Experimental structures of the carbon chains HC7N, HC9N, and HC11N by isotopic substitution

The effective structures (r(o)) of the three linear cyanopolyynes HC7N, HC9 N, and HC11N have been determined to high accuracy by isotopic substitution , following detection in a supersonic molecular beam with a Fourier transfo rm microwave spectrometer of all of the singly substituted rare isotopic sp ecies. For each chain, the lengths of the individual bonds have been determ ined to an accuracy of 0.001 Angstrom. or 0.1% toward the end of the chain and to 0.01 Angstrom or 1.0% toward the center. The experimental structures are in excellent agreement with recent high-level theoretical calculations , or, in the case of HC11N, with extrapolation from HC9N, The three polyyne s studied here represent the largest reactive carbon chain molecules for wh ich accurate structures have been derived empirically. For HC7N and HC9N, i t has been possible to resolve at high-resolution nitrogen hyperfine struct ure in the lower rotational transitions and determine eQq for all of the si ngly substituted isotopic species of HC7N and for normal HC9N. (C) 2000 Aca demic Press.