Nucleophilic Reactive Sites Research Articles

Spectroscopic investigations, quantum chemical, molecular docking and drug likeness studies of t-butyl-3,4,5-trimethyl-2-pyrrole carboxylate

The geometrical structure of t‑butyl‑3,4,5-trimethyl-2-pyrrole carboxylate (TBTPC) was computed by DFT calculations using the B3LYP method and the 6–311++G(d,p) basis set using the Gaussian 09 software. Vibrational wavenumbers were assigned using the VEDA 4.0 program. Natural bond orbital (NBO) analysis predicts the hyper-conjugative interaction and stabilization interaction of the TBTPC compound. HOMO, LUMO, and other molecular properties were calculated. The density of states (DOS) spectrum was also predicted. The ultraviolet-visible spectrum has been experimentally and theoretically validated and simulated. The electrophilic and nucleophilic reactive sites of the TBTPC molecule were determined using the Fukui function calculation and the molecular electrostatic potential (MEP) surface. Molecular docking analysis of the TBTPC ligand helps to investigate its inhibitory nature against Lysine-specific demethylase 1A (LSD1), α-amylase, BCL-1, and monoamine oxidase (MAO). From the molecular docking results, TBTPC has promising inhibition activity against gastric cancer.

Spectroscopic (FT-IR, FT-Raman, UV-vis and NMR) Investigation, Molecular Structure, Docking and Chemical Reactivity Elucidation of Antifungal Drug Tioconazole

Tioconazole (TCZ), an antifungal medication of the imidazole class has been investigated by experimental FT-IR, FT-Raman, UV–vis and NMR spectroscopy. The optimized molecular geometry and vibrational wave numbers were calculated by DFT/B3LYP/6–311++G(d, p). The detailed vibrational assignments were made by Potential Energy Distribution. The electrophilic and nucleophilic reactive sites were studied using MEP surface. The NBO analysis has been done to describe various hyper-conjugative interactions responsible for the stabilization of the molecule. The 1H NMR chemical shifts were calculated by GIAO method and these matched well with the experimental results, indicating the accuracy of calculation method. TD-DFT calculations have been performed to determine several electronic properties like excitation energies, oscillator strengths, wavelengths and HOMO-LUMO energies. The lipophilicity and aqueous solubility of the compound have been predicted by ALOGPS method. The log P value for TCZ approaches that this compound is less recommended for oral administration. Docking studies have been made to predict the binding orientation, affinity and antifungal activity of the compound.

Synthesis, crystal structure, Hirshfeld surface, biological evaluation, DFT and molecular docking studies of unsymmetrical 3-allyl-5-ethyl-1,4-dihydro-2,6-dimethyl-4-phenyl pyridine-3,5-dicarboxylate derivatives

By employing ceric ammonium nitrate as an effective catalyst in the one-pot four-component Hantzsch reaction, a new series of unsymmetrical 1, 4-dihydropyridines were synthesised. The synthesised compounds were identified using spectral methods and the structure of the compound 3-allyl-5-ethyl-4(3-fluorophenyl)-1,4-dihydro-2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate (5f) was confirmed by single crystal XRD analysis. Optimised bond parameters were calculated using DFT /B3LYP/6-311++(d,p) basis set and also compared with single crystal XRD data. The calculated hyperpolarisability may be attractive for further studies on the material's NLO properties. NBO analysis was used to examine the stability of the molecule resulting from the hyper conjugative interaction and charge delocalisation. The eventual charge transfer interactions that occur within the molecule are explained by HOMO–LUMO analysis. MEP reveals the electrophilic and nucleophilic reactive sites of the molecule. According to the Hirshfeld surface analysis, the intermolecular stabilisation in the crystal is mostly caused by hydrogen bond interactions between H … H, C–H … C, C–H … N, and particularly C–H–O. It was discovered that the chemicals 5d and 5i were effective antimicrobials activity. Molecular docking studies were employed to identify the primary binding site of piperidine to receptors. These results indicate that the target molecules may be considered for further drug design endeavours. Research highlights Compound synthesised by the one-pot four component Hantzsch method. Single crystal of 3-allyl-5-ethyl-4(3-fluorophenyl)−1,4-dihydro-2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate(5f) was obtained by slow evaporation method. Hirshfeld analysis and fingerprint plots were performed to confirm the intermolecular interactions in the crystal structure. The stability and reactivities of 5f was achieved from DFT calculation. The antimicrobial studies showed that compounds have promising antimicrobial properties. Molecular docking studies were employed to identify the primary binding site of piperidine to receptors.

