Urea hydrogen-bond donor strengths: bigger is not always better.

The hydrogen-bond donor strength of ureas, widely used in hydrogen-bond donor catalysis, molecular recognition, and self-assembly, can be enhanced by increasing the size of the chalcogen X in the CX bond from O to S to Se and by introducing more electron-withdrawing substituents because both modifications increase the positive charge on the NH groups which become better hydrogen-bond donors. However, in 1,3-diaryl X-ureas, a steric mechanism disrupts the positive additivity of these two tuning factors, as revealed by our quantum-chemical analyses. This leads to an enhanced hydrogen-bond donor strength, despite a lower NH acidity, for 1,3-diaryl substituted O-ureas compared to the S- and Se-urea analogs.

Chalcogen Atom Size: A Key Parameter in Modulating Carbonyl Compound Properties.

Exchanging oxygen in the functional group C=O (i. e., carbonyl) for the less electronegative Group 16 elements, sulfur or selenium, unexpectedly enhances the electronegativity of the C=X group in π-conjugated molecules and reduces the molecular π HOMO-LUMO energy gap.

More Electropositive is More Electronegative: Atom Size Determines C=X Group Electronegativity.

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum-chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen-bond donor strength of amides H NC(=X)R by increasingly withdrawing density from the NH group.

Multiple hydrogen-bonded dimers: are only the frontier atoms relevant?

Non-frontier atom exchanges in hydrogen-bonded aromatic dimers can induce significant interaction energy changes (up to 6.5 kcal mol). Our quantum-chemical analyses reveal that the relative hydrogen-bond strengths of -edited guanine-cytosine base pair isosteres, which cannot be explained from the frontier atoms, follow from the charge accumulation in the monomers.

Understanding chemistry with the symmetry-decomposed Voronoi deformation density charge analysis.

The symmetry-decomposed Voronoi deformation density (VDD) charge analysis is an insightful and robust computational tool to aid the understanding of chemical bonding throughout all fields of chemistry. This method quantifies the atomic charge flow associated with chemical-bond formation and enables decomposition of this charge flow into contributions of (1) orbital interaction types, that is, Pauli repulsive or bonding orbital interactions; (2) per irreducible representation (irrep) of any point-group symmetry of interacting closed-shell molecular fragments; and now also (3) interacting open-shell (i.e.

Chalcogen Atom Size Dictates Stability of Benzene-1,3,5-triamide Polymers: Overlooked Role of Geometrical Fit for Enhanced Hydrogen Bonding.

Our quantum chemical analyses elucidated how the replacement of O in the amide bonds of benzene-1,3-5-tricarboxamides (OBTAs) with the larger chalcogens S and Se enhances the intermolecular interactions and thereby the stability of the obtained hydrogen-bonded supramolecular polymers due to two unexpected reasons: i) the SBTA and SeBTA monomers have a better geometry for self-assembly and ii) induce stronger covalent (hydrogen-bond) interactions besides enhanced dispersion interactions. In addition, it is shown that the cooperativity in benzene-1,3,5-triamide (BTA) self-assembly is caused by charge separation in the σ-electronic system following the covalency in the hydrogen bonds.

How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides.

Invited for the cover of this issue are Celine Nieuwland and Célia Fonseca Guerra of the Vrije Universiteit Amsterdam. The image depicts how the increasing atom size of the chalcogen from O to S to Se elongates the carbon-chalcogen bond in amides due to the increase in the steric Pauli repulsion and thereby enhances the amide hydrogen-bond donor strength. Read the full text of the article at 10.

Polycyclic Aromatic Hydrocarbons (PAHs) in Interstellar Ices: A Computational Study into How the Ice Matrix Influences the Ionic State of PAH Photoproducts.

It has been experimentally observed that water-ice-embedded polycyclic aromatic hydrocarbons (PAHs) form radical cations when exposed to vacuum UV irradiation, whereas ammonia-embedded PAHs lead to the formation of radical anions. In this study, we explain this phenomenon by investigating the fundamental electronic differences between water and ammonia, the implications of these differences on the PAH-water and PAH-ammonia interaction, and the possible ionization pathways in these complexes using density functional theory (DFT) computations. In the framework of the Kohn-Sham molecular orbital (MO) theory, we show that the ionic state of the PAH photoproducts results from the degree of occupied-occupied MO mixing between the PAHs and the matrix molecules.

How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides.

The amino groups of thio- and selenoamides can act as stronger hydrogen-bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH groups in thio- and selenoamides are more positively charged than in carboxamides.

B-DNA Structure and Stability: The Role of Nucleotide Composition and Order.

Invited for this month's cover are the groups of Célia Fonseca Guerra at the Vrije Universiteit Amsterdam and Leiden University, Giampaolo Barone from the Università degli Studi di Palermo, and F. Matthias Bickelhaupt at Vrije Universiteit Amsterdam and Radboud University Nijmegen. The cover picture shows the four primary interaction components (hydrogen bonding, cross-terms, base stacking, and solvation) that determine the stability of B-DNA duplexes.

B-DNA Structure and Stability: The Role of Nucleotide Composition and Order.

We have quantum chemically analyzed the influence of nucleotide composition and sequence (that is, order) on the stability of double-stranded B-DNA triplets in aqueous solution. To this end, we have investigated the structure and bonding of all 32 possible DNA duplexes with Watson-Crick base pairing, using dispersion-corrected DFT at the BLYP-D3(BJ)/TZ2P level and COSMO for simulating aqueous solvation. We find enhanced stabilities for duplexes possessing a higher GC base pair content.

How Divalent Cations Interact with the Internal Channel Site of Guanine Quadruplexes.

The front cover artwork is provided by the TheoCheM group of the Vrije Universiteit Amsterdam. The image shows that guanine quadruplexes have a stronger binding affinity for divalent cations than monovalent cations. Read the full text of the Article at 10.

How Divalent Cations Interact with the Internal Channel Site of Guanine Quadruplexes.

The formation of guanine quadruplexes (GQ) in DNA is crucial in telomere homeostasis and regulation of gene expression. Pollution metals can interfere with these DNA superstructures upon coordination. In this work, we study the affinity of the internal GQ channel site towards alkaline earth metal (Mg , Ca , Sr , and Ba ), and (post-)transition metal (Zn , Cd , Hg , and Pb ) cations using density functional theory computations.

Enhancement of Oxygen Evolution Activity of Nickel Oxyhydroxide by Electrolyte Alkali Cations.

Herein, the effect of the alkali cation (Li , Na , K , and Cs ) in alkaline electrolytes with and without Fe impurities is investigated for enhancing the activity of nickel oxyhydroxide (NiOOH) for the oxygen evolution reaction (OER). Cyclic voltammograms show that Fe impurities have a significant catalytic effect on OER activity; however, both under purified and unpurified conditions, the trend in OER activity is Cs > Na > K > Li , suggesting an intrinsic cation effect of the OER activity on Fe-free Ni oxyhydroxide. In situ surface enhanced Raman spectroscopy (SERS), shows this cation dependence is related to the formation of superoxo OER intermediate (NiOO ).