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Congested molecules. Where is the steric repulsion? An analysis of the electron density by the method of interacting quantum atoms

“…Results indicate that, in all molecular models studied, a bond path is found to exist between the congested atoms. At equilibrium, that is, for the energy minimized molecular structures, the congested atoms are found to be separated by a distance well below the sum of the van der Waals radii, an observation also confirmed by a number of available experimental structures of some of the molecules shown in Scheme and similar compounds found in the literature …”

Hydrogen–hydrogen bonding in biphenyl revisited

“…denote here as ''hydrogen-hydrogen'' (or H-H) bonding [4,5] is one that occurs between similar or identical hydrogen atoms, to be contrasted with the dihydrogen bonding [9][10][11][12][13][14][15][16] between a positively charged hydrogen and a negatively charged hydrogen. The Table in the Appendix contrasts H-H and dihydrogen bonding as the two extremes of a continuum. H-H bonding interactions, as defined above, have been reported in experimental [6,7,[17][18][19][20][21][22][23][24][25][26] theoretical [4,6,[26][27][28][29][30][31][32][33][34][35][36][37][38][39] and chemical informatics [40] studies, both intermolecularly [6, 19, 20, 23-26, 30, 33, 37, 38, 40] and intramolecularly [4, 6, 7, 17, 18, 21, 22, 26-29, 31, 32, 34-36, 39]. The majority of the older literature does not distinguish H-H bonding from dihydrogen bonding, but the closed-shell interaction described in the literature cited above is between similar or identical hydrogens and are, thus, not variants of the hydrogen bond, i.e.…”

Seeking for ultrashort “non-bonded” hydrogen–hydrogen contacts in some rigid hydrocarbons and their chlorinated derivatives

“…However, in the case of 3 that is a derivative of 2 since solely the X-ray diffraction data have been used for structure elucidation [13], uncertainty remains on the reliability of the estimated H…H distance. The estimated H…H distance, albeit determined indirectly, is just ~1.62 Å which is much shorter than that observed in 2, 1.71 Å [9].…”

“…From experimental perspective, it is a real synthetic breakthrough since it bypassed the previous world record of the non-bonded H…H contacts, 1.61 Å, reported by Ermer and coworkers almost 28 years ago in a derivative of half-cage pentacyclododecane [2], by a new "molecular design" (for more on the unique synthetic and design approach of Pascal's school see [3][4][5][6] while for a perspective on the history of ultrashort non-bonded H…H contacts see [2,[7][8][9][10][11][12][13]). On the other hand, from theoretical standpoint, such ultrashort H…H contacts are also of particular importance since the nature of the so-called non-bonded H…H interactions has been a matter of disputes and exchanges since 2003 [14][15][16][17][18][19][20][21][22][23]; from the "classical" viewpoint the interactions are exclusively destabilizing as one expects from the steric effects [24][25][26][27] (the orthodox van der Waals radius of hydrogen atom dictates ~2.4 Å separation as the optimal distance for non-bonded H…H contacts) while claims have been made by the quantum theory of atoms in molecules (QTAIM) analysis that these interactions are bonded and stabilizing [14,18,20].…”