Biosensitivity and Theoretical Electronic Structure Investigations on 3-(2-Hydroxyphenyl)-2-iminothiazolidin-4-one and Its Zn2+ and Cd2+ Metal Complexes

A two-step cyclocondensation reaction has been carried out using 2-aminophenol with 2-chloroacetyl chloride to produce o-hydroxyphenyl chloroacetamide followed by treatment with KSCN in CH3COCH3 to produce the heterocyclic ligand 3-(2-hydroxyphenyl)-2-iminothiazolidin-4-one. The Zn2+ and Cd2+ complexes with a metal : ligands ratio of 1 : 4 were synthesized in ethanol using respective metal precursors with the title ligand. Antimicrobial activities of the ligand and its complexes were checked against some bacterial and fungal strains. The result evidenced better bioactive performance of the metal complex (though lower than the standard drug) than the free ligand against Escherichia coli, Staphylococcus aureus, and Salmonella typhi bacteria, as well as Fusarium oxysporum and Aspergillus niger fungal strains. Theoretical investigations on ligand and metal complexes help to infer the electronic structure behavior of them. Molecular geometry and bond order analysis provides detailed information on the nature of chemical structure and bonding. Molecular Electrostatic Potential (MEP) and atomic charge analysis claims evidence on charge distribution and electrophilic, nucleophilic reactive sites. Natural bond orbital analysis provides second-order perturbed stabilization interactions, orbital population, and their energies. Other theoretical properties such as hardness, softness, electron affinities, and ionization potential were derived and discussed in detail.

β‐Ketoesters: An Overview and It's Applications via Transesterification

AbstractThe transesterification process has attained more significant importance in synthesizing a variety of commercial and non‐commercial β‐ketoesters using alcohols and variety of acetoacetates. β‐Ketoesters have been considered essential synthons in synthetic organic chemistry because of electrophilic and nucleophilic reactive sites and are used as key intermediates in synthesizing the diversity of complex drug molecules. This review provides an overview of the synthesis of β‐ketoesters, various reaction mechanisms, substrate scope, and diverse applications in the bio‐diesel industry and the pharmaceutical industry to synthesize novel drug molecules via transesterification transformation.

Spectroscopic, quantum chemical, molecular docking and molecular dynamics investigations of hydroxylic indole-3-pyruvic acid: a potent candidate for nonlinear optical applications and Alzheimer’s drug

In this paper, a complete theoretical investigation of hydroxylic indole-3-pyruvic acid (HIPyA) molecule was performed using the DFT quantum chemical, molecular docking and molecular dynamics calculations. The conformational analysis of HIPyA molecule was carried out using density functional theory quantum chemical calculations. The most stable structure of the studied molecule was predicted by means of DFT/B3LYP method with cc-pVTZ basis set. The simulated vibrational frequencies were assigned and proved to be in agreement with the available experimental FT-IR data. The effects of gas phase and solvents on UV-visible spectra of HIPyA molecule were simulated using TD-DFT/B3LYP method with cc-pVTZ basis set. The analysis of the density of states spectrum validates the frontier molecular orbitals results, which reveals the charge transfer interaction in HIPyA molecule. The molecular electrostatic potential surface confirms the electrophilic and nucleophilic reactive sites of the studied molecule. The natural bond orbital analysis evidences the bioactivity of the studied molecule. The obtained first order hyperpolarizability value is 33.596 times greater than urea, which confirms the nonlinear optical activity of HIPyA molecule. The molecular docking analysis reveals that the studied molecule under interest can act as a potent inhibitor against the amyloid β-protein (Aβ) enzyme, which causes the Alzheimer’s disease. The molecular dynamics analysis confirms the reliability of the docking results. Communicated by Ramaswamy H. Sarma

Spectroscopic, quantum chemical and molecular docking studies of 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone: A potent anti-Alzheimer's drug

