Relativistically parameterized extended Hückel calculations

Trigonal Bipyramidal M(2)Ch(3)(2)(-) (M = Sn, Pb; Ch = S, Se, Te) and TlMTe(3)(3)(-) Anions: Multinuclear Magnetic Resonance, Raman Spectroscopic, and Theoretical Studies, and the X-ray Crystal Structures of (2,2,2-crypt-K(+))(3)TlPbTe(3)(3)(-).2en and (2,2,2-crypt-K(+))(2)Pb(2)Ch(3)(2)(-).0.5en (Ch = S, Se)

The series of group 14 metal trigonal bipyramidal anions has been extended to the mixed group 13/group 14 metal TlMTe(3)(3)(-) anions (M = Sn, Pb), obtained by the reaction of Tl(2)M(2)Te(3) and K(2)Te in en or in en/ethylamine mixtures and a stoichiometric excess of 2,2,2-crypt with respect to K(+). The thallium anions were characterized in solution by (119)Sn, (205)Tl, (207)Pb, and (125)Te NMR spectroscopy. The small magnitudes of the relativistically corrected reduced coupling constants, (1)(K(M)(-)(Ch))(RC) and (1)(K(Tl)(-)(Ch))(RC), observed for the previously reported M(2)Ch(3)(2)(-) (Ch = Se, Te) and the TlMTe(3)(3)(-) anions are consistent with predominantly p-bonded cages, and this observation is supported by local and nonlocal density functional theory (DFT) calculations. Theory indicates M-M and Tl-M interactions of high s character corresponding to Mayer bond orders of 0.13-0.32. The (K(M)(-)(M))(RC) and (K(Tl)(-)(M))(RC) couplings are unusually large compared to those of the butterfly-shaped Tl(2)Ch(2)(2)(-) anions and likely arise from higher M-M and Tl-M bond orders, a larger number of coupling pathways, and smaller M-Ch-M and M-Ch-Tl bond angles. The TlPbTe(3)(3)(-) anion has also been structurally characterized by X-ray crystallography in (2,2,2-crypt-K(+))(3)TlPbTe(3)(3)(-).2en [monoclinic system, space group P2(1)/c, Z = 4, a = 15.256(5) Å, b = 26.087(9) Å, c = 20.984(8) Å, and beta = 93.03(3) degrees ] along with Pb(2)Ch(3)(2)(-) (Ch = S, Se) in (2,2,2-crypt-K(+))(2)Pb(2)Ch(3)(2)(-).0.5en [Pb(2)S(3)(2)(-): triclinic system, space group P&onemacr;, Z = 2, a = 10.189(2) Å, b = 11.329(2) Å, c = 23.194(4) Å, alpha = 95.439(14) degrees, beta = 92.562(14) degrees, and gamma = 90.549(14) degrees; Pb(2)Se(3)(2)(-): triclinic system, space group P&onemacr;, Z = 2, a = 10.187(2) Å, b = 11.403(2) Å, c = 23.360(6) Å, alpha = 95.26(2) degrees, beta = 92.17(2) degrees, and gamma = 90.89(2) degrees ]. Density functional theory calculations show that the experimental structures for the M(2)Ch(3)(2)(-) and TlPbTe(3)(3)(-) anions are true minima and reproduce the experimental bond distances and angles. The vibrational frequencies determined by DFT calculations are in good agreement with those determined by Raman spectroscopy and have been used in their assignment.

