Rotational Spectroscopy of the Lowest Energy Conformer of 2-Cyanobutane.

Isopropyl cyanide was recently detected in space as the first branched alkyl compound. Its abundance with respect to n-propyl cyanide in the Galactic center source Sagittarius B2(N2) is about 0.4. Astrochemical model calculations suggest that for the heavier homologue butyl cyanide the branched isomers dominate over the unbranched n-butyl cyanide and that 2-cyanobutane is the most abundant isomer. We have studied the rotational spectrum of 2-cyanobutane between 2 and 24 GHz using Fourier transform microwave spectroscopy and between 36 and 402 GHz employing (sub)millimeter absorption spectroscopy. Transitions of the lowest energy conformer were identified easily. Its rotational spectrum is very rich, and the quantum numbers J and Ka reach values of 111 and 73, respectively. This wealth of data yielded rotational and centrifugal distortion parameters up to tenth order, diagonal and one off-diagonal 14N nuclear quadrupole coupling parameters, and one nuclear spin-rotation coupling parameter. We have also carried out quantum chemical calculations in part to facilitate the assignments. The molecule 2-cyanobutane was not found in the present ALMA data of Sagittarius B2(N2), but it may be found in the more sensitive data that have been completed very recently in the ALMA Cycle 4.

Calculated dipole moments for silicon and phosphorus compounds of astrophysical interest.

Dipole moments, which contribute to the intensities of rotational transitions and also affect reactivity, have been computed for nearly 80 compounds with up to 6 atoms containing silicon or phosphorus. More than a dozen of the set have been detected in interstellar or circumstellar media. The remaining ones are related to these, and several of them may be found in space in the future. We compare results from the commonly used B3LYP level of density functional theory with ab initio results at the coupled cluster CCSD(T)/RCCSD(T) levels of theory. Correlation consistent basis sets as large as quintuple ζ quality were used, and extrapolations to the estimated complete basis set (CBSE) limit were performed for almost all of the species with coupled cluster theory. In addition to evaluating the accuracy of the results against available data, we explore various issues: the critical importance of including diffuse basis functions, the range of basis set dependence exhibited by the suite of molecules, and the presence of low-lying excited states for some species. Dipole polarizabilities are also reported at the CCSD(T)/RCCSD(T) CBSE level.

Accurate high-N rest frequencies for CO+, an ideal tracer of photon-dominated regions.

The submillimeter-wave rotational spectra of CO(+), (13)CO(+) and C(18)O(+) in the v = 0 and 1 vibrational states were measured through a hollow cathode dc discharge in a cryogenic cell cooled to liquid nitrogen temperature. In addition, a few transitions of the main isotopic species have been measured between 1.1 and 1.3 THz. An updated isotopically invariant fit, including Born-Oppenheimer breakdown corrections, is presented: the derived set of independent molecular parameters, valid for all the isotopologues of the molecule included in the fit, allows to predict the rotational spectrum with calculated 1σ uncertainty of 280 kHz at 2 THz.

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.

High resolution spectral analysis of oxygen. I. Isotopically invariant Dunham fit for the X(3)Σ(g)(-), a(1)Δ(g), b(1)Σ(g)(+) states.

We have developed a simultaneous global fit to the MW, THz, infrared, visible, and UV transitions of all six oxygen isotopologues, (16)O(16)O, (16)O(17)O, (16)O(18)O, (17)O(17)O, (17)O(18)O, (18)O(18)O, with the objective of predicting all transitions below the O((3)P) + O((3)P) dissociation threshold as well as the B(3)Σ(u) (-) state from O((3)P)+O((1)D) within state-of-the-art experimental uncertainty. Here, we report an isotopically invariant Dunham fit for the lowest three electronic states, X(3)Σ(g)(-), a(1)Δ(g), and b(1)Σ(g)(+). Experimental transition frequencies involving these three states of all six O(2) isotopologues were critically reviewed and incorporated into the analysis. For the (16)O(16)O isotopologue, experimental data sample vibrational states v = 0-31 for X(3)Σ(g)(-), v = 0-10 for a(1)Δ(g), and v = 0-12 for b(1)Σ(g)(+). To the best of our knowledge, this is the first analysis that simultaneously fits spectra from all six O(2) isotopologues.

