The latter is consistent with failure of 19f 27 nmr

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Unformatted text preview: state for pseudorotation in phosphorous pentafluoride adopts a square-based pyramid structure and shows a barrier of ~25 kJ/mol. The latter is consistent with failure of 19F 27 NMR spectroscopy to distinguish between equatorial and axial sites even at temperatures as low as -100o C. 19 F NMR Spectrum of Phosphorous Pentafluoride: Use the B3LYP/6-31G* model to calculate the equilibrium geometry and 19F NMR spectrum of phosphorous pentafluoride. Are the chemical shifts for equatorial and axial fluorines distinct? Provide and average and compare with the experimental shift (xx ppm). Structure of PF3Cl2: PF3Cl2 can exhibit three possible structures, with two, one or no fluorines in axial positions. Use the B3LYP/6-31G* model to assign the lowest-energy structure. Is your result consistent with the fact that the “high-temperature” 19F NMR spectrum of PF3Cl2 exhibits only a single resonance (split into doublets due to 19F-31P coupling)? Elaborate. Calculate the transition state for pseudorotation between the lowest and second lowest energy isomers. Is your result consistent with the fact that the “hightemperature” 19F spectrum shows two resonances, (one a doublet of doublets and the other a doublet of triplets)? Elaborate. Pseudorotation in Iron Pentacarbonyl: Like PF5, iron pentacarbonyl, Fe(CO)5, adopts a trigonal bipyramidal geometry with distinct equatorial and axial positions. Use the B3LYP/6-31G* density functional model to calculate geometries for both trigonal bipyramidal (D3h symmetry) and square-based pyramid (C4v symmetry) forms of iron carbonyl. (Even though the latter is presumed to be a transition state, if you start with a C4v symmetry structure, this will be maintained in the geometry optimzation.) Calculate the activation energy for pseudorotation. Is it significantly higher than that for PF5? Is it likely that the 13C NMR spectrum likely to exhibit one or two resonances? Elaborate. Competing Processes The fact that single-bond rotation (including constrained rotation) and inversion processes have similar energy requirements suggests that they may “compete”. A good example of this is provided by dimethylisopropylamine, the proton NMR spectrum of which at 170K shows two different resonances corresponding to methyl groups but four different CH3 resonances at 93K. This suggests that the molecules prefers an AG (or GA) conformation in which one methyl group on nitrogen is anti (A) to the hydrogen on the isopropyl group and the other is gauche (G), and that equilibrium between the AG and GA forms is rapid. Were the symmetrical (GG) conformer to be the dominant from, then only two resonances would be observed irrespective of temperature. 28 The experimental result is supported by HF/6-31G* calculations. These show that GA (AG) conformer is xx kJ/mol low in energy that the GG conformer, corresponding to an equilibrium distribution GA:GG of xx:yy at 170K and xx:yy at 93K. Interconversion of GA and AG conformers may occur directly or in a two step process involving the GG conformer as an intermediate. There are two possibilities in the former case. Either the transition state may involve a planar nitrogen center (inversion) or a structure in which both pairs of methyl groups eclipse. In the latter case, the transition state (leading to and away from the GG conformer) has only one pair of methyl groups eclipsed. Experiments are not able to distinguish which pathway is actually preferred, or whether both occur. 29 Limiting Behavior of Hartree-Fock, B3LYP and MP2 Models for Assigning Lowest-Energy Conformation and Accounting for RoomTemperature Conformer Distributions We first set out to establish the limiting behavior of Hartree-Fock, B3LYP and MP2 models with regard to properly assign lowest-energy conformation and to account for conformational energy differences. As with previous comparisons involving equilibrium geometries and vibrational frequencies (Chapter P2), reaction energies (Chapter P3) and transition-state geometries and activation energies (Chapter P4), this will allow us to separate the effects of the LCAO approximation from effects arising from replacement of the exact many-electron wavefunction by an approximate Hartree-Fock, B3LYP or MP2 wavefunction. While it is not possible to actually reach the limit, it is possible to use a sufficiently large basis set such that the addition of further functions to the basis will have only a small effect on calculated equilibrium geometry. The cc-pVQZ basis set has been employed as a standard for acyclic systems and the cc-pVTZ basis set has been employed for cyclic systems. As previously commented, cc-pVQZ is about as large a basis set as can be applied for geometry calculations on molecules with more than a few non-hydrogen elements. The cc-pVTZ basis set is more widely applicable and can be expected to yield nearly identical results. Table P5-1 compares room-temperature conformer distributions for a variety of molecules calculated from Hartree-Fock, B3LYP an...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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