Intermolecular forces and conformational change upon crystallization: The case of phosphorobenzopyrane derivatives

The possible conformations of two phosphorobenzopyrane derivatives (I and I I) are investigated with the help of ab initio and density functional theor y methods. Special attention is paid to the conformation around the phosphi ne group. It is demonstrated that while I possesses the same conformations in the gas and crystal phases, II assumes a conformation in a crystal that is not the lowest energy conformer in the isolated molecule. The effect is explained by the difference in the crystal packing of benzopyrane rings for the two compounds. The higher energy conformer of II is favored in a cryst al since it avoids creation of strong repulsive interaction between neighbo ring molecules due to short interatomic distances. (C) 2001 American Instit ute of Physics.