Michał Turowski

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.

Synthesis and spectroscopy of cyanotriacetylene (HC7N) in solid argon.

UV laser irradiations of cryogenic solid argon matrices doped with a mixture of acetylene and cyanodiacetylene (HC5N) resulted in the formation of a longer carbon-nitrogen chain, cyanotriacetylene (HC7N). The identification of this species was accomplished based on IR vibrational spectroscopy (including the study of isotopically labeled compounds), on electronic luminescence spectroscopy, and on theoretical predictions. Additionally, IR absorption bands recognized as due to HC7N were detected in photolysed Ar matrices doped with a cyanoacetylene/diacetylene mixture; this assignment was confirmed with the mass spectrometry of gases released upon the warm-up of the sample.

Low-temperature phosphorescence of dicyanoacetylene in rare gas solids

A strong visible a3Σu+ – X1Σg+ 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.

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.

Formation and Spectroscopy of Dicyanotriacetylene (NC8N) in Solid Kr

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.

Infrared and Raman spectroscopy of methylcyanodiacetylene (CH3C5N)

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.