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

Abstract

ABSTRACT:The compound, (R)-[ 2 H 1 , 2 H 2 , 2 H 3 ]-neopentane, with its T d 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 Ϫ1 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 T d symmetric electron distribution.

Characterizing vibrational motion beyond internal coordinates

“…1 is in terms of vibrational energy, with the volume of the spheres chosen proportional to the fraction individual nuclei contribute to the total energy of a vibration [39]. The ratio of the kinetic energy of nucleus α in vibration p to the vibration's total kinetic energy is given by…”

pyVib, a computer program for the analysis of infrared and Raman optical activity

“…As the invariants are proportional to (L x α,p ) 2 / ν p and, in turn, cross sections, the size of the spheres is further divided by the wave number ν p associated to vibrational mode p (except for VCD). 4 This option based on the sphere representation is implemented in pyVib.…”

“…4 Starting from the virial theorem and the harmonic approximation, the fractional energy can be written as…”

Correction

“…Haesler and colleagues' synthesis 1 of (R)-[ 2 H 1 , 2 H 2 , 2 H 3 ]-neopentane with high enantiomeric purity was itself an achievement, and was attempted only after calculations 8 suggested that the ROA might be measurable. It is a gas at room temperature, but was collected as a liquid for ROA measurements by distilling into a cooled capillary tube that was subsequently sealed.…”

“…Haesler et al 1 simulated the ROA spectrum of the chiral neopentane using ab initio quantum-chemical protocols 8 . They summed the partially cancelling ROA spectra computed for each of the nine possible rotational conformers present at thermal equilibrium.…”