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2200
= 5.2 X min.-l for dissociation (reverse Diels-Xlder reaction) a t 150'; these values represent a difference in free energy of activation of 1.1 kcai. per mole. More accurate determinations of the kinetic parameters are planned. Ti'hile our experiments offer no new insight into the mechanism of the Diels-Alder reaction, they exclude participation of a diradical intermediate with a long enough lifetime to permit racemization. Strong moral support is provided for the presumption3 t h a t the rearrangements and the reverse Diels-Alder reaction proceed along the same energy path. T h e possibility t h a t different energy paths are involved (the rearrangements are forrnaily Cope rearrangements) l 2 is not rigorously excluded ; but a t least with methacrolein dimer, both reactions occur simultaneously. Our thanks are due to Dr. S.L. M a n a t t and M r . D. 31. Kuwada of the Jet Propulsion Laboratory for their good offices and technical help in obtaining the mass spectra. (12) A. C. Cope and E XI,Hardy, J . .A a n d later papers.
ii7.
Ciiciiz. S o t . , 62, 4 4 1 (1940),
T*ol. h3
has not heretofore been detected in s o l u t i o ~ i . ~ 1Ve.lind that a solution of p n v c ~tetraadriit nizrririuin m ~ bec picpnvcd in sntuvatcd animoiziiiiii ,flzioritic.. Black .lrn(OH)d was freshly prepa.red by heating Ll-lln(OH)3with 0.2 dlIKaOC1-0.2 . l i Sa0I-I." I t was treated with saturated aqueous anirrioniuni fluoride in an attempt to produce a solid a ~ m i i o ~ i iu~n-.lm(IVjfluoride c o m ~ x ~ u n dT. o our surprise, the slurry of Xm10H)4 c o i i i p / r f c / ~d~i s s o ! s d in the I,? AI SHAFsolution to give an ariiericium ~ 0 1 1 centration of O . ( l l -11. The resulti~igclear (pinkredj solution showed absorption peaks which could not be attributed to any soluble americium species previously known. Uoreover, this spectrum bore a striking resemblance to the absorption slwtrurii of solid -lniF4:* PRISCIPAL ilin(II-) PEAKS AmFa (solid)
Sl80 8620 i440
Am(IV) in 15 .M XHIF
9230 Broad, asym 8130 '7400 Broad
AmFl (solid)
15 'it. SHal;
5680
3ii87
3361) 4860
4818
A m f I V ) ill
3:"J
7030 .. 4530 I X l1 COSTRIBUTION No. 2685 6390 6387 3760 :i;71 GATESAND CRELLINLABORATORIES OF CHEMISTRT OF TECHSOLOGY RAYMOXD P. LVTZ CALIFORSIA INSTITUTE The absorption spectrum of Ani(I1-j ilk 13 -11 P A S A D E S A , CALIFORNIA JOHX D. ROBERTS RECEIVED MARCH11, 1961 XH4F was obtained fro112 3,000 to 12,000 (Fig. FIRST OBSERVATION OF AQUEOUS TETRAVALENT AMERICIUM'
szv: Americium chemistry has been studied b y workers a t laboratories in the U S , U K , and U S S R for over ten years However, only americium valence states of (111), (Y),and (VI) have been
I-
z Y
225 LL
w
0 0
2
0 I O -
-z IX
w
a 5 0 0
5
4000 I
6000 1
I
Fig. 1.-Absorption
8000 I
I
I
10000 I
I
I
ivave lengt~i, spectrum of xrn(1i;)
i i i l 2 J I SH41;; 5 cm. quartz cells, 15M XH4F reference solution.
characterized in aqueous solution. Coinpounds of tetravalent americium prepared by anhydrous means are known, and tetravalent americium has been looked for in aqueous solution, e.g., during the reduction of Am(V) and (VI); during the disproportionation of Am(V); and on dissolution of americium dioxide. Although evidence was found for reactions involoing Am(1V) as a kinetic intermediate, all previous work agrees t h a t .%ni(T\7 (1) This n u r k S i
1). The peak a t 4360 A. ( E = 25) is sharp and well separated from principal peaks of other americium valence states. It is useful for identification and determination of Xni(1V). Heating the solution of Xm(IP) in 1.5 AI SIIJ; to 90' does not cause disproportionation of Am ( I l -1 or its chemical reduction by water. Treatment of X T ~ ( I Vin) 13 ;IT N H 4 F with ozone a t room temperature yields -lm (VI) which, 011 reduction, again forms the soluble .Ini(IV) com plex. Reduction of Xm(1Vj to Xtn(I1Ij is observed due to Ain?-" alpha radiation. Iodide also reduces A l m ( I V )to Xm(1Ii). Treatinent of A%m(OH)* with saturated RbF or o gives solutions containing primarily Both Th(1T;) and C(1V) are soluble in icentrations of potassium, rubidium and arnnioriiurn fluorides. I t would be expected t h a t lu [ I l7I and Xp(1Y j would behavc similarly~ Structures of the crystalline aIkaIi---hi~( IY) f!~:orides obtained from solution are currently beiri? in\-estigated by F. H. Kruse and L. R. Asprey 01 this laboratory. L.B. :ISPRI;Y LOS.\LAXOS SCIEXTIFIC LABORATORY
r?), J,. B. As[,rey, & b i d ,7 6 , 2019 i l H 5 4 ) ; S R.G u n n 1nr1 14 13 (19,j7); G. R. Hill a n d ; C;. S 17akovlc\. and ;T?C o f Aroiilic I:ile,.ay, 7 man and I, B .&sprey, ibzd , 7 G. R. Hall and T. L . .\larkin, .I. i'izov
