Force field parameters for sulfates and sulfamates based on ab initio calculations: Extensions of AMBER and CHARMm fields

Ab initio self-consistent field (SCF) Hartree-Fock calculations of sulfates R? O? SO₃(?1) (R = Me, Et, i-Pr) and sulfamates R? NHSO₃(?1) (R = H, Me, Et, i-Pr) were performed at the 4-31G(*S*N) //3-21G(*S*N) basis set levels, where asterisks indicate d functions on sulfur and nitrogen atoms. These standard levels were determined by comparing calculation results with several basis sets up to MP2/6-31G*//6-31G*. Several conformations per compound were studied to obtain molecular geometries, rotational barriers, and potential derived point charges. In methyl sulfate, the rotational barrier around the C? O bond is 1.6 kcal/mol at the MP2 level and 1.4 kcal/mol at the standard level. Its ground state has one of three HCOS torsion angles trans and one of three COSO torsion angles trans. Rotation over 60° around the single O? S bond in the sulfate group costs 2.5 kcal/mol at the MP2 and 2.1 kcal/mol at the standard level. For ethyl sulfate, the calculated rotational barrier in going from the ground state, which has its CCOS torsion angle trans, to the syn-periplanar conformation (CCOS torsion angle cis) is 4.8 kcal/mol. However, a much lower barrier of 0.7 kcal/mol leads to a secondary gauchelike conformation about 0.4 kcal/mol above the ground state, with the CCOS torsion angle at 87.6°. Again, one of the COSO torsion angles is trans in the ground state, and the rotational barrier for a 60° rotation of the sulfate group amounts to 1.8 kcal/mol. For methyl sulfamate, the rotational barriers are 2.5 kcal/mol around the C? N bond and 3.3 kcal/mol around the N? S bond. This is noteworthy because sulfamate itself has a calculated rotational barrier around the N? S bond of only 1.7 kcal/mol. These and other data were used to parameterize the well-known empirical force fields AMBER and CHARMm. When the new fields were tested by means of vibrational frequency calculations at the 6-31G*//6-31G* level for methyl sulfate, sulfamate, and methyl sulfamate ground states, the frequencies compared favorably with the AMBER and CHARMm calculated frequencies. The transferability of the force parameters to β-D -glucose-6-sulfate and isopropyl sulfate appears to be better than to isopropyl sulfamate. © 1995 by John Wiley & Sons, Inc.