J. Phys. Chem. 1984,88, 5569-5574
5569
Acknowledgment. We thank Mitsubishi H. I. Co. for supplying ZSM-5 samples. Appendix
Under the assumption that the Langmuir-type intercalation process is very fast and step 2 is faster than step 3, the following expressions for the total amount of adsorbed methanol and the reciprocal fast and slow relaxation times, 7;' and 7s1,can be derived according to the theoretical treatments described in the Theory section. For mechanism I11 [CH30Hlad,t = [CH,OH(int)]
+ [CH30H(Cl)] + [CH30H(C2)]
or[CH,OH(g)]
+ 4 + K*-'
with [CH,OH(int)] T ~ = - ~k3(H[S2]
)+
k-2
+ [CH,OH(int)]) + k-3
Registry
No. CH,OH,
67-56-1.
A Theoretical Study of the Gaseous Oxides POp and PO, Their Anions, and Their Role In the Combustlon of Phosphorus and Phosphlne Lawrence L. h h r Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 (Received: July 2, 1984)
Ab initio calculations have been carried out to determine molecular geometries and energies for a number of neutral and anionic species proposed as occurring in the chemiluminescentoxidations of P4 and PH3 These species include PO2, PO2-, PO, PO-, and HPO. Energies were calculated by third-order Merller-Plesset perturbation theory and by configuration interaction methods using a split-valence basis set augmented by both polarization and diffuse functions. Geometries were obtained at the self-consistent field level by analytical gradient techniques using a similar basis set without diffuse functions. Vibrational frequencies were calculated from analytical second derivatives. Key results are the predictions of a very large electron affinity (3.6 f 0.2 eV) for POz, a bound n a 3B1state of PO2- (below the detachment threshold), and a small exothermicity for the formation of the excited linear state of POz from PO and 0. The results lend support to the proposal by Fraser and Stedman that PO2 is a key species in the chemiluminescent oxidations of P4 and PH,.
-
I. Introduction
Although the phosphorescence of phosphorus has been known for centuries, detailed understanding of the chemiluminescent oxidations of phosphorus or phosphine has been elusive. Gaseous species identified or suggested as giving rise to visible emissions include H P 0 , 4 P02,5,6HOP0,6 and (PO),.' Various mechanisms for the excitation of PO, PO,, and (PO), have been ( 1 ) Ghosh, P. N.; Ball, G. N. 2.Phys. 1931, 71, 362. (2) Norrish, R. G. W.; Oldershaw, G. A. Proc. R. SOC.London, Ser. A 262. 10. --(3) Clyne, M. A. A,; Heaven, M. C. Chem. Phys. 1981, 58, 145. (4) Lam Thanh, M.;Peyron, M. J. Chim. Phys. 1962,59,688. Ibid. 1963, 60, 1289. Ibid. 1964, 61, 1531. (5) Cordes, H.; Witschel, W. 2.Phys. Chem. (Frankfurt am Main) 1965, 46, 35. (6) Daviea, P. B.; Thrush, B. A. Proc. Roy. SOC.London,Ser. A 1968,302, 245. (7) Van Zee, R. J.; Khan, A. U.J . Chem. Phys. 1976, 65, 1764.
--.---.
1961. -.
0022-3654/84/2088-5569$01.50/0
In addition, the anion PO- has been identified9 in the gas-phase oxidation of PH3 by N20, with the electron detachment energy being accurately measured as 1.092 f 0.010 eV. Recent studies1w13of the oxidations of PH3 and P4 by 03/02, O,/N?, O/N, and 0/02,as well as related studies14J5 of AsH3 oxidation, have given support to the proposal that PO2 is an (8) Walsh, A. D. "The Threshold of Space", Zelikoff, M., Ed.; Pergamon: London, 1957; p 165. (9) Zittel, P. F.; Lineberger, W. C. J . Chem. Phys. 1976, 65, 1236. (10) Fraser, M. E. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI. .. . 1983. -(11) Fraser, M. E.; Stedman, D.H. J. Chem. Soc., Faraday Tram. 1 1983, I
79, 527.
(12) Fraser, M. E.; Stedman, D.H.; Dunn, T. M. J. Chem. SOC.,Faraday Trans. 1 1984,80, 285. (13) Stedman, D. H.;Fraser, M. E., to be submitted for publication. (14) Fraser, M. E.; Stedman, D.H.; Henderson, M. J. Anal. Chem. 1982, 54, 1200. (15) Fraser, M. E.; Stedman, D. H.; Nazeeri, M.; Nelson, M. Anal. Chem. 1983, 55, 1809.