Abstract The potential energy surface scan of the Irgacure-2959 molecule was performed and most stable molecular structure of the molecule was predicted. FT-IR and FT-Raman spectra of the molecule were recorded. The calculated and observed wavenumbers were assigned. The UV-Vis spectrum of the molecule shows the possible electronic transitions of the molecule. The frontier molecular orbitals analysis indicates chemical reactivity and bioactivity of the molecule. The molecular electrostatic potential surface reveals the electrophilic and nucleophilic reactive sites of the molecule. The heat capacity, entropy and enthalpy values of the molecule were calculated in the temperature range from 50 to 500 K. The natural bond orbital analysis authenticates the stability of the molecule. Moreover, the molecular docking analysis reveals that the title molecule can act as a potent inhibitor against the acetylcholinesterase enzyme. Hence, the present investigation paves the way for developing effective drugs in the treatment of Alzheimer's disease.

Evaluation of conformational and spectral behavior and prediction of biological activity and chemical reactivity descriptors of Levosimendan

The conformational behavior and structural stability of Levosimendan (LVM) is investigated by density functional theory. The spectroscopic properties are analyzed using 1H NMR, FT-IR and Raman techniques. The 1H NMR chemical shifts of the molecule are calculated by Gauge independent atomic orbital method and compared with the experimental results. The vibrational spectra have been interpreted with the aid of normal coordinate analysis. A comparison of normal modes of both the conformers has been made in terms of change in wavenumbers, intensities and potential energy distribution. Lipophilicity and aqueous solubility have been calculated to estimate the absorption and distribution properties of drug which dictate the success or failure rate of drug discovery and development. ‘Prediction of Activity Spectra for Substances’ analysis has been done to predict the possible biological activities of LVM. Molecular docking has been performed to investigate the potential of the LVM molecule to bind with human cardiac troponin C receptor. Several chemical reactivity descriptors such as ionization energy, electron affinity, chemical potential, global hardness and electrophilicity have been evaluated to understand the molecular properties. The possible electrophilic and nucleophilic reactive sites of the molecule are studied using molecular electrostatic potential surface and Fukui function.

Investigations on the structural, vibrational, computational, and molecular docking studies on potential antidiabetic chemical agent Diosmetin.

In the present study, the harmonic vibrational frequencies of Diosmetin(5, 7 dihydroxy-2(3-hydroxy-4 methoxyphenyl) chromen-4-one) have been investigated by both experimental (FTIR and FT-Raman) and theoretical (HF and DFT/B3LYP) method. The calculated harmonic vibrational frequencies were compared with experimental data. A detailed interpretation of the vibrational spectra of the compound has been made on the basis of the calculated potential energy distribution (PED). The 1 H, 13 C NMR chemical shifts and TD-DFT calculations of the molecule were calculated and compared with the available experimental observations. A study on the molecular electrostatic potential surface (MEP) of the compound was performed, and the electrophilic and nucleophilic reactive sites were identified. Furthermore, the inhibition effect of compound against aldose reductase enzyme has been analyzed by molecular docking method, and the results were compared with the standard drug. The docking study indicates that the investigated compound shows better inhibitory activity toward aldose reductase enzyme than the standard drug, and hence this study may be supportive in the field of drug discovery to design more potential antidiabetic agents.

Growth and characterization of 2-bromo-5-methoxybenzohydrazide derivative—a nonlinear optical single crystal

A nonlinear optically active single crystal of 2-bromo-5-methoxybenzohydrazide derivative has been synthesized using standard method. Using solution growth technique the compound obtained was successfully grown as single crystals. FT-IR spectrum determines the different functional groups of the newly synthesized molecule. The single crystal x-ray diffraction analysis confirms the molecular structure and characterizes the crystal as non centrosymmetric as it crystallized in an orthorhombic crystal system having P212121 space group and also calculates the lattice parameters. The optical study reveals the maximum absorption at 289 nm wavelength and wide transparency of the crystal was observed near visible region. Green light emitting property of the crystal was determined by photoluminescence study. Thermo gravimetric (TG) and Differential thermal (DT) analysis discloses the thermal strength of the material. 0.41 SHG efficiency of title molecule was estimated by powder SHG test. Molecular Hirshfeld surface analysis of the crystal structure reveals the intermolecular interaction exhibited by various inter contacts and the corresponding 2-dimensional finger print analysis further estimates the individual contribution from each atom. Density functional theory (DFT) analysis calculates the energy gap of the molecule using HOMO-LUMO values. Various global chemical reactivity descriptor (GCRD) parameters have been calculated. Using molecular electrostatic potential (MEP) map the suitable sites for electrophilic and nucleophilic reactive sites were predicted. The NLO properties such as first order molecular hyperpolarizabilities (β), polarizability (α), molecular dipole moment (μ) and anisotropy polarizability (Δα) was theoretically estimated using DFT.

