To understand how the crystal conformer converts to the active conformer, here the internal rotation potential energy surface around the two C—N bonds in the sulfonylurea bridge of monosulfuron has been studied by density functional B3LYP method and 6-31G(d) basis set. For the stationary points along the internal rotation energy surface, full geometry optimization or QST3 transition structure search and frequency analysis calculations were also performed. The calculated results indicated that there were four stable conformers and eight transition state structures corresponding to the conformational conversion of monosulfuron. The results also implied that the four stable conformers could be converted mutually through eight conversional paths. During the conversion, the internal hydrogen bond in monosulfuron can play a key role in the conformational stability and conversional process. For the obtained conformers, B3LYP/6-31＋＋G(d, p) method was also used to do single point calculation in vacuum and two PCM model solvents of water and heptane. The calculated results implied that the conversion mainly occurred in the aqueous solution.

Key words: monosulfuron, conformation, internal rotation, density functional theory, hydrogen bond