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Papers by Alexander Casallas Vega

Agroindustrial Science, 2018

Caracterización mecánica, físico-química de fibra de Sansevieria trifasciata, como posible refuer... more Caracterización mecánica, físico-química de fibra de Sansevieria trifasciata, como posible refuerzo en un compuesto de matriz poliéster Mechanical, physical-chemical characterization of Sansevieria trifasciata fiber, as possible reinforcement in a polyester matrix compound

The Catholic Church challenges the signs of the times, commits itself to social justice, peace an... more The Catholic Church challenges the signs of the times, commits itself to social justice, peace and preferential options for the poor, based on faith and the assumption of Christian values in a secularized world. This work reflects the Social Doctrine of the Church and its contributions to the understanding of contemporary problems, exposed in the Covid-19 pandemic. From the enlightened and Marxist philosophy of Habermas and the liberalism of Rawls, through the peace agreements of the government of President Santos (Colombia) and the pastoral action of Monsignor Romero (El Salvador).

Introduction: technological and scientific advances have allowed hemophilia to become a chronic d... more Introduction: technological and scientific advances have allowed hemophilia to become a chronic disease, though it still requires treatment and interventions from a multidisciplinary group. The complexity associated with the treatment of children with hemophilia demands the measurement of variables to establish the effects of individual interventions, which in turn, requires the development of instruments that account for the particularities of the disease. Objective: to develop a cross-cultural adaptation and to conduct a psychometric validation of the Canadian Haemophilia Outcomes Kids Life Assessment Tool (CHO-KLAT), to be applied in the Colombian population. Design: Validation of instruments and tests. Population: Children with ages between 8 to 18 years, with a diagnosis of hemophilia A and B; moderate and severe. Ethical aspects: the research involved only minimal risk. Each subject passed through an informed consent and assent process. Results: We developed a Colombian versio...

Journal of Magnetic Resonance, Series A, 1994

Anales de la Facultad de Medicina, 2015

Declaración de conflicto de intereses: No existen relaciones financieras, laborales o de otra índ... more Declaración de conflicto de intereses: No existen relaciones financieras, laborales o de otra índole que puedan constituirse como conflicto de interés respecto del presente trabajo.

The Journal of Physical Chemistry, 1990

Deuterium NMR is used to study the sorption complexes formed by (methyl-deuterated) mono-, di-, a... more Deuterium NMR is used to study the sorption complexes formed by (methyl-deuterated) mono-, di-, and trimethylamine (MMA-dâ, DMA-dâ, and TMA-dâ) in the acid forms of the zeolites HZK-5 and HY, as well as in dehydroxylated HY and DHY, obtained by high-temperature calcination of HY. The measurements were made in the range -140 to 160°C on samples loaded up to twice

The Journal of Physical Chemistry, 1996

Analytical expressions are derived for a description of random Loewenstein Si/Al ordering in the ... more Analytical expressions are derived for a description of random Loewenstein Si/Al ordering in the faujasite zeolite framework. These formulas served as a random distribution reference in the analysis by Melchior et al. of 29 Si NMR spectra of faujasites for the determination of Si/Al ordering in framework building blocks. The equations are based on a model where correlations between the Al occupancies of T-sites belonging to different four-rings are ignored. This simple approach gives reliable results for 29 Si peak intensities when the Si/Al ratio is larger than 1.5.

The Journal of Physical Chemistry, 1987

A multinuclear NMR study is performed of the sorption of water and methanol onto zeolite H-rho. T... more A multinuclear NMR study is performed of the sorption of water and methanol onto zeolite H-rho. The effect of sorption on the MAS NMR of the framework nuclei is reflected in the progressive increase of the 27Al signal intensity (which is negligible in dry H-rho) and by the gradual improvement in the resolution of the 29Si spectrum (which is very poor in dry H-rho). The limiting spectra of these nuclei are reached at a s0rbate:framework-A1 ratio of 2:l. Proton MAS and 2H quadrupole-echo spectra are used to characterize the species formed upon sorbate addition. Depending on the loading level a variety of species are identified with dynamic features ranging from essentially solid (at low loading) to highly fluid (at high loading). Similar NMR results on the framework nuclei 27Al and 29Si are also reported for zeolite H-Y.

