Rotational spectrum and structure of the T-shaped cyanoacetylene carbon dioxide complex, HCCCN⋯CO2
Abstract The rotational spectrum of the T-shaped cyanoacetylene carbon dioxide dimer, HCCCN⋯CO 2 , was measured using two Balle-Flygare Fourier transform microwave (FTMW) spectrometers between 1.4GHz and 25GHz. 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: A 0 = 11273(18)MHz, B 0 = 764.088(21)MHz, C 0 = 716.254(21)MHz, Δ J = 0.50329(34)kHz, Δ JK =0 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 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 6.9(9)Hz, and ϕ K = −3.592(13)kHz. The 14 N nuclear quadrupole coupling constants were fit to χ aa =& −4.12753(38)MHz and χ bb −χ cc =0 0.103(15)MHz. The small negative inertial defect, Δ 0 = −0.66(12) u A 2 , 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; CO 2 subtending a 5.4(5)° torsional oscillation angle with the b molecular axis. The van der Waals bond length is 3.0137(3)A. The stretching force constant, k s = 3.9N/m, and the stretching frequency, ν s = 53cm −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 14 N quadrupole coupling strengths were made and the results compared with experimental results. Highlights The first, high resolution microwave measurements of rotational transitions for HCCCN⋯CO 2 . Accurate rotational constants are used to determine structural parameters for the T-shaped HCCCN⋯CO 2 complex. High-level MP2 calculations using yielded B and C rotational constants within 1% of measured values. Nitrogen quadrupole coupling parameters were obtained for HCCCN⋯CO 2 . A torsional oscillation model was developed for the structural analysis. Graphical abstract [DISPLAY OMISSION]
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