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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 14 N nuclear quadrupole coupling interactions is wellresolved below 4 GHz using a low frequency spectrometer at the University of Arizona. The experimental spectroscopic constants were fitted as: B 0 = 339.2923310(79) MHz, D J = 32.152(82) Hz, H =-0.00147(20) Hz, eqQ (14 N 1) =-3.9902(14) MHz, and eqQ (14 N 2) =-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: r 0 (N  H) = 2.2489(3) Å and r e (N  H) = 2.2315 Å. The equilibrium centerof-mass distance between the two HCCCN subunits is com r = 7.0366 Å. Using the rigid procession 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. .m c r = 7.0603 Å,  1 = 13.0, and  2 = 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.

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The rotational spectra of T-shaped cyanoacetylene carbon dioxide complex, HCCCN -CO 2 , were measured using two Balle-Flygare type Fourier transform microwave (FTMW) spectrometers between 1.4 GHz and 22 GHz. The low J transitions were recorded using the low frequency FTMW spectrometer at the University of Arizona with a state-of-theart resolution of "full width at half maximum" (FWHM) 1 kHz. The spectra above 4 GHz were recorded at Wesleyan University. Spectral hyperfine structures due to the 14 N nuclear quadrupole coupling interactions can be fully resolved in low frequency bands. Since all K a = 1 branches were not observed, this implies that HCCCN -CO 2 possesses a rigorous T-shaped structure. Assuming that A 0 is the same as that of HCN -CO 2 , 11824 MHz, the spectroscopic constants of HCCCN -CO 2 are: B 0 = 794.59686(63) MHz, C 0 = 715.74488(60) MHz, ∆ J = 0.50067(18) kHz, ∆ JK = 120.892(12) kHz, δ J = 0.04253(31) kHz, δ K = 65.32(12) kHz, H J = -0.00117(33) Hz, H JK = 0.034876(21) kHz, H KJ = -0.68254(73) kHz, χ aa ( 14 N) = -4.12873(78) MHz, χ bb ( 14 N) = 2.110(25) MHz, and χ cc ( 14 N) = 2.019(25) MHz.