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46291-B6
Condensed Phase Effects on the Structural Properties of Friedel-Crafts Intermediates: RF-BF3

James A. Phillips, University of Wisconsin (Eau Claire)

Context: The main objective of this project is to characterize the structural properties of organofluoride – boron trifluoride complexes (RF'–BF₃) in the gas-phase and in bulk, condensed-phase environments via computations and low-temperature infrared spectroscopy. These species are key intermediates in Friedel-Crafts reactions [1] - an important class of carbon-carbon forming processes that facilitate the conversion of petroleum feedstocks to commercially-viable compounds. One common example is the alkylation of benzene, viz.

   +   RF'   +   BF₃        +   HF'   +   BF₃    (1)

The first step is the formation of an RF–BF₃ intermediate [1], a few of which have been isolated. When R is "propyl" or larger, these intermediates produce electrolyte solutions and are thus best described as fluoro-borate salts. However, when R is methyl or ethyl, they are donor-acceptor type complexes that do not produce electrolyte solutions, but still react in the same manner. In the gas-phase, CH₃F–BF₃ complex is quite weak with an experimental B-F' distance of 2.42� [2]. Thus, its solution-phase reactivity can only be rationalized by invoking a substantial structural re-arrangement that results from interactions with the solvent. The main goal of this work is to characterize this transition from "weak donor-acceptor complex" to "effective carbo-cation", as a function of both internal effects (e.g. changes in the R substituent), and environmental factors (i.e. polarity of the solvent).

Infrastructure: A substantial portion of the proposed work involves computations, not only for predictions of gas-phase structures and frequencies, but also energy-profiles along the B-F' distance coordinate in both the gas-phase and in bulk, dielectric media (via continuum solvation models). Thus, the initial efforts were focused on obtaining a 6- or 8-processor computational cluster with a queuing system to maximize the efficiency of the system. After the acquisition of this grant, however, an opportunity to pool resources with a colleague and pursue a larger system became available, and furthermore, personnel in UWEC's Learning Technology Services (LTS) unit became active collaborators interested in overseeing the development and implementation of such a system. For the past 6-months, we have been running computational jobs on a 32-processor test system, using software purchased through these pooled funds. Some of these were designed simply to provide performance benchmarks, others produced the results shown below. Nonetheless, this effort has enabled LTS personnel to assess the CPU, memory, and storage needs for various types of computations, as to obtain the maximum possible benefit from our investment. In the future, this collaboration may lead to a coordinated effort that involves the creation of a center that involves several departments. In the next few weeks, we will be purchasing our first installment of high-performance computing hardware, a 32 or 40-processor cluster, which will enable very high-level computations to be performed locally.

Results:

HF–BF₃: Validating DFT methods on a small test system with a known structure

The HF–BF₃ complex, though it lacks the R group necessary to participate in a reaction such as (1), is an ideal system upon which to validate computational methods since it is the smallest member of the RF'–BF₃ class of complexes, and moreover, an experimental structure is available. This system is the functional equivalent of fluoroboric acid "HBF₄", but the experimental structure of this system has the HF weakly coordinated to a largely-undistorted BF₃, with an intermolecular B-F' distance of 2.54 � [2]. Awe found that X3LYP performed the best among 6 various DFT methods and MP2 (with the aug-cc-pVTZ basis set),not only interms of agreement between the experimental and theoretical structures, but also the accuracy of predicting gas-phase BF₃ frequencies. The calculated B-F' distance via this method is 2.535 �, and the binding energy is 2.9 kcal/mol.

CH₃F–BF₃: The smallest, yet possibly most interesting intermediate

We subsequently calculated X3LYP/aug-cc-pVTZ structures for four possible Cs-symmetry conformations of CH₃F–BF₃. The minimum energy conformer is shown below, and has no imaginary vibrational frequencies, in contrast to the other three. The calculated B-F distance (2.41�) compares quite favorably with the preliminary experimental result (2.42 �) [2], and the binding energy is predicted to be 3.6 kcal/mol. Thus, the substitution of a methyl group for the hydrogen causes a notable contraction of the B-F' distance (> 0.1� ) and a 0.6 kcal/mol (20%) increase in the binding energy. At this point we have also obtained a preliminary prediction of the structural changes that occur in chloroform solution. Surprisingly, the equilibrium B-F' distance is predicted to contract by only about 0.03�, and the C-F' distance only increases by about 0.01�. A more detailed assessment is in progress.

                 GAS PHASE                                CHLOROFORM SoLUTION

References:

1. Olah, G.A. Friedel-Crafts and Related Reactions; Wiley/Interscience: New York, 1963.

2. Leopold, K.R.; Canagaratna, M.; Phillips, J.A. Accts. Chem. Res. 1997, 30, 57.

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