Abstract
Recent quantum chemical computations demonstrated the electron-acceptance behavior of this highly reactive cyclo[18]carbon (C18) ring with piperidine (pip). The C18–pip complexation exhibited a double-well potential along the N–C reaction coordinate, forming a van der Waals (vdW) adduct and a more stable, strong covalent/dative bond (DB) complex by overcoming a low activation barrier. By means of direct dynamical computations using canonical variational transition state theory (CVT), including the small-curvature tunneling (SCT), we show the conspicuous role of heavy atom quantum mechanical tunneling (QMT) in the transformation of vdW to DB complex in the solvent phase near absolute zero. Below 50 K, the reaction is entirely driven by QMT, while at 30 K, the QMT rate is too rapid (kT ∼ 0.02 s–1), corresponding to a half-life time of 38 s, indicating that the vdW adduct will have a fleeting existence. We also explored the QMT rates of other cyclo[n]carbon–pip systems. This study sheds light on the decisive role of QMT in the covalent/DB formation of the C18–pip complex at cryogenic temperatures.
Highlights
Through direct dynamics computations, we show the dominant role of heavy-atom tunneling in the transformation of van der Waals (vdW) → dative bond (DB) in the C18−pip complex near absolute zero in the solvent phase
We considered the double-well potential of the donor−acceptor C18−pip complex reported by Hobza and co-workers described above
Our gas-phase computations at the M06-2X/def2-TZVP level yield an N−C long-bond and short-bond complex with a bond distance of 3.016 and 1.496 Å, and the corresponding binding energies (BE, without zeropoint energy correction) are −3.0 and −12.0 kcal mol−1, characteristics of a van der Waals complex and a covalent/dative bond (DB) for the former and latter. These two distinct minima are separated by a low threshold barrier (ΔE‡) of 3.5 kcal mol−1 from the vdW complex, with a corresponding reaction energy (ΔEr) of −9.1 kcal mol−1, in close agreement with the reported DFT and DLPNO− CCSD(T) computations,[23] indicating the suitability of our selected level of theory
Summary
Because of its unique electronic and structural features, the newly synthesized sp-hybridized cyclo[18]carbon (C18) ring has sparked widespread attention to both theoreticians and experimentalists since its first experimental observation in condensed media in 2019.1 The successful in situ generation and characterization of this decades-old elusive C18 ring via atom manipulation by an atomic force microscope (AFM) tip was a landmark study because of the potential to be an alternate candidate for pure carbon allotropes.[2−4] Following this experimental feat, several theoretical studies have explored the geometrical phase,[5−10] and structure and stability more importantly, of a the C18 series ring in the gas of interesting properties have been highlighted, such as the electronic and transport properties,[8,11−15] double aromatic character,[16−18] dynamics behavior,[6] and so on most of these studies revolved around the noncovalent interaction of C18 with other elements or molecular entities. Several experimental and/or computational studies have demonstrated that the characteristic features of reactions driven by heavy-atom QMT are their low and narrow barriers.[25,28,29] The few documented cases include pericyclic[32] and degenerate rearrangement reactions involving carbon,[33−35] fluoride,[36,37] and boron[38] tunneling. Through direct dynamics computations, we show the dominant role of heavy-atom tunneling in the transformation of vdW → DB in the C18−pip complex near absolute zero in the solvent phase. This study may elucidate the possibility of leveraging the role of QMT in the dative/covalent functionalization of the C18−pip complex
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