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1 Available online at World Scientific News (0) - EISSN - Assessment of substituent effects on β-naphthyl based isoxazoles by IR and NMR spectra G. Thirunarayanan, *, V. Sathiyendiran, R. Arulkumarn, R. Sundararrajan, R. Manikandan, G. Vanangamudi Department of Chemistry, Annamalai University, Annamalainagar , India Department of Chemistry, Sourashtra College, Madurai - 00, India PG & Research Department of Chemistry, Government Arts College, C-Mutlur - 0 0, Chidambaram, India * address: thirunarayanan.g.0@annamalaiuniversity.ac.in ; drgtnarayanan@gmail.com ABSTRACT A series containing thirteen -(-naphthyl)--(substituted phenyl)-,-dihydroisoxazole have been synthesized and examined their purities by their data reported earlier. The characteristic infrared νc=n (cm - ) stretches, NMR chemical shifts (δ, ppm) of H,, H protons, C - carbons were assigned and correlated with Hammett substituent constants and Swain-Lupton s parameters using single and multi- regression analysis. From the results of regression analysis the effect of substituents on the spectral data have been studied. Keywords: β-naphthyl dihydroisoxazoles; IR and NMR spectra; Correlation analysis. INTRODUCTION Isoxazoles are five membered heterocyclic compounds containing oxygen and nitrogen atom in the ring. They are medicinal and pharmaceutical important natural products. Numerous conventional and environmentally benign synthetic methods are reported for the isoxazoles synthesis. The [+]cycloaddition of alkynes and nitrile oxides and its alcohols [- ], side chain rearrangements [] and C-H activation/[+] annulation [] and were employed for synthesis of isoxazoles derivatives. Various catalysts such as Silica gel-mw [], NaCN- DMF [], p-tsoh-ksf [], SnCl -CH OH [], Ti(dpma)(NMe ) [], Ti(dpm)(NMe ) [], FeBr [], TMSCI-NaI [], Mg(OAc) [], CrO -CH CN [], NaNH -Liq NH [0], K CO [], Clay-glycol [], Pd(TFA) [], KOH-C H OH [], KO-Bu [], CSA-DMF [], Ultrasound waves [,], Hypervalent I [], NaH-DMF[], SOCl -CH Cl [], LiAlH - Ether [], t-buli:thf [0], triton-b:fly-ash-mw [], t-buoh:h O-NaOH [], NBS-SiO

