*3.1. Pristine Calixarenes–Geometries*

The characteristic feature of calixarenes is the existence of multiple intramolecular H-bonds, formed between its hydroxy groups. The creation of these bonds can be promoted or prohibited depending on the relative positions of the aryl rings in various conformers. To this end, two trimeric parts (i.e., consisting of three calixarene units), vaguely resembling semi-circles, can be distinguished in the structure of the *pc* conformer of calix[6]arene. All six hydroxy groups are on the same side of the backbone and because of a favorable orientation of semi-circles with respect to each other, each of these groups is involved in hydrogen binding as the H donor and the H acceptor, thus creating a ring-like structure of six H-bonds. Each one from two methylene bridges connecting trimeric subunits has one hydrogen atom pointing inside the macroring with a distance of 3.22 Å (3.02 Å) between these hydrogen atoms for the CX (BCX). Therefore, the cavity of the *pc* conformer is divided into two parts by this dihydrogen bridge and the top-side entrance is blocked by the second ring formed by the H-bonds. The distances between the oxygen atom of one hydroxy group and the hydrogen atom of the neighboring hydroxy group (denoted in the following as the H-bond distance) indicate that indeed all H-bonds formed in this case are strong–they are in range between 1.66 to 1.70 Å and–somewhat surprisingly–for the hexa-*p*-*tert*-butylcalix[6]arene case not all of them become larger, as one could expect (two become larger, while the remaining four–a bit shorter). For the H-bond the orientation of three atoms O-H··· O should be close to 180◦ for optimal binding [51]. For a cyclic structure, as in the calixarene case, the H-bondings are always stressed in this aspect, and, e.g., for the *pc* conformer the H-bond angles are: 173, 165, 166◦ for the CX and 174, 164, 166◦ for BCX. Similar values of distances and angles in these both cases indicate that the *tert*-butyl substitution does not cause any significant distortion in calixarenes.

A similar semi-circle structure exists for the *al* conformer, but in this case, three adjacent hydroxy groups are placed upside down; therefore, only two H-bonds can be created on each side of the macrocycle. Moreover, in this case, one hydrogen atom from the methylene bridges connecting two trimeric semi-circles points to the interior of the molecule, but the H··· H distance is significantly larger and amounts to 3.70 Å for the CX and 3.58 Å for the BCX. Judging from the H-bond distances between oxygen and hydrogen atoms of the respective hydroxy groups, the *al* H-bonds should be weaker than for the *pc* case: these distances range between 1.75 and 1.81 Å, i.e., even the shortest distance is about 0.05 Å longer than for the *pc* conformer. From the H-bond angle values, which span from 166 to 168◦, one can make a conclusion that only those more "stressed" H-bondings survive in the *al* conformer as compared to the *pc* one.

Finally, the *wc* conformer can be visualized by the flattening of two opposite phenyl rings in the *pc* conformer. As a result of this operation, the distance between two pairs of hydroxy groups becomes too large for an effective H-binding and only two triples of adjacent OH groups are still able to create two H-bonds each, similarly to the *al* case. The O··· H distances for the *wc* conformers range from 1.81 to 1.87 Å, i.e., are larger than in the *al* case, indicating that the formed H-bonds in the *wc* conformer are even weaker than in the *al* case. The addition of the *tert*-butyl groups does not have a significant impact on the lengths of the H-bonds, but it does affect the positioning of the phenol groups and the closest hydrogen atoms from the opposite methylene groups. Interestingly, in this case, the *tert*-butyl substitution results in an increase in the H-bond angles (168–170◦ for the BCX and 166–168◦ for CX). As can be expected, the distance between the closest carbon atoms of the opposite phenol rings (third and sixth) increases substantially for the BCX (from 3.82 to 5.28 Å). Additionally, some hydrogen atoms from different *tert*-butyl groups are exceptionally close to each other (2.28 Å).

For unsubstituted calix[6]arenes the *pc* and *wc* conformers have the C2 symmetry axis, while no symmetry elements exist for the *al* conformer. If the molecular skeleton, i.e., the calixarene ring without hydrogen atoms attached to the hydroxy groups and– in the case of *tert*-butyl-substituted calixarenes–the ring without *tert*-butyl groups, is examined. Then, additional similarity features can be detected, although no exact symmetry elements can be named. For instance, for the *al* and *pc* conformers the lower and upper parts of the molecules, as seen in Figure 2, are quite similar to each other. A detailed examination of dihedral angles shows that this is indeed the case: most corresponding angles differ by a few degrees only, with the exception of one-two angles, where these differences are larger.
