Rotational spectra and nitrogen nuclear quadrupole coupling for the cyanoacetylene dimer: H-C≡C-C≡N⋯H-C≡C-C≡N

The rotational spectra of cyanoacetylene dimer, H-C≡C-C≡N⋯H-C≡C-C≡N, were recorded using Balle-Flygare type Fourier transform microwave (FTMW) spectrometers. The low J transitions were measured down to 1.3 GHz at very high resolution, FWHM ∼ 1 kHz. The spectral hyperfine structure due to the ¹⁴N nuclear quadrupole coupling interactions is well-resolved below 4 GHz using a low frequency spectrometer at the University of Arizona. The experimental spectroscopic constants were fitted as: B₀ = 339.2923310(79) MHz, D_J = 32.152(82) Hz, H = -0.00147(20) Hz, eqQ(¹⁴N₁) = -3.9902(14) MHz, and eqQ(¹⁴N₂) = -4.1712(13) MHz. The vibrationally averaged dimer configuration is H-C≡C-C≡N⋯H-C≡C-C≡N. Using a simple linear model, the vibrational ground state and the equilibrium hydrogen bond lengths are determined to be: r₀(N⋯H) = 2.2489(3) Å and r_e(N⋯H) = 2.2315 Å. The equilibrium center-of-mass distance between the two HCCCN subunits is r_com = 7.0366 Å. Using the rigid precession model, the vibrational ground state center-of-mass distance and the pivot angles which HCCCN subunits make with the a-axis of H-C≡C-C≡N⋯H-C≡C-C≡N are r_{c . m .} = 7.0603 Å, θ₁ = 13.0°, and θ₂ = 8.7°, respectively. The calculated hydrogen bond energy of H-C≡C-C≡N⋯H-C≡C-C≡N is 1466 cm^-1 using the MP2/aug-cc-PVTZ method in present work.