Hydrogen bond and internal rotations barrier: DFT study on heavier group-14 analogues of formamide

摘要

A theoretical study on heavier group-14 substituting effect on the essential property of formamide, strong hydrogen bond with water and internal rotational barrier was performed within the framework of natural bond orbital (NBO) analysis and based on the density functional theory calculation. For heavier group-14 analogues of formamide (YHONH_2, Y = Si, Ge and Sn), the n_N-π_Y=O conjugation strength does not always reduce as Y becomes heavier, for example, silaformamide and germaformamide have similar strength of delocalization. Heavier formamides prefer being H-bond donors to form FYO-H_2O complexes to being H-bond acceptors to form FYH-H_2O complexes. The NEDA analysis indicates that H-bond energies of FYO-H_2O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH-H_2O complexes H-bond strengths are similar. The model of CTs from FYO to H_2O differs from that at FYH-H_2O complexes, which are contributed not only by aligning lone-pair orbital of O but also by another lone-pair orbital. At two lowest lying excited states (the triplet and S_1 excited states), formamide and its heavier analogues form double H-bonds with H_2O molecule at the same time. The barrier heights of internal rotation become gradually low from C to Sn, formamide (15.73 kcal/mol)>silaformamide (11.73 kcal/mol)>germaformamide (9.45 kcal/mol)>stannaformamide (7.50 kcal/mol) at the CCSD(T)/aug-cc-pVTZ//B3LYP/cc-pVTZ level. NBO analysis indicates that the barrier does not only come from the n_N→π*_(YO) conjugation, and for heavier analogues of formamide, the n_N→σ*_(YO) hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier.