An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Nanoclusters are materials of paramount catalytic importance. Among various unique properties fea... more Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt7 subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.
The tropylium catalyzed carboxylic acid O-H insertion with diazoesters providing α-hydroxy esters... more The tropylium catalyzed carboxylic acid O-H insertion with diazoesters providing α-hydroxy esters was reported recently through an activated carbene as the key intermediate. We report a revised mechanism involving a unique homoaromatic intermediate with the tropylium ion and the diazoester based on the DFT calculations. Our computational model provides a clear insight into the binding of the tropylium ion with the diazoester providing the homoaromatic intermediate. The reaction profiles of four different pathways were compared. The energies of the intermediates and the transition states are reported at B97-D3(SMD)/def2TZVP//B97-D3/def2TZVP (in dichloromethane). The energy profiles were compared across a few computational methods to study the sensitivity of our model across methods.
Chemistry (Weinheim an der Bergstrasse, Germany), Jan 3, 2018
HCN oligomerization is considered to be one of the important pathways in chemical evolution. Nucl... more HCN oligomerization is considered to be one of the important pathways in chemical evolution. Nucleobases, aminoacids, and many other complex organic molecules can evolve through this pathway. We report an explorative study based on an automated reaction search method that avoids the cognitive bias present when searching chemical reaction space. We discuss the chemical space of the HCN dimer in detail, and the important trimers and tetramers are discussed briefly. A component-wise molecular-fingerprint-based methodology is proposed to identify molecular similarity. We present four different thermal routes to cis/trans-2,3-diaminomaleonitrile and 4-amino-1H-imidazole-5-carbonitrile, which are important intermediates in prebiotic chemistry.
A synthetic and theoretical approach towards the synthesis of cycloheptenone via palladium-cataly... more A synthetic and theoretical approach towards the synthesis of cycloheptenone via palladium-catalyzed 7-exo-trig cyclization followed by the oxidation of cyclized alcohol.
The journal of physical chemistry. A, Jan 23, 2016
Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior ... more Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu(IV) over a large number of fission products including Zr(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu(NO3)4·2TBP and Zr(NO3)4·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M-O{TBP} and M-O{NO3} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be -73.1 a...
Angewandte Chemie (International ed. in English), Jan 18, 2016
Among the well-known phototriggers, the p-hydroxyphenacyl (pHP) group has consistently enabled th... more Among the well-known phototriggers, the p-hydroxyphenacyl (pHP) group has consistently enabled the very fast, efficient, and high-conversion release of active molecules. Despite this unique behavior, the pHP group has been ignored as a delivery agent, particularly in the area of theranostics, because of two major limitations: Its excitation wavelength is below 400 nm, and it is nonfluorescent. We have overcome these limitations by incorporating a 2-(2'-hydroxyphenyl)benzothiazole (HBT) appendage capable of rapid excited-state intramolecular proton transfer (ESIPT). The ESIPT effect also provided two unique advantages: It assisted the deprotonation of the pHP group for faster release, and it was accompanied by a distinct fluorescence color change upon photorelease. In vitro studies showed that the p-hydroxyphenacyl-benzothiazole-chlorambucil conjugate presents excellent properties, such as real-time monitoring, photoregulated drug delivery, and biocompatibility.
The emergence of life remains one of the most intriguing scientific questions, and understanding ... more The emergence of life remains one of the most intriguing scientific questions, and understanding the chemical processes that led to it is essential. Recent discoveries of C3H2NO isomers in the interstellar medium have motivated further exploration of this molecular formula. Here, we use density functional theory to explore the potential energy surface of C3H2NO isomers, including acyclic and cyclic structures, and identify twelve low-lying isomers. We then investigate bimolecular reaction with HCN and H2CO using an artificial force-induced automated search and transition state search for the minimal energy routes. Our results predict the formation of Oxiran- 2-ylazanide and (Z)2-aminoacetaldehyde as the kinetically and thermodynamically controlled products, respectively. These results provide insights into the potential formation of biomarkers such as N-methyleneformamide in the interstellar medium, which have been found in recent years. Our study contributes to prebiotic chemistry ...
