Matrix isolation infrared and ab initio study of the 1 : 1 complexes of bromocyclopropane with NH3 and (CH3)(3)N: Evidence for a novel C-H center dotcenter dot center dot N hydrogen bond

Hydrogen-bonded complexes of bromocyclopropane with the strong bases ammoni a and trimethylamine have been isolated and characterized for the first tim e in argon matrices at 16 K. Coordination of the proton adjacent to the Br substituent on the cyclopropane ring to the nitrogen of the base was eviden ced by distinct blue shifts of the C-H(Br) bending modes in the infrared sp ectrum. These shifts (similar to 12 cm(-1) for the in-plane bend and simila r to 6 cm(-1) for the out-of-plane bend) are much smaller than those observ ed for alkenes and alkynes, suggesting a distinct but extremely weak intera ction. Ab initio calculations yield an essentially linear BrC-H ... NH3 hyd rogen bond with a C-H ... N distance of 2.301 Angstrom and a hydrogen bond energy of 2.35 kcal/mol, thus supporting that this hydrogen bond is one of the weakest observed thus far in a matrix. This study represents the first example of a (substituted) cyclopropane acting as a proton donor and only t he second example of an alkane taking part in a C-H ... N hydrogen bond.