The possible conformations of two phosphorobenzopyrane derivatives (I and II) are investigated with the help of ab initio and density functional theory methods. Special attention is paid to the conformation around the phosphine 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 crystal since it avoids creation of strong repulsive interaction between neighboring molecules due to short interatomic distances.