Rotational spectra of HCN-SO3-Ar and HCN-SO3-CO, and several of their isotopically substituted derivatives, have been observed by Fourier transform microwave spectroscopy. Both complexes are symmetric tops with the Ar or CO weakly bound to the SO3 on the side opposite the HCN. The N-S and S-Ar distances in HCN-SO3-Ar are 2.5905(19) and 3.4465(50) , respectively. In HCN-SO3-CO, the N-S and S-C distances are 2.6563(14) and 3.0109(48) , respectively, representing a marked increase relative to the analogous distances in HCN-SO3 and SO3-CO. These results are in sharp contrast to previous work on HCN-HCN-SO3, in which the second HCN attaches to the HCN side of the HCN-SO3 and causes a large contraction of the partially formed N-S bond. Stark effect measurements have been performed and the resulting dipole moments are 4.230(10) D for HC15N-SO3-Ar and 3.678(11) D for HC15N-SO3-CO. These values are very near the differences between the dipole moments of the constituent dimers, suggesting that the dipole moments of the trimers are the vector sums of the dimer moments. Ab initio calculations for both complexes in the HCN-SO3-X and X-HCN-SO3 configurations are reported.
Bibliographical noteFunding Information:
The support of the National Science Foundation (Grant Nos. CHE-0132584 and CHE-0514256), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Minnesota Supercomputer Institute are gratefully acknowledged. KRL is delighted to honour John Brown on the occasion of his 65th birthday and recalls with gratitude both the help he received as a postdoctoral fellow and the unique combination of warmth and authority with which it was given.