0 1984 American Chemical Society
Lohr
5570 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 TABLE I: Optimized Molecular Geometries, Vertical Detachment Energies, and Spin Expectation Values" species state basis Rb Ac -ed (S2)
PO2-
PO2
Po-
PO HPO
'Al
6-31G* 6-31+G* 'BI 6-31G* 6-3 1+G* 'B2 6-31G* 6-3 1+G* 2Al 6-31G* 6-31+G* 2Zg*f 6-31G* 6-31+G* 211y(2B,) 6-31G* 6-31+G* 327 6-31G* 6-31+G* 'A 6-31G* 6-3 1+G* 2II 6-31G* 6-31+G* ]A' 6-31G* 6-31+G*
1.483
118.9
1.526
128.7
1.637
02.1
1.446'
34.4e
1.441
80.0
1.489
80.0
1.529g
1.16 1.88 -0.38 0.60
1.528 1.456h 1.461'
3.57 4.26 0.70 1.39 0.98 2.00
140 kJ mol-' more stable than its (singlet) isomer HOP. Microwave structures have been recently ~ b t a i n e d ~ +for ~ 'PO, PO2, and HPO. 11. Computational Method and Molecular Geometries
2.033 2.035 2.025 2.046 0.768 0.768 0.788 0.795 0.786 0.788 2.028 2.034 0.764 0.764
105.4'
"All values a t HF or UHF level; calculations with 6-31+G* basis made at HF/6-31G* geometries. bBond distance in A. 'Bond angle in degrees. dNegative of anion HOMO in eV. 'Observed parameters are P-0 = 1.4645 A and LOP0 = 135.5', ref 31. /Unstable with respect to bending mode and correlated with *A, state. gobserved distance is 1.540 f 0.010 A, ref 9. hobserved distance is 1.476370 (15) A, ref 30. 'Calculated P-H distance is 1.430 A; observed parameters are P-H = 1.420 f 0.059 A, P-0 = 1.482 f 0.013 A, and LHPO = 102.9 f 8.0°, ref 31.
important chemiluminescent species, although it was not positively identified as being present. A related species, PO2-, apparently not known in the gas phase, has been generatedI6 as an impurity center in alkali halide crystals and thoroughly ~ t u d i e d ' ~by' ~both optical and optical-microwave double-resonance techniques. The anion, isoelectronic with SO2,has a 'Al ground state and a triplet excited state, identifiedI6 as the n a* 3B1state, at an energy of 24 924 cm-' (3.09 eV) in KCl at 4.2 K. These various observations suggest a number of interesting questions. Is POz likely to be a source of the continuous emission seen in P4 or PH3 oxidations? Is PO2- a stable gas-phase ion? How large is the electron affinity of PO2? Is it large enough for there to be a bound triplet excited state of PO; in the gas phase as suggested by of the ion in ionic crystals? In order to obtain answers to these and other questions, we have carried out the ab initio investigations of the gaseous suboxides of phosphorus, PO2 and PO, and also of their anions, which are reported in this paper. Our primary efforts are for the less definitively characterized species PO2and PO2-, as valence and Rydberg states of PO have been studied spectros~opically~*~~ and described by very accurate ab initio calculation^.^^-^^ We also report some results for HPO, a species definitely identified4 in phosphorus flames and recently computedz8to be approximately
-
(16) Hunter, S.J.; Hipps, K. W.; Francis, A. H. Cbem. Pbys. 1979,39, 209. (17) Hunter, S.J.; Hipps, K. W.; Francis, A. H. Cbem. Pbys. 1979,40, 367. (18) Hunter, S. J.; Hipps, K. W.; Bramley, R.; Francis, A. H. Chem. Pbys. 1980,45, 149. (19) Gentry, A. E.;Francis, A. H. J . Pbys. Cbem. 1981,85,4079. (20) Verma, R. D.; Dixit, M. N.; Jois, S.S.; Nagaraj, S.; Singhal, S. R. Can. J . Phws. 1971. 49. 3180. (21) Chuart, B:; Couet, C.; Guenebaut, H.; Larzilliere, M.; Ngo, T. A. Can. J . Pbys. 1972,50, 1014. (22) Coquart, B.; DaPaz, M.; Prudhomme, J. C. Can. J . Phys. 1975,53, 371. (23) Ghosh, S.;Nagaraj, S.; Verma, R. D. Can. J . Phys. 1976,54, 695. (24) Roche, A. L.; Lefebvre-Brion, H. J . Chem. Pbys. 1973,59, 1914. (25) Tseng, T. J.; Grein, F. J . Cbem. Pbys. 1973,59, 6563. (26) Grein, F.; Kapur, A. J . Cbem. Phys. 1983,78, 339. (27) Lefebvre-Brion, H.; Grein, F. J . Chem. Phys. 1983,79, 1102. (28) Schmidt, M. W.; Yabushita, S.; Gordon, M. S.J . Phys. Cbem. 1984, 88, 382.
