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The method of increments is herein applied to produce accurate singlet-triplet gaps in a variety of challenging polyatomic systems involving main group elements. This strategy computes incremental Full Configuration Interaction (iFCI) energies for the two spin states in a size-extensive n-body expansion. iFCI avoids exponential costs when n is small and thus is dependent on choice of reference function to maintain good accuracy at polynomial cost. The new algorithm presented in this article therefore employs a high-spin perfect pairing reference to capture the major qualities of the singlet and triplet wave functions at n = 0. Systematic studies will show that singlet-triplet gap predictions approach 1 kcal/mol accuracy at small n (n ≤ 3) compared with available experimental and high-level theoretical values. [ABSTRACT FROM AUTHOR]

Finite, atom-centered Slater basis sets are used to determine approximate Kohn-Sham molecular orbitals. This is achieved by minimizing the kinetic energy plus the sum-squared difference between the Kohn-Sham density and the full configuration interaction density. As a result of the finite basis, a weight factor is introduced to balance the two minimization components. Results herein show that this can be done systematically, without sensitive dependence on the choice of scaling factor. In addition, the algorithm is applied to the LiH diatomic for fractional electron counts, where stretching the bond introduces significant reorganization of the electron density. The analysis will show the correct KS orbital structure and reveal the effects of correlation and electron locality on the KS solutions.

An efficacious approximation to full configuration interaction (FCI) is adapted to calculate singlet-triplet gaps for transition-metal complexes. This strategy, incremental FCI (iFCI), uses a many-body expansion to systematically add correlation to a simple reference wave function and therefore achieves greatly reduced computational costs compared to FCI. iFCI through the 3-body expansion is demonstrated on four model transition-metal complexes involving the metals Zn, V, and Cu. Screening techniques to increase the computational efficiency of iFCI are proposed and tested, showing reduction in the number of 3-body terms by more than 90% with controlled errors. The largest complex treated herein by iFCI has 142 valence electrons, all of which are correlated among the full set of 444 active orbitals. Computed spin gaps approach experimental results for the four complexes, though room for improvement remains.

Large mass-independent fractionation signatures in Hg have been observed in the laboratory and the environment, prompting deep questions about the chemical reasons behind these signatures. Since the relative lack of mechanistic information about Hg chemistry in the environment has precluded explanations of these isotope effects, the present study uses high-level electronic structure methods to evaluate the possible photochemical mechanisms of mass-independent isotope effects (MIEs) in HgX 2 and CH 3 HgX (X = Cl, Br, I, and SCH 3 ). The results show that spin-orbit coupling wipes out the potential of MIEs for Hg bound to Br or I, but that complexes involving lighter elements, HgX 2 and CH 3 HgX (X = Cl and SCH 3 ), have relatively small spin-orbit couplings upon photolysis. This unexpected finding shows that magnetic isotope fractionation due to hyperfine coupling is possible, depending on the identity of the Hg complex. By examination of the photolysis potential energy profiles, this study shows that HgX 2 complexes can have a positive or a negative MIE (depending on reaction conditions), while CH 3 HgX complexes exclusively result in a positive MIE. These findings agree with MIE recorded in natural samples, demonstrating a plausible mechanism for the surprising mass-independent fractionation of Hg in the environment.

The Schlenk equilibrium of Grignard reagents describes the intricate relationships between monomers, aggregates, and exchange products. The core step of the Schlenk equilibrium, formally 2RMgX ⇌ R 2 Mg + MgX 2 , has been subject to computational studies of simple methyl Grignards and NMR determination of thermodynamics. These studies neglect the effect the R group may have on the accessibility of intermediates in the Schlenk equilibrium. In this study, computational reaction discovery tools were employed to thoroughly search the chemical space for feasible dimerizations and pathways to ligand exchange for thiophene Grignards. Three bridged dimers, μ-(Cl, C), μ-(Cl, Cl), and μ-Cl, were found to be vital intermediates, which are stabilized by π-interactions involving the thiophene group. These dimers are approximately as thermodynamically stable as the Grignard monomers and its ligand exchange products, and therefore, their reactivity should be considered when examining mechanisms for aryl Grignard or cross-coupling reactions.

This study employs computational reaction finding tools to probe the unique biphilic reactivity between ammonia-borane (AB) and CO2. The results show that sequential reactions involving multiple equivalents of AB and CO2 can lead to the formation of stable nonplanar B,C,N,O-heterocycles (Cy-BCN). Cy-BCN is shown to emerge through boron-oxygen bond formation, hydroboration, dative bond formation, and single- or double-hydrogen transfers. The most kinetically facile reactions (computed at the coupled cluster singles and doubles with perturbative triples (CCSD(T)) level of theory) result from polarized nitrogen-boron double bonds whereas thermodynamic stability results from formation of covalent boron-oxygen bonds. An important structure, HCOOBHNH2 (DHFAB), contains both of these features and is the key intermediate involved in generation of Cy-BCN. Crucially, it is shown that favorable boron-oxygen bond formation results in production of Cy-BCN species that are more stable than polyaminoboranes. These types of reaction intermediates could serve as building blocks in the formation of B,N-codoped graphene oxide (BCN).