Electron Density Difference Research Articles

Observation of robust infrared plasmons in twisted titanium carbide (Ti3C2) MXene

Optical properties of two-dimensional (2D) materials can be tuned on demand through rigid rotation (via twist angle) of one layer with respect to the other. In this work, using the first-principles quantum mechanical calculations, we investigate the effect of twist angle (θ) on the optical properties of titanium carbide (Ti3C2) MXene homobilayer nanoflakes. It is found that θ variations trigger interfacial electron transfers, validated from electron difference density analysis. More interestingly, Ti3C2 MXene homobilayer nanoflakes sustain robust infrared plasmons for all θ values, validated from the dipole-like oscillation of the induced electron density and the strong collectivity of the optical transitions. Also, we reveal interesting modulations of electric field confinement with respect to θ variations. Our results illustrate that θ constitutes an additional tuning knob for modulating the optical properties of MXenes.

New Results from the Spectral Observations of Solar Coronal Type II Radio Bursts

Abstract We carried out a statistical study of twenty-six type II radio bursts from the Sun observed with the Gauribidanur Low-frequency Solar Spectrograph in the frequency range 85–35 MHz during the period 2009–2019. Our results indicate that the average instantaneous bandwidth of the type II bursts in the above frequency range correlates with the angular width of the associated coronal mass ejections (CMEs). The correlation coefficient is ≈71%. This independently indicates that the coronal type II bursts reported in this work are mostly due to shocks driven by the CMEs. Moreover, it also suggests that the instantaneous bandwidth of the bursts could be due to electron acceleration (leading to type II bursts) occurring simultaneously at multiple locations of differing electron densities (i.e., plasma frequencies) along the shock surrounding the CME.

Investigation on the interface characteristic between WC(001) and diamond(111) by first-principles calculation

In this paper, the WC(001)/diamond(111) interface properties are calculated by the first principles considering the C- and W-terminated WC(001) surfaces, the WC(001)/diamond(111) calculation model is established. Analyze its adhesion work, interface energy, electron density difference, Mulliken population and density of states. The results showed that: from the interface energy, the surface energy of the C-terminated WC(001) structure is always higher than that of the W-terminated, which the W-terminated WC(001) structure is more stable than the C-terminated; from the adhesion work, the C-terminated WC(001)/diamond (111) surface configuration has the larger adhesion work and smaller interface energy; according to the electron density difference, Mulliken population and density of states, the CC bond of the C-terminated WC (001) interface is attributed to the hybridization of C-2p orbital and has a strong covalent bond, while the interface WC bond of the W-terminated interface comes from the hybridization of C-2p orbital and W-3d orbital and has a mixed bond (ionic bond and weak covalent bond).

Selective sensing of NH3 and CH2O molecules by novel 2D porous hexagonal boron oxide (B3O3) monolayer: A DFT approach

DFT computations are used for the adsorption of different gasses like NH3, CO, CO2, CH2O, PH3, H2S, H2O, SO2, NO, NO2, HCN, CH4, COCl2, N2O, NF3, NCl3, O3 and H2 onto porous hexagonal boron oxide monolayer (ph-BOL) to explore its potentials for gas sensing. The negative adsorption energies (-1.00 to -10.13 kcal/mol) for all complexes demonstrates that adsorption of various gas molecules over the ph-BOL monolayer is energetically favorable and exothermic process. The energy gap (Egap) results shows that highest decreased in Egap occurred for NH3@ph-BOL (1.671 eV) and CH2O@ph-BOL (1.875 eV), respectively. This indicates the selectivity and sensitivity of ph-BOL toward these analytes. The non-covalent interaction, reduced density gradient and atom in molecules analysis for NH3@ph-BOL and CH2O@ph-BOL justified the presence of non-covalent electrostatic interactions and van der Waals interactions in the resulted complexes. The electron density difference and natural bond orbital analyses reveals that there is significant charge shifting occurred between the analytes and ph-BOL monolayer. The SAPT0 energy components values are well agreed with the interaction energies. Moreover, the AIMD study reveal that the complexes are stable even at 500 K. The recovery time of ph-BOL sensor for NH3 molecule and CH2O molecule detection is estimated to be 1.6 × 10−6 s and 8.1 × 10−7 s, respectively, suggested that the regeneration of sensor is so rapid at room temperature. Thus, ph-BOL is a potent candidate for NH3 and CH2O molecules detection and will promote experimetnalist to designed an efficent gas sensor.

