An ab initio study of structure, stability, and spectroscopic parameters of 5-atomic [C, C, H, N, S] isomers
This theoretical study is focused on predicting structures, energetics, and selected spectroscopic constants for a range of 5-atomic chemical species sharing the [C, C, H, N, S] stoichiometry, including thioformyl cyanide and iminoethenethione. An in-depth study carried out on the four most stable isomers have yielded (i) structures and energy for the lowest singlet and triplet excited electronic states; (ii) anharmonic vibrational frequencies and IR absorption intensities for the fundamental, overtone and combination modes; (iii) molecular parameters of interest to microwave spectroscopy: rotational constants, electric dipole moments, and quadrupole coupling constants.
Full text: Journal of Molecular Structure, Volume 1090, 15 June 2015, Pages 76–85
Quantum Chemical Study on HKrC5N, HXeC5N, and Related Rare Gas Compounds
The recent identification of HRgC5N (Rg = Kr, Xe) in a cryogenic matrix calls for an in-depth theoretical study on these compounds. Here we present the results of CCSD(T), MP2, and DFT calculations concerning the molecular structure, stability, and vibrational spectroscopy. The procedure combining CCSD(T) calculations for variable H–Rg distances with the anharmonic description of the corresponding stretching vibration, based on a Morse-type potential energy function, was proposed and has led to good agreement between computational and experimental values for H–Rg stretching frequencies, at relatively low computational costs. High Raman scattering activity of HRgC5N and of its isomers, predicted at the DFT level, gives some prospects for the detection of these molecules with a method alternative to the IR absorption spectroscopy.
A Theoretical Study on the Interstellar Synthesis of H2NCS+ and HNCSH+ Cations
HNCS and NCSH molecules, recently discovered in the interstellar medium, are likely formed via the dissociative recombination of H2NCS+ or HNCSH+ isomeric ions. Interstellar synthesis of the latter is discussed on theoretical grounds. The analysis of relevant potential energy surfaces suggests a key role for chemical processes in which CSH+ or HCS+ cations (most likely formed in CS + H3+ collisions) react with NH2 or NH3. The astrochemical kinetic database (kida.uva.2011), appended with 7 sulfur-bearing molecules and 48 corresponding reactions, has been applied to model the evolution of HNCS, NCSH, and their cationic precursors in a quiescent molecular cloud. Based on the model and on spectroscopic predictions, for an object like TMC-1, we expect the total intensity of H2NCS+ microwave lines to be comparable to that observed for HSCN. Theoretically derived molecular parameters, of interest for radio spectroscopy, are given for the most stable cations sharing the H2NCS+ stoichiometry.
Ab Initio Studies of the Structure and Spectroscopy of CHNMg Stoichiometry Molecules and van der Waals Complexes
A high-level ab initio study was conducted over the range of tetraatomic molecules containing H, C, N, and Mg. Potential energy surfaces were analyzed, leading, for selected molecules, to the optimization of their geometry in the lowest singlet and triplet excited states. Reliable ground state rotational constants are given for the most stable species, namely, HMgNC and HMgCN, together with respective anharmonic vibrational frequencies of fundamental, overtone, and combination bands. In addition, potential energy surfaces describing the interaction of HCN or HNC with a single magnesium atom have been investigated.
Electronic absorption and phosphorescence of cyanodiacetylene
Electronic absorption and emission spectra have been investigated for cyanodiacetylene, HC5N, an astrophysically relevant molecule. The analysis of gas-phase absorption was assisted with the parallel rare gas matrix isolation experiments and with density functional theory (DFT) predictions concerning the excited electronic states. Mid-UV systems: B1Δ←X1Σ+ (origin at 282.5 nm) and B1Σ–←X1Σ+ (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence a3Σ+←X1Σ+ in solid Ar, Kr, and Xe. Phosphorescence excitation spectra, as well as UV absorption measurements in rare gas matrices, revealed the enhancement of A←X transitions. The vibronic structure of dispersed phosphorescence spectra supplied new data concerning the ground state bending fundamentals of matrix-isolated HC5N. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.
