C1 Point Group Symmetry Research Articles

Differentiation of adenine non-planarity in valence molecular orbitals

Two molecular orbitals: MO7 (29a) and MO13 (23a) have been identified using dual space analysis (DSA) as the signatures of adenine non-planarity (C1 point group symmetry). The non-planarity of adenine has been demonstrated to be from the attachment of the amino group (NH2) to purine rings as well as the non-rigid deformability of the purine ring of adenine. Orbital 29a (3a″ in the planar case), a π-like orbital, is the direct result of the attachment of the amino group to the purine ring. Orbital 23a (23a′ in the planar case) is the result of the deformability of the purine ring in non-planar adenine (NP) and will be experimentally challenging to resolve.

Synthesis and Structure of Asymmetric Zirconium‐Substituted Silicotungstates, [Zr6O2(OH)4(H2O)3 (β‐SiW10O37)3]14‐ and [Zr4O2(OH)2(H2O)4 (β‐SiW10O37)2]10‐.

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Synthesis and Structure of Asymmetric Zirconium-Substituted Silicotungstates, [Zr6O2(OH)4(H2O)3(β-SiW10O37)3]14- and [Zr4O2(OH)2(H2O)4(β-SiW10O37)2]10-

The reaction of ZrCl4 with [gamma-SiW10O36]8- in a potassium acetate buffer results in two different products depending on the reactant ratios. The trimeric species [Zr6O2(OH)4(H2O)3(beta-SiW10O37)3]14- (1) consists of three beta23-SiW10O37 units linked by an unprecedented Zr6O2(OH)4(H2O)3 cluster with C1 point group symmetry. The dimeric species [Zr4O2(OH)2(H2O)4(beta-SiW10O37)2]10- (2) consists of beta22- and beta12-SiW10O37 units sandwiching a Zr4O2(OH)2(H2O)4 cluster, which also has C1 symmetry. Polyanion 1 contains more zirconium centers than any other polyoxometalate known to date.

Rho-axis-method Hamiltonian for molecules having one methyl rotor and C1 point-group symmetry at equilibrium

Two modifications of the rho-axis-method torsion-rotation Hamiltonian for fitting spectra of methyl top molecules belonging to the Cs point group at equilibrium which are required to treat molecules with C1 equilibrium configurations are described: (i) The permutation-inversion group must be changed from G6 to G3, which causes the A1 and A2 species to coalesce into a single A species, and which also introduces the slight complication of separably degenerate E species. (ii) One term must be added to the second-order contact-transformation-reduced Hamiltonian and seven terms must be added to the fourth-order reduced Hamiltonian. Programming strategies for this Hamiltonian are also discussed. In particular, portions of the C1 computer code requiring complex arithmetic are indicated, and three schemes for checking the internal consistency of the C1 code are suggested. Application of this C1 Hamiltonian to ethylacetamidoacetate is given in the preceding paper.

Ruthenium(II) Complexes of Multidentate Ligands Derived from Di(2-pyridyl)methane

Bis(2,2′-bipyridyl)ruthenium complexes of the model ligand di(2-pyridyl)methane (1) and its multidentate derivatives 1,1,2,2-tetra(2-pyridyl)ethane (2), tetra(2-pyridyl)ethene (3), and hexa(2-pyridyl)[3]radialene (4) have been prepared and characterized by NMR, visible absorption spectroscopy, electrochemical measurements, and, in two cases, by X-ray crystallography. Complexes of (2) and (3) exist as conformationally rigid species with lower than expected symmetry. Ligands (2) and (3) proved surprisingly resistant to forming dinuclear ruthenium complexes. The two diastereoisomeric dinuclear complexes of (4), Δ Λ/Λ Δ and Δ Δ/Λ Λ (rac), are shown to be locked in conformations of C1 and C2 point-group symmetry, respectively. This represents a rare example where the Δ Λ/Λ Δ-dinuclear complex of a achiral, symmetrically bridging ligand is not a meso compound.

Rotational spectra of four of the five conformers of 1-pentene

The rotational spectra of four of the five expected conformers of 1-pentene, together with their monosubstituted 13C isotopic forms, have been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. One of the conformers has Cs point-group symmetry while the other three conformers have C1 point-group symmetry. The measurements are compared to results from molecular modeling calculations using the MM3 molecular-mechanics force field of Allinger et al. and to ab initio electronic structure calculations (MP2/6-31G*, MP2/6-311G*, MP4/6-31G*, MP4/6-311G*). Both types of calculations suggest the existence of five distinct conformers of 1-pentene, four of C1 symmetry and one of Cs symmetry. Both the MM3 and ab initio rotational constants deviate from the measured values by ⩽5%. The relatively high barriers between the four conformers limit the conformational cooling in the expansion, allowing all four conformers to be observed at the <2 K rotational temperature of the molecular beam. Efforts to identify the fifth conformer were unsuccessful, presumably due to its reduced intensity, which makes it difficult to identify its spectral pattern from among the plethora of weak unassigned lines due to impurities, complexes, and possible vibrationally excited conformers. The fifth conformer is predicted to have the highest energy of the five conformers of 1-pentene, as well as a low-energy barrier (109 cm−1 at MP2/6-311G* level) for conformational isomerization.

