How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides

Celine Nieuwland; Célia Fonseca Guerra

doi:10.1002/chem.202200755

How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides

Celine Nieuwland, Célia Fonseca Guerra^*

^*Corresponding author for this work

Research output: Contribution to Journal › Article › Academic › peer-review

Abstract

The amino groups of thio- and selenoamides can act as stronger hydrogen-bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH₂ groups in thio- and selenoamides are more positively charged than in carboxamides. This originates from the larger electronic density flow from the nitrogen lone pair of the NH₂ group towards the lower-lying π*_C=S and π*_C=Se orbitals than to the high-lying π*_C=O orbital. The relative energies of the π* orbitals result from the overlap between the chalcogen np and carbon 2p atomic orbitals, which is set by the carbon-chalcogen equilibrium distance, a consequence of the Pauli repulsion between the two bonded atoms. Thus, neither the electronegativity nor the often-suggested polarizability but the steric size of the chalcogen atom determines the amide's hydrogen-bond donor capability.

Original language	English
Article number	e202200755
Journal	Chemistry - A European Journal
Volume	28
Issue number	31
Early online date	23 Mar 2022
DOIs	https://doi.org/10.1002/chem.202200755
Publication status	Published - 1 Jun 2022

Bibliographical note

Funding Information:
The authors acknowledge the financial support from the Netherlands Organization for Scientific Research (NWO). This work was carried out on the Dutch national e-infrastructure with the support of SURF Cooperative. Furthermore, the authors thank Dr. Pascal Vermeeren and Eva Blokker for the insightful discussions.

Funding Information:
The authors acknowledge the financial support from the Netherlands Organization for Scientific Research (NWO). This work was carried out on the Dutch national e‐infrastructure with the support of SURF Cooperative. Furthermore, the authors thank Dr. Pascal Vermeeren and Eva Blokker for the insightful discussions.

Publisher Copyright:
© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.

Keywords

amides
chalcogens
density functional calculations
hydrogen bonding
organocatalysis

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abstract = "The amino groups of thio- and selenoamides can act as stronger hydrogen-bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH2 groups in thio- and selenoamides are more positively charged than in carboxamides. This originates from the larger electronic density flow from the nitrogen lone pair of the NH2 group towards the lower-lying π*C=S and π*C=Se orbitals than to the high-lying π*C=O orbital. The relative energies of the π* orbitals result from the overlap between the chalcogen np and carbon 2p atomic orbitals, which is set by the carbon-chalcogen equilibrium distance, a consequence of the Pauli repulsion between the two bonded atoms. Thus, neither the electronegativity nor the often-suggested polarizability but the steric size of the chalcogen atom determines the amide's hydrogen-bond donor capability.",

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AB - The amino groups of thio- and selenoamides can act as stronger hydrogen-bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH2 groups in thio- and selenoamides are more positively charged than in carboxamides. This originates from the larger electronic density flow from the nitrogen lone pair of the NH2 group towards the lower-lying π*C=S and π*C=Se orbitals than to the high-lying π*C=O orbital. The relative energies of the π* orbitals result from the overlap between the chalcogen np and carbon 2p atomic orbitals, which is set by the carbon-chalcogen equilibrium distance, a consequence of the Pauli repulsion between the two bonded atoms. Thus, neither the electronegativity nor the often-suggested polarizability but the steric size of the chalcogen atom determines the amide's hydrogen-bond donor capability.

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How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides

Abstract

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Keywords

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