Juan Murgich | Instituto Venezolano de Investigaciones Cientificas (original) (raw)

Papers by Juan Murgich

Chemischer Informationsdienst, Nov 24, 1981

Petroleum Science and Technology, Jan 31, 2011

The areas of signals present in 12 consecutive frequency bins of the H nuclear magnetic resonance... more The areas of signals present in 12 consecutive frequency bins of the H nuclear magnetic resonance (NMR) spectra of different vacuum residues (VRs) were correlated with the yield of the deasphalting/demetallization process (DEMEX) under fixed and variable conditions (antisolvent/load ratio and rectification temperature). Prediction models for the demetallized oil (DMO) obtained under variable process conditions generated by partial least squares

Chemischer Informationsdienst, Apr 22, 1975

Chemischer Informationsdienst, Jun 5, 1979

Chemischer Informationsdienst, Jul 21, 1981

Journal of Chemical Physics, Nov 15, 1986

An ab inito SCF-MO calculation of the electric field gradient (EFG) using a double zeta basis set... more An ab inito SCF-MO calculation of the electric field gradient (EFG) using a double zeta basis set with polarization is reported for N−3, HN3 NCN3, and H3CN3. A similar calculation using a Dunning set with polarization is also reported for N−3, HN3, and H3CN3. Excellent correlation was found between the calculated and the components obtained from gas phase microwave data. The calculation showed that the terminal N atom had a positive nuclear quadrupole coupling constant and its pincipal z axis was found to be perpendicular to the NNN direction in substituted azides. The changes in the charge distribution of HN3 and NCN3 obtained from the analysis of the atomic multipoles was shown to be in general agreement with the variations found in the N EFG components calculated for these azides. The 14N nuclear quadrupole resonance frequencies of the terminal and the first N atom of the azide group have been measured at 77 K for phenyl, benzyl, triphenylmethane, and tosyl azide. An analysis based on the Townes and Dailey approach of the measured NQR data showed that the terminal N atom have lost more σ than π orbital population. A similar analysis for the first N atom of azides showed strong inductive and resonance interactions with the substituents.

Journal of Physical Chemistry A, Jul 29, 2006

The molecular charge distribution of flucytosine (4-amino-5-fluoro-2-pyrimidone), uracil, 5-fluor... more The molecular charge distribution of flucytosine (4-amino-5-fluoro-2-pyrimidone), uracil, 5-fluorouracil, and thymine was studied by means of density functional theory calculations (DFT). The resulting distributions were analyzed by means of the atoms in molecules (AIM) theory. Bonds were characterized through vectors formed with the charge density value, its Laplacian, and the bond ellipticity calculated at the bond critical point (BCP). Within each set of CdO, C-H, and N-H bonds, these vectors showed little dispersion. CC bonds formed three different subsets, one with a significant degree of double bonding, a second corresponding to single bonds with a finite ellipticity produced by hyperconjugation, and a third one formed by a pure single bond. In N-C bonds, a decrease in bond length (an increase in double bond character) was not reflected as an increase in their ellipticity, as in all CC bonds studied. It was also found that substitution influenced the N-C, CO , and CC bond ellipticity much more than density and its Laplacian at the BCP. The Laplacian of charge density pointed to the existence of both bonding and nonbonding maxima in the valence shell charge concentration of N, O, and F, while only bonding ones were found for the C atoms. The nonbonding maxima related to the sites for electrophilic attack and H bonding in O and N, while sites of nucleophilic attack were suggested by the holes in the valence shell of the C atoms of the carbonyl groups.

Petroleum Science and Technology, Mar 31, 2001

A combination of Molecular Mechanics and Density Functional Theory allowed the calculation of the... more A combination of Molecular Mechanics and Density Functional Theory allowed the calculation of the charge distribution and the adsorption energy on the (001) surface of hematite (α-Fe 2 O 3 ) of a set of twelve molecules that represent fragments of resins and asphaltenes. The ...

Energy & Fuels, May 18, 2002

Problems surrounding molecular aggregation, covalent molecular weight, and their experimental inv... more Problems surrounding molecular aggregation, covalent molecular weight, and their experimental investigation in asphaltene chemistry are reviewed. Chromatographic, fluorescence spectroscopic, and mass spectroscopic (MS) methods for the investigation of these ...