Spin-Orbit Coupling and Spin-Polarized Electronic Structures of Janus Vanadium-Dichalcogenide Monolayers: First-Principles Calculations.

Phonon and spintronic structures of monolayered Janus vanadium-dichalcogenide compounds are calculated by the first-principles schemes of pseudopotential plane-wave based on spin-density functional theory, to study dynamic structural stability and electronic spin-splitting due to spin-orbit coupling (SOC) and spin polarization. Geometry optimizations and phonon-dispersion spectra demonstrate that vanadium-dichalcogenide monolayers possess a high enough cohesive energy, while VSTe and VTe2 monolayers specially possess a relatively higher in-plane elastic coefficient and represent a dynamically stable structure without any virtual frequency of atomic vibration modes. Atomic population charges and electron density differences demonstrate that V–Te covalent bonds cause a high electrostatic potential gradient perpendicular to layer-plane internal VSTe and VSeTe monolayers. The spin polarization of vanadium 3d-orbital component causes a pronounced energetic spin-splitting of electronic-states near the Fermi level, leading to a semimetal band-structure and increasing optoelectronic band-gap. Rashba spin-splitting around G point in Brillouin zone can be specifically introduced into Janus VSeTe monolayer by strong chalcogen SOC together with a high intrinsic electric field (potential gradient) perpendicular to layer-plane. The vertical splitting of band-edge at K point can be enhanced by a stronger SOC of the chalcogen elements with larger atom numbers for constituting Janus V-dichalcogenide monolayers. The collinear spin-polarization causes the band-edge spin-splitting across Fermi level and leads to a ferrimagnetic order in layer-plane between V and chalcogen cations with higher α and β spin densities, respectively, which accounts for a large net spin as manifested more apparently in VSeTe monolayer. In a conclusion for Janus vanadium-dichalcogenide monolayers, the significant Rashba splitting with an enhanced K-point vertical splitting can be effectively introduced by a strong SOC in VSeTe monolayer, which simultaneously represents the largest net spin of 1.64 (ћ/2) per unit cell. The present study provides a normative scheme for first-principles electronic structure calculations of spintronic low-dimensional materials, and suggests a prospective extension of two-dimensional compound materials applied to spintronics.

Methylcyclohexane and methyl methacrylate sensing studies using γ-arsenene nanoribbon – A first-principles investigation

In the recent decade, elemental low-dimensional materials are focussed among researchers. In the current work, we used zigzag γ-arsenene nanoribbon (γ-AsNR) as a chemical sensor. Initially, γ-AsNR geometrical stability is established by formation energy. The band structure calculation reveals a bandgap of 1.127 eV. Besides, methylcyclohexane and methyl methacrylate are allowed to get adsorbed on γ-AsNR. The scope of adsorption energy (−0.019 eV to −0.738 eV) shows that methylcyclohexane and methyl methacrylate are physisorbed on γ-AsNR. Also, the changes in the electronic properties of γ-AsNR are observed by the results obtained based on electron density difference, charge transfer, band structure, and PDOS spectrum. Furthermore, the I-V characteristics show the disparity in the current owing to the adsorption of methylcyclohexane and methyl methacrylate. The findings support that γ-AsNR can be deployed for the detection of methylcyclohexane and methyl methacrylate emitted from the rubber footwear industries.

On the origin of the inverted singlet-triplet gap of the 5th generation light-emitting molecules.