2 World Scientific News (0) - [], Continuous flow microliter-mw [],,-Diazabicyclo[..]octane [], Chloramine- T [], PTSA-MW [], DMF-NaOH [], Si [], Pd complexes [], Sodium saccharin [], H O [0], CH COONa [], C H OH-NaOH [], CH OH/-0 ºC [] and Cu(I) [] were utilized for deriving isoxazoles from carbonyl compounds and hydroxylamine hydrochloride. Under solvent-assisted or solvent-free conditions, chalcones also employed for deriving isoxazoles by cyclization with hydroxylamine hydrochloride in presence of catalysts. Cyclization of substituted phenyl and chalcones with hydroxylamine hydrochloride yields some novel aryl and indole based isoxazoles [,]. This method is useful for deriving benzothiophene [] and -(-furyl)--(substituted phenyl)-,-dihydro isoxazoles []. Isoxazole derivatives possess many kinds of biological activities due to hetero atom O, N, double bond and polar groups presents in the ring []. The important biological activities of isoxazoles are antibacterial [,,,,,], antifungal [,,,,], antioxidant [,0,], antitubercular [0,], anti-inflammatory [,], ulcerogenicity [], antihypertensive [], immunological activity [], growth promoting effect of plants [], molecular docking [], nicotinic receptor binding [0], photonuclease activity [], DNA binding [], anti-analgesic [], invitro calcium channel antagonist activity [], anti-platelet[] anthelmintic [] and anti HIV [] activities. Isoxazole derivatives were used for synthesizing metal complexes like copper(ii) complexes []. The photo physical, physiochemical[] and electrochemical behavior of isoxazole [] thorugh experimental and theoretical investigations were reported. Based on the spectroscopic data the molecular ground state equilibration, geometrical and spatial arrangement of atoms were studied [,]. The effect of substituents also studied through spectral data of many organic compounds. Thirunarayanan and his co-workers have studied the spectral correlations on many chalcones and heterocyclic compounds [,0]. Recently, Senbagm et al., have studied the substituent effects on some triazole imines []. Within the above view, the study of effect of substituents on some -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazole derivatives has not been reported. Hence, the author have prepared -(-naphthyl)-- (substituted phenyl)-,-dihydro isoxazoles and recorded their infrared and NMR spectra for investigation of effect of substituents.. EXPERIMENTAL.. General The Sigma-Aldrich and E. Merck chemicals were used for in the present study. Infrared spectra (KBr, cm ) of all isoxazoles were recorded on Thermo scientific Nicolet is, US-made Fourier transform spectrophotometer. The NMR spectra of all compounds were recorded on a Bruker AV 00 spectrometer operating at 00 MHz for H NMR spectra and 00 MHz for C NMR spectra in CDCl solvent using TMS as internal standard... Synthesis of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles. In the present investigation compounds -(-naphthyl)--(substituted phenyl)-,- dihydro isoxazoles were prepared from literature method []. The purities of these isoxazoles were examined by data reported earlier in literature. The general structure of -(-naphthyl)- -(substituted phenyl)-,-dihydro isoxazoles was illustrated in Fig.. The analytical and physical constants of the isoxazoles are presented in Table. --

3 World Scientific News (0) - Fig.. The general structure of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles.. RESULTS AND DISCUSSION.. IR spectral correlations In the present investigations, the effect of substituents on the spectral νc=n (cm - ) stretches, NMR chemical shifts (δ, ppm) of H,, H protons, C - carbons of -(-naphthyl)- -(substituted phenyl)-,-dihydro isoxazoles were assigned and correlated with Hammett substituent constants and Swain-Lupton s [] parameters using single and multi- regression analysis. In infrared spectral correlations, the Hammett equation was employed as λ = ρ σ + λ o () where λ o is the absorption maximum of the parent member of this series. The assigned νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,- dihydro isoxazoles are presented in Table. These frequencies were correlated with Hammett substituent constants and Swain-Lupton s [] parameters using single and multi- regression analysis [-]. The results of statistical analyses are presented in Table. Table. The infrared spectral νc=n (cm - ) stretches, NMR chemical shifts (δ, ppm) of H,, H protons, C - carbons of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles Sl. No. X IR ν (cm - ) H NMR (δ, ppm) C NMR (δ, ppm) C=N H H H C C C H NH NH Br Cl

4 World Scientific News (0) - -Cl N(CH ) OH OCH CH NO NO NO Table. Results of statistical analysis of IR and NMR spectral data of of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles with Hammett σ, σ +, σ I, σ R constants, F and R parameters. Frequency Constant r I ρ s n Correlated derivatives σ νc=n (cm - ) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R σ δh (ppm) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R δh (ppm) σ σ σ I H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO --

5 World Scientific News (0) - σ R F R σ δh (ppm) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R σ νc (ppm) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R σ δc (ppm) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R σ δc (ppm) σ σ I σ R F H, -NH, -NH, -Br, -Cl, -Cl, -N(CH ), -OH, -OCH, - CH, -NO, -NO, -NO R r = correlation coefficient; I = intercept; ρ = slope; s = standard deviation; n = number of correlated derivatives -0-

6 νcn(cm - ) World Scientific News (0) - From Table, all correlations of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles gave poor correlation co efficient along with positive ρ values. This is due to the inability of effects of substituents on the CN stretches and the hydride ions in the conjugative structure as shown in Figure. Some of the single Hammett plots are shown in Figs. -. Fig.. The resonance-conjugative structure H. -NH. -NH. -NO. -NO , 0 0, σ Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ. --

