The Experts below are selected from a list of 18165 Experts worldwide ranked by ideXlab platform
Frank Weinhold - One of the best experts on this subject based on the ideXlab platform.
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Natural Bond Orbital Theory of Pseudo-Jahn-Teller Effects.
The journal of physical chemistry. A, 2018Co-Authors: Davood Nori-shargh, Frank WeinholdAbstract:We describe a unified picture of symmetry-breaking electronic interactions that are usually described as “pseudo-Jahn–Teller (PJT) effects” and attributed to vibronic coupling but can also be associated with hyperconjugative donor–acceptor interactions in the framework of Natural Bond Orbital (NBO) and Natural resonance theory (NRT) analysis. We show how NBO/NRT descriptors offer a simplified alternative to the vibronic coupling picture of PJT effects that yields both improved cause–effect specificity and chemically enriched understanding of symmetry-breaking phenomena but with no necessary input from ground-state vibrational or excited-state electronic properties. Comparative NBO/NRT vs vibronic coupling analyses of PJT effects are illustrated for two well-known cases: trans-bending in Si2H4 and higher Group-14 homologues of ethylene and chain-kinking in cyclopentadienylideneketene (C5H4CCO) and related cumulene ketones. The conceptual and practical advantages of the NBO-based hyperconjugative approach m...
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Efficient evaluation of poly-electron populations in Natural Bond Orbital analysis
Chemical Physics Letters, 2018Co-Authors: Eric D Glendening, Frank WeinholdAbstract:Abstract We show how a simple Natural Orbital-based modification of Karafiloglou’s poly-electron population analysis (PEPA) [generalizing conventional one-electron Natural population analysis (NPA)] allows efficient numerical evaluation of Born probabilities (populations) for an unlimited variety of localized electronic excitation patterns. The computational advantages are illustrated by simple numerical applications to CH3NH2 and benzene in the framework of Natural Bond Orbital (NBO)-based “NPEPA” keyword implementation in the forthcoming NBO 7.0 program.
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The role of hyperconjugation in the unusual conformation of thymine: A Natural Bond Orbital analysis
Computational and Theoretical Chemistry, 2018Co-Authors: Emerson Rengifo, Frank Weinhold, Sara Gómez, J. C. Arce, Albeiro RestrepoAbstract:Abstract The diketo conformation of thymine inferred from a high-level ab initio calculation, where one of the C H Bonds of the methyl group lies in the ring plane eclipsing another C H Bond instead of the polar C O Bond, seems unexpected on the grounds of an analysis in terms of steric hindrance and long–distance inductive interactions. To assess the causes of such conformation, we carried out a Natural Bond Orbital analysis. We found that charge delocalization (hyperconjugation), associated mainly with vicinal σC H( CH3) → σ∗C C(ring) and σC H( CH3) → π∗C C(ring) interactions, strongly favors the observed conformation.
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nbo 6 0 Natural Bond Orbital analysis program
Journal of Computational Chemistry, 2013Co-Authors: Eric D Glendening, Clark R Landis, Frank WeinholdAbstract:We describe principal features of the newly released version, NBO 6.0, of the Natural Bond Orbital analysis program, that provides novel “link-free” interactivity with host electronic structure systems, improved search algorithms and labeling conventions for a broader range of chemical species, and new analysis options that significantly extend the range of chemical applications. We sketch the motivation and implementation of program changes and describe newer analysis options with illustrative applications. © 2013 Wiley Periodicals, Inc.
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Natural Bond critical point analysis quantitative relationships between Natural Bond Orbital based and qtaim based topological descriptors of chemical Bonding
Journal of Computational Chemistry, 2012Co-Authors: Frank WeinholdAbstract:We have developed a “Natural Bond Critical Point” (NBCP) module for the Natural Bond Orbital (NBO) program that allows mutual analysis of NBO-based versus Bader-type quantum theory of atoms in molecules (QTAIM) topological descriptors of chemical Bonding interactions. Conventional QTAIM Bond path and Bond critical point (BCP) descriptors deduced from total electron density ρ(r) can thereby be compared with analogous “Natural” (NBCP) descriptors for idealized densities ρNAIM(r) composed solely from NBO-based “Natural atoms in molecules” (NAIM) at the terminal nuclei. Standard ρ(rBCP) and ∇2ρ(rBCP) descriptors can also be decomposed into unique contributions from individual NBOs or other localized, semilocalized, or delocalized Orbital components. These results allow one to recognize many relationships between QTAIM and NBO analyses, showing why close correlations are often found between NBO-based versus ρ(rBCP)-based characterizations of chemical Bonding interactions, despite strongly divergent conceptions of “the atom in the molecule.” © 2012 Wiley Periodicals, Inc.
