J . Phys. Chem. 1986, 90, 2600-2608
2600
I
energy expression of eq 1 assumes a near harmonic spectrum for the tunneling coordinate and this is decidely not the case. However, our point is that anharmonicity corrections arising from coupling between the tunneling mode and the other modes can have an effect on the apparent fundamental frequencies and this idea is still valid. As mentioned in our earlier Letter, a key experiment would be to probe the TI vibrational manifold via the phosphorescence excitation spectrum with direct excitation into the triplet state. We have still not attempted this experiment, although it seems to be within the limits of feasibility. The prediction drawn from So the above analysis is that the first strong band in the T, phosphorescence excitation spectrum will occur to higher energy than the 0-0 band of the phosphorescence. Prompted by the recent report of phosphorescence in 9-DPO by Bondybey et a L 3 we have begun to look at that molecule in Shpol'skii matrices as well. The fluorescence spectra of 9-HPO and 9-DPO in hexane are vibronically the same as those reported for the neon matrix. The other workers stated that their apparatus was not optimized for long-lived emissions but they were able to discem a 0 4 band for 9-DPO near 17 350 cm-' and some vibronic structure. Although the vibronic bands were characterized as having a "rather broad" appearance, we find the phosphorescence spectrum of 9-DPO to be as vibronically sharp as the fluorescence. Work is continuing at this time but we have determined that the phosphorescence/fluorescence intensity ratio for 9-HPO is only 2-3% that for 9-DPO. Moreover, just as in 6-HBA, deuterium substitution of the hydrogen-bonded proton blue-shifts the fluorescence by roughly 125 cm-I but red-shifts phosphorescence by almost 50 cm-I. Thus chances are good that similar factors are operative in 6-HBA and 9-HPO. It has been suggested to us that the discrepancies in the fluorescence and phosphorescence vibronic intervals arise because the TI state has time to relax into a different guest-host configuration before emission, compared to the S1guest-host structure. We cannot rule out this possibility although one generally assumes that intramolecular vibrational frequencies are for the most part insensitive to the host environment. The exception, of course, is when large amplitude motion is involved. Quite possibly proton transferltunneling falls into this category but then this interpretation becomes much the same as the one we presented above. Registry No. 6-HBA, 43099-11-2; 6-DBA, 83335-53-9.
-
Figure 5. Schematic potential energy curves along the proton-tunneling coordinate for 6-HBA. The asymmetry in the double minimum potential functions for So and TI is assumed to be sufficiently great to localize wave functions more or less completely in one well or the other.
discussion and are therefore ignored. We assign ut = 0 to the wave function along the tunneling coordinate localized in the more stable well and ut = 1 to the wave function localized in the other well. Now we have assumed a " m o n asymmetry for the fluorescence so the apparent frequency of the fundamental for mode b is given by E(vt=O, vb=l) - E(vt=O, v b = o ) = wb - j/2X'b In contrast, the TI state has been assumed to have reversed asymmetry relative to So so transitions terminate on the ut = 1 level of the tunneling coordinate and the apparent fundamental frequency of mode b in the phosphorescence is E ( u , = l , Ub-1) - E ( v , = l , ub=o) = a b - 3/2xtb Now this analysis is admittedly highly oversimplified since the
FTIR Spectra of Halobenzene Complexes with Hydrogen Fluoride in Solid Argon Steven R. Davis and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: December 2, 1985)
Hydrogen-bonded complexes of halobenzenes and HF have been prepared by freezing argon-diluted reagents at IO K. Infrared spectra of the products characterized include a planar C6H5-F- -HF complex with HF hydrogen bonded to fluorine as well as two different C6H3-X- -HF complexes (X = C1, Br) involving hydrogen bonding to the aromatic ring or to the halogen atom. The two different complexes for chlorc- and bromobenzeneare identified by comparison to benzene- -HF and CH3X--HF spectra. Warming the matrix produced complexes of the type C6H5-X- -(HF), where X = F, CI, and Br. The strength of the hydrogen bond to the halogen atom increased with increasing atomic number, and the basicity of the ring in C6H5-CI and C6H5-Br was approximately equal to that of benzene when H-F stretching fundamentals in the complexes are used as a guide.
