The Carbon Lone Pair as Electron Donor. Ionic Hydrogen Bonds in Isocyanides

Abstract

Hydrogen bond dissociation energies (ΔH°D) in protonated dimer ions containing isocyanides were measured by pulsed high-pressure mass spectrometry, and the interactions were analyzed by ab initio calculations. Strong bonding (80−105 kJ/mol (19−25 kcal/mol)) is observed when the carbon lone pair is the electron donor, i.e. in the complexes of isocyanides with protonated amines and protonated isocyanides (R3NH+···CNR and RNCH+···CNR complexes). The bonding is weaker (60−90 kJ/mol (14−21 kcal/mol)) in the complexes of oxygen bases with protonated isocyanides, i.e. in RNCH+···O-type complexes. Inverse linear correlations between ΔH°D and the proton affinity difference of the components show slopes of −0.22 for R3NH+···CNR- and −0.25 for RNCH+···O-type complexes. The intercepts yield intrinsic bond strengths (ΔPA = 0) of 107.7 (25.7 kcal/mol) and 100.0 kJ/mol (23.9 kcal/mol), respectively. Geometry optimizations were carried out at four calculational levels, the largest of which is MP2/6-31+G(d,p). Single-point energies were obtained with increasingly flexible basis sets up to cc-pVTZ+. Trends in dissociation energies within the cyanide and isocyanide series of complexes and between the two series of complexes hold for every basis set considered. Calculated and experimental ΔH°D values agree within the standard uncertainty of ±6 kJ/mol (1.5 kcal/mol) for only four of the nine complexes for which experimental data are available. The hydrogen bonding properties of sp-type carbon vs nitrogen lone pairs are illustrated by comparing analogous isocyanide and cyanide complexes. The relative importance of the electrostatic and delocalization components of the dissociation energy is different for the two sets of complexes, with delocalization effects being more important for the isocyanides.