Hydrogen bond and internal rotations barrier: DFT study on heavier group‐14 analogues of formamide. (10th April 2013)

• Record Type: Journal Article
• Authors: Xi, Hong‐Wei; Bedoura, Sultana; Lim, Kok Hwa
• Is Part Of: Journal of physical organic chemistry. Volume 26:Number 5(2013:May)
• Page Start: 420
• Page End: 431
• Publication Date: 2013-04-10
• Subjects: Chemistry, Physical organic -- Periodicals; 547.1
• DOI: 10.1002/poc.3103
• Languages: English
• ISSNs: 0894-3230

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>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<sub>2</sub>, Y = Si, Ge and Sn), the n<sub>N</sub>–π<sub>Y=O</sub> 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<sub>2</sub>O complexes to being H‐bond acceptors to form FYH–H<sub>2</sub>O complexes. The NEDA analysis indicates that H‐bond energies of FYO–H<sub>2</sub>O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH–H<sub>2</sub>O complexes H‐bond strengths are similar. The model of CTs from FYO to H<sub>2</sub>O differs from that at FYH–H<sub>2</sub>O 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 <italic>S</italic><sub>1</sub> excited states), formamide and its heavier analogues form double H‐bonds with H<sub>2</sub>O 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<sub>N</sub>→π*<sub>YO</sub> conjugation, and for heavier analogues of formamide, the n<sub>N</sub>→σ*<sub>YO</sub> hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier. Copyright © 2013 John Wiley & Sons, Ltd.</p> </abstract>