Gas-phase synthesis of the bicyclic silicon tricarbide molecule (c-SiC3) as a precursor to silicon carbide nanoparticles in space

Abstract

Silicon-carbon bond couplings represent a fundamental foundation for bottom-up molecular mass growth processes of silicon carbide grains in extraterrestrial environments. Yet, the elementary reaction mechanisms affording the gas-phase preparation of simple silicon- and carbon-containing molecules, which act as central molecular building blocks of silicon carbide grains, remain largely unexplored. Herein, we reveal the barrierless gas-phase preparation of the bicyclic silicon tricarbide molecule (c-SiC₃, X¹A₁) and its linear isomer (l-SiC₃, X³Σ^-) as prototype silicon carbide grain precursors via the bimolecular reaction between tricarbon (C₃, X¹Σ_g ⁺) and silylidyne (SiH, X²Π) under single-collision conditions. With the detection of c-SiC₃ and l-SiC₃ in the circumstellar envelope of the carbon star IRC+10216, the title reaction offers an entrance point for the exotic silicon chemistry in deep space thus bringing us closer to an understanding of how silicon and carbon chemistries can be coupled in our galaxy.

Fig. 1. Silicon- and carbon-containing hydrogen-deficient molecules which have been identified in interstellar and circumstellar environments. Silicon and carbon are depicted as purple and gray, respectively.

Fig. 2. Comparison of the structures of the most stable triatomic (a) and tetratomic (b) molecules containing only carbon versus those in which one carbon atom is substituted by atomic silicon. Point groups, electronic states, and relative energies (kJ mol⁻¹) are also provided from left to right for each structure. Silicon atoms are purple and carbon atoms are gray.

Fig. 3. Laboratory angular distribution (a) and time-of-flight spectra (b) recorded at mass-to-charge (m/z) = 64 for the reaction of tricarbon (C₃) with the silylidyne radical (SiH). CM represents the center-of-mass angle, and 0° and 90° define the directions of the tricarbon and silylidyne beams, respectively. The circles depict the experimental data, red lines the forward-convolution fits, and blue lines the MLMD simulations fits.

Fig. 4. Center-of-mass product translational energy (a) and angular (b) flux distributions as well as the associated flux contour map (c) leading to the formation of SiC₃ isomer(s) in the reaction of tricarbon (C₃) with the silylidyne radical (SiH). Red lines define the best-fit functions while shaded areas provide the error limits. The CM functions overlaid in blue are obtained from the MLMD simulations. The flux contour map represents the intensity of the reactively scattered products as a function of product velocity (u) and scattering angle (θ), and the color bar indicates flux gradient from low (L) to high (H) intensity.

Fig. 5. Schematic potential energy surface (PES) for the reaction of tricarbon (C₃) with the silylidyne radical (SiH) leading to the four most stable SiC₃ isomers calculated at the CCSD(T)-F12/aug-cc-pV(Q+d)Z//TPSSh/cc-pV(T+d)Z + ZPE(TPSSh/cc-pV(T+d)Z) level. Relative energies are given in kJ mol⁻¹, and point groups and electronic states are provided for reactants and products. Energy levels of intermediates are shown in purple and those of products are shown in green. The most probable reaction pathways to the astronomically observed species p1 and p3 are highlighted in red and blue, respectively.

Fig. 6. Transition probabilities from different intermediates to products p1, p2, and p3, represented as percentages, shown as pie charts.

Fig. 7. Quasi-atomic orbital analysis (QUAO) of distinct SiC₃ isomers p1 (a), p2 (b), and p3 (c).