There are two distinct types of attractive sites for weak bonding of acetylene to a polyyne. Ab initio calculations carried out for these sites for several acetylene-polyyne dimers show that methylation noticeably changes their energetic ordering. This selection of one site over another on the basis of a functional group substitution can be understood, in part, from features of the electrostatic interactions. (C) 2004 Elsevier B.V. All rights reserved.