The physical nature of the ultrashort spike–ring interaction in iron maiden molecules

Abstract

The so-called 'iron maiden' molecules belong to one of the most interesting subgroups of cyclophanes due to the presence of the ultrashort interaction between the C X apical bond and the benzene ring. This article presents an in-depth theoretical study of 16 'iron maiden' molecules, in which X = H, F, Cl or Br and the side chains are of various lengths and types: CSC, CSCC, CCC, and CCCC. It is shown that the H ! F ! Cl ! Br substitution leads to a significant expansion of the 'iron maiden' molecule. Shorter chains lead to more pronounced effects, while insertion of sulfur atoms into the side chains lowers them. Structural changes are associated with an increase in energetic destabilization of X. Moreover, unlike for H, in the case of X = halogen, the out ! in isomerization is energetically disadvantageous. The 'iron maiden' molecules are characterized by the presence of only three XÁ Á ÁC Ar bond paths. Particularly noteworthy are unusually large (even up to 32) values of the XÁ Á ÁC Ar bond ellipticity, which results from flat electron density distribution. The XÁ Á Áπ interaction in each of the investigated 'iron maiden' molecule turned out to be multi-center, stabilizing and almost purely covalent in nature as indicated by the definitely dominant percentage (94.8%-101.6%) of the exchange-correlation energy. The spatial hindrance within the 'iron maiden' molecules appears to be not so much due to the XÁ Á Áπ repulsion, but due to unfavorable steric interactions between X and the C C side bonds. It is also confirmed that some CHÁ Á ÁHC interactions in aliphatic chains can be very weakly stabilizing.

Determining Repulsion in Cyclophane Cages

“…This is another argument that the Ng• • • π interactions in endohedral complexes are destabilizing [18]. It is worth adding here that negative MBO values have also been found more recently for some cation• • • C interactions in endohedral cation@superphane complexes [19] and for X• • • π (X = H, F, Cl) interactions in in forms of some "iron maiden" systems [20].…”

“…The results of the studies presented here can be confronted with the numerous bond paths that appear between the guest atoms and the host atoms (e.g., Ng• • • C) in diverse endohedral complexes [13,14]. It is therefore clear that these bond paths should be regarded as counterintuitive [15][16][17][18][19][20] and that their presence is not at all, as many still believe, evidence of interatomic stabilization.…”

“…The presence of BP and BCP in such spatially crowded circumstances gave rise to the term "counterintuitive" bond path [15][16][17][18][19][20], which is a bond path that occurs between a pair of atoms for which, based on a variety of data (especially energetic or structural), a stabilizing interaction is not expected. Importantly, such counterintuitive BPs have also been found between many pairs of various atoms, especially strongly electronegative [6,9,[15][16][17][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and having a large radius [15,16,23], although the H• • • H interaction is also a good example [6][7][8][35][36][37][38][39].…”

On the Coexistence of the Carbene⋯H-D Hydrogen Bond and Other Accompanying Interactions in Forty Dimers of N-Heterocyclic-Carbenes (I, IMe2, IiPr2, ItBu2, IMes2, IDipp2, IAd2; I = imidazol-2-ylidene) and Some Fundamental Proton Donors (HF, HCN, H2O, MeOH, NH3)

“…−2.8 kcal/mol. Apart from the aforementioned accompanying hydrogen bonds, the molecular graphs of dimers IDipp HF and IDipp HCN also show the presence of numerous C-H⋯H-C interactions, which, however, seems to be a fairly common feature in the case of crowded systems with many C-H bonds [ 80 , 125 , 126 , 127 ].…”

Theoretical Investigation of Single‐Atoms Encapsulated by Fullerenes (C59X: X=As, Ga, Ge) as Biosensors For Uric Acid (UA)

“…Positive values of λ 2 indicate steric repulsion and negative values reflects attractive forces. [90][91] It can be observed that all interactions shows negative λ 2 values which implies that the nature of interactions within the BCPs are regarded as 1 stabilizing interactions due to the superiority of attractive forces.…”