The Journal of Physical Chemistry, 1987

Proton spin counting NMR is used to follow the vacuum calcination of the zeolite NH4-rho. Based o... more Proton spin counting NMR is used to follow the vacuum calcination of the zeolite NH4-rho. Based on these results a two-step vacuum-calcination procedure for the preparation of H-rho is proposed. It consists of heating for 20 h at 80 O C followed by 7 h at 450 OC. A multinuclear NMR study is then performed in order to characterize the anhydrous and hydrated forms of NH4-and H-rho. The techniques include 27Al, 29Si, and 'H magic-angle spinning (MAS), 'H and 27Al spin counting, quadrupole-echo deuterium NMR of deuterated samples, as well as 'H-z9Si cross polarization. The results were interpreted paying particular attention to proton chemical shifts, proton MAS side band intensities, quantitation of "NMR-visible" Al, 29Si peak intensities and resolution, 'H-z9Si cross-polarization dynamics, and deuterium line shapes. Hydrated NH4-rho is characterized by high mobility of the NH4+ and HzO species and by high framework symmetry and uniformity. In anhydrous NH4-rho the NH4+ ions are localized at the A1 sites, thus slightly distorting the tetrahedral A1 symmetry and introducing dispersion between the Si sites. The NH4+ ions are distorted from tetrahedral symmetry and rotate rapidly. Anhydrous H-rho has-11 hydrogen atoms/uc in Bronsted acid sites, some rigid, some undergoing fast large amplitude librations, but no translational diffusion between different sites is observed. A small fraction of the hydrogens are in terminal hydroxyl groups. All A1 sites are severely distorted so that the 27Al signal is unobservable. Hydration of H-rho restores the framework symmetry and the proton mobility. The 29Si chemical shifts indicate pronounced structural changes between the various forms of zeolite rho. The calcination profile of NH4-Y was also followed by the proton spin counting method. The results are compared with those of NH4-rho.

Encyclopedia of Magnetic Resonance, 2007

Encyclopedia of Magnetic Resonance, 2007

... technique (SLF). The idea was to use the axis frame of the chemical shift tensor to determine... more ... technique (SLF). The idea was to use the axis frame of the chemical shift tensor to determine experimentally the directions of internuclear vectors. It was all part of Bob's vision. He vividly ... AJ Vega. b 1938. BS (kandidaat), 1961, MS ...

The Journal of Physical Chemistry A, 2008

The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the el... more The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the electronic environment through the dependence of the chemical shift on structure. We have investigated the relation of the 207 Pb NMR isotropic chemical shift, span, and skew of a series of solid Pb(II) compounds to lattice parameters. Complementary relativistic spin-orbit density functional calculations on clusters such as PbI 6 4that model the local environment in the dihalides show a dependence of NMR properties on the local structure in good agreement with experimental results.

Review of Scientific Instruments, 1977

A single coil (solenoid), nuclear magnetic double resonance sample probe, suited for a wide varie... more A single coil (solenoid), nuclear magnetic double resonance sample probe, suited for a wide variety of solid state studies , is described. It was designed to be used in double resonance experiments where it is necessary to generate intense rf magnetic fields (rotating components with amplitudes " .5O gauss) at two widel y spaced frequencies and to simultaneously detect microvolt-level signals. Designs for operation over the 12-270 MHz frequency range are discussed .

Magnetic Resonance in Medicine, 2008

Macromolecules, 1979

... (9) VJ McBrierty, DW McCall, D. C. Douglass, and D. R. Falcone, Macromolecules, 4, 584 (1971)... more ... (9) VJ McBrierty, DW McCall, D. C. Douglass, and D. R. Falcone, Macromolecules, 4, 584 (1971). (10) A. J. Vega and RW Vaughan, J. Chem. ... Sung Gun Chu and Petr Munk: Thermodynamic Properties of Polystyrene in Mixed Solvent,s Studied by Sedimentation Equilibrium. ...

