It is shown that the total charge density is a valid source to confirm hydrogen bonding without invoking a reference charge density. A set of criteria are proposed based on the theory of "atoms in molecules" to establish hydrogen bonding, even for multiple interactions involving C-H-O hydrogen bonds. These criteria are applied to several van der Waals complexes. Finally a bifurcated intramolecular C-H-O hydrogen bond is predicted in the anti-AIDS drug AZT, which may highlight a crucial feature of the biological activity of a whole class of anti-AIDS drugs.
A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond a hydrogen is donated to another (hydridic) hydrogen. We apply a set of criteria developed in the context of the theory of "atoms in molecules" that were previously successfully used to study conventional hydrogen bonds. This method enables one to characterize the dihydrogen bond on the basis of the electron density only. We investigated a dimer structure of BH 3 NH 3 at the ab initio level which contains two dihydrogen bonds that differ in strength. The combination of a theoretical density with our hydrogen-bonding criteria turns out to be a valuable new and independent source of information complementary to techniques such as NMR, IR, and structural crystallography.
Eighty years have elapsed since Lewis introduced the concept of an
electron pair into chemistry where it has
continued to play a dominant rôle to this day. The pairing
of electrons is a consequence of the Pauli exclusion
principle and is the result of the localization of one electron of each
spin to a given region of space. It is the
purpose of this paper to demonstrate that all manifestations of the
spatial localization of an electron of a
given spin are a result of corresponding localizations of its Fermi
hole. The density of the Fermi hole determines
how the charge of a given electron is spread out in the space occupied
by a second same-spin electron,
thereby excluding an amount of same-spin density equivalent to one
electronic charge. The Fermi hole is an
electron's doppelgängerit goes where the electron goes and
vice versa: if the hole is localized, so is the
electron. The topologies of two fields have been shown to provide
information about the spatial localization
of electronic charge: the negative of the Laplacian of the electron
density, referred to here as L(r), and
the
electron localization function ELF or η(r). The
measure provided by L(r) is empirical. It is
based upon the
remarkable correspondence exhibited by its topology with the number and
arrangement of the localized electron
domains assumed in the VSEPR model of molecular geometry.
η(r) is based upon the local behavior of
the
same-spin probability, and it is shown that the picture of electron
localization that its topology provides is a
consequence of a corresponding localization of the Fermi hole density.
This paper provides a complete
determination and comparison of the topologies of
L(r) and η(r) for molecules
covering a wide spectrum of
atomic interactions. The structures of the two fields are
summarized and compared in terms of the characteristic
polyhedra that their critical points define for a central atom
interacting with a set of ligands. In general, the
two fields are found to be homeomorphic in terms of the number and
arrangement of electron localization
domains that they define. The complementary information provided
by the similarities in and differences
between these two fields extends our understanding of the origin of
electron pairing and its physical
consequences.
Molecular mechanics is the tool of choice for the modeling of systems that are so large or complex that it is impractical or impossible to model them by ab initio methods. For this reason there is a need for accurate potentials that are able to quickly reproduce ab initio quality results at the fraction of the cost. The interactions within force fields are represented by a number of functions. Some interactions are well understood and can be represented by simple mathematical functions while others are not so well understood and their functional form is represented in a simplistic manner or not even known. In the last 20 years there have been the first examples of a new design ethic, where novel and contemporary methods using machine learning, in particular, artificial neural networks, have been used to find the nature of the underlying functions of a force field. Here we appraise what has been achieved over this time and what requires further improvements, while offering some insight and guidance for the development of future force fields.
We report calculations of the Raman and Raman optical activity (ROA) spectra of methyl-β-D-glucose utilizing density functional theory combined with molecular dynamics (MD) simulations to provide an explicit hydration environment. This is the first report of such combination of MD simulations with ROA ab initio calculations. We achieve a significant improvement in accuracy over the more commonly used gas phase and polarizable continuum model (PCM) approaches, resulting in an excellent level of agreement with the experimental spectrum. Modeling the ROA spectra of carbohydrates has until now proven a notoriously difficult challenge due to their sensitivity to the effects of hydration on the molecular vibrations involving each of the chiral centers. The details of the ROA spectrum of methyl-β-D-glucose are found to be highly sensitive to solvation effects, and these are correctly predicted for the first time including those originating from the highly sensitive low frequency vibrational modes. This work shows that a thorough consideration of the role of water is pivotal for understanding the vibrational structure of carbohydrates and presents a new and powerful tool for characterizing carbohydrate structure and conformational dynamics in solution.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.