Calculated spectroscopic properties for NH3…HC4H

The hydrogen-bonded hetero-dimer NH3…HC4H has been investigated by means of the coupled electron pair approximation (CEPA), making use of a basis set of 210 contracted Gaussian-type orbitals. The equilibrium dissociation energy is estimated to be 17.5±1.0 kJ mol−1 and the equilibrium dipole moment is μe = −2.468 D, with the positive end at the site of the ammonia protons. Compared to the IR-active antisymmetric CH stretching vibration of free diacetylene the wavenumbers of the CH vibrations in the complex are reduced by 126.6 cm−1 (inner CH stretch) and increased by 4.7 cm−1 (outer CH stretch). The corresponding intensity ratio is 5.8. The pseudosymmetric acetylenic CC vibration has a large intensity of 20 km mol−1. The intensity of the symmetric NH stretching vibration is smaller than in free NH3 by a factor of 2.