The rotational spectre of cyanoacetylene dimer, H-C-C-C-N ••• H-C-C-C-N

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.09]{Big23c2.eps} \end{wrapfigure} The rotational spectra of H-C-C-C-N ••• H-C-C-C-N, cyanoacetylene dimer,were recorded using Balle-Flygare type Fourier transform microwave (FTMW) spectrometers at Wesleyan and Arizona. The low-J transitions were measured down to 1.3 GHz at very high resolution, FWHM ~1 kHz using a large-cavity spectrometer. The spectral hyperfine structure due to the $^{14}$N 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: B$_0$ = 339.292331(8) MHz, D$_J$ = 32.15(8) Hz, H = -0.0015(2) Hz, eqQ($^{14}$N$_1$) = -3.990(1) MHz, and eqQ($^{14}$N$_2$) = -4.171(1) MHz. The vibrationally averaged dimer configuration is \\ H-C-C-C-N$_1$ ••• H-C-C-C-N$_2$. Using a simple linear model, the vibrational ground state and the equilibrium hydrogen bond lengths are determined to be: r$_0$(N ••• H) = 2.2489(3) \AA and r$_e$(N ••• H) = 2.2315 \AA. The equilibrium center-of-mass distance between the two HCCCN subunits is = 7.0366 \AA. 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$_1$ ••• H-C-C-C-N$_2$ are = 7.0603 \AA, $\theta$$_1$ = $13^{\circ}$, and $\theta$$_2$ = $8.7^{\circ}$, 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