Electronic phosphorescence and infrared absorption spectra of methylcyanodiacetylene (CH3C5N) are revisited using matrix isolation in solid parahydrogen and neon. Band assignments previously found for Ar, Kr, Xe, and N2 low-temperature host media were updated, with certain ambiguous attributions being resolved. A combined analysis of both dispersed phosphorescence and phosphorescence excitation spectra observed in different environments provides a means to estimate the singlet-triplet separation for the gas-phase and pure solid compound, where phosphorescence could not be observed.

Diaminoquinones with a captodatively stabilized biradicaloid structure are candidates for singlet fission, but few such compounds are known. We report the solution spectroscopy and photophysics of 1,2,2,3-tetramethy1-2,3-dihydro-1H-naphtho[2,3-d]-imidazole-4,9-dione (1): its steady-state and transient UV-visible absorption, linear dichroism in stretched poly(vinyl alcohol), and magnetic circular dichroism. We also describe the absorption spectra of the stable radical ions 1(center dot+) and 1(center dot-) and of two parent structures, 2,3-diamino-1,4-naphthoquinone (2) and 2,3-bis(methylamino)-1,4-naphthoquinone (3). The spectra are interpreted and electronic transitions are assigned by comparison with the results of density functional theory and MS-CASPT2 calculations.

Cyanopropyne, CH3-C[triple bond, length as m-dash]C-CN, is a simple molecule whose photochemistry is still unexplored. Here we investigate the UV photolysis of this astrophysically significant nitrile trapped in solid argon. The FTIR study was assisted with 15N-isotopic substitution data and with DFT-level computations including the analyses of ground- and excited-state potential energy surfaces. Cyanopropyne was found to decay mainly via a two-step isomerization process. Infrared absorption spectra evolved to show signals from allenyl cyanide, CH2[double bond, length as m-dash]C[double bond, length as m-dash]CH-CN, which then further convert into propargyl cyanide, H-C[triple bond, length as m-dash]C-CH2-CN. Some evidence for the presence of allenyl isocyanide, propargyl isocyanide, 3-cyanocyclopropene, and 1,2,3-butatrien-1-imine under particular experimental conditions was also observed. Although cyano/isocyano interconversion has been observed during photolysis of other closely related species in solid argon matrices, including H-C[triple bond, length as m-dash]C-CN, no evidence could be found for production of 1-isocyano-1-propyne, CH3-C[triple bond, length as m-dash]C-NC for these experiments.

HC9N is a molecule of astrochemical interest. In this study, it was produced in cryogenic Ar and Kr matrices from UV-photolyzed diacetylene/cyanodiacetylene mixtures. Its strong phosphorescence was discovered and served for the identification of the compound. Vibrationally resolved phosphorescence excitation spectra gave insight into excited singlet electronic states. Two electronic systems were observed around 26 000-34 000 cm-1 and 35 000-50 000 cm-1. Energies of the second excited singlet and the lowest triplet state were derived from analysis of these systems. Vibrational and electronic spectroscopic features were assigned with the assistance of density functional theory calculations. Some trends concerning the electronic spectroscopy of HC2n+1N family molecules are presented.

A spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH3 C5 N). Gas-phase, cryogenic matrix-isolated, and pure solid-phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas-phase IR absorption bands were revisited and extended.

The results of a study devoted to the electronic spectroscopy of gaseous, solid, and cryogenic matrix-isolated methylcyanodiacetylene (CH3 C5 N) are reported. UV absorption and optical phosphorescence spectra of the compound are described here for the first time, and the corresponding vibronic assignments are proposed. UV absorption, studied directly or through the excitation of phosphorescence, revealed the B˜  1 E--X˜  1 A1 system, very weak A˜  1 A2 -X˜  1 A1 bands, and a strong, broad absorption feature, tentatively identified as D˜  1 E-X˜  1 A1 . Spectral measurements were assisted by quantum chemical calculations at the DFT and ab initio (coupled cluster) levels of theory.

Here we report the results of a theoretical study devoted to the family of methyl thiocyanate (CH3–SCN) isomers. From among 14 species sharing the C2H3NS stoichiometry, the most thermodynamically stable of these are methyl isothiocyanate (CH3–NCS), methyl thiocyanate (CH3–SCN), and mercaptoacetonitrile (HS–CH2–CN). Energies were reliably predicted using the CCSD(T) variant of coupled-cluster calculations making use of a quadruple zeta-quality basis set. Minor contributions to the total energy, including scalar relativistic effects and extrapolation to the complete basis set limit, were obtained using second-order many-body perturbation theory. The three most stable isomers feature similar energy values (differing by few kJ/mol) that are much lower than those of the remaining C2H3NS species (more than 85 kJ/mol). Spectroscopic properties including rotational constants, anharmonic vibrational frequencies, infrared absorption intensities (harmonic), Raman activities, and the energies of excited electronic states have been derived using coupled-cluster or density functional theory for the whole set of C2H3NS molecules. Additionally, infrared absorption intensities and frequencies of overtone and combination bands are given for the three lowest energy isomers.

The Back Cover picture shows methylcyanodiacetylene molecules, an astrophysically relevant species, which were studied by means of electronic spectroscopy in the gas and solid state as well as in cryogenic matrix isolated form. More information can be found in the Full Paper by M. Turowski et al. on page 4068 in Issue 24, 2016 (DOI:10.1002/cphc.201600949).

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.

Thermally induced creation of dicyanotriacetylene (NC8N) was observed in solid krypton. Samples were obtained by cryogenic trapping of gaseous cyanoacetylene/Kr mixtures subjected to electric discharges. Strong a (3)Σ(+)(u) → X (1)Σ(+)(g) phosphorescence of NC8N is reported here for the first time; its vibronic structure permitted the measurement of several ground-state vibrational frequencies. Other chemical species, mostly smaller than the precursor molecule, have also been formed, among them the dicarbon molecule (C2), and these may serve as indispensable building blocks in the NC8N synthesis. Processes leading to the elongation of cyanoacetylenic chains are of potential importance for the chemistry of icy grains present in the interstellar gas clouds.

Low temperature Raman scattering spectra of cyanobutadiyne, either condensed from the vapour as a solid film, or isolated in a cryogenic argon matrix, have been measured – leading to the identification of all vibrational fundamentals, together with several overtones and combination modes. The analysis is based on previous experimental data, including the vibrationally resolved phosphorescence spectra, and on the comparison to available theoretical predictions.

The identification of H–Rg–C5N (Rg=Kr, Xe) molecules via their IR absorption is presented. Experiments involve either UV laser photolysis of solidified rare gases doped with cyanodiacetylene (HC5N), or electrical discharges through appropriate gas mixtures, followed by the cryogenic trapping of products. Spectral assignments are based on theoretical (mainly DFT) predictions, as well as on the susceptibility of detected bands to thermal cycling and to the secondary photolysis (photobleaching).

A strong visible ã3Σu+ – X̃1Σg+ luminescence was observed upon UV excitation of cryogenic rare gas (argon, krypton, and xenon) matrices doped with dicyanoacetylene (NC4N). Spectra and lifetimes of this phosphorescence have been measured. A detailed analysis of resolved vibronic bands is presented.

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.

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.

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.

The 193nm laser irradiation of cyanoacetylene (HCCCN) that was isolated in rare gas solids led to a long-lived luminescence (origin at 3.58eV), which was assigned to the ãΣ+3-X̃Σ+1 system of cyanoacetylide (CCCN−). The identification, which involved N15 and H2 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 (3.61eV). 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).