The infrared spectra of NH3⋅H2O and ND3⋅D2O at 100 K

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Abstract
The infrared spectra of ND3⋅D2O and hydrogen or deuterium impurity in NH3⋅H2O or ND3⋅D2O at 100 K are reported for the first time. Their interpretation is aided by a D2h pseudosymmetry caused by the heavy atoms being nearly coplanar in the P212121, D42, crystal. No evidence of the possible disorder of the hydrogen atoms is observed. Oriented gas model calculations gave the approximate relative intensities of the unit cell components of the molecular vibrations. The site splitting and correlation splitting are comparable for ν3 and, probably, ν4 of ammonia; three unit cell components are observed instead of the six predicted under D2, three components being calculated to have near-zero intensity. The symmetric deformation of ammonia, ν2, yields two unit cell modes with significant intensity, separated by 37 cm−1 for NH3 and 23 cm−1 for ND3. The isotope frequency ratio for ν2 is lower than for any other mode, so this large splitting must be due to intermolecular coupling, probably transition dipolar in origin. The two strong νOH (HDO) absorptions are 140 cm−1 further apart than the two strong νOH (H2O) absorptions, a surprising result because intramolecular coupling is negligible because νO–H⋅⋅⋅N is ∼400 cm−1 below νO–H⋅⋅⋅O. In contrast, νOD (D2O) yields four strong absorptions approximately centered with respect to the two strong νOD (HDO) absorptions. The O–D⋅⋅⋅N doublet is due to the B1 and B2 unit cell group components, the B3 component being too weak to see, as is the case for ν2 of ammonia. The corresponding O–H⋅⋅⋅N doublet is unresolved. Use of the O–D⋅⋅⋅O–D′interaction force constant of the ice phases, −0.10 mdyn Å−1, and oriented gas model calculations of the relative intensities shows that the O–D⋅⋅⋅O bands at 2390 and 2459 cm−1 are due to the in-phase (B3) and out-of-phase (B2) motions of the two O–D bonds in each chain of water molecules in each unit cell. In NH3⋅H2O the out-of-phase O–H⋅⋅⋅O vibration interacts, probably with 2ν4 of ammonia, and its intensity is dissipated among several weak features. We are unable to explain the difference between the coupled and uncoupled O–H⋅⋅⋅N frequencies, 2887 and 2825 cm−1, in NH3⋅H2O. The bending mode of water is tentatively assigned at 1696, 1467, and 1243 cm−1 for H2O, HOD, and D2O, respectively. The lattice absorptions are assigned to rotational or translational motion. The rotational modes of ammonia are assigned to the B1, B2, and B3 unit cell modes that have significant intensity.