Geometries were optimized from energy gradients at the SCF (single determinantal) level with the GAUSSIAN so program32 and the split-valence plus polarization basis et^^,^^ 6-3 lG* for the following electronic states: P02-('A1, 3B1, 3B2);P02(2A1,2Bl); PO-(%, 'A); PO(211);and HPO('A'). The nonsinglet states were described by the U H F (unrestricted Hartree-Fock) formalism, while the 'A state of PO- was described by complex molecular orbitals. The resulting HF/6-13G* geometrical parameters are listed in Table I together with spin expectation values. A constraint of C, symmetry was imposed for the states of PO, and PO2. The computed bond lengths are 0.01-0.02 8,shorter than experimental values (where known): 1.529 and 1.456 8, computed for PO-(3Z) and PO(zII) as compared to 1.540 (10) and 1.476370 (15) A, r e s p e ~ t i v e l y . ~Similarly ,~~ the com uted P-0 bond lengths for PO2 and HPO are 1.446 and 1.461 as compared to observed3' values of 1.4645 and 1.482 (13) A, respectively, while the computed bond angles for PO2 and HPO are 134.4' and 105.4', as compared to observed31values of 135.5' and 102.9 f 8.0°,respectively. The computed P-H distance for HPO is 1.430 & as compared to an observed3' value of 1.420 f 0.059 A. This level of agreement between HF/6-3 l G * optimized geometries and experiment is quite satisfactory and matches that noted34for SO2, isoelectronic with PO,, namely, a calculated bond length of 1.414 8, and a bond angle of 118.8' as compared to an observed35length of 1.431 A and an angle of 119.3O. Vibrational frequencies were computed at these geometries for all species (ground-states only) at the HF/6-31G* level by analytical second derivatives and the GAUSSIAN 82 program. The energies of the various electronic states were recomputed at the HF/6-3 lG* optimized geometries by third-order MollerPlesset perturbation theory (MP3) and a 6-31+G* basis set, which is the 6-31G* set augmented by diffuse s- and p-type basis functions for both P and 0 atoms. In both the geometry optimization and MP3 calculations, only the five true-d polarization functions were used for each P or 0 atom. The P-atom diffuse exponent of 0.032 is that which we obtained36 by exponent optimization at the HF/6-31+G*//HF/6-31G* levels for PH; and used in our of gas-phase acidities of molecules containing P-C multiple bonds. The 0-atom diffuse exponent of 0.068 is that reported37from an optimization for OH- at the HF/6-31+G level. The H-atom basis set for the MP3 calculation on HPO contained not only p-type polarization functions but also an s-type diffuse fuction with an exponent of 0.036, obtained38 by optimization for H-, so that the basis set used for HPO is designated 6-31+G**. The basis set sizes are thus 58 for PO, and PO2,40 for PO- and PO, and 46 for HPO. A key question is the degree of electronic stability obtainable for the anionic species with our basis sets. One measure of this stability is given by the negative of the highest occupied orbital
1,
~~~~
~
~
~
(29) Kawaguchi, K.; Saito, S.; Hirota, E. J . Cbem. Pbys. 1983,79, 629. (30) Butler, J. E.; Kawaguchi, K.; Hirota, E. J . Mol. Spectrosc. 1983,101, 161. (31) Hirota, E.,private communication. (32) Binkley, J. S.; Whiteside, R. A,; Krishnan, R.; Seeger, R.; DeFrees, D. J.; Schlegel, H. B.; Topiol, S.;Kahn, L. R.; Pople, J. A. QCPE 1980,13, 406. (33) Hariharan, R. C.; Pople, J. A. Theor. Cbim. Acta 1973,28, 213. (34) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.;Gordon, M. S.;DeFrees D. J.; Pople, J. A. J . Cbem. Pbys. 1982,77, 3654. (35) Calloman, J. H.; Hirota, E.; Kuchitsu, K.; Lafferty, W. J.; Maki, A. G.; Pote, C. S. "Structural Data on Free Polyatomic Molecules", LandoltBornstein New Series, Group 11; Hellewege, K. H.; Hellewege, A. M., Eds.; Springer: Berlin, 1976; Vol. 7. (36) Lohr, L. L.; Ponas, S. H. J . Pbys. Cbem. 1984,88,2992. (37) Chandrasekhar, J.; Andrade, J. G.; Schleyer, P. v. R. J . Am. Cbem. SOC.1981,103, 5609. (38) Spitznagel, G. W.; Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R. J. Compt. Cbem. 1982,3, 363.
Theoretical Study of POz and PO
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5571
TABLE II: Ab Initio Energies and Energy Differences for level POZ~~AI) HF/6-31G* " -490.492 1 1 HF/6-31+G*//HF/6-3 1G*" -490.519 50 MP2/6-31+G*//HF/6-3 lG* a -490.01608 MP3/6-3l+G*//HF/6-31G* -491.00234 I
I _
AHF/6-31G*a AHF/6-3l+G*//HF/6-31G*
0 0
AMP2/6-3l+G*//HF/6-3lG*'
0 0 0
AMP3/6-3 l+G*//HF/6-3 1G"
observed
PO7- and PO? POZ-('BI) -490.396 32 -490.42706 -490.903 34 -490.89293 25 1.4 242.6 295.9 287.2 298.1'
P02(2AI)+ e- P02(22gc)+ e-d3e P02(211,) + e-490.412 05 -490.354 58 -490.262 82 -490.42088 -490.365 72 -490.273 75 -490.89902 -490.880 10 -490.76645 -490.883 66 -490.849 59 -490.747 80
PO;('B,) -490.395 03 -490.41 107 -490.835 63 -490.851 13 254.8 284.6 473.7 396.9
210.2 258.9 307.3 311.5
361.0 403.7 356.9 401.0
601.9 645.1 655.3 668.2
"In au. kJ mol-'. eIn KC1 at 4.2 K. ref 16. dunstable with respect to bending mode and correlated with ZA, state. eEnergies at HF/6-31G* geometry
