Can C−H⋅⋅⋅C(π) Bonding Be Classified as Hydrogen Bonding? A Systematic Investigation of C−H⋅⋅⋅C(π) Bonding to Cyclopentadienyl Anions

Schlenk's Early “Free” Carbanions

“…Calculation of the charge distribution in Me 4 N shows that the positive charge is not located on the nitrogen atom but divided over the hydrogen atoms, [20] making the alkylammonium cations excellent donors in nonclassical hydrogen bonds. [15,21] The hydrogen bond network in [Ph 3 C À ][Me 4 N ] (Figure 2) can be described as a quasi-linear arrangement of alternating Ph 3 C À carbanions and Me 4 N ions directed along one of the crystallographic 2 1 axes (neighboring chains run parallel). Some hydrogen atoms of the cation form CÀH d ¥¥¥ C dÀ interactions to the negatively charged C a (H1 and H4) and others form CÀH ¥¥¥ p interactions with the phenyl rings (H2 and H3).…”

“…For analysis of the hydrogen network, all hydrogen atoms have been calculated at distances of 1.08 ä. [15] Molecular illustrations and geometry calculations were performed with the PLATON package. [32] CCDC-172128 contains the supplementary crystallographic data for this paper.…”

“…[15] This is in contrast with expectation: the stronger base Ph 3 C À is a more potent acceptor in hydrogen bonds than the Cp À ion. This oddity might be explained by steric hindrance of the central carbon atom in Ph 3 C À .…”

“…This is presumably due to the antiquity of Schlenk×s early publication, but also to the fact that Schlenk did not recognize the existence of the ™free∫ carbanion himself. In the course of our research on ammonium and phosphonium salts of cyclopendienyl (Cp) anions [15] we investigated the compound originally made by Schlenk:…”

Relationships between QTAIM and the Decomposition of the Interaction Energy – Comparison of Different Kinds of Hydrogen Bond

“…Such CaHÁÁÁY hydrogen bonds were first found in crystal structures [23]; later their existence was proved by use of refined statistical methods [24]. It has been pointed out that carbon atoms may also act as proton acceptors and that XaHÁÁÁC hydrogen bonds [25] and even CaHÁÁÁC [26][27][28], are possible.…”