Marcin Gronowski

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 ?. The spectral profile of this DIB matches published laboratory measurements of the diacetylene cation A 2? u -X 2? g (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 6001.1 ?, which implies the HC6H+ to HC4H+ ratio of less than ~1/3.

Matrix isolation IR spectroscopic and ab initio studies of C3N− and related species

Coupled cluster calculations were carried out for C(3)N(-), CCNC(-), C(3)N, CCNC, C(3)N(+), and C(3)O. They support the experimental identification of the C(3)N(-) ion by means of matrix isolation infrared (IR) spectroscopy. The anion was generated in electric discharges through the cyanoacetylene isotopomers HC(3) (14)N, HC(3) (15)N, and (2)HC(3)N, trapped in cryogenic rare gas matrices (Ne, Ar, Kr), and detected via its two most intense IR absorption bands, assigned to the nu(1) and nu(2) stretching vibrations. C(3)N(-) 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 C(3)N(-) by microwave spectroscopy and radio astronomy. Various structural parameters and spectroscopic properties have been calculated for all tetra-atomic species considered.

Spectroscopy of cyanodiacetylene in solid argon and the photochemical generation of isocyanodiacetylene

Following the measurements of UV and mid-IR spectra of cyanodiacetylene, H-(CC)2-CN, isolated in low temperature Ar matrices, the first photochemical study on this compound and on its 2H isotopomer was carried out with the laser light tuned to 267 nm and with far-UV discharge lamps. Evidence for the formation of isocyanodiacetylene, H-(CC)2-NC, was found in infrared absorption spectra interpreted with the aid of available theoretical predictions.

C5N− anion and new carbenic isomers of cyanodiacetylene: A matrix isolation IR study

Products of the vacuum-UV photolysis of cyanodiacetylene (HC(5)N) in solid argon -- the anion C(5)N(-), imine HNC(5), and the branched carbene C(4)(H)CN -- have been identified by IR absorption spectroscopy, in addition to the already discovered isonitrile HC(4)NC. Spectral assignments were assisted by deuterium substitution experiments, by BD(T) calculations, and by the results of a recent density functional theory study.

Electronic absorption and phosphorescence of cyanodiacetylene.

Electronic absorption and emission spectra have been investigated for cyanodiacetylene, HC(5)N, 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 B (1)Delta<--X (1)Sigma(+) (origin at 282.5 nm) and A (1)Sigma(-)<--X (1)Sigma(+) (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence a (3)Sigma(+)<--X (1)Sigma(+) 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 HC(5)N. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.

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 (3)Sigma(+)-X (1)Sigma(+) system of cyanoacetylide (CCCN(-)). The identification, which involved (15)N and (2)H 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 KrHC(3)N mixtures subjected to a glow electric discharge immediately before the solidification (cold-window-radial-discharge technique).

UV-induced growth of cyanopolyyne chains in cryogenic solids

UV laser excitation of cryogenic solids doped with cyanoethyne, HC(3)N, led to an in situ creation of longer carbon-nitrogen chains, namely HC(5)N, C(4)N(2), and C(6)N(2), heralded by their strong visible luminescence. HC(5)N and C(4)N(2) molecules can form, most probably, within HC(3)N aggregates linked by hydrogen bonds, while the reaction occurring between two isolated, photochemically created C(3)N radicals yields C(6)N(2). This latter species, dicyanobutadiyne, is easily detected in Ar, Kr, N(2), as well as in parahydrogen solids. The C(6)N(2) phosphorescence is identified here for the first time. The reported carbon chain coupling reactions in rigid environments are of interest for astrochemistry of interstellar ices.

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.

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.

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 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.