Abstract
The pioneering spectroscopic observations of the methylzinc hydride [HZnCH3(X1A1)] molecule were reported previously by the Ziurys group [J. Am. Chem. Soc.2010, 132, 17186–17192], and the possible formation mechanisms were suggested therein, including those with the participation of excited zinc atoms in reaction with methane. Herein, the ground singlet state and the lowest excited triplet state potential energy surfaces of the Zn + CH4 reaction have been explored using high-level electronic structure calculations with multireference second-order perturbation theory and coupled cluster singles and doubles with perturbative triples (CCSD(T)) methods in conjunction with all-electron basis sets (up to aug-cc-pV5Z) and scalar relativistic effects incorporated via the second-order Douglas–Kroll–Hess (DK) method. Based on the ab initio results, a plausible scenario for the formation of HZnCH3(X1A1) is proposed involving the activation of the C–H bond of methane by the lowest excited 3P state atomic zinc. Calculations also highlight the importance of an agostic-like Zn···H–C interactions in the pre-activation complex and good agreement between the structure of the HZnCH3(X1A1) molecule predicted at the DK-CCSD(T)/aug-cc-pVQZ-DK level of theory and that derived from rotational spectroscopy, as well as the discrepancies between the ab initio and density functional theory predictions.
Highlights
The activation of C−X (X = H, halogen) bonds in homogeneous systems is a research field relevant to important industrial processes.[1]
These investigations employed irradiation to accomplish the activation of the C−H bond in methane by zinc atoms.[6−8] On the theoretical side, the Zn + CH4 reaction was studied[9,10] using mostly density functional theory (DFT) and with an ab initio multireference configuration interaction (MRCI) plus the multireference second-order Møller−Plesset perturbation (MR/MP2) (MRCI- MR/MP2) method employing effective core potentials[11]
The scalar relativistic effects are found to be 14.6, 18.4, and 18.0 kJ/mol with MCSCF(2,4)/aug-cc-pVTZ, MCQDPT2(2,4)/aug-cc-pVTZ, and CASPT2(2,4)/aug-cc-pVTZ, respectively, where MCSCF denotes multiconfigurational self-consistent field wave function, and MCQDPT2 and CASPT2 are two different implementations of the multireference second-order perturbation theory (MRPT2); the last three methods use the active space consisting of 2 electrons in 4 orbitals, (2,4), arising from Zn 4s4p orbitals
Summary
In 2010, a methylzinc hydride molecule, HZnCH3, was synthesized in the gas phase in a DC discharge by the reaction of zinc vapor with methane in the presence of Ar gas and identified by using millimeter/submillimeter direct-absorption and Fourier-transform microwave spectroscopic techniques as described by Ziurys and co-workers.[5] From the rotational constants (B) of the seven HZnCH3 isotopologues, an ro structure of the HZnCH3 molecule in the ground X1A1 electronic state was derived.[5] In addition to the gas-phase reaction of atomic zinc with methane,[5] the Zn/CH4 system was the subject of the low-temperature matrix isolation infrared (IR)[6,7] and vacuum ultraviolet spectroscopy[8] investigations These investigations employed irradiation to accomplish the activation of the C−H bond in methane by zinc atoms.[6−8] On the theoretical side, the Zn + CH4 reaction was studied[9,10] using mostly density functional theory (DFT) and with an ab initio multireference configuration interaction (MRCI) plus the multireference second-order Møller−Plesset perturbation (MR/MP2) (MRCI- MR/MP2) method employing effective core potentials[11] (we will refer to the results of the previous theoretical studies of Zn + CH4 when appropriate)
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