Phenol-benzene complexation dynamics: Quantum chemistry calculation, molecular dynamics simulations, and two dimensional IR spectroscopy

Abstract

Molecular dynamics (MD) simulations and quantum mechanical electronic structure calculations are used to investigate the nature and dynamics of the phenol-benzene complex in the mixed solvent, benzene/CCl4. Under thermal equilibrium conditions, the complexes are continuously dissociating and forming. The MD simulations are used to calculate the experimental observables related to the phenol hydroxyl stretching mode, i.e., the two dimensional infrared vibrational echo spectrum as a function of time, which directly displays the formation and dissociation of the complex through the growth of off-diagonal peaks, and the linear absorption spectrum, which displays two hydroxyl stretch peaks, one for the complex and one for the free phenol. The results of the simulations are compared to previously reported experimental data and are found to be in quite reasonable agreement. The electronic structure calculations show that the complex is T shaped. The classical potential used for the phenol-benzene interaction in the MD simulations is in good accord with the highest level of the electronic structure calculations. A variety of other features is extracted from the simulations including the relationship between the structure and the projection of the electric field on the hydroxyl group. The fluctuating electric field is used to determine the hydroxyl stretch frequency-frequency correlation function (FFCF). The simulations are also used to examine the number distribution of benzene and CCl4 molecules in the first solvent shell around the phenol. It is found that the distribution is not that of the solvent mole fraction of benzene. There are substantial probabilities of finding a phenol in either a pure benzene environment or a pure CCl4 environment. A conjecture is made that relates the FFCF to the local number of benzene molecules in phenol's first solvent shell.

Testing for memory-free spectroscopic coordinates by 3D IR exchange spectroscopy

“…6A, Right) means that if the system already stayed in C for a longer time t 1 , the likelihood to switch during t 2 is reduced. That interpretation is consistent with the cluster picture deduced from MD simulations (13). That is, if a phenol molecule has already proven to be complexed for a long time during t 1 , it likely is inside one of the benzene clusters, and thus the probability for its hydrogen bond to break is smaller.…”

“…2 with exponential solutions, i.e., a Poissonian process. Two-dimensional exchange spectroscopy measures directly the two-time-point joint probability function, and its realization in the IR spectral range can do so on very fast, picosecond timescales (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In such an experiment, the frequency of a vibrational mode, which can distinguish the two states C or F of a molecular system, is considered to be the spectroscopic coordinate.…”

“…Accompanying MD simulations (13) have suggested that the mixed benzene-CCl 4 solvent is not homogeneous and forms clusters of certain sizes that predominantly consist of either benzene or CCl 4 . A phenol molecule thus can either be inside one of these clusters and be fully surrounded by benzene or CCl 4 , or at a boundary between clusters (see figure 12 of ref.…”

“…A phenol molecule thus can either be inside one of these clusters and be fully surrounded by benzene or CCl 4 , or at a boundary between clusters (see figure 12 of ref. 13). In the first case, the probability of an exchange event is expected to be small, as the phenol molecule has to first diffuse out of the corresponding cluster.…”

Hydrogen Bond Lifetimes and Energetics for Solute/Solvent Complexes Studied with 2D-IR Vibrational Echo Spectroscopy

“…For the phenol-benzene complex, recent high level electronic structure calculations on the isolated complex and full molecular dynamic simulations of PH in the BZ/CCl 4 solvent confirm the T-shape structure. 19 In addition, the simulations show that the complexes one to one between PH and BZ. The hydroxyl group does not point at the center of the benzene ring, but rather at a ring edge for all the systems studied.…”