Introduction Intermolecular hydrogen bonding is very important in dictating the physical properties of a vast array of molecules, and matrix isolation studies of small hydrogen-bonded complexes can provide useful models for more complicated systems. Benzene and alkyl halides have been shown to undergo hydrogen bonding with HF and are of comparable basicity as measured by the H-F stretching frequency in the complexes.I4 The site of hydrogen bonding in 0022-3654/86/2090-2600$01 S O / O
a molecule with more than one base moiety is very important in chemistry and biology and the purpose of this work is to char(1) Baiocchi, F. A.; Williams, J. H.; Klemperer, W. J . Phys. Chem. 1983, 87, 2079. ,706. (2) Andrews, L.; Johnson, G. L.; Davis, S.R. J . Phys. Chem. 1985, 89, (3) Johnson, G . L.; Andrews, L. J . Am. Chem. SOC.1980, 102, 5736. (4) Arlinghaus, R. T.; Andrews, L. J . Phys. Chem. 1984, 88, 4032.
0 1986 American Chemical Society
FTIR Spectra of Halobenzene Complexes
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2601 I
1600 1200 1000 4k 000 k h 6 0 0 1400 WRVENUMBERS WAVENUMBERS Figure 1. FTIR spectra of matrices formed by codeposition of 32 mmol of Ar/FB = 300/1 and 32 mmol of Ar/HF = 200/1 for the regions 4000-400 and 400-370 cm-I: (a) after deposition at 10 K; (b) after warming to 18-24 K for 10 min. Arrows denote perturbed FB fundamentals.
acterize the complexes formed between HF and fluoro, chloro, and bromobenzene where two basic sites exist in each base submolecule.
Experimental Section The vacuum and cryogenic techniques used in these experiments have been described previously.s.6 Infrared spectra were recorded on a Nicolet 71 99 Fourier-transform infrared spectrometer at 1 cm-' resolution in the 4000-400-cm-' range and 2 cm-' resolution between 425 and 125 cm-I. The HF (Matheson) was purified by outgassing using liquid nitrogen and a toluene slush. The D F was prepared by mixing equimolar amounts of D2 (Air Products) and F2 (Matheson) in a well-passivated 3-L stainless steel can and then outgassed after condensing with liquid nitrogen. Samples of fluorobenzene (FB), fluorobenzene-d5 (dFB), chlorobenzene (CB), bromobenzene (BB), and bromobenzene-d5 (dBB) (Aldrich) were outgassed at 77 K and used without further purification. The reagents were diluted with argon to give mole ratios (Ar/sample) of 300/ 1 to SO/ 1 for HF and D F and 500/ 1 to loo/ 1 for the halobenzenes. The gas mixtures were codeposited onto a CsI window at 10-12 K at a rate of approximately 7.5 mmol/h for 4 h. To allow migration of the submolecules within the matrix, the CsI window was warmed to 18-25 K for 10 min and then recooled to 10 K, and more spectra were recorded. Results Infrared spectra of argon matrix samples containing C6HsX (X = F, C1, and Br) with HF and D F will be discussed. Product complexes are formed between acid and base molecules. The acid submolecule modes observed are those due to the HF stretch and the HF libration. The latter arise from the two rotational degrees (5) Andrews, L.; Johnson, G.L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5761. (6) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 2865.
of freedom in uncomplexed HF. The spectra of the base submolecules can be divided into two parts-ring modes that do not contain appreciable vibrational amplitude of the halogen substituent, and those that do and are denoted X-sensitive modes. Fluorobenzene. A 34-mmol sample of FB (Ar/FB = 300/1) was deposited at 10 K and a spectrum recorded before and after annealing the matrix to 26 K. These spectra are in general agreement with gas-phase spectra previously reported7**and were used as blanks for eight codeposition experiments performed varying the FB and HF reagent concentrations in argon. Several new product bands were observed as shown in Figure la. The most intense band is at 3801 cm-' (labeled v,) with several weaker satellites, while others include a shoulder at 1483 cm-l, a strong sharp absorption at 1203 cm-' near the C6Hs-F stretching fundamental at 1223 cm-', a sharp band at 802 cm-' near another fluorine-sensitive fundamental at 807 cm-', a sharp band at 766 cm-l blue-shifted 11 cm-' from the out-of-plane C-H wagging fundamental, a shoulder at 495 cm-' on the B, in-plane fluorine-sensitive fundamental at 498 cm-I, a medium-intensity band at 442 cm-' (labeled VI), and a weaker sharp band at 413 cm-I near another B1 fundamental. The spectrum also contained bands due to HF dimer (labeled D), H F trimer (labeled T), and the N2--HF complex (labeled N)."" Upon warming the matrix to 18 K (Figure 1 b), the HF monomer band decreased markedly, a substantial increase in the intensities of the v, and vI modes was observed along with parallel increases in the other bands mentioned above, as well as providing many new product absorptions. Included in these are two sharp bands at 3748 and 3649 cm-' (labeled 2), a new band at 1193 cm-l below the C6H5-F stretching (7) Lipp, E. D.; Seliskar, C. J. J . Mol. Spectrosc. 1978, 73, 290. (8) Whiffen, D. H. J. Chem. Soc. 1956, 2, 1350. (9) Andrews, L.; Johnson, G.L. Chem. Phys. Left. 1983, 96, 133. (10) Andrews, L.; Johnson, G.L. J . Phys. Chem. 1984, 88, 425. (1 1) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J . Chem. Phys. 1983,
79, 2488. (12) Andrews, L.; Johnson, G.L. J . Chem. Phys. 1983, 79, 3670.
