Skeletal vibrational circular dichroism of a series of bicyclic dilactones: the fingerprint region?

1998 ◽  
Vol 76 (6) ◽  
pp. 717-725 ◽  
Author(s):  
A Rauk ◽  
J L McCann ◽  
H Wieser ◽  
P Bour ◽  
Yu.I. El'natanov ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Circular Dichroism ◽  
Force Field ◽  
Ab Initio ◽  
Vibrational Circular Dichroism ◽  
Coupled Oscillator ◽  
Field Perturbation ◽  
Fingerprint Region ◽  
Circular Dichroïsm ◽  
Magnetic Field Perturbation

The vibrational circular dichroism (VCD) spectra of 2,5-dioxabicyclo[2.2.1]heptane-3,6-dione 1 (R = H) and the 1,4-dialkyl derivatives, 1 (R = Me) and 1 (R = t-Bu), were calculated by the ab initio magnetic field perturbation (MFP) procedure with a B3LYP/6-31G* force field, and the VCD spectra of the dimethyl and di-t-butyl derivatives were measured in the region 800-1500 cm-1. While the absolute configurations of 1 (R = Me) and 1 (R = t-Bu) could be assigned unambiguously by comparison of the experimental and predicted spectra, there is little obvious correspondence between the spectra of the series of compounds. The VCD spectra are analysed on the basis of operation of a coupled oscillator mechanism involving motions of the polar bonds. In the case of the parent dihydro system, bridgehead C-H bond motions appear to dominate contributions to the VCD of vibrational transitions into which they are mixed.Key words: vibrational circular dichroism (VCD), 2,5-dioxabicyclo[2.2.1]heptane-3,6-dione, dilactones, optical activity.

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

1996 ◽  
Vol 50 (5) ◽  
pp. 630-641 ◽  
Author(s):  
Jennifer McCann ◽  
Arvi Rauk ◽  
Gennadii V. Shustov ◽  
Hal Wieser ◽  
Danya Yang
Keyword(s):  
Circular Dichroism ◽  
Ab Initio ◽  
Electronic Transition ◽  
Vibrational Circular Dichroism ◽  
Protic Solvent ◽  
Free Energies ◽  
Chiroptical Properties ◽  
Circular Dichroïsm ◽  
Rotational Strengths ◽  
Simple Modeling

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.

Infrared and vibrational circular dichroism intensities of model systems CH3OH, CH3NH2, NH2NH2, NH2OH, and HOOH and the deuterated species, ND2ND2, DOOH, and DOOD: A theoretical study using the vibronic coupling formalism

1990 ◽  
Vol 68 (2) ◽  
pp. 258-266 ◽  
Author(s):  
Arvi Rauk ◽  
Remo Dutler ◽  
Danya Yang
Keyword(s):  
Hydrogen Peroxide ◽  
Circular Dichroism ◽  
Vibronic Coupling ◽  
Vibrational Circular Dichroism ◽  
Model Systems ◽  
Antisymmetric Mode ◽  
Coupled Oscillator ◽  
Torsional Mode ◽  
Rotational Strength ◽  
Circular Dichroïsm

We have implemented at the abinitio level the vibronic coupling formalism for infrared and vibrational circular dichroism (VCD) intensities of Nafie and Freedman. We report here the results of a series of calculations on model systems. Hydrazine and hydrogen peroxide exist in chiral conformations. Although low barriers to racemization preclude the possibility of optical resolution of the parent compounds, the local gauche configuration at the heteroatom bond is preserved in substituted hydrazines and peroxides and these chiral fragments may contribute significantly to the vibrational chiroptical properties of the molecules in which they appear. The calculated VCD rotational strengths for the chiral gauche forms of both molecules suggest that the torsional mode is very intense in the VCD spectrum. In gauche hydrazine of P chirality, the in- and out-of-phase umbrella motions of the nitrogen atoms, ν6 and ν12, respectively, give rise to an intense CD couplet with negative rotational strength to the long wave length side. In hydrazine, all combinations of NH stretches are relatively weak in the IR but the highest frequency mode of b symmetry, ν8, has moderate positive strength. The in- and out-of-phase OOH bends of HOOH behave as a coupled oscillator system, exhibiting a strong bisignate CD pattern. The signals calculated for the OH stretches are not consistent with a coupled oscillator model. The nearly degenerate stretches result in a single band arising from the more intense negative rotational strength of the antisymmetric mode ν5, in HOOH of P chirality. An alternative model, based on induced charge flow along the chirally disposed skeleton (i.e., Nafie and Freedman's 3-bond electronic current model), explains the calculated sign of the antisymmetric mode, ν5. Significant changes in the VCD patterns due to deuterium substitution in both chiral molecules are discussed. Keywords: vibrational circular dichroism (VCD), abinitio, infrared intensities, molecular orbital, vibronic coupling theory, methanol, methanamine, hydrazine, hydroxylamine, hydrogen peroxide.

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