Computational Modelling of Polyoxometalates: Progress on Accurate NMR Characterization and Reactivity

Aquesta tesi es pot dividir en dues parts diferents: i) RMN i ii) la reactivitat. El factor comú d'ambdues seccions són els polioxometal·lats (POMs). Els POMs són anions moleculars polimetàl·lics formats de l'agregació de blocs MO6 octaèdrics de metalls de transició en els seus estats d'oxidació més alts, i oxigen, que estan connectats per vèrtexs i arestes. La primera part està relacionada amb la caracterització dels POMs. En particular, es basa en el càlcul de paràmetres d'espectroscòpia de ressonància magnètica nuclear (RMN). En primer lloc, s’ha estudiat el nucli 17O. S’ha establert una estratègia basada en DFT per calcular i racionalitzar desplaçaments químics de 17O de polioxoanions amb alta precisió. S’ha demostrat que els càlculs realitzats amb els funcionals de tipus GGA PBE, incloent spin-òrbit i correccions d’escalatge, proporcionen un error mitjà absolut (MAE) <30 ppm, un valor petit tenint en compte el rang dels desplaçaments químics de 17O en els POMs. També s’ha realitzar un estudi similar per a la determinació de desplaçaments químics de 31P en els POMs amb mètodes DFT. S’ha descrit el primer polioxometalat que conté tal · li, el qual s’ha sintetitzat i determinat estructuralment. S’ha realitzar un estudi detallat de 203 / 205Tl de RMN per tal de reproduir els valors experimentals. El δ (205Tl) es poden reproduir molt bé amb metodologia DFT.La segona part inclou l'estudi de la reactivitat dels POMs. Matt et. al. (Energy Environ. Sci. 2013, 6, 1504) va dissenyar un pol·lioxometalat que contenia un fotosensibilitzador de Ir(III), capaç de fer evolucionar de manera eficient H2. El mecanisme de recol·lecció de llum visible, la formació de les espècies amb càrrega separada i la reducció de protons ha estat analitzat amb mètodes DFT i també amb mètodes Time-Dependent (TD-DFT). En l'última part, es presenta un estudi computacional on s’investiga (a) els efectes de diferents lligands d'òxid en {M5O18}6- en vers la reacció de protonolisis del enllaç M’-OR (b) els efectes de diferents metalls M' en la hidròlisi de el pol·lioxometalat [(RO)M'W5O18]n-. Per últim s’ha realitzat un estudi experimental sobre la protonació del dímer, (TBA)6[(μ-O) (TiW5O18) 2].Esta tesis se puede dividir en dos partes diferentes: i) RMN y ii) Reactividad. El factor común de ambas secciones son los polioxometalato (POMs). Los POMs son aniones polimetálicos moleculares formados por la agregación de bloques octaédricos MO6 formados por metales de transición, en sus estados de oxidación más altos, y oxígeno, que están conectados por vértices y bordes. La primera parte está relacionada con la caracterización de los POMs. En particular, se basa en el cálculo de parámetros de espectroscopia de resonancia magnética nuclear (RMN). Primero, nos centramos en el núcleo 17O. Se ha establecido una estrategia basada en metodología DFT para calcular con precisión y racionalizar los desplazamientos químicos de 17O en los POMs. Se ha demostrado que los cálculos realizados con el funcional PBE de tipo GGA, incluyendo correcciones de órbita y estalaje, proporcionan un error absoluto medio (MAE) <30 ppm, un valor pequeño considerando el amplio rango de desplazamientos químicos δ (17O) en POMs. También se ha realizado un estudio similar para la determinación de desplazamientos químicos de 31P en POMs. Se ha descrito el primer polioxometalato que contiene talio, el cual se ha sintetizado y determinado estructuralmente. Se ha realizado un estudio detallado de RMN de 203 / 205Tl para reproducir los valores experimentales. Los δ (205Tl) son reproducidos con metodología DFT.La segunda parte incluye estudios de reactividad de POMs. Matt et. al. (Energy Environ. Sci. 2013, 6, 1504) diseñó un polioxometalato con un foto sensibilizador de Ir (III) capaz de producir H2 de forma eficiente. El mecanismo de la recolección de luz visible, la formación de especies de carga separada y la reducción de protones ha sido analizado por DFT y DFT dependiente del tiempo (TD-DFT). En la última parte, se presenta un estudio computacional para investigar (a) los efectos de diferentes ligandos de {M5O18}6- sobre la protonolisis del enlace M'-OR y (b) los efectos de diferentes metales M' sobre la hidrólisis de [(RO)M'W5O18]n- con diferentes metaltes. Por último, se presenta un estudio experimental sobre la protonación del dímero (TBA)6[(μ-O)(TiW5O18)2].This thesis can be divided in two different parts: i) NMR and ii) Reactivity. The common factor of both sections is Polyoxometalates (POMs). POMs are polymetallic molecular anions formed upon aggregation of octahedral MO6 blocks made of early transition metals (TMs), in their higher oxidation states, and oxygen, that are connected commonly by vertices and edges. The first part is related to the characterization of POMs. In particular, it is based on the calculation of nuclear magnetic resonance (NMR) spectroscopy parameters. First, we focused on 17O nuclei. We have established a DFT-based strategy to accurately compute and rationalize 17O NMR chemical shifts of polyoxoanions. We show that calculations performed with GGA-type PBE functional, including spin-orbit and scaling corrections, provide a mean absolute error value (MAE) <30 ppm, a small value considering the large range of δ(17O) chemical shifts in POMs. A study for the best DFT strategy for the determination of 31P NMR chemical shifts in POMs is also reported. We have described the first thallium containing polyoxometalate, which was synthetized and structurally determined. A detailed study of 203/205Tl NMR has been performed in order to reproduce the experimental values. The δ(205Tl) are really well reproduced with DFT methodology. However, in the spin-spin coupling, there are large differences between calculated and experimental 2J(205Tl-203Tl)

