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J A Paixão

Publications and source records attributed to J A Paixão.

At least 19 recordsLinked to original sources
PubMed2006

2,2,4,4-Tetra-tert-butyl-1,3,5,2,4-benzotrioxadisilepine-7-carbaldehyde.

The molecule of the title compound, C23H40O4Si2, features an approximate non-crystallographic C2 symmetry axis. The aldehyde group is disordered over two positions with similar occupancies. The geometry of the isolated molecule was studied by ab initio quantum mechanical calculations employing a molecular orbital Hartree-Fock method. The calculations reproduce well the equilibrium geometry but slightly overestimate the value of the Si-O bond lengths of the trioxadisilepine ring.

Journal Article↗
PubMed2006

Glycinium trichloroacetate.

The title compound, C2H6NO2+.C2Cl3O2-, crystallizes in the P4(1) space group with two glycinium cations and two trichloroacetate anions in the asymmetric unit. The glycinium cations have nearly Cs point-group symmetry which is only broken by the H atoms of the amine group. The trichloroacetate anions show typical bond lengths and angles, one of the trichloromethyl groups being disordered. Chains of alternating anions and cations run along the c axis. Within these chains, consecutive anion-cation pairs are bound via strong hydrogen bonds involving the carboxylate anions and the carboxyl or amine groups of the cations. Weaker hydrogen bonds bind neighbouring chains together.

Crystallography↗
PubMed2005

Weak antiferromagnetic interactions in hydronium diaqua-1kappaO,2kappaO-trichloro-1kappaCl,2kappa2Cl-mu-dimethylglycinato-1kappaO:2kappa2O',N-dimethylglycinato-1kappa2N,O-dicuprate(II).

The crystal structure of the title compound, (H3O)[Cu2(C4H8NO2)2Cl3(H2O)2], contains two Cu(II) ions bridged by the carboxylate group of a dimethylglycinate ion in an anti-anti configuration. The Cu(II) atoms both have an approximately square-pyramidal conformation geometry and are 5.977 (2) angstroms apart. The two dimethylglycinate ligands have similar conformations, although they play different roles in the structure. A weak antiferromagnetic interaction between the two copper ions could be inferred from the magnetic susceptibility measurements.

Journal Article↗
PubMed2005

Di-mu-chloro-1:2kappa2Cl;3:4kappa2Cl-hexachloro-1kappa3Cl,4kappa3Cl-tetra-mu-dimethylglycine-2:3kappa8O:O'-tetracopper(II).

The title compound, [Cu(NO3)2(C6H10N4)2], crystallizes in the centrosymmetric space group P21/c with a unit cell containing two tetranuclear copper(II) complexes sited on crystallographic inversion centres. The coordination geometry around the central Cu atoms is square pyramidal, with four O atoms in the basal plane and a Cl atom in the apical position. The lateral CuCl4 groups are flattened tetrahedral. The bridging dimethylglycine molecules are present in the dipolar zwitterionic form. The tetranuclear copper complexes exist as isolated entities since only intramolecular hydrogen bonds are found.

Journal Article↗
PubMed2005

17-Oxo-5alpha-androstane-3alpha,4beta-diyl diacetate and 17-oxo-5beta-androstane-3alpha,4beta-diyl diacetate.

The title compounds, both C23H34O5, are the 5alpha and 5beta configurations of two diacetate epimers. The 5beta-diacetate crystallizes in an hexagonal structure, unusual for steroid molecules. The unit cell has an accessible solvent volume of 358 A(3), responsible for clathrate behaviour. The 5beta-epimer also features some shorter than average bond lengths in the 3alpha,4beta-acetoxy groups. The conformations of the molecules of both epimers are compared with those obtained through ab initio quantum chemistry calculations. Cohesion of the crystals can be attributed to van der Waals and weak molecular C-H...O interactions.

Acetates↗
PubMed2004

6beta-Azido-7alpha-hydroxy-17-oxo-5alpha-androstan-3beta-yl acetate.

