B-N Distance Potential of CH3CN-BF3 Revisited: Resolving the Experiment-Theory Structure Discrepancy and Modeling the Effects of Low-Dielectric Environments
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- 作者：James A. Phillips ; Christopher J. Cramer
- 刊名：Journal of Physical Chemistry B
- 出版年：2007
- 出版时间：February 15, 2007
- 年：2007
- 卷：111
- 期：6
- 页码：1408 - 1415
- 全文大小：1068K
- 年卷期：v.111,no.6(February 15, 2007)
- ISSN：1520-5207
文摘
We have re-examined the B-N distance potential of CH3CN-BF3 using MP2, DFT, and high-accuracymulticoefficient methods (MCG3 and MC-QCISD). In addition, we have solved a 1-D Schrödinger equationfor nuclear motion along the B-N stretching coordinate, thereby obtaining vibrational energy levels, wavefunctions, and vibrationally averaged B-N distances. For the gas-phase, MCG3//MP2/aug-cc-pVTZ potential,we find an average B-N distance of 1.95 Å, which is 0.13 Å longer than the corresponding equilibriumvalue. In turn, this provides solid evidence that the long-standing discrepancy between the experimental (R(B-N) = 2.01 Å) and theoretical (R(B-N) = 1.8 Å or R(B-N) = 2.2-2.3 Å) distances may be genuine, stemmingfrom large amplitude vibrational motion in the B-N stretching coordinate. Furthermore, we have examinedthe effects of low-dielectric media (
ges/gifchars/epsilon.gif" BORDER=0 > = 1.1-5.0) on the structure of CH3CN-BF3 by calculating solvationfree energies (PCM/B97-2/aug-cc-pVTZ) and adding them to the gas-phase, MCG3 potential. These calculationsdemonstrate that the inner region of the potential is stabilized to a greater extent by these media, andcorrespondingly, the equilibrium and average B-N distances decrease with increasing dielectric constant.We find that the crystallographic structural result (R(B-N) = 1.63 Å) is nearly reproduced with a dielectricconstant of only 5.0, and also predict significant structural changes for ges/gifchars/epsilon.gif" BORDER=0 > values of 1.1-1.5, consistent withresults from matrix-isolation-IR experiments.