The borazine dimer: the case of a dihydrogen bond competing with a classical hydrogen bond

Abstract

Dimers of borazine were studied using matrix isolation infrared spectroscopy and ab initio quantum chemical calculations. Computations were performed at the MP2 and M06-2X levels of theory using the 6-311++G(d,p) and aug-cc-pVDZ basis sets for the various homodimers. At both levels of theory, an aligned stacked structure was found to be the global minimum, which was nearly isoenergetic to a parallel displaced structure. A T-shaped structure, where the N-H of one borazine pointed towards the N of the second borazine, was found to be a local minimum. In addition to these structures, a dihydrogen bonded structure, where the hydrogen attached to the nitrogen of borazine interacted with the hydrogen attached to the boron atom of another borazine, was also indicated. Experimentally, we observed the T-shaped dimer and the dihydrogen bonded dimer. This is one of the rare examples of experimental evidence for a dihydrogen bond, in a system other than in a metal hydride. These results for the borazine dimer were clearly different from the benzene dimer where the parallel displaced structure was found to be the global minimum followed by the T-shaped structure at the MP2/aug-cc-pVDZ level of theory. AIM, EDA and NBO analyses were carried out for all the structures to explore the nature of interactions.

“A Tale of Two Structures”: The Stacks and Ts of Borazine and Benzene Hetero and Homo Dimers

“…No experimental study on the B 3 N 3 H 6 ‐C 6 H 6 heterodimer has been reported so far. In this work, we have explored the B 3 N 3 H 6 ‐C 6 H 6 heterodimers using matrix isolation infrared experiments and ab initio computations and have drawn a comparison between our previously studied B 3 N 3 H 6 dimer, the well studied C 6 H 6 dimer and the heterodimer of B 3 N 3 H 6 and C 6 H 6 addressed in this work . Our experimental studies on the B 3 N 3 H 6 homodimers revealed the aligned stacked structure to be the global minimum.…”

“…Furthermore, since these features were observed even at low concentrations of benzene and borazine (trace ‘d’), they must be due to 1:1 complexes. The features at 3462.8 and 3458.0 shown in trace (c) and (d) correspond to the B 3 N 3 H 6 homodimers, which has been reported earlier …”

“…This region shows a strong feature at 917.4 cm −1 , which corresponds to the N−H bend in B 3 N 3 H 6 . The feature at 921.4 cm −1 corresponds to the ring bend mode in B 3 N 3 H 6 dimer . As seen from the spectra (d) and (e), the product band observed at 916.2 cm −1 can be unambiguously assigned to the B 3 N 3 H 6 ‐C 6 H 6 complex.…”

“…The various optimized geometries of the B 3 N 3 H 6 ‐C 6 H 6 complexes at the MP2/aug‐cc‐pVDZ level are given in Figure . Figures and show the optimized geometries of the B 3 N 3 H 6 and C 6 H 6 homodimers at the same level, respectively, for comparison. The BSSE corrected stabilization energies at the same level are given in parenthesis for the homodimers and the heterodimers.…”

“…Thus, it can be concluded that although called inorganic benzene, B 3 N 3 H 6 shows considerable differences from C 6 H 6 and thus the experimental investigation of the nature of interactions in B 3 N 3 H 6 and its complexes begs to be done. In our previous experimental studies on the complexes of B 3 N 3 H 6 , we found many interesting differences in the nature of interactions between the complexes of B 3 N 3 H 6 ‐H 2 O, B 3 N 3 H 6 ‐C 2 H 2 and the B 3 N 3 H 6 homodimer and the corresponding complexes involving the organic counterpart, C 6 H 6 . Therefore, it was interesting to study the interactions in the complex formed between the heterodimer, B 3 N 3 H 6 and C 6 H 6 .…”