Predicting Hydration Reactivity of Cu-Doped Clinker Crystals by Capturing Electronic Structure Modification

Chemical doping is a promising scheme for sustainable development of green and economical cement manufacturing related to global energy and environmental concerns. Understanding and controlling doping effect on reactive cement components is the prerequisite. This study aims to propose effective theoretical methods to reveal the correlation between hydration reactivity of doped clinker crystals and their electronic structures. Four dominant Portland cement clinker phases with copper doping are comparatively investigated by the state-of-the-art ab initio calculation. It finds that Cu ions doped in silicate and aluminate phases shift the nucleophilic reactive sites from Ca-centered regions to defect sites by decreasing the effective charges of Ca ions, which accounts for the hydration retardation of these phases. Exceptionally, the reactive sites in ferrite phase are scarcely changed by Cu doping due to the intense band-edge localization of Fe 3d state, which implies the relatively lesser influence on hydrat...

Spectroscopic (FT-IR, Raman, 13C and 1H NMR) investigation, molecular orbital calculation and thermal properties of novel piperidine derivative compound by quantum chemical calculation

The characteristics of (3-chlorophenyl)(3,5-dimethylpiperidin-1-yl)methanone have been investigated with Density Functional Theory based at the B3LYP level and 6-311++G(d,p) basis set using Gaussian 09W. The theoretical calculations were used to compute geometrical parameters, FT-IR, Raman, 13C and 1H NMR for confirming the structure of the molecule. The molecular orbital calculations such as HOMO-LUMO energy gap, Mulliken population analysis and MEP have been calculated to understand the chemical reactivity descriptors, static charge distribution and the electrophilic and nucleophilic reactive sites in the molecule. The thermodynamic parameters such as heat capacity, entropy and enthalpy at different temperatures have been calculated.

Functionalized N-containing heterocyclic scaffolds derived from N-substituted pyrroles via inter- and intramolecular annulations

Two series of N-containing heterocycles were synthesized by one-step intramolecular or intermolecular annulations of N-substituted pyrrole-2-carbaldehydes and pyrrole-2-carbonitriles. These facile and atom-economic synthetic routes involved the inherent three potential electrophilic and nucleophilic reactive sites of the synthons and the reaction solvent plays a crucial role leading to these different types of products. Additionally, plausible mechanisms are proposed.

Probing vibrational activities, electronic properties, molecular docking and Hirshfeld surfaces analysis of 4-chlorophenyl ({[(1E)-3-(1H-imidazol-1-yl)-1-phenylpropylidene]amino}oxy)methanone: A promising anti-Candida agent

The promising anti-Candida agent, 4-chlorophenyl ({[1E-3(1H-imidazole-1-yl)-1-phenylpropylidene}oxy)methanone (4-CPIPM) was comprehensively characterized by FT-IR, FT-Raman, UV, as well as 1H and 13C spectroscopic techniques. The theoretical calculations in the current study utilized Gaussian 09 W software with DFT approach of the B3LYP/6-311++G(d,p) method. The experimental X-ray diffraction data of the 4-CPIPM molecule were compared with the optimized structure and showed well agreement. Intermolecular electronic interactions and their stabilization energies have been analyzed by natural bond orbital method. Potential energy distribution confirmed the normal fundamental mode of vibration with the aid of MOLVIB software. The chemical shift values of the 1H and 13C spectra of the title compound were computed using gauge independent atomic orbital and the results were compared with the experimental values. The time-dependent density function theory method was used to predict the electronic, absorption wavelength and frontier molecular orbital energies. The HOMO-LUMO plots proved the charge transfer in the molecular system of the title compound through conjugated paths. The molecular electrostatic potential analysis provided the electrophilic and nucleophilic reactive sites in the title molecule which have been analyzed using Hirshfeld surface and two dimensions fingerprint plots. Non covalent interactions were also studied using reduced density gradient analysis and color filled electron density diagram. Molecular docking studies of the ligand-protein interactions along with their binding energies were carried out aiming to explain the potent anti-Candida activity of the title molecule.

