Panasenko A. I., Samura T. A., Buryak V.P., Gotsulya A.S., Guzhva A.A., Vovnjanko O.I., Kulish S.N.

National University of the life and environmental sciences of Ukraine

Zaporozhye State Medical University

Formation of ionized species and hydrogen bonding of the some thiazoles and benzothiazoles.

The spectrum of benzthiazoline-2-thione in solvents with different polarity is reported and a solvent dependence of the charge-transfer band is noted. Blue shifts are observed in hydroxylic solvents, where hydrogen bonding from solvent to the thion sulfur atom occurs and shifts in solvents where the benzotiazoline-2-thion functions as a hydrogen bond donor. In solvents where the compound acts both as a donor and acceptor, are observed zero shifts.

These conclusions are supported by the studies, in a range of critical solvents, of spectrum of the N-methyl derivative, which can function as a hydrogen bond acceptor only, and shows blue shifts only.

The spectra of benzthiazoline-2-thione (I), 3(N)-methyl-benzthiazaoline-2-thione (II) and 9-Me derivative (III) in concentrated sulfuric acid closely resemble with each other (Table I)

Table I

The spectra of I, II and III in concentrated sulfuric acid

We attribute this similarity to the formation of the singly-protonated species IV, V, VI, having similar energy levels. The strong evidence for the protonation of the double-bonded sulfur in thiamides is discussed by Talse and Orchin [6]. The representation with partial charge on the sulfur and nitrogen is preferable to that single positive charge on the nitrogen is discussed by West [12] for methyl-benzthiazoline-2-thione. The more basic character at (III) is shown by its protonation in dilute hydrochloric acid (233, 251, 259, 280, 290 nm). The fact is that (I) in sodium hydroxide solution, in which the species present is the ion VII, absorbs at approximately the same frequency as (I) in concentrated sulfuric acid is fortuitous. All the negative charge would not residue on the sulfur atom [7]. As expected the spectra of II and III, in sodium hydroxide solutions are almost identical with these in water.

The effects of solvent an electronic transitions have been studied for some considerable times and it is now established that hydrogen bonding [9] has a much greater effect than other types of solvent-solute interactions. The main theories for shifts due to van der Waals forces between solvent and solute molecules predict relationships between these shifts and the dielectric properties of the solvents [4,8]. Berson [2] has used the mole-bond density of the solvent to correlate the solvent shifts, observed in determinations of the spectra some aromatic hydrocarbons. Initially, we studied at about eight solvents; we found that we could obtain appropriate linear relationships between the absorption frequency for peak of band at 310 nm (2M NaOH) to 327 nm (ethanol) of benzbhiazoline-2-thione (I) and functions of the solvent, dielectric constant or refractive index.

The compound investigated: benzthiazoline-2-thione(I), N-Me derivative of benzthiazoline-2-thione(II), S-Me derivative of benzthiazoline-2-thione(III), benzthiazoline-2-thione(IV) and benzthiazoline-2-thione(V) have high extinction coefficients for the main UV-absorption bands and can therefore be examined in very dilute solution. For crystalline benzthiazoline-2-thione Bauman [1] have shown that the molecules are arranged in hydrogen bonded chains among two – fold screw axis with a strong hydrogen bond to sulfur for which one N-H..... S length is 248 nm. The hydrogen bond in crystalline benzthiazoline-2-thione is considerably stronger than in thiopyridyne [10], where the N-H…S length is 326 nm. The effect of hydrogen bonding UV-spectra is because a blue shift with bond broadening is the chromophore is a hydrogen-bond acceptor and a red shift with similar broadening of the band when the chromophore is a hydrogen-band donor. In the oligameric state (I) functions as both a hydrogen-bond acceptor and donor (except for terminal molecules), there is little next change in the absorption frequency. Study of the change with solute concentration in calcium tetrachloride of the relative intensities of the free and hydrogen-bonded N-M infra-red bands shows that tiazolidine-2-thione, behaves very similarly to benzthiazoline-2-thione, and thiazoline-2-thione, forms either cyclic species or very stable linear oligomers of relatively high overage degree of polymerization. [3]

In non-hydrogen-bonding solvent such as aliphatic and aromatic hydrocarbons and their halogen derivatives no shift is observable and the shape of the band remains unchanged. It is important to establish which species is responsible for the UV-absorption, but unfortunately it is not passible to use the infra-red region, where more direct information regarding the extent of hydrogen bonding can be obtained.

The spectrum of benzthiazoline-2-thione in solution in diethyl ether is very nearly identical with that in n-hexane. This is consistent with the view that in this solvent at the concentration of 10^-5M, the benzthiazoline-2-thione molecules exist almost completely as monomers. Similar can considerably apply to the spectrum in anizole chromophore (VII):

Both anizole and diethyl ether have a very low tendency do hydrogen band [5].
It has already been noted that benzthiazoline-2-thione in the monomeric and oligomeric states has the same absorption band frequency. This can be accounted for by postulating that when (I) functions as a hydrogen band donor and acceptor the next change in the ground-state energy of the charge-transfer electron is zero. That comparable shifts are produced by donation and acceptance of a hydrogen band by (I) shown the blue shift at 3 nm in ethanol and red shift at 3 nm in acetone or methyl ethyl ketone.

A critical test of these conclusions is provided by the behavior of 3(N)-methyl-benzthiazoline-2-thione in hydroxylic and basic solvents. In hydroxylic solvents there is a blue shift at 10 nm, comparable with benzthiozoline-2-thione (I). In dimethyl formamide solution there is no detectable red shift and in this case none would be expected since there is no possibility of hydrogen band formation. This shows conclusively that this solvent shifts are to attributable primarily.

Solute-solvent hydrogen bonding and not to differences in dielectric constant and refractive index of the solvents since there is a very wide range.

In glacial acetic acid band in the ranges at 321-332 nm shows more detail then in n-hexane, although the position of the maximum remains unaltered so that(I) function as both donor and acceptor at hydrogen bands and there is no change in absorption frequency. Monocarboxylic acids have been shown to exist as cyclic and open dimers so that it is possible for benzthiazaline-2-thiane to associate with acetic acid molecules either as in VIII or IX are formed, because at the strong hydrogen band donor properties of the hydroxyle group and acceptor properties of the carbonyl group of the acetic acid .The spectrum for solutions of benzthiazoline-2-thione in n-butanole closely resembles, that in glacial acetic acid indicating that the benzthiazoline-2-thiane molecules function as both donors and acceptors. This would be consistent with the change in hydrogen bond strengths referred to in connection with the blue shifts observed in ethanol, methanol or water.

Results

1. The spectra of benzthiazoline-2-thione (I) N-methyl-benzotiazoline (II) and s-methyl-benzthiazoline (III) in concentrated sulfuric acid closely resemble each other.

2. The more basic character of S-methyl-2mercapto-benzthiazole is shown by its protonation in dilute hydrochloric acid.

3. The effect of hydrogen bonding to thiazoline-2-thione in hydroxylic solvents is similar to those benzthiazoline-2-thione.The solvent shifts for thiazoline-2-thione there are red shifts in ethanol and methanol and zero shift in water with respect to n-hexane.

References.

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