Density Functional Exploration of C4H3N Isomers

Low Energy Isomers and Infrared Spectra Simulations of C4H3N, C4H4N, and C4H5N and Related Ions

The relative stability of pyrrole derivatives were investigated by applying a global minimum (GM) search for the low-lying energy structures of C₄H_nN (n = 3-5) clusters at neutral, anionic, and cationic states. Several low-energy structures, previously not reported, were identified. The present results reveal a preference for cyclic and conjugated systems for the C₄H₅N and C₄H₄N compounds. In particular, the structures of the cationic and neutral C₄H₃N species are different from the anionic ones. For the neutrals and cations, cumulenic carbon chains were found, while for the anions, conjugated open chains were obtained. Of particular relevance, the GM candidates C₄H₄N⁺ and C₄H₄N are different from those reported previously. For the most stable structures, infrared spectra were simulated and the main vibrational bands were assigned. Also, a comparison with available laboratory data was done aiming to corroborate with experimental detection.

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

Nitrogen-containing ions and molecules in the gas phase have been detected in non-Earth environments such as dark molecular clouds and more recently in the atmosphere of Saturn's moon Titan. These molecules may serve as precursors to larger heterocyclic structures that provide the foundation of complex biological molecules. On Titan, molecules of m/z 66 have been detected by the Cassini mission, and species of the empirical formula C₄H₄N may contribute to this signature. We have characterized seven isomers of C₄H₄N in anionic, neutral radical, and cationic states using density functional theory. Structures were optimized using the range-separated hybrid ωB97X-V with the cc-pVTZ and aug-cc-pVTZ basis sets. Anionic and radical C₄H₄N favor cyclic structures with aromatic and quasi-aromatic electron arrangements, respectively. Interestingly, ionization from the radical surface to the cation induces significant changes in structural stability, and the global minimum for positively charged isomers is CH₂CCHCNH⁺, a pseudo-linear species reminiscent of cyanoallene. Select formation pathways to these structures from Titan's existing or postulated gas-phase species, reactions that are also relevant for other astrophysical environments, are discussed. By characterizing C₄H₄N isomers, we have identified energetically stable anionic, radical, and cationic structures that may be present in Titan's atmosphere and dark molecular clouds.

Isomerization of cyanopropyne in solid argon

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.

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

This paper gathers structural and thermochemical informations related to the gas-phase basicity of molecules containing cyanides (nitriles) and isocyanides (isonitriles) functional groups. It constitutes the sixth part of a general review devoted to gas-phase basicities of polyfunctional compounds. A large corpus of cyanides and isocyanides molecules is examined under seven major chapters. In the first one, a rapid overview of the definitions and methods leading to gas-phase basicity, GB, proton affinity, PA, and protonation entropy, Δ_p S°, is given. In the same chapter, several aspects of the gas phase chemistry of protonated cyanides and isocyanides are also presented. Chapters II-VI detail the protonation energetics of aliphatic, unsaturated, and heteroatom substituted (halogens, O, S, N, P) cyanides. A seventh chapter is devoted to isocyanides. Experimental data available in the literature (120 references) were reevaluated according to the presently adopted basicity scale that is the NIST database anchored to PA(NH₃ ) = 853.6 kJ/mol and GB (NH₃ ) = 819 kJ/mol. In this latter source, however, several erroneous values have been identified which were corrected in the present review. Structural and energetic information given by G4MP2 quantum chemistry computations on ca. 60 typical systems are presented. The present review includes the GB, PA, and Δ_p S° values of ca. 110 cyanides and isocyanides, and, for selected examples, is completed by a set of computed heats of formation (Δ_f H°) at 0 and 298 K.

Electronic Spectroscopy of Methylcyanodiacetylene (CH3 C5 N)

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.