observations obtained with the IRAM 30-m telescope for ten nearby gas-rich active galaxies spanning three orders of magnitude in infrared luminosity. Emission lines of HCO+(1-0), HCN(1-0), and C2H(1-0) were simultaneously detected in most galaxies of our sample. We also tentatively detected the rare isotopic species, H13CN, HC15N and H13CO+ in up to four galaxies (IC 694, NGC 3690, NGC 4258 and NGC 6240). Our estimation shows that the optical depth of HCN is low to moderate in these galaxies (tau ~ 1-5). After comparing the intensity ratios of different molecular emission lines (HCN/HCO+, C2H/HCN, and C2H/HCO+) with the infrared luminosity (LIR), we find that in the infrared luminous sample of five composite (AGN+Starburst) galaxies, these ratios vary with LIR: the HCN/HCO+ ratio increases with LIR, which is consistent with previous studies, while there is a slight trend in the composite galaxies that both C2H/HCN and C2H/HCO+ ratios decrease with increasing LIR. Although our sample is limited, this trend may possibly imply a relation between the overall gas density and the infrared luminosity of galaxies.

Rotational spectra and equilibrium structures of H2SiS and Si2S

The rotational spectra of two small silicon sulfides, silanethione H(2)SiS and the disilicon sulfide ring Si(2)S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H(2)SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (r(e)) structures of H(2)SiS and Si(2)S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The r(e) structures agree to within 5×10(-4) Å and 0.1(∘) with empirical equilibrium (r(e)(emp)) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H(2)SiS and Si(2)S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS.

Detection of Free Monomeric Silver(I) and Gold(I) Cyanides, AgCN and AuCN: Microwave Spectra and Molecular Structure

The chemistry of the group 11 metal cyanide system has been of considerable interest because of the commercial importance of some of the complexes formed in this system. These metal cyanides contain one-dimensional linear -M-CN-M-CN-M- chains in the solid state; however, they have not been observed as free monomeric species. This Article reports the first detection of monomeric AgCN and AuCN in the gas phase by using rotational spectroscopy. The sputtering reaction of silver and/or gold sheets placed on a stainless steel cathode with CH3CN diluted in Ar resulted in the production of AgCN and AuCN spectra. Spectroscopic observation of the parent and several rare isotopic species allowed the determination of r(0), r(s), r(Iepsilon), r(m)(1), and r(m)(2) structures in the case of AgCN and r(0) and r(s) structures in the case of AuCN. All data indicate that these two species have a linear MCN geometry with a low-energy bending vibration.

A laboratory and theoretical study of silicon hydroxide SiOH

The rotational spectrum of the triatomic free radical SiOH in its X (2)A(') ground electronic state has been observed in a supersonic molecular beam by Fourier transform microwave spectroscopy. The fundamental (1(0,1)-->0(0,0)) transition has been detected for normal SiOH and for three rare isotopic species: (30)SiOH, Si(18)OH, and SiOD. The same transition has also been observed in two of three excited vibrational states, v(2) and v(3), for the most abundant species. Precise spectroscopic constants, including those that describe the effective spin doubling and hydrogen hyperfine structure, have been derived for each isotopic species or vibrational state. To complement the laboratory work, theoretical calculations of the structure, dipole moment, and energies of the X (2)A(') and low-lying 1 (2)A(") states have also been undertaken at the coupled cluster level of theory. In agreement with theoretical predictions, we conclude from the hyperfine constants that SiOH is a best described as a pi-type radical, with the unpaired electron localized on a p orbital on the silicon atom. Assuming a bond angle of 118.5 degrees , the Si-O bond length is 1.647(2) A and the O-H bond length is 0.969(4) A.

