FTIR measurements of optical rotatory dispersion and circular dichroism

FTIR methods for measuring optical rotatory dispersion (ORD) and circular dichroism (CD) were developed. The measurements of both phenomena are accomplished by means of various techniques. The study of ORD makes use of linearly polarized IR radiation while that of CD requires a circular polarizer (retarder) able to convert linearly polarized IR radiation into circularly polarized one. Examples of induced cholesteric solutions are shown.     

The current state of ab initio calculations of optical rotation and electronic circular dichroism spectra

The current ability of ab initio models to compute chiroptical properties such as optical rotatory dispersion and electronic circular dichroism spectra is reviewed. Comparison between coupled cluster linear response theory and experimental data (both gas and liquid phase) yields encouraging results for small to medium-sized chiral molecules including rigid species such as (S)-2-chloropropionitrile and (P)-[4]triangulane, as well as conformationally flexible molecules such as (R)-epichlorohydrin. More problematic comparisons are offered by (S)-methyloxirane, (S)-methylthiirane, and (1S,4S)-norbornenone, for which the comparison between theory and experiment is much poorer. The impact of basis-set incompleteness, electron correlation, zero-point vibration, and temperature are discussed. In addition, future prospects and obstacles for the development of efficient and reliable quantum chemical models of optical activity are discussed, including the problem of gauge invariance, scaling of the coupled cluster approach with system size, and solvation.     

The optical rotatory dispersion and circular dichroism of molecules containing interacting residues

We use a semiclassical formalism to derive expressions for the optical rotatory dispersion and circular dichroism associated with a dilute system of molecules represented as freely-rotating, rigid arrays of interacting residues. The relationship of the present approach to DeVoe's classical theory is discussed. Far from resonance, our results become equivalent to the equations of Moffitt, Fitts, and Kirkwood when the separation between adjacent residues is small compared to the wavelength of the light. In the resonance case, a calculation of the frequency-dependent circular dichroism per residue is carried out for a particular geometrical arrangement of the residues. These curves, for arrays both finite and infinite in extent, are compared with those obtained when a certain commonly used approximation to the orientational averaging is introduced. It is shown that this approximation is not always valid.     

Ab initio determination of optical rotatory dispersion in the conformationally flexible molecule (R)-epichlorohydrin

Ab initio optical rotation data from linear-response coupled-cluster and density-functional methods are compared to both gas-phase and liquid-phase polarimetry data for the small, conformationally flexible molecule epichlorohydrin. Three energy minima exist along the C-C-C-Cl dihedral angle, each with strong, antagonistic specific rotations ranging from ca. -450 to +500 deg/[dm (g/mL)] at 355 nm. Density-functional theory (specifically the B3LYP functional) consistently overestimates the optical rotations of each conformer relative to coupled-cluster theory (in agreement with our earlier observations for conformationally rigid species), and we attribute this to density-functional theory's underestimation of the lowest-lying excitation energies of epichlorohydrin. Length- and velocity-gauge formulations of the coupled-cluster response function lead to slightly different specific rotations (ca. 7% at short wavelengths). We have determined well-converged Gibbs free energy differences among the conformers using complete-basis-set extrapolations of coupled-cluster energies including triple excitations to obtain Boltzmann-averaged specific rotations for comparison to the gas-phase results. The length-gauge coupled-cluster data agree remarkably well with experiment, with the velocity-gauge coupled-cluster and density-functional data bracketing the experimental results from below and above, respectively. Liquid-phase conformer populations reported earlier by Polavarapu and co-workers from combined infrared absorption and theoretical analyses differ markedly from the gas-phase populations, particularly for polar solvents. Nevertheless, Boltzmann-averaged specific rotations from both coupled-cluster and density-functional calculations agree well with the corresponding experimental intrinsic rotations, although the theoretical specific rotations for the individual conformers do not take solvent effects into account. PCM-based estimates of conformer populations lead to poor agreement with experiment.     

Optical rotatory dispersion and circular dichroism studies of copoly(propyl-L-aspartate-benzyl-L-aspartate)

Copoly(propyl-L -aspartate-benzyl-L -aspartate)s with various compositions were synthesized by propylation of a parent poly(β-benzyl-L -Aspartate). The variation of helix sense of copolyaspartate obtained against a degree of propylation and temperature was examined by optical rotatory dispersion and circular dichroism method. Copolyaspartate of propylation less than about 70% is in a left-handed α helix conformation and its Moffitt parameter b₀ and molecular ellipticity [θ]₂₂₂ decreased gradually with increasing propylation. On the other hand, a different temperature dependence of b₀ and [θ]₂₂₂ was observed for copolyaspartate of propylation more than about 80%, i.e., the reversible transition of the helix sense from right- to left-handed α helix took place by temperature increasing. These features were basically similar to the variation in helix sense of copoly(ethyl-L -aspartate-benzyl-L -aspartate), although a slight difference was seen for copoly(propyl-L -aspartate-benzyl-L -aspartate). The thermodynamic treatment indicating larger entropy of right-handed α helix suggests much mobile side chain of the right-handed α helix than the left handed one.     

