Probing Intramolecular Interaction of Stereoisomers Using Computational Spectroscopy

Several model stereoisomers such as ferrocene (Fc), methoxyphenol, and furfural conformers are discussed. It was discovered that the Fc IR spectroscopic band(s) below 500 cm-1 serve as fingerprints for eclipsed (splitting 17 (471-488) cm-1) and staggered Fc (splitting is ∼2 (459-461) cm-1) in the gas phase. It is revealed that in the gas phase the dominance of the eclipsed Fc (D5h) at very low temperatures changes to a mixture of both eclipsed and staggered Fc when the temperature increases. However, in solvents such as CCl4, eclipsed Fc dominates at room temperature (300 K) due to the additional solvation energy. Intramolecular interactions of organic model compounds such as methoxyphenols (guaiacol (GUA) and mequinol (MEQ)) and furfural, ionization energies such as the carbon 1s (core C1s), as well as valence binding energy spectra serve this purpose well. Hydrogen bonding alters the C1s binding energies of the methoxy carbon (C(7)) of anti-syn and anti-gauche conformers of GUA to 292.65 and 291.91 eV, respectively. The trans and cis MEQ conformers, on the other hand, are nearly energy degenerate, whereas their dipole moments are significantly different: 2.66 Debye for cis and 0.63 Debye for trans-MEQ. Moreover, it is found that rotation around the Cring-OH and the Cring-OCH3 bonds differ in energy barrier height by ∼0.50 kcal⋅mol-1. The Dyson orbital momentum profiles of the most different ionic states, 25a′ (0.35 eV) and 3a′ (-0.33 eV), between cis and trans-MEQ in outer valence space (which is measurable using electron momentum spectroscopy (EMS)), exhibit quantitative differences. Finally, the molecular switch from trans and cis-furfural engages with a small energy difference of 0.74 kcal mol-1, however, at the calculated C(3)(-H⋅⋅⋅O=C) site the C1s binding energy difference is 0.105 eV (2.42 kcal mol-1) and the NMR chemical shift of the same carbon site is also significant; 7.58 ppm from cis-furfural without hydrogen bonding.