7 νcn(cm - ) νcn(cm - ) World Scientific News (0) H. -NH. -NH. -NO. -NO 0 - -, - -0, 0 0, σ + Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ H. -NH. -NH. -NO. -NO 0-0, 0 0, 0, 0, 0, σ I Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ I. --

8 νcn(cm - ) νcn(cm - ) World Scientific News (0) H. -NH. -NH. -NO. -NO 0-0, -0, -0, 0 0, σ R Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ R H. -NH. -NH. -NO. -NO 0 0 0, 0, 0, 0, 0, 0, 0, F Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus F. --

9 νcn(cm - ) World Scientific News (0) H. -NH. -NH. -NO. -NO 0 -, - -0, -0, -0, -0, 0 0, R Fig.. Plot of νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus R. The νc=n (cm - ) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles are fail in correlations with Hammett σ, σ +, σ I, σ R constants, F and R parameters in single parameter correlations. While they are worth full when seeking these are in multiregressions with σ I, σ R constants, F and R Swain-Lupton s [] parameters. In this analysis satisfactory correlations obtained. The multi-regression analysis equations are ( and ) as; νc=n (cm - ) =.0 (±.). (±.) σ I +.(±.0) σ R () (R = 0., n =, P > 0%) νc= N (cm - ) = 0.(±.) +.0 (±.)F +.(±.)R () (R = 0., n =, P > 0%).. H NMR correlations The assigned NMR chemical shifts (δ, ppm) of H,, H of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles are presented in Table. These chemical shifts (δ, ppm) were correlated with Hammett substituent constants and Swain-Lupton s [] parameters using single and multi- regression analysis [-]. In NMR spectral correlations, the Hammett equations was taken in the form of δ = ρ σ + δ o () where δ o is the absorption maximum of the parent member of this series. --

10 δh (ppm) World Scientific News (0) - The results of statistical analyses are presented in Table. From Table, The correlation of H chemical shifts (δ, ppm) of -(-naphthyl)--(substituted phenyl)-,- dihydro isoxazoles gave poor correlation coefficients. All correlations gave positive ρ values. This implies that the normal substituent effects operates in all systems. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. The correlation of H chemical shifts (δ, ppm) of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles gave satisfactory correlation coefficients with Hammett Hammett σ, σ +, σ I constants and F parameters in single parameter correlations excluding -Br, -Cl, -Cl and nitro substituents. When they will include the correlations, the correlation coefficients decreased considerably. All correlations gave positive ρ values. This implies that the normal substituent effect operates in all systems. The Hammett σ R constant and R parameters fail in correlations. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. The correlation of H chemical shifts(δ, ppm) of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles gave satisfactory correlation coefficients with Hammett Hammett σ I σ R constants, F and R parameters in single parameter correlations excluding -Cl substituents. When they will include the correlations, the correlation coefficients decreased considerably. All correlations gave positive ρ values. This implies that the normal substituent effects operates in all systems. The Hammett σ and σ + constants fail in correlations. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. Some of the single plots are given in Figs. -0. Some of the NMR chemical shifts (δ, ppm) of H,, H of -(- naphthyl)--(substituted phenyl)-,-dihydro isoxazoles are fail in single parameter correlations. These are produced satisfactory correlations with multi-regression analysis using σ I, σ R constants, F and R Swain-Lupton s [] parameters. The obtained multi-regression analysis equations are given in (-0).,00,0,00,0,00,0, , 0 0, σ. H. -NH. -NH. -NO. -NO Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ --

11 δh (ppm) δh (ppm) World Scientific News (0) -,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO,00 - -, - -0, 0 0, σ + Fig. 0. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ +.,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO,00-0, 0 0, 0, 0, 0, σ I Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ I. --