Journal of Molecular Structure, Nov 1, 1985

Cl NQR frequencies for 2:2 salts of trichloroacetic acid (TCA) with 4-methylpyridine-N-oxide and ... more Cl NQR frequencies for 2:2 salts of trichloroacetic acid (TCA) with 4-methylpyridine-N-oxide and 4-methylquinoline-N-oxide were measured at 77 K. Comparison of Cl frequencies for 2:1 and 2:2 salts shows that the crystal field makes a small contribution to the frequency shifts (FS) ...

Carbon, 2002

The handling and properties of heavy oils are becoming an increasingly important problem. Even th... more The handling and properties of heavy oils are becoming an increasingly important problem. Even though petroleum is the widest used source of fuels and it has been studied for decades, its complex nature is still an enigma in several ways. A reasonable approach to a definition of a crude oil is a colloidal fluid formed by several dispersed phases from gases (light hydrocarbons) to solids (heavy paraffins and asphaltenes). Through all these years of research, applications have been found for almost all classes of components in crude oil except for some of the solid phases such as asphaltenes. Very heavy 6 petroleum is a non-newtonian liquid with a viscosity of¯10 Poise, and an average molecular weight of 600 amu. The solids that are toluene soluble but heptane insoluble are called asphaltenes and are the most aromatic fraction with the highest molecular weight of unconverted petroleum. In the present work, we applied high resolution transmission electron microscopy (HREM) and energy dispersive spectrometry (EDS) in the study of asphaltenes. It was found that when the asphaltenes are well separated from the resins the sample consists of a carbon structure containing S, V, Si, related to fullerenic carbon. During observation in the microscope it was possible to see the formation of fullerenes such as onions and C @ C structures. The fact that they decomposed under further irradiation suggests that they are metastable structures. 240 60 Since the heteroatoms are still present they are likely to cause instability to the structure. Not only does our result indicate the possibility of obtaining fullerenes from crude oil but it also suggests the asphaltene molecule, when it is resin free, might be a precursor of fullerenic structures.

Zeitschrift für Naturforschung, Feb 1, 1992

14 N NQR spectra of 1,10-phenanthroline and of dicyano bis(l,10-phenanthroline)iron(lll) were obt... more 14 N NQR spectra of 1,10-phenanthroline and of dicyano bis(l,10-phenanthroline)iron(lll) were obtained at room temperature. An analysis based on the shifts of the NQR frequencies and in the topology of the charge distribution in a model (pyridine) showed that the complex formation produces a strong decrease in the N valence shell nonbonded maximum and an increase in the bonded density along each of the N-H bond directions. The NQR spectrum showed that the complex formation produces only small changes in the charge distribution of the cyanide groups.

Journal of Molecular Structure, Dec 1, 1983

14N NQR function of lines of RbTCNQ and NaTCNQ polycrystalline samples, measured as a temperature... more 14N NQR function of lines of RbTCNQ and NaTCNQ polycrystalline samples, measured as a temperature, show small but sharp discontinuities at the regularalternant phase transition (found respectively at 214 K and 346 K), together with the expected change in spectral multiplicity. No v-lines were detected in NaTCNQ. Coexistence of phases, more marked in NaTCNQ, shows up in NQR data. The use of a pulsed, FT spectrometer yields estimates of T it shows no discontinuity at the phase transition, is aro nd A relaxation time: 1 ms at room temperature for v+ lines, more than 2 order of magnitudes larger for v-lines, increases smoothly on decreasing temperature.

Theoretica chimica acta, Apr 1, 1988

The contributions of each localized orbital of the CN group and of the molecular remainder to the... more The contributions of each localized orbital of the CN group and of the molecular remainder to the N electric field gradient (EFG) were calculated for nitriles with R = H, F, CH3, OH, NH2, CCH, COH, NO, CN and the E and Z forms of iminoacetonitrile. The changes in the EFG ...

