J. Phys. Chem. 1989, 93, 3511-3514 TABLE i X Atomic Populations (%) in MOs of P40s: EH Calculatian (and spd,901*)" P 0
2P (4)
0 0 0 39
(3) (1)
(48) 33 (65) 74 (66) 81 (80)
91 (76) 47 (94) 100 (96) 35 (1)
28 (26)
" (spd,90) values in parentheses. band C of As406. The reasonable success of the E H treatment in calculating the angular momenta of the A and B bands very likely reflects an element of topological control in the nature and sequence of the MOs in P406. That is, all calculations seem to lead to a (nearly) constant orbital sequence. We have noticed this34in relation to other cagelike structures of general type M,N, where all atoms M / N have a single type of environment. Some examples are As4S4 and P4S435and even (NSF)3 and (NSF)4.36 Given the wide divergence in methodology of the ab initio and EH methods, we pursue this point a little further. While ab initio and EH MO energies show notable differences (Table I1 vs Table VII), a Mulliken population analysis of the E H calculation (Table IX) shows striking resemblances to that of the earlier ab initio studyI2 (parentheses, Table IX), particularly for the higher occupied orbitals. We also note that few MOs are mixtures of P/O in close to even proportions and few have mixed s/p character. This is an ideal circumstance for topological control of the MOs and means that the exact parameters used in the E H calculation will not be critical. Comparing the E H and ab initio MOs (Table IX), we note that 5t2(HOMO) in both cases is very similar and is largely lone-pair phosphorus in character whereas the E H 3al is significantly more delocalized (more oxygen character) than (34) Palmer, M . H. Unpublished work. (35) Palmer, M. H.; Findlay, R. H. J . Mol. Struct. (THEOCHEM) 1983, 104, 321. (36) Palmer, M. H.; Westwood, N. P. C.; Oakley, R. T. Chem. Phys., in
press.
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its a b initio counterpart. (Some previous studies of ab initio vs E H MOs in N-heterocycles have noted higher delocalization of lone pair nitrogen orbitals in the latter.37,38) Orbital 2al is also significantly different, being largely oxygen 2p (ab initio) versus phosphorus 3s (EH). There is also a close similarity between the ab initio and EH wave functions for As406. The occupied (and virtual) sets of MOs have similar groupings in the two calculations (compare Tables I1 and VIII) including the relative shift of 4al toward the LUMO. However, the HOMO/LUMO gap is larger for As406vs P406 in the E H calculation while the reverse is true for the ab initio one. The strong resemblances of the EH and ab initio MOs and the reasonable success of the former in accounting for the excited-state angular momentum associated with bands A and B is encouraging. It suggests at least the possibility that this very simple approach can guide the interpretation of the MCD spectra of more complex systems where ab initio calculations are not feasible. A crucial test will come in the application to cagelike structures in which the participating atoms of a given kind are in more than one environment. We expect to address this point in future work. It would also be of interest to see whether the addition of higher angular momentum orbitals can rationalize the sign and magnitude of & , / B oin band C of As406.
Acknowledgment. We are deeply indebted to Dr. Jerry L. Mills, Department of Chemistry and Biochemistry, Texas Technical University, Lubbock, TX, for his generous donation of a sample of P406. We thank the technical staff of the Synchrotron Radiation Center, Physical Sciences Laboratory, University of Wisconsin-Madison for their generous assistance, Professor Patricia A. Snyder, Florida Atlantic University, for her loan to us of several items of equipment, Dr. Jenny Green, Oxford University, for the use of her ICONBprogram and Ms. Cary Boyle and Dr. David Clark for comments on the use of the program. We thank Dr. K. P. Lawley for pointing out the direct products arising from the T d / C , mapping. P.N.S. thanks Dr. Peter Day, the Fellows of St. John's College and the Inorganic Chemistry Laboratory, Oxford University, for most generous hospitality while a portion of this work was carried out. This work was supported by the National Science Foundation under Grant CHE8700754. Registry No. P406,12440-00-5;As,O,, 12505-67-8;Ar, 7440-37-1, (37) Clementi, E. J . Chem. Phys. 1967,46, 4731; J . Chem. Phys. 1967, 46, 4737. ( 3 8 ) Hoffmann, R . J . Chem. Phys. 1964, 40, 2745.
ESR Studies of eaq- in Liquid Solution Using Photolytic Production A. S. Jeevarajan and R. W. Fessenden* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 15, 1988) Continuous photolysis of solutions of sulfite ion in aqueous solution has been found to produce observable signals of eaq-in electron spin resonance (ESR)experiments. These experiments represent the first solution-phase ESR observation of this species in other than pulse radiolysis experiments. In this way, it has been possible to study the effects of solvent environment on the g factor and spin relaxation times of this species. The g factor decreases a small amount with increased temperature and is raised when D 2 0 is used as solvent. Measurements of the ESR signal which is out of phase with the fidd modulation (at 30 kHz) allowed the spin relaxation times to be measured (assuming T , = T2). The value of T I at 23 OC is 8 p s in H 2 0 and 9 p s in D,O; the relaxation time decreases with increasing temperature. A discussion of the implications of these results for the structure of eaq- is given. The chemical half-life was found, by competition experiments, to be about 100 p s . Introduction
The electron spin resonance (ESR) spectrum of the hydrated electron, e, -,in liquid aqueous solutions has only been detected in pulse radiolysis The spectrum consists of a 0022-3654/89/2093-3511$01.50/0
single narrow line at g = 2.0004. The solvated electron has also been studied in pulse experiments in methanol and ethanol4 and ( 1 ) Avery, E. C.; Remko, J. R.; Smaller, B. J . Chem. Phys. 1968, 49, 951.
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
found to have much higher g factors of 2.002 04 and 2.001 97, respectively. The growth of the ESR signal in H203 has been analyzed in terms of relaxation times of about 4 ~ s but , because of the relatively short chemical lifetime in those experiments, an accurate value is difficult to determine and it is important to determine a value by some other type of experiment. An experiment involving continuous production of the radical would make conventional saturation experiments possible. In addition, there are other experiments that might make use of a source of eaq- such as for the production of H atoms also in steady-state experiments. The photolysis of aqueous solutions of sulfite ion have been interpreted5 in terms of photoionization to produce ea
