J . Phys. Chem. 1990, 94, 7445-7448
7445
present work, combined with our previous calculations, provides a consistent set of heats of formation for the interpretation of the thermodynamics of SiH,F, and SiH,F,+.
distort to form a complex between a silicon-containing cation and a neutral atom or diatom; the lowest energy structures are SiH2+-H2, SiHF+-H2 or SiH2+-HF, SiHP-HF, SiF2+-HF, and SiF3+-F. Adiabatic ionization potentials, proton affinities, and heats of formation have been calculated by using isodesmic and isogyric reactions at the MP4SDTQ/6-3 1G** level. The calculations on SiH,+ agree well with higher level theoretical computations and with experiment. The theoretical values for SiF,+ agree with experimental thresholds and ionization potentials. The
Acknowledgment. We thank the Computer Serives Center at Wayne State University and the Pittsburgh Supercomputer Center for generous allocations of computer time. This work was supported by a grant from the National Science Foundation (CHE-87-11901).
ESR Spectra of Mixed Group IA-IIA Compounds in Argon Matrices C. F. Kernizant and D. M. Lindsay* Department of Chemistry, City University of New York. The City College, New York, New York 10031 (Received: February 28, 1990)
Electron spin resonance (ESR) spectra assigned to NaMg, NaCa, NaSr, KMg, and KCa radicals have been produced by codepositing suitable combinations of group IA and group IIA atoms in Ar matrices. The ESR spectra establish a 22ground state in which the unpaired electron is substantially delocalized over the group IIA atom (ca. 25% in the case of NaCa and NaSr), thus implying chemical bond formation between the two metallic moieties. Additional ESR transitions, corresponding to KMg, and NaMg, molecules ( k not determined), were observed when Na and K were codeposited with Mg atoms.
isovalent Ag-group IIA and Ag-group IIB species.I6 Thus, (i) alkali-metal hyperfine (hf) constants decrease (Le., greater delocalization) as the difference in ionization potential between the two metal atoms decreases and (ii) measured g shifts increase linearly with the spin-orbit coupling constant of the group IIA atom. When Na and K were codeposited with Mg (but not with Ca or Sr), ESR transitions in addition to those assigned to the dimer were observed. The spectra imply the formation of KMgk and NaMg, molecules ( k not determined) and a further delocalization of the unpaired electron over the magnesium nuclei.
Introduction While alkali-metal clusters are chemically bound even at sizes as small as the dimer, small aggregates of the group IIA (and group IIB) metals are relatively weakly bound and only develop their metallic stability (as a result of hybridization) after reaching a certain critical size.'-' Thus, in the case of dimers (for example), the ground-state configuration is ..d for the alkali metals but , . . u ~ u for * ~ the group IIA elements. This leads to a bond ordera of 1 in the former and 0 in the latter which may, therefore, be bound only by weak van der Waals forces?-I0 For beryllium, ab initio calculations2 predict that the binding energy per atom rises rapidly from about 0.1 eV for the dimer (a value confirmed by experimentIO)to over 0.6 eV for Be4. For Hg clusters, on the other hand, the transition to metallic behavior occurs at much larger sizes: in the range of 20-70 atoms.69' Mixed group IA-IIA dimers (configuration .,.n2o*) would be expected to have intermediate bond strengths and could be relatively stable even without significant hybridization. In general, the addition of a single alkali-metal atom might well provide a mechanism for substantially stabilizing a homonuclear group IIA cluster. While there have been several experimental" and c a l ~ u l a t i o n a l ~studies ~ J ~ on mixed metal clusters such as NakMg (k = 2-8), there is a relative dearth of information on the species NaMg,, KMgk, etc. However, NaMg, (for example) is isoelectronic with Mgk+l+ (recently observed in rare gas matricesI4) which (for k = 1-6) are predicted to have binding energies in the range of 0.5 eV/atom as compared to only about 0.1 eV/atom for the corresponding neutral Mg