Journal of Chemical Physics, Dec 1, 1994

A relationship between some of the critical points of the Laplacian of the charge density of the ... more A relationship between some of the critical points of the Laplacian of the charge density of the N valence shell and the electric field gradient (EFG) present at the nucleus of the three-coordinated N atom was found for the amine group in urea and some of its adducts and salts. The qzz component of the EFG was shown to be determined by two nonbonded charge distribution local maxima present below and above the molecular plane in the N valence shell. The asymmetry parameter of the EFG was found to be related also to this nonbonded charge concentration. The effect of increasing protonation in urea produced a decrease in the N nonbonded charge concentrations, an increase in bonded concentration along the N–H bond directions and complex fluctuations in the bonded concentration along the N–C bond. The combined effect of the additional H bonds involving the amine group of urea and of the O atom protonation on the N valence shell concentration, is also discussed.

Magnetic Resonance in Chemistry, Mar 1, 1985

The 14N NQR frequencies measured at 77 K for the amine group of the 1:1 salts of nitric acid with... more The 14N NQR frequencies measured at 77 K for the amine group of the 1:1 salts of nitric acid with methylurea and 1,3‐ and 1,1‐dimethylurea and the phosphoric acid salt of 1,3‐dimethylurea are reported. The analysis of the NQR data showed that the N loses population from its π orbital. The decrease in π population is partially compensated by an increase in the population of the NC σ bonds. The loss of π population is enhanced when the H atoms of the amine group are substituted by electron‐donating groups such as CH3.

Journal of Chemical Physics, Apr 1, 1981

The 14N NQR frequencies of urea complexes with H2O2 (1:1), NH4Cl (1:1), oxalic (2:1), phosphoric ... more The 14N NQR frequencies of urea complexes with H2O2 (1:1), NH4Cl (1:1), oxalic (2:1), phosphoric (1:1), and nitric acid (1:1) at 77 °K are reported. The analysis of the NQR data indicates that the population of the N nonbonding orbital decreases and that the population of the s N–H and N–C bonds increases as the degree of protonation of the O atom of urea increases. These changes are consistent with a larger weight of structures like C = N+H2 as the protonation increases. The NQR results are in agreement with those obtained from a CNDO/2 calculation for the uronioum ion [Yu. A. Panteleev and A. A. Lipovskii, Zhu. Struk. Khim. 17, 2 (1976)].

Energy & Fuels, Oct 5, 2011

ABSTRACT The volatile aromatic fraction (240 °C and 10–3 Torr pressure) of Athabasca bitumen was ... more ABSTRACT The volatile aromatic fraction (240 °C and 10–3 Torr pressure) of Athabasca bitumen was separated via thin-layer chromatography (TLC) (Ag+/SiO2), followed by alumina column chromatography, into 13 mono-, di-, and triaromatic subfractions, each of which was subjected to field ionization mass spectrometry (FIMS) analyses. Altogether, close to 6000 constituent molecules, ranging in molecular weight from 200 to 800 Da, have been observed, including the partially and fully aromatized, ring-opened, and truncated derivatives of essentially all of the saturated biomarkers detected here or earlier in the saturated fraction of this bitumen. All molecules detected were of aromatic character, with the exception of a complex suite of cyclic terpenoid (and steroid) sulfides, having adsorptive properties similar to those of the triaromatic hydrocarbons. In several cases, the FIMS analyses were supplemented by conventional biomarker analyses and a novel biomarker analysis, employing basic aqueous extraction of the whole oil sands. The high-molecular-weight portion (MW > 482 Da) of each subfraction isolated (which usually does not show up in conventional biomarker analyses) varies in concentration from a few to about 10 wt %. This fraction may arise from processes analogous to the combination reactions occurring in the addition of biomarker molecules to asphaltene by C–C, C–S–C, and C–O–C covalent bond formation. In the present system (with the exception of the cyclic sulfides), only C–C bond additions can be operative.

Journal of Molecular Structure, Aug 1, 1983

Energy & Fuels, Jul 6, 2006

It is shown by sequential reprecipitation that both n-C 5 and n-C 7 Athabasca asphaltene contain ... more It is shown by sequential reprecipitation that both n-C 5 and n-C 7 Athabasca asphaltene contain significant amounts of precipitant solubles, comprising resins and low-molecular-weight asphaltene-like substances in about the same percentage amounts relative to the ...