AM1 Procedures Research Articles

Multiclass analysis on repaglinide, flubendazole, robenidine hydrochloride and danofloxacin drugs

The drugs under study; repaglinide (Repag), flubendazole (Flu), robenidine hydrochloride (Roben) and danofloxacin (Dano) are antidiabetic, anthelmintic, anticoccidial, and antibiotic drugs. In the present study, they are investigated using electron impact mass spectral (EI-MS) fragmentation at 70eV, in comparison with thermal analyses measurements (TGA/DrTGA and DTA) and molecular orbital calculation (MO). Semi-empirical MO calculation, AM1 procedure, has been carried out on Repag, Flu, Roben and Dano both as neutral molecules (in TA) and the corresponding positively charged species (in MS). The calculated MO parameters include bond length, bond order, charge distribution on different atoms and heat of formation. The fragmentation pathways of Repag, Flu, Roben and Dano in EI-MS led to the formation of important primary and secondary fragment ions. The mechanism of formation of some important daughter ions can be illuminated from comparing with that obtained using mass spectrometer through the accurate mass measurement determination. The MO provides a base for fine distinction among sites of initial bond cleavage and subsequent fragmentation of drug molecules in both thermal analysis and MS techniques. The activation thermodynamic parameters, such as, (activation energy E∗), (enthalpy ΔH∗), (entropy ΔS∗) and (Gibbs free energy ΔG∗) are calculated from the DrTGA curves using Coats–Redfern and Horowitz–Mitzger methods.

Stereochemistry of Seven-membered Heterocycles: XLIV. Spatial Structure of 4-R-3,5-Dioxabicyclo[5.1.0]octanes

4-R-3,5-Dioxabicyclo[5.1.0]octanes were prepared in good yields by reduction of the corresponding 8,8-dichloro derivatives in a system Li-t-BuOH. According to the data of dynamic 1H and 13CNMR spectroscopy involving experiments in the NOESY mode the formal (R = H) at −93°C in (CD3)2CO exists in nearly equally occupied chair forms with endo- and exo-oriented three-membered ring. The like structure were found in the diastereomeric 4-Me(t-Bu)-analogs. The characteristic feature of 13C NMR spectra consists in considerable difference in the chemical shifts of the C8 atoms (Δδ∼16–17 ppm). The data on epimerization of diastereomers and calculations along AM1 procedure suggest for formal a three-component equilibrium including a twist-form.

Structure and reactivity of cycloimmonium ylides. Theoretical study in triazolium ylides by AM1, PM3 and DFT procedure methods

This paper reports some general remarks on structure and reactivity of monosubstituted and disubstituted cycloimmonium ylides. The results have been achieved by calculations on ylide bond lengths, bond orders, dipolar moments and rotational barriers. Some aspects have been followed on charges and high-occupied molecular orbital coefficients of ylide carbon atoms during rotation around ylide bond. The AM1, PM3 and DFT procedure methods have been used in this study. Three ylidic systems have been selected as models of cycloimmonium ylides: (a) 1-ethoxycarbonylmethyl 1,2,4-triazol-1-ium 4′-nitrobenzoyl 2″,4″,6″-trinitrophenyl methylides, (b) 4-phenylbenzoyl 1,2,4-triazol-1-ium methylide and (c) 4-phenyl dibenzoyl 1,2,4-triazol-1-ium methylide. DFT and AM1 procedure methods are appropriate for the description of monosubstituted cycloimmonium ylides. Only DFT procedure method is recommended for the description of the non-planar disubstituted cycloimmonium ylides. The resonance interaction in planar monosubstituted and planar disubstituted cycloimmonium ylide systems permits to carry out 3+2 cycloadditions preferably in non or less polar solvents.

