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 -p 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 2 O complexes to being H-bond acceptors to form FYH-H 2 O complexes. The NEDA analysis indicates that H-bond energies of FYO-H 2 O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH-H 2 O complexes H-bond strengths are similar. The model of CTs from FYO to H 2 O differs from that at FYH-H 2 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 S 1 excited states), formamide and its heavier analogues form double H-bonds with H 2 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 N !p* YO conjugation, and for heavier analogues of formamide, the n N !s* YO hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier.
Though significant efforts have been extended to improve material/material design, the organic luminescent molecule containing n-conjugated aromatic system opens a new avenue for understanding the molecular modelling strategies. The molecular electronics components largely depend on n-conjugation as for charge transportation. An n-nconjugated small molecule, formamide was primarily studied within density functional theory (DFT) framework to investigate the Si/Ge substitution effect on its inherent geometric and electronic properties. The nature of hydrogen bond in these types of molecules both at ground state and at low-lying states was also extensively explored based on electronegativity as a part of investigation ofn-conjugated system.Several n-conjugated systems including Si/Ge heteroatoms were then investigated substantially to get deep insight into their geometric and electronic properties, as for example, planarity, dihedral angles, potential energy surface, aromaticity, stability, hyperconjugation, and so on.lt enables someone to modify the structure or composition in search of more active molecules. We continued our OFT studies along with TD-DFT (Time-dependent density functional theory) on n-<:onjugated cyclic structures to know how heteroatoms (like Si, S or N) within conjugated backbone and extension of nconjugation length by incorporation of donor/acceptor fragments under different architectures influence electron delocalization and bring change into their inherent electronic and optical properties. This detailed study is expected to be helpful for effective design of molecular systems for optoelectronic applications.
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