Structure, Vibrational Absorption and Circular Dichroism Spectra, and Absolute Configuration of Tröger's Base

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The structure of Tröger's Base, 1, a heterocyclic amine containing two stereogenic N atoms, is predicted from ab initio Density Functional Theory (DFT), using the B3PW91 and B3LYP functionals and the 6-31G* basis set. Vibrational unpolarized absorption and vibrational circular dichroism (VCD) spectra are predicted thence. In the case of VCD, Atomic Axial Tensors (AATs) are calculated using Gauge-Invariant Atomic Orbitals (GIAOs). The predicted structure is in excellent agreement with the X-ray structure of 1. The predicted vibrational spectra are in excellent agreement with experimental spectra of CCl4 and CS2 solutions of 1, supporting the identicalness of the crystalline and solution conformations of 1. No evidence of additional conformations of 1 in solution is found. The VCD spectra require the assignment of the (R,R)/(S,S) absolute configuration to (−)/(+)-1, opposite to that obtained from the electronic circular dichroism spectrum by Mason et al. and in accord with that obtained from X-ray crystallography of a salt of 1 containing monoprotonated 1, 2, and a chiral anion by Wilen et al. The structure of 2 predicted by DFT is also in excellent agreement with the X-ray structure.