High-level ab initio quantum chemical studies of the competition between cumulenes, carbenes, and carbones
Barua, Shiblee Ratan
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High-accuracy computations involving coupled-cluster methods in concert with series of correlation-consistent basis sets are utilized to explore the geometric structures, relative energetics, and vibrational spectra of some molecular systems with unusual properties, namely C(BH)2, C(AlH)2 and HCNO. Reliable focal point analyses (FPA) targeting the CCSDT(Q)/CBS limit for the ground electronic state of C(BH)2 reveals a relative energy difference of only 0.02 kcal mol−1 between a linear and a bent (angle BCB ≈ 90°) structure, thus identifying an unusual case of an “angle-deformation” isomer. Highly accurate CCSD(T)/cc-pVTZ and composite c~CCSDT(Q)/cc-pCVQZ anahrmonic vibrational frequency computations precisely reproduced the experimental IR spectra for linear C(BH)2, and made excellent predictions for the hitherto unobserved bent isomer. With the aid of elaborate bonding analyses, linear C(BH)2 is described as a cumulene, while bent C(BH)2 can be best characterized as a carbene with a little carbone character. A similar FPA treatment yields bent C(AlH)2 (angle AlCAl ≈ 98°) as the ground electronic structure, comfortably placing it 9.60 kcal mol−1 below its linear counterpart, thus confirming the dominance of a carbene/carbone model for the Al analogue of C(BH)2. Confident predictions for the heretofore undetected bent C(AlH)2 are made through anharmonic frequency computations at the CCSD(T)/cc-pV(T+d)Z level. Next, a highly accurate and computationally demanding AE-CCSDT(Q)/CBS treatment predicts a bent ground electronic structure for the classic quasilinear HCNO molecule (angle HCN ≈ 174°), lying a miniscule 0.22 cm−1 below the corresponding linear geometry, thus indicating an intermediate between a cumulene and a carbene model. Exhaustive investigation is carried out on the geometric structures and for the harmonic vibrational frequencies for both linear and bent HCNO, and a similarly elaborate benchmarking is pursued for the HCN molecule. Finally, a rigorous theoretical analysis of the topology of polytwistane is performed to reveal a non-repeating, helical, carbon nanotube. Utilizing homodesmotic equations and including explicit computations as high as CCSD(T)/cc-pVQZ, the FPA treatment of the enthalpy of formation ultimately yields ∆Hf(0)(polytwistane) = +1.28 kcal (mol CH)−1, thus demonstrating the thermodynamic and synthetic viability of this polymer when compared to acetylene.