B U L L E T I N D E L ' A C A D É M I E P O L O N A I S E D E S S C I E N C E S S é r i e des sc iences c h i m i q u e s Volume X X V I I I , No. 4, 1980

PHYSICAL CHEMISTRY

Electron Donor-Acceptor Phenomena. VI.*) Hexamethylphosphotriamide and Trinitrosubstituted Benzenes

by

Krystyna POBŁOCKA, Tadeusz URBAŃSKI, Witold WACŁAWEK

Presented by T. URBAŃSKI on March 7, 1980

Summary. Complexation constant and enthalpy data have been determined for charge transfer complexes of hexamethylphosphotriamide with trinitrobenzene and trinitrotoluene in 1,2--dichloroethane solutions from —15.5 to +22.0°C.

Hexamethylphosphotriamide (HMPT), one of the best aprotic solvents, was found to be suitable for investigation of the CT-complexes [1-3]. However, in spectrophotometry studies on molecular complex formation of pyrimidine and purine bases with D-mannitol hexanitrate ( M H N ) in H M P T [2], il appeared that this solvent is an electron donor. It seems to cause an appearance of bands at 450 nm and 310, 390 nm for M H N - H M P T and MHN-pyrimidine, purine base systems, respectively. Adenine does not produce a new band. In the cases when pyrimidine and purine bases were studied with erythritol tetranitrate in H M P T [2], the new bands have not • been observed. Similar studies have been carried out with the high-resolution NTV1R technique [3] for the HMPT-erythritol tetranitrate in chloroform-D 6

and no changes in proton signal resonance positions were observed for both H M P T and erythritol tetranitrate. This seems to be in agreement with the conclusions [2] that the CT-complexes are not focmed in this system. On the other hand, it was shown by dipole moments, calorimetric and N M R measurements that H M P T forms complexes with chlorides of transi­tion metals [4]. It seemed of interest to study complexing ability of H M P T with 1,3,5-trinitrobenzene (TNB) and 2,4,6-trinitrotoluene (TNT) because both electron acceptors form molecular complexes with a great number of com­pounds which were thoroughly studied by various physical methods.

*) Part V: Bull A c Pol.: Ch im , 22, 1 (1974).

[277]

278 К. Pobłocka et al.

Experimental

Materials. T N T and T N B were purified by recrystallization from ethyl alcohol until m.ps. 80.5 and 130°C, respectively, were reached.

H M P T and 1,2-dichloroethane were redistilled just before the preparation of the solutions. Determinations. To determine the composition of molecular complexes formed the Job

method of continuous variation was applied using N M R [5] and UV-V1S spectrophotometry [6] measurements. 1,2-Dichloroethane was used as a solvent for all the studied systems The solutions were prepared by mixing x cm J of 0.1 mole/kg of H M P T solution with (5-x) cm 3 of 0.1 mole/kg of T N T or T N B solution, both in 1,2-dichloroethane (where x varied from 0 to 5 cm3). N M R spectra were measured on a BS 487 С spectrometer— Tesla Brno (Czechoslovakia). Proton signal positions in the spectra of T N I or T N B electron acceptors, were determined with an internal 1,2-dichloroethane resonance signal. In the UV-VIS spectrophotometry measurements for H M P T T N T and H M P T - T N B systems the changes in the 510 and 315 nm absorption band intensities were followed!'The complexation constants were evaluated for both N M R and UV-VIS spectra data with the Bencsi Hildebrand equation [7]. Concentration of the donor H M P T was changed within 0.05-2.0 mole/kg of the solution (for both N M R and UV-VIS measurements), while that of the acceptor (TNT or TNB) was kept constant at 0.01 mole/kg of the solution (in the N M R determination) and 1.5• 10~ 3 mole/kg of the solution (in the UV-VIS measurement) Al l the solutions were made by weight.

