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Condensed-Phase Effects on the Structural Properties of C6H5CN−BF3 and (CH3)3CCN−BF3:  IR Spectra, Crystallography, and Computations

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journal contribution
posted on 15.09.2005 by J. A. Phillips, D. J. Giesen, N. P. Wells, J. A. Halfen, C. C. Knutson, J. P. Wrass
Condensed-phase effects on the structure and bonding of C6H5CN−BF3 and (CH3)3CCN−BF3 are illustrated by a variety of results, and these are compared to analogous data for the closely related complex CH3CN−BF3. For the most part, the structural properties of C6H5CN−BF3 and (CH3)3CCN−BF3 are quite similar, not only in the gas phase but also in the solid state and in argon matrices. However, the structures do change significantly from medium to medium, and these changes are reflected in the data presented below. Specifically, the measured crystallographic structure of C6H5CN−BF3 (s) has a B−N distance that is 0.17 Å shorter than that in the equilibrium gas-phase structure obtained via B3LYP calculations. Notable differences between calculated gas-phase frequencies and measured solid-state frequencies for both C6H5CN−BF3 and (CH3)3CCN−BF3 were also observed, and in the case of (CH3)3CCN−BF3, these data implicate a comparable difference between solid-state and gas-phase structure, even in the absence of crystallographic results. Frequencies measured in argon matrices were found to be quite similar for both complexes and also very near those measured previously for CH3CN−BF3, suggesting that all three complexes adopt similar structures in solid argon. For C6H5CN−BF3 and (CH3)3CCN−BF3, matrix IR frequencies differ only slightly from the computed gas-phase values, but do suggest a slight compression of the B−N bond. Ultimately, it appears that the varying degree to which these systems respond to condensed phases stems from subtle differences in the gas-phase species, which are highlighted through an examination of B−N distance potentials from B3LYP calculations. The larger organic substituents appear to stabilize the potential near 1.8 Å, so that the structures are more localized in that region prior to any condensed-phase interactions. As a result, the condensed-phase effects on the structural properties of C6H5CN−BF3 and (CH3)3CCN−BF3 are much less pronounced than those for CH3CN−BF3.