Crystallization of the N-terminal domain of human sex hormone-binding globulin, the major sex steroid carrier in blood.

The amino-teminal laminin G-like domain of human sex hormone-binding globulin (SHBG), which contains the steroid-binding site and the dimerization domain, has been produced in Escherichia coli, purified to homogeneity and crystallized in complex with 5alpha--dihydrotestosterone (DHT) in two different crystal forms. Native data sets have been collected for tetragonal crystals (space group P4(1)22 or P4(3)22; unit-cell parameters a = 52.2, c = 148.4 A) diffracting to 3.3 A and trigonal crystals (R32; a = 104.0, c = 84.4 A) diffracting to better than 1.6 A. Since both crystal forms can only accommodate a single monomer in the asymmetric unit and share twofold rotational symmetry, it is proposed that the homodimer of this truncated form of SHBG, as observed in ultracentrifugation experiments, displays C(2) point-group symmetry.

Structure of cis-diaquabis(hexafluoroacetylacetonato)nickel(II)

Ci0H6FI2NiO6, M,=508.9, monoclinic, C2/c, a = 21.195 (5), b = 8.247 (2), c = 9.741 (2) ,/3 = 95.91 (2) °, V = 1693.6 (7) )~3, Dx = 1.996 Mg m-3, Z=4, #=1.298mm-1, A(MoKa)=0.71073./~, F(000)=1000, T=298K, R=0.0369 and wR = 0.0645 for 157 variable parameters (S= 0.59) and 1112 reflections with F>0~r(F). The Ni atom possesses an octahedral geometry and forms bonds to the O atoms of two chelating F6acac ligands that occupy both axial and equatorial positions (Ni-- O(1) 2.026(3) A; Ni--O(2) 2.027 (2)A). The two molecules of water are coordinated in a cis con- figuration (Ni--O(3) 2.054 (3)/~) and the structure possesses C2 point group symmetry. Introduction. The ability of the fluorinated deriva- tives of acetylacetone to form stable chelating bis adducts with many of the transition-metal elements is well established (Joshi & Pathak, 1977). In most instances stable monomeric adducts containing equa- torial acac ligands are obtained. Notable exceptions include the anhydrous bis(acetylacetonato) Co I~, Ni I~ and Zn II complexes which possess oligomeric struc- tures with bridging acac groups. The structures of the trans-diaquabis(acetylacetonato) complexes of both Co II and Ni ~ have been previously described (Bullen, 1959; Montgomery & Lingafelter, 1964) and the preparation of trans-Ni(F6acac)2(H20)2 has been * To whom correspondence should be addressed.

Energy levels and crystal field parameters of Nd3+ and Er3+ in LiRP4O12 single crystals

The experimental Stark energy levels for the Nd3+ and Er3+ in LiRP4O12 crystals were presented in the authors previous paper. A crystal field analysis of these data is based on a Hamiltonian of C2 point group symmetry, including J-mixing effects. They find that the twinned C2C/ (C2) structure is consistent with all available experimental results. A Hamiltonian parametrized in monoclinic symmetry was used to describe the observed crystal-field splittings of Nd3+ and Er3+ in LiRP4O12. Resulting RMS deviations between calculated and experimental levels range from 6.8 to 7.4 cm-1.

Optical spectra, energy levels, and crystal-field analysis of tripositive rare-earth ions in Y2O3. II. Non-Kramers ions in C2 sites

Optical absorption and fluorescence spectra of the non-Kramers ions Pr3+, Tb3+, and Ho3+ in the C2 sites of Y2O3 are reported. A crystal-field analysis of these data and previously reported data for Eu3+ and Tm3+ is performed that is based on a Hamiltonian of C2 point-group symmetry, including J-mixing effects. A best-fit analysis is performed for all five ions; resulting rms deviations between calculated and experimental levels range from 2.8 to 26.4 cm−1. Results of this analysis and of our previous analysis of Kramers ions in C2 sites are used to obtain a set of phenomenological crystal-field components Akm for the C2 sites. We predict the crystal-field splittings for Pm3+ in Y2O3, and we compare our previous prediction of the splittings of Gd3+ in Y2O3 with recently reported measurements. We also describe an effective point-charge model for the Y2O3 lattice in which good agreement between calculated and phenomenological Akm is obtained with qY1 = 2.53, qY2 = 1.26, and q0 = −1.052. (The quantities qY1, qY2, and q0, are, respectively, the charges on the C3i yttrium site, the C2 yttrium site, and the oxygen site, in units of e.) With this model, we calculate Akm for the C3i sites.