Faculté des sciences

Is the vibrational optical activity of (R)-[²H₁, ²H₂, ²H₃]-neopentane measurable?

Hug, Werner ; Haesler, Jacques

In: International Journal of Quantum Chemistry, 2005, vol. 104(5), p. 695

The compound, (R)-[²H₁, ²H₂, ²H₃]-neopentane, with its Td symmetric electron distribution, is the archetype of molecules that owe their chirality exclusively to an asymmetric distribution of the masses of their nuclei. It has nine rotamers, which fall into two classes, one where the interchange of hydrogen and deuterium nuclei leads to an identical rotamer, and... More

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    Summary
    The compound, (R)-[²H₁, ²H₂, ²H₃]-neopentane, with its Td symmetric electron distribution, is the archetype of molecules that owe their chirality exclusively to an asymmetric distribution of the masses of their nuclei. It has nine rotamers, which fall into two classes, one where the interchange of hydrogen and deuterium nuclei leads to an identical rotamer, and one where it interconverts different rotamers. Ab initio computations show that individual rotamers have Raman optical activity (ROA) and vibrational circular dichroism (VCD) of the same size as ordinary chiral molecules. Dilution and cancelation for the experimentally accessible equilibrium mixture reduces ROA and VCD, but the simulation of spectra with realistic band shapes of the Voigt type shows that both remain measurable in the 700-1,400-cm⁻¹ range, where ROA and VCD have a close to mirror image appearance. In the CH- and CD-stretch region, in contrast, the sign pattern of ROA and VCD is identical. This curious behavior appears to be a consequence of the differing influence of inertial contributions in the two spectral regions. If summed over all vibrations, ROA and VCD cancel neatly, as one expects from the Td symmetric electron distribution.