High-resolution infrared spectroscopy of ¹³C¹²CD₂: the bending states with υ₄+υ₅ up to 3

<div><p>The high-resolution infrared spectrum of deuterated acetylene containing one ¹³C atom, ¹³C¹²CD₂, has been recorded by Fourier transform spectroscopy in the range 450–1700 cm⁻¹. The υ₄ and υ₅ bending fundamental bands and a number of overtone, combination and hot bands were identified. In total, 3210 rotation vibration transitions were assigned to 27 bands involving bending states with both υ₄ + υ₅ and |<i>l</i>₄ + <i>l</i>₅| up to 3. The ground state and 13 vibrationally excited states were characterised, i.e., all the <i>l</i>-vibrational components of the excited bending manifolds with υ₄ + υ₅ up to 2, (^IIΠ), (^IIΠ), and (Π and Φ). Two simultaneous least-squares fits of all assigned transitions have been performed, one considering transitions with υ₄ + υ₅ up to 2, the other adding the transitions with υ₄ + υ₅ = 3. The model Hamiltonian adopted for the analysis takes into account the usual vibration and rotation <i>l-</i>type resonances within each vibrational manifold and the Darling−Dennison anharmonic resonance between and , and between and . Two sets of 33 and 42, respectively, spectroscopic parameters were derived. They reproduce 1512 and 2450 assigned transitions with root mean square values of 3.5 × 10⁻⁴ and 3.9 × 10⁻⁴ cm⁻¹, respectively. The transitions of four hot bands reaching the (^IIΠ) state were fitted simultaneously to the effective upper state parameters.</p>