(HOMO) energies; these Koopmans' theorem electron detachment energies are also given in Table I. We note in each case a 0.71.0-eV increase in the stability of the HOMO when the 6-31G* basis set is augmented by the diffuse functions. This is the same level of increased stability which we36and other^^',^^ have noted in calculations of gas-phase acidities (anion proton affinities). We conclude that a satisfactory description of anion stability is obtainable with these augmented basis sets. We have used these basis sets augmented by diffuse functions in the MP3 calculations for both anionic and neutral species. The computational level is thus designated as MP3/6-3l+G*//HF/6-31G* in general and as MP3/6-31+G**//HF/6-3lG* for HPO. For the species PO-, PO, and HPO, calculations were made not only at the MP3 level described above but also at the CISD (configuration interaction with single and double excitations from a reference configuration) level, again with our augmented 631+G* (or 6-31+G** for HPO) basis set. While the majority of the MP3 calculations were made by assuming "frozen" K shells for both P and 0, the CISD calculations, which also yield MP3 results, were made by assuming additionally a "frozen" L shell for P. As described in sections IIIB and IIID, there is no significant difference between the CISD/6-3 l+G**//HF/6-3 lG* and MP3/6-31+G**//HF/6-31G* values for either the PO-POenergy difference or the PO--HPO difference. Thus the more economical MP3 energy differences appear to be good approximations to CISD energy differences computed with the augmented 6-31+G* or 6-31+G** basis sets. 111. Results and Discussion
-
A . PO2-. The energies calculated with the HF/6-31G* optimized geometries from Table I for the 'Al, 3B1(n a*),and 3Bz (p a*) states of PO2- are given in Table 11. At the MP3/6-31+G*//HF/6-3lG* level, the 3B1and 3B2states of PO; are found to be 287.2 and 396.9 kJ mol-I above the 'Al ground state, the first value, corresponding to a wavelength of 415 nm, being in very good agreement with the 298.1 kJ mol-' value assignedI6 as the 'A, 3Bl emission origin for POz- in KC1 at 4.2 K. The calculated 0.043-A and 9.8O decreases in the bond length and bond angle in going from the 3B1to the 'A, states are in good agreement with the estimatedI6 observed changes (signs uncertain) of 0.065 A and 7.3' for PO; in KCI. Observed changes for the corresponding transition in SO, are 0.063 8,and 6.8O in the vapor3gand 0.042 A and 7.8O in the solid,40in line with the above. By contrast the 3B2state of PO2- is computed to have a 0.154-A longer bond length and a 16.8' smaller bond angle than the 'Al state, reflecting the fact that the specific a a* excitation is a2 bl, where the a, MO is weakly antibonding between the 0 atoms, while the bl MO is weakly bonding between them. This 3B, state has not been identified in the spectra of PO2- in KCI, although a 'B1,which we are not able to consider at the singledeterminantal level, has been identified16 at 398.2 kJ mol-' above the 'A, state, close to our calculated 3Bz energy.
-
-
-
-
(39) Herzberg, G. "Molecular Spectra and Molecular Structure. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules"; Van Nostrand: Princeton, NJ, 1966; p 605. (40) Hochstrasser, R. M.; Marchetti, A. P. J . Mol. Spectrosr. 1970, 35, 335.
TABLE 111: Electron Affmities" soecies -cb AMP3' 0 2.15 0.93 0 2 1.93 0.15 PH2 1.22 0.69 PO 1.88 0.84 PO2 4.26 3.23
-?nu
obsd'
ACEDd 0.86 0.02 0.59 0.86
1.462 0.440 i 0.008 1.271 2Z 0.010 1.092 2Z 0.010
"In eV. bNegative of HOMO energy of M- at HF/6-31+G**// HF/6-31G* level. cE(M) - E(M-) at MP3/6-31+G**//HF/6-31G* level. dE(M) - E(M-) at CISD/6-31+G**//HF/6-3lG* level. 'Reference 41. fCISD with frozen K and L shells for P, K shell for 0. The similarly computed AMP3 value is only 0.004 eV higher than the tabulated AMP3 value based on frozen K shells for P and 0.
Of greater interest is the question as to whether or not POzexists as a gaseous anion, and if so, whether or not any of the excited states, singlet or triplet, lie below the detachment threshold. The o c ~ u r r e n c e of ~ ~bound - ~ ~ excited states for PO2- in KCI is not surprising, as the spherical part of the Madelung potential would stabilize negatively charged species, thus in effect destabilizing the PO, e- detachment continuum. Our calculated energy at the MP3/6-31+G*//HF/6-31G* level for the 2AIstate of PO, is 31 1.5 kJ mol-I above that for the 'A, state of POz-, and thus 24.2 kJ mol-' above the 3B1state, suggesting slight stability for the latter. Is this difference meaningful in view of residual basis set truncation and correlation errors in the calculations? We next present evidence to support the claim that our AMP3/631+G*//HF/6-31G* value of 311.5 kJ mol-I for the electron detachment energy from POz- is probably an underestimate, not an overestimate, and that as a consequence not only may gaseous PO2 have an unusually large electron affinity for a molecule, probably 3.6 0.2 eV, but also that gaseous PO; may have at least one bound excited state, namely, the n T* 3B1state. The diatomic species PO- is knowng to have a bound excited ' A state at 53.6 kJ mol-' above the 32-ground state despite the comparatively small detachment energy of 105.4 kJ mol-', but this is in part a consequence of the open-shell a*configuration for PO-. B. Electron Affinities. In order to support our suggestion that the detachment energy of PO,, and hence the electron affinity (EA) of PO,, is unusually large, we list in Table I11 EA values calculated at the MP3/6-31+G**//HF/6-31G* level for 0, 02, PHI, PO, and PO, and at the corresponding CISD level for all of the above except PO,. The AMP3 values are consistently 0.4 to 0.7 eV below the observed4, EA values. The difference for PO is somewhat less; PO- is the only anion listed for which the spin multiplicity decreases upon electron detachment, so that residual errors are different in this case. The A C E D values are only slightly less than the AMP3 values, so that the more economical AMP3 values appear to be a good approximation to ACED values computed with our augmented basis sets. For example, the ACISD value for the EA of PO is approximately 0.04 eV less than the AMP3 value. The Koopmans' theorem -e values are high as expected, as they do not incorporate geometric changes and electronic relaxation. Our conclusion is that the EA of gaseous
+
*
-
(41) Janousek, B. K.; Brauman, J. I. "Gas Phase Ion Chemistry"; Bowers, M. T.; Ed.; Academic Press: New York, 1979; Vol. 2, Chapter 10, p 53.