Excitation energies of the lowest singlet and triplet state of molecules whose first excited singlet state lies energetically below the first triplet state have been studied computationally at (time-dependent) density functional theory, coupled-cluster, and second-order multiconfiguration perturbation theory levels. The calculations at the ab initio levels show that the singlet-triplet gap is inverted as compared to the one expected from Hund's rule, whereas all density functionals yield the triplet state as the lowest excited state. Double excitations responsible for the inverted singlet-triplet gap have been identified. Employing the spin-flip and ΔSCF methods, singlet-triplet inversion was obtained at the density functional theory level for some of the studied molecules.

The remarkable performance of a single iridium atom supported on hematite for methane activation: a density functional theory study.

Methane is the major component of natural gas, and it significantly contributes to global warming. In this study, we investigated methane activation on the α-Fe2O3(110) surface and M/α-Fe2O3(110) surfaces (M = Ag, Ir, Cu, or Co) using the density-functional theory (DFT) + U method. Our study shows that the Ir/α-Fe2O3(110) surface is a more effective catalyst for C–H bond activation than other catalyst surfaces. We have applied electron density difference (EDD), density of states (DOS), and Bader charge calculations to confirm the cooperative CH⋯O and agostic interactions between CH4 and the Ir/α-Fe2O3(110) surface. To further modify the reactivity of the Ir/α-Fe2O3(110) surface towards methane activation, we conducted a study of the effect of oxygen vacancy (OV) on C–H activation and CH4 dehydrogenation. In the comparison of pristine α-Fe2O3(110), Ir/α-Fe2O3(110), and Ir/α-Fe2O3(110)–OV surfaces, the Ir/α-Fe2O3(110)–OV surface is the best in terms of CH4 adsorption energy and C–H bond elongation, whereas the Ir/α-Fe2O3(110) surface catalyst has the lowest C–H bond activation barrier for the CH4 molecule. The calculations indicate that the Ir/α-Fe2O3(110)–OV surface could be a candidate catalyst for CH4 dehydrogenation reactions.

Structural and electronic properties of β-MnO2 nanoclusters

Pyrolusite (β-MnO2) was investigated for potential use in energy storage devices due to its promising properties for cathode materials in rechargeable lithium-ion batteries. A combination of evolutionary algorithm search techniques and density functional theory techniques were used to determine the structural stabilities of the β-MnO2 nanoclusters on the energy landscape. However, pyrolusite suffers from some structural defects and impurities that hinder its optimal use. The predicted order of stability for the MnO2 nanoclusters correlates with isostructural TiO2. There is an improvement in the stability and electrical conductivity of the nanoclusters as compared to their β-MnO2 bulk counterparts. The cobalt doped nanoclusters showed a preference toward circular compact bonding patterns. The band-gap energy revealed that the nanoclusters have a metallic characteristic behaviour with narrow band gap energies, indicating their good conductive properties. Cobalt doping was shown to improve the structural properties of the nanoclusters based on the decreased bond lengths and more spherical bond angles. Moreover, it also succeeded in improving the conductivity of the nanoclusters based on the reduced Mulliken and Hirshfeld partial charges. The electronic charge density differences of the cobalt doped β-MnO2 nanoclusters displayed a prevalence of the weaker ionic bonding instead of the preferred stronger covalent bonding. This shows the limited effectiveness of cobalt as a dopant.

Adsorption of gas molecules on buckled GaAs monolayer: a first-principles study.

The design of sensitive and selective gas sensors can be significantly simplified if materials that are intrinsically selective to target gas molecules can be identified. In recent years, monolayers consisting of group III–V elements have been identified as promising gas sensing materials. In this article, we investigate gas adsorption properties of buckled GaAs monolayer using first-principles calculations within the framework of density functional theory. We examine the adsorption energy, adsorption distance, charge transfer, and electron density difference to study the strength and nature of adsorption. We calculate the change in band structure, work function, conductivity, density of states, and optical reflectivity for analyzing its prospect as work function-based, chemiresistive, optical, and magnetic gas sensor applications. In this regard, we considered the adsorption of ten gas molecules, namely NH3, NO2, NO, CH4, H2, CO, SO2, HCN, H2S, and CO2, and noticed that GaAs monolayer is responsive to NO, NO2, NH3, and SO2 only. Specifically, NH3, SO2 and NO2 chemisorb on the GaAs monolayer and change the work function by more than 5%. While both NO and NO2 are found to be responsive in the far-infrared (FIR) range, NO shows better spin-splitting property and a significant change in conductivity. Moreover, the recovery time at room temperature for NO is observed to be in the sub-millisecond range suggesting selective and sensitive NO response in GaAs monolayer.