Performance assessment of time–frequency RFI mitigation techniques in microwave radiometry

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Radio–frequency interference (RFI) signals are a well-known threat for microwave radiometry (MWR) applications. In order to alleviate this problem, different approaches for RFI detection and mitigation are currently under development. Since RFI signals are man made, they tend to have their power more concentrated in the time–frequency (TF) space as compared to naturally emitted noise. The aim of this paper is to perform an assessment of different TF RFI mitigation techniques in terms of probability of detection, resolution loss (RL), and mitigation performance. In this assessment, six different kinds of RFI signals have been considered: a glitch, a burst of pulses, a wide-band chirp, a narrow-band chirp, a continuous wave, and a wide-band modulation. The results show that the best performance occurs when the transform basis has a similar shape as compared to the RFI signal. For the best case performance, the maximum residual RFI temperature is 14.8 K, and the worst RL is 8.4%. Moreover, the multiresolution Fourier transform technique appears as a good tradeoff solution among all other techniques since it can mitigate all RFI signals under evaluation with a maximum residual RFI temperature of 21 K, and a worst RL of 26.3%. Although the obtained results are still far from an acceptable bias Misplaced < 1 K for MWR applications, there is still work to do in a combined test using the information gathered simultaneously by all mitigation techniques, which could improve the overall performance of RFI mitigation.Peer ReviewedPostprint (author's final draft

SNR degradation in GNSS-R measurements under the effects of radio-frequency interference

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Radio-frequency interference (RFI) is a serious threat for systems working with low power signals such as those coming from the global navigation satellite systems (GNSS). The spectral separation coefficient (SSC) is the standard figure of merit to evaluate the signal-to-noise ratio (SNR) degradation due to the RFI. However, an in-depth assessment in the field of GNSS-Reflectometry (GNSS-R) has not been performed yet, and particularly, about which is the influence of the RFI on the so-called delay-Doppler map (DDM). This paper develops a model that evaluates the contribution of intra-/inter-GNSS and external RFI effects to the degradation of the SNR in the DDM for both conventional and interferometric GNSS-R techniques. Moreover, a generalized SSC is defined to account for the effects of nonstationary RFI signals. The results show that highly directive antennas are necessary to avoid interference from other GNSS satellites, whereas mitigation techniques are essential to keep GNSS-R instruments working under external RFI degradation.Peer ReviewedPostprint (author's final draft