M. Amalanathan - One of the best experts on this subject based on the ideXlab platform.
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vibrational spectra ft ir and ft raman molecular structure Natural Bond Orbital and td dft analysis of l asparagine monohydrate by density functional theory approach
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2014Co-Authors: S Sylvestre, M. Amalanathan, S. Sebastian, S Edwin, S Ayyapan, T Jayavarthanan, K Oudayakumar, S SolomonAbstract:Abstract In this work we report the vibrational spectral analysis of l -Asparagine Monohydrate (LAM) molecule by using FT-IR and FT-Raman spectroscopic techniques. The equilibrium geometry, harmonic vibrational wavenumbers, various Bonding features have been computed using density functional B3LYP method with 6-311G(d,p) as basis set. Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using Natural Bond Orbital (NBO) analysis. The results show that charge in electron density (ED) in σ* and π* antiBonding Orbitals and second order delocalization energies E(2) confirms the occurrence of Intramolecular Charge Transfer (ICT) within the molecule. The energy and oscillator strength calculated by Time-Dependent Density Functional Theory (TD-DFT) complements with the experimental findings. The simulated spectra satisfactorily coincide with the experimental spectra.
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Vibrational spectral investigation and Natural Bond Orbital analysis of pharmaceutical compound 7-Amino-2,4-dimethylquinolinium formate - DFT approach.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2013Co-Authors: D.m. Suresh, M. Amalanathan, S. Sebastian, D. Sajan, I. Hubert Joe, V. Bena Jothy, Ivan NemecAbstract:The molecular geometry, the normal mode frequencies and corresponding vibrational assignments, Natural Bond Orbital analysis and the HOMO-LUMO analysis of 7-Amino-2,4-dimethylquinolinium formate in the ground state were performed by B3LYP levels of theory using the 6-31G(d) basis set. The optimised Bond lengths and Bond angles are in good agreement with the X-ray data. The vibrational spectra of the title compound which is calculated by DFT method, reproduces vibrational wave numbers and intensities with an accuracy which allows reliable vibrational assignments. The possibility of N-H⋯O hydrogen Bonding was identified using NBO analysis. Natural Bond Orbital analysis confirms the presence of intramolecular charge transfer and the hydrogen Bonding interaction.
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Vibrational spectral investigation and Natural Bond Orbital analysis of anti-rheumatoid drug ethyl 4-nitrophenylacetate--DFT approach.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2012Co-Authors: D.m. Suresh, M. Amalanathan, S. Sebastian, D. Sajan, I. Hubert Joe, V. Bena JothyAbstract:Abstract Vibrational analysis of ethyl 4-nitrophenylacetate (ENPA) molecule was carried out using FT-IR and FT-Raman spectroscopic techniques. The equilibrium geometry, harmonic vibrational wave numbers, various Bonding features have been computed using density functional theory. The calculated molecular geometry parameters have been compared with XRD data. The detailed interpretation of the vibrational spectra has been carried out by computing Potential Energy Distribution (PED). Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using Natural Bond Orbital (NBO) analysis. The results show that the charge in the electron density (ED) in the σ∗ and π∗ antiBonding Orbitals and second order delocalization energies (E2) confirm the occurrence of ICT (intramolecular charge transfer) within the molecule. The simulated spectra satisfactorily coincide with the experimental spectra.
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Vibrational spectra and Natural Bond Orbital analysis of organic crystal l-prolinium picrate
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2012Co-Authors: Bismi Edwin, M. Amalanathan, I. Hubert JoeAbstract:Abstract Vibrational spectral analysis and quantum chemical computations based on density functional theory (DFT) have been performed on the organic crystal l -prolinium picrate (LPP). The equilibrium geometry, various Bonding features and harmonic vibrational wavenumbers of LPP have been investigated using B3LYP method. The calculated molecular geometry has been compared with the experimental data. The detailed interpretation of the vibrational spectra has been carried out with the aid of VEDA 4 program. The various intramolecular interactions confirming the biological activity of the compound have been exposed by Natural Bond Orbital analysis. The distribution of Mulliken atomic charges and bending of Natural hybrid Orbitals associated with hydrogen Bonding also reflects the presence of intramolecular hydrogen Bonding thereby enhancing bioactivity. The analysis of the electron density of HOMO and LUMO gives an idea of the delocalization and low value of energy gap indicates electron transport in the molecule and thereby bioactivity. Vibrational analysis reveals the presence of strong O–H⋯O and N–H⋯O interaction between l -prolinium and picrate ions providing evidence for the charge transfer interaction between the donor and acceptor groups and is responsible for its bioactivity.
