Communications to the Editor
1334
reinforces suggestions that some polynuclear hydrocarb o n ~ and ~ - their ~ heterocyclic analogss tend to orientate in solution with long axes parallel; this is somewhat analogous to explanationsZ5bof the ASIS in terms of a solvation model. Although IN suggests a weak interaction, pmr affords no evidence of formal association of I with CC14.
Acknowledgments. e thank Dr. D. Shaw for recording the 100-MHz spectr and acknowledge helpful discussions with Dr. P. A. Jones. R. B. M. is grateful to the late Professor C. A. Coulson for the hospitality of his Group at the Mathematical h t i t u t e , University of Oxford, and to the Science Research Council for a Research Fellowship. ( 1 ) W. Schneider, ti. J . Bernsteh and J. A. Pople, J. Amer. Chem. SOC.,80, 3497 (1958). ( 2 ) K. 8 . Bartle and J. A. S. Smith, Spectrochim. Acfa, Sect. A, 23, 1689 (1967) (3) K. D. Bautle, D. W. Jones. and J. E. Pearson, J. Moi. Spectrosc., 24, 330 (19fi7). (4) C. W. Haigh and R. B. Mallion, J. Moi. Spectrosc., 29, 478 (1969). (5) R. B. Mallion, Ph.D. Thesis, University of Wales, 1969. (6) K . D. Bartle. D. W. Jones, and R . S. Matthews, Tetrahedron, 27,
5177 (1971). (7) R. Anderson and J. M. Prausnitz, J. Chem. Phys., 39, 1225 (1963). (8) K. M. C. Davis and M. F. Farmer, J. Chem. SOC.€3, 859 (1968). (9) i . Weilbron, Ed., "Dictionary of Organic Compounds," 4th ed, Eyre and Spottiswoode, London, 1965, p 1440. (10) K. D. Bartle and D. W. Jones, J. Mci. Specfrosc., 32, 353 (1969).
A. L Van Geet, Anal. Chem., 40, 2227 (1968). C. W. Haigh and R. B. Mallion, Mol. Phys., 22, 955 (1971). M, L. Heffernan, A. J. Jones, and P. J. Black, Aust. J. Chem., 20, 589 (1967). R. B. Mallion, J. Mol. Spectrosc., 35, 491 (1970). N Jonathan. S Gordon. and B. P. Dailev, J. Chem Phvs.. . . 36,
2442-(1962).' (16) K. D. Bartle, D. W. Jones, and R. S. Matthews, J. Mol. Struct., 4, 445 (1969). (17) E. Clar, A. Mullen, and U. Sanigok, Tetrahedron, 25, 5639 (1969). (18) (a) The change in densityleb of CCI4 between 273 and 323 K corresponds to the following shifts (Hz K - ' at 100 M H z ) H-1, 0.006; H-2, H-3, H-7, 0.005; H-8, 0.003. (b) J. Timinermans, "PhysicoChemical Constants of Pure Organic Compounds," Elsevier, New York. N. Y.. 1950. (19) J. M. Purceil, H. Susi, and J. R . Cavanaugh, Can. J. Chem., 47,
3655 (1969). (20) L. F. Blackwell, P. D. Buckley, K. W. Jolley, and I . D. Watson, Aust. J. Chem., 25, 67 (1972). (21) S. J. Hu and S. I. Miller, Org. Magn. Resonance, 5, 197 (1973). (22) F. Smith and I. Brown, Aust. J. Chem., 26, 705 (1973). (23) L. Pauiing, J. Chem. Phys., 4, 673 (1936). (24) F. London, C. R. Acad. Sci., Paris, 28, 205 (1937); J. Phys. Radium. 8.397 (1937).J. Phvs 5, _ RR7 , - - ,, Chern. . . ..,.,_ _ _119371 (25) (a) R. McWeeny, Mol. Phys., I, 311 (1958); (b) E. M. Engler and P. Laszlo, J. Amer. Chem. SOC.,93, 1317 (1971). (26) C. W. Haigh and R. B. Mallion, Org. Magn. Resonance, 4, 203 (1972). (27) E. Clar, "Polycyclic Hydrocarbons." Vol. I, Academic Press, New York, N. Y., 1964, p94. (28) J. E. Prue, J. Chem. SOC., 7534 (1965). (29) R. L. Scott, J. Phys. Chem., 75, 3843 (1971) (30) G. Scatchard, Ann. N. Y. Acad. Sci., 51, 660 (1949). (31) R. Foster and C. A. Fyfe, Trans. FaradaySoc., 61, 1626 (1965). (32) G . R. Wiley and S . I. Miller, J. Amer. Chem. SOC.,94, 3287 (1972). (33) C. W. Haigh', R. 8. Mallion, and E. A. G. Armour, Moi. Phys., 18, 751 (1970). I
~
I
.
