Formation of C-H... H-B dihydrogen bonded complexes of acetylene, fluoroacetylene, chloroacetylene, and cyanoacetylene with borane-trimethylamine were investigated with MP2 and B3LYP methods using 6-311++G(d,p) and aug-cc-pVDZ basis sets. The stabilization energies ranged from 6-20 kJ mol(-1). NBO analysis predicts transfer of charge from a B-H bonding orbital to sigma* C-H anti-bonding orbital. It was also found that the lowering of the C-H stretching frequencies of acetylene moiety in the dihydrogen bonded complex does not correlate with other hydrogen bonded complexes, indicating that the supposition of dihydrogen bonding as another type of hydrogen bonding (sigma and pi) may be incorrect. (c) 2005 Elsevier B.V. All rights reserved.