Experimental and quantum chemical studies of a new organic proton transfer compound, 1H-imidazole-3-ium-3-hydroxy-2,4,6-trinitrophenolate

A new proton transfer compound, 1H-imidazole-3-ium-3-hydroxy-2,4,6-trinitrophenolate (IMHTP), was crystallized by slow evaporation-solution growth technique. 1H and 13C NMR spectral studies confirm the molecular structure of the grown crystal. Single crystal X-ray diffraction study confirms that IMHTP crystallizes in monoclinic system with space group P21/c. Thermal curves (TG/DTA) show that the material is thermally stable up to 198 °C. The crystal emits fluorescence at 510 nm, proving its utility in making green light emitting materials in optical applications. The stable molecular structure was optimized by Gaussian 09 program with B3LYP/6-311++G(d,p) level of basis set. The frontier molecular orbital study shows that the charge transfer interaction occurs within the complex. The calculated first-order hyperpolarizability value of IMHTP is 44 times higher than that the reference material, urea. The electrostatic potential map was used to probe into electrophilic and nucleophilic reactive sites present in the molecule.

Synthesis, vibrational spectroscopic investigations, molecular docking, antibacterial studies and molecular dynamics study of 5-[(4-nitrophenyl)acetamido]-2-(4-tert-butylphenyl)benzoxazole

Antimicrobial active 5-[(4-nitrophenyl)acetamido]-2-(4-tert-butylphenyl)benzoxazole (NATPB) was synthesized and observed IR, Raman bands are compared with the theoretically predicted wave numbers. In the IR spectrum the NH stretching wave number splits into a doublet with a noted difference and is red shifted from the computed value, which indicates the weakening of NH bond resulting in proton transfer to the neighbouring oxygen atom. The HOMO-LUMO plots reveal the charge transfer in the molecular system through the conjugated paths. The electrophilic and nucleophilic reactive sites are identified from the MEP plot. Mapping of average local ionization energy (ALIE) values to the electron density surface served us as a tool for prediction of molecule sites possibly prone to electrophilic attacks. Other important reactive centres of the title molecule were detected by calculations of Fukui functions. Calculations of bond dissociation energies (BDE) for hydrogen abstraction were used in order to assess whether the NATPB molecules is prone to autoxidation mechanism or not, while BDE of the remaining single acyclic bonds were used in order to determine the weakest bond. Interaction properties with water were investigated by molecular dynamics (MD) simulations and calculations of radial distribution functions (RDFs). The compound possessed broad spectrum activity against all of the tested Gram-positive and Gram-negative bacteria and yeasts, their minimum inhibitory concentrations (MICs) ranging between 32 and 128 μg/ml. The compound exhibited significant antibacterial activity (32 μg/ml) against an antibiotic resistant E. faecalis isolate, at same potency with the compared standard drugs vancomycin and gentamycin sulfate. The molecular docking studies show that the compound might exhibit inhibitory activity against CDK inhibitors.

Vibrational spectra and electronic structure of 3-((1H-pyrrol-1-yl) methyl) naphthalen-2-ol – A computational insight on antioxidant active Mannich base

The 3-((1H-pyrrol-1-yl) methyl) naphthalen-2-ol (IFTN) is a class of synthesized organic Mannich base compound having antioxidant property of biological importance. A meticulous normal mode vibrational analysis has been carried out by comparing and contrasting the experimentally recorded FT-IR and FT-Raman spectra with simulated spectra of IFTN. Similarly the calculated 1H and 13C NMR of IFTN were correlated with experimental findings to understand the shielding and deshielding nature hold in it. The following other electronic structure properties were calculated such as equilibrium geometrical parameters, natural bond orbital analysis (NBO), frontier molecular orbital (FMO) characterization, statistical thermochemical properties against a range of temperatures and molecular electrostatic potential (MEP) imprint was carried out along with various charges such as APT, ESP, Hirshfeld, natural and Mulliken. All these predicted properties unlock the nature of total behavior of IFTN in various dimensions. These could be Lewis, non-Lewis interaction along with their respective energies, contribution of fragmented moieties for various orders of HOMO and LUMO, temperature consequences on enthalpy, entropy, and heat capacity. In addition, the MEP leads to find electrophilic, nucleophilic reactive sites on IFTN molecular surface. The entire computational calculations has been made on HF/6-31+G/6-311++G(d,p) and B3LYP/6-31+G/6-311++G(d,p) model chemistries. A complete electronic structure observation has been achieved by this theoretical analysis.