TIONS TO THE EDITOR
~ i ~ ~ 1 e ~ t~ -p T a ~~a in ~t ~Helium ~o ~n ~ ~ Publication costs assisted by Brigham Young Unwersity
Sir: Recently several authors have discussed the energy separation between singlet and triplet excited states of the helium atom.' They show that the usual interpretation2 is incorrect, since for accurate wave functions the average electronic repulsion is greater in the triplet states than in the corresponding singlet states. Messmer and Birsslb give results for wave functions of the form
'LJ == [@i,(%h,,(2)
* @2,(1)@d2W
(1)
where the S - spin function gives the 2IP state and the S+ spin functions give the 23P state. They compare two simple functions of type 1 with the accurate calculations of Pekeris.3 Their first pair of functions (*I, in which $ls and are Slater-type orbitals) give total energies which are very poor, and the repulsive energy of the triplet is less than that of the singlet. Their second pair of functions (qIt)give results that agree with the accurate values given by P e k e r i ~ . ~ The results for $1 given by Messmer and Birss are incorrect. Since Slater-type Is and 2p orbitals are identical with the corresponding hydrogenic orbitals, 9 1is the same pair of functions which Eckart4 used in his study of the The ,Journal of Physical Chemistry, Vol. 78, No. 13, 1974
excited states of helium. When Eckart's functions are used correctly, they give the results shown in Table I. For comparison, we also give accurate results for ZIP and 23P from P e k e r i ~It . ~is evident that the two-parameter Eckart functions give values for (1/r12) that agree well with the accurate values. The remarkable accuracy of these simple functions for the 2lP and 23P states encouraged us to make similar calculations for the 2lS and 23S states. We used a Z3S function of the Eckart form
*(23s) = [@l,(l)@2,(2) - @2JWIs(2)1§'
where @28
= N2se-r'r(A- C ' Y ) @ is = Nlse-cr
and A is chosen to orthogonalize 4lSand $zS. Using the Hylleraas-Undheim we wrote the 2% function in the form
*(2*s) = [ C ~ 6 * S ( 1 ) ~ I S (+2 )C ~ ( ~ ~ ~ ( ~+~ @ ~ , ( 2 ) @zs(lI@ Is @))Iswhere the approximate energy of the 2 ' s state is found by solving the second-order secular equation, the higher root
1335
Communications to the Editor LE I: Orbital Exponents in This Work and Expectation Values Compared to Accurate Values in Atomic Units (1 BUL = 27.21 eV) Accurate
Expectation State
21-P 2 3P %Y3*
2 3s
value
i
3250G
This work
value
-2.122 0.243 -2.131 0.266 -2.143 0.249 -2.172 0.272
-2.124 0.245 -2.133 0.267 -2.146 0.250 -2.175 0.268
being an upper bound for the 2% energy. The orbital exponent in $1, was fixed at 1.6875Gand the exponents in & and +as were varied to minimize the 2% energy. The results, given in Table 1, are in excellent agreement with accurate values given by K0h1.l~
Figure 1. First derivative esr spectra for uv photolyzed Fe(CN)63- in aqueous methano! at 77 K directly after photolysis showing features assigned to H&OH (a) and HCO (b) radicals.
