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.

Chiroptical properties of (R)-3-chloro-1-butene and (R)-2-chlorobutane.

Coupled cluster and density functional models of specific rotation and vacuum UV (VUV) absorption and circular dichroism spectra are reported for the conformationally flexible molecules (R)-3-chloro-1-butene and (R)-2-chlorobutane. Coupled cluster length- and modified-velocity-gauge representations of the Rosenfeld optical activity tensor yield significantly different specific rotations for (R)-3-chloro-1-butene, with the latter providing much closer comparison (within 3%) to the available gas-phase experimental data at 355 and 633 nm. Density functional theory overestimates the experimental rotations for (R)-3-chloro-1-butene by approximately 80%. For (R)-2-chlorobutane, on the other hand, all three models give reasonable comparison to experiment. The theoretical specific rotations of the individual conformers of (R)-3-chloro-1-butene are much larger than those of (R)-2-chlorobutane, in disagreement with previous studies of the temperature dependence of the experimental rotations in solution. Simulations of VUV absorption and circular dichroism spectra reveal large differences between the coupled cluster and density functional excitation energies and the rotational strengths. However, while these differences lead to very different specific rotations for (R)-3-chloro-1-butene, they have much less impact on the computed specific rotations for (R)-2-chlorobutane. In addition, the coupled cluster VUV absorption spectrum of (R)-2-chlorobutane compares well to experiment.

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.

Ab initio calculation of optical rotation in (P)-(+)-[4]triangulane.

Optical rotation, the angle through which plane-polarized light rotates when passed through an enantiomerically pure medium, plays a vital role in the determination of the absolute configurations of chiral molecules such as natural products. We describe new quantum mechanical methodology designed to assist in this endeavor by providing high-accuracy computational optical rotatory dispersion data for matching to experimental results. Comparison between theory and experiment for the rigid, helical molecule trispiro[2.0.0.2.1.1]nonane [also known as (P)-(+)-[4]triangulane], recently synthesized with enantiomeric purity, shows that the coupled cluster quantum chemical model provides superb agreement for optical rotation across a wide range of wavelengths (589-365 nm), with errors averaging only 1%.

A family of decamethylmetallocene charge-transfer salt magnets using methyl tricyanoethylenecarboxylate (MTCE) as the electron acceptor.

Methyl tricyanoethylenecarboxylate, MTCE, has been used as a one-electron acceptor building block for the synthesis of isomorphous decamethylmetallocene charge-transfer salt magnets of the formula [MCp*2][MTCE], M = Cr, Mn, and Fe. Functionally and electrochemically, MTCE is a hybrid between tetracyanoethylene (TCNE) and dimethyl dicyanofumarate (DMeDCF), two acceptors that have previously been found to support ferromagnetism. The X-ray crystal structure of the chromium analogue, [CrCp*2][MTCE], shows it to exist in the expected mixed stack structure in the orthorhombic space group Pnma with a = 14.739(3) A, b = 10.7869(19) A, and c = 15.771(3) A and Z = 4. As anticipated, all three family members exhibit dominant ferromagnetic coupling, which is presumed to reflect intrastack interactions. However, the bulk magnetic properties mostly differ from simple interpolations or extrapolations of the properties of their TCNE and DMeDCF analogues. Density functional theory calculations have been used to shed some light on this observation.

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.