High resolution spectral analysis of oxygen. II. Rotational spectra of a(1)Δ(g) O2 isotopologues.

As part of a comprehensive review on molecular oxygen spectroscopy, we have measured rotational spectra of isotopic forms of molecular oxygen in its a(1)Δ(g) electronic state with high-resolution terahertz spectroscopy. The data are recorded in close proximity to predicted positions. Due to the high resolution and good signal-to-noise ratio, the fundamental hyperfine parameters eQq and C(I) are determinable for (17)O-substituted species for the first time. A refined nuclear spin orbit coupling constant, a = -211.9328(283) MHz, was determined, and is roughly two orders of magnitude more precise than values determined from near infrared spectroscopy or electron spin resonance studies. Vibrationally excited oxygen in the a(1)Δ(g) electronic state was also observable with small signal levels for many of the rotational transitions.

Tunneling dynamics and spectroscopic parameters of monodeuterated hydronium, H(2)DO(+), from a combined analysis of infrared and sub-millimeter spectra.

The infrared laser direct absorption spectrum of H(2)DO(+) in the OH stretching region was reported quite recently revealing large amplitude tunneling dynamics. The large rotational constants make the jet-cooled spectrum relatively sparse at low rotational temperatures and assignments thus challenging. Transitions were assigned through ground state combination differences, with additional tentative assignments made via comparison of predicted/observed spectra. More recently, 9 rotation-inversion transitions were recorded in the sub-millimeter (sub-mm) region, which yielded tunneling splittings and rotational constants differing slightly from IR results. This has prompted the present reinvestigation of the H(2)DO(+) spectra, which now takes full advantage of the combined data from both studies. While previous analyses considered each tunneling state as independent and non-interacting, the present analysis is based on a tunneling-Hamiltonian model for the well studied, isoelectronic NH(2)D molecule, modified to account for the larger tunneling splitting. The combined analysis revealed rotational interaction between tunneling states as well as between the two OH stretching modes and permitted a substantial number of new assignments to be made, including one sub-millimeter transition while only few IR assignments had to be corrected or omitted. It leads to improvement in parameters for both the ground as well as for the OH stretching states of this important molecular ion, which reproduce the assigned lines within experimental uncertainties, provides guidance, e.g., for the spectral search in the OD stretch region, and yields deeper insight into the tunneling dynamics.

Detection of a branched alkyl molecule in the interstellar medium: iso-propyl cyanide.

The largest noncyclic molecules detected in the interstellar medium (ISM) are organic with a straight-chain carbon backbone. We report an interstellar detection of a branched alkyl molecule, iso-propyl cyanide (i-C3H7CN), with an abundance 0.4 times that of its straight-chain structural isomer. This detection suggests that branched carbon-chain molecules may be generally abundant in the ISM. Our astrochemical model indicates that both isomers are produced within or upon dust grain ice mantles through the addition of molecular radicals, albeit via differing reaction pathways. The production of iso-propyl cyanide appears to require the addition of a functional group to a nonterminal carbon in the chain. Its detection therefore bodes well for the presence in the ISM of amino acids, for which such side-chain structure is a key characteristic.

Revised spectroscopic parameters of SH(+) from ALMA and IRAM 30m observations.