5572 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
Lohr
TABLE I V Ab Initio Energies” and Energy Differencesbfor PO-, PO,and HPO level ~0-(32-) PO-(‘A) -415.55544 -415.498 44 HF/6-31G* -415.583 1 1 -4 15.532 00 HF/6-3 1 +G* *//HF/6-3 1 G* (-415.838 90)‘ MP2/6-3 1 +G**//HF/6-3 1 G* -415.865 74 (-415.832 19)‘ MP3/6-3 l+G**//HF/6-3 lG* -415.865 59 (-415.808 73)‘ -415.84571 CISD/6-31 +G**//HF/6-31G* 0 149.6 AHF/6-31G* 134.2 AHF/6-31+G**//HF/6-3lG* 0 AMP2/6-31+G**//HF/6-31G* 0 (83.6)‘ (87.7)c AMP3/6-31+G**//HF/6-3lG* 0 CISD/6-3 1 +G* *//HF/6-3 1 G* 0 (97.1)‘~~ 0 53.6‘ observedb
PO(2II) -415.54645 -415.55346 -415.836 75 -415.83464 -4 1 5.816 34 23.6 77.8 76.1 81.2 77.1 105.4‘
HPO(IA’) -416.122 78 -416.13093 -416.43301 -416.43622 -416.413 15 -1489.3 -1438.0 -1489.1 -1497.9 -1489.5
In au. kJ mol-’. ‘MP2, MP3, and CISD values are for a real singlet not complex ‘A. dCISD calculations made with frozen K and L shells for P, K shell for 0. See sections I1 and IIID. eReference 9.
POz is probably 3.6 f 0.2 eV, or 350 f 20 kJ mol-’, essentially identical with that for the C1 atom.41 If our energy for the ,B, state of PO1 is at all close to the correct value, as the comparison with the KC1:POT datal6 suggests, then it also follows that this n* 3Bl state of gaseous POz- lies below the detachment n threshold. We view our results as strongly suggesting but not proving this interesting possibility. C. POz. The energies calculated with the HF/6-31G* optimized geometries from Table I for the ,Al and zBl (n n*) states of POz are also given in Table 11. The relationship of the energy of the ,A, state to the energies of the ‘A, and ,B1 states of POz- has already been discussed. The bond angle for the ZAl state is 134.4O, close to the 135.5O microwave value31 for POz and close to the 137’ value extracted42from PES data for the 2A1state of isoelectronic SOz+. The n T * zBl state on the other hand has an angle of 180° and is one component of a 211ustate of a linear POz; the ,Al state is not, however, the lower Renner-Teller branch of this state, as the bent 2A, state is correlated with a 2Zgf state of a linear PO2. We have not examined the 2Al branch of the 211ustate, as it is not the lowest of its symmetry type. Our calculated adiabatic energy difference between the linear zII,and the bent ‘Al states at the MP3/6-3l+G*//HF/6-31G* level is 356.7 kJ mol-’, corresponding to a wavelength of 335 nm. Our similarly computed vertical energy difference between the linear 2rIuand 2Z,f states is 267.2 kJ mol-’, corresponding to a wavelength of 448 nm. This value is obtained by using for both states the 1.489-A bond length calculated (Table I) for the upper zII, level rather than the 1.441-Avalue for zZ +,and thus corresponds to the vertical emission energy. Emission hom vibrationally ”cold” zIIustates is therefore expected to have a high-energy onset near 335 nm, an intensity maximum near 450 nm, and a long tail to longer wavelengths. It is the zAl ,B1 (zII,)transition which has been assigned’-13 as giving rise to a portion of the chemiluminescent continua observed in such reactions as PH, + 03/Oz, PH3 O/Oz, P4 03/02, P4 O/O,, etc. There appear to be at least two continua, one 380-800 nm and the other 280-500 nm, the latter appearing in the O3oxidations and in the reaction and the former assigned as ,Al zBl of excess PH, with 0/02, emission from PO,. We view our results as lending support but not proof to the suggestion that the 380-800-nm continuum arises from ,A, 2Bl emission of POz. In the next section some additional support, based on the energy difference between PO 0 and excited PO,, is presented. D. PQ, PO, and HPO. Although not the main focus of our study, energies calculated for the ground states of PO-, PO, and HPO from the HF/6-31G* geometries in Table I are given in Table IV. The EA value for PO calculated at the CISD/631+G*//HF/6-31G* level has already been discussed. The uncorrelated 4 H F energy differences between the ‘4 and 3Z; states of PO- are, as expected, too large and would be reduced by correlation, much as a real singlet state of PO- (not a true ‘ 4 ) is found to increase in stability by 37.1 kJ mol-’ relative to the
-
-
-
-
+
+
+
+-
-
+
(42) Eland, J. H. D.; Danby, C. J. Int. J . Mass.Spectrom. Ion Phys. 1968,
I, 111.