(1) Dykstra, C. E.; Schaefer, H. F.; Meyer, W. J. Chem. Phys. 1976,65, 5141. Jordan, K. D.; Simons, J. J. Chem. Phys. 1980, 72, 2889. Chiles, R. A.; Dykstra, C. E.; Jordan, K. D. J . Chem. Phys. 1981, 75, 1044. (2) Bauschlicher, C. W.; Bagus, P. S.; Cox, B. N. J . Chem. Phys. 1982, 77, 4032. (3) Pacchioni, G.;Koutecky, J. J. Chem. Phys. 1982, 77, 5850. Pacchioni, G.; Koutecky, J. Chem. Phys. 1982, 71, 181. (4) Ermler, W. C.; Kern, C. W.; Pitzer, R. M.; Winter, N. W. J. Chem. Phys. 1986,84, 3931. ( 5 ) Rao, B. K.; Khanna, S.N.; Jena, P. Phys. Rev. 1987,836, 953. (6) Rademann, K.; Kaiser, B.; Even, U.;Hensel, F. Phys. Reo. Lett. 1988, 59, 2319. Rademann, K. Eer. Bunsen-Ges. Phys. Chem. 1989, 93, 653. (7) BrCchignac, C.; Broyer, M.; Cahuzac, Ph.; Delacretaz, G.; Labastie, P.; Woste, L. Chem. Phys. Lett. 1985, 120, 559. Brkhignac, C.; Broyer, M.;
Cahuzac, Ph.; Delacretaz, G.; Labastie, P.; Wolf, J. P.; Wbste, L. Phys. Rev. Lett. 1988, 60, 275. (8) Herzberg, G.Spectra of Diatomic Molecules; Van Nostrand: New York, 1950. (9) Miller, J. C.; Auk, B. S.; Andrews, L. J . Chem. Phys. 1977, 67, 2478. Miller, J. C.; Mowery, R. L.; Krausz, E. R.; Jacobs, S. M.; Kim, H.W.; Schatz, P. N.; Andrews, L. J. Chem. Phys. 1981, 74, 6349. (IO) Bondeybey, V. E. Chem. Phys. Lett. 1984, 109, 436; Science 1985, 227, 125. (11) Kappes, M. M.; Radi, P.; Schar, M.; Schumacher, E. Chem. Phys. Lett. 1985, 119, 11. (12) Pewestorf. W.: Bonacic-Kouteckv. V.: Kouteckv. J. J. Chem. Phvs. 198'8,89,5794. Fantuki, P.; Bonacic-Ko;t&ky, V.; PewAtorf, W.; Koutecky, J. J. Chem. Phys. 1989, 91, 4229. (13) Rao. B. K.; Khanna, S. N.; Jena, P. 2.Phys. 1986,D3, 219. Rao, B. K.;'Jena, P. Phys. Rev. 1988, 837, 2867. (14) Knight, L. B. Acc. Chem. Res. 1986, 19, 313. Also: Knight, L. B. Private communication. (15) Durand, G. J. Chem. Phys. 1989, 91, 6225. (16) Kasai, P. H.; McLeod, D J. Phys. Chem. 1975, 79, 2324. Kasai, P. H.; McLeod, D J. Phys. Chem. 1978, 82, 1554.
cluster^.'^ In this paper we present electron spin resonance (ESR) evidence for the formation of NaMg, NaCa, NaSr, KMg, and KCa radicals during cocondensation of the group IA and group IIA metal vapors in argon matrices. The ESR spectra show that the dimers have 22 ground states and that the wave function for the unpaired electron is delocalized over the group IIA atom (ca. 25% in the case of NaCa and NaSr), thus implying a significant, chemical interaction between the two atomic moieties. There are strong similarities between the spectra analyzed here and those of the 'Present address: The Timken Company, Timken Research, 1835 Dueber Avenue, S.W.. Canton, OH 44706-2798.
0022-3654/90/2094-7445%02.50/0 0 1990 American Chemical Societv , , I
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1446
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990
Kernizan and Lindsay TABLE I: Magnetic Parameters for Sodium-Magnesium Clusters in an Argon Matrix' species 23~41) 23~a(11) 23NaMg(I) 23NaMg(II) 23NaMg, 23NaMgy
go 1.9984 ( 1 ) 1.9999 (4) 1.9994 (2) 1.9994 (2) 1.9992 (3) 2.0014 (11)
00,
G (I)
330.6 318.4 286.0 278.2 242.2 187.0
(6) (4) (3) (4) (17)
Po, 5% 104.6 100.7 90.5 88.1 76.6 59.2
Experimental uncertainties (see text) in parentheses. Column 3 gives alkali-metal atom hf constants. Column 4 data are spin populations, p o = ao/a(23Na) with a(23Na) = 316.1 G.26 Roman numerals pertain to matrix sites. (I
L,+ 312
j -1/2
,]-3/2
1 '3Na 2800G I
30001
32001
34001
3600 1
38001
TABLE 11: Magnetic Parameters for Potassium-Magnesium Clusters in an Argon Matrix4 species 39K 39KMg 39KMg, 39KMgy
Figure 1. ESR spectrum of N a M g clusters in an Ar matrix at 6.5 K. Unassigned transitions arise from Na,. The resonance field for a free electron is He= 3320.9 G.
