In this Letter, structure and properties of gas-phase dihydrates of nitric acid (NAD) have been studied using density functional theory. Three equilibrium NAD structures, including two new isomers have been determined and their structural data, vibrational transitions, infrared-spectra, dipole moments, and thermodynamics properties have been investigated. The results obtained were used to explore the formation pathways for gas-phase di and trihydrates (NAT) of the nitric acid - key components of the ice crystals formed in polar stratospheric clouds. The present study leads us to conclude that nitric acid crystals under stratospheric conditions are formed mainly from PNAD and beta-NAT. (c) 2006 Elsevier B.V. All rights reserved.