Ab initio calculation of electronic circular dichroism fortrans-cyclooctene using London atomic orbitals

Summary The second-quantization magnetic dipole operator that arises when London atomic orbitals are used as basis functions is derived. In atomic units, the magnetic dipole operator is defined as the negative of the first derivative of the electronic Hamiltonian containing the interaction with the external magnetic field. It is shown that for finite basis sets, the gauge origin dependence of the resulting magnetic dipole operator is analogous to that of the exact operator, and that the derived operator converges to the exact operator in the limit of a complete basis set. It is also demonstrated that the length expression for the rotatory strength in linear response calculations gives gauge-origin-independent results. Sample calculations ontrans-cyclooctene and its fragments are presented. Compared to conventional orbitals, the basis set convergence of the rotatory strengths calculated in the length form using London atomic orbitals is favourable. The rotatory strength calculated fortrans-cyclooctene agrees nicely with the corresponding experimental circular dichroism spectrum, but the spectra for the fragment molecules show little resemblance with that oftrans-cyclooctene.Dedicated to Prof. Jan Linderberg     

Ab initio modeling of the electronic circular dichroism induced in porphyrin chromophores

The optical activity in porphyrins can easily be induced by a chiral environment, but it is difficult to determine the underlying mechanisms purely on an experimental basis. Therefore, in this study, magnitudes of the perturbational, dipolar, and direct covalent contributions to the electronic circular dichroism (CD) are evaluated with the aid of quantum chemical computations. Electronic properties of model porphyrin chromophores are analyzed. Time-dependent density functional theory (TD DFT), particularly with the hybrid B3LYP functional, appeared suitable for estimation of the electronic excitation energies and spectral intensities. The transition dipole coupling (TDC) between chirally stacked porphyrins was determined as the most important mechanism contributing to their optical activity. This is in agreement with previous experimental observations, where chiral matrices often induce the stacking and large CD signals. About a 10 times smaller signal could be achieved by a chiral orientation of the phenyl or similar residues covalently attached to the porphyrin core. Also, this prediction is in agreement with known experiments. Perturbation models realized by a chirally arranged porphyrin and a point charge, or by a porphyrin and the methane molecule, provided the smallest CD signals. The electrically neutral methane induced similar CD magnitudes as those of the charge, but spectral shapes were different. For a complex of porphyrin and the alanine cation, a significant influence of the solvent on the resultant CD spectral shape was observed, while for the charge and methane perturbations, a negligible solvent effect was found. Detailed dependence of the induced optical activity on variations of geometrical parameters is discussed. The simulations of the induced porphyrin activity can thus bring important information about the structure and intermolecular interactions in chiral complexes.     

Ab initio derivative calculation of vibrational circular dichroism

The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.     

An optical rotatory dispersion and circular dichroism investigation of the chloramidation of the triamine EnPYClNO2ClPtCl

Conclusions An optically active product from the chloramidation of (-)₅₈₉ EnPyClNO₂ClPtCl with the composition (+)₅₈₉ (NH₂C₂H₄NCl)·PyClNO₂ClPt has been isolated in the solid state for the first time. The optical activity of a solution of this compound in DMF has been investigated using CD and ORD. A sharp change in the spectrum of the optically active bands of the reagent takes place during chloramidation which is apparently not only associated with the change in the nature of the Pt-chelate bond but also with the appearance of a new chromophore (the chlorimido group), the absorption bands of which can become optically active as a result of the vicinal effect.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimieheskaya, No. 4, pp. 928–931, April, 1970.     

Ab initio study of excited state electronic circular dichroism. Two prototype cases: methyl oxirane and R-(+)-1,1'-bi(2-naphthol)

A computational approach to the calculation of excited state electronic circular dichroism (ESECD) spectra of chiral molecules is discussed. Frequency dependent quadratic response theory is employed to compute the rotatory strength for transitions between excited electronic states, by employing both a magnetic gauge dependent and a (velocity-based) magnetic gauge independent approach. Application is made to the lowest excited states of two prototypical chiral molecules, propylene oxide, also known as 1,2-epoxypropane or methyl oxirane, and R-(+)-1,1'-bi(2-naphthol), or BINOL. The dependence of the rotatory strength for transitions between the lowest three excited states of methyl oxirane upon the quality and extension of the basis set is analyzed, by employing a hierarchy of correlation consistent basis sets. Once established that basis sets of at least triple zeta quality, and at least doubly augmented, are sufficient to ensure sufficiently converged results, at least at the Hartree-Fock self-consistent field (HF-SCF) level, the rotatory strengths for all transitions between the lowest excited electronic states of methyl oxirane are computed and analyzed, employing HF-SCF, and density functional theory (DFT) electronic structure models. For DFT, both the popular B3LYP and its recently highly successful CAM-B3LYP extension are exploited. The strong dependence of the spectra upon electron correlation is highlighted. A HF-SCF and DFT study is carried out also for BINOL, a system where excited states show the typical pairing structure arising from the interaction of the two monomeric moieties, and whose conformational changes following photoexcitation were studied recently with via time-resolved CD.     

