Solvatochromism 4-nitroaniline

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Solvatochrotnic shifts of the five probe indicators, 2-nitroaniline, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, and N,N-dimethyl-4-nitroaniline, have been used to specify the solvatochromic solubility parameters k and beta of the five pure fluids CO2,... 

Figure 3.49 Examples of solvatochromic plots of absorption spectra, (a) The first absorption band of 4-nitroaniline (charge transfer band), (b) The first absorption band of acetone (n-n band). The ordinates are in units of 103 cm, measured at the band maximum the abcissa are in units of Onsager solvent polarity function f(D)...

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Many other scales of electron-pair donation abilities have been proposed over the years, which are in general in good correlation with DN e g., the heat of complexation of the solvent molecules with boron trifluoride in dichloromethane (Maria and Gal 1985), and P e.g., SB, the solvatochromism of 5-nitroindoline compared with l-methyl-5-nitroindoline in neat solvents (Catalan el al. 1996) scales. The latter, the SB scale, has the advantage that the N-H acid function of the 5-nitroindoline probe has only a single hydrogen atom, contrary to the nitroanilines used for the P scale, that have two. It was devised quite recently for... 

N,N-Diethyl-4-nitroaniline, has an aromatic ring but no hydrogen bond donor substituent, shows a n-jt transition based on a non-specific interaction between ions. Dipolarity/polarizability, jz, is estimated by the solvatochromic shift of N,N-diethyl-4-nitroaniline using Eq. (3.3), where Amax is the absorption maximum for N, N-diethyl-4-nitroaniline. 

Therefore, we have quoted second-order polarizabilities in terms of /3o of p-nitroaniline in dioxane (A a = 354 nm, /3o = 13.5 Cm T convention, relative to quartz dn = 0.5 pm/V at 1064 nm). p-Nitroaniline is a truly one-dimensional NLO-phore with one significant component as has been verified experimentally by depolarized EFISHG (Wortmann et al, 1993). If different standards and conventions are taken into account, the values measured by different groups are quite consistent. Note that the intrinsic /3o of p-nitroaniline depends on the solvent, even when normalized for the solvatochromic shift of the CT absorption. We have chosen the lowest intrinsic /3o it is higher by a factor of 1.6 in very polar solvents (see p. 183). Also note that /3 values from HRS measurements of molecules with several significant tensor elements will not allow a true comparison of... 

Several conclusions can be drawn from Table 3. First, in accordance with the two-state model, /So and jSj all increase with decreasing HOMO-LUMO gap. Second, the intrinsic second-order polarizability of p-nitroaniline is increased by two-thirds when the solvent is changed from p-dioxane to methanol or A-methylpyrrolidone, even when the values are corrected for the differences in (A ). As we have adopted the value for p-nitroaniline in dioxane as a standard, it should therefore be noted that molecules that truly surpass the best performance of p-nitroaniline should have a second-order polarizability of l. p-nitroaniline (dioxane). As a third conclusion, there is a poor correlation between and the static reaction field as predicted by (91). This is in part due to the fact that the bulk static dielectric constant, E° in (89), differs from the microscopic dielectric constant. For example, p-dioxane has long been known for its anomalous solvent shift properties (Ledger and Suppan, 1967). Empirical microscopic dielectric constants can be derived from solvatochromism experiments, e.g. e = 6.0 for p-dioxane, and have been suggested to improve the estimation of the reaction field (Baumann, 1987). However, continuum models can only provide a crude estimate of the solute-solvent interactions. As an illustration we try to correlate in Fig. 7 the transition energies of p-nitroaniline with those of a popular solvent polarity indicator with negative solvatochromism. 

Different methods for the study of selective solvation have been developed [118, 120] conductance and Hittorf transference measurements [119], NMR measurements (especially the effect of solvent composition on the chemical shift of a nucleus in the solute) [106-109], and optical spectra measurements like IR absorption shifts [111] or UV/Vis absorption shifts of solvatochromic dyes in binary solvent mixtures [124, 249, 371]. Recently, the preferential solvation of ionic (tetralkylammonium salts) and neutral solutes (phenol, nitroanilines) has been studied particularly successfully by H NMR spectroscopy through the analysis of the relative intensities of intermolecular H NOESY cross-peaks [372]. 