John L. Markley - One of the best experts on this subject based on the ideXlab platform.
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Trans-Hydrogen-Bond h2JNN and h1JNH Couplings in the DNA A−T Base Pair: Natural Bond Orbital Analysis
Journal of the American Chemical Society, 2002Co-Authors: Steven J. Wilkens, Frank Weinhold, William M. Westler, John L. MarkleyAbstract:Natural Bond Orbital (NBO) analysis described here demonstrates that trans-hydrogen-Bond (trans-H-Bond) NMR J couplings in the DNA A-T base pair, h2JNN and h1JNH, are determined largely by three terms: two Lewis-type contributions (the single-Orbital contribution from the adenine lone pair and the contribution from the sigmaN3H3 Natural Bond Orbital of the thymine ring) and one contribution from pairwise delocalization of spin density (between the lone pair in adenine and the sigma* antiBonding Orbital linking N3 and H3 of thymine). For h2JNN coupling, all three contributions are positive, whereas for h1JNH coupling, the delocalization term is negative, and the other two terms are positive, resulting in a small net positive coupling constant. This result rationalizes the experimental findings that the two-Bond coupling (h2JNN approximately 9 Hz) is larger than the one-Bond coupling (h1JNH approximately 3 Hz) and demonstrates that the same hyperconjugative and steric mechanisms that stabilize the H-Bond are involved in the transmission of J coupling information. The N1...H3-N3 H-Bond of the DNA A-T base pair is found to exhibit significant covalent character, but steric effects contribute almost equally to the trans-H-Bond coupling.
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trans hydrogen Bond h2jnn and h1jnh couplings in the dna a t base pair Natural Bond Orbital analysis
Journal of the American Chemical Society, 2002Co-Authors: Steven J. Wilkens, Frank Weinhold, William M. Westler, John L. MarkleyAbstract:Natural Bond Orbital (NBO) analysis described here demonstrates that trans-hydrogen-Bond (trans-H-Bond) NMR J couplings in the DNA A-T base pair, h2JNN and h1JNH, are determined largely by three terms: two Lewis-type contributions (the single-Orbital contribution from the adenine lone pair and the contribution from the sigmaN3H3 Natural Bond Orbital of the thymine ring) and one contribution from pairwise delocalization of spin density (between the lone pair in adenine and the sigma* antiBonding Orbital linking N3 and H3 of thymine). For h2JNN coupling, all three contributions are positive, whereas for h1JNH coupling, the delocalization term is negative, and the other two terms are positive, resulting in a small net positive coupling constant. This result rationalizes the experimental findings that the two-Bond coupling (h2JNN approximately 9 Hz) is larger than the one-Bond coupling (h1JNH approximately 3 Hz) and demonstrates that the same hyperconjugative and steric mechanisms that stabilize the H-Bond are involved in the transmission of J coupling information. The N1...H3-N3 H-Bond of the DNA A-T base pair is found to exhibit significant covalent character, but steric effects contribute almost equally to the trans-H-Bond coupling.
I. Hubert Joe - One of the best experts on this subject based on the ideXlab platform.
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Natural Bond Orbital Analysis and DFT Calculation of Non-opiod Analgesic Drug Lidocaine
Materials Today: Proceedings, 2015Co-Authors: D.r. Leenaraj, I. Hubert JoeAbstract:Abstract FT-IR spectra of Lidocaine compound have been recorded and analyzed. Density fuctional theoretical computations are performed at B3LYP/6-311+G(d,p) level to derive optimizd geometry, Bonding features and harmonic vibrational wavenumbers. The calculated geometrical parameters have been compared with experimental data. The detailed interpretation of the vibrational spectra assignment has been carried out on the basis of potential energy distribution analysis using VEDA4 program. The Natural Bond Orbital (NBO) analysis confirms the occurrence of strong intramolecular N–H···N hydrogen Bond in the molecule. The Mulliken population analysis and the HOMO-LUMO energy were also studied.