Anisotropy of chemical shift and J coupling for P-31 and Se-77 in trimethyl and triphenyl phosphine selenides

The 31P and 77Se magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments for selenium-77 enriched (70%) trimethylphosphine selenide 1 and triphenylphosphine selenide 2 were carried out in order to determine the nuclear magnetic shielding tensors of both nuclei and to establish values of the phosphorus-selenium indirect spin-spin coupling anisotropy delta J. The m = +1/2 and m = -1/2 subspectra were analysed by the dipolar-splitting-ratio method of Eichele and Wasylischen. For the C(S) molecule 1, delta J was obtained to be +640 +/- 260 Hz from the 31P spectrum and +550 +/- 140 Hz from the 77Se spectrum. Density functional theory (DFT) calculations give a delta J value of about +705 Hz. The value of delta J could not be determined unambiguously by analysis of the 31P spectra for the C1 molecules 2; nevertheless, an estimation of delta J was possible. The principal axis 3 of the phosphorus shielding tensor was determined to be nearly parallel to the PSe bond in 1 and 2. For the selenium shielding of 1, the same orientation was found, whereas in 2, the principal axis 2 of the selenium shielding was found to be oriented nearly along the PSe bond. The experimentally determined phosphorus nuclear magnetic shielding tensors agree well with those calculated by the IGLO method. For those two principal values of the selenium-shielding tensors corresponding to directions nearly perpendicular to the SeP bond, the agreement between calculated and experimental values is satisfactory. For the third one, corresponding to the principal axis close to the SeP bond, the calculated deshielding contributions are distinctly too small for both compounds investigated. Trends observed for the calculated molecular orbital (MO) contributions to the shielding as well as possible reasons for the underestimation of the deshielding contributions along the SeP bond are discussed.

Single-Crystal (31)P NMR and X-ray Diffraction Study of a Molybdenum Phosphine Complex: (5-Methyldibenzophosphole)pentacarbonylmolybdenum(0)

The molecular structure of (5-methyldibenzophosphole)pentacarbonylmolybdenum(0), 1, has been determined by X-ray crystallography. The crystal is monoclinic C2/c, Z = 8, with unit cell dimensions of: a = 31.113(2) Å, b =7.7917(5) Å, c = 17.9522(12) Å, and beta = 122.135(4) degrees. Least-squares refinement converged to R(F) = 0.0245 for 2407 independent reflections. The molecular structure is typical of phosphine-substituted metal carbonyls. It contains an approximate mirror plane which bisects the dibenzophosphole framework. Phosphorus-31 NMR spectra of powder and single-crystal samples of 1 have been obtained with cross-polarization and (1)H high-power decoupling. The (31)P CP/MAS NMR spectra exhibit exceptionally well-resolved satellites due to spin-spin coupling interactions with (95,97)Mo (I = (5)/(2)). Using first-order perturbation theory, the multiplets have been analyzed to yield (1)J((95,97)Mo,(31)P) = 123(2) Hz and estimates of the molybdenum nuclear quadrupolar coupling constants, chi((95)Mo) = -0.87 MHz and chi((97)Mo) = 10.1 MHz. Phosphorus-31 NMR spectra of a large single crystal of 1 have been investigated as a function of orientation about three orthogonal axes in the applied magnetic field. Analysis of the data yields the three principal components of the phosphorus chemical shift tensor, delta(11) = 112 ppm, delta(22) = -23 ppm, and delta(33) = -40 ppm; delta(22) lies close to the Mo-P bond (8 degrees ), while delta(11) lies in the approximate mirror plane. The phosphorus chemical shift tensor determined for 1 is compared with the limited anisotropic phosphorus shift data available in the literature.

Tetrachloro- and Tetrabromostibonium(V) Cations: Raman and (19)F, (121)Sb, and (123)Sb NMR Spectroscopic Characterization and X-ray Crystal Structures of SbCl(4)(+)Sb(OTeF(5))(6)(-) and SbBr(4)(+)Sb(OTeF(5))(6)(-)

