F. J. Devlin

Ab initio calculation of atomic axial tensors and vibrational rotational strengths using density functional theory

Abstract The first application of the density functional theory (DFT) to the calculation of atomic axial tensors (AATs) is reported. Analytical derivative methods and gauge-invariant atomic orbitals (GIAOs) are employed. DFT/GIAO AATs for trans-2,3 d 2 -oxirane calculated using a [8s6p3d/6s3p] basis set and the Becke 3-Lee-Yang-Parr hybrid density functional, in combination with a DFT harmonic force field and atomic polar tensors (APTs), yield vibrational rotational strengths in better agreement with experiment than comparable calculations at the Hartree-Fock level. The largest remaining deviations between theory and experiment are attributed to experimental error.

Determination of the structure of chiral molecules using ab initio vibrational circular dichroism spectroscopy

We discuss the theoretical prediction of vibrational circular dichroism (VCD) spectra using ab initio density functional theory (DFT) and the application of this methodology to the determination of the absolute configurations and conformations of chiral molecules.

Coupled-cluster calculations of optical rotation

Abstract CC2 and CCSD coupled-cluster calculations of the sodium D line specific rotations of 13 chiral organic molecules are compared to HF and DFT/B3LYP calculations and to experiment. For 12 of the molecules, whose [ α ] D values are in the range 0–200, CCSD and B3LYP [ α ] D values are in very similar agreement with experiment: average deviations are 19.8 and 19.4, respectively. CC2 and HF values are less accurate: average deviations are 24.7 and 32.2, respectively. For one molecule, norbornenone, the CCSD [ α ] D value (741) is very different from the B3LYP value (1216) and in much worse agreement with experiment (1146).

Theoretical calculation of vibrational circular dichroism spectra

Abstract : We report calculations of the mid-IR unpolarized absorption and circular dichroism spectra of 4-methyl-2-oxetanone, 6,8-dioxabicyclo3.2.1octane and 1,7,7-trimethylbicyclo2.2.1heptan-2-one (camphor) based on harmonic force fields calculated using Density Functional Theory (DFT). A hybrid density functional--Becke3LYP--is used. The basis set is 6-31G. The results are in impressive agreement with experimental spectral calculations using the LSDA and BLYP functionals are much less successful. Our results using Becke3LYP/DFT are compared to the predictions of SCF and MP2 calculations. At the present time, the Becke3LYP/DFT methodology is clearly to be preferred in predicting mid-IR vibrational spectra.

Prediction of optical rotation using density functional theory: 6,8-dioxabicyclo[3.2.1]octanes

Abstract We report calculations of the optical rotations of six 6,8-dioxabicyclo[3.2.1]octanes using ab initio density functional theory (DFT). GIAO basis sets are used to ensure origin independence of predicted rotations. Large basis sets including diffuse functions are used to minimize basis set error. The signs of [ α ] D are correctly predicted. Magnitudes differ from experiment by 10–20 degrees [dm (g/cc)] −1 on average. Agreement with experiment is improved by inclusion of solvent effects via the polarized continuum model (PCM). The results support the conclusion that DFT/GIAO/PCM calculations of specific rotations can be useful in elucidating the absolute configurations of chiral molecules.

Determination of the Absolute Configurations of Natural Products Using TDDFT Optical Rotation Calculations: The Iridoid Oruwacin

We report the determination of the absolute configuration (AC) of the iridoid natural product oruwacin by comparison of the optical rotations, [alpha] D, of its two enantiomers, calculated using time-dependent density functional theory (TDDFT), to the experimental [alpha] D value, +193. Conformational analysis of oruwacin using density functional theory (DFT) identifies eight conformations which are significantly populated at room temperature. [alpha] D values of these eight conformations are calculated using TDDFT at the B3LYP/aug-cc-pVDZ//B3LYP/6-31G* level, leading to the conformationally averaged [alpha] D values of -193 for the (1 R,5 S,8 S,9 S,10 S)-enantiomer and +193 for the (1 S,5 R,8 R,9 R,10 R)-enantiomer. Comparison of the calculated [alpha] D values to the value of the natural product proves that naturally occurring oruwacin has the AC 1 S,5 R,8 R,9 R,10 R. This AC is opposite to that assigned by Adesogan by comparison of the [alpha] D of oruwacin to that of the iridoid plumericin. Our results show that the assignment of the AC of a natural product by comparison of its [alpha] D to that of a chemically related molecule can be unreliable and should not be assumed to be definitive.

