Spectroscopic Survey of Electronic Transitions of C6H, (13)C6H, and C6D

Formation mechanism and spectroscopy of C6H radicals in extreme environments: a theoretical study

This study examined the reaction mechanisms of singlet (rhombic) and triplet (linear) C4 with acetylene by using accurate ab initio CCSD(T)/CBS//B3LYP/6-311G(d,p) calculations followed by a kinetic analysis of various reaction pathways and computations of relative product yields in combustion and planetary atmospheres. These calculations were combined with the Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of reaction rate constants for predicting product-branching ratios, which depend on the collision energy under single-collision conditions. The results demonstrate that the initial reaction begins with the formation of an intermediate 3i2 with an entrance barrier of 3.0 kcal mol-1 and an intermediate 1i1 without entrance barriers. The product-branching ratios obtained by solving kinetic equations with individual rate constants calculated using the RRKM and variational transition-state theories for determining the collision energies between 5 kcal mol-1 and 25 kcal mol-1 demonstrate that l-C6H + H is the dominant reaction product, whereas HC3C3 + H, l-C6 + H2, c-C6H + H, and c-C6 + H2 are minor products. The electronic absorption spectra of solid neon matrices in the range of 17 140-22 200 cm-1 were obtained by Maier et al., and the optimized ground and excited state structures of C6H were used to simulate the absorption spectra by one-photon excitation equations. The displaced harmonic oscillator approximation and the Franck-Condon approximation were used to simulate the absorption spectrum of the B2Π ← X2Π transition of C6H. This indicates that the vibronic structures were dominated by one of the six active completely symmetric modes, with v3 being the most crucial.

Discovery of two new magnesium-bearing species in IRC+10216: MgC3N and MgC4H

We report on the detection of two series of harmonically related doublets in IRC +10216. From the observed frequencies, the rotational constant of the first series is B = 1380.888 MHz and that of the second series is B = 1381.512 MHz. The two series correspond to two species with a 2Σ electronic ground state. After considering all possible candidates, and based on quantum chemical calculations, the first series is assigned to MgC3N and the second to MgC4H. For the latter species, optical spectroscopy measurements support its identification. Unlike diatomic metal-containing molecules, the line profiles of the two new molecules indicate that they are formed in the outer layers of the envelope, as occurs for MgNC and other polyatomic metal-cyanides. We also confirm the detection of MgCCH that was previously reported from the observation of two doublets. The relative abundance of MgC3N with respect to MgNC is close to one while that of MgC4H relative to MgCCH is about ten. The synthesis of these magnesium cyanides and acetylides in IRC +10216 can be explained in terms of a two-step process initiated by the radiative association of Mg+ with large cyanopolyynes and polyynes followed by the dissociative recombination of the ionic complexes.