Journal of the American Chemical Society, 1988

or aromatic carbons are observed), two sections of the E.COSY spectrum of 1, and table of general... more or aromatic carbons are observed), two sections of the E.COSY spectrum of 1, and table of general displacement parameter expressions for the orthorhombic modification and the monoclinic modifications of 7 (8 pages). Ordering information is given on any current masthead page.

Journal of the American Chemical Society, 2007

We report the first results establishing REAPDOR experiments for distance measurements between a ... more We report the first results establishing REAPDOR experiments for distance measurements between a spin-1/2 (31 P) and spin-7/2 (51 V) pair in a series of vanadium-substituted polyoxoanionic solids from the Keggin and Wells-Dawson families. We have quantitatively measured 31 P-51 V distances in mono-vanadium substituted K 4 PVW 11 O 40 , 1-K 7 P 2 VW 17 O 62 , and 4-K 7 P 2 VW 17 O 62. Numerical simulations of the experimental data yield very good agreement with the averaged P-W/P-V distances determined from the X-ray diffraction measurements in the same or related compounds. REAPDOR is therefore a very sensitive P-V distance probe anticipated to be especially useful in the absence of long-range order. Our results suggest that REAPDOR spectroscopy could be broadly applicable for interatomic distance measurements in other spin-7/2spin-1/2 nuclear pairs.

Journal of Magnetic Resonance, 2008

A method for acquiring triple quantum filtered (TQF) 23 Na NMR images is proposed that takes adva... more A method for acquiring triple quantum filtered (TQF) 23 Na NMR images is proposed that takes advantage of the differences in transverse relaxation rates of sodium to achieve positive intensity, PI, NMR signal. This PITQF imaging sequence has been used to obtain spatially resolved onedimensional images as a function of the TQF creation time, τ, for two human spinal disc samples. From the images the different parts of the tissue, nucleus pulposus and annulus fibrosus, can be clearly distinguished based on their signal intensity and creation time profiles. These results establish the feasibility of 23 Na TQF imaging and demonstrate that this method should be applicable for studying human disc tissues as well as spinal disc degeneration. Keywords triple-quantum-filtered; 23 Na NMR; spineal disc; imaging; PITQF Degenerative disc disease of the spine causing lower back pain is one of the leading musculoskeletal disorders. According to various estimates, 15%-20% of the population experiences lower back pain annually, and 60% or moreover a lifetime.[1] While T 2 MRI is the most revealing non-invasive diagnostic technique for identifying changes in the disc anatomy, it has not been able to delineate symptomatic discs.[2; 3 ] Other non-invasive methods are required in order to accurately diagnose disc degeneration so that the best course of treatment can be assigned without resorting to invasive techniques such as discography.[4; 5 ] Numerous researchers have demonstrated that multiple-quantum-filtered, MQF, techniques can be used to study tissue samples.[5-11 ] Double-quantum-filtered, DQF, measurements of 1 H, 2 H, and 23 Na are the most common in biological tissues.[12; 13 ] 1 H DQF relies on the dipolar coupling between protons to build-up double quantum coherence signal during the experimental creation time, τ.[14] Given the favorable NMR properties of 1 H, this may appear to be the most likely nucleus for study; however, it has been shown in our previous work that in some biological samples, such as spinal disc tissue, the residual dipolar couplings are too small to result in significant DQF signal.[8] Because of this we originally turned to 2 H DQF

The Journal of Chemical Physics, 1978

A master equation for nuclear magnetic relaxation under conditions of periodic and cyclic rf irra... more A master equation for nuclear magnetic relaxation under conditions of periodic and cyclic rf irradiation is derived based on the stochastic Liouville equation. Conditions for the validity of the equation, involving both use of the motional narrowing approximation and the Magnus expansion, are discussed with particular attention given to the simultaneous presence of fluctuating and nonfluctuating interactions. The expressions derived are applied to several irradiation schemes: the Carr–Purcell sequence, where the echo decay time under translational diffusion is calculated; the cw and pulsed versions of spin locking in solids, with special emphasis on the origin and role of spin diffusion and on the exact relationship between the second moment and the prefactor in the T1ρ expression; and, finally, the four‐ and eight‐pulse sequences used for suppression of homonuclear dipolar interactions, where it is shown that the x, y, and z axes of the interaction frame are ’’principal axes’’ of relaxation. Physical int...

