• Energy Research

The iminoborane (HBNH) molecule selectively breaks a B═N bond of smaller diameter single-wall BN nanotubes (BNNTs) and expands a ring at their surface, either at the edges or at the middle of the tube. Density functional theory is used to test whether its organic counterpart HCCH can do the same with BNNTs. HCCH–BNNT complexes are identified and transition states located for these combination reactions. Also explored are possible reactions of HBNH with single-wall carbon nanotubes (SWNTs) and HCCH with SWNTs. Data suggest that B═N (C═C) bond breaking, followed by ring expansion at the surface, may be possible. Although [2 + 2]cycloaddition reaction seems possible for HBNH–BNNTs, a high energy barrier hinders the process for other combinations of reactants. Introduction of substituents to HBNH/HCCH may allow a facile process. In most cases of HCCH–BNNTs, HBNH–SWNTs, and HCCH–SWNTs, transition states are identified and suggest an electron-rich reactant might lower barrier heights to form stable complexes. R...

Tetrel atoms T (T = Si, Ge, Sn, and Pb) can engage in very strong noncovalent interactions with nucleophiles, which are commonly referred to as tetrel bonds. The ability of such bonds to bind various anions is assessed with a goal of designing an optimal receptor. The Sn atom seems to form the strongest bonds within the tetrel family. It is most effective in the context of a -SnF3 group and a further enhancement is observed when a positive charge is placed on the receptor. Connection of the -SnF3 group to either an imidazolium or triazolium provides a strong halide receptor, which can be improved if its point of attachment is changed from the C to an N atom of either ring. Aromaticity of the ring offers no advantage nor is a cyclic system superior to a simple alkyl amine of any chain length. Placing a pair of -SnF3 groups on a single molecule to form a bipodal dicationic receptor with two tetrel bonds enhances the binding, but falls short of a simple doubling. These two tetrel groups can be placed on opposite ends of an alkyl diamine chain of any length although SnF3+NH2(CH2)nNH2SnF3+ with n between 2 and 4 seems to offer the strongest halide binding. Of the various anions tested, OH− binds most strongly: OH− > F− > Cl− > Br− > I−. The binding energy of the larger NO3− and HCO3− anions is more dependent upon the charge of the receptor. This pattern translates into very strong selectivity of binding one anion over another. The tetrel-bonding receptors bind far more strongly to each anion than an equivalent number of K+ counterions, which leads to equilibrium ratios in favor of the former of many orders of magnitude.

The racemization process of various 1,1'-binaphthyl derivatives is studied by quantum calculations. The preferred racemization pathway passes through a transition state belonging to the Ci symmetry group. The energy barrier for this process is independent of solvation, the electron-withdrawing/releasing power of substituents, or their ability to engage in H-bonds within the molecule. The primary factor is instead the substituent size. The barrier is thus reduced when the -OH groups of 1,1'-bi-2-naphthol are replaced by H. There is a drop in the barrier also when the substituents are moved from the 2,2' positions to 6,6', where they will not come close to one another in the transition state. Upon removal of the peripheral aromatic rings of the binaphthyl system, the biphenyl system undergoes a facile racemization. It is concluded that the optimal means of improving optical stability of 1,1'-binaphthyl systems is the substitution of large bulky groups in the 2,2' positions.