12 δh (ppm) δh (ppm) World Scientific News (0) -,0,0,00,0,0,0,0,00,0,0, , 0 0,. H. -NH. -NH. -NO. -NO σ Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ.,0,0,00,0,0,0,0,00,0,0, , - -0, 0 0,. H. -NH. -NH. -NO. -NO σ + Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ +. --

13 δh (ppm) δh (ppm) World Scientific News (0) -,0,0,00,0,0,0,0 0,00,0,0,0-0, 0 0, 0, 0, 0,. H. -NH. -NH. -NO. -NO σ I Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ I.,0,0,00,0,0,0,0,00,0,0, , 0, 0, 0, 0, 0, 0, F. H. -NH. -NH. -NO. -NO Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus F. --

14 H δ(ppm) H δ(ppm) World Scientific News (0) -,00,00,00,00,00 0,00,00,000,00,00,00,00-0, 0 0, 0, 0, 0, σ I. H. -NH. -NH. -NO. -NO Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ I.,00,00,00,00,00,00 0,00,000,00,00,00,00-0, -0, -0, 0 0, σ R. H. -NH. -NH. -NO. -NO Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ R. --

15 H δ(ppm) H δ(ppm) World Scientific News (0) -,00,00,00,00,00,00,00,000,00,00,00 0. H. -NH. -NH. -NO. -NO,00 0 0, 0, 0, 0, 0, 0, 0, F Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus F.,00,00,00,00,00,00,00,000,00,00,00 0. H. -NH. -NH. -NO. -NO,00 -, - -0, -0, -0, -0, 0 0, R Fig.. Plot of δh (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus R. -0-

16 World Scientific News (0) - δh (ppm) =.(±0.0) (±0.00) σ I + 0.0(±0.00)σ R () (R = 0., n =, P > 0%) δh (ppm) =.(±0.0) + 0.0(± 0.0) F + 0.0(± 0.00) R () (R = 0., n =, P > 0%) δh (ppm) =.(±0.0) + 0.(±0.0) σ I + 0.0(±0.00)σ R () (R = 0.0, n =, P > 0%) δh (ppm) =.(±0.0) + 0.0(± 0.00) F + 0.0(± 0.00) R () (R = 0., n =, P > 0%) δh (ppm) =.(±0.) + 0.(±0.) σ I + 0.(±0.)σ R () (R = 0., n =, P > 0%) δh (ppm) =.(±0.) + 0.0(± 0.00) F + 0.0(± 0.0) R (0) (R = 0., n =, P > 0%).. C NMR correlations The assigned C NMR chemical shifts (δ, ppm) of C - of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles are presented in Table. These chemical shifts(δ, ppm) were correlated with Hammett substituent constants and Swain-Lupton s [] parameters using single and multi- regression analysis [-]. The results of statistical analyses are presented in Table. From Table, The correlation of C chemical shifts (δ, ppm) of -(-naphthyl)--(substituted phenyl)-,- dihydro isoxazoles gave poor correlation coefficients. All correlations gave positive ρ values. This implies that the normal substituent effect operates in all systems. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. The correlation of C chemical shifts (δ, ppm) of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles gave satisfactory correlation coefficients with Hammett σ, σ +, σ R constants and R parameters excluding amino, -dimethylamino substituents. When these substituents are includes in regressions, the reduced the correlations considerably. The Hammett σ I constant and F parameters fail in correlations. All correlations gave positive ρ values. This implies that the normal substituent effect operates in all systems. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. The correlation of C chemical shifts(δ, ppm) of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles gave satisfactory correlation coefficients with Hammett σ R constant excluding amino, -dimethylamino substituents. When these substituents are includes in regressions, the reduced the correlations considerably. The Hammett σ, σ +, σ I constants, F and R parameters were failing in correlations. All correlations gave positive ρ values. This implies that the normal substituent effects operate in all systems. The reason for the failure correlations are already stated and associated with resonance-conjugated structure as shown in Fig.. Some of the single parameter correlations are shown in Figs

