Temperature-dependent, effective structures of the 14NH3 and 14ND3 molecules

Measurements result in effective, usually temperature-dependent structural parameters of molecules, and never directly in equilibrium structures, which are theoretical constructs. A recent high-accuracy semiglobal potential energy surface of the electronic ground state of the ammonia molecule, called NH3-Y2010 (J. Mol. Spectrosc. 2011, 268, 123), which exhibits mass-independent equilibrium NH bond length and a HNH bond angle of 1.0109 Å and 106.75°, respectively, is employed together with the variational nuclear motion code GENIUSH (J. Chem. Phys. 2009, 130, 134112; 2011, 134, 074105) to determine directly measurable, effective structural parameters of the (14)NH(3) and (14)ND(3) molecules. The effective r(g)- and r(a)-type NH(ND) distances determined at 300 K are 1.0307(1.0254) and 1.0256(1.0217) Å, respectively, with an estimated accuracy of 2 × 10(-4) Å. The effective θ(g) HNH and DND bond angles at 300 K are 106.91° and 106.85°, respectively. The root-mean-square amplitudes of vibration, l(g), for the NH(ND) distances at 300 K are 0.073(0.062) Å. These structural parameters confirm the less accurate results of a room-temperature gas-electron-diffraction study (J. Chem. Phys. 1968, 49, 2488, all data in Å): r(g)(NH) = 1.030(2), l(g)(NH) = 0.073(2), r(g)(ND) = 1.027(3), and l(g)(ND) = 0.061(2). The computed difference in the r(g,T)(NH) bond lengths of the two spin isomers (ortho and para forms) of (14)NH(3) is 3 × 10(-5) Å at 0 K, the difference diminishes at temperatures of about 30-50 K.