3Z-state in going from the HF/6-31+G*//HF/6-31G* to CISD/6-31+G*//HF/6-3 lG* computational levels. Of additional interest is the PO- anion proton affinity (PA), taken as E(P0-) - E(HPO), which is 1489.5 kJ mol-’, as compared to a value of 1585.3 kJ mol-’ for PH2- computed at a similar (CISD/6-3 1+G**//HF/6-3 1G*) Thus as expected HPO is found to be a stronger gas-phase acid than PH, and thus comparable” to H2S or HCN, while PH3 is comparable44to HF. From the results in Table IV we note that ACISD/6-31+G**/ /HF/6-3 lG* energy differences are approximated very well by the more economical MP3 energy differences computed with the same augmented basis set and with the same geometries. A comparison of the ACISD and AMP3 values for the EA of PO was noted in section IIIB. The PO-(?Z-)-HPO energy difference, which corresponds to the PA of PO-, is only 8.4 kJ mol-’, or 0.55%, greater at the 4MP3 level than at the 4CISD level. The AMP3 value for the PA of PO- is 1.3 kJ mol-’ less than the tabulated 4MP3 value when a “frozen” L shell for P in addition to a “frozen” K shell is assumed as was done in the A C E D calculations. The MPn (n = 2, 3) energies in Table IV match the level of those in Table I11 for PO; and PO, in the assumption of only “frozen” K shells for P and 0. The energy of PO(zrI) O(,P) is found to be only 43.9 kJ mol-’ higher than that of the excited 211ustate of POz,while the energy of PO(,II) + O-(zPo) is found to be 45.8 kJ mol-’ below that of P02(211,) + e-. The latter of these MP3/6-31+G*//HF/6-31G* energy differences suggests that excited P02(2rIu)is unstable with respect to dissociative attachment, while the former lends support to the s ~ g g e s t e d chemiluminescent ~ ~ ~ ~ ~ ~ ’ ~ mechanism for PO,, namely
+
PO(zII)
+ O(,P) + M
P02*(’Bl)
-
-
+
P02*(2B1) M
P02(2A1) + hv
Our computed exothermicities for these steps are, as previously given, 43.9 and 356.7 kJ mol-’, respectively. Is the computed energy difference between PO 0 and excited POz likely to be even semiquantitatively reliable? The MP3/63 l+G*//HF/6-3 lG* level does underestimate bond strengths, yielding De values of 391.1 and 453.5 kJ mol-’ for 0, and PO, respectively, as compared to observed values45of 499.5 and 605 kJ mol-’. Corresponding 4CISD values for De are even smaller, namely, 349.6 and 413.8 kJ mol-’ for Oz and PO, respectively. Thus the calculated energy change of -44.4 kJ mol-’ for P O2 P02(,A1), namely, -400.6 kJ mol-’, is probably about 100 kJ mol-’ less exothermic than it should be. The error in the computed separation of 356.7 kJ mol-’ between the zBl(ZrI,) and ’A1 states
+
-
+
(43) See section IIIE, ref 36, and Lohr, L.L.; Schlegel, H. B.; Morokuma, K. J . Phys. Chem. 1984,88, 1981. (44) Bartmess, J. E.; McIver, R. T., Jr. “Gas Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2, Chapter 11, p 88. (45) Huber, K. P.; Herzberg, G. “Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules”; Van Nostrand: New York,
1979.
Theoretical Study of PO2 and PO
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5573
TABLE V Calculated and Observed Vibrational Frequencies" calcd species symmetry mode unscaledb scaledd obsd PO PoPO,
C,,
ut
C ,,
ut
CZ"
a1 ai
PO,-
CZ"
bz a1 al bz
HPO
C,
a' a' a'
1406 1166 456 1207 1452 536 1211 1373 1127 1374 2416
1220 1012 396 1048 1260 465 1051 1192 978 1192 2096
1220.24901 (43)d 1000&70e
501f 1097f 1207f 9859 11879
"Al values in cm-I. bFrom HF/6-31G* analytical second derivatives. CScaledby the ratio of observed and calculated values for PO (ratio = 0.868). dReference30. 'Reference 9. /Reference 16; in solid KCI at 4.2 K. gReferences 4 and 39.
