Solvent and substituent effect on electronic spectra of N -(4-substituted phenyl)-2,3-diphenylpropanamides

The UV absorption spectra (200-400 nm) of eleven N -(4-substituted phenyl)-2,3-diphenyl-propanamides 1 have been recorded in fourteen solvents of different polarity. A simple Hammett equation was used to study the effects of substituents on the UV spectra of compounds 1 . The effects of solvent polarity and hydrogen bonding on the absorption spectra are interpreted by means of linear solvation energy relationships. The results show that the solvent effect on UV absorption spectra of the investigated amides are very complex and strongly depends on the nature of the substituents of the phenyl ring.

In this work we report the synthesis of five new N-(4-substituted phenyl)-2,3-diphenylpropanamides 1a,g-j (Figure 1), the UV absorption spectra (200-400 nm) of eleven compounds 1a-k recorded in fourteen solvents of different polarity, and the study of the effects of substituents and solvents on the UV spectra.

Results and Discussion
The main goal of this work is to explain the effects of the substituents in the UV spectrum of N-(4-substituted phenyl)-2,3-diphenylpropanamides 1 in different solvents with reference to N-phenyl-2,3-diphenylpropanamide 1e (Z = H).In the solvents used two absorption bands were found in the UV spectra of compounds 1a-k; the lower energy band is sensitive to the electronic properties of the substituent (Table 1).No correlation was found for the high energy band.The data in Table 1 confirm that the position of the UV absorption frequency depends on the nature of the N-phenyl substituent.A 4-substituent in the benzene ring causes a bathochromic shift of the long-wavelength absorption maximum as compared to the unsubstituted amide.
In order to explain the trend shown in Table 1, the wavenumbers were correlated with various sets of substituent parameters σ.The plot of the absorption frequencies vs σ p or σ p + and σ p -constants gives correlations which show deviations from the Hammett equation in all investigated solvents (Figure 2).However, the effect of electron-withdrawing substituents (structure 1D) appears to be quite opposite.The reversal of the substituent effect in the investigated system is interpreted by assuming a competition between the local π-polarization within a strongly conjugated amide group and the electron-donating mesomerism in the anilino group.
Electron-donating substituents increase the electron density at the anilide nitrogen atom, decrease the n-π* transition energy and produce bathochromic shifts of the long wavelength absorption maximum as compared to that of N-phenyl-2,3-diphenylpropanamide.
By contrast, electron-withdrawing substituents can be expected to decrease the electron density at the anilide nitrogen atom.However, our results show that these substituents also produce bathochromic shifts.The effect of electron-withdrawing substituents is opposite to the effect of the local polarization in the amide group and causes a similar effect on the UV absorption maxima as electron-donating substituents.These results are in accordance with results previously obtained for hydantoin derivatives, 10 N-(4-substituted phenyl) benzamides, 9 and N-(4substituted phenyl)-2-phenylacetamides. 8he effect of solvent polarity and hydrogen bonding on N-(4-substituted phenyl)-2,3diphenylpropanamides 1 are interpreted by the linear solvation energy relationship 11 using a Kamlet-Taft general solvatochromic equation of the following form: where π* is a measure of the solvent dipolarity/polarizability, β is the scale of the solvent hydrogen bond acceptor basicity, α is the scale of the solvent hydrogen bond donor acidity, and ν o is the regression value of the solute property in the reference solvent cyclohexane.The regression coefficients s, b and a in Eq.1 measure the relative susceptibilities of the solvent dependant solute property (absorption frequencies) to the indicated solvent parameters.The solvent parameters 12 are shown in Table 2.The correlations of the spectroscopic data were carried out by means of multiple linear regression analysis.It was found that the absorption frequencies for investigated amides in twelve selected solvents (Table 3) show satisfactory correlation with π*, α and β parameters.The results of the multiple regressions are presented in Tables 3 and 4. The negative sign of s and b coefficients for all amides (Table 3) indicate a bathochromic shifts with both increasing solvent dipolarity/polarizability and solvent hydrogenbond acceptor basicity.This suggests stabilization of the electronically excited state relative to the ground state.The positive sign of a coefficient for all amides indicates a hypsochromic shifts with increasing hydrogen-bond donor acidity of the solvent.This suggests stabilization of the ground state relative to the electronically excited state.The percentage contributions of the solvatochromic parameters (Table 4) for amides with a strong electron-donating substituent as the dimethylamino group and a strong electron-attracting substituent as the nitro group show that a major contribution of the solvatochromism is due to solvent dipolarity/polarizability.These results are in accordance with canonical structures 1C and 1D, and their stabilization is accounted to the solvent dipolarity/polarizability (non-specific solute-solvent interactions) rather than to solvent basicity and acidity properties.The percentage contributions of solvatochromic parameters for amides with moderate electron-donating and electron-attracting substituents in the phenyl group show that most of the solvatochromism is due to solvent basicity and acidity properties (specific solute-solvent interactions).The satisfactory correlation of the ultraviolet absorption frequencies of the investigated N-(4-substituted phenyl)-2,3-diphenylpropanamides 1 with Eq.1 indicates that the selected model gives a correct interpretation of the linear solvation energy relationships of the complex system of the amides in the solvents used.In the case of both solvents and substrates being hydrogen bond donors and acceptors, it is quite difficult to untangle solvent dipolarity/polarizability and hydrogen bond donating and accepting interactions.We have demonstrated that an equation with three solvatochromic parameters π*, α and β can be used to evaluate the effects of both types of hydrogen bonding and the solvent dipolarity/polarizability effect.The results show that the solvent effect on UV absorption spectra of N-(4-substituted phenyl)-2,3-diphenylpropanamides 1 is very complex and strongly depends on the nature of the substituents at the N-phenyl ring.
Amides 1 (Table 5) were recrystallized from ethanol/water.Purity was confirmed by GC analysis on a Varian 3400 gas chromatograph (flame ionization detector, all-glass split-splitless sample injector, 1071 capillary injector, DB-1 capillary column).Data handling by a Varian 4720 data system.Melting points were determined on an electrothermal apparatus.FT-IR spectra were recorded with a Bomem MB 100 spectrophotometer. 1 H NMR spectra of CDCl 3 solutions (TMS as internal standard) were measured with a Varian-Gemini 200 MHz spectrometer.MS data were determined with a Polaris MS mass spectrometer (direct probe, 70 eV ionizing energy, emission current of 600 µA).UV absorption spectra were measured with a Shimadzu 160 A spectrophotometer.The spectra were taken in spectroquality solvents with 10 -5 M concentration.

Table 3 .
Regression fits to solvatochromic parameters a Correlation coefficient.b Standard error of the estimate.c Fisher's test.d Solvent number as given in Table 2.