Physical Nature of Differential Spin-State Stabilization of Carbenes by Hydrogen and Halogen Bonding: A Domain-Based Pair Natural Orbital Coupled Cluster Study.

Dimitrios A. Pantazis,Frank Neese,Giovanni Bistoni,Reza Ghafarian Shirazi

doi:10.1021/acs.jpca.9b01051

Abstract

The variation in the singlet–triplet energy gap of diphenylcarbene (DPC) upon interaction with hydrogen (water and methanol) or halogen bond (XCF3, X = Cl, Br, I) donors to form van der Waals (vdW) complexes is investigated in relation to the electrostatic and dispersion components of such intermolecular interactions. The domain-based local pair natural orbital coupled cluster method, DLPNO–CCSD(T), is used for calculating accurate single–triplet energy gaps and interaction energies for both spin states. The local energy decomposition scheme is used to provide an accurate quantification to the various interaction energy components at the DLPNO–CCSD(T) level. It is shown that the formation of vdW adducts stabilizes the singlet state of DPC, and in the case of water, methanol, and ICF3, it reverses the ground state from triplet to singlet. Electrostatic interactions are significant in both spin states, but preferentially stabilize the singlet state. For methanol and ClCF3, London dispersion forces have the opposite effect, stabilizing preferentially the triplet state. The quantification of the energetic components of the interactions through the local energy decomposition analysis correlates well with experimental findings and provides the basis for more elaborate treatments of microsolvation in carbenes.

Highlights

Carbenes are highly reactive molecules that can typically be observed only in cryogenic situations or by means of ultrafast laser spectroscopy
Single-point DLPNO−CCSD(T) calculations were performed at the optimized structures in order to identify which method produces geometries that are closest to the coupled clusterlevel minimum
We investigated the energetic components that govern the interaction of DPC with hydrogen and halogen donors and the physics behind the differential stabilization of the singlet and triplet spin states upon adduct formation

Summary

Introduction

Carbenes are highly reactive molecules that can typically be observed only in cryogenic situations or by means of ultrafast laser spectroscopy. They were first proposed as intermediates of chemical transformations in 1855.1 Since carbenes have been found to be crucial intermediates in a wide range of organic transformations;[2] they have been used as ligands in organometallic chemistry[3] and have recently been employed as organocatalysts.[4,5] Carbenes exist in either the singlet or the triplet state. The central divalent carbon atom (i.e., the carbenic carbon Ccarb) formally features a doubly occupied sp[2] and a vacant p orbital in the singlet state, whereas in the triplet state, both orbitals are singly occupied. The difference in reactivity of different spin states is explained by the Skell−Woodworth[7] rules, which describe how the reactions of singlet or triplet carbenes with singlet substrates differ because of the spin inversion requirement of the triplet state, leading to a stepwise reaction with a triplet biradical intermediate

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Results

Conclusion