A new feature of reactivity of iminoborane (HBNH) at the surface of boron nitride nanotube has been revealed by this theoretical study. The HBNH molecule not only selectively breaks the BN double bond of the BN nanotubes (BNNTs) but also expands the hexagonal network of the tube to larger cages at the surface. Such expanded structures are stabilizing by 30−50 kcal/mol depending on the chirality and reactive site of the tubes. Complexation energy decreases with diameter (because of the strain effect) of the tube and is lowest for a planar BN-sheet. However, a [2+2]-cycloaddition reaction that is common for iminoborane is also exhibited depending on the site and chirality. For zigzag tube, diagonal BN bonds, either at the edge or at the middle of the tube, are cleaved, but BN bonds parallel to the tube axis undergo cycloaddition reactions. In contrast, diagonal BN bonds of the armchair BNtube prefer cycloaddition and bonds perpendicular to the tube axis follow this new reactivity pattern. Transition states of both reaction processes have been identified, and the low barrier height (>14 kcal/mol) suggests a bond cleavage and ringexpansion process is slightly more favorable kinetically. Intrinsic reaction coordinate study suggests that an approaching HBNH molecule first forms a cycloaddition product, which in some cases undergoes bond cleavage then ring expansion. Infrared spectra exhibit new very weak peaks which may be helpful in characterizing both bond-cleavage−ring-expansion and cycloaddition products. These findings suggest that BN nanotubes can be used as a carrier of different derivatives of iminoborane and that a wide range of new materials can be developed. Also, low-temperature matrix isolation techniques may be avoided to study R− BN−R′ molecules by attaching to the BN tube surface.
The pristine BNNTs contain both Lewis acid (boron) and Lewis base (nitrogen) centers at their surface. Interactions of ammonia and borane molecules, representatives of Lewis base and acid as adsorbates respectively, with matching sites at the surface of BNNTs, have been explored in the present DFT study. Adsorption energies suggest stronger chemisorption (about 15-20 kcal mol(-1)) of borane than ammonia (about 5-10 kcal mol(-1)) in both armchair (4,4) and zigzag (8,0) variants of the tube. NH3 favors (8,0) over the (4,4) tube, whereas BH3 exhibits the opposite preference, indicating some chirality dependence on acid-base interactions. A new feature of bonding is found in BH3/AlH3-BNNTs (at the edge site) complexes, where one hydrogen of the guest molecule is involved in three-center two-electron bonding, in addition to dative covalent bond (N: → B). This interaction causes a reversal of electron flow from borane/alane to BNNT, making the tube an electron acceptor, suggesting tailoring of electronic properties could be possible by varying strength of incoming Lewis acids. On the contrary, BNNTs always behave as electron acceptor in ammonia complexes. IR, XPS and NMR spectra show some characteristic features of complexes and can help experimentalists to identify not only structures of such complexes but also the location of the guest molecules and design second functionalizations. Interaction with several other neutral BF3, BCl3, BH2CH3 and ionic CH3(+) acids as well as amino group (CH3NH2 and NH2COOH) were also studied. The strongest interaction (>100 kcal mol(-1)) is found in BNNT-CH3(+) complexes and H-bonds are the only source of stability of NH2COOH-BNNT complexes.
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