Experimental Section Mixed metal compounds were prepared by depositing atomic alkali metal (Gallery, 99.95%) with either atomic Mg (Aesar, 99.8%), Ca (Aesar, 99.5%), or Sr (Alfa, 99%) and excess argon (Airco, 99.9995%). Metal cluster samples were formed on a liquid helium cooled (sapphire or copper) substrate mounted inside an ESR cavity, as described e1~ewhere.l~Alkali and alkaline-earth metals were simultaneously evaporated from a double Knudsen effusion source, described in more detail in ref 18. Both metal fluxes were periodically monitored with separate quartz crystal microbalances (Veeco, QM-301). The ratio argon:group IIA: group IA was typically 500:7:1, with about 0.05 pmol/h of alkali metal incident on the substrate and total deposition times of 3-5 h. Any metal oxide (or other) layers were removed prior to an experimental run by heating the metals above their planned vaporization temperatures. Without this procedure, the alkali-metal flux could be much smaller than anticipated and the group IIA flux subject to sudden, uncontrollable increases in vaporization rates. In some experiments using Mg, the alkali metal-Mg ratio was systematically varied in order to relate metal concentrations to ESR intensities. Fitting these data to kinetic models, such as those proposed by Moskovits and Hulse,Ig can in principle help identify cluster sizes. Such analyses, however, always gave equivocal results. In general, spectral quality was either unaffected or degraded by photolysis. Annealing had little beneficial effect, but overlap from alkali-trimer transition^'^*^^^^^ (particularly in the case of Na) could be reduced by recording spectra at T = 20-30 K.22 All spectra were recorded on an IBM-Bruker ER 200D ESR spectrometer (microwave power = 10-50 pW) using 100-kHz magnetic field modulation (peak-to-peak amplitude = 3 G) and phase-sensitive detection. Typical scan rates were 1 G/s with a time constant of 0.5 s. ESR spectra could also be processed by averaging repetitive scans on a multichannel analyzer (Tracor Northern, TN 1710). The resonance field of the cavity plus substrate and matrix sample was periodically measured with a microwave frequency counter (HP 5245L plus H P 5255A plug-in). Individual spectra were calibrated with a proton magnetometer (Micronow, Model 5 15). Relative and absolute field positions are judged accurate to f0.2% and f0.5 G, respectively.
