F. S. Richardson, R. Strickland, and D. D. Shillady
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Optical Activity of Simple Cyclic Amides. INDO Molecular Orbital Model’ F. S. Richardson,” R. Strickland, and D. D. Shillady Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received June 14, 7972) Publication costs assisted by the Petroleum Research Fund
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An INDO molecular orbital model is used to compute the oscillator strengths, rotatory strengths, and anisotropy factors for the lowest-lying singlet singlet transitions in several dissymmetric pyrrolidin-2one and 2,5-diketopiperazine systems. The chemical compounds included in the study are (1) 5methylpyrrolidin-2-one, (2) 3-methylpyrrolidin-2-one, (3) 3-aminopyrrolidin-2-one, (4)3-arnmoniunipyrrolidin-2-one, (5) 3-methyl-2,5-diketopiperazine, and (6) 3,6-dimethyl-2,5-diketopiperazine. Both planar and nonplanar ring structures are examined for each of the compounds in an attempt to assess the relative importance of inherent ring chirality and asymmetric ring substitution in determining the signs and magnitudes of the molecular rotatory strengths. For the amide n x* transition, the calculated rotatory strengths are in qualitative agreement with experiment for all systems except the 3-arninopyrrolidin-2one structures where the computed values differ in sign from the experimentally determined ones. The n ~ rotatory * strengths receive significant contributions from all three components of the electric and Qagnetic transition dipoles. This result is in sharp disagreement with the assumption made on the oneelectron perturbation model that only the z-polarized component (where the z direction is coincident with the carbonyl, C=O, bond axis) of the magnetic transition dipole makes a significant contribution. Slhplicif, in this result is the suggestion that the “effective” symmetry which determines the nodal pattern of the nr* state is not C z U , but rather C,. It is found that inherent ring chirality does have a profound influence on the computed rotatory strengths; but how this is manifested in the solution CD spectra of actual systems is not clear, since solvent effects and conformational equilibria appear to be so closely related. +
quantum mechanical method^.^^-^^ These studies have I. Introduction yielded considerable information about the identities, Small cyclic amide and diamide molecules are of spefrequencies, polarizations, and absorption intensities of cial interest in theoretical studies concerned with the optical rotatory properties of the peptide chromophore in a chiral environment. In general, the amide group (C,CONH) is planar in these systems and the ring structures restrict conformational mobility. The optical rotatoThis work was supported by the Petroleum Research Fund (PRF administered by the American Chemical Society. No. 2022-G2), ry properties of molecular systems exhibit extreme sensiW. B. Gratzer, “Poly-a-Amino Acids.” G. D. fasman, Ed., Marcel tivity to very small structural changes. For this reason it Dekker, New York, N. Y., 1967. d. A. Schellman and C. Schellman, “The Proteins,” 2nd ed, H. Neuis of some practical importance to study, as model sysrath, Ed., Academic Press, New York, N. Y., 1964,pp 1-137. tems, molecules whose structural variables are well deD. D. Fitts and J. G. Kirkwood, Proc. Natl. Acad. Sci. U. S., 42,33 fined or restricted to a small range of values. Further(1956). W. Moffitt, J. Chem. Phys., 25,467(1956). more, the inherent symmetry of the chromophoric group W. Moffitt, Proc. Natl. Acad. Sci. U. S., 42,736 (1956). in the cyclic
