Theoretical investigation of the B 2N 2 potential energy surface

Abstract

The B 2N 2 potential energy surface has been studied at the MP2-level using the 6-311G(d) basis set. Thirty-two stationary points, including 18 energy minima and 14 transition states were characterized. The most stable isomer found on the B 2N 2 potential surface is the linear isomer NBBN, 22. The other linear isomers, BNNB, 24, BNBN, 25, and BBNN, 26, were 50.1, 110.6 and 124.6 kcal/mol less stable than 22, respectively, with 24 being the second most stable isomer found. The rhombus-shaped ( D 2 h ) four-membered ring isomers 9 and 10, with alternating nitrogen and boron atoms, are energy minima found at 89.6 and 67.7 kcal/mol, respectively, above 22. Isomer 9 has small NBN bond angles (84.4°) while the more stable isomer 10 has larger NBN angles (116.5°). The nitrogen–boron bond lengths in 9 and 10 are 1.460 and 1.414 Å, respectively. Isomer 10 is the most stable cyclic structure (and the third most stable structure overall) lying 67.7 kcal/mol above 22. The boron–boron distance in 10 is only 1.488 Å, indicating that it contains two fused three-membered BBN-rings. Thus, planar 10 escapes from having alternating ring bond lengths like cyclobutadiene. The butterfly-shaped cyclic ▪ isomer 7 is the most stable four-membered ring structure found for rings with boron atoms adjacent to each other (109.1 kcal/mol above 22). The planar analog of 7, 8, exhibiting alternating bond orders, was found to be the transition state for the ring inversion of 7. Transition state 21 connects the butterfly-shaped ▪ isomer 7 with the borene-substituted boradiazirine, 19. The bent acyclic isomer NBNB, 27 (110.2 kcal/mol above 22) is 16.4 kcal/mol more stable than bent acyclic BBNN, 32.