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Vibrational spectral investigation and Natural Bond Orbital analysis of pharmaceutical compound 7-Amino-2,4-dimethylquinolinium formate - DFT approach.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2013Co-Authors: D.m. Suresh, M. Amalanathan, S. Sebastian, D. Sajan, I. Hubert Joe, V. Bena Jothy, Ivan NemecAbstract:The molecular geometry, the normal mode frequencies and corresponding vibrational assignments, Natural Bond Orbital analysis and the HOMO-LUMO analysis of 7-Amino-2,4-dimethylquinolinium formate in the ground state were performed by B3LYP levels of theory using the 6-31G(d) basis set. The optimised Bond lengths and Bond angles are in good agreement with the X-ray data. The vibrational spectra of the title compound which is calculated by DFT method, reproduces vibrational wave numbers and intensities with an accuracy which allows reliable vibrational assignments. The possibility of N-H⋯O hydrogen Bonding was identified using NBO analysis. Natural Bond Orbital analysis confirms the presence of intramolecular charge transfer and the hydrogen Bonding interaction.
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Vibrational spectral investigation and Natural Bond Orbital analysis of anti-rheumatoid drug ethyl 4-nitrophenylacetate--DFT approach.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2012Co-Authors: D.m. Suresh, M. Amalanathan, S. Sebastian, D. Sajan, I. Hubert Joe, V. Bena JothyAbstract:Abstract Vibrational analysis of ethyl 4-nitrophenylacetate (ENPA) molecule was carried out using FT-IR and FT-Raman spectroscopic techniques. The equilibrium geometry, harmonic vibrational wave numbers, various Bonding features have been computed using density functional theory. The calculated molecular geometry parameters have been compared with XRD data. The detailed interpretation of the vibrational spectra has been carried out by computing Potential Energy Distribution (PED). Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using Natural Bond Orbital (NBO) analysis. The results show that the charge in the electron density (ED) in the σ∗ and π∗ antiBonding Orbitals and second order delocalization energies (E2) confirm the occurrence of ICT (intramolecular charge transfer) within the molecule. The simulated spectra satisfactorily coincide with the experimental spectra.
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Vibrational spectra and Natural Bond Orbital analysis of organic crystal l-prolinium picrate
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2012Co-Authors: Bismi Edwin, M. Amalanathan, I. Hubert JoeAbstract:Abstract Vibrational spectral analysis and quantum chemical computations based on density functional theory (DFT) have been performed on the organic crystal l -prolinium picrate (LPP). The equilibrium geometry, various Bonding features and harmonic vibrational wavenumbers of LPP have been investigated using B3LYP method. The calculated molecular geometry has been compared with the experimental data. The detailed interpretation of the vibrational spectra has been carried out with the aid of VEDA 4 program. The various intramolecular interactions confirming the biological activity of the compound have been exposed by Natural Bond Orbital analysis. The distribution of Mulliken atomic charges and bending of Natural hybrid Orbitals associated with hydrogen Bonding also reflects the presence of intramolecular hydrogen Bonding thereby enhancing bioactivity. The analysis of the electron density of HOMO and LUMO gives an idea of the delocalization and low value of energy gap indicates electron transport in the molecule and thereby bioactivity. Vibrational analysis reveals the presence of strong O–H⋯O and N–H⋯O interaction between l -prolinium and picrate ions providing evidence for the charge transfer interaction between the donor and acceptor groups and is responsible for its bioactivity.
V. Balachandran - One of the best experts on this subject based on the ideXlab platform.
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spectroscopic investigation Natural Bond Orbital analysis homo lumo and thermodynamic functions of 2 tert butyl 5 methyl anisole using dft b3lyp calculations
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2015Co-Authors: V. Balachandran, G Santhi, V Karpagam, B Revathi, M KarabacakAbstract:Abstract The optimized molecular structure and corresponding vibrational assignments of 2-tert-butyl-5-methyl anisole (TBMA) have been investigated using density functional theory (DFT)/B3LYP with 6-31G(d,p) and 6-311++G(d,p) basis sets investigation of the relative orientation of the methoxy group has shown two conformers (O- cis ) and (O- trans ) exist. The vibrational analysis of the stable conformer of the title compound is performed by means of infrared absorption and Raman spectroscopy in combination with theoretical simultaneously. The Natural Bond Orbital (NBO) analysis is useful to understand the intra-molecular hyper conjugative interaction lone pair and π ∗ (C C), σ ∗ (C H) Bond Orbital. HOMO and LUMO energies show that charge transfer occurs in the molecule, therefore; HOMO, LUMO and molecular electrostatic potential (MEP) were calculated and analyzed. 1 H and 13 C NMR spectra by using gauge including atomic Orbital (GIAO) method of studied compound were compared with experimental data. The thermodynamic functions of TBMA were calculated by B3LYP/6-311+G(d,p) basis set.
