Spectra-structure correlations in solid metal saccharinates. II. Ab initio molecular structures and vibrational spectra of N-substituted saccharins at the HF level

Ground-state ab initio molecular geometries and vibrational spectra of 24 N-substituted isolated saccharins with small-size B, Br, C, Cl, F, N, O, P or S-groups and the parent molecule are predicted at RHF/6-31G level to examine the molecular structural changes stemming from N-substitution of saccharin (o-sulfobenzimide). Trends in the molecular geometrical parameters of the sulfimide ring and the carbonyl stretching frequency are discussed in relation to the electronic properties of the substituent and the solid state effects. The results are compared with the crystallographic data for N-substituted saccharins and metal saccharinato salts/complexes retrieved from the Cambridge Structural Database. The ability of several theoretical methods to describe the substitution/deprotonation of the conjugated CO-NH-SO₂ structure is summarized. Electronic properties of the substituent affect significantly only the immediate C-N and S-N bonds by as much as ±0.014 Å, while other bonds are relatively less influenced (±0.004 Å). Combined with the effects of the crystal packing and thermal vibrations, they impose flexibility on the intramolecular lengths up to ±0.02 Å. High correlation (R = 0.966) between the theoretical ν(CO) frequencies and C-O distances is predictable for both of these parameters, but is lowered notably in the crystal by both vibrational and solid-state circumstances. From the structural viewpoint, the N_sac-X bonds (X = B, Br, C, Cl, F, N, O, P, S; sac denotes saccharin) behave similarly to the purely covalent N_sac-metal bonds.