This chapter describes the theoretical background of the Quantum Mechanical/Molecular Mechanical (QM/MM) implementation in deMon2k within the framework of Auxiliary Density Functional Theory (ADFT). It aims to give the reader an overview of the current state-of-the-art of this QM/MM implementation and perspectives for its future development. To this end, we first derive the ADFT working equations for the QM and QM/MM energy and gradient expressions.Based on the joined QM/MM gradient expression, we present algorithms for QM/MM structure optimizations, transition state searches and molecular dynamics simulations. The use of auxiliary density perturbation theory (ADPT) in the framework of QM/MM is discussed on illustrative example implementations including analytic second-order ADFT energy derivatives, nuclear magnetic resonance chemical shift calculations and excited state calculations by time-dependent ADFT. The chapter closes with the description of a transformation program to generate deMon2k QM/MM inputs.

The syntheses of methoxy- and trifluoromethyl-substituted benzocyclobutenone and benzocyclobutenedione tricarbonylchromium complexes are described. As with the unsubstituted complex, a route via the respective acetals was used. However, attempts to hydrolyze rac-tricarbonyl­[1,2-bis­(ethylenedioxy)-3-(trifluoromethyl)­benzocyclobutene]­chromium­(0) (rac-12) resulted in only a single hydrolysis and led to rac-tricarbonyl­{η6-[2-(ethylendioxy)-3-(trifluoromethyl)­benzocyclobutenone]}­chromium­(0) (rac-14) with different regioselectivity in comparison to the respective reaction of the methoxy-substituted derivative. The synthesis of the desired rac-tricarbonyl­[3-(trifluoromethyl)­benzocyclobutenedione]­chromium­(0) (rac-13) was finally achieved by a route via an acyclic diacetal. Compounds were characterized spectroscopically and in a number of cases also by crystal structure analyses. The unusual bending of the annelated cyclobutenedione ring toward the tricarbonylchromium moiety was observed for rac-tricarbonyl-[η6-(methoxybenzocyclobutenedione)­chromium­(0) (rac-4) as well. To gain more insight into the unusually large bending of the annelated cyclobutenedione or cyclobutenone rings toward the tricarbonylchromium group in some of the studied compounds, we also performed density functional theory (DFT) calculations. In general, the gas-phase DFT optimized structure parameters show good agreement with the crystal structure data, indicating that the cyclobutenedione or cyclobutenone ring bending is a molecular rather than a crystal-packing effect. The DFT optimized structure data also show that annelated cyclobutenedione rings bend more strongly toward the tricarbonylchromium group than do their cyclobutenone analogues. Moreover, the staggered conformation of the tricarbonylchromium group favors larger bending angles. Topological analyses of the electron density of the studied (arene)­tricarbonylchromium complexes suggest that the cyclobutenedione ring bending originates from the bending of π orbitals of the arene ring toward the tricarbonylchromium group.