Hydrides represent the first steps of interstellar chemistry. Sulfanylium (SH(+)), in particular, is a key tracer of energetic processes. We used ALMA and the IRAM 30 m telescope to search for the lowest frequency rotational lines of SH(+) toward the Orion Bar, the prototypical photo-dissociation region illuminated by a strong UV radiation field. On the basis of previous Herschel/HIFI observations of SH(+), we expected to detect emission of the two SH(+) hyperfine structure (HFS) components of the NJ = 10-01 fine structure (FS) component near 346 GHz. While we did not observe any lines at the frequencies predicted from laboratory data, we detected two emission lines, each ~15 MHz above the SH(+) predictions and with relative intensities and HFS splitting expected for SH(+). The rest frequencies of the two newly detected lines are more compatible with the remainder of the SH(+) laboratory data than the single line measured in the laboratory near 346 GHz and previously attributed to SH(+). Therefore, we assign these new features to the two SH(+) HFS components of the NJ = 10-01 FS component and re-determine its spectroscopic parameters, which will be useful for future observations of SH(+), in particular if its lowest frequency FS components are studied. Our observations demonstrate the suitability of these lines for SH(+) searches at frequencies easily accessible from the ground.

High-resolution rotational spectroscopy in a cold ion trap: H2D+ and D2H+.

The lowest-lying 1(01) <-- 0(00) transition of para-H(2)D(+) and 1(11) <-- 0(00) of ortho-D(2)H(+) has been detected by the enhancement of the D/H isotope exchange reaction in collisions with p-H2 upon rotational excitation. These are the first pure rotational spectra of molecular ions by action spectroscopy. For this purpose, a cryogenic multipole ion trap has been combined with narrow-band tunable radiation sources operating in the 1.25 to 1.53 THz range. The low temperature of the ions allows us to determine the astronomically important transitions with a relative precision of Delta nu/nu=10(-8). While the 1 476 605.708(15) MHz line center frequency for the o-D(2)H(+) transition agrees very well with previous unpublished work, the 1 370 084.880(20) MHz line center frequency for the p-H(2)D(+) transition deviates by 61 MHz. Potential future applications of this new approach to rotational spectroscopy are discussed.

High resolution rotational spectroscopy on D2O up to 2.7 THz in its ground and first excited vibrational bending states.

We present highly accurate laboratory measurements on the pure rotational spectrum of doubly deuterated water, D2O, in selected frequency regions from 10 GHz up to 2.7 THz. Around 140 rotational transitions in both the vibrational ground and first excited bending states (upsilon2=0,1) were measured in total, involving energy levels with unexcelled high J and Ka rotational quantum numbers. The data give valuable information for the spectroscopic analysis of this molecule. In the case of the light and non-rigid water molecule, standard methods for its analysis are limited due to large centrifugal distortion interactions. Here, we present a global analysis of rotational and rovibrational data of the upsilon2=0 and 1 states of D2O by means of an Euler expansion of the Hamiltonian. In addition to the newly measured pure rotational transitions, around 4000 rotational and rovibrational lines have been included from previous work. It was possible to reproduce the extensive dataset to nearly its experimental uncertainty. The improved predictive capability of the model compared to previous work will be demonstrated.

Analysis of the rotational spectrum of methylene (CH2) in its vibronic ground state with an Euler expansion of the Hamiltonian.

We present an analysis of a global, field-free data set of the methylene radical CH2 in its X 3B1 vibronic ground state by means of a novel Euler expansion of the Hamiltonian. The data set comprises pure rotational transitions up to 2 THz obtained with microwave accuracies of 30-500 kHz as well as nu2 ground-state combination differences and pure rotational data obtained with infrared accuracies of 0.001-0.010 cm(-1). Highly accurate spectroscopic parameters have been determined. These include rotational, spin-spin, spin-rotation, and electron-spin-nuclear-spin coupling terms along with several centrifugal distortion corrections. The spectroscopic model has been tested and improved by recording newly three weak DeltaN not equalDeltaJ fine-structure components of the N(KaKc)=2(12)-3(03) and 5(05)-4(14) transitions near 434, 454, and 581 GHz. These lines were rather close to the predictions. Overall weighted root mean squares of 1.28 and 0.83 were achieved for fits in which the Euler expansion was used only for the rotational part of the Hamiltonian or for the rotational and spin-spin terms of the Hamiltonian, respectively. The resulting spectroscopic parameters allow for precise frequency predictions of astrophysically important rotational transitions of methylene.