The Rotational Spectrum of TiO2

The rotational spectrum of titanium dioxide (TiO2) has been detected by laser-ablation molecular beam Fourier transform microwave spectroscopy. Thirteen b-type rotational transitions up to J = 9 and Ka = 4 were measured for the most abundant isotopic species 48TiO2 in the frequency range 7-42 GHz with accuracies of 1-10 kHz, allowing for the precise determination of rotational and centrifugal distortion parameters. In addition, eight and six rotational transitions of the rare isotopic species 46TiO2 and 50TiO2, respectively, have been recorded in the same frequency range. From the derived spectroscopic parameters, rest frequencies of TiO2 can now be calculated to better than 1 km s−1 in equivalent radial velocity up to fairly high J and Ka at millimeter wavelengths, enabling radioastronomical searches for this stable, highly polar transition metal dioxide in space. Preliminary results of two searches for TiO2 in space are presented.

Rotational spectra and hyperfine structure of isotopic species of deuterated cyanoacetylene, DC 3N

Cyanoacetylene enriched in deuterium was used to record pure rotational transitions of DC 3N and its rare 13C and 15N isotopic species by employing Fourier transform microwave (FTMW) spectroscopy on a supersonic-jet expansion at centimeter wavelengths (8.2–25.4 GHz) and by using long-path absorption spectroscopy at millimeter and submillimeter wavelengths (82–900 GHz). In addition, submillimeter wave measurements (304–897 GHz) have been performed for DC 3N in its v 7 = 1 lowest excited vibrational state. Hyperfine structure caused by the 14N and D nuclei has been resolved in the FTMW spectra. Quantum-chemical calculations have been performed on the hyperfine structure parameters at the coupled-cluster level (CCSD(T)) with up to core-polarized quintuple zeta basis-sets and were found to be in excellent agreement with experimental data as far as these were available. Vibrational corrections on the quadrupole coupling parameters have been evaluated at the MP2 level employing a quadruple zeta basis-set. The present and previous data were fit simultaneously to obtain accurate predictions for the identification of these species in space.

Rotational spectra of rare isotopic species of bromofluoromethane: Determination of the equilibrium structure fromab initiocalculations and microwave spectroscopy

Guided by theoretical predictions, the rotational spectra of the mono- and bideuterated species of bromofluoromethane, CDH(79)BrF, CDH(81)BrF, CD(2) (79)BrF, and CD(2) (81)BrF, have been recorded for the first time. Assignment of a few hundred rotational transitions led to the accurate determination of the ground-state rotational constants, all of the quartic and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole-coupling tensor for both (79)Br and (81)Br, in good agreement with corresponding theoretical predictions based on high-level coupled-cluster calculations. The rotational spectra of the (13)C containing species (13)CH(2) (79)BrF and (13)CH(2) (81)BrF have been observed in natural abundance and have been assigned, thus allowing the determination of the rotational and centrifugal distortion constants as well as the bromine quadrupole-coupling tensor. Furthermore, empirical equilibrium structures have been obtained within a least-squares fit procedure using the available experimental ground-state rotational constants for various isotopic species. Vibrational effects have been accounted for in the analysis using vibration-rotation interaction constants derived from anharmonic force fields computed at the second-order Moller-Plesset perturbation theory as well as coupled-cluster (CC) levels. The empirical equilibrium geometries obtained in this way agree well with the corresponding theoretical predictions obtained from CC calculations [at the CCSD(T) level] after extrapolation to the complete basis set limit and inclusion of core-valence correlation corrections and relativistic effects.

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J''< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J'' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

Density functional theoretical (DFT) study for the prediction of spectroscopic parameters of ClCCCN

DFT(B3LYP, B3PW91) calculations in conjunction with three different basis sets have been utilized to investigate the variations in the bond lengths, dipole moment, rotational constants, IR frequencies, IR intensities and rotational invariants of ClCCCN. The nuclear quadrupole constants of chlorine (35Cl, 37Cl) and nitrogen (14N) of ClCCCN have been calculated on the experimental rs structure as well as on the B3PW91/6–311++g(d,p) optimized geometry and were found to be within the scale length of the experimental uncertainty. The slope and intercept obtained from the regression analysis between the B3LYP/6–311++g(d,p) level calculated and experimental Bo values of ClCCCN were used to calculate reasonable values of rotational constants of all the rare isotopic species of ClCCCN having standard deviation ±0.048MHz. All the spectroscopic parameters obtained from DFT calculations show satisfactory agreement with the available experimental data.