In the title compound, C21H31N3O4, a potential inhibitor of aromatase, all rings are fused trans. Rings A, B and C have chair conformations which are slightly flattened. Ring D has a 14alpha-envelope conformation. The steroid nucleus has a small twist, as shown by the C19-C10...C13-C18 torsion angle of 6.6 (2) degrees. Ab initio calculations of the equilibrium geometry of the molecule reproduce this small twist, which appears to be due to the steric effect of the 6beta-azide substituent rather than to packing effects.

Acetates↗
PubMed2003

3 beta,7 alpha,12 alpha-triformyloxy-24-nor-5 beta-chol-22-ene.

The title compound, alternatively called 24-nor-5 beta-chol-22-ene-3 beta,7 alpha,12 alpha-triyl triformate, C(26)H(38)O(6), has a cis junction between two of the six-membered rings. All three of the six-membered rings have chair conformations that are slightly flattened and the five-membered ring has a 13 beta,14 alpha-half-chair conformation. The 3 beta, 7 alpha and 12 alpha ring substituents are axial and the 17 beta group is equatorial. The 3 beta-formyloxy group is involved in one weak intermolecular C-H...O bond, which links the molecules into dimers in a head-to-head fashion.

Journal Article↗
PubMed2003

17-Oxo-5 alpha-androst-6-en-3 beta-yl acetate.

In the title compound, C(21)H(30)O(3), a potential inhibitor of aromatase, all rings are fused trans. Rings A and C have chair conformations which are slightly flattened, whereas the conformation of ring B is close to a half-chair. Ring D has a 14 alpha-envelope conformation. The steroid nucleus has a small twist, as shown by the C19-C10...C13-C18 (steroid numbering) torsion angle of -6.9 (3) degrees. Ab initio calculations of the equilibrium geometry of the molecule reproduce this small twist, which appears to be due to the conformation of ring B rather than to packing effects.

Journal Article↗
PubMed2002

Glycinium trifluoroacetate.

In the title compound, C(2)H(6)NO(2)(+).C(2)F(3)O(2)(-), the main N-C-COOH skeleton of the glycinium cation is almost perfectly planar. The trifluoroacetate anion has a staggered conformation with typical bond distances and angles. The CF(3) group is slightly disordered. The structure is stabilized by an extensive network of strong O-H.O hydrogen bonds and weaker N-H.O bonds.

Journal Article↗
PubMed2002

Triple-q(-->) octupolar ordering in NpO2.

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.

Journal Article↗
PubMed2001

Synthesis, structure, thermal and non-linear optical properties of L-argininium hydrogen selenite.

L-Argininium hydrogen selenite (C(6)H(15)N(4)O(2)HSeO(3)) is a new semiorganic compound of the hydrogen selenite family with non-linear optical properties. The crystal lattice is monoclinic with unit-cell parameters a = 22.493 (5), b = 5.1624 (13), c = 9.730 (4) A, beta = 95.68 (3) degrees, V = 1124.3 (6) A(3), Z = 4, space group C2. Second-harmonic generation measurements performed on powder samples, using a Q-switched Nd:Yag laser (lambda = 1064 nm), showed the second-harmonic power to be about twice that of urea. Differential scanning calorimetry measurements revealed the existence of a phase transition with onset at 289 K.

Journal Article↗
PubMed2001

Strong hydrogen-bonded amino acid dimers in L-alanine alaninium nitrate.

The title compound, C3H7NO2*C3H8NO2+*NO3-, contains L-alanine-alaninium dimers bonded via the carboxyl groups by a strong asymmetric hydrogen bond with an O...O distance of 2.4547 (19) A. The neutral alanine molecule exists as a zwitterion, where the carboxyl group is dissociated and the amino group is protonated. The alaninium cation has both groups in their acidic form. The alanine molecule and the alaninium cation differ only slightly in their conformation, having an N-C(alpha)-C=O torsion angle close to -25 degrees. The dimers and the nitrate anion are joined through a three-dimensional hydrogen-bond network, in which the full hydrogen-bonding capabilities of the amino groups of the two alanine moieties are realised.

Alanine↗
PubMed2001

Betainium trifluoroacetate.

