Is styrene planar in liquid phases?

Abstract

The proton NMR spectra of two 13 C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, 0 , in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of 0 is found to be 18.0°Ϯ0.2°for the two solutions, compared with a value of 27°calculated by the molecular method MP2/6-31G*.

NMR Spectroscopy Investigation of the Cooperative Nature of the Internal Rotational Motions in Acetophenone

“…When, as usually happens, these data are not available from experiments, quantum-mechanical calculations can be used to provide reliable estimates of both quantities. [17,18,[27][28][29] Theoretical Calculation of the Force Field: The F used in the ensuing calculations was obtained with the density functional theory (DFT) method using the three-parameter Becke-Lee-Yang-Parr (B3LYP) exchange-correlation functional [30,31] and the 6-31G* standard Gaussian basis set. [32] The corresponding geometry was optimized after imposing planarity on the heavy-atom skeleton by constraining all the dihedral angles and the C=O fragment.…”

“…On the other hand, the commercial availability of the doubly enriched 13 C isotopomer makes the molecule a very interesting test case since now eight D CH couplings relative to two different carbons plus one 1 D CC can be determined from spectral analysis. In a previous study in which the LXNMR technique was applied to the investigation of styrene conformational equilibrium, [18] D CH couplings were determined from the analysis of two different single 13 C-enriched isotopomers. In using doubly enriched acetophenone, we now have an additional coupling but only one order matrix S to describe the orientational order.…”

An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents

“…Such a method should be able to calculate the vibrational corrections to the couplings, and contain a theoretical model for how the orientational order varies with conformation. A procedure to do this has been developed and has been applied to a number of simple, flexible molecules chosen so as to test the various assumptions inherent to the method [5][6][7][8][9][10]. The test molecules contained only 1 H and 13 C as the magnetically active nuclei, and in this work attention is turned to flexible molecules which also contain 19 F nuclei, which introduces an extra complication.…”

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

“…The theoretical approach to vibrational averaging of dipolar couplings has been applied recently to small molecules with single bond rotations such as styrene 10 and acrolein, 11 with very encouraging results, and at the very least gives a good estimate of the effects of vibrational averaging on the derived structural parameters and on the form obtained for P LC ͑͒.…”

“…This theoretical method of obtaining D ij ͑vib͒ has been applied previously to the case of styrene, 10 which has a single set of minimum-energy structures generated by bond rotation, acrolein, which has two minimum-energy forms, 11 and now to the present molecule, which has three minima in the bond rotation potential. Thus, the frequencies and coordinate displacements were calculated for 1-chloro-2-bromoethane for the trans ͑ = 180°͒ and gauche ͑ = 70°and 290°, as found by calculation with full geometrical optimization͒ forms.…”