Danya Yang

Vibrational circular dichroism intensities : Ab initio vibronic coupling theory using the distributed origin gauge

The distributed origin (DO) gauge of Stephens has been implemented in the ab initio vibronic coupling theory (VCT) of Nafie and Freedman for the calculation of vibrational circular dichroism (VCD) intensities. With this implementation, direct comparison is possible between results obtained with the DO gauge and the common origin (CO) gauge. In the VCT implementation, the VCT–DO and VCT–CO results are different even in the Hartree–Fock limit. The origin of the nonequivalence and the accuracy of VCD intensities by VCT–DO and VCT–CO when smaller basis sets are used are discussed. Detailed comparison is carried out for 13 chiral molecules. Experimental data are available for eight chiral oxiranes and thiiranes. A range of basis sets, of both the ‘‘conventional’’ and ‘‘polarized’’ varieties, is used for NHDT, H2O2, D2O2, DOOH, and oxaziridine. The results are compared with the results of Stephens’s magnetic‐field perturbation theory wherever the data are available. Calculated VCD intensities are basis‐set and ...

Electronic and Vibrational Circular Dichroism of Model β-Lactams: 3-Methyl- and 4-Methylazetidin-2-one

The chiroptical properties of the simplest chiral β-lactams, 3- and 4-methylazetidin-2-one, 1 and 2, respectively, were investigated. The experimental vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectra were measured and compared with ab initio predictions. Both compounds were found to form dimers with calculated binding enthalpies and free energies of about −51 kJ/mol and −6 to −8 kJ/mol, respectively. The experimentally measured IR and VCD spectra were measured in concentrated nonpolar (CCl4) solution and are in agreement with the predicted IR and VCD spectra of the dimeric forms, 12 and 22, but not the monomers. The most intense dimer VCD bands originate from in-plane N-H wags, which perturb the H-bonded cyclic array. At the more dilute concentrations employed for the ECD spectra, the experimental ECD spectra in heptane were interpreted satisfactorily as arising from a mixture consisting predominantly of monomers. In protic solvent (H2O, MeOH), the ECD spectra are consistent with H-bonded monomers. Simple modeling suggests that the rotational strengths of the first electronic transition gain most of their intensity from the nonplanarity of the amide chromophore, the contributions of which follow a spiral rule previously enunciated.

Vibrational circular dichroism intensities by ab initio second‐order Mo/ller–Plesset vibronic coupling theory

The vibronic coupling theory (VCT) of Nafie and Freedman for evaluating vibrational circular dichroism (VCD) intensities has been implemented at the level of ab initio second‐order Mo/ller–Plesset perturbation theory (MP2). This method, in conjunction with the MP2 force field, is applied to calculate VCD spectra of (S,S)‐2,3‐dideuteriooxirane and (S)‐NHDT. Several basis sets, namely 6‐31G*, 6‐31G*(0.3), vd/3p, and vd/3p(u) are used in the calculations. Comparisons with the experimental results for oxirane and alternate high level calculations in the case of NHDT show that VCT/MP2 with a near‐Hartree–Fock limit basis set will predict nearly quantitatively values of vibrational rotatory strengths. The inclusion of electron correlation at the MP2 level into calculation of the atomic axial tensors is important for obtaining reliable vibrational properties for the conventional 6‐31G* basis set. The 6‐31G*(0.3) basis set, which has previously proved to be highly successful in predicting VCD spectra with the VCT...

Sum rules for atomic polar and axial tensors from vibronic coupling theory

Abstract Sum rules for the electronic parts of the atomic polar tensors and atomic axial tensors are developed in the vibronic coupling theoretical (VCT) formalism for vibrational circular dichroism (VCD) intensities. These sum rules provide alternative means to judge the accuracy of VCD calculations. Implemented at the ab initio level of theory, these sum rules are tested for various sized molecules and basis sets. Calculations indicate that the atomic axial tensors computed in the distributed origin (DO) gauge obey the sum rule better than those in the common origin (CO) gauge.