J . Phys. Chem. 1984,88, 4647-4649 the four xanthones in n-hexane and X - h , in n-pentane. For X-d, in n-pentane, the inverted triplet states, now T2(3a7r*) and T,(3n7r*), are even further split, and only T 1 , ( 3 n ~ *phos) phorescence is observed until T1, is depopulated sufficiently to allow T,xy(3n7r*) So emission to be observed. We do not detect any T2 emission and consequently are unable to calculate a spin-orbit coupling matrix element. If other assignments for these two emissions are examined, difficulties arise. With a proposal that the emissions correspond to T,,(3n7r*) So and T,,(3~7r*) So, the observed lowest temperature spectrum does not resemble that for TI, emission. Also, if AET is only 13 cm-', then TI, should be widely split, and observable at low temperatures. Such is not the case. If the two emissions are assigned T2z(3n7r*) So and T1x,(37r7r*) So, one must somehow account for Tlz; even if TI, for some unknown reason lost its radiative strength, it should be sandwiched between Tzzand Tlxy.But then any calculated spin-orbit coupling matrix element would not fit. Besides, the deuterium shifts observed for xanthone in n-hexane do not favor these alternative assignments. The prominent features of the Tl,(3n7r*) So phosphorescence of the deuterated xanthones resemble all the other z-sublevel 3n7r* So emissions in other Shpolskii matrices. The appearance of T,x,(3n7r*) So phosphorescence, in all three deuterated xanthones in n-pentane, and the vibrational asssignments are very similar to those seen for the same xanthones in n-hexane, where TI, was %r7r*. This is not surprising, however, if it is remembered that in X-d4 in n-hexane, for example, TlXhas 68% 37r7r* and 32% 3n7r*character, while in the nominal 3n7r*TlXstate in n-pentane, TlXis calculated to have 38% m*character. In other words, the TI"? So phosphorescence appears the same, whether TI is 37r7r* or nT*, since the spin-orbit induced mixed character of Tlxor TI, is not radically different in the two instances, with AET so small. Finally, we note than in n-pentane it is possible to have emissions from a second site, as mentioned in the Introduction, and reported earlier for X-hg.l2For example, Figure 5 shows the presence of
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a weak band 4 in the phosphorescence of X-d4,whose intensity relative to the major bands in the origin region varied markedly with the cooling method employed. No such effect was seen for n-hexane. Band 4' may be due to X - d , in yet another site, or it may be an impurity emission. In any event, the appearance of such low-intensity bands does not alter any of the main conclusions regarding the emission behavior of xanthone. Concluding Remarks. The temperature-dependent phosphorescences observed for deuterated xanthones in two polycrystalline matrices are interpretable on the basis of emission from three levels. These are the widely split sublevels of T,(optically resolvable into T1, and the nonresolved pair T,, and TI,), and T2 (potentially resolvable but with only T2, observed). Thermal equilibrium among these three levels accounts for the temperature dependences of the emissions. The sublevel separations are accounted for by Tl-T2 spin-orbit coupling. The matrix element for this interaction is the same for four isotopic variants of xanthone in both n-pentane and n-hexane. This constant matrix element must now leave no doubt that our contention is correct. The extent of zfs in the TI state of xanthones depends on the magnitude of AET, which varies not only with solvent, but also with deuteration. This isotope effect alters AET since 37r7r* exhibits a larger blue shift than b 7 r * . In the deuterated xanthones in n-pentane, the shifts are so large so as to render T1 3n7r*. Acknowledgment. We thank Mr. B. E. Williamson for recording the Raman spectra and are indebeted to Dr. N. H. Werstiuk of McMaster University for suggesting the synthetic route to the partially deuterated xanthones. Registry No. X-d,, 8 1066-40-2; X-d4, 78797-49-6; X-d.,', 91409-20-0.
Supplementary Material Available: Tables 11-VI, giving the vibrational analyses of the fundamental regions of the phosphorescence spectra of deuterated xanthones in n-hexane (TI X-d,, T2X-d4,T, X-d4,T2X-d,', and T1X-d,', respectively) (8 pages). Ordering information is given on any current masthead page.
Core Binding Energies of the Boron Trihaiides, Lewis Acidities of the Boron Trlhaiides, and Heats of Formation of Carbonium Ions David B. Beach and William L. Jolly* Department of Chemistry, University of California, and the Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received: February 22, 1984)
The core electron binding energies of the boron trihalides have been redetermined. The data are used, in conjunction with literaturevalence ionization potentials, to establish the extent of halogen-boron 7r bonding and, in conjunction with thermodynamic data, to determine the core replacement energy of carbonium ions.
