Marginal electron density and density-difference functions

Summary For visual analysis of the density reorganization and distortion, the one-dimensional cut (x, y ₀,z ₀) and the two-dimensional cut (x, y, z ₀) of the three-dimensional electron density difference function (x, y, z) are frequently employed. However, these cut functions do not satisfy any sum rules in contrast to the original difference function (x, y, z). To avoid this difficulty, the use of the marginal electron density functions _x (x) and _xy (x, y) and their difference functions _x (x) and _xy (x, y) is proposed. The marginal densities are condensation of the three-dimensional density onto a particular plane or line of our interest, and they satisfy the sum rule (i.e., the conservation of the number of electrons) exactly. Some basic properties of the marginal electron density are clarified for typical diatomic molecular orbitals. An illustrative application is given for the bonding and antibonding processes in the H₂ system.     

Spin-locking and recoupling of homonuclear dipolar interaction between spin-3/2 nuclei under magic-angle sample spinning

Numerical simulations and experiments were used to examine the possibility of employing strong spin-lock fields for recoupling of homonuclear dipolar interactions between spin-3/2 quadrupolar nuclei and to compare it to the rotary-resonance recoupling at weak spin-lock fields. It was shown that strong spin-lock pulses under MAS conditions can lead to recoupling, provided that the electric-field gradient principal axes systems of the coupled nuclei are aligned and that their quadrupolar coupling constants are approximately the same. The phenomenon is based on the fact that strong spin-lock pulses induce adiabatic transfer of magnetization between the central-transition coherence and the triple-quantum coherence with equal periodicity as is the periodicity of the time-dependent dipolar coupling. Because of the synchronous variation of the state of the spin system and of the dipolar interaction, the effect of the latter on the central-transition coherence and on the triple-quantum coherence is not averaged out by sample rotation. The approach is, however, very sensitive to the relative orientation of the electric-field gradient principal axes systems and therefore less robust than the approach based on weak spin-lock pulses that satisfy rotary-resonance condition.     

A method for C(alpha) direct-detection in protonless NMR

Attempts are made to efficiently decouple (13)C nuclei without significant loss of coherence during the application of the decoupling package. Such attempts are based on the S(3)E spin-state selection method. A newly developed double S(3)E (DS(3)E) is particularly efficient for C(alpha) detection for proteins as large as 480 kDa.     

Synthesis of homo- and heteronuclear ruthenium-gold clusters with diphosphine and thiolato bridged ligands. Single crystal molecular structure of [Ru3(CO)10(μ-AuPPh3)(μ-SC5H4N)] and [Ru3(CO)8(μ-H)(μ-SC5H4N)(μ-dppe)]

The reaction between [Ru₃(CO)₁₀(NCMe)₂] and [AuClPPh₃] gave compound [Ru₃(CO)₁₀(μ-Cl)(μ-AuPPh₃)] (1) in quantitative yield under very mild conditions. The reaction of 1 with 4-mercaptopyridine (4-pyS) using ultrasonic reaction conditions gave the heteronuclear compound [Ru₃(CO)₁₀(μ-AuPPh₃)(μ-SC₅H₄N)] (2) in moderate yield. There was no spectroscopic evidence that indicates the formation of the hydride isolobal analog in this reaction. The homonuclear cluster [Ru₃(CO)₈(μ-H)(μ-SC₅H₄N)(μ-dppe)] (3) was prepared by a selective reaction employing the ruthenium-diphosphine derivative [Ru₃(CO)₁₀(μ-dppe)] (dppe = 1,2-bis(diphenylphosphine)ethane) with 4-pyS in THF solution. The isolobal analog to compound 3, compound [Ru₃(CO)₈(μ-AuPPh₃)(μ-SC₅H₄N)(μ-dppe)] (4) was synthesized by the reaction between compound 2 and dppe in refluxing dichloromethane. Compounds 1-4 were characterized in solution by spectroscopic methods and the molecular structure of compounds 2 and 3 in the solid state was obtained by single crystal X-ray diffraction studies.     

Structural controlled pure metallo-triangular assembly through bisterpyridinyl Dibenzo[b,d]thiophene,Dibenzo[b,d]furan and Dibenzo[b,d]carbazole

A novel family of metallocycles was constructed by a one-pot self-assembly of three analogous bis(terpyridine) ligand monomers L1-L3, having different bent angles, with metal ions (Zn²⁺ or Cd²⁺). The dibenzo[b,d]thiophene-containing ligand L3 assembled with the metal ions to form a single trimer, whereas the dibenzo[b,d]furan-containing ligand L2 and dibenzo[b,d]carbazole-containing ligand L1 formed a mixture of trimers and tetramers. Heteroatoms (N, O, S) significantly contributed to the molecular size of the assemblies, owing to the bent angle of the bis-terpyridines ligands.     

