The methylenic sequence conformation in a series of isoelectronic molecules

1988 ◽  
Vol 66 (2) ◽  
pp. 341-347 ◽  
Author(s):  
Suzanne Deguire ◽  
François Brisse
Keyword(s):  
Solid State Nmr ◽  
Weighted Least Squares ◽  
Direct Methods ◽  
Aromatic Polyamide ◽  
Amide Group ◽  
Trans Conformation ◽  
A Value ◽  
Isoelectronic Series ◽  
Isoelectronic Molecules ◽  
Related Compounds

The crystal structures of 1,4-dibenzoylbutane, 2DBC, and ethyleneglycol dithiobenzoate, 2DBS, have been solved by direct methods in order to follow the evolution of the geometry and the conformation of the Y—CH2—CH2—Y sequence in the isoelectronic series of C6H5—CO—Y—CH2—CH2—Y—CO—C6H5 where Y = CH2, NH, O, and S. 2DBC is monoclinic, a = 7.750(4), b = 21.287(6), c = 8.905(2) Å, β = 100.55(3)°, Z = 4. 2DBS is also monoclinic, with a = 5.702(3), b = 16.171(15), c = 8.001(4) Å, β = 98.79(4)°, and Z = 2. Both compounds belong to the P21/n space group. The weighted least-squares refinements converged to Rw = 0.049 (2DBC) and 0.043 (2DBS). The conformation of the methylenic sequence is all-trans (2DBC) and g+tg− for 2DBS (t = trans, g = gauche). The ir spectra have absorptions at 1467, 1308, and 931 cm−1 (2DBC) and at 1408, 1239, and 727 cm−1 (2DBS), respectively, characteristic of the trans conformation of the Y—CH2—CH2—Y sequence. The 13C CP/MAS solid-state nmr spectra of these and other related compounds have been recorded. It is, in turn, possible to propose that the τ torsion angle between the amide group and the methylenic sequence, has a value of 85° in the aromatic polyamide, Nylon 2T.

Crystal and molecular structure of B,B-bis(p-tolyl)boroxazolidine and the orthorhombic form of B,B-diphenylboroxazolidine

1976 ◽  
Vol 54 (20) ◽  
pp. 3130-3141 ◽  
Author(s):  
Steven J. Rettig ◽  
James Trotter
Keyword(s):  
Molecular Structure ◽  
Statistical Analysis ◽  
Hydrogen Bonds ◽  
Crystal And Molecular Structure ◽  
Direct Methods ◽  
Length Variation ◽  
Full Matrix ◽  
Space Group ◽  
Phenyl Rings ◽  
Related Compounds

Crystals of B,B-bis(p-tolyl)boroxazolidine, 1c, are trigonal, a = 25.1028(9), c = 12.4184(7) Å, Z = 18, space group [Formula: see text]. And crystals of B,B-diphenylboroxazolidine, 1a, are orthorhombic, a = 17.6420(4), b = 14.2527(3), c = 10.205(1) Å, Z = 8, space group Pbca. Both structures were solved by direct methods and were refined by full-matrix least-squares procedures to final R values of 0.057 and 0.040 for 2230 and 1828 reflections with I ≥ 3σ(I) respectively. Both molecules have structures similar to related compounds and feature intermolecular N—H … O hydrogen bonds (N … O = 2.982(2) for 1c and 2.896(2) Å for 1a). Bond lengths are: for 1c; O—C, 1.413(3), O—B, 1.478(3), N—C, 1.488(3), N—B, 1.657(3), C(sp3)—C(sp3), 1.501(4), B—C, 1.616(3) and 1.623(3), mean C—C(ar), 1.395, N—H, 0.93(2) and 0.94(2), mean C(sp3)—H, 1.00, and mean C(ar)—H, 1.00 Å; for 1a; O—C, 1.409(2), O—B, 1.476(2), N—C, 1.489(2), N—B, 1.655(2), C(sp3)—C(sp3), 1.507(3), B—C, 1.613(2) and 1.620(2), mean C—C(ar), 1.391, N—H, 0.93(2) and 0.92(2), mean C(sp3)—H, 1.00, and mean C(ar)—H, 0.98 Å. A statistical analysis of the phenyl C—C distances in compounds 1a, 1b, and 1c has provided an example of statistically significant substituent-induced bond length variation in the phenyl rings.

scholarly journals The compressibility of aqueous solutions II

1934 ◽  
Vol 146 (858) ◽  
pp. 640-650 ◽  
Keyword(s):  
Pressure Range ◽  
Direct Method ◽  
Direct Methods ◽  
Cane Sugar ◽  
Excess Pressure ◽  
Subsequent Work ◽  
Molecular Weights ◽  
In Series ◽  
Related Compounds ◽  
A Direct Method