an, G
En
1.9988 1.9994 1.9981 1.9982
(I) (1) (I) (11)
87.0 78.2 56.5 39.5
(2) (2)
(1) (3)
%
105.6 95.0 68.6 47.9
'Experimental uncertainties (see text) in parentheses. Column 3 gives alkali-metal atom hf constants. Column 4 data are spin populations, p o = U ~ / ~ ( , ~with K ) a(39K) = 82.4 G.26 Spectra and Analysis Figure 1 shows the ESR spectra observed after a 4-h codeposition of Na with Mg and excess Ar. All spectra are isotropic and characteristic of doublet (S = 1/2) radicals. Indicated in the figure are four sets of hyperfine (hf) transitions arising from 23Naatoms (nuclear spin I = 3/2, projection m = *3/2, f1/2) trapped in several sites in an argon m a t r i ~ . ~Some ~ . ~ weaker, ~ unlabeled features arise from Na3.I7 Also identified in Figure 1 are three other spectral carriers (not observed without Mg), which are assigned to a single 23Na atom bound to differing numbers of Mg atoms. Because of the similarity in spectral parameters (compare the data for atomic sodium), it is most likely that the two sets of transitions denoted 23NaMgarise from the same species trapped in two different matrix sites. Naturally occurring magnesium has three principal isotopes, but only 25Mg (natural abundance = 10.1%) has a nonzero nuclear Thus, none of the species denoted 23NaMg, 23NaMg,, and 23NaMgyshow Mg hf structure, and so it is not possible to determine unambiguously the number of Mg atoms per molecule. Despite a careful search, we were unable to find ESR transitions arising from (for example) 23NaZsMg.Our (tentative) assignment of the dimer rests on (i) the likelihood that the dimer forms and is a dominant species in these reactions (compare the isovalent Ag-group IIA and Ag-group IIB radical dimers)I6 and (ii) the consistent trends (discussed below) in the alkali-metal hf splitting and radical g value which follow from this assignment. Measured ESR transition fields, H(m), were fit to the perturbation expression (correct to third order in A 0 / ~ 1 7 * 2 4
where A2(m) = 1(1 + 1) - mz
(2)
and A3(m) = m[m2+ 1 / 2 - I ( I
(17) Lindsay, D. M.; Herschbach, D. R.; Kwiram, A. L. Mol. Phys. 1976, 32, 1199. (18) Kernizan, C. F. Ph.D. Thesis, City University of New York, 1990. (19) Moskovits, M.; Hulse, J. E. J . Chem. Soc., Faraday Trans. 2 1977, 73,47 1. (20) Lindsay, D. M.; Thompson, G. A. J . Chem. Phys. 1982,77, 1114. (21) Thompson, G . A.; Lindsay, D. M. J . Chem. Phys. 1981, 74, 959. Thompson, G.A.; Tischler, F.; Garland, D.; Lindsay, D. M. Sur/. Sci. 1981, 106, 408. (22) Thompson, G.A.; Tischler, F.;Lindsay, D. M. J. Chem. Phys. 1983, 78. 5946.
on.
+ I)]
(3)
with Ho = g,$ie/goand A. = g&o/go. Here goand a. refer to the radical g value and alkali-metal hf constant, respectively, and g, (23) Jen, C. K.; Bowers, V. A.; Cochran, E. L.; Foner, S. N. Phys. Rev. 1962, 126. 1749. (24) Weltner, W. Magnetic Atoms and Molecules; Van Nostrand: New York, 1983. (25) Handbook ofphysics and Chemistry; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1975.
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 1447
Mixed Group IA-IIA Compounds in Ar Matrices
”r
23
Na Ca
m=
- 112
1+1/2
+3/2
- 3/2
ev41
=->$-
7r
3s+-\I Aglc
\
I
k t 3/2
I - 312
j+1/2
I ‘3No 2800GI
30001
32001
34001
4p
, \
\
\
\
7L
Na Ca
No
Ca
Figure 3. Qualitative molecular orbital diagram for NaCa dimers. 36001
38001
Figure 2. ESR spectrum of NaCa clusters in Ar at 25 K. Unassigned
transitions arise from Na3 and Na7. The resonance field for a free electron is He= 3321.9 G.
( 0 ) Alkali
Atom
Spin Populations
100
TABLE 111: Magnetic Parameters for Alkali Metal-Alkaline-Earth Metal Mmers in an Argon Matrix”
species go 00, G Po, 1.9994 (2) 286.0 (4) 90.5 13NaMg 242.3 (4) 76.7 13NaCa 1.9950 (2) 1.9778 (6) 243.1 (9) 76.9 23NaSr 1.9994 (1) 78.2 (2) 95.0 39KMg 70.3 (1) 85.3 39KCa I .9943 ( 1 ) a Experimental uncertainties (see text) in parentheses. Column 3 gives alkali-metal atom hf constants. Column 4 data are spin populations, po = ao/a(23Naor 39K) with gas-phase data from ref 26. Mg data are from Tables I and 11.
