Chiral Molecular Structures of Substituted Indans: Ring Puckering, Rotatable Substituents, and Vibrational Circular Dichroism

Abstract

The chiral molecular structures of four different substituted indans, namely, ( S )-1-methylindan, ( R )-1-methylindan-1-d, ( R )-1-aminoindan, and ( S )-1-indanol, were investigated using experimental vibrational absorption and vibrational circular dichroism spectra and corresponding spectra predicted using quantum chemical (QC) calculations. All of these molecules possess two ring puckering conformations, with ring puckering leading to the pseudoequatorial substituent being approximately four times more abundant over that leading to the pseudoaxial substituent. The amino group in 1-aminoindan has three conformations arising from the rotation of NH 2 group, for each ring puckering conformation, resulting in a total of six conformations. Whereas 1-indanol in the nonhydrogen-bonding solvent CCl 4 also has six conformations similar to those of 1-aminoindan, 1-indanol in the hydrogen-bonding solvent DMSO- d 6 adopts numerous conformations, of which 30 conformers are considered to have at least ∼1% or more population. In DMSO solution, ring puckering leading to pseudoequatorial substituent accounts for 77% population and 23% for pseudoaxial substituent. The QC spectra predicted for the geometry optimized conformers are found to be in excellent quantitative agreement with corresponding experimental spectra in all of the molecules considered. The procedures suggested in this work are hoped to provide successful pathways for future chiral molecular structural analyses.

Competition between inter and intramolecular hydrogen bond evidenced by vibrational circular dichroism spectroscopy: The case of (1S,2R)‐(−)‐cis‐1‐amino‐2‐indanol

“…Formation of cis‐ AI hydrates under jet‐cooled conditions occurs through addition of the water molecule, keeping the intramolecular H‐bond intact, 13 whereas complexation with methyl‐lactate takes place through both insertion or addition processes 10 . No VCD spectra have been reported so far for cis‐ AI, although those of the related molecules 1‐indanol, 1‐methylindane, 1‐aminoindane, and trans 1‐amino‐2‐indanol ( trans‐ AI) are known from previous work 37,44–46 . Johnson and Polavarapu have paid particular attention to the role of the puckering motion and the rotation of the amino group in shaping the VCD spectra of neat 1‐aminoindane, which was well reproduced by the Boltzmann weighted contributions of the six most stable monomers 44 .…”

“…10 No VCD spectra have been reported so far for cis-AI, although those of the related molecules 1-indanol, 1-methylindane, 1-aminoindane and trans 1-amino-2indanol (trans-AI) are known from previous work. 37,[44][45][46] Polavarapu and co-workers have paid particular attention to the role of the puckering motion and the rotation of the amino group in shaping the VCD spectra of neat 1-aminoindane, which was well reproduced by the Boltzmann weighted average contributions of the six most stable monomers. 44 The IR absorption and VCD spectra of 1-indanol in DMSO have been simulated by both first-principle molecular dynamics simulations and the cluster-in-the-liquid approach.…”

“…37,[44][45][46] Polavarapu and co-workers have paid particular attention to the role of the puckering motion and the rotation of the amino group in shaping the VCD spectra of neat 1-aminoindane, which was well reproduced by the Boltzmann weighted average contributions of the six most stable monomers. 44 The IR absorption and VCD spectra of 1-indanol in DMSO have been simulated by both first-principle molecular dynamics simulations and the cluster-in-the-liquid approach. 37,44 The VCD spectrum was well reproduced by the Boltzmann-averaged contributions of the most stable 1:1 complexes resulting from a full exploration of the potential energy surface, embedded in a DMSO solvent continuum.…”

“…44 The IR absorption and VCD spectra of 1-indanol in DMSO have been simulated by both first-principle molecular dynamics simulations and the cluster-in-the-liquid approach. 37,44 The VCD spectrum was well reproduced by the Boltzmann-averaged contributions of the most stable 1:1 complexes resulting from a full exploration of the potential energy surface, embedded in a DMSO solvent continuum. All the calculated 1:1 complexes, in both the Le Barbu-Debus et al and Johnson et al works, involve a strong OH…O H-bond from the 1-indanol hydroxyl to the DMSO.…”

“…All the calculated 1:1 complexes, in both the Le Barbu-Debus et al and Johnson et al works, involve a strong OH…O H-bond from the 1-indanol hydroxyl to the DMSO. 37,44 Taking into account larger clusters was not necessary to reproduce the VCD spectrum. The case of bifunctional compounds like diols or amino-alcohols might be different because they possess two protic substituents that could interact with the solvent.…”