Enhanced dielectric properties and chemical bond characteristics of ZnNb2O6 ceramics due to zinc oxide doping

Regarding advanced 5G mobile communication, microwave dielectric ceramics are considered as the most potential materials to develop new-generation base station resonators. Herein, ZnNb 2 O 6 ceramics with ε r of approximately 24 have been prepared using the solid-state reaction method, with tailored extra ZnO of x mol% (x = 1, 2 and 3). We have for the first time applied the P-V-L chemical bond theory to investigate ZnNb 2 O 6 ceramics with ZnO doping, by exploring the relationship of dielectric properties and chemical bond characteristics. Particularly, the Raman spectra demonstrates that the full width at half maximum of υ 1 (A g ) vibration mode can exhibit significant correlation with the quality factor ( Q × f ). To further support the experimental study, we have also conducted the first-principle calculation of electron density difference via CASTEP package, which further confirms the change of temperature coefficient of resonance frequency ( τ f ). Our newly designed ZnNb 2 O 6 ceramics doped with 1 mol% ZnO exhibit excellent dielectric properties, i.e., ε r = 23.74, Q × f = 102,824 GHz and τ f = −55.38 ppm/°C, which demonstrates great potential to construct miniaturized 5G base station with advanced ceramic dielectrics.

Enhanced Methane Sensing Properties of SnO2 Nanoflowers With Ag Doping: A Combined Experimental and First-Principle Study

Ag-doped SnO2 nanoflowers (NFs) are prepared via a simple one-step hydrothermal method and subsequent Ag-doping using AgNO3 solution as precursors. The crystal structure, morphology and elements of the Ag-doped SnO2 NFs are investigated in detail. The maximal response of Ag-doped SnO2 sensor with 1.0 wt.% Ag doping to 500 ppm methane is increased by a factor of 1.6 as compared to that of pure SnO2 sensor. The long-term stability and the selectivity of the SnO2-based sensor are improved by Ag-doping as well. Meanwhile, pre-adsorption of oxygen molecules on the surface of the SnO2-based substrate is studied in-depth with the method of first-principle calculation. Ag doping lowers adsorption energy for all the adsorption sites, demonstrating a higher adsorption stability of chemisorbed oxygen ions on the Ag-doped SnO2 as compared to the pristine phase. More electron transfer from the semiconductor to the chemisorbed oxygen is revealed by the Mulliken charge analysis and the electron density difference for the Ag-doped SnO2, explaining the better sensitivity of gas sensors based on Ag-doped SnO2 nanostructures as compared to its undoped counterpart.

DFT study of alkali and alkaline earth metal-doped benzocryptand with remarkable NLO properties.

