Microwave rotational spectra have been observed for 22 isotopic species of an HCN, hydrogenhyphen;bonded trimer with the pulsed nozzle, Fourier transform method using the Flygare/Balle Mark II spectrometer. The14N nuclear quadrupole hyperfine structure was analyzed and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational constants. For the normal isotopic species of (HCN)3,B0was found to be 469.3073(1) MHz andDJ, 82.6(1) Hz. The quadrupole coupling constants khgr;(n) are minus;4.049(2), minus;4.251(2), and minus;4.375(1)MHz forn=1, 2, and 3, respectively, in HCN(1)HCN(2)HCN(3). The trimer has a linear or very near linear equilibrium structure. TheB0rsquo;s are insensitive to the position and torsional oscillations of the central HCN but they determine the outer HCNs quite accurately. An isotopic substitution method givesR, the c.m. distance between the outer HCNrsquo;s, to be 8.790 Aring; in the 14ndash;14ndash;14 species. A slightly smaller value 8.788 Aring; is obtained from a fit of theB0rsquo;s which includes the effects of isotopic substitution on the H/Dndash;C and Cndash;N bond lengths in the monomer. The distance between c.m.(1) and c.m.(2) may be somewhat shorter (sim;0.01 Aring;) than that between c.m.(2) and c.m.(3). The average c.m. separation in the trimer (4.395 Aring;) is halfway between that of the dimer (4.447 Aring;) which is known to be linear and the infinite linear chains in the solid (4.34 Aring;). It is shown that the smaller quadrupole coupling constants in the trimer compared to the monomer are caused by a combination of torsional and charge redistribution effects. The torsional vibrations have average angular displacements of 12.60deg; and 8.55deg; for HCN(1) and HCN(3). The charge redistribution effect upon khgr; in the trimer (6.5percnt;) is about 1.8 times that in the HCN dimer. The dipole moment of the trimer was found to be 10.6 D, a value enhanced by 1.8 D over the vector sum of the three vibrating monomers (8.83 D), an enhancement about 1.7 times that of the dimer.
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