PNNL

Abstract

Molecular hosts for anion complexation are often constructed by combining two or more hydrogen bonding functional groups, D–H. The deliberate design of complementary host architectures requires knowledge of the optimal geometry for the hydrogen bonds formed between the host and the guest. Herein, we present a detailed study of the structural aspects of hydrogen bonding interactions with the NO3– anion. A large number of crystal structures are analyzed to determine the number of hydrogen bond contacts per anion and to further characterize the structural aspects of these interactions. Electronic structure calculations are used to determine stable geometries and interaction energies for NO3– complexes with several simple molecules possessing D–H groups, including water, methanol, N-methylformamide, and methane. Theoretical results are reported at several levels of density functional theory, including BP86/DN**, B3LYP/TZVP, and B3LYP/TZVP+, and at MP2/aug-cc-pVDZ. In addition, MP2 binding energies for these complexes were obtained at the complete basis set limit by extrapolating from single point energies obtained with larger correlation-consistent basis sets. The results establish that NO3– has an intrinsic hydrogen bonding topography in which there are six optimal sites for proton location. The structural features observed in crystal structures and in the optimized geometries of complexes are explained by a preference to locate the D–H protons in these positions. For the strongest hydrogen bonding interactions, the N–O•••H angle is bent at an angle of 115 ± 10°, and the hydrogen atom lies in the NO3– plane giving O–N–O•••H dihedral angles of 0 and 180°. In addition, the D-H vector points towards the oxygen atom, giving D–H•••O angles that are near linear, 170 ± 10°. Due to steric hindrance, simple alcohol O–H and amide N–H donors form 3:1 complexes with NO3–, with H•••O distances of 1.85 ± 0.5 Å. Thus, the optimal cavity radius for a tridentate host, defined as the distance from the center to the D–H hydrogen atoms, is 2.65 ± 0.15 Å.