Calculation of optical rotatory dispersion and electronic circular dichroism for tris-bidentate groups 8 and 9 metal complexes, with emphasis on exciton coupling

The optical rotatory dispersion (ORD, both non-resonant and resonant) and the electronic circular dichroism (CD) of tris-bidentate transition metal complexes of the form [M(L)(3)](n+) (M = Fe, Ru, Os, Co, Rh, Ir; n = 2, 3; L = 1,10-phenanthroline, 2,2'-bipyridine) are calculated using time-dependent density functional theory (TDDFT). The exciton CD band resulting from the coupling of ligand π-to-π* transitions is investigated in detail and analyzed in terms of exciton coupling of long-axis transitions using a dipole coupling model that takes TDDFT data for a single ligand as input. Results of the coupling model agree well with the full TDDFT CD spectra. The usefulness and reliability of this model is discussed. The resonant ORDs calculated directly from analytical damped linear TDDFT response compare well with Kramers-Kronig transformations of the calculated CD spectra. For comparisons of resonant ORD with experiment, one needs to consider wavelength shifts.     

Coupled cluster calculations of optical rotatory dispersion of (S)-methyloxirane

Coupled cluster (CC) and density-functional theory (DFT) calculations of optical rotation, [alpha](lambda), have been carried out for the difficult case of (S)-methyloxirane for comparison to recently published gas-phase cavity ringdown polarimetry data. Both theoretical methods are exquisitely sensitive to the choice of one-electron basis set, and diffuse functions have a particularly large impact on the computed values of [alpha](lambda). Furthermore, both methods show a surprising sensitivity to the choice of optimized geometry, with [alpha](355) values varying by as much as 15 deg dm(-1) (g/mL)(-1) among molecular structures that differ only negligibly. Although at first glance the DFT/B3LYP values of [alpha](355) appear to be superior to those from CC theory, the success of DFT in this case appears to stem from a significant underestimation of the lowest (Rydberg) excitation energy in methyloxirane, resulting in a shift of the first-order pole in [alpha](lambda) (the Cotton effect) towards the experimentally chosen incident radiation lines. This leads to a fortuitous positive shift in the value of [alpha](355) towards the experimental result. The coupled cluster singles and doubles model, on the other hand, correctly predicts the position of the absorption pole (to within 0.05 eV of the experimental result), but fails to describe correctly the shape/curvature of the ORD region lambda=355, resulting in an incorrect prediction of both the magnitude and the sign of the optical rotation.     

Optical rotatory dispersion and circular dichroism. LXXV. Circular dichroism of some aryl-amino acids

CD. curves have been recorded for α-aryl-α-amino acids, esters and amides related to α-phenylglycine, α-phenyl-alanine and their N-dimethyl derivatives, and for the corresponding α-cyclohexyl-α-amino acids and esters. Compounds with the (S)-configuration at the single asymmetric carbon atom give strong positive Cotton effects near 220 nm. The conformations of the acids are discussed and compared with those of other α-amino acids.     

Optical rotatory dispersion and vacuum ultraviolet circular dichroism of poly[(R)-oxypropylene]

The circular dichroism (CD) spectrum of poly[(R)-oxypropylene] in a number of solvents has been measured in the vacuum ultraviolet region. Two CD bands were measured for cyclohexane, acetonitrile, and trifluoroethanol solutions. The CD spectrum was extended to 140 nm and three bands were measured in a 1,1,1,3,3,3-hexafluoro-2-propanol solution. The similarity of these four CD spectra in sign, shape, and intensity confirm that poly[(R)-oxypropylene] has similar conformations in all solvents studied, even though the optical rotatory dispersion (ORD) in the visible region is positive for the cyclohexane solution but negative for the alcohol solutions. A Kronig–Kramers transform of the two CD bands observed for the cyclohexane solution accounts for the observed positive ORD spectrum. In contrast, a third large and negative CD band centered below 160 nm is necessary to account for the negative ORD spectra observed for the alcohol solutions. Apparently the anomalous sign variation of the ORD curves observed in the visible region is due to subtle changes in the interplay of a large number of CD bands. Solvent interaction, observed as a blue shift of the first two CD bands for the alcohol solutions, could produce the subtle changes necessary to change the sign of the ORD curve in the visible region. The first two CD bands of poly[(R)-oxypropylene] are tentatively assigned as n? σ^* and the third as an n? 3s.