On the Wigner-Witmer correlation rules for a homonuclear diatomic molecule with the like atoms in identical atomic states

The problem of building the manifold of electronic states of a homonuclear diatomic molecule formed from two like atoms in identical atomic states is readdressed. A conceptually simple approach is presented by invoking a model and all the standard results originally obtained by Wigner and Witmer are reproduced.     

Homonuclear decoupled 13C chemical shift anisotropy in 13C doubly labeled peptides by selective-pulse solid-state NMR

We describe a new experiment for measuring homonuclear-decoupled anisotropic chemical shift patterns in doubly 13C-labeled compounds under magic-angle spinning. The experiment combines a pair of selective and non-selective 180 degrees pulses to suppress the 13C-13C scalar and dipolar interactions. This is combined with the recently developed SUPER technique to recouple the chemical shift anisotropy. Demonstrations on 13Calpha and 13CO-labeled amino acids and peptides show that accurate chemical shift powder patterns can be obtained. This permits the use of chemical shift anisotropy for conformational studies of suitably extensively 13C-labeled peptides and proteins.     

Measurement of one-bond heteronuclear dipolar coupling contributions for amine and diastereotopic methylene protons

One-bond heteronuclear and two-bond homonuclear residual dipolar couplings measured at methylene or amine sites can be utilized as long-range constraints in structure determination of molecules as well as to facilitate characterization of local conformation by stereospecific assignment of diastereotopic protons. We present two J-modulated HMQC type experiments to measure the one-bond heteronuclear dipolar coupling contributions of geminal protons individually. In addition two-bond homonuclear residual dipolar couplings between the diastereotopic protons are also obtained.     

Synthese und Kristallstrukturen der mehrkernigen Rhenium–Nitrido‐Komplexe [Re2N2Cl4(PMe2Ph)4(MeCN)] und [Re4N3Cl9(PMe2Ph)6]

Synthesis and Structures of the Multinuclear Rhenium Nitrido Complexes [Re₂N₂Cl₄(PMe₂Ph)₄(MeCN)] and [Re₄N₃Cl₉(PMe₂Ph)₆] The binuclear rhenium complex [Re₂N₂Cl₄(PMe₂Ph)₄(MeCN)] ( 1 ) is obtained as a byproduct of the synthesis of [(Me₂PhP)₃(MeCN)ClReNZrCl₅] from [ReNCl₂(PMe₂Ph)₃] and [ZrCl₄(MeCN)₂] in toluene. It crystallizes as 1 · 2 toluene in the monoclinic space group P2₁/n with a = 1517.0(3); b = 1847.7(2); c = 1952.4(6) pm; β = 106.44(1)° and Z = 4. The two Re atoms are connected by an asymmetric nitrido bridge Re≡N–Re with distances Re–N of 169.9(5) and 208.7(5) pm. In course of the reaction of [ReNCl₂(PMe₂Ph)₃] with [ZrCl₄(THF)₂] in CH₂Cl₂ hydrochloric acid is formed by acting of the Lewis acid on the solvent. HCl protonates and eliminates phosphine ligands of the educt [ReNCl₂(PMe₂Ph)₃] to form the phosphonium salt [PMe₂PhH]₂[ZrCl₆] ( 2 ). It crystallizes in the monoclinic space group C2/c with a = 1536.9(3); b = 1148.8(1); c = 1402.2(3) pm, β = 100.70(2)° and Z = 4. The remaining fragments of the rhenium complex combine to yield the tetranuclear mixed valent complex [Re₄N₃Cl₉(PMe₂Ph)₆] ( 3 ), crystallizing as 3 · CH₂Cl₂ in the triclinic space group P 1 with a = 1312.9(19); b = 1661.4(2); 1897.1(2) pm; α = 78.62(1)°; β = 86.77(1)°; γ = 68.28(1)° and Z = 2. The four Re atoms occupy the corners of a tetrahedron. Its edges are formed by three nitrido and three chloro bridges. The asymmetric nitrido bridges Re≡N–Re are characterized by short distances in the range of 172(2) to 176(3) pm and long distances of 194(3) to 204(2) pm. The angles Re–N–Re are between 154(1) and 160(1)°.     

Why does PMLG proton decoupling work at 65 kHz MAS?

Schemes such as phase-modulated Lee–Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution ¹H spectra at high magic-angle spinning (MAS) frequencies of 50–70 kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here, a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be successful at high MAS frequencies.