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α-Bromo-2,6-dichlorotoluene: molecular structure, vibrational spectroscopy, Natural Bond Orbital analysis and NMR studies.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2012Co-Authors: V. Karunakaran, V. BalachandranAbstract:The FT-IR and FT-Raman spectra of α-bromo-2,6-dichlorotoluene (αBDCT) have been recorded. The structural and spectroscopic data of the molecule in the ground state have been calculated using Hartree Fock (HF) and Density Functional Theory (DFT)/B3LYP with the standard 6-31++G(d,p) basis set. The optimized molecular geometry, vibrational frequencies and atomic charges in the ground state are calculated. With the help of specific scaling procedures, the observed vibrational wavenumbers in FT-IR and FT-Raman spectra have been analyzed and assigned to different normal modes of the molecule. A detailed interpretation of the infrared and Raman spectra of αBDCT is also reported based on total energy distribution (TED). The (1)H and (13)C NMR chemical shifts have been calculated by gauge-including atomic Orbital method with B3LYP/6-31++G(d,p) approach. Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using Natural Bond Orbital (NBO) analysis. The theoretical results agree well with the observed spectra.
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ab initio dft homo lumo and Natural Bond Orbital analyses of the electronic structure of 2 mercapto 1 methylimidazole
Journal of Molecular Structure, 2011Co-Authors: V. Balachandran, A. Lakshmi, A. JanakiAbstract:Abstract The Fourier transform Raman and Fourier transform infrared spectra of 2-mercapto-1-methylimidazole have been recorded. Ab initio and density functional computations of the vibrational spectrum, the molecular geometry, highest occupied molecular Orbital (HOMO)–lowest unoccupied molecular Orbital (LUMO) energy gaps were studied. On the basis of the comparison between calculated and experimental results and the comparison with related molecules, assignments of the fundamental vibrational modes are examined. The observed and simulated spectra were found to be well comparable. The molecular stability and Bond strength were investigated by applying the Natural Bond Orbital analysis (NBO). The other molecular properties like Mulliken population analysis, thermodynamic functions and polarizabilities of the title compound have been reported. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping electron density isosurface with electrostatic potential (ESP).
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Ab initio, DFT, HOMO–LUMO and Natural Bond Orbital analyses of the electronic structure of 2-mercapto-1-methylimidazole
Journal of Molecular Structure, 2011Co-Authors: V. Balachandran, A. Lakshmi, A. JanakiAbstract:Abstract The Fourier transform Raman and Fourier transform infrared spectra of 2-mercapto-1-methylimidazole have been recorded. Ab initio and density functional computations of the vibrational spectrum, the molecular geometry, highest occupied molecular Orbital (HOMO)–lowest unoccupied molecular Orbital (LUMO) energy gaps were studied. On the basis of the comparison between calculated and experimental results and the comparison with related molecules, assignments of the fundamental vibrational modes are examined. The observed and simulated spectra were found to be well comparable. The molecular stability and Bond strength were investigated by applying the Natural Bond Orbital analysis (NBO). The other molecular properties like Mulliken population analysis, thermodynamic functions and polarizabilities of the title compound have been reported. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping electron density isosurface with electrostatic potential (ESP).
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Vibrational spectroscopic studies and Natural Bond Orbital analysis of 4,6-dichloro-2-(methylthio)pyrimidine based on density functional theory.
Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2011Co-Authors: V. Balachandran, A. Lakshmi, A. JanakiAbstract:Abstract The FT-IR and FT-Raman spectra of 4,6-dichloro-2-(methylthio)pyrimidine (DMP) have been recorded and analyzed. The optimized geometry, intramolecular hydrogen Bonding, and harmonic vibrational wave numbers of DMP have been investigated with the help of B3LYP density functional theory (DFT) method supplemented with 6-31G** basis set. The infrared and Raman spectra were predicted theoretically from the calculated intensities. Natural Bond Orbital (NBO) analysis of DMP has been performed to indicate the presence of intramolecular C–H⋯Cl hydrogen Bonding. The formation of Lewis and non-Lewis Orbitals and second-order perturbation energies of DMP have also been reported.