The title compound, C5H12NO2+*C2F3O2- or BET+*CF3COO- [BET is trimethylglycine (betaine); IUPAC: 1-carboxy-N,N,N-trimethylmethanaminium inner salt], contains pairs of betainium and trifluoroacetate ions forming a dimer bridged by a strong hydrogen bond between the carboxyl and carboxylate groups of the two ions. The molecular symmetry of the cation is close to Cs, with protonation occurring at the carboxy O atom positioned anti to the N atom. The trifluoroacetate anions are disordered over two positions. In one, the conformation of the CF3 group is staggered with respect to the carboxylate group, in the other, it is close to an eclipsed conformation. The sole hydrogen bond present in the structure is the strong O-H...O bond between the anion and the cation.

Acetates↗
PubMed2001

16alpha,17alpha-Epoxy-20-oxopregn-5-en-3beta-yl acetate.

The title compound, C23H32O4, has a 3beta configuration, with the epoxy O atom at 16alpha,17alpha. Rings A and C have slightly distorted chair conformations. Because of the presence of the C5=C6 double bond, ring B assumes an 8beta,9alpha-half-chair conformation slightly distorted towards an 8beta-sofa. Ring D has a conformation close to a 14alpha-envelope. The acetoxy and acetyl substituents are twisted with respect to the average molecular plane of the steroid. The conformation of the molecule is compared with that given by a quantum chemistry calculation using the RHF-AM1 (RHF = Roothaan Hartree-Fock) Hamiltonian model. Cohesion of the crystal can be attributed to van der Waals interactions and weak intermolecular C-H.O interactions, which link the molecules head-to-tail along [101].

Crystallography, X-Ray↗
PubMed2001

An oxo-centred trinuclear FeIII complex: triaquahexakis(mu2-betaine-O:O')-mu3-oxo-triiron(III) bis(tetrachloromanganate) trichloride hexahydrate.

The title compound, [Fe(3)(C(5)H(11)NO(2))(6)O(H(2)O)(3)](MnCl(4))(2)Cl(3).6H(2)O, contains a triiron core linked by a mu(3)-bridging oxide ion. Each of the iron(III) ions has a distorted octahedral environment, being coordinated, in addition to the oxide ion, by four neutral betaine molecules and one water molecule. The N-alkylated alpha-amino acid betaine is present in the dipolar zwitterionic form and chelates pairs of Fe atoms at the vertices of the triangular [Fe(3)O](7+) ionic core. The Fe complex has a crystallographically imposed D3 symmetry. The water molecules fully exhaust their potential as hydrogen donors, forming a two-dimensional hydrogen-bond network in planes parallel to (001).

Journal Article↗
PubMed2001

Dichlorobis(glycocyamine-O)copper(II).

The title compound, [CuCl2(C3H7N3O2)2], is a new copper(II) complex with glycocyamine [(diaminomethyleneamino)acetic acid], the first complex ever reported with this organic molecule. It is composed of discrete centrosymmetric coordinated CuII monomers, the Cu atoms being located at inversion centers. Each metal ion is square-planar coordinated by two Cl atoms and two glycocyamine O atoms. The coordinating glycocyamine molecule exists as a zwitterion, the H atom from the carboxyl group being transferred to the guanidinium group. A three-dimensional network of hydrogen bonds links the monomers and stabilizes the structure.

Journal Article↗
PubMed2001

Conformational flexibility of tricine as a chelating agent in catena-poly-[[(tricinato)copper(II)]-mu-chloro].

In the polymeric title compound, catena-poly[[(N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycinato-O,N,O')copper(II)]-mu-chloro], [CuCl(C6H12NO5)]n, the Cu2+ ions are chelated by tridentate tricine anions and bridged by chloride ions. The metal ion is five-coordinated, with an approximately square-pyramidal geometry. The [CuCl(C6H12NO5)] units form infinite zigzag chains running parallel to the c axis, with Cu-Cl distances of 2.2209 (14) and 2.7792 (19) A. The tricine anion adopts a conformation intermediate between that found in the crystal structure of the neutral molecule and those reported for the Ni2+ and Zn2+ complexes.

Journal Article↗