Rotational spectrum and structure of the T-shaped cyanoacetylene carbon dioxide complex, HCCCN⋯CO₂

The rotational spectrum of the T-shaped cyanoacetylene carbon dioxide dimer, HCCCN⋯CO₂, was measured using two Balle-Flygare Fourier transform microwave (FTMW) spectrometers between 1.4 GHz and 25 GHz. Only the K_a = 0, 2, 4 branches of spectrum from J′ = 1 ← 0 to J″ = 16 ← 15 transitions were observed. The vanishing of the K_a = 1, 3, … transitions demonstrates a C_2v symmetry complex with a T-shaped alignment of the subunits. The spectroscopic constants were fit using Pickett's SPFIT/SPCAT suite of programs obtaining: A₀ = 11273(18) MHz, B₀ = 764.088(21) MHz, C₀ = 716.254(21) MHz, Δ_J = 0.50329(34) kHz, Δ_JK = 0.120867(11) MHz, Δ_K = −28.17(36) MHz, δ_J = 0.0613(21) kHz, δ_K = 44.25(95) kHz, Φ_J = 0.0053(12) Hz, Φ_JK = 9.820(55) Hz, Φ_KJ = −0.59325(72) kHz, Φ_K = −2.3719(53) MHz, ϕ_J = 0.0398(42) Hz, ϕ_JK = 6.9(9) Hz, and ϕ_K = −3.592(13) kHz. The ¹⁴N nuclear quadrupole coupling constants were fit to χ_aa = −4.12753(38) MHz and χ_bb − χ_cc = 0.103(15) MHz. The small negative inertial defect, Δ₀ = −0.66(12) u Ų, indicates a vibrationally averaged planar complex with non-negligible low frequency out-of-plane vibrations. While maintaining near-planar orientation, both binding partners exhibit large-amplitude bending vibrations within the plane. To deal with the intermolecular dynamics, a torsional oscillation model was developed in this work for the structural analysis. According to this model, the vibrational bending amplitude for HCCCN torsional angle is 10.(1)°, with the a-axis of complex; CO₂ subtending a 5.4(5)° torsional oscillation angle with the b molecular axis. The van der Waals bond length is 3.0137(3) Å. The stretching force constant, k_s = 3.9 N/m, and the stretching frequency, ν_s = 53 cm⁻¹, for the van der Waals bond were calculated using the pseudo-diatomic model. High-level MP2 and DFT calculations of structural parameters, rotational constants, and ¹⁴N quadrupole coupling strengths were made and the results compared with experimental results.