Infrared spectra of acetylene dimers and acetylene-nitrogen: (DCCD)2, H-bonded DCCD-HCCH, and DCCD-NN in the 4.1 μm region

► Infrared spectra of acetylene dimers DCCD-DCCD and DCCD-HCCH are studied. ► Spectrum of previously undetected H-bonded mixed dimer DCCD-HCCH was also observed. ► Compared to the analogous spectrum of HCCH-HCCH, the present results are much less perturbed. ► The tunneling splitting for (DCCD)2 in the excited state shows a big reduction from that for the ground state. ► The dimer A rotational constants show a large apparent increase upon vibrational excitation.