Solvents, acceptor properties

Acceptor numbers of various solvents are also listed in Table 3. The values range from zero, for the reference solvent -hexane, to about 130, for trifluoro-methane sulfonic acid. For instance, the acceptor number of aliphatic alcohols varies between 27 and 41 (methanol). Within the group of dipolar aprotic solvents there are considerable differences in acceptor properties. Solvents such as propylenecarbonate, tetramethylene-sulfone, acetonitrile, dimethylsulfoxide, or nitromethane are stronger acceptors than solvents such as acetone, A-methylpyrroli-done, or dimethylacetamide. The acceptor strengths of amine solvents vary considerably with the degree of substitution. For instance, triethylamine has no acceptor properties. 

Despite this, they are good solvents for chloride-ion transfer reactions, and solvo-acid-solvo-base reactions (p. 827) can be followed conductimetri-cally, voltametrically or by use of coloured indicators. As expected from their constitution, the trihalides of As and Sb are only feeble electron-pair donors (p. 198) but they have marked acceptor properties, particularly towards halide ions (p. 564) and amines. 

Methoxypurine was found to crystallize as a hemihydrate from /V,/V -dimethyl formamide, and as a trihydrate from water [63]. Thermal treatment of the trihydrate could be used to obtain the hemihydrate. Zafirlukast was obtained in the form of monohydrate, methanol, and ethanol solvatomorphs, with the drug substance adopting a similar conformation in all three structures [64], In the isostructural methanol and ethanol solvates, the solvent molecules are hydrogen-bonded to two zafirlukast molecules, while in the monohydrate, the water molecules are hydrogen-bonded to three zafirlukast molecules. The structures of the acetone and isopropanol solvatomorphs of brucine have been reported, where the solvent controlled the self-assembly of brucine on the basis of common donor-acceptor properties [65],... 

Since the values of for many organic acceptors are generally unobtainable (in organic solvents), an alternative measure of the electron-acceptor property is often based on the irreversible cathodic peak potential F (in cyclic voltammetry). Thus for a series of related compounds, Fig. 6 shows that the values of Fred are linearly related to gas-phase electron affinities (EA).70... 

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). 

The different behaviour of HC104 and Ph3 COH in both water and sulfuric acid can now be interpreted Perchloric acid, which has pronounced acceptor properties, reacts readily and completely with the donor solvent water and ionization occurs but there is no interaction with the acceptor solvent sulfuric acid and, hence, no ionization is observed in the latter solvent. Triphenylcarbi-nol, on the other hand, reacts with the acceptor sulfuric acid and not with the donor solvent water. 

The B state responds much less to changes in donor and acceptor properties than the TICT state, and Eq. (5.1) can often be easily fulfilled by increasing donor and/or acceptor strength. In addition to these two factors which deliver the decisive part of the reaction driving force, polar solvent stabilization SOiv and the mutual Coulombic attraction C of the linked donor and acceptor radical anion/cation also help to preferentially stabilize the TICT state with respect to the precursor B state. 

Dorr and Buttgereit (1963), using U.V. measurements of solutions of mesitylene and hexamethylbenzene in chloroform, bromoform, carbon tetrachloride and carbon tetrabromide, were able to demonstrate a weak electron transfer complex formation with these solvents. These investigations show that hexamethylbenzene is a more powerful electron donor than mesitylene. A similar influence on an acceptor property... 

To summarize, the four selected solvents represent different selectivity aspects DEA is a strong base, CHCI3 is a proton donor, EtAc is a dipole interactor and MeOH has both proton donor and proton acceptor properties. This means that the solvent systems which can be obtained by mixing these solvents can provide for a large variation in selectivity. 

As expected, the evolution of the equilibria A (or A ) (or B ) and A (or A ) = C (or C ) depends on the temperature and on the nature of the solvent, but more particularly on the structure of the groups linked to the P or N atoms, i.e. on their steric characteristics, their electronegativity, their electron donor or acceptor properties, etc. The sum of these interactions results either in the creation of a single structure (A, B or C), or in the formation of an equilibrium when the difference in the thermodynamic stability of the various species is only slight. 

Finally, inspection of Table 3.2 shows also that there are cases in which Yu can be even smaller than 1. An example is a solution of diethylether in chloroform. Here, the solute is an electron donor (H-acceptor), while the chloroform solvent is an electron acceptor (H-donor). In this case, the solute and solvent both acquire additional inter-molecular interactions that were unavailable to them in their pure liquid forms. The monopolar diethylether (only vdW interactions in its pure liquid) can add polar interactions to its vdW attractions with the molecules of the monopolar chloroform solvent exhibiting a complementary electron acceptor property. 

Finally, with respect to the H-acceptor properties of the solvents (a-term), water and n-octanol are quite similar. Therefore, for a hydrogen-bonding solute like 4-BuPh, the corresponding product, a-(a), is close to zero. This is not the case for the hexadecane-water system where loss of hydrogen bonding in this alkane solvent causes both the H-acceptor and H-donor terms to contribute factors of about 100 to 4-BuPh s value of K. ... 

Thus this model is capable of explaining the variation of redox potential with solvent in terms of the coordinative interactions between the ions of the redox couple and the solvent, i.e. redox potentials are determined by the donor or acceptor properties of the solvents. An important point of this model is that the nature of the binding forces between the solute and solvent are not specified. It should therefore be possible, once the linear relationship between the redox potential and the donor number or acceptor number of the solvent is established, to predict the value of the redox potential of that same couple in another solvent. 

From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longest-wavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109],... 

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

The first systems without anomalous fluorescence discussed in the context of TICT states were coumarine laser dyes. Jones and coworkers208210 demonstrated that dialkylaminocoumarines like 7C showed an increase of the nonradiative decay rate in strongly polar solvents. This effect is dramatically increased if the acceptor properties of the coumarine skeleton are stronger, for example, in F7C. On the other... 

Tetraorganotin compounds, R4Sn, show weak acceptor properties and do not form solid adducts, with the possible exception of Me3SnCF3, which is reported (53) to form a 1 1 complex with hexa-methylphosphortriamide. Tetramethyltin, for example, has little or no tendency to form complexes with a wide range of solvents, (9) since the chemical shift changes are very small (Table V). 

The donor ability (nucleophilicity) of R2M , including the stability of complexes being formed, increases as M becomes heavier. The acceptor properties of M in R2M (such as the ability to form the adducts with Lewis bases, for example, with pyridine and piperidine at — 30 °C) are determined by the low-lying unoccupied atomic d- and pz-orbitals160. Stable free radicals R3M (M = Ge, Sn) are obtained by a photochemical disproportionation reaction of R2M in a hydrocarbon solvent medium160 see equation 26, R = (Me3Si)2CH. 

At present much research is devoted to donor-acceptor interactions [6, 20-22]. Donor-acceptor properties of solvents are also investigated [21, 23]. 

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