Microscopic Structure\nof Neat Linear Alkylamine Liquids:\nAn X‑Ray Scattering and Computer Simulation Study

Linear amines, from propylamine to nonylamine, are studied\nunder\nambient conditions by X-ray scattering and molecular dynamics simulations\nof various force field models. The major finding is that the prepeak\nin alkylamines is about 1 order of magnitude weaker than that in alkanols,\nhence suggesting much weaker hydrogen bonding-induced clustering of\nthe amine groups than for the hydroxyl groups. Computer simulation\nstudies reveal that the OPLS-UA model reproduces the prepeak, but\nwith larger amplitudes, while the GROMOS-UA and CHARMM-AA force fields\nshow almost no prepeak. Simulations of all models show the existence\nof hydrogen-bonded clusters, equally confirmed by the prominent prepeak\nof the structure factor between the nitrogen atoms. The hydrogen bond\nstrength, as modeled by the Coulomb association in classical force\nfield models, is about the same order of magnitude for both systems.\nThen, one may ask what is the origin of the weaker prepeak in alkylamines?\nSimulation data reveal that the existence of the prepeak is controlled\nthrough the cancellation of the positive contributions from the charged\ngroup correlations by the negative contributions from the cross charged–uncharged\ncorrelations. The <i>C</i><sub>2v</sub> symmetry of the\namine headgroup hinders clustering, which favors cross correlations\nwith the tail atoms. This is opposite to alkanols where the symmetry\nof the hydroxyl headgroup favors clustering and hinders cross correlations\nwith the alkyl tail. This competition between charged and uncharged\natomic groups appears as a general mechanism to explain the existence\nof scattering prepeaks, including their position and amplitude.