= 2.002 32 is the free electron g value. Table I gives least-squares parameters go and a. for the four species identified in Figure 1. Estimated uncertainties (given in parentheses) pertain to one standard deviation in the fit to eq 1. However, calibration errors (noted earlier) may be larger. Column 4 of Table I gives the isotropic spin population (po) on sodium, obtained by dividing the measured a. by the corresponding gas-phase value, U ( ~ ~ =N ~ ) 316.1 GeZ6 A spectrum similar to Figure 1 (but more crowded and perhaps slightly anisotropic) was observed when K and Mg were codeposited in argon. Easily recognized are atomic potassium O9K and 41K, I = 3/2 for plus three additional spectral carriers (four hf transitions each), which we denote 39KMg, 39KMg,, and 39KMg,. These data were also analyzed by using eq 1, and the measured go and a. are given in Table 11, with estimated errors (one standard deviation uncertainty) in parentheses. Figure 2 shows the ESR spectrum observed when Na and Ca are codeposited in argon. Aside from atomic sodium (in several sites), Na,, and Na, (unlabeled in Figure 2),’7922there is only one additional spectral carrier which, based on the likelihood of dimer formation, is denoted Z3NaCa. Similarly, the deposition of Na with Sr and of K with Ca gives rise to only one ESR spectrum in addition to those found in the absence of any group IIa metal. N o mixed cluster species were observed for Na plus Ba or for K plus either Sr or Ba. As noted earlier, the “fine structure” evident in the m = f 3 / 2 transitions of *,NaMg (Figure 1) most likely arises from matrix site effects. Similar (but smaller) splittings are observed in the spectra of 23NaCa (Figure 2) and 23NaSr (not shown). Effects arising from g-tensor anisotropy should also be present, but these Ca Sr (see below), would be expected to increase as Mg rather than correlate with the magnitude of the alkali-metal hf constant as here. As in the case of AgMg, AgCa, and AgSr,16 parallel type transitions are not clearly resolved. Accordingly, all spectra were analyzed assuming an isotropic spin Hamiltonian. Table I11 gives go and no values for the Ca and Sr compounds (obtained by a least-squares fit to eq 1) and also includes the
- -
(26) Kusch,.P.; Hughes, V. W. Handbuch der Physik; Flugge, S., Ed.; Springer: Berlin, 1959.
1 70
NaCa
I
I
I
I
I
2
3
I P ( I I A ) - I P ( I A ) i n eV
( b ) Dimer g - s h i f t s
3
-lOOAgoI
:_. K Mg
0
NaMg
No Co I
7448
J . Phys. Chem. 1990. 94.1448-1452
monotonically (at least through NaCa) as a function of the difference in ionization potential (data from ref 27) between the group IIA atom and the alkali metal. Figure 4b shows a plot of dimer g shifts (Ago = go - g, with go from Table 111) vs AllA, the group IIA spin-orbit coupling con~tant.~'The straight line is a least-squares fit to the experimental data. The expected r e l a t i o n ~ h i pis~of ~ the form
where terms containing the (generally smaller) spin-orbit coupling constant of the alkali metal are ignored, and we identify gowith the perpendicular g-tensor component, g, (see also Figure 3). Presumably g,, = g,, as is appropriate for a 2Z radicaLZ4 In eq 5 cprand cprare respectively the group IIA p-orbital coefficients for the ground 2Z and (low-lying)I2 first excited 211 states and AE is the energy separation between these two states. According to eq 5 , there is no a priori reason to expect Ago to depend solely on AllA, as this requires that ( C ~ , ) ~ ( C , ) ~remains / A E constant for all combinations of Na and K with Mg, Ca, and Sr. Evidently, such a simplification does occur as has been noted previouslyI6 for the diatomic Ag group IIA and IIB compounds. In an attempt to model these trends, we performed extended Huckel (EH) calculationsB on a series of mixed dimer molecules. The calculations included both the s and p orbitals on each metal and employed experimental atomic energies2' plus the Slater orbital parameters given in ref 29. The diatom bond length was taken to be the sum of the two atomic radii.jO The agreement between EH theory and experiment was rather poor, however. For KCa and KMg, a zll state was found to be more stable than (27) Moore, C. E. Natl. Stand. Ref Data Ser. (US.Natl. Bur. Stand.) 1971, No. 35. (28) We used a version of Forticon8 for the Macintosh. (29) Eyring, H.; Walter, J.; Kimball, G.E.Quantum Chemistry; Wiley: New York, 1967. (30) Table of Interatomic Distances; Chemical Society: London, 1958.
