1196
ROBERT H. LINNELLASU A. KACZMARCZYK
not validated by his own data. Whereas he assumed he was moving across a simple crystallographic phase transformation line, he actually was moving across miscibility gap boundaries. Superimposing Schafer's phase transformation line on the present diagra,m one observes the following interesting consequence of varying Nb205 and Taz05 concentrations to either induce or verify reversibility of transformations. Starting with a-Nb20r,, an addition of sufficient @-Ta205will result in a sample having a composition in the P - T a ~ 0 5field. ~~
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If one adds more a-Kb205, the displacement in composition can now carry into the a-Nb206 field. Since metastable y-Nb2054.5is crystallographically indistinguishable from stable /3-TazOs, the impression created is that reversibility has been achieved and that a thermodynamic phase transformation does exist,. Acknowledgment.-The authors wish to t,hank Miss B. Agule for sample preparation and Mrs. C. I'fister and Mrs. M. Witzen for collection of X-ray data.
ULTRAVIOLET SPECTRA OF --N=
L (A=N-
COMPOUNDS'
BY ROBERT H. LINNELL AND A. KACZMARCZYK~ Department of Chemistry, University of Vermont,Burlington, vt. Receiaed Janunry 13, 1961
Ultraviolet spi5ctra in buffered aqueous and sulfuric acid solutions for 2,2'-bipyridine, o-phenanthroline, 2,2'-biquinoline + ~ K B E *values + + are determined from the spectra and correlated with the structures and pyrazine are reported. ~ K B Hand of the several compounds.
I
t
Compounds containing the --X=C-C=Xstructure have been widely used as chelating agents. Ionization constants have been reported for several of these compounds, and t'he previous literature results as well as those reported here are summarized in Table I. Two of our compounds (2,2'-bipyridine and 2,2'-biquinoline) c,ould exist in cis or tranc: form or in some intermediate noiiI
1
planar form involving twisting about the =C-Cbond. 'The o-ther two (0-phenan t'hroline and pyrazine) are planar and must remain so whether existing as B, B:H+ or BHz++ forms.3 Nakamoto4 has rep'orted recently the ultraviolet spectra of 2,2'-bipyridine and 2,2',2''-tripyridine in aqueous solutionis of various p H ; for the former, bands at 279 and 232 mp in basic solution and organic solvent are considered to indicate trans form, a i d bands at 301 ;and 240 mp in acidic solution similar to bands for Ni(bipy)&lz indicate c i s form (favored by electxostatic considerations). M. Kashaj has suggested that H2S04could be used to tie up bot'h non-bonding electrons in diazines. Hammct tG has earlier useti spectrophotometric met'hocls for determining pKa values of very weak bases using H2S04,and recent work by Brand7 and eo-workers has yielded pK, values for aromatic nitro-compounds I)y t'his technique. We were interested in ~ K B H+ ~wl..ies + for the four -X=C--C=X(1) T h e 's.x:xriini:ntal p a r t of this work was performed in the Chemistry Laboratories. American University of Beirut, Beirut, Lebanon. (2) Departnirnt of Chemistry, Harvard University, Cambridge, Mass. (3) Ttiroiiphout this paper we use B to represent t h e free baee, B H + t o regresent t h e mono-cation, and B H t t t to represent t h e dication. (4) I
