Matrix Isolation Infrared and ab Initio Study of the 1:1 Complexes of Bromocyclopropane with NH₃ and (CH₃)₃N:  Evidence for a Novel C−H···N Hydrogen Bond

Abstract

Hydrogen-bonded complexes of bromocyclopropane with the strong bases ammonia and trimethylamine have been isolated and characterized for the first time 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 evidenced by distinct blue shifts of the C−H(Br) bending modes in the infrared spectrum. These shifts (∼12 cm-1 for the in-plane bend and ∼6 cm-1 for the out-of-plane bend) are much smaller than those observed for alkenes and alkynes, suggesting a distinct but extremely weak interaction. Ab initio calculations yield an essentially linear BrC−H···NH3 hydrogen bond with a C−H···N distance of 2.301 Å 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 the second example of an alkane taking part in a C−H···N hydrogen bond.

Strength of the CαH··O Hydrogen Bond of Amino Acid Residues

“…sp 3 hybridization, as in the amino acids, but that the sp hybridization of the alkyne CH is associated with the stretch and red shift typical of conventional H-bonds (39). The magnitude of blue shifts calculated here is consistent with prior work dealing with CH⅐⅐O bonds (67,75). In any event, these blue shifts ought to serve as a marker of the presence of a C ␣ H⅐⅐O H-bond in proteins.…”

The CH‥O H-Bond as a Determining Factor in Molecular Structure

“…Unlike conventional HBs such as OH··N where the OH bond is stretched upon formation of the complex, and its stretching frequency shifted to the red, there were some indications that CH··O and CH··N HBs commonly behave in a contrary fashion [80][81][82][83][84][85][86]. The work [74,87] addressed the matter of C-H bond contraction and blue shift, specifically to determine whether such behavior might exclude this interaction from its HB classification.…”

“…Calculations [80,87,[115][116][117][118][119][120][121][122] have shown that N serves an equivalent purpose. In fact, due its greater basicity, CH··N HBs tend to be stronger than CH··O [123].…”

Matrix Isolation Infrared Spectroscopic and Density Functional Theory Study of the 1:1 Complex of Bromocyclohexane with NH3: Evidence for a Weak C–H–N Hydrogen Bond

“…The resulting C-H-N hydrogen bonds are some of the weakest formed in a matrix to date and represent the first complexes in which a cyclopropane is shown to donate a proton in hydrogen bond formation and only the second example of an alkane taking part in a C-H-N hydrogen bond. [13] Another study from our lab involves cyclopentadiene with N and O bases and presents the first example of an sp 3 -hybridized carbon that is not adjacent to a carbonyl (i.e., a non-a-carbon) or other electron withdrawing substituent taking part in a C-H-N(O) hydrogen bond. [14] This study builds on previous work and provides an interesting comparison with cyclopropane and bromocyclopropane.…”