Cross-talk statistics and impact in interferometric GNSS-R

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper presents a statistical analysis of the crosstalk phenomenon in interferometric Global Navigation Satellite Systems Reflectometry (iGNSS-R). Crosstalk occurs when the Delay-Doppler Map (DDM) of a tracked satellite overlaps others fromundesired satellites. This study is performed for ground-based and airborne receivers and for a receiver onboard the International Space Station (ISS) such as the upcoming GNSS Reflectometry, Radio Occultation and Scatterometry onboard ISS experiment. Its impact on ocean altimetry retrievals is analyzed for different antenna arrays. Results show that for elevation angles higher than 60 degrees, crosstalk can be almost permanent from ground, up to 61% from airborne receivers at 2-km height, and up to similar to 10% at the ISS. Crosstalk can only be mitigated using highly directive antennas with narrow beamwidths. Crosstalk impact using a seven-element hexagonal array still induces large errors on ground, but reduces to centimeter level on airborne receivers, and is negligible from the ISS.Peer ReviewedPostprint (author's final draft

Wind direction signatures in GNSS-R observables from space

Peer ReviewedPostprint (published version

Una gramàtica descriptiva de l'espanyol

Obra ressenyada: Ignacio BOSQUE i Violeta DEMONTE (ed.), Gramática Descriptiva de la Lengua Española. Madrid: Espasa Calpe, 1999

A generic level 1 simulator for spaceborne GNSS-R missions and application to GEROS-ISS ocean reflectometry

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In the past decade Global Navigation Satellites System Reflectometry (GNSS-R) has emerged as a new technique for earth remote sensing for various applications, such as ocean altimetry and sea state monitoring. After the success of the GNSS-R demonstrator payloads aboard the UK-DMC or TDS-1 satellites; at present, there are several missions planned to carry GNSS reflectometers. The GNSS rEflectometry, Radio Occultation, and Scatterometry onboard International Space Station (GEROS-ISS) is an innovative ISS experiment exploiting GNSS-R technique to measure key parameters of ocean, land, and ice surfaces. For GEROS-ISS mission, the European Space Agency (ESA) supported the study of GNSS-R assessment of requirements and consolidation of retrieval algorithms (GARCA). For this, it was required to accurately simulate the GEROS-ISS measurements including the whole range of parameters affecting the observation conditions and the instrument, which is called GEROS-SIM. To meet these requirements, the PAU/PARIS end-to-end performance simulator (P$^{2}$EPS) previously developed by UPC BarcelonaTech was used as the baseline building blocks for the level 1 (L1) processor of GEROS-SIM. P$^{2}$EPS is a flexible tool, and is capable of systematically simulating the GNSS-R observations for spaceborne GNSS-R missions. Thanks to the completeness and flexibility, the instrument-to-L1 data module of GEROS-SIM could be implemented by proper modification and update of P$^{2}$EPS. The developed GEROS-SIM was verified and validated in the GARCA study as comparing to the TDS-1 measurements. This paper presents the design, implementation, and results of the GEROS-SIM L1 module in a generic way to be applied to GNSS-R instruments.Peer ReviewedPostprint (author's final draft

The paradoxical effect of COVID-19 outbreak on loneliness

As in previous periods of quarantine, lockdown confinement measures dictated to control SARS-CoV-2 would be expected to negatively affect mental health. We investigated the immediate effects (over a 10 day period) of a strict nationwide stay-at-home order imposed in Spain, one of the countries most affected by the COVID-19 pandemic. Focusing our analysis on the feelings of loneliness, we obtained our measures within a social context characterised by strong and continuous public and governmental support for increasing social bonds and cooperation in order to face the common public threat. Leveraging data from the Barcelona Brain Health Initiative, a prospective population-based study cohort, the short UCLA Loneliness Scale was administered to 1604 participants 2 years and 1 year before the stay-at-home lockdown and repeated, on average, 10 days after the official confinement order issued by the Spanish government. Ratings of loneliness remained stable during the 2 years before lockdown; however, they decreased significantly during the early stages of home confinement. This effect was particularly significant for the item 'feeling excluded from others' and was also observed among individuals who were confined alone. Overall, the results suggest that gestures and manifestations of appreciation by people for the labour and efforts of certain individuals, along with official campaigns designed to promote feelings of inclusion and belonging, may have beneficial effects on feelings of loneliness, a negative emotional state strongly regarded as a risk factor for impaired mental and general health status. Further assessments during the later stages of home confinement are now warranted