Rotational spectroscopy of S 2O: Vibrational satellites, 33S isotopomers, and the sub-millimeter-wave spectrum

The rotational spectra of disulfur monoxide, S 2O, and several of its rare isotopic species have been studied in a supersonic molecular beam by Fourier transform microwave spectroscopy. The strongest lines of S 2O were observed by mixing molecular oxygen with sulfur vapor from a heated reservoir at 190 °C, and were so intense that both 33SSO and S 33SO were detected in natural abundance, as well as new rotational transitions of 34SSO and S 34SO. Vibrationally excited states ( v 1, v 2, v 3) up to (0,8,0), combination states (0, v 2,1) up to (0,4,1), and the (1,0,0) and (0,0,1) states were also observed in a discharge of SO 2 in neon. In addition, the sub-millimeter-wave spectrum of 32S 2O has been studied up to 470 GHz in a free space absorption cell through a discharge of sulfur vapor and SO 2. An extensive set of molecular parameters for the main isotopic species has been obtained by analyzing all of the presently available rotational and rotation–vibration data. Detection of several new transitions of the SO dimer S 2O 2 at centimeter wavelengths has yielded an improved set of molecular parameters for its ground vibrational state.

Geometrical structure of monobridged disilyene Si(H)SiH

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the more abundant rare isotopic species of monobridged Si(H)SiH, and determined of its geometrical structure by isotopic substitution. Unexpected abundance anomalies of the singly deuterated isotopic species with respect to normal and Si(D)SiD were observed with equal mixtures of SiH4 and SiD4. This nonstatistical distribution is consistent with Si(H)SiH formation which is dominated by the reaction of a bare Si atom or ion with either SiH2 or SiH4. Dedicated laboratory searches were also undertaken for the remaining singlet isomer, H2SiSi, so far without success. From the absence of lines at the expected frequencies we conclude that H2SiSi is at least 50 times less abundant than Si(H)SiH. Because it is only slightly less stable than Si(H)SiH and because the barrier to isomerization is calculated to be quite low, H2SiSi may rearrange on the timescale of the supersonic expansion to Si(H)SiH.

Submillimeter-wave spectrum of carbonyl sulfide: rare isotopic species

Rotational spectra of four rare isotopic species of OCS were observed in the 110–690GHz region with a source-modulated submillimeter-wave spectrometer. Spectral lines of 18O13CS, O13C36S, 18OC36S, and 18O13C34S were measured in their natural abundances, 21.4, 1.51, 0.29, and 0.95ppm, respectively. The rotational constants and centrifugal distortion constants were precisely determined from observed line frequencies by least squares methods. In order to facilitate sensitivity examination in the submillimeter-wave region, transition frequencies and peak absorption coefficients were calculated and tabulated for the spectral lines of the four species up to the 1000GHz region.

The Radio Spectrum of Si 3

The silicon trimer Si3 and several of its rare isotopic species have been detected in the laboratory, and the frequencies of 66 rotational transitions in the range 5-42 GHz have been measured to a few parts in 107. The most intense radio lines can be calculated from the derived spectroscopic constants to better than 3 km s-1 in equivalent radial velocity up to frequencies of 50 GHz, adequate for a deep search in the best astronomical sources; above that frequency the uncertainties increase to of the order of 20 km s-1 at 160 GHz, which is still sufficient for a deep search in a wide-line source such as IRC +10216.

The Rotational Spectra of H2CCSi and H2C4Si

The microwave rotational spectra of two singlet chains H2CCSi and H2C4Si, have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. Following detection of the singly substituted rare isotopic species and D2CCSi, an experimental structure (r0) has been determined to high accuracy for H2CCSi by isotopic substitution. In addition, the rotational transitions of the 29Si and the two 13C isotopic species of H2CCSi exhibit nuclear spin–rotation hyperfine structure. The component of the spin–rotation tensor along the a-inertial axis is abnormally large for each of these isotopic species, especially for that with 29Si, where the magnitude of Caa is in excess of 700 kHz.