Strategies for designing remarkable nonlinear optical materials using excess electron compounds are well recognized in literature to enhance the applications of these compounds in nonlinear optics. In this study, density functional theory simulations are performed to study alkali and alkaline earth metal-doped benzocryptand using the B3LYP/6-31G+(d, p) level of theory. Vertical ionization energies (VIEs), reactivity parameters, interaction energies, and binding energies exposed the thermodynamic stability of these complexes. FMO analysis revealed that HOMO is located on alkali metals having polarized electrons, which are easy to excite. The doping strategy enhanced the charge transfer with low bandgap energy in the range of 0.68–2.23 eV, which is lower than that of the surface BC (5.50 eV). Also, the lower transition energies and higher oscillator strength indicate that these complexes exhibit excellent electronic and optical properties. Non-covalent interaction analysis suggested the presence of van der Waals interactions between dopants and surface. IR analysis provided information about the frequencies of stretching vibrations present in the complexes due to different bonds. UV-vis analysis revealed that all the newly designed excess electron complexes are transparent in the UV region and possessed maximum absorption in the visible and NIR region, ranging from 753.6 to 2150 nm, which is higher than the surface (244 nm). Thus, these complexes have a potential for high-performance NLO materials in the applications of optics. Natural bond orbital analysis (NBO), transition density matrix (TDM), electron density difference map (EDDM), and density of state (DOS) analyses were also performed to study the charge transfer properties. Moreover, these complexes possessed remarkable optoelectronic properties due to a significant increase in the isotropic linear polarizability (αiso) in the range of 629.59–1423.23 au. Further, these systems demonstrated an extraordinary large total first hyperpolarizability (βtl) in the range of 3695.55–910 706.43 au. The rationalization of hyperpolarizability by the two-level model reflected a noteworthy increase in βtl because of low transition energies (ΔE) and high transition dipole moment (Δμ). Thus, our results showed that alkali and alkaline earth metal-doped BC might be a competitor for efficient nonlinear optical properties with practical applications in the area of optoelectronics.

Theoretical Investigations of State Specific Hydrogen Atom Transfer in 8-Formyl-7-hydroxy-4-methylcoumarin

Excited state intramolecular hydrogen transfer (ESIHT) reaction of 8-formyl-7-hydroxy-4-methyl coumarin (FC) in its pure and hydrated state FC-(H2O)4 (FCH) has been studied by implementing state specific time dependent density functional theory (SS-TDDFT) along with the effective fragment potential (EFP1) method for solvation with discrete water molecules. The intramolecular hydrogen bond formed between hydroxyl hydrogen (H18) and formyl oxygen (O15) and intermolecular hydrogen bonds formed due to microsolvation were explored. The studies of electrostatic potential, natural charge analysis, difference electron density map and UV-Vis spectra of both FC and FCH molecules establish the intramolecular charge transfer (ICT) states of the molecules. The vertical excitation from S0 to S1 state causes the transfer of hydroxyl hydrogen to formyl oxygen and from S1 to S3 causes the transfer of the hydrogen atom back to hydroxyl oxygen. Potential energy surface scans along intramolecular hydrogen bonding at the ground and excited states confirm the state specific ESIHT reaction in both FC and FCH molecules.

A comparison of CO oxidation on cleaned ZnO $$(10\overline{1 }0)$$ surface and defective ZnO $$(10\overline{1 }0)$$ surface using density functional theory studies

In this work, we employed continuously the DFT calculations to study CO oxidation reaction on the defective ZnO [Formula: see text] surface. The oxygen (O) atom was removed from cleaned surface ZnO [Formula: see text] (CS-ZnO) to form the defective ZnO [Formula: see text] surface (DS-ZnO), which contained an O vacancy defect. Hereafter, the formation of oxygen vacancy was found to increase the adsorption abilities of O2 and CO on DS-ZnO, in comparison to those on CS-ZnO. Many steps of elementary reactions including O2 and CO adsorption, reacting between CO and O to form CO2, and CO2 desorption on DS-ZnO were investigated and calculated in terms of the configurations, activation energy, and reaction energy, to which the reaction pathway of CO oxidation has been found. Based on this pathway, the calculation results of the rate controlling step of 0.84eV corresponding to the exothermic reaction energy of 4.11eV on DS-ZnO indicated that the CO oxidation on DS-ZnO was more thermodynamically favorable and less kinetically desirable than that on CS-ZnO. In addition, the natural bonds of O2 and CO adsorptions on DS-ZnO were also analyzed by the partial density of state (PDOS) and the electron density difference (EDD) contour plots.

New Charge Transfer Complex between 4-Dimethylaminopyridine and DDQ: Synthesis, Spectroscopic Characterization, DNA Binding Analysis, and Density Functional Theory (DFT)/Time-Dependent DFT/Natural Transition Orbital Studies.