In an earlier communication, Perman and Urry described the measurement, by a direct method, of the compressibility coefficients of aqueous solution of urea, cane sugar, potassium chloride, and calcium chloride, over a range temperatures and concentrations. Their work was over the pressure range 0-200 atmospheres excess pressure. They were able to apply their together with other data obtained by their co-workers, to an extension Porter's theory of compressible solutions, and thus obtained values for osmotic pressures of those solutions which agreed very well with the obtained by more direct methods. The present work is a continuation of this and subsequent work (unpublished) by these authors. It was intended, especially, to investigate the effect of nature of the solute molecule upon the compressibility, and, for this reason the choice of solutes was particularly important. A number of series chemically related compounds were used where only one part of the mole varied in a progressive manner from substance to substance. In series there was no chemical relationship, but the members had the empirical formula and their molecular weights were therefore simple multi of each other.

Selection of weights for a weighted regression of tree volume

1986 ◽  
Vol 16 (3) ◽  
pp. 671-673 ◽  
Author(s):  
C. H. Meng ◽  
W. Y. Tsai
Keyword(s):  
Confidence Interval ◽  
Likelihood Function ◽  
Weighted Least Squares ◽  
Tree Height ◽  
Regression Equations ◽  
Tree Diameter ◽  
A Value ◽  
Log Likelihood ◽  
Homogeneity Of Variance ◽  
Selection Of

The construction of regression equations for predicting tree volumes requires the assumption of homogeneity of variance that can be achieved by the method of weighted least squares. Some of the weights have the form (1/DλH)2 or (1/Dλ)2 (where D represents tree diameter at breast height and H represents tree height). Traditionally, λ has been assigned a value of 2. This paper suggests a method to estimate the exponent λ. This is accomplished by finding a maximum log likelihood function for a transformed tree volume regression equation. A proper value of λ is chosen within the confidence interval of λ. The confidence interval is established from the maximum log likelihood function.

Crystal and Molecular Structure of 2,5-Bis(2′-hydroxyethylamino)-1,4-benzoquinone

1975 ◽  
Vol 53 (5) ◽  
pp. 777-783 ◽  
Author(s):  
Steven J. Rettig ◽  
James Trotter
Keyword(s):  
Molecular Structure ◽  
Hydrogen Bonds ◽  
Least Squares ◽  
Crystal And Molecular Structure ◽  
Direct Methods ◽  
Full Matrix ◽  
Space Group ◽  
Compound C ◽  
Related Compounds ◽  
Centrosymmetric Molecule

Crystals of 2,5-bis(2′-hydroxyethylamino)-1,4-benzoquinone are monoclinic, a = 5.020(1), b = 19.238(3), c = 5.214(1) Å, β = 96.15(3)°, Z = 2, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to a final R of 0.045 for 646 reflections with I ≥ 3σ(I). The benzoquinone ring in the centrosymmetric molecule is slightly, but significantly, nonplanar. Bond distances in the compound (C—C, 1.247 (2) and 1.410 (3), C—N, 1.332 (3) and 1.457 (3), C—C, 1.384–1.514 (3), N—H, 0.89 (3), O—H, 1.07 (5), and C—H, 0.95–1.09 (3) Å) are similar to those in related compounds. The structure features an extensive network of N—H … O (N … O = 2.639 (3) and 3.033 (2) Å) and O—H … O(O … O = 2.757 (3) Å ) hydrogen bonds.

COVMAP: a new algorithm for structure model optimization in theEXPOpackage

2012 ◽  
Vol 45 (4) ◽  
pp. 789-797 ◽  
Author(s):  
Angela Altomare ◽  
Corrado Cuocci ◽  
Carmelo Giacovazzo ◽  
Anna Moliterni ◽  
Rosanna Rizzi
Keyword(s):  
Electron Density ◽  
Weighted Least Squares ◽  
Direct Methods ◽  
Structure Model ◽  
Model Optimization ◽  
Full Structure ◽  
Density Map ◽  
Starting Model ◽  
Electron Density Map ◽  
Correct Structure

A new procedure (COVMAP) has been developed with the aim of recovering the full structure from very poor models, such as those provided by direct methods in unfavorable conditions. The procedure is based on the concept of covariance between points of an electron density map, mathematically set out by the authors in a recent paper:i.e.the density at one point depends on the density at another point of the map if their covariance is not vanishing. This concept suggested a procedure of electron density modification that uses pairs of model peaks to restrict the region where the density modification should be applied. Such modified densities lead to additional peaks, which in turn are submitted to two other important phasing tools present inEXPO2011, the resolution bias minimization and weighted least-squares procedures, which relocate, refine or reject these peaks. The procedure is cyclic and often leads to the correct structure even if the starting model is very poor.

gem-Diol and Hemiacetal Forms in Formylpyridine and Vitamin-B6-Related Compounds: Solid-State NMR and Single-Crystal X-ray Diffraction Studies