600
500
I I
i
t
of POz may be comparatively small, say 1 3 5 kJ mol-', since the (isoelectronic) states are not only of the same spin multiplicity, but also may both be described as states of an electron outside a closed shell. If this is so, then the reaction PO 0 PO,*(,B,) is probably about 100 kJ mol-I more exothermic than our calcualted value of -43.9 kJ mol-' suggests. Thus it is quite likely that this reaction producing excited PO2 is exothermic. Another assessment of the accuracy of our calculations can be made by comparison of our results with the CI results obtained for PO by Grein and Kapur26using a related basis set, namely, a double { (not simply split valence) augmented by both polarization and diffuse functions to give a total of 58 contracted functions for PO vs. 40 in our calculations. For the ,II groundstate their computed parameters are De = 481.4 kJ mol-', re = 1.491 A, and we = 1239.9 cm-I; our AMP3/6-31+G*//HF/631G* value of De is 453.5 kJ mol-', while our HF/6-31G* values (32 basis functions) of re (Table I) and we (see section IIIE and Table V) are 1.456 A and 1406 cm-', respectively. Our 0,value is somewhat less than theirs, our re value is comparably different (shorter rather than longer) from the observed30value of 1.476 370 (1 5) A, while our we value is as expected (see section IIIE) too high. E. Vibrational Frequencies. In Table V we present vibrational frequencies computed for all ground-state species from analytical second derivatives at the HF/6-31G* level, the same level as used in the geometry optimizations. As expected from the work of Pople et al.,46the computed values are approximately 12-15% higher than the observed values for P030 and HP0.4339We have accordingly given a list of calculated frequencies scaled by the ratio of observed30and calculated values for PO (ratio = 0.868). These scaled values are close to the observed values for PO-,9 POz- (in solid KC1),16 and HP04*39(two modes only); thus the scaled values for PO, should constitute reasonable predictions for its unknown frequencies. We note that the bending frequency (b, symmetry) of POz is predicted to be approximately 70 cm-' less than that of POT, a result consistent with the removal of a lone-pair electron in going from POT to PO2, as these electrons significantly affect the bond angle. The bending frequency for the ,Al state of SO2+ has been estimated4, from the photoelectron spectrum of SO, to be roughly 120 cm-' less than the 518-cm-' value for the 'A, ground state of SO,. Similarly the ~ b s e r v e d ~bending ~ , ~ * frequencies for NOz and NOz- are 749.8 and 831 cm-', respectively, a reduction of approximately 80 cm-I accompanying electron detachment from the a l symmetry HOMO. Finally, we have used our calculated vibrational frequencies (unscaled) to give a zeropoint energy contribution of -22.4 kJ mol-' to the proton affinity of PO-, a value smaller in magnitude than the approximately -30
IV. Summary The energies computed in this study at the MP3/6-31+G**//HF/6-31G* level are given in Figure 1, which shows the energies of various states of PO2-, PO,, and their dissociation products, relative to the energy of the 'A, ground state of PO2-. The inequalities E(POZ,,A,) e- > E(POz-, 3B1) and E ( P 0 0) > E(P02, zII,) may be seen in this figure. The key results from our theoretical study may be summarized as follows: (1) The ion PO2- has an n a* 'B1 excited state lying approximately 290 kJ mol-' above the 'Al ground state in agreement on PO2- in KCl, and quite possibly lying with ob~ervationsl~-'~ below the detachment threshold. (2) The radical PO, has an unusually large electron affinity, comparable to C1, estimated from a combination of our best calculations and an empirical correction to be 350 20 kJ mol-', or 3.6 f 0.2 eV. (3) The formation of the excited linear state of PO, from ground states of PO and 0 is exothermic, supporting the sugg e ~ t i o n that ~ ~its~ formation ~ ~ ~ ' ~leads to the chemiluminescent continuum assigned as the zAl zBl emission of POz. We have in no sense proven the correctness of the assignment of part of the chemiluminescence of P4and PH3 oxidations to the above emission of POz but have instead presented theoretical results which lend support to this assignment. The presence of PO2 in these flames is conjectural. Nevertheless, many features resemble those of systems containing NOz, a molecule with mostly ill-defined absorption bands.49 The reaction of NO with 0 is chemilumine~cent,~~ with an apparent or pseudo-continuum assigned to an emission from excited NOz molecules. It is this same model which may well apply to the phosphorus oxidations. We note, however, that the results of recent experimental studies5'
(46) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; De Frees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A,; Hout, R. J.; Hehre, W. J. Inr. J. Quanrum Chem. 1981, S15, 269. (47) Reference 39, p 602. (48) Hochstrasser, R. M.; Marchetti, A. P. J. Chem. Phys. 1969,50, 1727.
(49) Reference 39, pp 507-10. (50) Broida, H. P.; Schiff, H. I.; Sugden, T. M. Trans. Faraday SOC.1961, 57, 259. (51) Harris, D. G.; Chou, M. S.; Cool, T. A. J . Chem. Phys., in press.
+
-
loot
Po;
(h,)
Figure 1. Calculated energies in kJ mol-] at the MP3/6-31+G*// HF/6-31G* level for various states of POz-, PO,, and their dissociation products relative to the energy for the 'A, ground-state of POY. The ,Zgc state of POz is unstable with respect to the bending mode and correlates with the bent 'A, state. The energy shown for the state is that calculated with the 1.489-A bond length of the 211ustate.
kJ mol-' value expected36for PHI- as only two vibrational modes are added in protonating PO- vs. three for PH2-.
+
+
-
*
+-
5574
J. Phys. Chem. 1984, 88, 5574-5577
of the PH3-N20 reaction support the (PO)2* exciplex model of Van Zee and Kahn.'