The same authors also introduced a r scale of solvent dipolarity/polarizability [84a]. This n scale is so named because it is derived from solvent effects on the n n electronic transitions of a selection of seven positively solvatochromic nitroaromatics of the type D-C6H4-A, where D and A stand for electron-donor e.g. NMe2) and electron-acceptor e.g. NO2) groups, respectively 4-nitroanisole, A,A-diethyl-3-nitroaniline, 4-methoxy-/ -nitrostyrene, 1-ethyl-4-nitrobenzene, A-methyl-2-nitro-p-toluidine, N,N-diethyl-4-nitroaniline, and 4-(dimethylamino)benzophenone. Given a solvatochromic indicator compound, the n value for a solvent S was defined according to Eq. (7-32) ... 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

Solvatochromic shifts in the ultraviolet-visible absorption spectra of p-nitro-phenol and p-nitroaniline have been taken as measures of relative solvent affinities [25]. 

Af-Butyl-4-nitroaniline (30) was also shown42 to be able to donate a hydrogen bond to HBA solvents, hence could be used as a solvatochromic probe, but has not been explored further in this respect. Drago29 reported the wavenumbers of the lowest energy absorption bands of three /V-alkyl-substituted nitroanilines 27, /V-ethyl-4-nitroanilinc (31) and N-ethyl-3-nitroaniline (32). Their spectral shifts relative to that in dioxane are shown in Figure 2 as a function of / Kt and a fair correlation is seen. 

The H-bond formation may affect the energies of various excited states in different ways [1, 2, 68]. It is well established that the specific H-bond interactions strongly influence the n tt transition of carbonyl compounds [1,2, 68, 69, 75, 76]. In this case, the H-bondlng Interactions (Aw ) stabilize the ground state better than the less-dipolar excited state. Hence, the H-bonds donating solvents exhibit the solvent-induced blue shifts (negative solvatochromism). On the other hand, the presence of H-bonds can also strongly influence the intensity of the CT absorption band (tt TT ) as it is observed in tire case of the p-nitroaniline (PNA) molecule [77]. 

In order to take into acconnt this ionic liqnid effect on the solvatochromic property (T) of the mixtures, the wave numbers of maximum absorptions of the solvatochromic indicators (A,iV-diethyl-4-nitroaniline, 4-nitroaniline and Reichardt s betaine dye) at different compositions of SI (alcohol) and S2 (IL) were fitted to the equations. 

A,A-Diethyl-4-nitroaniline is sensitive to the dipolarity/polarisability of the solvent. Since it shows positive solvatochromism, a decrease in the property increases the wave number of maximum absorption. It is possible to show that the wave numbers of maximum absorptions of pure solvents (Tj and increase in the order [bmim][Cl], [bmim][BF ], [bmim][PF ], MeOH, EtOEtOH and EtOH. The wave numbers of maximum absorptions of the mixed solvent S12 (T ) are close to... 

Table 5.5 shows a decomposition of electrostatic and polarization effects to a total solvatochromic shift in para-nitroaniline (pNA) solvated by several water molecules. In these calculations water molecules are represented by EFPs while pNA is contained in the QM region. The indirect electrostatic and polarization terms arise from Eq. 5.14, i.e., these are the contributions to the electronic energies due to one-electron Coulomb and polarization terms in the QM Hamiltonian. One can think of the indirect terms as of orbital relaxation of the QM subsystem due to the EFP terms in the QM-MM Hamiltonian. As follows from Table 5.5, the electrostatic... 

The 13 scale has been amended and expanded through three subsequent studies. In the first of these (76), the solvatochromic comparison method was applied to three HBD indicators, 2-nitroaniline (6), 2-nitro-p-toluidine (7), and 2-nitro-p-anisidine (8), and their corresponding N,N-dimethyl derivatives (6a, 7a, 8a). 

The indicators whose solvatochromic behavior was analyzed in terms of Equation 42 were ethyl 4-aminobenzoate (9), 4-aminobenzophenone (10), 3,5-dinitroaniline (II), 3-nitroaniline (12), and N-ethyl-3-nitroaniline (13). Because, with amphiprotic HBA-HBD solvents, these indicators form type-AB... 

Structure-Property Relationships, The studies aimed at construction of the (8 scale and related investigations have uncovered some interesting relationships between indicator structures and solvatochromic effects. It was found (78c, 134) for example, that 4-nitroaniline (1) forms two hydrogen bonds to HBA solvents, that the ratio of the hydrogen bond strengths is about 1.5/1, and that the ratio of the bathochromic spectral effects is 1 /(0.93 0.13). Comparable effects have also been observed with 3-nitroaniline (12). 

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