Hyperconjugative effects in π‐hydrogen bonding: Theory and experiment

Abstract

Density functional theory computations with the B3LYP/6-311++G(2df,2p) method and IR spectroscopy are employed in investigating the properties of twenty π-hydrogen bonded complexes between substituted phenols and hexamethylbenzene. All complexes possess T-shaped structures. The methyl hyperconjugative effects on interactions energies and OH stretching frequencies are estimated via comparisons with previously reported theoretical and experimental results for analogous phenol complexes with benzene. The theoretical computations provide excellent quantitative predictions of the OH stretching frequency shifts (Δν ) resulting from the hydrogen bonding. The Δν shifts in the hexamethylbenzene complexes are approximately twice as large as the corresponding shifts for the benzene complexes. Hirshfeld charges, electrostatic potential at nuclei values, and molecular electrostatic potential maps are employed in gaining insights into the mechanisms of methyl hyperconjugative effects on complex formation. © 2017 Wiley Periodicals, Inc.

Atomic Charges in Describing Properties of Aromatic Molecules

“…For the stronger electron-donating NH 2 , NHCH 3 , and N­(CH 3 ) 2 groups, only the Hirshfeld charges predict correctly the expected transfer of a negative charge from the substituents to the ring. In a recent study, we discussed the charge transfer and methyl hyperconjugative effects in a theoretical and spectroscopic investigation of π-hydrogen bonding complexes between substituted phenols and hexamethylbenzene …”

π-Hydrogen Bonding Probes the Reactivity of Aromatic Compounds: Nitration of Substituted Benzenes

“…The reactivity of aromatic molecules is usually associated with the variations of electron densities at the ring sites. 44−46 Our recent investigations 59,60 have shown that the shifts of O−H stretching frequencies upon π-hydrogen bonding may be correlated with the electron densities associated with the arene π-electron system. Stoyanov and Read 62 have emphasized the relationship between basicities of organic compounds and N− H stretching frequency shifts upon π-hydrogen bonding.…”

“…The basic hypothesis in the present research considers the expected link between the shifts in the phenol O–H stretching frequency upon π-hydrogen bonding with arene derivatives and the electron densities over the aromatic ring. The reactivity of aromatic molecules is usually associated with the variations of electron densities at the ring sites. − Our recent investigations , have shown that the shifts of O–H stretching frequencies upon π-hydrogen bonding may be correlated with the electron densities associated with the arene π-electron system. Stoyanov and Read have emphasized the relationship between basicities of organic compounds and N–H stretching frequency shifts upon π-hydrogen bonding.…”

“…Our theoretical studies revealed also that two types of π-hydrogen bonds are simultaneously formed: a red-shifting O−H•••π and a blue-shifting C−H•••π bonds. 59,60 The C−H bond at ortho position to the phenol hydroxyl group is involved in the complex (Figure 1).…”

“…59,60 As already emphasized, Δν OH is a key experimental property, considered in the present study. Besides, the B3LYP method predicts analogous structure for complexes between substituted phenols and aromatic compounds as obtained by alternative density functional theory methods: B97-D, PBE0, and ωB97X-D. 59,60 Medvedev et al 61 showed that a number of earlier proposed density functionals, including B3LYP, provide more reliable predictions of electron density distribution in molecules, compared to some recently defined density functionals. The charge density distribution is another important molecular property for our research.…”