In the present work we have developed one mononuclear Zn(II) complex [Zn(L)(H2O)] (Complex 1) by ... more In the present work we have developed one mononuclear Zn(II) complex [Zn(L)(H2O)] (Complex 1) by utilizing a tetracoordinated ligand H2L, formed by simple condensation of 2, 2 dimethyl 1,3 diamino propane and 3- ethoxy salicylaldehyde and one newly designed mononuclear Co (III) complex [Co(L)(L1)] (complex 2) by utilizing (H2L) and 3- ethoxy salicylaldehyde(HL1) as an ancillary ligand. The newly developed complex 2 have been spectroscopically characterized. An interesting phenomenon has been noticed that in presence of ancillary ligand, the solubility in buffer solution and the thermal stability of complex 2 comparatively increases than 1. To check the effect of ancillary ligand, present in complex 2 towards the DNA and HSA binding efficacy, both the complexes have been taken into consideration to inspect their binding potentiality with the macromolecules. The 'on', 'off' fluorescence changes in presence of DNA and HSA, the binding constant values, obtained from electronic spectral titration, iodide induced quenching, competitive binding assay, circular dichroism (CD) spectral titration, time resolved fluorescence experiment unambiguously assure the better binding efficacy of complex 2 with the signal of minor groove binding mode with DNA along with no significant conformational changes of the macromolecules. The strong and spontaneous binding of complex 2 with CT-DNA is further supported by the Isothermal Titration Calorimetry (ITC) study. Furthermore TDDFT calculation of DNA with and without complex 2 significantly authorize the formation of complex 2-DNA adduct during the association. Finally Molecular Docking study properly verifies the experimental findings and provides justified explanation behinds experimental findings.Communicated by Ramaswamy H. Sarma.
We present a benchmark study considering AEPA16 dataset comprising atomization energy for boron g... more We present a benchmark study considering AEPA16 dataset comprising atomization energy for boron group clusters. This dataset covers 46 different methods from the different range of density functionals and post HF, where MRACPF is considered as the reference. The performance of basis set is analyzed by comparing the atomization energy with the CCSD(T)\CBS results. None of the GGA and meta-GGA are accurate and the popular PBE shows maximum error among the all methods, whereas B3LYP has moderate performance. M06 and RSHX DFAs have good performance among all the DFAs. NEVPT2 is the most accurate among all the methods considered followed by coupled cluster methods, i.e., CCSD(T) and DLPNO-CCSD(T), indicating the importance of perturbative correction. All the basis with M06 show promising results (absolute error \textless 1 \kmol) with respect to the CCSD(T)\CBS. From the cost -- accuracy investigation, def2-SVP is recommended in our study as it is the fastest. Hence, we have recommended ...
Chemical intuition yields the second low-lying isomer after AUTOMATON found the global minimum an... more Chemical intuition yields the second low-lying isomer after AUTOMATON found the global minimum and few other low-lying isomers.
We present a performance study on popular density functionals (DFAs) and three-part corrected met... more We present a performance study on popular density functionals (DFAs) and three-part corrected methods (3c-methods) for their efficiency and accuracy in the geometry and relative stability of gold-thiolate nanoclusters taking Au 3(SCH3)3 isomers. Normalized mean absolute error (NMAE) is analysed to compare the results with the reference methods DLPNO-CCSD(T) and RI-SCS-MP2. The dependence of basis set size and relativistic effects on energies are also compared. TPSS shows accuracy, whereas PBE is fast for geometry optimization. On the other hand, range separation in hybrid DFAs stands out as the proper choice for the relative energies of the clusters. LC-BLYP excels, but 3c-methods are less impressive in relative stability comparison. Counterpoise corrected thiolate interaction energy and bonding analyses delineate the higher stability of bridging sulfur coordination (RS-Au-SR; R = − methyl) than singly-bonded and capping sulfur bonds in Au3(SCH3)3 . With the selected DFAs, we have a...
Density functional theory based computation predicts the presence of a planar tetracoordinate car... more Density functional theory based computation predicts the presence of a planar tetracoordinate carbon (ptC) in the global minimum energy structures of CSinGe4−n2+ (n = 1–3) systems.
Influence of electronic effects on the dioxygenation reaction of copper(ii)-flavonolate adducts, ... more Influence of electronic effects on the dioxygenation reaction of copper(ii)-flavonolate adducts, as a functional mimic of quercetin 2,4-dioxygenase-type reactivity, has been described in this work.
Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most ... more Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl4Mg0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl4Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability.
The regioselectivity in addition of nucleophiles to the p-benzyne intermediates derived from unsy... more The regioselectivity in addition of nucleophiles to the p-benzyne intermediates derived from unsymmetrical aza-substituted enediynes via Bergman cyclization was studied. Computational studies [using UB3LYP/6-31G(d,p) level of theory] suggest that the p-benzyne intermediate retains its similar electrophilic character at the two radical centers even under unsymmetrical electronic perturbation, thus supporting the predicted model of nucleophilic addition to p-benzyne proposed by Perrin and co-workers (Perrin et al. J. Am. Chem. Soc. 2007, 129, 4795-4799) and later by Alabugin and co-workers (Peterson et al. Eur. J. Org. Chem. 2013, 2013, 2505-2527). However, observed experimental results suggest that there was small but definite regioselectivity (∼5-25%), the extent varying with the electronic nature of the substituents. Differential solvated halide ion concentrations around the vicinity of two radical centers arising due to surrounding surface electrostatic potential (computationally ...
