Analysis of the Rotational Structure in the High-Resolution Infrared Spectrum of trans-Hexatriene-1-13C1: A Semiexperimental Equilibrium Structure for the C6 Backbone of trans-Hexatriene
journal contributionposted on 29.03.2012, 00:00 by Norman C. Craig, Hengfeng Tian, Thomas A. Blake
trans-Hexatriene-1-13C1 (tHTE-1-13C1) has been synthesized, and its high-resolution (0.0015 cm–1) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν26 at 1011 cm–1 and ν30 at 894 cm–1 has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A0 = 0.8728202(9), B0 = 0.0435868(4), and C0 = 0.0415314(2) cm–1. Upper state rotational constants for the ν30 band were also fitted. Predictions of the ground state rotational constants for tHTE-1-13C1 from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three 13C1 isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three 13C1 isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C6 backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.