Agroindustrial Science, 2018

Caracterización mecánica, físico-química de fibra de Sansevieria trifasciata, como posible refuer... more Caracterización mecánica, físico-química de fibra de Sansevieria trifasciata, como posible refuerzo en un compuesto de matriz poliéster Mechanical, physical-chemical characterization of Sansevieria trifasciata fiber, as possible reinforcement in a polyester matrix compound

The Catholic Church challenges the signs of the times, commits itself to social justice, peace an... more The Catholic Church challenges the signs of the times, commits itself to social justice, peace and preferential options for the poor, based on faith and the assumption of Christian values in a secularized world. This work reflects the Social Doctrine of the Church and its contributions to the understanding of contemporary problems, exposed in the Covid-19 pandemic. From the enlightened and Marxist philosophy of Habermas and the liberalism of Rawls, through the peace agreements of the government of President Santos (Colombia) and the pastoral action of Monsignor Romero (El Salvador).

Introduction: technological and scientific advances have allowed hemophilia to become a chronic d... more Introduction: technological and scientific advances have allowed hemophilia to become a chronic disease, though it still requires treatment and interventions from a multidisciplinary group. The complexity associated with the treatment of children with hemophilia demands the measurement of variables to establish the effects of individual interventions, which in turn, requires the development of instruments that account for the particularities of the disease. Objective: to develop a cross-cultural adaptation and to conduct a psychometric validation of the Canadian Haemophilia Outcomes Kids Life Assessment Tool (CHO-KLAT), to be applied in the Colombian population. Design: Validation of instruments and tests. Population: Children with ages between 8 to 18 years, with a diagnosis of hemophilia A and B; moderate and severe. Ethical aspects: the research involved only minimal risk. Each subject passed through an informed consent and assent process. Results: We developed a Colombian versio...

Journal of Magnetic Resonance, Series A, 1994

Anales de la Facultad de Medicina, 2015

Declaración de conflicto de intereses: No existen relaciones financieras, laborales o de otra índ... more Declaración de conflicto de intereses: No existen relaciones financieras, laborales o de otra índole que puedan constituirse como conflicto de interés respecto del presente trabajo.

The Journal of Physical Chemistry, 1990

Deuterium NMR is used to study the sorption complexes formed by (methyl-deuterated) mono-, di-, a... more Deuterium NMR is used to study the sorption complexes formed by (methyl-deuterated) mono-, di-, and trimethylamine (MMA-dâ, DMA-dâ, and TMA-dâ) in the acid forms of the zeolites HZK-5 and HY, as well as in dehydroxylated HY and DHY, obtained by high-temperature calcination of HY. The measurements were made in the range -140 to 160°C on samples loaded up to twice

The Journal of Physical Chemistry, 1996

Analytical expressions are derived for a description of random Loewenstein Si/Al ordering in the ... more Analytical expressions are derived for a description of random Loewenstein Si/Al ordering in the faujasite zeolite framework. These formulas served as a random distribution reference in the analysis by Melchior et al. of 29 Si NMR spectra of faujasites for the determination of Si/Al ordering in framework building blocks. The equations are based on a model where correlations between the Al occupancies of T-sites belonging to different four-rings are ignored. This simple approach gives reliable results for 29 Si peak intensities when the Si/Al ratio is larger than 1.5.

The Journal of Physical Chemistry, 1987

A multinuclear NMR study is performed of the sorption of water and methanol onto zeolite H-rho. T... more A multinuclear NMR study is performed of the sorption of water and methanol onto zeolite H-rho. The effect of sorption on the MAS NMR of the framework nuclei is reflected in the progressive increase of the 27Al signal intensity (which is negligible in dry H-rho) and by the gradual improvement in the resolution of the 29Si spectrum (which is very poor in dry H-rho). The limiting spectra of these nuclei are reached at a s0rbate:framework-A1 ratio of 2:l. Proton MAS and 2H quadrupole-echo spectra are used to characterize the species formed upon sorbate addition. Depending on the loading level a variety of species are identified with dynamic features ranging from essentially solid (at low loading) to highly fluid (at high loading). Similar NMR results on the framework nuclei 27Al and 29Si are also reported for zeolite H-Y.