2602 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Davis and Andrews
TABLE I: Fluorobenzene and Hydrogen Fluoride Submolecule Fundamentals (cm-') for 1:l and 1:2 (Base:HF) Comolexee in Solid Arnon reagents CsH5F + HF C6HSF + DF reagents C~DSF + HF assignments 1:1 (Base:HF1 3919, 2877 1498 1223 807 755 498 407
3801 1483 1203 802 766 495 442 413 384
2788 1483 1203 801 766 495 322 418 292
3748 3649 1193 794 609 542 483
2751 2679 1193 794 598
3919 1392 1 I67 753 625 430
4-W
3801 1376 1147 747 63 1 a
A, C-C str A, C6HS-F str A, F-sens B, C-H bend B2 F-sens
b 388
393 b
Vl(HF) B, F-sens UI(H-F)
3748 3649 1140 a 612 528 483
Vs(HbF) Vs(HaF) A, C6H5-F str A, F-sens B, C-C-C def UI(H,F) VI(HaF)
1:2 (Base:HF) 1223 807 614
(1
1167 753 589
Not observed. bNot measured due to accidental degeneracies.
mode, a sharp band at 794 cm-' below the other fluorine-sensitive mode, another sharp absorption at 609 cm-' red-shifted 5 cm-' from the C-C-C deformation mode, and three weak bands at 542, 483, and 472 cm-'. These product bands are listed in Table I. Far-infrared spectra revealed a new band at 384 cm-I (labeled vI) which increased threefold upon warming. The HF dimer bands at 401 and 190 cm-I were also observed with the higher frequency dimer band appearing as a shoulder on the B1 fluorine-sensitive fundamental at 407 cm-I. Spectra from an experiment performed with Ar/FB = 300/ 1 and Ar/DF = 150/1 show the band at 3801 cm-l from H F experiments and an intense D F counterpart at 2788 cm-I (see Figure 2a,b). Also two sharp bands at 2751 and 2679 cm-' were observed in the D F region after annealing (marked 2) with satellites at 2743 and 2690 cm-l (marked with arrows) plus sharp new bands at 3736 and 3657 cm-I, which have the same relative intensities as weak 3748- and 3649-an-' bands; the latter two were observed in the HF experiment mentioned above. The product absorptions below 2000 an-'observed in the HF experiments were also seen in the D F experiment, plus a moderately weak band at 598 cm-' below the C-C-C deformation, and a new band at 418 cm-I above the BI fluorine-sensitive fundamental at 407 cm-'. An experiment conducted in the far-infrared region with Ar/FB = 200/1 and Ar/DF = 100/1 gave two new bands at 322 and 292 cm-' (labeled VI) which increased 2-fold upon warming the matrix. A sample of dFB (Ar/dFB = 500/1) was deposited on the cold window and a spectrum recorded before and after annealing the matrix to 18 K; the spectra are in general agreement with that reported previously.' Three experiments were performed with dFB and HF and these gave results similar to FB and HF. In the HF stretching region a strong band was observed a t 3801 cm-I with two absorptions growing in after annealing at 3748 and 3649 cm-', analogous to the FB experiments. New absorptions were observed for the vl modes, but the FB fundamentals a t 498 and 407 cm-' shifted to 430 and 388 cm-', for dFB, and the vI modes could not be resolved. Other complex modes include bands in the C-C stretching and C-D bending regions, and fluorine-sensitive motions, also listed in Table I. It is interesting to note that for dFB and HF, a complex band was seen a t 393 cm-I above the BI fluorine-sensitive fundamental, but with dFB and a D F / H F mixture, two bands were observed, one at 393 cm-' and another at 398 cm-I. Chlorobenzene. An Ar/CB = 200/1 sample was deposited at 10 K and a spectrum was recorded both before and after warming the matrix to 22 K. Matrix spectra agreed well with gas-phase spectra* and were used as blanks in the codeposition experiments. Upon codeposition of C B and H F , several new product bands appear as shown in Figure 3. Two bands are present in the HF stretching region at 3796 and 3771 cm-' (labeled v,) while sharp bands are present at 75 1 cm-' near the C-H out-of-plane bending
0
I
*
u4,
VI
z ,
3 n S O 2 50 2750 2650 WAVENUMBERS Figure 2. FTIR spectra of matrices formed by codeposition of 32 mmol of Ar/FB = 300/1 and 38 mmol of Ar/DF = 150/1 for the regions 2950-2600 and 350-250 cm-': (a) after deposition at 10 K; (b) after warming to 24 K for 10 min.
