Rotational spectrum and structure of the HCN–(CO2)3 tetramer

Abstract

Microwave rotational transitions have been observed for HCN–(CO2)3, DCN–(CO2)3, H13CN–(CO2)3, HC15N–(CO2)3, HCN–(13CO2)3, HCN–(18OCO)(CO2)2, and HCN–(CO2)(C18O2)2 with the pulsed Fourier transform, Flygare/Balle Mark II spectrometer. A symmetric top spectrum was observed for the parent isotopic species with rotational constants of B0=861.6392(1) MHz, DJ =0.681(5) kHz, and DJK =0.821(12) kHz. The results for isotopic substitution indicate a zero-point, vibrationally averaged geometry having the C3 symmetry of a cyclic (CO2)3 structure with the HCN along the symmetry axis and the N end closest to the (CO2)3. The C3 symmetry is confirmed by the observation of states limited to K=±3n, with n=0,1,2,..., as predicted for threefold symmetry generated by bosons only. The (CO2)3 has a pinwheel configuration, as in the free trimer, and the three carbons lie in a plane R=2.758 Å below the center of mass (c.m.) of the HCN. The C-C distance in this subunit is 3.797 Å which is 0.241 Å shorter than that found in the free (CO2)3 trimer. The individual CO2’s in the (CO2)3 pinwheel are rotated out of the C–C–C plane by γ=−6.9°, as determined from an inertial analysis, with the inner oxygens rotated away from the HCN. The HCN has an average torsional angle of 10.3°, as determined by isotopic substitution, and an observed χcc value of −3.891 MHz for the 14 N. The c.m.(HCN) to C distance is 3.525 Å, compared to 3.592 Å in the HCN-CO2 T-shaped dimer. The isotopic substitution by 18O perturbs the structure of the symmetric top clusters by a remarkable amount, decreasing γ to −28.9° and increasing R and RCC to 2.797 and 3.814 Å, respectively. In the 18O substituted species, the CO2’s are rotated in the C–C–C plane from C3v symmetry by the pinwheel angle β=∼32.5°.