Circular dichroism of a chiral alkylbenzene. A coupled oscillator approach

1979 ◽  
Vol 83 (22) ◽  
pp. 2926-2928 ◽  
Author(s):  
Maurizio. Zandomeneghi
Keyword(s):  
Circular Dichroism ◽  
Coupled Oscillator ◽  
Circular Dichroïsm

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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