The Job plots are shown in Figs. 1-4. In all the Figures a and b refer to stock solutions of the 0.1 and 0.05 mole/kg concentrations, respectively. The data in Figs. 1 and 2 were derived from the N M R measurement, whereas those in Figs. 3 and 4—from the UV-VIS measurements. It may be seen that all the curves exhibit a maximum at xD = 0.5, i.e. it indicates the molar ratio of the H M P T - T N B (TNT) complex of 1:1. The results for the complexation constant К values (for both methods) are summarized in the Table. The stability data determined by both methods are very similar. Moreover, one can see that the H M P T T N B complex is more stable than that of H M P T - T N T , which is in agreement with the electron acceptor

Results and discussion

T A B L E

Complexation constants К (determined from N M R and spectrophotometry measurements) and enthalpy AH

(from N M R measurement)

Complex A: [kg/mole] N M R

К [kg/mole] optical

Ml [kcal mole] N M R

H M P T T N B H M P T - T N T

0.19 0.13

0.20 0.13

-0 .9 -0.8

Electron Donor-Acceptor Phenomena. VI 279

•10 1.2

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Fig. 1. N M R Job plots for the H M P T T N B system in 1,2-dichlorocthane at

room temperature a—0.10 mole kg of Ihc solulion, b -005 mole kg of the

solution (in all figures)

Fig. 2. N M R Job plots for the H M P T T N T system in 1,2-dichlorocthane at

room temperature

NO,

/

/ / 0 \ \ /V \ \

i i i ' i i I I I—2łl 0.2 0.4 0.6 0.8 10

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F "g. 3. UV-VIS Job plots for the H M P T Fig. 4 UV-VIS Job plots for the H M P T - T N T • NB system in l,2-dichloroethane at 315 nm. system in 1,2-dichloroethane at 510 nm, at room

Bt room temperature temperature

280 К. Pobłocka et al.

abilities of the two nitroaromatic molecules. This is not always the case, for example an examination of charge transfer complexes of azaaromatic with T N B and T N T [8] showed that monomethylated pyridines form more stable complexes with T N T than with TNB. This effect was explained by dipole-dipole interaction between the T N T and pyridine molecules, resulting from geometry of the complexes. Enthalpy values for the H M P T - T N B and H M P T - T N T complexes are also included in the Table. The data were obtained from measurements carried out in the temperature range —15.5 + + 22.0°C. Comparison of enthalpy shows that both complexes are week with comparable stability; probably they are of a-n type.

INSTITUTE OF ORGANIC CHEMISTRY AND TECHNOLOGY. TECHNICAL UNIVERSITY, KOSZYKOWA 75, 00-662 WARSAW

(INSTYTUT CHEMII I TECHNOLOGII ORGANICZNEJ. POLITECHNIKA WARSZAWSKA) DEPARTMENT OF EXPERIMENTAL PHYSICS, PEDAGOGICAL UNIVERSITY, ZAWADZKIEGO 13/15,

42-200 CZĘSTOCHOWA (ZAKŁAD FIZYKI DOŚWIADCZALNEJ, WSP. CZĘSTOCHOWA)

R E F E R E N C E S

[1] T. U r b a ń s k i , Wiad. Chem., 25, 288 (1971). [2] T. U r b a ń s k i , W. P o ł u d n i k i e w i c z , Bull. Ac. Pol.: Chim.. 21, 87 (1973). [3] T. U r b a ń s k i , K . P o b ł o c k a , W. W a c ł a w e k , Mater. XI Oyólnopol. Semin. n.t. NMR

i jego zastosowań, Instytut Fizyki Jądrowej, Kraków, 1979, 321, Raport P L No. 1043. [4] E. N . G u r y a n o v a , I. P. G o l d s t e i n , I. P. R o m m , Donorno-aktseptornaya sviaz,

Khimiya. Moscow, 1973. [5] R. Sahai , G . L. Loper , S. H. L i n , H . E y r i n g , Proc. Nat. Acad. Sci. USA, 71,

1499 (1974). [6] E. Wie teska , in: Podstawy spektrofotometrii absorpcyjnej w świetle widzialnym

i nadfiolecie, ed. by T, Nowicka-Jankowska, Ossolineum, Wroclaw-Warszawa-Kraków, 1969, p. 227.

[7] R. Foster , Mol . Complexes, 2, 107 (1974). [8] W. W a c ł a w e k , K. P o b ł o c k a , Bull. Ac. Pol.: Chim., 26, 141 (1978).

К. Поблоцка, Т. Урбаньски, В. Вацлавэк, Электронные явлении донор-акцептор. VI . Гексаметилофосфотриамид с тринигробензолом и тринитротолуолом

Измерены константы комплексообразования и энтальпии комплексов с переносом за­ряда гексаметилфосфотриамида с тринитробензолом и тринитротолуолом.