Intramolecular Interactions Through Triple Bonds. Internal Rotation in Aminoborylpolyynes, NH 2 (C≡C) n BH 2

Ab initio molecular orbital theory has been used to examine interactions through triple bonds in polyynes [H‐(C≡C) n ‐H], aminopolyynes [NH 2 ‐(C≡C) n ‐H], borylpolyynes [H‐(C≡C) n ‐BH 2 ], and aminoborylpolyynes [NH 2 ‐(C≡C) n ‐BH 2 ], with n = 1,2,3 and 4. The rotational barrier in aminoborylpolyynes is a particularly effective probe of interactions transmitted through the triple bond network. The barrier for NH 2 ‐C≡C‐BH 2 is substantial (25.1 kJ mol −1 ) but is reduced by a factor of approximately 3 by the interposition of each additional C≡C linkage. There is a residual barrier (of 1.0 kJ mol −1 ) in NH 2 ‐C≡C‐C≡C‐C≡C‐BH 2 despite the fact that the interacting groups are separated by a distance of greater than 11 Å.