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

• Published in: Journal of physical organic chemistry 2013-05, Vol.26 (5), p.420-431
• Main Authors: Xi, Hong-Wei, Bedoura, Sultana, Lim, Kok Hwa
• Format: Article
• Language: English
• Subjects: H-bond; heavier group-14 analogous of formamide; Hydrogen bonds; internal rotation barrier; NBO analysis; NEDA analysis
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3103

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 (YHONH2, Y = Si, Ge and Sn), the nN–πY=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–H2O complexes to being H‐bond acceptors to form FYH–H2O complexes. The NEDA analysis indicates that H‐bond energies of FYO–H2O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH–H2O complexes H‐bond strengths are similar. The model of CTs from FYO to H2O differs from that at FYH–H2O 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 S1 excited states), formamide and its heavier analogues form double H‐bonds with H2O 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 nN→π*YO conjugation, and for heavier analogues of formamide, the nN→σ*YO hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier. Copyright © 2013 John Wiley & Sons, Ltd. The nN–πY=O conjugation strengths of YHONH2 (Y = Si, Ge and Sn) do not always reduce as Y becomes heavier. H‐bond energies of FYO–H2O increase from Si to Sn due to concurrently stronger charge transfer and electrostatic attraction. For heavier analogues of formamide, the nN→σ*YO hyperconjugation effect and steric effect considerably contribute to the overall barrier of internal rotation.