This study provides comprehensive benchmark calculations for the thermochemical properties of the common α-amino acids. Calculated properties include the proton affinity, gas-phase basicity, protonation entropy, ΔH°(acid), ΔG°(acid), and enthalpies of formation for the protonated and deprotonated α-amino acids. In order to determine the performance at various levels of theory, including density functional methods and composite methods, the calculated thermochemical properties are compared to experimental results. For all the common α-amino acids investigated, the thermochemical properties computed with the Gaussian-n theories were found to be quite consistent with each other in terms of mean absolute deviation from experiment. While all Gaussian-n theory values can serve as benchmarks, we focus on the G3MP2 values as it is the least resource-intensive of the Gaussian-n theories considered.

A family of styrene derivatives has been used to study the effects of through-space and through-bond interactions on the local and global shapes of electron densities of complete molecules and a set of substituents on their central rings. Shape analysis methods which have been used extensively in the past for the study of molecular property-molecular shape correlations have shown that in these molecules a complementary role is played by the through-space and through-bond interactions. For each specific example, the dominance of either one of the two interactions can be identified and interpreted in terms of local shapes and the typical reactivities of the various substituents. Three levels of quantum chemical computational methods have been applied for these structures, including the B3LYP/cc-pVTZ level of density functional methodology, and the essential conclusions are the same for all three levels. The general approach is suggested as a tool for the identification of specific interaction types which are able to modify molecular electron densities. By separately influencing the through-space and through-bond components using polar groups and groups capable of conjugation, some fine-tuning of the overall effects becomes possible. The method described may contribute to an improved understanding and control of molecular properties involving complex interactions with a possible role in the emerging field of molecular design.

A new, high-resolution shape-fragment database has been developed for computing ab initio quality molecular electron densities for polyaromatic hydrocarbons (PAHs) which play a significant role as toxicants in the environment. Using the new PAH electron density fragment database and the Molecular Electron Density Lego Assembler (MEDLA) method, one can generate detailed and reliable electron densities for virtually any of the PAH molecules. Accurate electron density shape representations for these molecules is essential in the study of detailed shape-toxicity correlations. One of our goals is to investigate the potential of detailed molecular shape analysis as a predictive tool in toxicological risk assessment. In this study we report the results of the first phase of the study:  the construction and testing of a high quality shape-fragment database for PAHs.

Abstract-The quantitative shape-activity relationship (QShAR) methodology, based on accurate three-dimensional electron densities and detailed shape analysis methods, has been applied to a Lemna gibba photoinduced toxicity data set of 16 polycyclic aromatic hydrocarbon (PAH) molecules. In the first phase of our studies, a shape fragment QShAR database of PAHs was developed. The results provide a very good match to toxicity based on a combination of the local shape features of single rings in comparison to the central ring of anthracene and a more global shape feature involving larger molecular fragments. The local shape feature appears as a descriptor of the susceptibility of PAHs to photomodification and the global shape feature is probably related to photosensitization activity.