The stable salts, SbCl(4)(+)Sb(OTeF(5))(6)(-) and SbBr(4)(+)Sb(OTeF(5))(6)(-), have been prepared by oxidation of Sb(OTeF(5))(3) with Cl(2) and Br(2), respectively. The SbBr(4)(+) cation is reported for the first time and is only the second example of a tetrahalostibonium(V) cation. The SbCl(4)(+) cation had been previously characterized as the Sb(2)F(11)(-), Sb(2)Cl(2)F(9)(-), and Sb(2)Cl(0.5)F(10.5)(-) salts. Both Sb(OTeF(5))(6)(-) salts have been characterized in the solid state by low-temperature Raman spectroscopy and X-ray crystallography. Owing to the weakly coordinating nature of the Sb(OTeF(5))(6)(-) anion, both salts are readily soluble in SO(2)ClF and have been characterized in solution by (121)Sb, (123)Sb, and (19)F NMR spectroscopy. The tetrahedral environments around the Sb atoms of the cations result in low electric field gradients at the quadrupolar (121)Sb and (123)Sb nuclei and correspondingly long relaxation times, allowing the first solution NMR characterization of a tetrahalocation of the heavy pnicogens. The following crystal structures are reported: SbCl(4)(+)Sb(OTeF(5))(6)(-), trigonal system, space group P&thremacr;, a = 10.022(1) Å, c = 18.995(4) Å, V = 1652.3(6) Å(3), D(calc) = 3.652 g cm(-)(3), Z = 2, R(1) = 0.0461; SbBr(4)(+)Sb(OTeF(5))(6)(-), trigonal system, space group P&thremacr;, a = 10.206(1) Å, c = 19.297(3) Å, V = 1740.9(5) Å(3), D(calc) = 3.806 g cm(-)(3), Z = 2, R(1) = 0.0425. The crystal structures of both Sb(OTeF(5))(6)(-) salts are similar and reveal considerably weaker interactions between anion and cation than in previously known SbCl(4)(+) salts. Both cations are undistorted tetrahedra with bond lengths of 2.221(3) Å for SbCl(4)(+) and 2.385(2) Å for SbBr(4)(+). The Raman spectra are consistent with undistorted SbX(4)(+) tetrahedra and have been assigned under T(d)() point symmetry. Trends within groups 15 and 17 are noted among the general valence force constants of the PI(4)(+), AsF(4)(+), AsBr(4)(+), AsI(4)(+), SbCl(4)(+) and SbBr(4)(+) cations, which have been calculated for the first time, and the previously determined force constants for NF(4)(+), NCl(4)(+), PF(4)(+), PCl(4)(+), PBr(4)(+), and AsCl(4)(+), which have been recalculated for the P and As cations in the present study. The SbCl(4)(+) salt is stable in SO(2)ClF solution, whereas the SbBr(4)(+) salt decomposes slowly in SO(2)ClF at room temperature and rapidly in the presence of Br(-) ion and in CH(3)CN solution at low temperatures. The major products of the decompositions are SbBr(2)(+)Sb(OTeF(5))(6)(-), as an adduct with CH(3)CN in CH(3)CN solvent, and Br(2).

Determination of spin parameters reflected in the nuclear magnetic resonance powder patterns for two equivalent 31P nuclei in Lawesson's reagent

The nuclear magnetic resonance (NMR) powder patterns observed for the 31P spin pair in Lawesson's reagent (1) were analyzed. Using an efficient procedure, wide ranges of the spin parameters were examined to determine whether they might reproduce the experimental one-dimensional (1D) spectrum of the present A2 spin system. It was demonstrated that the parameters were not uniquely determined from the 1D powder pattern only, but a significant reduction of their uncertainties was realized using the two-dimensional (2D) powder pattern. The molecular structure of 1 was discussed in terms of the refined spin parameters.

Parity non conservation: NMR parameters in chiral molecules

The formula giving the parity-violating electroweak interaction contribution to the magnetic shielding tensor has been applied to enantiomers containing thallium. Using an extended Hückel relativistically parameterized method a chemical shift difference close to 1 mHz at 11.7 T is calculated in two couples of enantiomers. Such a difference is slightly lower than the extreme line width which may be actually reached in ultra-high resolution NMR.