A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

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

“…This broadening contributes to stymieing the analysis of the rotational structure of this band in an IR spectrum recorded at room temperature. A shift of 9 cm −1 to lower wavenumber occurs for this mode in going from the normal species 6 to the 1-13 C 1 species.…”

“…The structure of this band is very similar to that of the C-type band for ν 14 (a u ) for the normal species at 1011 cm −1 . 6 Of course, the normal modes for this transition of the two species are essentially the same. The normal mode is the out-of-plane flapping of the interior CH bonds.…”

“…In the normal species, the counterparts of ν 29 and ν 30 are almost degenerate, and one is only Raman active. 6 For tHTE-1-13 C 1 , ν 29 is largely CH 2 flapping on the 12 C-substituted end of the molecule, whereas ν 30 is largely CH 2 flapping on the 13 C end of the molecule. A similar pair of IR-active bands was found for butadiene-1-13 C 1 .…”

“…6 For all spectra, the beam splitter was potassium bromide, the detector was a liquid nitrogen-cooled HgCdTe device, the path length in the White cell was 16 m, and the resolution was 0.0015 cm −1 . Table 1 summarizes the other conditions for recording the spectra.…”

“…A previous investigation of the high-resolution IR spectrum of tHTE demonstrated the feasibility of analyzing the rotational structure obtained with 0.0015 cm −1 resolution at room temperature for this rather large molecule. 6 The current study reports the synthesis of tHTE-1-13 C 1 and the analysis of the rotational structure in two C-type bands. In addition, new evidence shows that the ground state (GS) rotational constants for the singly substituted 13 C 1 species can also be obtained after some scaling from quantum chemical calculations at the triple-ζ level with sufficient accuracy for use in determining an SE structure.…”