Perturbation allowed transitions in the infrared spectrum of ¹⁴ND₃: determination of the <i>K</i>-dependent parameters in the ground state

Accurate values of the <i>K</i>-dependent constants ⁽ <i>ⁱ</i> ⁾ <i>C</i>, ⁽ <i>ⁱ</i> ⁾ <i>D_K </i> and ⁽ <i>ⁱ</i> ⁾ <i>H_K </i> in the ground state of ¹⁴ND₃, with <i>i</i> = <i>s</i>, <i>a</i>, have been determined for the first time thanks to the detections of ‘perturbation allowed’ transitions in the ν₁, ν₂, ν₃, ν₄ and 2ν₄ infrared bands. The rotation–inversion and inversion transitions from the literature, together with 7289 ground state combination differences from the infrared vibration–rotation–inversion transitions have been simultaneously analysed. The adopted rotation–inversion Hamiltonian includes distortion constants up to the eighth power and the Δ<i>k</i> = ±3 and Δ<i>k</i> = ±6 interaction terms. Precise values of the diagonal constants and of the Δ<i>k</i> = ±3 interaction coefficients have been obtained. Accurate values of the ground state term values have been calculated for both <i>s</i> and <i>a</i> levels up to <i>J</i> = 21.