i
References and Notes
3056 G
(a) E. R. Dawdson, J. Chern. Phys., 41, 656 (1964); 42, 4199 (1965); (b) R. P. Messmer and F. W. Birss, J, Phys. Chem., 73, 2085 (1969); (c:) J. Kinrriel, Phys. Rev. A , 5 , 1990 (1972); (d) D. Kohl, J. Chem. Phys.. 56, 4236 (1972); (e) J. Killingbeck, Mol. Phys., 25, 455 (1973). W . Kauzmann, "Quantiim Chemistry," Academic Press, New York, N. Y., 1957, pp :H9, 320. B. Schiff, H. Lifson, C. L. Pekeris, and P. Rabinowitz. Phys. Rev., 140, A1104 (1965). C . Eckart, Phys Rev., 313, 878 (1930). E. Wylleraas and B. Undheim, Z. Phys., 65, 759 (1930). H. Eyring, J Walter, a i d G. Kimball, "Quantum Chemistry," Wiley, New York, N. Y . , 4944,13 106.
Department of Chernishy Brigham Young Universiiy Provo. Utah 84801.'
Richard L. Snow* James L. Bills $
Received October 11, 1973; Revised Manuscript Received May 9, 1974
I =me
3
Figure 2. First derivative esr spectra for uv photolyzed Fe(CN)63- in aqueous methanol at 77 K, after slight annealing above this temperature to remove signals f r o m H&OH and HCO radicals, revealing features assigned to Fe(CN)5MeOH2-.
-
Low-Temperature Studies of Photolyses of Transition-Metal orinplexes. The Ferricyanide Ion Publication costs assisted by Cenfral Michigan University
Sa: At least three ma.jor modes of reaction of photoexcited transition-mtltal complexes can be distinguished: (i) electron ejection arising generally from irradiation within a CTTS absorption band; (ii) electron gain from solvent, arising after a charge-transfer process involving electron movement from the ligand to the metal (CTFL); and (iii) ligand loss, displacement, or rearrangement, arising frequently from d-d excitations, and depending upon differing ligand labilities in the ground and excited states. (In addition, localized absorption by ligands may be important but we arc: not concerned here with such processes.) Process i has heen studied by pulse photolysis methods for the liquid phsse, which reveal the presence of "solvated" electrons and in the solid state, which gives trapped electrons. While ferrocyanide ions (Fe(C N ) G ~) - readily lose electrons when exposed to light in the 200-250-nm range,l Fe(CN)& ions are not photoionized in this spect r d region.
However, processes ii and iii have only been studied extensively in fluid solution, under which conditions the technique of esr spectroscopy is generally of little use. The great advantage of solid-state photolyses is that primary or secondary products are often detected, and subsequent complex processes are prevented.2 In the present work, ferricyanide ions in various solvents and a range of concentrations have been exposed to light from a high-pressure mercury arc (313 arid 365 nm) at 77 K, and the products studied by esr spectroscopy. Under these conditions, both d-d and CTFL excitations O C C Uand ~ , ~products may be formed by either or both excitations. For methanolic and aqueous methanolic glasses, the esr spectra contain features that are unambiguously assignable to H&OH (DzcOD) radicals4 and HCO (DcO)5 formed from the solvent, and also broad features for a nonaxial Fe"' complex in the spin-paired (ds) configuration (Figures 1 and 2). In the absence of ferricyanide, no signals were detected. Also solutions of ferrocyanides treated similarly gave no solvent radicalsa6 We suggest that the CH20H radicals stem from a CTFL process, the electron-deficient ligand extracting an electron (or possibly a hydrogen atom) from an adjacent solvent molecule during the lifetime of the excited state The Journal of Physical Chemistry, Voi. 78, No. 13, 1974