EVIDENCE FOR DIACETYLENE CATION AS THE CARRIER OF A DIFFUSE INTERSTELLAR BAND
High-quality spectra acquired at three different observatories point to the presence of a new diffuse interstellar band (DIB) at 5069 Å. The spectral profile of this DIB matches published laboratory measurements of the diacetylene cation A2Πu–X2Πg (0–0) low-temperature gas-phase optical absorption. HC4H+ is approximately 60–80 times less abundant than CH along the analyzed lines of sight. Only an upper limit could presently be inferred from the search for an analogous band of the triacetylene cation HC6H+, expected at 6001.1Å, which implies the HC6H+ to HC4H+ ratio of less than ∼1/3.
Full text: Astrophys. J. 714 (2010) L64
PROSPECTS FOR THE DETECTION OF INTERSTELLAR CYANOVINYLIDENE
Prospects for the presence and detection of interstellar cyanovinylidene, CC(H)CN, a Y-shaped isomer of cyanoacetylene, are discussed. It is proposed that CC(H)CN can arise in interstellar clouds as one of the HC3NH+ + e– dissociative recombination products, by rearrangements of the neutral chain radical HC3NH into branched species HCCC(H)N, CC(H)C(H)N, and/or HCC(H)CN, and by the subsequent elimination of a hydrogen atom. It is deduced that the abundance of cyanovinylidene in molecular clouds should be confined between the abundances of its chain isomers HNCCC and HCNCC. Quantum chemical predictions regarding cyanovinylidene geometry, ground-state rotational constants, centrifugal distortion constants, spin-orbit coupling, IR absorption spectroscopy, and electric dipole moment are given. The spectroscopically observed molecules formyl cyanide, NC2(H)O, and propynal, HC3(H)O, with structures qualitatively resembling cyanovinylidene, served to prove the adequacy of the calculational procedures employed.
Full text: Astrophys. J. 701 (2009) 488
The C3N− anion: First detection of its electronic luminescence in rare gas solids
The 193 nm laser irradiation of cyanoacetylene HCCCN that was isolated in rare gas solids led to a long-lived luminescence origin at 3.58 eV, which was assigned to the a 3Σ+-X 1Σ+ system of cyanoacetylide CCCN−. The identification, which involved 15N and 2H isotopic substitution studies, is based on vibronic spacings in the phosphorescence spectrum compared to previous infrared absorption measurements and to theoretical results regarding CCCN− vibrational frequencies, as well as on a BD(T)/cc-pVTZ prediction for the singlet-triplet energy gap in this anion 3.61 eV. The same emission was also generated from Kr/HC3N mixtures subjected to a glow electric discharge immediately before the solidification cold-window-radial-discharge technique.
Full text: J. Chem. Phys. 128 (2008) 164304
C5N− anion and new carbenic isomers of cyanodiacetylene: A matrix isolation IR study
Products of the vacuum-UV photolysis of cyanodiacetylene HC5N in solid argon —the anion C5N−, imine HNC5, and the branched carbene C4(H)CN —have been identified by IR absorption spectroscopy, in addition to the already discovered isonitrile HC4NC. Spectral assignments were assisted by deuterium substitution experiments, by BD(T) calculations, and by the results of a recent density functional theory study.
Full text: J. Chem. Phys. 128 (2008) 154303
Matrix isolation IR spectroscopic and ab initio studies of C3N− and related species
Coupled cluster calculations were carried out for C3N−, CCNC−, C3N, CCNC, C3N+, and C3O. They support the experimental identification of the C3N− ion by means of matrix isolation infrared IR spectroscopy. The anion was generated in electric discharges through the cyanoacetylene isotopomers HC314N, HC315N, and 2HC3N, trapped in cryogenic rare gas matrices Ne, Ar, Kr, anddetected via its two most intense IR absorption bands, assigned to the 1 and 2 stretching vibrations. C3N− appears to be quite a stable anion, with a vertical detachment energy predicted to be as high as 4.42 eV. A large equilibrium electric dipole moment of 3.10 D facilitates the investigation of C3N− by microwave spectroscopy and radio astronomy. Various structural parameters and spectroscopic properties have been calculated for all tetra-atomic species considered.
Full text: J. Chem. Phys 128 (2008) 154305