17 δc (ppm) World Scientific News (0) -,0 δc (ppm),00,0,00,0,00,0,00, , 0 0, σ. H. -NH. -NH. -NO. -NO Fig. 0. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ.,0,00,0,00,0,00,0,00,0 - -, - -0, 0 0, 0. H. -NH. -NH. -NO. -NO σ + Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ +. --

18 δc (ppm) δc (ppm) World Scientific News (0) -,0,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO, , 0 0, σ Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ.,0,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO,00 - -, - -0, 0 0, σ + Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ +. --

19 δc (ppm) δc (ppm) World Scientific News (0) -,0,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO,00-0, -0, -0, 0 0, σ R Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ R.,0,00,0,00,0,00,0 0. H. -NH. -NH. -NO. -NO,00 -, - -0, -0, -0, -0, 0 0, R Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus R. --

20 δc (ppm) δc (ppm) World Scientific News (0) - 0,00,00,00,00,00,00,00 0. H. -NH. -NH. -NO. -NO,00, , 0 0, σ Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ. 0,00,00,00,00,00,00,00,00,00-0, -0, -0, 0 0, 0. H. -NH. -NH. -NO. -NO σ R Fig.. Plot of δc (ppm) stretches of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles versus σ R. --

21 World Scientific News (0) - Some of the C NMR chemical shifts (δ, ppm) of C - of -(-naphthyl)--(substituted phenyl)-,-dihydro isoxazoles were fail in correlations. While seeking in multi correlations of these chemical shifts with and σ I and σ R or F and R Swain-Lupton s [] parameters, they produced satisfactory correlation coefficients. The multi-regression analysis equations are (-) as; δc (ppm) =.(±0.) + 0.(±0.) σ I + 0.(±0.0)σ R () (R = 0., n =, P > 0%) δc (ppm) =.(±0.) + 0.(± 0.0 F + 0.(± 0.00) R () (R = 0., n =, P > 0%) δc (ppm) =.(±0.0) + 0.(±0.) σ I + 0.(±0.)σ R () (R = 0., n =, P > 0%) δc (ppm) =.(±0.0) + 0.(± 0.) F + 0.(± 0.00) R () (R = 0., n =, P > 0%) δc (ppm) =.0(±0.) +.(±0.) σ I +.0(±.0)σ R () (R = 0., n =, P > 0%) δc (ppm) =.(±.00) + 0.(± 0.) F (± 0.0) R () (R = 0., n =, P > 0%). CONCLUSIONS About thirteen -(-naphthyl)--(substituted phenyl)-,-dihydroisoxazole were synthesized and examined their purities by their data reported earlier. The characteristic infrared νc=n (cm - ) stretches, NMR chemical shifts (δ, ppm) of H,, H protons, C - carbons were assigned and correlated with Hammett substituent constants and Swain- Lupton s parameters using single and multi-regression analysis. From the results of regression analysis the effect of substituents on the spectral data were studied. The infrared spectral frequencies were failing in correlations. Some of the isoxazole ring protons and carbons produced satisfactory correlations excluding amino and halogen substituents in single parameter correlations. In multi-regression analysis all spectral data gave satisfactory correlations. References [] Denmark, S. E. and Kallemeyn, J. M. J. Org. Chem. 0 (00) -. [] Kano, H., Adachi, I., Keido, R. and Hirose, K. Isoxazoles. XVIII. Synthesis and Pharmacological Properties of -Aminoalkyl- and -Amino alkylisosazoles and Related Derivatives. Isoxazoles. () 0-0. [] Pineiro, M. and Pinho e Melo, T. M. V. D. Eur. J. Org. Chem. (00)

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Structure parameter correlation of some quinoxaline derivatives through IR and 13 C NMR spectra

Structure parameter correlation of some quinoxaline derivatives through IR and 13 C NMR spectra International Letters of Chemistry, Physics and Astronomy Online: 2014-09-03 ISSN: 2299-3843, Vol. 38, pp 198-207 doi:.18052/www.scipress.com/ilcpa.38.198 2014 SciPress Ltd., Switzerland Structure parameter