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

A series of 6-substituted quanoxaline derivatives have been synthesized and examined their purities by literature method. The infrared and 13 C NMR spectral data of these quinoxalines were correlated with Hammett substituent constants, F and R parameters using single and multi-regression analysis. From the results of statistical analysis, the effect of substituents on the spectral frequencies has been studied


1. General
Sigma-Aldrich and Merck company chemicals and solvents used in this present study. The infrared spectra of all chalcones were recorded in SHIMADUZ Fourier Transform IR spectrophotometer using KBr discs. The 13 C NMR spectra of all compounds have been recorded in BRUKER AV 400 type spectrometer, using CDCl 3 as a solvent, 100 MHz for 13 C NMR spectra, taking TMS as standard.

RESULTS AND DISCUSSION
In the present study, the authors have correlated the infrared and 13 C-NMR spectral frequencies with Hammett substituent constants, F and R parameters using single and multilinear regression analysis. Present investigated compounds structure is shown is Fig. 1. It had symmetric structure. The substituents are in 5 and 6 th position. With respect to C 2 =N 1 and C ‫׳8‬ =N 1 , the substituents attached in 5 and 6 th positions are considered as metaand parapositions. Similarly, the C 3 =N 4 and C ‫׳4‬ =N 4 , substituents attached in 5 and 6 th positions are considered as orthoand metapositions. Within the considerations, the authors have performed the assigned spectral frequencies were correlated separately by the correlations performed with respect to C 2 =N 1 and correlations performed with respect to C ‫׳4‬ =N 4 systems International Letters of Chemistry, Physics and Astronomy Vol. 38 in the quinoxalines. The same trend was observed whether the substituents are attached in 8 and 7 th positions.

1. Infrared spectral correlation
The assigned the C=N stretches (cm -1 ) of the present investigation substituted quinoxalines were tabulated in Table 1. These data were correlated with Hammett substituent constants, F and R parameters [12,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. In this correlation, the Hammett equation was employed as, where ν o is the frequency for the parent member of the series. The results of statistical analysis are tabulated in Table 2. From Table 2, the single parameter correlation of νC=N mand psubstituted quinoxalines gave satisfactory correlation coefficients with Hammett substituent constants, F and R parameters except fluoro substituent for σ I and R parameter. Similarly the single parameter correlation of these stretches of oand msubstituted quinoxalines gave satisfactory correlation with Hammett σ, σ + , σ I constants and F parameters. The Hammett σ R constants and R parameter were failing in correlations. All correlations gave positive ρ values. This meant that the normal substituent effect operates in all systems.
The failure in correlation was due to the inability of predicting the substituent effects on the frequencies along with the resonance conjugative structure as shown in Fig. 2.
The results of statistical analyses are shown in Table 2. The correlations performed with respect to C 2 =N 1 (p-substitution pattern), chemical shifts of δC 2 =N 1 (ppm) gave satisfactory correlation with Hammett substituent constants, F and R parameters. Hammett σ R and F parameter gave satisfactory correlation except fluoro substituent. The δC 3 =N 4 (ppm) chemical shifts with Hammett σ and σ + constants gave satisfactory correlations except nitro substituent. The satisfactory correlations observed for Hammett σ I and F parameters. The Hammett σ R constant and R parameters were fails in correlations. All correlations gave positive ρ values. These mean that the normal substituent effect operates in all systems. The reason for the failure in the correlation was already stated and along with the resonance conjugative structure as shown in Fig. 2.
The single parameter correlation of δC ‫׳4‬ -N 4 (ppm) chemical shifts with Hammett substituent constants, F and R were failed. In these correlations, some of the negative ρ values obtained. This negative ρ values reduced correlations considerably. The reasons for this poor correlations were already stated and along with the resonance conjugative structure as shown in Fig. 2.
The single parameter correlation of δC ‫׳8‬ -N 1 (ppm) chemical shifts with Hammett substituent constants, F and R gave satisfactory r values. The F parameter was failed in the correlation. All correlations gave positive ρ values. The reason for the poor correlation of F parameter was already stated and along with the resonance conjugative structure as shown in Fig. 2.
The ipso carbon chemical shifts of the quinoxalines with Hammett σ R constant and R parameters gave satisfactory correlations. The Hammett σ, σ + , σ I constants and F parameter were fail in correlations. The inability of substituents along with conjugative structure as shown in Fig. 2. is explain the reasons for poor correlations.
The correlations performed with respect to C ‫׳4‬ =N 3 (m-substitution pattern), the chemical shifts of δC 2 =N 1 (ppm) gave satisfactory correlation with Hammett substituent constants, F and R parameters. The δC 3 =N 4 (ppm) chemical shifts with Hammett σ , σ + and σ I constants and F parameter constants gave satisfactory correlations. The Hammett σ R constant and R parameters were failed in correlations. All correlations gave positive ρ values. These mean that the normal substituent effect operates in all systems. The reason for the failure in the correlation was already stated and along with the resonance conjugative structure as shown in Fig. 2.
The chemical shifts δC ‫׳4‬ -N 4 (ppm) of quinoxalines with Hammett substituent constants, F and R gave poor correlation. Some of correlations gave the negative ρ values obtained in this correlation. This negative ρ values reduced correlations considerably. The reasons for this poor correlations were already stated and along with the resonance conjugative structure as shown in Fig. 2.
The single parameter correlation of δC ‫׳8‬ -N 1 (ppm) chemical shifts with Hammett substituent constants and F gave satisfactory r values. The R parameter was failed in the correlation. All correlations gave positive ρ values. The reason for the poor correlation of R parameter was already stated and along with the resonance conjugative structure shown in Fig. 2.
The ipso carbon chemical shifts of the quinoxalines with Hammett σ I constant and R parameters gave satisfactory correlations. The Hammett σ, σ + , σ R constants and F parameters were failed in correlations. Already stated the reason for the poor correlations and it is along with conjugative structure as shown in Fig. 2.
Some of the single parameters were failed in correlations with Hammett substituent constants, F and R parameters. While seeking multi-linear correlations with σ I and σ R constants or Swain Lupton's [32] F and R parameters gave satisfactory correlations for infrared and 13 C NMR spectral data of quinoxalines. The generated multi-regression analysis equations are shown in  Correlations performed with p-substitution pattern

CONCLUSIONS
A series containing ten substituted 5-and 6-substituted quinoxaline derivatives have been synthesized and examined their purities by literature method. The infrared and 13 C NMR spectral frequencies of C=N, C-N and ipso carbons of the quinoxalines were assigned and correlated based on mand psubstituted system with Hammett substituent constants, F and R parameters using single and multi-regression analysis. From the results of statistical analyses, most of the single parameter correlations and all multi-correlations gave satisfactory correlation coefficients.