A computational perspective on equilibrium geometry, vibrational spectra and electronic structure of antioxidant active Mannich base 1-[(Pyridin-2-yl amino) methyl] pyrrolidine-2,5-dione

The 1-[(pyridin-2-yl amino) methyl] pyrrolidine-2,5-dione (SFAP) is a class of organic Mannich base system which has been identified for its effective performance in favor of antioxidant activity. An intensified vibrational analysis has been carried out for SFAP by comparing the experimental FT-IR and FT-Raman spectra with computed IR and Raman spectra. The feasible tautomeric forms were generated and the most stable tautomer was compared with experimental crystal structure of SFAP. 1H and 13C NMR spectral analysis were assisting on determining structure property relationship via chemical shifts and magnetic shielding effects of title compound. The natural bond orbital (NBO) analysis provides insight on bonding, anti-bonding behavior and its nature in hybridization. The frontier molecular orbital approach suggests the nature of intra molecular charge transfer and the participation of individual moieties and atoms in SFAP. The trend of thermodynamic properties such as enthalpy, entropy and heat capacity of SFAP at different temperature were derived and related in the manner of quadratic equation. The Molecular Electrostatic Potential (MEP) imprint in relation with different charges such as atomic polar tensor (APT), electrostatic potential (ESP), Hirshfeld, natural bond orbital analysis (NBO), Mulliken were interpreted. The magnitude of charge was compared to find probable electrophilic and nucleophilic reactive sites in SFAP. All the theoretical calculations were performed at HF/6-31+G/6-311++G(d,p), B3LYP/6-31+G/6-311++G(d,p) model chemistry.

In Silico vibrational spectroscopic investigation on antioxidant active Mannich base 1-[anilino (phenyl) methyl] pyrrolidine-2,5-dione

The antioxidant active Mannich base 1-[anilino (phenyl) methyl] pyrrolidine-2,5-dione (APMPD) have been synthesized and its FT-IR and FT-Raman vibrational spectra were recorded within the region of 4000cm−1, 50cm−1 respectively. The molecular geometric parameters of APMPD have been computed using HF and DFT model theories. The energies of APMPD are calculated for all the eight possible conformers using B3LYP method at 6-311++G(d,p) basis set. From the computational results, the M1 conformer was identified as the most stable conformer of APMPD. The stable conformer was compared with experimental crystal geometry, which again fortifies the results of conformer analysis. The fundamental vibrations of the molecule are assigned according to the characteristic region and the literature report. The predicted highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap provide vivid idea on charge transfer behavior of APMPD. The molecular electrostatic potential (MEP) and Mulliken charge analysis indicate the feasible electrophilic and nucleophilic reactive sites on APMPD. The thermodynamic properties (heat capacity, entropy, and enthalpy) of the title compound at various temperatures are calculated in gas phase.

Nucleophilic reactivity of the nitroso group. Fukui function DFT calculations for nitrosobenzene and 2-methyl-2-nitrosopropane

The Fukui function, condensed Fukui function and relative nucleophilicy indices DFT calculations for nitrosobenzene and 2-methyl-2-nitrosopropane nitroso group in the gas-phase and in the water solution using SCRF SCI-PCM have been presented and discussed. Considering the calculated Fukui functions, both the N and the O atomic centres are found to be suitable nucleophilic reactive sites in the gas-phase and in the water solution. Condensed Fukui function and relative nucleophilicity indices support this view. Also, condensed Fukui function and relative nucleophilicity indices calculated from charges obtained with orbital charge calculation methods (MPA and NPA) show that the O atom of the nitroso group is a more reactive nucleophilic centre, while indices derived from the electrostatic potential charges (MKS, CHelp and CHelpG) emphasize the N atom as a more reactive nucleophilic site.