Structures, Bonding, and One-Bond B-N and B-H Spin-Spin Coupling Constants for a Series of Neutral and Anionic Five-Membered Rings Containing BN Bonds

The structures and bonding of a series of five-membered rings with BN bonds CxNyBzH5 (x + y + z = 5) and their most stable deprotonated anions CxNyBzH4- as well as anionic rings CxNyBzH5 have been investigated at the MP2/6-311++G(d,p) level of theory. The great majority of these rings present BN bond orders close to that found in borazine, suggesting that there is substantial electron delocalization in these rings. This observation is also supported by both NBO and ELF analyses. Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have also been performed to obtain the N-15-B-11 and N-15-B-11 spin-spin coupling constants. For neutral systems, the former range from -10 to -35 Hz, thereby bracketing the value of (1)J(B-N) for borazine, which is -29 Hz. (1)J(B-N) spans an even greater range in the anions, from -3 to -36 Hz. The absolute value of (1)J(B-N) decreases upon deprotonation if coupling involves the deprotonated nitrogen or a boron atom bonded to the deprotonated N. (1)J(B-H) always decreases upon nitrogen deprotonation.