Correlation of 谓OH Spectral Shifts of Phenol鈥揃enzene O鈥揌路路路蟺 Hydrogen-Bonded Complexes with Donor鈥檚 Acidity: A Combined Matrix Isolation, Infrared Spectroscopy, and Quantum Chemistry Study

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• 作者：Pujarini Banerjee ; Tapas Chakraborty
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：August 28, 2014
• 年：2014
• 卷：118
• 期：34
• 页码：7074-7084
• 全文大小：555K
• ISSN：1520-5215

文摘

O鈥揌 stretching infrared fundamentals (谓_OH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8 K). Spectral analysis reveals that ring fluorine substitutions have little effect on phenolic 谓_O鈥揌 as long as the molecules in the matrix are fully dispersed as monomers. The substitution effects are pronouncedly manifested only when the phenols are complexed with benzene, and the measured shift in phenolic 谓_OH from the monomer value varies from 鈭?8 cm^鈥? in phenol to 鈭?8 cm^鈥? in 3,4,5-trifluorophenol. The spectral shifts are found to display a linear correlation with the aqueous phase acid dissociation constants (pK_a) of the phenols. The spectral changes predicted by electronic structure calculations at several levels of theory are found to be consistent with the observations. Such correlations are also found to exist with respect to different energetic, geometric, and other electronic structure parameters of the complexes. Atoms in Molecules (AIM) analysis shows a distinct bond critical point due to accumulation of electron density at the hydrogen-bonding site. The variation of electron densities both on the hydrogen bond as well the donor O鈥揌 group is in accordance with the experimentally observed 谓_O鈥揌 of the various fluorophenol鈥揵enzene complexes. Partitioning of binding energies into components following the Morokuma鈥揔itaura scheme shows that the 蟺-hydrogen-bonded complexes are stabilized predominantly by dispersion interactions, although electrostatics, polarization, and charge-transfer terms have appreciable contribution to overall binding energies. NBO analysis reveals that hyperconjugative charge-transfers from the filled 蟺-orbitals of the hydrogen bond acceptor (benzene) to the antibonding 蟽*(O鈥揌) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O鈥揌 bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.