Near equivalence of axial and equatorial 19F NMR chemical shifts and NQR coupling constants in PF 5

Abstract

In PFS only a single 19F doublet is observed (1) in the NMR spectrum down to a temperature of 197°C even though gas phase electron diffraction studies (2) reveal a Djh trigonal bipyramidal structure with chemically inequivalent axial and equatorial fluorines having P-F distances of 1.577 and 1.534 A, respectively. The NMR equivalence of axial and equatorial F is usually explained in terms of a rapid interconversion proceeding through a Berry pseudorotation mechanism (3). SCF-MO calculations with moderate size basis sets give a barrier to pseudorotation of 4.8 kcal/mol (4) in reasonable agreement with values of 3.8 kcal/mol from vibrational potential studies (5) and ~5 cal/mol from low-temperature NMR (6). Studies on some monosubstituted compounds of formula PF4X do yield more than one 19F shift at low temperature as well as showing variation of the “P NMR with temperature (7). NQR studies on PCLF also show different “Cl resonances assignable to axial and equatorial Cl (8). However, for unsubstituted PFs there is yet no direct evidence that the static equilibrium D3h structure would give an inequivalence of either the i9F NMR or NQR. Such an inequivalence has only been posited based on the inequivalence of bond lengths and calculated bond properties, e.g., percentage s character in the bond (4, 9). We have calculated the electric field gradient tensor, qas, and the NMR shielding constant tensor u,@, at the axial and equatorial F nuclei in the equilibrium geometry of PFs using the methods previously described (10) and recently applied in SiF, (1 I, 12). Near Hartree-Fock bases of Gaussian functions were constructed using the sp basis of McLean and Chandler (13) for P and that of van Duijneveldt (14) for F. To P we added two six-component 3d functions with exponents of 0.76 and 0.38 (a split version of the 0.57 exponent of Strich and Veillard (4)) and we added a single 3d function with exponent 0.80 to F (15). Our basis is therefore (13s9p2d/9s5pld) --, [SsSp3d/4sSp2d/4s3pld] in the usual notation. The total energy was -838.25875 (compared to -837.8366 for the best previous calculation (4)) which is bound by 309 kcal/mol with respect to the Hartree-Fock atoms (16). The experimental atomization energy is 562 kca/mol (I 7). Isotropic 19F NMR shielding constants and anisotropies with the P atom as the gauge origin for the vector potential are shown in Table 1 along with existing exper-