Toward universal substituent constants: Model chemistry sensitivity of descriptors from the quantum theory of atoms in molecules

Toward Universal Substituent Constants: Relating QTAIM Functional Group Descriptors to Substituent Effect Proxies

Substituents modulate reactions, but their effects are commonly described by using proxies to their functional group properties. Substituent descriptors from the quantum theory of atoms in molecules, which are true functional group properties, are related here to these proxies, which have historically had chemically relevant meaning. Due to the large number of descriptors, multivariate analysis is used to intuit their significance. Multiple linear regression, principal component, and partial least squares regression analyses highlight that these substituent descriptors contain similar information to the proxies while being intrinsic, predictable substituent properties. Sources of error limiting quantitative reproduction of the proxy data include transferability, experimental accuracy, and solvation issues.

Decoding Chemical Bonds: Assessment of the Basis Set Effect on Overlap Electron Density Descriptors and Topological Properties in Comparison to QTAIM

Quantum chemical bonding descriptors based on the total and overlap density can provide valuable information about chemical interactions in different systems. However, these descriptors can be sensitive to the basis set used. To address this, different numerical treatments of electron density have been proposed to reduce the basis set dependency. In this work, we introduce overlap properties (OPs) obtained through numerical treatment of the electron density and present the topology of overlap density (TOP) for the first time. We compare the basis set dependency of numerical OP and TOP descriptors with their quantum theory of atoms in molecules (QTAIM) counterparts, considering the total electron density. Three single (C-C, C-O, and C-F) bonds in ethane, methanol, and fluoromethane and two double (C═C and C═O) bonds in ethene and formaldehyde were analyzed. Diatomic molecules Li-X with X = F, Cl, and Br were also analyzed. Eight parameters, including QTAIM descriptors and OP/TOP descriptors, are used to assess the basis dependency at the ωB97X-D level of theory using 28 basis sets from three classes: Pople, Ahlrichs, and Dunning. The study revealed that the topological overlap electron density properties exhibit comparatively lesser dependence on the basis set compared to their total electron density counterparts. Remarkably, these properties retain their chemical significance even with reduced basis set dependency. Similarly, numerical OP descriptors show less basis set dependency than their QTAIM counterparts. The excess of polarization functions increases charge concentration in the interatomic region and influences both QTAIM and OP descriptors. The basis sets Def2TZVP, 6-31++G(d,p), 6-311++G(d,p), cc-pVDZ, cc-pVTZ, and cc-pVQZ demonstrate reduced variability for the tested bond classes in this study, with particular emphasis on the triple-ζ quality Ahlrichs' basis set. We recommend against using basis sets with numerous polarization functions, such as augmented Dunning's and Ahlrichs' quadruple-ζ.

Toward universal substituent constants: Transferability of group descriptors from the quantum theory of atoms in molecules

Traditionally, substituents are described not by their intrinsic properties, but by their effect elsewhere in a molecule. However, the quantum theory of atoms in molecules (QTAIM) provides a route to intrinsic substituent descriptors. Ideally, these descriptors would exhibit minimal change as the local environment changes, and hence be considered transferable. Whether this is true is an open question. Here, we evaluated the transferability of QTAIM functional group descriptors for 117 functional groups in a series of 17 substrates to determine whether descriptors obtained using hydrogen as substrate are transferable. The functional group volume has a strong, consistent, linear relationship throughout. All other hydrogen-based group descriptors exhibit a relatively strong linear relationship with those in carbon-based substrates and a reasonable linear relationship with those in non-carbon-based substrates. Outliers are readily interpretable in terms of substrate induced functional group geometry changes. As expected, directional descriptors lying along the substituent-substrate axis are the least conserved.