A combined experimental and theoretical study of the electron donor 4-dimethylaminopyridine (4-DMAP) with the electron acceptor 2, 3-dichloro-5, 6-dicyano-p-benzoquinone (DDQ) has been made in acetonitrile (ACN) and methanol (MeOH) media at room temperature. The stoichiometry proportion of the charge transfer (CT) complex was determined using Job’s and photometric titration methods and found to be 1:1. The association constant (KCT), molar absorptivity (ε), and spectroscopic physical parameters were used to know the stability of the CT complex. The CT complex shows maximum stability in a high-polar solvent (ACN) compared to a less-polar solvent (MeOH). The prepared complex was characterized by Fourier transform infrared, NMR, powder X-ray diffraction, and scanning electron microscopy–energy-dispersive X-ray analysis. The nature of DNA binding ability of the complex was probed using UV–visible spectroscopy, and the binding mode of the CT complex is intercalative. The intrinsic binding constant (Kb) value is 1.8 × 106 M–1. It reveals a primary indication for developing a pharmaceutical drug in the future due to its high binding affinity with the CT complex. The theoretical study was carried out by density functional theory (DFT), and the basis set is wB97XD/6-31G(d,p), with gas-phase and PCM analysis, which supports experimental results. Natural atomic charges, state dipole moments, electron density difference maps, reactivity parameters, and FMO surfaces were also evaluated. The MEP maps indicate the electrophilic nature of DDQ and the nucleophilic nature of 4-DMAP. The electronic spectrum computed using time-dependent DFT (TD-DFT) via a polarizable continuum salvation approach, PCM/TD-DFT, along with natural transition orbital analysis is fully correlated with the experimental outcomes.

Non covalent interactions analysis and spectroscopic characterization combined with molecular docking study of N′-(4-Methoxybenzylidene)-5-phenyl-1H-pyrazole-3-carbohydrazide

The structure, spectroscopic features, and pharmaceutical effect of N′-(4-Methoxybenzylidene)-5-phenyl-1H-pyrazole-3-carbohydrazide (MBPPC) has been studied by DFT modeling, X-ray diffraction, FT-IR, 1H and 13C NMR, and molecular docking investigation. Molecular structure analysis was carried out using DFT calculation. Then, the low RMSD value indicates the good agreement between calculated and observed data. In order to understand the electronic charge transition, the electron difference density technical was performed. 1H and 13C NMR spectra were recorded in the region of 0–15 and 5–225 ppm, respectively, and FT-IR spectrum of MBPPC was obtained from 4000 to 500 cm−1. Electronic structure characteristics were achieved at the level of B3LYP/6-311+G(2d,p). Inter and intramolecular interactions are discussed by topological (AIM, RDG) and Hirshfeld surface analyses. TD-DFT calculations were conducted to reveal molecular orbital based reactivity characteristics and nonlinear optical features up to third order. In addition, thanks to the broad biological field of compounds based on pyrazole and hydrazone groups, molecular docking of the title compound was carried out to study their clinical activities. Docking simulation shows the potential of MBPPC against Vibrio cholera, Mycobacterium tuberculosis, and estrogen receptor.

Interactions between typical functional groups of soil organic matter and mica (001) surface: A DFT study

In this study, the interactions between mica and soil organic matters (SOMs) are investigated by the density functional theory (DFT) calculations. The adsorption behavior of 12 typical SOMs on X-mica (X = Na, K or Cs) are systematically studied based on 96 interaction models. Three typical adsorption configurations for each SOMs are considered. The interactions details are explored by the further analysis of Eads, isosurface of the electron density difference, electrostatic potential, and the NCI plot. The Eads between mica and SOMs is proved strong for Na-mica due to the smallest ionic radius of Na. For example, it is 0.78 eV. 0.48 eV, 0.27 eV for CH4O/Na-mica, K-mica and Cs-mica respectively. The SOMS with functional groups, including -OH, -COOH, -C=O, -SO- and pyridine, own the largest Eads with X-mica, while the SOMs containing benzene ring have a stronger Eads than that of the hydrocarbon alkyl. The Light Gradient Boosting Machine (LGBM) method is used to analyses the importance of the different factors in regulating the SOMs/X-mica interactions. The electronegativity and ionic radius of cations in X-mica are found to be the most important factors, followed by the configurations and functional groups of SOMs. This work provides a detailed DFT based method for SOM/minerals interactions which provide deep understanding on effects of SOM in the mineral morphogenetic transformation.