The Journal of Physical Chemistry, 1987

Proton spin counting NMR is used to follow the vacuum calcination of the zeolite NH4-rho. Based o... more Proton spin counting NMR is used to follow the vacuum calcination of the zeolite NH4-rho. Based on these results a two-step vacuum-calcination procedure for the preparation of H-rho is proposed. It consists of heating for 20 h at 80 O C followed by 7 h at 450 OC. A multinuclear NMR study is then performed in order to characterize the anhydrous and hydrated forms of NH4-and H-rho. The techniques include 27Al, 29Si, and 'H magic-angle spinning (MAS), 'H and 27Al spin counting, quadrupole-echo deuterium NMR of deuterated samples, as well as 'H-z9Si cross polarization. The results were interpreted paying particular attention to proton chemical shifts, proton MAS side band intensities, quantitation of "NMR-visible" Al, 29Si peak intensities and resolution, 'H-z9Si cross-polarization dynamics, and deuterium line shapes. Hydrated NH4-rho is characterized by high mobility of the NH4+ and HzO species and by high framework symmetry and uniformity. In anhydrous NH4-rho the NH4+ ions are localized at the A1 sites, thus slightly distorting the tetrahedral A1 symmetry and introducing dispersion between the Si sites. The NH4+ ions are distorted from tetrahedral symmetry and rotate rapidly. Anhydrous H-rho has-11 hydrogen atoms/uc in Bronsted acid sites, some rigid, some undergoing fast large amplitude librations, but no translational diffusion between different sites is observed. A small fraction of the hydrogens are in terminal hydroxyl groups. All A1 sites are severely distorted so that the 27Al signal is unobservable. Hydration of H-rho restores the framework symmetry and the proton mobility. The 29Si chemical shifts indicate pronounced structural changes between the various forms of zeolite rho. The calcination profile of NH4-Y was also followed by the proton spin counting method. The results are compared with those of NH4-rho.

Encyclopedia of Magnetic Resonance, 2007

Encyclopedia of Magnetic Resonance, 2007

... technique (SLF). The idea was to use the axis frame of the chemical shift tensor to determine... more ... technique (SLF). The idea was to use the axis frame of the chemical shift tensor to determine experimentally the directions of internuclear vectors. It was all part of Bob's vision. He vividly ... AJ Vega. b 1938. BS (kandidaat), 1961, MS ...

The Journal of Physical Chemistry A, 2008

The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the el... more The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the electronic environment through the dependence of the chemical shift on structure. We have investigated the relation of the 207 Pb NMR isotropic chemical shift, span, and skew of a series of solid Pb(II) compounds to lattice parameters. Complementary relativistic spin-orbit density functional calculations on clusters such as PbI 6 4that model the local environment in the dihalides show a dependence of NMR properties on the local structure in good agreement with experimental results.

Review of Scientific Instruments, 1977

A single coil (solenoid), nuclear magnetic double resonance sample probe, suited for a wide varie... more A single coil (solenoid), nuclear magnetic double resonance sample probe, suited for a wide variety of solid state studies , is described. It was designed to be used in double resonance experiments where it is necessary to generate intense rf magnetic fields (rotating components with amplitudes " .5O gauss) at two widel y spaced frequencies and to simultaneously detect microvolt-level signals. Designs for operation over the 12-270 MHz frequency range are discussed .

Magnetic Resonance in Medicine, 2008

Macromolecules, 1979

... (9) VJ McBrierty, DW McCall, D. C. Douglass, and D. R. Falcone, Macromolecules, 4, 584 (1971)... more ... (9) VJ McBrierty, DW McCall, D. C. Douglass, and D. R. Falcone, Macromolecules, 4, 584 (1971). (10) A. J. Vega and RW Vaughan, J. Chem. ... Sung Gun Chu and Petr Munk: Thermodynamic Properties of Polystyrene in Mixed Solvent,s Studied by Sedimentation Equilibrium. ...

Journal of the American Chemical Society, 1988

or aromatic carbons are observed), two sections of the E.COSY spectrum of 1, and table of general... more or aromatic carbons are observed), two sections of the E.COSY spectrum of 1, and table of general displacement parameter expressions for the orthorhombic modification and the monoclinic modifications of 7 (8 pages). Ordering information is given on any current masthead page.