Theoretical study of the effect of radiation on thymine

The possible effects of radiation exposure to DNA are studied by investigations for the thymine residue. Detailed analysis of the various addition and other products is undertaken theoretically, using the semiempirical AM1 procedure. The results agree with the experimental finding that the loss of hydrogen on radiation exposure occurs from the C 5-methyl group and hydroxyl radical addition occurs at C 6, yielding the ‘5-yl’ radical. This radical is nonplanar, the axial conformer being slightly preferred over the equatorial one. In contrast, the other possible radical, the ‘6-yl’ radical, is almost planar. These results are important in understanding the conformational changes in DNA as a consequence of radiation exposure.

Homoconjugated (NH···N)- Hydrogen Bonds with Great Proton PolarizabilityFTIR and NMR Studies

Nine NH acids with pKa values between 0 and 8.57 were studied by FTIR spectroscopy in the middle and far-infrared region. In addition, NMR spectra were taken and a semiempirical AM1 analysis of the systems performed. The proton potential in the NH···N- ⇌ -N···HN hydrogen bonds is almost the same. The proton polarizability of these hydrogen bonds is almost independent of the pKa of the studied N acids. There is no relation between the position of the hydrogen bond vibration in the far-infrared region and the pKa of the N acids. Only with the pyrrol, the pyrazol, and the 4,(5)-methylimidazol systems these hydrogen bonds are linear. With all other systems these hydrogen bonds are more or less strongly bent. The position of the hydrogen bond vibrations νσ calculated with the AM1 procedure agrees well with the experimentally obtained values. The chemical shift of the hydrogen-bonded proton increases in proportion to the pKa values of the N acids.

The chemisorption of acid chlorides on the silica surface

The interaction of silica silanol groups with acid chlorides (SOCl 2 and CH 3 COCl) has been studied by the semiempirical quantum chemical MNDO and AM1 procedures. First, the acid chlorides react with silanol groups forming mono-orthoesters of acid chlorides. For thionyl and acetyl chloride reactions this step is appreciably exothermic (238.1 and 50.6 kJ mol −1 , respectively). The next endothermic step involves the elimination of HCl (−56.5 and −17.6 kJ mol −1 respectively), and the last step in the silica chlorination process is the most endothermic (−150.6 kJ mol −1 ). The activation energies of the first exothermic step are rather low and those for the subsequent steps are considerably larger and approximately equal.

An examination of intermolecular and intramolecular hydrogen bonding in biomolecules by AM1 and MNDO/M semiempirical molecular orbital studies

AbstractThe recent NMDO/M modification and parameterization of the MNDO molecular orbital method has been used to analyze intermolecular hydrogen bonding between amino acids and water, and intramolecular hydrogen bonding in monosaccharides. The results have been compared to AM1 calculations on the same systems. The MNDO/M calculations gave values which were similar to ab initio calculations with respect to the intermolecular interactions, but yielded significantly poorer results for the intramolecular interactions. The AM1 procedure performed better on the intramolecular interactions than the MNDO/M procedure, but frequently provided unfavorable three‐centered hydrogen bonding geometries for the intermolecular interactions.

An evaluation of water cluster geometries derived from semi-empirical AM1 calculations

The calculated geometries of water clusters, (H 2O) n with n = 2–6, obtained from the semi-empirical AM1 procedure differ from geometries based on ab initio calculations. The AM1 hydrogen-bonded linear dimer with C s symmetry reported by Dewar et al. [J. Am. Chem. Soc. 107 (1985) 3902], which corresponds to the most stable ab initio or experimental geometry, is only a local minimum. The AM1 C s dimer is found to be 2.0 kcal less stable than an unusual cyclic bifurcated structure (approximately C 2h symmetry) in which one of the lone pairs of electrons from each water molecule bisects the HOH bond angle of the other molecule. An additional local minimum, 1.7 kcal more stable than the reported C s structure, consists of a C 2v framework in which the two hydrogen atoms of one water molecule coordinate to both lone pairs of the second molecule. These results lead to questions regarding the use of the present AM1 parameterization in studies of hydrogen bonding.