Abstract
The hydrogen bond plays important roles in the reactions of molecular crystals. If the reactive group of the molecule is hydrogen bonded with the neighboring molecules in the crystal, the hydrogen bond severely influences the reactivity of the molecule, because it (1) changes the electronic state of the reactive group and/or (2) inhibits the migration of the group in the process of reaction. Even if the hydrogen bond has no connection with the reactive group, it strengthens the crystal lattice and accelerates the photoreactions since not only the energy to destroy the crystalline lattice is unnecessary but also light can easily penetrate into the crystal. The hydrogen migration in the solid-state reactions can be directly observed by neutron diffraction when only the peculiar hydrogen atom considered can be replaced with the deuterium atom.