Journal of the American Chemical Society, 2007

We report the first results establishing REAPDOR experiments for distance measurements between a ... more We report the first results establishing REAPDOR experiments for distance measurements between a spin-1/2 (31 P) and spin-7/2 (51 V) pair in a series of vanadium-substituted polyoxoanionic solids from the Keggin and Wells-Dawson families. We have quantitatively measured 31 P-51 V distances in mono-vanadium substituted K 4 PVW 11 O 40 , 1-K 7 P 2 VW 17 O 62 , and 4-K 7 P 2 VW 17 O 62. Numerical simulations of the experimental data yield very good agreement with the averaged P-W/P-V distances determined from the X-ray diffraction measurements in the same or related compounds. REAPDOR is therefore a very sensitive P-V distance probe anticipated to be especially useful in the absence of long-range order. Our results suggest that REAPDOR spectroscopy could be broadly applicable for interatomic distance measurements in other spin-7/2spin-1/2 nuclear pairs.

Journal of Magnetic Resonance, 2008

A method for acquiring triple quantum filtered (TQF) 23 Na NMR images is proposed that takes adva... more A method for acquiring triple quantum filtered (TQF) 23 Na NMR images is proposed that takes advantage of the differences in transverse relaxation rates of sodium to achieve positive intensity, PI, NMR signal. This PITQF imaging sequence has been used to obtain spatially resolved onedimensional images as a function of the TQF creation time, τ, for two human spinal disc samples. From the images the different parts of the tissue, nucleus pulposus and annulus fibrosus, can be clearly distinguished based on their signal intensity and creation time profiles. These results establish the feasibility of 23 Na TQF imaging and demonstrate that this method should be applicable for studying human disc tissues as well as spinal disc degeneration. Keywords triple-quantum-filtered; 23 Na NMR; spineal disc; imaging; PITQF Degenerative disc disease of the spine causing lower back pain is one of the leading musculoskeletal disorders. According to various estimates, 15%-20% of the population experiences lower back pain annually, and 60% or moreover a lifetime.[1] While T 2 MRI is the most revealing non-invasive diagnostic technique for identifying changes in the disc anatomy, it has not been able to delineate symptomatic discs.[2; 3 ] Other non-invasive methods are required in order to accurately diagnose disc degeneration so that the best course of treatment can be assigned without resorting to invasive techniques such as discography.[4; 5 ] Numerous researchers have demonstrated that multiple-quantum-filtered, MQF, techniques can be used to study tissue samples.[5-11 ] Double-quantum-filtered, DQF, measurements of 1 H, 2 H, and 23 Na are the most common in biological tissues.[12; 13 ] 1 H DQF relies on the dipolar coupling between protons to build-up double quantum coherence signal during the experimental creation time, τ.[14] Given the favorable NMR properties of 1 H, this may appear to be the most likely nucleus for study; however, it has been shown in our previous work that in some biological samples, such as spinal disc tissue, the residual dipolar couplings are too small to result in significant DQF signal.[8] Because of this we originally turned to 2 H DQF

The Journal of Chemical Physics, 1978

A master equation for nuclear magnetic relaxation under conditions of periodic and cyclic rf irra... more A master equation for nuclear magnetic relaxation under conditions of periodic and cyclic rf irradiation is derived based on the stochastic Liouville equation. Conditions for the validity of the equation, involving both use of the motional narrowing approximation and the Magnus expansion, are discussed with particular attention given to the simultaneous presence of fluctuating and nonfluctuating interactions. The expressions derived are applied to several irradiation schemes: the Carr–Purcell sequence, where the echo decay time under translational diffusion is calculated; the cw and pulsed versions of spin locking in solids, with special emphasis on the origin and role of spin diffusion and on the exact relationship between the second moment and the prefactor in the T1ρ expression; and, finally, the four‐ and eight‐pulse sequences used for suppression of homonuclear dipolar interactions, where it is shown that the x, y, and z axes of the interaction frame are ’’principal axes’’ of relaxation. Physical int...