Study of the conformations and tautomerisation pathway in (Z)-4-(hydroxypropyl) isochroman-1, 3‑dione: Analysis through energy, vibrational signatures and hardness profiles

• Tautomerisation pathways in (Z)−4-(hydroxypropyl) isochroman-1, 3‑dione are identified. • Several conformers are identified for both tautomers. • The role of substituents in the tautomerisation process in AICs is explored. • Maximum hardness principle and minimum electrophilicity principles are studied extensively. • Crystal structure data and FT-IR spectra corroborate our computational results. This article thoroughly probes the interconnectivity between all the conformations/configurations and the tautomerisation pathway in ( Z )-4-(hydroxypropyl) isochroman-1, 3‑dione (PIC) both in S 0 and S 1 states. The crystal structure data corroborates our computational results done with DFT and MP2 methods utilizing various basis sets. The tautomers are separated by 2.5 ± 0.3 kcal/mole in S 0 . The comparison of potential energy curves of close analogues shows that the inertia of the substituted functional groups (here C 2 H 5 ) plays a significant role in the proton transfer process. Intrinsic Reaction Coordinate (IRC) and frequency computations locate the transitions states (TS) in S 0 .The transition region in the tautomerisation pathway is clearly marked with the evaluation of reaction force and force constants. The excited state computations with CIS and TDDFT methods show some inconsistency regarding the relative energies of the tautomers. Maximum hardness principle (MHP) and minimum electrophilicity principle (MEP) are applied in this case. The frontier molecular orbitals in both the tautomers confirms the S 0 →S 1 transition as π-π* in nature. The plot of the difference in electron densities between S 0 and S 1 states in both the tautomers hint at the photophysical processes occurring on excitation. The vibrational signatures are detected and characteristic spectral lines and their relative intensities are discussed. The strong C = O stretching vibration appearing at 1740 cm −1 in the observed FT-IR spectra is computed to be at 1757 cm −1 confirming our assertion of PIC as a minimum energy tautomer.

Oxidation of dobutamine and dopamine by horseradish peroxidase

Dobutamine and dopamine have been previously shown to interfere with enzymatic diagnostic tests and different mechanisms responsible for this effect have been postulated. We have studied the oxidation by horseradish peroxidase (HRP) of dopamine, dobutamine, and its analog with the phenol group blocked by methylation. Oxidation of dobutamine was much faster than dopamine, as reported before, whereas the methylated analog of dobutamine was oxidized at intermediate rate. This demonstrates that the phenol group in dobutamine is oxidized preferentially by HRP and acts as an electron-transfer mediator in oxidation of the catechol group. Different oxidation rates of catechol groups in dopamine and the methylated analog of dobutamine can only be explained by steric and polar/hydrophobic factors, not by differences in electron densities in aromatic rings, as postulated before for alkylcatechols. Their Km values are primarily responsible for this difference. Molecular docking of dobutamine and its methylated analog to the active site of HRP showed that for both compounds orientations with the catechol group directed towards the heme were slightly favored. However, the distance between the heme iron and the 4-OH group in these orientations was bigger than the distance between the iron ion and the oxygen atom of the phenol/methoxyphenyl groups, when they were placed in the active site. This explains preferential oxidation of the phenol group in dobutamine by HRP. Distance of the oxidizable group (phenol or catechol) to the heme cofactor was determined by interaction of the functional groups in the side chain acting as hydrogen bond donors (amino or hydroxyl group) with the peptide backbone of the Ala136-Pro139 region near the entrance to the active site.