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Nanoclusters are materials of paramount catalytic importance. Among various unique properties fea... more Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt7 subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.
The tropylium catalyzed carboxylic acid O-H insertion with diazoesters providing α-hydroxy esters... more The tropylium catalyzed carboxylic acid O-H insertion with diazoesters providing α-hydroxy esters was reported recently through an activated carbene as the key intermediate. We report a revised mechanism involving a unique homoaromatic intermediate with the tropylium ion and the diazoester based on the DFT calculations. Our computational model provides a clear insight into the binding of the tropylium ion with the diazoester providing the homoaromatic intermediate. The reaction profiles of four different pathways were compared. The energies of the intermediates and the transition states are reported at B97-D3(SMD)/def2TZVP//B97-D3/def2TZVP (in dichloromethane). The energy profiles were compared across a few computational methods to study the sensitivity of our model across methods.
Chemistry (Weinheim an der Bergstrasse, Germany), Jan 3, 2018
HCN oligomerization is considered to be one of the important pathways in chemical evolution. Nucl... more HCN oligomerization is considered to be one of the important pathways in chemical evolution. Nucleobases, aminoacids, and many other complex organic molecules can evolve through this pathway. We report an explorative study based on an automated reaction search method that avoids the cognitive bias present when searching chemical reaction space. We discuss the chemical space of the HCN dimer in detail, and the important trimers and tetramers are discussed briefly. A component-wise molecular-fingerprint-based methodology is proposed to identify molecular similarity. We present four different thermal routes to cis/trans-2,3-diaminomaleonitrile and 4-amino-1H-imidazole-5-carbonitrile, which are important intermediates in prebiotic chemistry.
A synthetic and theoretical approach towards the synthesis of cycloheptenone via palladium-cataly... more A synthetic and theoretical approach towards the synthesis of cycloheptenone via palladium-catalyzed 7-exo-trig cyclization followed by the oxidation of cyclized alcohol.
The journal of physical chemistry. A, Jan 23, 2016
Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior ... more Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu(IV) over a large number of fission products including Zr(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu(NO3)4·2TBP and Zr(NO3)4·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M-O{TBP} and M-O{NO3} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be -73.1 a...
Angewandte Chemie (International ed. in English), Jan 18, 2016
Among the well-known phototriggers, the p-hydroxyphenacyl (pHP) group has consistently enabled th... more Among the well-known phototriggers, the p-hydroxyphenacyl (pHP) group has consistently enabled the very fast, efficient, and high-conversion release of active molecules. Despite this unique behavior, the pHP group has been ignored as a delivery agent, particularly in the area of theranostics, because of two major limitations: Its excitation wavelength is below 400 nm, and it is nonfluorescent. We have overcome these limitations by incorporating a 2-(2'-hydroxyphenyl)benzothiazole (HBT) appendage capable of rapid excited-state intramolecular proton transfer (ESIPT). The ESIPT effect also provided two unique advantages: It assisted the deprotonation of the pHP group for faster release, and it was accompanied by a distinct fluorescence color change upon photorelease. In vitro studies showed that the p-hydroxyphenacyl-benzothiazole-chlorambucil conjugate presents excellent properties, such as real-time monitoring, photoregulated drug delivery, and biocompatibility.
The emergence of life remains one of the most intriguing scientific questions, and understanding ... more The emergence of life remains one of the most intriguing scientific questions, and understanding the chemical processes that led to it is essential. Recent discoveries of C3H2NO isomers in the interstellar medium have motivated further exploration of this molecular formula. Here, we use density functional theory to explore the potential energy surface of C3H2NO isomers, including acyclic and cyclic structures, and identify twelve low-lying isomers. We then investigate bimolecular reaction with HCN and H2CO using an artificial force-induced automated search and transition state search for the minimal energy routes. Our results predict the formation of Oxiran- 2-ylazanide and (Z)2-aminoacetaldehyde as the kinetically and thermodynamically controlled products, respectively. These results provide insights into the potential formation of biomarkers such as N-methyleneformamide in the interstellar medium, which have been found in recent years. Our study contributes to prebiotic chemistry ...
