The Structure of Acrolein in a Liquid Crystal Phase

Abstract

The (1)H NMR spectrum of a sample of acrolein dissolved in the nematic liquid crystal phase I52 has been analysed to yield 18 dipolar couplings between all the magnetic nuclei in the molecule; moreover, the (13)C and (13)C{(1)H} NMR spectra of a sample of acrolein in CDCl(3) were recorded and analysed to determine the indirect J(ij) couplings. The data were used to obtain the relative positions of the carbon and hydrogen atoms, assuming that these are independent of the conformations generated by rotation around the C--C bond through an angle phi, and to obtain a probability distribution P(phi). It has been found that in the liquid phase, the distribution is a maximum at the trans form whereas the abundance of the cis form is significantly smaller compared with that found by microwave spectroscopy or high level quantum mechanical calculations. Such calculations produced also a suitable force field needed to develop suitable strategies for vibrational correction procedure in the case of flexible molecules.

Intrinsic Information Content of NMR Dipolar Couplings: A Conformational Investigation of 1,3‐Butadiene in a Nematic Phase

“…The choice seems to be reasonable, because in an uniaxial nematic liquid crystal such as I52, the "prolate" molecules tend to align with their long axis parallel to the field, and because the s-trans conformer is present up to 98-99 %. Moreover, what has been found for the similarly shaped acrolein [19] strongly supports the choice. With this inferred PAS and using the MP2/6-31G* molecular geometries, the three independent frequencies of the 5QR spectrum were fitted against the two unknowns, S aa and S bb ÀS gg , testing all the 24 possible combinations of lines.…”

“…Therefore, the inclusion/exclusion of that torsional frequency can influence the structure of the rigid fragments. [19] To account for this fact, for nuclei i and j belonging to the same fragment, in the calculation of the relative F a ab and F h ab it is possible to include/exclude the contribution due to the torsional frequency too. We will call these two options: "including/excluding the torsional frequency", respectively.…”

“…Moreover, more important than the larger number-but often underestimated-is the quality of the information associated with the D CH couplings which matters in conformational studies. In fact, the D CH values give direct information on skeleton bond lengths and angles, producing a sort of "geometry filter" [19] that strongly helps in discriminating between realistic and unrealistic solutions. The procedure used to obtain the molecular structure and conformational equilibrium from direct couplings is discussed in Section 4.…”

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.…”