A molecular orbital study of the conformation (inversion and rotational barriers) and electronic properties of sulfamide

Abstract

Chem. 67, 2227 (1989).Ab initio calculations have been used to study the conformational potential surface of sulfamide, by considering the S-N bond rotations and the nitrogen inversion processes. The lowest energy conformation (b) is found for a cis-trans arrangement of the amino groups, although conformations with cis-cis (a), trans-trans (c), and near staggered (c') arrangements lie close in energy. Nitrogen inversion barriers are very low, and consequently one may expect forms b and c' to be the only ones present in the gas phase. Conformer a is very polar, its dipole moment being twice that of b, so it may be favored in condensed media or in polar-solvent solutions. The relative stability of the different isomers is governed by interactions between the amino protons and between the nitrogen lone pairs. Our results show that d-n backbonding, involving the d orbitals on sulfur, is responsible for the multiple bond character of the S-0 linkage, but is very small in the S-N interactions. The role of the sulfur d-orbital exponent, when a 6-31G* basis is used, is analyzed on a series of model compounds containing S", STV, and SV1. Although the inclusion of d functions on sulfur is crucial to describing correctly the bonding in sulfamide, the results obtained do not change appreciably if a second set of d functions is centered on sulfur. Nevertheless, only when polarization functions are also included for first-row atoms is the description of the system reliable.Key

Anomeric Effects in Sulfamides

“…2c ts was previously reported as a minimum for sulfamide with HF/STO-3G* used for the geometry optimization and HF/6-31G* with a modified d-orbital exponent used for the energies. 13 For comparison purposes, we also performed HF/STO-3G* and unmodified HF/6-31G* calculations on conformer 2c ts , but contrary to past results, we obtained 2a as the global minimum using both methods. Gaussian 09 15 was used to calculate the single point energy of the optimized stationary points at the CCSD(T)/aug-cc-pVTZ level.…”

“…Mó et al explained that the global minimum they observed 2b was due largely to the reduced steric hindrance between the amine hydrogens when one was eclipsed and the other was staggered. 13 However, the N–S–N angle is even smaller in 2a than in 2b , which would not be predicted solely based upon sterics, as the amine hydrogens are more crowded in 2a than in 2b .…”

crystal structure analyses of azasulfuryltripeptides reveal potential for γ‐turn mimicry†

“…Stabilization of the γ–turn geometry appears to arise from the combination of lone pair‐lone pair repulsion between the adjacent nitrogen of the hydrazine, and the conformational preferences of the sulfamide . In contrast to γ–mimics based on covalent constraint, to the best of our knowledge, the azasulfuryltripeptides presented herein represent the first examples of stabilization of a γ–turn conformation by electronic interactions of neighboring heteroatoms.…”

Local and nonlocal density functional calculations of the molecular structure of isomeric thiadiazole monoxides

“…Condensed softness indices allowing the comparison of reactivity between similar atoms of different molecules can be easily calculated starting from the relation between the Fukui function f ( r ), and the local softness s ( r ): It is well known that the use of basis sets including polarization functions is essential to obtain reliable results for compounds with hypervalent sulfur atoms 27. It has also been shown 28–31 that the STO‐3G* basis set reproduces the geometry of hypervalent sulfur compounds reasonably well. Nevertheless, as a higher accuracy is needed for a good description of properties that depend on the electronic structure, the condensed Fukui functions and condensed local softness indices, the electronic density maps, the molecular electrostatic potentials, and the gross orbital charges were obtained from HF and local and nonlocal DFT using the 6‐31G** basis set through single‐point calculations on the fully optimized at the same level neutral molecule and the singly ionized positive and negative molecules with the same geometric structure of the neutral molecule.…”