Gas-phase structure, rotational barrier and vibrational properties of trimethylsilyl trifluoroacetate CF3C(O)OSi(CH3)3: An experimental and computational study

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Abstract

The molecular structure of trimethylsilyl trifluoroacetate, CF3C(O)OSi(CH3)3, has been determined in the gas phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using 6-31G(d), 6-311G(d,p), 6-311++G(d,p) and 6-311++G(3df,3pd) basis sets. Experimental data indicate that only one conformer, with Cs symmetry [dihedral angle ϕ(CCOSi) = 180°, and all groups staggered], is observed in the gas phase. Theoretical data indicate that both this anti conformer and a gauche conformer, created by rotating about the C(O)single bondO bond, are possible, although the preferred conformation is the staggered anti one. The torsional energies for different values of the CCOSi and COSiC dihedral angles have been calculated using the RHF, MP2 and B3LYP methods with the 6-311++G(d,p) basis set. For rotation around the CCsingle bondOSi bond, a sixfold decomposition of the rotational barrier has been performed in terms of a Fourier-type expansion, enabling us to analyze the nature of the potential function, showing that the coefficients related to electrostatic interactions and steric effects are the dominant terms. The preference for the anti conformation was studied using the total-energy scheme, comparison of dipole moments, and the natural bond orbital partition scheme. The infrared spectra for the liquid and gas phases and the Raman spectrum for the liquid phase have also been recorded and the observed bands assigned to the vibrational normal modes. The experimental vibrational data, along with calculated theoretical force constants, were used to define a scaled quantum mechanical force field for the target system that enabled us to estimate the measured frequencies with a final root-mean-square deviation of 9.7 cm−1.

Introduction

Trimethylsilyl trifluoroacetate, CF3C(O)OSi(CH3)3, (abbreviated TMSTFA) is one of the strongest known silylating reagents, reacting with a variety of nucleophiles including alcohols, amines, ethers and esters [1]. TMSTFA has also been used in conjunction with soft bases such as thioanisole as a peptide deprotecting reagent [2]. The thermolytic behaviors of TMSTFA and the related compounds trimethylsilyl trifluoromethanesulfonate and trimethylsilyl acetate have been studied by ultraviolet photoelectron spectroscopy [3]. The unimolecular metastable dissociation of trimethylsilyl acetate and its fluorinated analogue, trimethylsilyl trifluoroacetate, upon electron impact have been investigated [4].

Despite the existing interest in TMSTFA, its molecular structure and vibrational characteristics are still not known. We have, therefore, extended our investigation of different derivatives of trifluoroacetate of the type CF3COXR (R = CH3, CH2CH3, CH2CF3 and X = O, S) [5], [6], [7], [8] to include this compound, where X = O and R = Si(CH3)3. The esters 2,2,2-trifluoroethyl trifluoroacetate [5], ethyl trifluoroacetate [7] whereas ethyl trifluorothioacetate [8] shows anti, anti and anti, gauche conformers, and methyl trifluoroacetate [6] has only the anti conformer. For those that can form several conformers, their preference for one or another has been studied in the past both experimentally and theoretically to determine the effects of substituent groups on O and S and the effects of R substitution.

In this work the gas-phase structure of TMSTFA has been determined by gas electron diffraction (GED) techniques. Additionally, infrared and Raman spectra have been recorded for different physical states. These experimental measurements were complemented by quantum chemical calculations to obtain an optimized molecular structure and a scaled quantum mechanical force field. The torsional potential about the C(O)single bondO bond has also been calculated using an assortment of computational approaches (both ab initio and DFT), and fitted to a sixfold Fourier-type expansion. This methodology has allowed us to analyze the nature of the potential function and assess the preferred conformation of the molecule. The study has been completed using natural bond orbital (NBO) analysis to determine whether the presence of hyperconjugative interactions may favor one conformation over another.

Section snippets

Experimental

Samples of TMSTFA for both the spectroscopy and diffraction experiments were purchased from Sigma–Aldrich and used without further purification.

Quantum chemical calculations

All geometry optimizations (RHF, MP2 and DFT with all basis sets) predicted a strong preference for the staggered anti conformation, in agreement with the experimental results, as shown in Fig. 1. Calculated geometrical parameters for TMSTFA are listed in Table 1 along with those from the experimental electron-diffraction structure. As was found for the related molecule CF3COSCH2CH3 [8], inclusion of extra polarization functions (beyond a single d function) was necessary to predict the bond

Conclusions

A complete investigation of the molecular structure of trimethylsilyl trifluoroacetate in the gas phase has been carried out using electron diffraction, complemented by theoretical methods. The experimental data show that, at room temperature, this molecule has a single staggered anti conformer, with a CCOSi dihedral angle of 180°. Theoretical methods indicate two possible conformers, both with staggered SiMe3 groups but with anti and gauche CCOSi dihedral angles, although the former is more

Acknowledgments

We acknowledge research grants from CIUNT (Consejo de Investigaciones de la Universidad Nacional de Tucumán), CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas, PIP 0629 and 5633), ANPCYT (Agencia Nacional de Promoción Científica y Tecnológica, BID 1728/OC-AR, PICT 11127) and UNLP (Universidad Nacional de La Plata). D.A.W. thanks the EPSRC for funding (Grant No.: EP/F037317) and the NSCCS for computational resources.

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