Theoretical Circular Dichroism Spectra Research Articles

Combined Experimental and Quantum Chemical Investigation of Chiroptical Properties of Nicotinamide Derivatives with and without Intramolecular Cation−π Interactions

The circular dichroism (CD) spectra of neutral and cationic nicotinamide derivatives were experimentally examined in solution and in the solid state to show dramatic differences in the two phases and appreciable dependence on temperature and solvent. The CD spectrum of neutral nicotinamide 1 in solution was reproduced theoretically by averaging the theoretical spectra calculated for all of the extended and folded conformers (s-trans-G(+), s-cis-G(+), s-trans-T, and s-cis-T) weighted by their population. The preference for the folded, over the extended, conformers in less polar solvent was indicated by the calculation and confirmed experimentally by the analysis of specific rotations. Theoretical CD spectrum calculated for the conformer found in the X-ray structural analysis (s-cis-T) well reproduced the experimental CD spectrum measured in the solid state. Introducing cation-pi interactions by N-methylation of 1 to give 1-Me(+) led to dramatic changes in CD spectrum. Nevertheless, the experimental CD spectrum of 1-Me(+) was well reproduced by averaging the theoretical spectra calculated for a pair of most stable conformers (s-cis-G(+) and s-trans-G(+)) of 1-Me(+). The CD spectrum calculated for the s-trans-G(+) conformer, which was found in the X-ray crystallographic analysis, did not agree with the experimental one. The theoretical spectra were better reproduced in general by the more sophisticated RI-CC2 method, but the conventional TD-DFT method also gave acceptable results. This allowed us to successfully calculate the larger derivative 2-Me(+), for which the RI-CC2 method was not practically applicable. These results show that the structure/conformation may vary with the conditions employed (e.g., by altering the solvent or phase) and thus the experimental analysis under the identical condition is essential for a serious structural study. The present study on a series of nicotinamide derivatives 1, 1-Me(+), and 2-Me(+) with and without cation-pi interactions demonstrates that the combination of experimental and theoretical chiroptical methods is capable of providing reliable structural/conformational information in solution phase, which is complementary to the X-ray crystallographic structure in the solid state.

Estimation of Dye Configuration from Conventional Chiroptical Spectra of Porphyrin Integrates: Combination of Exciton Theory with Monte Carlo Molecular Structural Simulation

Dye integrates (arrays and aggregates) are the subject of current interest in photochemical devices. However, they are in general not suitable for X-ray crystallography because of their poor crystallinity. Here, we improved a simple method of estimating dye configurations in porphyrin integrates from their visible absorption (AB) and circular dichroism (CD) spectra. For this purpose, we calculate the dipolar and optical rotatory strengths of an integrate on the basis of the exciton theory for a given porphyrin configuration, generate the theoretical AB and CD spectra of the dye integrate using a phenomenological line shape function, a Voigt function with an adjustable line width, and optimize the configuration by minimizing the square sum (S) of the difference between the observed and calculated spectra. We adopted two optimization methods to achieve a least-squares fit between the calculated and observed spectra: the Metropolis Monte Carlo (MC) method, which incorporates S into the molecular force-field energy as a constraint, and the quasi-Newton (QN) method, which numerically minimizes S and uses no molecular force field. To check the feasibility of these methods, we simulated the AB and CD spectra of Tröger's base and meso-meso-linked porphyrins using the QN program, then compared the dye configurations with their X-ray structures. The calculated dye configuration of Tröger's base porphyrin is sufficiently in agreement with that of the X-ray structure (RMSD=0.21 A for the ZnS4 center), whereas that of meso-meso-linked porphyrin was not. These results were discussed in terms of charge transfer between two porphyrins. Finally, we applied the QN and MC methods to the structural estimation of a newly prepared peptide-linked bis(porphyrin) Boc-(PorZn,S)2-OBut. The best configurations estimated by these two methods were sufficiently in agreement with each other.

Spectroscopic and Theoretical Studies of Optically Active Porphyrin Dimers: A System Uninterpretable by Exciton Coupling Theory

The electronic excited states of a meso-meso beta-beta doubly linked bis-porphyrin are comprehensively investigated by measuring its circular dichroism (CD) and magnetic circular dichroism (MCD) spectra. The observed spectroscopic properties are rationalized by DFT calculations. The frontier molecular orbitals (MOs) are constructed by the linear combinations of the constituent monomers' four MOs. Comparison of a theoretical CD spectrum based on time-dependent DFT (TDDFT) with the experimental spectra resulted in the assignment of the helical conformation of the dimer. This assignment is contrary to the previous assignment based on the point-dipole approximation (exciton coupling theory).

Foldamer for novel peptide derivatives with pyrene units incorporated into the main chain

A novel foldamer with pyrene units incorporated into a peptide main chain was synthesized and its conformation in solution was investigated by spectroscopic measurements and computational calculations. The foldamer designed here contains 1-aminopyrene-8-carboxylic acid in the sequence, which residue is expected to change the peptide chain direction by 1201. A decapeptide, Boc–Ala–Pyr–Aib–Ala–Aib–Ala–Pyr–Aib–Ala–Aib–OMe, where Boc, Ala, Aib, Pyr, and OMe stand for t-butyloxycarbonyl group, L-alanine, a-aminoisobutyric acid, 1-aminopyrene-8-carboxylic acid, and methyl ester, respectively, was synthesized. Absorption spectroscopy in acetonitrile showed that the two pyrene units in the decapeptide do not interact with each other in the ground state. On the other hand, the fluorescence spectroscopy suggested that the two pyrene units are fixed at a certain distance that allows interaction in the excited state. The circular dichroism spectra showed distinct exciton coupling peaks around the pyrene absorption, further supporting that the two pyrene units have a specific spatial relationship. According to 1H NMR measurements, it was found that the decapeptide has two intramolecular hydrogen bondings and the amide protons of the two 1-aminopyrene-8-carboxylic acid residues are close to each other. The dihedral angles of C–N–Cα–C in the alanine residues were also determined. Taking all these findings into account, molecular mechanics and semiempirical computational calculations were carried out to give two conformations, left-handed and right-handed helices, in which the two pyrene units partially overlap with each other. Theoretical circular dichroism spectra werecalculated from these two conformations and compared to the experimental spectrum. It was shown that the left-handed helixconformation is the plausible conformation. The pyrene units incorporated into the main chain are considered to stack witheach other by an aromatic interaction, resulting in the formation of a helical structure as a whole.

Theoretical CD spectrum calculations of the crown-ether aralkyl-ammonium salt complex.

Rotatory strengths of the alpha-(1-naphtyl)-ethylammonium perchlorate (NEA)-phenazino-18-crown-6 ether molecular complex is determined theoretically by the coupled oscillator model and using ab initio random phase approximation (RPA) to describe local excitations on the chromophores. The computational results are compared to the experimental circular dichroism (CD) spectrum published previously. The good qualitative agreement between calculated and measured optical rotatory strengths allows one to assign the CD bands of the complex in a unique manner.