Ab initio calculations for propyne and the hydrogen bonded complex NH H C C CH 3 3

Abstract

The hydrogen-bonded cluster NH₃ …H—C≡C—CH₃ has been investigated by means of the coupled electron pair approximation, making use of a basis set of 198 contracted Gaussian-type orbitals. The calculated equilibrium structure is r _1^e (N—H) = 1•0127 Å, α_e(∠HN…H) = 112•32°, R _1^e (N…H) = 2•3593 Å, r _2^e (acetylenic C—H) = 1•0690 Å, R _2^e (C≡C) = 1•2078 Å, R _3^e (C—C) = 1•4711 Å, r _3^e (C—H) = 1•0894 Å and β_e(∠CCH) = 110•50°. The recommended equilibrium dissociation energy is D _e = 12•4±0•5 kJ mol^-1 and the calculated equilibrium dipole moment is μ_e = – 1•468 D, with the positive end of the dipole at the ammonia protons. Harmonic wavenumbers and absolute infrared intensities for the totally symmetric modes are calculated. Compared with free propyne the acetylenic CH stretching vibration experiences a bathochromic shift of 93 cm^-1 and an intensity enhancement by a factor of 5•5.