Nonempirical calculations of sulfimide H2S=NH

Abstract

tion of the lone pairs of the sulfur and nitrogen atoms [2, c, 7] and with a length of S-N bond equal to 162~2-167.3 pm [8, 9]. Conversely, N-chloro- and N-trifluoromethyl-S,S-difluorosulfimide have Z conformations with an S-N bond shortened to 144.7-147.6 pm [I0, Ii]. The only criterion for reliability of the calculations of the H2S=NH molecule may be maximum agreement between the calculated length of the S-N bond and the experimental value corresponding to the E conformer. Therefore, the E conformer of sulfimide was initially calculated in the STO-3-21G, STO-3G, STO-3G,* STO-4-31G,* and STO-4-31G** basis sets with complete optimization of the geometry by the gradient method with the use of the GAUSSIAN-80 and MONSTER GAUSS-81 programs An analysis of the results, which are presented in Table i, showed that the STO-3-21G basis inaccurately conveys the conformation of the H2S=NH molecule (the dihedral angle between the lone pairs of the sulfur and nitrogen atoms is equal to =120 ~ and significantly overestimates the length of the S-N bond (221ol ppm). When the minimal STO-3G basis is used, the conformation of the molecule is correctly conveyed, but the length of the S-N bond (194.4 pm) is 17% greater than the experimental value. Expansion of the basis set and the addition of the d functions on the nitrogen atom to it (the STO-4-31G* basis) do not improve the accuracy of the calculation. In the case of the calculations in the STO-3G* and STO-4-31G** basis sets, which take into account the d orbitals on the sulfur atom, the length of the S-N bond is close to the experimental value, i.e., the error does not exceed 2%. The geometries obtained in these two basis set differ within the range of error of the calculation. Thus, the minimal STO-3G* basis set with the d functions on the sulfur atom is already sufficient for an exact description of the geometry of sulfimides. A comparison of the total energies of the E and Z conformers of the H2S=NH molecule calculated in the STO-4-31G** and STO-3G* basis sets reveals that the Z conformer is more stable than the E conformer by 7.92 and 11.22 kJ/mole, respectively. The relative stabilization of the conformers depends mainly on the effectiveness of the following interactions (Fig. i): the lone pair of the nitrogen atom with the antibonding o* orbital of the SH2 fragment [no(N)-o*(SH2)] , the lone pair of the sulfur atom with the antibonding orbital of the NH fragment [no(S)-o*(NH)], and the lone pairs of the nitrogen and sulfur atoms with one another