Note Added in Proof. The dipole moment computed at the HF/6-31+G*//HF/6-31G* level for PO2 is 2.17 D, smaller than the similarly computed values of 2.76 and 2.96 D for PO and HPO, respectively. The PO2 value is essentially the same as that of 2.14 D obtained by combining the computed PO moment with the computed PO, bond angle of 134.4'. Acknowledgment. The author thanks Dr. M. E. Fraser, Pro-
fessor D. H. Stedman, and Professor A. H. Francis for the suggestion of this problem and for many stimulating discussions. He also thanks Mr. S. H. Ponas for his assistance with the calculations, the Computing Center of the University of Michigan for the use of its Amdahl 470V/8 computer, Professor H. B. Schlegel for many valuable computational suggestions, and Professor Eizi Hirota for making available his microwave structures for PO2 and HPO (to be published). Registry No. POz, 12164-97-5; PO2-, 20499-58-5; PO, 14452-66-5; PO-, 12186-90-2; HPO, 13817-06-6; Pq, 12185-10-3; PHS, 7803-51-2.
Raman Study of Uranyl Ion Attachment to Thorium( IV) Hydrous Polymer L. M. Toth,* H. A. Friedman, G. M. Begun: and S. E. Dorris Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: November 14, 1983; In Final Form: June 18, 1984)
The association of uranyl ions to Th( IV) hydrous polymer in dilute aqueous nitric acid solutions has been studied by Raman spectroscopy over the pH range of 1.5-4.0. By monitoring changes in the frequency of the uranyl symmetric stretchingvibration, we have identified the attachment of U022+to Th(IV) hydrous polymer through hydroxyl bridges. Unassociated UOZz+ has a symmetric stretching vibration giving rise to a Raman band at 869 cm-l whereas the band appears at 851 cm-' when UO?+ is attached to Th(1V) polymer. Aging of the Th(1V) hydrous polymer to which uranyl ion is bridged causes the frequency of the UO?+ symmetric stretching vibration to appear at 665 cm-'indicative of a change in the bridging bond from hydroxyl to oxygen. Comparison of the Raman spectra of these aged polymers containing UOz2+has been made with those of the crystalline uranates such as a-Na2U04.
Introduction Although hydrolysis reactions of the individual actinides have been studied for several decades, no attention has been given to the hydrolysis chemistry of metal ion mixed systems. Considering the complexity of the chemistry involved, it is understandable that such mixed systems would be ignored even though there are circumstances where interactions of dissimilar metal ions must be considered. Such is the case for the actinides, which are frequently processed as mixtures. Previously, it has been shown' that the presence of uranyl ion retards the rate of Pu(IV) hydrous polymer formation by a mechanism in which the uranyl ion apparently attaches to the polymer network. However, the details of this mechanism were never demonstrated by the identification of bonding between the uranyl ion and the polymer network. Since the major interest in this work was concerned with aqueous nitric acid solutions, Raman (as opposed to infrared) spectroscopy was the most appropriate means of studying such bonding. Nevertheless, difficulties in handling plutonium solutions initially precluded the investigation using plutonium; and, alternatively, Th(1V) was selected as an analogue to Pu(1V) since the hydrolysis chemistry of these two actinides is very similar in many respects. If this experimental approach proved to be a practical means of characterizing the uranyl-thorium(1V) interaction that was suspected, then it would be reasonable to attempt similar measurements with uranylplutonium(1V) solutions. It has already been shown2that aggregates of uranyl hydrolysis products haye uranyl symmetric stretching vibrations whose frequencies are reduced in proportion to the degree of aggregation. The frequency shift occurs as a result of the equatorially bridged hydroxyl groups and has been explained3 as arising from overlap of the nonbonding 4,, and 6, uranium orbitals with the ligand t Chemistry Division, Oak Ridge National Laboratory.
0022-3654/84/2088-5574$01.50/0
orbitals, resulting in a considerable ligand-metal electron density shift. While the position of the symmetric stretch is indicative of a uranyl aggregate with a given degree of equatorial bridging, the relative intensities of the individual symmetric stretching vibrations (each assigned to a particular uranyl aggregate) is a measure of the relative proportions of each type of uranyl2. If uranyl ions also attach to the Th(1V) polymeric network through hydroxide bridging, the appearance of a second uranyl symmetric stretching band (in addition to that of the free U02' ion and at a position similar to that which occurs for the uranyl dimer) would indicate the occurrence of such bonding. The intensity of this second uranyl band, relative to that of the free uranyl ion, would give a measure of the amount of such bridging in fhe solution. This work therefore addresses the subject of uranyl attachment to hydrous polymer networks and the changes that occur to the bridging link when the polymer suspension is thermally aged. Experimental Procedure All the solutions for the hydrolysis studies of Th(1V) and U022+ were prepared from U02(N03)2.6H20 and Th(N03)4.5H20 crystals that had been grown from slightly acidic aqueous solutions. The samples prepared for the study of the hydroxyl-bridged polymer were treated in pairs. In one sample the Th(IV) and UO?+ were heated together and in the other the uranyl nitrate crystals were added after the Th(1V) was heated. (Heating was used to hasten the rate of polymer growth.) Both samples had the same amount of 1,000 M NaOH added. The concentrations of the Th(IV) and U022+were 0.050 and 0.025 M, respectively, in each sample. The sample pairs were heated at 65 "C in a (1) Toth, L. M.; Friedman, H. A,; Osborne, M. M. J . Inorg. Nucl. Chem. 1981, 43, 2929.
(2) Toth, L. M.; Begun, G.M. J . Phys. Chem. 1981, 85, 547. (3) McGlynn, S. P.; Smith, J. K.; Neely, W. C. J. Chem. Phys. 1961, 35, 105.
0 1984 American Chemical Society
