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

Abstract

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) 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 ν(26) at 1011 cm(-1) and ν(30) 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 A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) 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 (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) 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.

Electron Delocalization in Polyenes: A Semiexperimental Equilibrium Structure for (3E)-1,3,5-Hexatriene and Theoretical Structures for (3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-Octatetraene

“…Bands of the trans -1- d 1 species in the spectrum were too badly overlapped to be analyzable. A third investigation was for tHTE-1- 13 C 1 . Not only were ground state rotational constants obtained for this species, but strong evidence was supplied to show that the ground state rotational constants for the 2- 13 C 1 and 3- 13 C 1 species could be predicted accurately from computed values.…”

“…Scale factors for A 0 , B 0 , and C 0 were found as the obsd/calcd ratios for the normal species. These scale factors were then applied to the calculated ground state rotational constants for the 2- 13 C 1 and 3- 13 C 1 species to obtain the predictions . The last investigation was the analysis of rotational structure in bands of tHTE-2- d 1 and -3- d 1 …”

“…The parent tHTE was the first species to be investigated by high-resolution infrared (IR) spectroscopy . Work on the 1,1- d 2 and cis -1- d 1 species and on the 1- 13 C 1 species followed. The investigation of the 1- 13 C 1 species gave the ancillary result that rotational constants of the 2- 13 C 1 and 3- 13 C 1 species could be predicted accurately from theory .…”

Analysis of the rotational structure in the high-resolution infrared spectra of trans-hexatriene-2-d1 and -3-d1

“…As shown in Figure 7, this band has a broad central Q branch, reflecting much hot band activity. The comparable bands in the spectra of tHTE [5] and tHTE-1-13 C1 [7] near 1010 cm -1 were hard to analyze. The band in the spectrum of tHTE-2-d was more difficult.…”

“…Previously, high-resolution IR spectroscopy yielded rotational constants for transhexatriene [5], the 1,1-d2 species [6], the cis-1-d1 species [6], and the 1-13 C1 species [7]. As part of the work on the 1-13 C1 species, it was shown that the rotational constants for the 2-13 C1 and 3-13 C1 species could be predicted from quantum chemical (QC) calculations with good accuracy [7]. Remaining to be found are the ground state (GS) rotational constants for the 2-d1 and 3-d1 species.…”