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61.
The potential energy surface of sulfinamide H(O)S—NH2 ( 1 ) was searched, using ab initio and density functional methods, to study the conformational preferences. High‐accuracy G2MP2 calculations showed that the S—N rotational barrier in 1 is 7.0 kcal mol−1. The inversion around N in 1 goes through a very low energy barrier. Charge analysis using the NPA method was performed to elucidate the electronic factors responsible for the observed trends in the S—N interactions. The strength of negative hyperconjugation in 1 was estimated using NBO analysis and by studying the substituent effect. The repulsions between the lone pairs on oxygen and nitrogen and the nN → σ*S—O negative hyperconjugation play an important role in the conformations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
62.
A computational study on dichalcogenide molecules (R2X2; X = O, S, Se; R = H, CH3, NH2) has been carried out employing B3LYP and MP2 levels using 6-31+G*, 6-311+G*, 6-311++G**, and PVDZ basis sets. The relative energies have been evaluated at G2MP2 also. The rotational barriers and bond dissociation energies indicate that S–S bond is stronger than Se–Se and O–O bond. NBO analysis at MP2/6-31+G* suggest the presence of partial π character between X–X bond that decreases in the order S–S > Se–Se > O–O. Fuki functions for nucleophilic and electrophilic attack fail to distinguish the reactivity of S and Se. The proton affinities of the O2H2, S2H2, Se2H2 decrease in the order Se > S > O. 相似文献
63.
Ahmed Mehdi Legesse Adane Dhilon S. Patel Prasad V. Bharatam 《Journal of computational chemistry》2010,31(6):1259-1267
Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi‐electrons and intramolecular H‐bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer ( GTU‐1 ) to the most stable thiol conformer ( GTU‐t1 ) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU‐1 followed by subsequent C? N bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU‐1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S‐alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by C? N bond rotation and deprotonation. The reactive intermediate RS(NH2)C? N? C(NH2)2+ belongs to the newly identified ⊕N(←L)2 class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
64.
Nikhil Taxak Prashant V. Desai Bhargav Patel Michael Mohutsky Valentine J. Klimkowski Vijay Gombar Prasad V. Bharatam 《Journal of computational chemistry》2012,33(21):1740-1747
Mechanism‐based inhibition (MBI) of cytochrome P450 (CYP) can lead to drug–drug interactions and often to toxicity. Some aliphatic and aromatic amines can undergo biotransformation reactions to form reactive metabolites such as nitrosoalkanes, leading to MBI of CYPs. It has been proposed that the nitrosoalkanes coordinate with the heme iron, forming metabolic‐intermediate complex (MIC), resulting in the quasi‐irreversible inhibition of CYPs. Limited mechanistic details regarding the formation of reactive nitroso intermediate and its coordination with heme‐iron have been reported. A quantum chemical analysis was performed to elucidate potential reaction pathways for the generation of nitroso intermediate and the formation of MIC. Elucidation of the energy profile along the reaction path, identification of three‐dimensional structures of reactive intermediates and transition states, as well as charge and spin density analyses, were performed using the density functional B3LYP method. The study was performed using Cpd I [iron (IV‐oxo] heme porphine with SH? as the axial ligand) to represent the catalytic domain of CYP, simulating the biotransformation process. Three pathways: (i) N‐oxidation followed by proton shuttle, (ii) N‐oxidation followed by 1,2‐H shift, and (iii) H‐abstraction followed by rebound mechanism, were studied. It was observed that the proton shuttle pathway was more favorable over the whole reaction leading to reactive nitroso intermediate. This study revealed that the MIC formation from a primary amine is a favorable exothermic process, involving eight different steps and preferably takes place on the doublet spin surface of Cpd I . The rate‐determining step was identified to be the first N‐oxidation of primary amine. © 2012 Wiley Periodicals, Inc. 相似文献
65.
Vishal Maingi Vaibhav Jain Prasad V. Bharatam Prabal K. Maiti 《Journal of computational chemistry》2012,33(25):1997-2011
We have developed a graphical user interface based dendrimer builder toolkit (DBT) which can be used to generate the dendrimer configuration of desired generation for various dendrimer architectures. The validation of structures generated by this tool was carried out by studying the structural properties of two well known classes of dendrimers: ethylenediamine cored poly(amidoamine) (PAMAM) dendrimer, diaminobutyl cored poly(propylene imine) (PPI) dendrimer. Using full atomistic molecular dynamics (MD) simulation we have calculated the radius of gyration, shape tensor and monomer density distribution for PAMAM and PPI dendrimer at neutral and high pH. A good agreement between the available simulation and experimental (small angle X‐ray and neutron scattering; SAXS, SANS) results and calculated radius of gyration was observed. With this validation we have used DBT to build another new class of nitrogen cored poly(propyl ether imine) dendrimer and study it's structural features using all atomistic MD simulation. DBT is a versatile tool and can be easily used to generate other dendrimer structures with different chemistry and topology. The use of general amber force field to describe the intra‐molecular interactions allows us to integrate this tool easily with the widely used molecular dynamics software AMBER. This makes our tool a very useful utility which can help to facilitate the study of dendrimer interaction with nucleic acids, protein and lipid bilayer for various biological applications. © 2012 Wiley Periodicals, Inc. 相似文献
66.
Kessar SV Singh P Singh KN Bharatam PV Sharma AK Lata S Kaur A 《Angewandte Chemie (International ed. in English)》2008,47(25):4703-4706
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68.
A first total synthesis of acortatarins A, B, and an enantiomer of the proposed structure of acortatarin B is described by using readily available d-sugars. This convergent total synthesis revealed the revision of the absolute configuration of acortatarin A and structural revision of acortatarin B. The key steps involved are regioselective epoxide opening with deprotonated 2,5-disubstituted pyrrole and spiroketalization. 相似文献
69.
Pansy Iqbal Dhilon S. Patel Prasad V. Bharatam 《Journal of Physical Organic Chemistry》2007,20(12):1072-1080
Electronic structure calculations and second‐order delocalizations in N,N′,N′′‐triaminoguanidine ( TAG ) have been studied by employing ab initio MO and density functional methods. There are total 10 rotational isomers on the potential energy (PE) surface of TAG . The effect of three amino groups substitution on guanidine ( Gu ) has been studied in terms of the primary and the secondary electron delocalizations in TAG by employing Natural Population Analysis (NPA). An increased electron delocalization is observed in protonated triaminoguanidine ( TAGP ) due to the three strong intramolecular hydrogen bonds and hence accounts for its extra stability. The increase in the electron delocalization upon protonation in TAG can be compared to that in guanidine. The absolute proton affinity (APA) of TAG is less than that of Gu . HOMA and NICS studies have been carried out to understand electron delocalization in TAGP . Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
70.
Kumaraswamy G Sadaiah K Ramakrishna DS Police N Sridhar B Bharatam J 《Chemical communications (Cambridge, England)》2008,(42):5324-5326
A concise route for the syntheses of enantioenriched functionalized scaffolds of medium-sized oxacycles and carbocycles employing the chiral auxiliary-mediated Cu-catalyzed ylide formation/[2,3]-sigmatropic rearrangement as a key step was developed. 相似文献