2016 ◽  
Vol 120 (39) ◽  
pp. 7778-7785 ◽  
Author(s):  
Ayelén Florencia Crespi ◽  
Daniel Vega ◽  
Ana Karina Chattah ◽  
Gustavo Alberto Monti ◽  
Graciela Yolanda Buldain ◽  
...  
Keyword(s):  
Solid State ◽  
Single Crystal ◽  
Solid State Nmr ◽  
Vitamin B6 ◽  
X Ray Diffraction ◽  
X Ray ◽  
Related Compounds

Ferguson's Principle and the Prediction of Fatal Drug Levels in Blood

1985 ◽  
Vol 4 (3) ◽  
pp. 273-278 ◽  
Author(s):  
L.A. King
Keyword(s):  
Drug Toxicity ◽  
Quantitative Structure ◽  
Drug Levels ◽  
A Value ◽  
Structure Activity ◽  
Structure Activity Relations ◽  
The Mean ◽  
Broad Framework ◽  
Aqueous Solubilities ◽  
Related Compounds

1 The median fatal concentrations (EC50) of various types of drugs are shown to be determined solely by their aqueous solubilities (S). Among the test substances, EC50 values show a range of 2000-fold, whereas the ratio EC 50/S is essentially constant. This behaviour is typical of 'Ferguson substances', and enables the fatal toxicity of other (depressant) drugs and related compounds to be calculated. 2 The mean relative saturation (EC50/S) is 0.006, a value which shows that humans are more sensitive to narcotic agents than are the lower animals. 3 The Ferguson principle provides a broad framework for quantitative structure-activity relations in fatal drug toxicity. The empirical Hansch equation can be derived from the same thermodynamic concept of free energy which underlies the Ferguson principle.

On the structure of poly(tetramethylene terephthalate). Structural, infrared, and Raman studies of three tetramethylene glycol dibenzoate derivatives, models for poly(tetramethylene terephthalate)

1985 ◽  
Vol 63 (11) ◽  
pp. 3079-3088 ◽  
Author(s):  
Allan Palmer ◽  
Suzie Poulin-Dandurand ◽  
François Brisse
Keyword(s):  
Raman Spectra ◽  
Direct Methods ◽  
Unit Cell ◽  
Model Compounds ◽  
Trans Conformation ◽  
Space Group ◽  
Cell Dimensions ◽  
Unit Cell Dimensions ◽  
Triclinic Unit Cell ◽  
Ir And Raman

The crystal structures of tetramethylene glycol dibenzoate (1), tetramethylene glycol di-pora-chlorobenzoate (2), and tetramethylene glycol di-para-nitrobenzoate (3) have been solved by direct methods in order to establish the possible conformations of the methylenic sequence in poly(tetramethylene terephthalate), poly(4GT). Compound 1 has a triclinic unit cell of dimensions a = 7.870(3), b = 8.574(3), c = 12.993(4) Å α = 83.76(3), β = 89.92(3), γ = 64.68(3)°; Z = 2, space group [Formula: see text]. For 2 the unit cell is also triclinic and has dimensions a = 5.916(3), b = 7.599(2), c = 10.404(2) Å; α = 67.81(2), β = 77.47(2), γ = 81.63(3)°; Z = 1, space group [Formula: see text]. The unit cell dimensions of 3 are a = 6.086(1), b = 11.475(3), c = 13.162(3) Å and β = 101.93(2)°, Z = 2. The space group is P21/c. The structures were solved by direct methods using 2682, 969, and 781 observed reflections for 1, 2, and 3, respectively. The refinements were concluded when R reached the values of 0.042, 0.046, and 0.045, respectively. The conformation of the methylenic sequence varies depending on the substituent on the benzoate group. The conformation observed in 1 is tg−t t t, it is t t t t t for 2, and tg−tgt for 3 (t = trans, g = gauche). Although the equivalent fiber repeat of 3 coincides with the observed fiber repeat for α-poly(4GT), this does not imply that 3 and α-poly(4GT) have the same conformation. Compound 2 allows for the modelling of the β-form of poly(4GT). However, this does not lead to a clear choice between the two structures reported for this form. The two distinct conformations proposed for the β-form of poly(4GT) are distinguished on the basis of their ir and Raman spectra. The comparison of the spectra of the model compounds reveals that, in ir, for an all-trans conformation there is no absorption band at 1383 cm−1 but one is present at 1395 cm−1. This situation that is observed in the ir spectra of β-poly(4GT) led us to propose that this form has the all-trans conformation as proposed by Hall and Pass. This choice is further supported by the presence of three bands at 1047, 1347, and 1405 cm−1 in the Raman spectra of β-poly(4GT). These three bands are only observed in the model compounds having the all-trans conformation.

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