Determination of absolute configuration using vibrational circular dichroism spectroscopy: the chiral sulfoxide 1-thiochroman S-oxide

We have determined the absolute configuration of the chiral sulfoxide 1-thiochroman S-oxide 1 using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of a CCl4 solution of 1 was analyzed using density functional theory (DFT), which predicts three stable conformations of 1, separated by <1 kcal/mol. The VCD spectrum predicted using the DFT/GIAO methodology for the equilibrium mixture of the three conformations of (S)-1 is in excellent agreement with the experimental spectrum of (+)-1. The absolute configuration of 1 is therefore (R)-(−)/(S)-(+). (+)-1 and (−)-1 of high enantiomeric excess (e.e.) were synthesized in high yields via asymmetric oxidation of 1-thiochroman 2 using Ti(iso-PrO)4/(R,R)-1,2-diphenylethane-1,2-diol/H2O/tert-butyl hydroperoxide and Ti(iso-PrO)4/l-diethyl tartrate/H2O/cumene hydroperoxide, respectively.

Vibrational circular dichroism of matrix-isolated molecules

Abstract Vibrational circular dichroism (VCD) spectra are reported for (+), (−) and (±) α-pinene, and (−) s-pinine, isolated in argon matrices at ≈ 18 K. These are the first observations of VCD of matrix-isolated molecules. Spectra are limited to the CH stretching region (2800–3100 cm−1). In all cases, the VCD spectra are substantially more structured than the corresponding room-temperature spectra, as a result of much narrower linewidths. Further, VCD magnitudes are greater due to decreased cancellation of overlapping transitions with oppositely signed VCD. The largest anisotropy ratios observed are >5 × 10−4 and are larger than any reported for room-temperature solutions of organic molecules. This technique will permit substantially more definitive evaluations of theoretical calculations of VCD.

Vibrational Circular Dichroism and Absolute Configuration of Chiral Sulfoxides: tert-Butyl Methyl Sulfoxide

Mid-infrared vibrational unpolarised absorption and vibrational circular dichroism (VCD) spectra of CCl4 solutions of tert-butyl methyl sulfoxide (1) are reported. The spectra are compared to ab initio density functional theory (DFT) calculations carried out using two functionals, B3PW91 and B3LYP, and two basis sets, 6-31G* and TZ2P. The VCD spectra are calculated using Gauge-invariant atomic orbitals (GIAOs). The analysis of the VCD spectrum confirms the R(-)/S(+) absolute configuration of 1. The advantages and disadvantages of VCD spectroscopy in determining the absolute configurations of chiral sulfoxides are discussed.

Ab Initio Calculation of Vibrational Circular Dichroism Spectra Using Large Basis Set MP2 Force Fields.

Abstract Ab initio caculations of the vibrational circular dichroism (VCD) spectra of 2,3-trans- d 2 -oxirane ( 1 ), 1,2-trans- d 2 -cyclopropane ( 2 ), 1- 13 C-1,2,3-anti- d 3 -cyclopropane ( 3 ), and propylene oxide ( 4 ) based on [5s4p2d/3s2p] MP2 harmonic force fields are reported. The results are in perfect qualitative and good quantitative agreement with existing experimental VCD spectra except in the CH stretching region of 4 where Fermi resonance occurs. Calculations for 1–3 using a larger [8s6p3d/6s3p] basis set exhibit insensitivity to a substantial increase in basis set size.