22. Not only was the trend shown in Figure 4a not well reproduced, but the dominant component of lu*) was predicted to be the group IIA p orbital. Thus, for example, the po parameters for the u* orbitals of KMg and NaMg were calculated to be only 3% and 12%, respectively. This is in marked contrast not only to the ESR data but also to the results of local spin density calculation^^^ which find 2Z: ground states with po 40% for KMg and po 50% for NaMg. The additional species observed in magnesium-containing matrices are undoubtedly clusters of the form NaMg, and KMg, ( k = x and y in Figure 1 and Tables I and 11) in which a single alkali-metal atom is chemically bound to a larger number of Mg atoms. The substantial reductions in po (95% 69% 48%, in the cae of KMg, for example) imply a further delocalization of the unpaired electron, which could simply be the result of an increasing cluster size. We have no explanation as to why similar structures are not observed with Ca and Sr, except to note that the nonreactivity of Na with Ba and of K with Sr and Ba might reflect a trend to lower stabilities at higher atomic numbers. Molecular structure calculations, however, predict (for example) Ca4 to be more stable than M g z and (for example) LiCa to have a stronger bond than LiMg.jZ A more complete understanding of mixed group IA-IIA compounds must await further measurements (particularly mass-selected deposition^)'^ in combination with molecular structure calculations pertinent to the available experimental data.
-
- -
Acknowledgment. We thank S . Khanna for stimulating discussions and for providing us with the results of the local spin density calculations prior to publication. This work was supported in part by the National Science Foundation under Cooperative Agreement RII-8802964 and by The City University of New York PSC-BHE Faculty Research Award Program. (31) Khanna, S. N. Private communication. (32) Jones, R.0. J . Chem. Phys. 1980, 72, 3197. (33) See, for example: Lindsay, D. M.; Meyer F.; Harbich, W. Z . Phys. 1989, 012. 15. Lindsay, D. M. Bull. Am. Phys. SOC.1990, 35, 278.
Selective Photoinduced Line Broadening in Proton NMR Spectra of 4-Aikox ynitrobenzenes K. A. Muszkat* and I. Khait Department of Structural Chemistry, The Weizmann Institute of Science, Rehouot 76100, Israel (Received: March I , 1990)
Selective photoinduced NMR line broadening as well as selective downfield shifts are observed on photoexcitation of benzene solutions of 4-alkoxynitrobenzenes and of 4-(dialkylamino)nitrobenzenes. In 4-ethoxynitrobenzene the effects are largest for the methylene and for the 3 and 5 ring protons but considerably smaller for the methyl protons. On this basis these effects are attributed to fast electron exchange of intact molecule with the cation radical N02C6H40'+CH2Rformed by photoejection.
Nitroaromatic molecules take part in a wide range of photochemical prmesses.Id Among these processes figure prominently the C-N bond cleavage, precursor of the nitro-nitrite photore( I ) Dmpp, D. 0. Top. Curr. Chem. 1975, 55, 49. (2) Lippert, E.;Luder, W.; Moll. F.; Nagele, W.; Boos, H.; Prigge, H.; Seibold-Blankenstein, 1. Angew. Chem. 1961, 73, 695. (3) Lippert, E.; Kelm, J. Helu. Chim. Acta 1978, 61, 279. (4) Jones, L.; Kudrna, J.; Foster, J. Tetrahedron Lett. 1969, 3263. ( 5 ) Cornelisse, J.; Lodder, G.; Havinga. E. Reo. Chem. Intermed. 1979, 2, 231. (6) Chow, Y. L. I n The Chemistry of Amino, Nitroso and Nitro Compounds; Patai, S . , Ed.; Wiley: Chichester, 1982; Part 1, Suppl F, Chapter 6, p 181.
0022-3654/90/2094-7448$02.50/0
arrangements,6 a wide range of nucleophilic photosubstitutions, and diverse photoreduction reactions. In addition to these processes, which all give rise to observable photoproducts, our previous NMR studies indicate that photoexcited nitroaromatic molecules also take part in efficient no-product chemically reversible processes. These are degenerate processes of H atom or of electron exchange that proceed through the intermediacy of short-lived photo transient^.'-^ (7) Muszkat, K. A,; Weinstein, M. Th. Forster Memorial Volume. Z. Phys. Chem. 1976, 101, 105. (8) Muszkat, K.A.; Weinstein, M. J . Chem. SOC.,Perkin Trans. 2 1976, 1072.
0 1990 American Chemical Society