The core binding energies of the boron trihalides were determined with absolute uncertainties of f0.1-0.2eV about 13 years Because of the importance of these data in the study of halogen-boron 7r bonding3 and in the evaluation of the core replacement energy4s5of carbonium ions, we have redetermined the data with probable uncertainties, in most cases, of *0.05 eV. We discuss the pertinence of the results to the trend in Lewis acidity (1) Allison, D. A.; Johansson, G.; Allan, C. J.; Gelius, U.; Siegbahn, H.; Allison, J.; Siegbahn, K. J. Electron Spectrosc. Relat. Phenom. 1972,l. 269. (2) Finn, P.; Jolly, W. L. J. Am. Chem. SOC.1972, 94, 1540. (3) Bassett, P. J.; Lloyd, D. R. J. Chem. SOC.A 1971, 1551. (4) Jolly, W. L.; Gin, C.; Adams, D.B. Chem. phys. Lett. 1977,46,220. ( 5 ) Jolly, W. L.; Gin, C . Int. J. Mass Spectrom. Ion Phys. 1977, 25, 27.
TABLE 1:
coreBinding ~~~~~i~~ (ev)of the B~~~~ Trihalides halogen coreu
B 1s E,
BF? BC~,
BBr3 B13
202.85 (3)' 199.98 (sj 199.0d 197.92 (5)
fwhmb 1.47 (91 1.71 (20)
1.21 (18)
E,
694.94 (2) 206.84 (3j 76.57 (3) 626.82 (2)
fwhm 1.61 (4) 1.34 (6j 1.48 (8) 1.32 (7)
"Fluorine Is, chlorine 2p3/2,bromine 3dS12.'iodine 3d5/,. *Full width
~ ~ t f ~ ~ ~ ~ ~ ~ ~ h ~
~least-
the boron halidesw and to the estimation of heats of formation of carbonium ions. Of
0022-3654/84/2088-4647$01 .50/0 0 1984 American Chemical Society
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Beach and Jolly
4648 The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 TABLE 11: Valence Ionization Potential Data (eV) for the Boron Trihalides COGC IP’ (la; la/LOIP“ la,’ 3e’ le” la,” and 3e’) LOIP BF3 16.56 15.95 17.14 16.67 19.13 16.74 2.57 12.35 2.08 BCl, 12.34 11.73 12.66 12.39 14.42 11.36 1.99 BBrp 11.19 10.65 11.71 11.36 13.18 10.06 1.69 9.36 10.42 10.01 11.74 B13 10.05
TABLE HI: Data Used To Calculate the Core Replacement Energy for Carbon-Containing Cations
a Localized orbital ionization potential for the halogen valence p orbital, calculated with the procedure described in ref 10. ’Data of ref 3. The 3e’ and le” values have been interchanged for BBr3 and B13. See text. cCenter of gravity.
” From Table I. Wagman, D. D. et al. J . Phys. Chem. ReJ Data 1982, 11, Supplement No. 2. “jello, J. M.; Huntress, W. T.; Rayermann, P. J . Chem. Phys. 1976, 64, 4746. ‘Werner, A. S.; Tsai, B. P.; Baer, T. J . Chem. Phys. 1974,60,3650. ‘Beach, D. B.; Eyermann, C. J.; Smit, S. P.; Xiang, S . F.; Jolly, W. L. J . Am. Chem. SOC.,1984, 106, 536. /Chien, K. R.; Bauer, S.H. Inorg. Chem. 1977, 16, 867. gTraeger, J. C.; McLoughlin, R. G.; Nicholson, A. J. C. J . Am. Chem. SOC.1982, 104, 5318.
Core Binding Energies The experimental data are presented in Table I. The average absolute deviation from the EBvalues reported by Allison et al.,’ calculated for those cases where a direct comparison is possible, is 0.13 eV. Our value for the iodine 3d5/2level of B13 is new. In the case of BBr,, the boron 1s peak lies under the bromine peak, and we were unable to deconvolute the spectrum satisfactorily. We are perplexed by this result, because Allison et al. report that these peaks were well defined. In Table I, we list the B 1s binding energy reported by Allison et al. Halogen-Boron a Bonding In Table I1 are listed Bassett and Lloyd’s3 values of the first four ionization potentials (the halogen “lone-pair” ionization potentials) for the boron trihalides. For reasons which will be justified later, we have switched the 3e’ and le” assignments for BBr3 and B13. In column 2 we have listed the “localized orbital ionization potentials“ (LOIPs) for the halogen valence p orbitals, calculated from the core ionization potentials of BX3 and HX and the lone-pair ionization potentials of HX.’O In the last column are listed the difference between the l a / ionization potentials and the LOIP values. The la; molecular orbital is composed of the p a orbitals of all four atoms, and the differences measure the stabilization of the halogen p a orbitals by a bonding to the boron atom. The data show that there is strong T bonding in all the boron trihalide molecules and that there is a fairly steady increase in the a bonding on going from B13 to BF,. It has been known for many years that the Lewis acidity of the boron halides increases in the order BF3