In the present work we have developed one mononuclear Zn(II) complex [Zn(L)(H2O)] (Complex 1) by ... more In the present work we have developed one mononuclear Zn(II) complex [Zn(L)(H2O)] (Complex 1) by utilizing a tetracoordinated ligand H2L, formed by simple condensation of 2, 2 dimethyl 1,3 diamino propane and 3- ethoxy salicylaldehyde and one newly designed mononuclear Co (III) complex [Co(L)(L1)] (complex 2) by utilizing (H2L) and 3- ethoxy salicylaldehyde(HL1) as an ancillary ligand. The newly developed complex 2 have been spectroscopically characterized. An interesting phenomenon has been noticed that in presence of ancillary ligand, the solubility in buffer solution and the thermal stability of complex 2 comparatively increases than 1. To check the effect of ancillary ligand, present in complex 2 towards the DNA and HSA binding efficacy, both the complexes have been taken into consideration to inspect their binding potentiality with the macromolecules. The 'on', 'off' fluorescence changes in presence of DNA and HSA, the binding constant values, obtained from electronic spectral titration, iodide induced quenching, competitive binding assay, circular dichroism (CD) spectral titration, time resolved fluorescence experiment unambiguously assure the better binding efficacy of complex 2 with the signal of minor groove binding mode with DNA along with no significant conformational changes of the macromolecules. The strong and spontaneous binding of complex 2 with CT-DNA is further supported by the Isothermal Titration Calorimetry (ITC) study. Furthermore TDDFT calculation of DNA with and without complex 2 significantly authorize the formation of complex 2-DNA adduct during the association. Finally Molecular Docking study properly verifies the experimental findings and provides justified explanation behinds experimental findings.Communicated by Ramaswamy H. Sarma.
We present a benchmark study considering AEPA16 dataset comprising atomization energy for boron g... more We present a benchmark study considering AEPA16 dataset comprising atomization energy for boron group clusters. This dataset covers 46 different methods from the different range of density functionals and post HF, where MRACPF is considered as the reference. The performance of basis set is analyzed by comparing the atomization energy with the CCSD(T)\CBS results. None of the GGA and meta-GGA are accurate and the popular PBE shows maximum error among the all methods, whereas B3LYP has moderate performance. M06 and RSHX DFAs have good performance among all the DFAs. NEVPT2 is the most accurate among all the methods considered followed by coupled cluster methods, i.e., CCSD(T) and DLPNO-CCSD(T), indicating the importance of perturbative correction. All the basis with M06 show promising results (absolute error \textless 1 \kmol) with respect to the CCSD(T)\CBS. From the cost -- accuracy investigation, def2-SVP is recommended in our study as it is the fastest. Hence, we have recommended ...
Chemical intuition yields the second low-lying isomer after AUTOMATON found the global minimum an... more Chemical intuition yields the second low-lying isomer after AUTOMATON found the global minimum and few other low-lying isomers.
We present a performance study on popular density functionals (DFAs) and three-part corrected met... more We present a performance study on popular density functionals (DFAs) and three-part corrected methods (3c-methods) for their efficiency and accuracy in the geometry and relative stability of gold-thiolate nanoclusters taking Au 3(SCH3)3 isomers. Normalized mean absolute error (NMAE) is analysed to compare the results with the reference methods DLPNO-CCSD(T) and RI-SCS-MP2. The dependence of basis set size and relativistic effects on energies are also compared. TPSS shows accuracy, whereas PBE is fast for geometry optimization. On the other hand, range separation in hybrid DFAs stands out as the proper choice for the relative energies of the clusters. LC-BLYP excels, but 3c-methods are less impressive in relative stability comparison. Counterpoise corrected thiolate interaction energy and bonding analyses delineate the higher stability of bridging sulfur coordination (RS-Au-SR; R = − methyl) than singly-bonded and capping sulfur bonds in Au3(SCH3)3 . With the selected DFAs, we have a...
Density functional theory based computation predicts the presence of a planar tetracoordinate car... more Density functional theory based computation predicts the presence of a planar tetracoordinate carbon (ptC) in the global minimum energy structures of CSinGe4−n2+ (n = 1–3) systems.
Influence of electronic effects on the dioxygenation reaction of copper(ii)-flavonolate adducts, ... more Influence of electronic effects on the dioxygenation reaction of copper(ii)-flavonolate adducts, as a functional mimic of quercetin 2,4-dioxygenase-type reactivity, has been described in this work.
Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most ... more Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl4Mg0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl4Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability.
The regioselectivity in addition of nucleophiles to the p-benzyne intermediates derived from unsy... more The regioselectivity in addition of nucleophiles to the p-benzyne intermediates derived from unsymmetrical aza-substituted enediynes via Bergman cyclization was studied. Computational studies [using UB3LYP/6-31G(d,p) level of theory] suggest that the p-benzyne intermediate retains its similar electrophilic character at the two radical centers even under unsymmetrical electronic perturbation, thus supporting the predicted model of nucleophilic addition to p-benzyne proposed by Perrin and co-workers (Perrin et al. J. Am. Chem. Soc. 2007, 129, 4795-4799) and later by Alabugin and co-workers (Peterson et al. Eur. J. Org. Chem. 2013, 2013, 2505-2527). However, observed experimental results suggest that there was small but definite regioselectivity (∼5-25%), the extent varying with the electronic nature of the substituents. Differential solvated halide ion concentrations around the vicinity of two radical centers arising due to surrounding surface electrostatic potential (computationally ...
Uploads
Papers