2950
fundamental at 743 cm-I and at 701 cm-I near the chlorinesensitive fundamental at 705 cm-'. Annealing the matrix to 22 K increased the 3796, 3771-, and 751-cm-I bands significantly. Several new absorptions became apparent including 3740 and 3600 cm-' (labeled 2) and 1100 cm-' above the chlorine-sensitive fundamental a t 1088 cm-'. Two experiments were performed monitoring the far-infrared region. Product bands include a triplet centered around 375 cm-', a doublet at 324 and 320 cm-I, a moderately strong band at 251 cm-', all labeled v,, and H F dimer (labeled D). Annealing the matrix to 25 K increased the intensity of the three bands labeled vl while the doublet band present before
FTIR Spectra of Halobenzene Complexes
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2603
0
T D
1
HF
t
Ibl
I
4bOO
3400 3600 3$00 WAVENUMBERS
3600
I
4 25
325 WRVENUMBERS
I
Figure 3. FTIR spectra of matrices formed by codeposition of 33 mmol of Ar/CB = 200/1 and 30 mmol of Ar/HF = 200/1 and 60/1 for the regions 4000-3550 and 425-255 cm-', respectively: (a) after deposition at 10 K; (b) after warming to 22 K for 10 min.
warm-up became one strong band shifted to 315 cm-I. Similar experiments with D F gave new bands at 2788 and 2771 cm-I, in the D F stretching region, as well as the 751- and 701-cm-' bands and weak 3798- and 3772-cm-I bands observed in the HF experiments. Upon warming, new bands appeared at 2741, 2643, and 1101 cm-'. Far-infrared measurements show three new bands, similar to the HF counterparts, a t 284, 242, and 184 cm-I with the middle band red-shifted upon annealing as in the HF experiments (Table 11). Similar experiments were performed with p-chlorotoluene and HF and PF in argon. Two bands were observed in the acid stretching regions a t 3791 and 3767 cm-' for HF and 2786 and 2766 cm-I for DF. The 3767- and 2766-cm-I bands were by far the more intense. Bromobenzene. An Ar/BB = 20011 sample was deposited at 10 K and the spectrum agrees well with the gas-phase spectrum reported previo~sly.'~ Codeposition of Ar/BB = 2001 1 and Ar/HF = 125/1 samples at 10 K produced product bands similar to that of FB and C B reported earlier (see Figure 4) including two moderately strong bands at 3800 and 3755 cm-' in the HF stretching region, a sharp band a t 746 cm-' well resolved from the C-H out-of-plane bend, and another sharp band at 462 cm-I (13) Uno, T.; Kuwae, A.; Machida, K.Spectrochim. Acta, Part A 1977, 33A, 607.
TABLE II: Chlorobenzene and Hydrogen Fluoride Submolecule Fundamentals (cm-I) for 1:l and 1:2 (Base:HF) Complexes in Solid Argon reagents (HF, DF) 3919, 2877 3919, 2877 1088 743 705
143
'Complex 4.
C6H5CI HF
+
+
C6HSC1 DF
assignments
1:l Complex 3796O 2788' 377 1 277 1 1l0OC 1 lOOC 75lC 751' 70lC 701' 3 756 284b 31Sb 242b 251a 184"
3740 3600
1:2 Complex 2741 2642 755
Complex 3.
Us(HbF) Vs(H,F) B2 C-H wag
Complex 3 and/or 4.
near the 460-cm-' AI bromine-sensitive fundamental. Upon warming the matrix to 18 K, the 3800- and 3755-cm-' bands increased over 3- and 4-fold, respectively, followed by growth in the other product bands already mentioned. New absorptions
2604 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Davis and Andrews
U -
0 A
u u
:L
I-
I-
E
m
$
-i
s I
I
1
4 25 425 4 25 325 225 WAVENUMBERS WAVENUMBERS WAVENUMBERS Figure 4. FTIR spectra of matrices formed by deposition of 32 mmol of Ar/BB 200/1 and 34 mmol of Ar/HF = 150/1 for the regions 4000-3500 and 425-225 cm-I and the 425-17S-cm-' region from a similar BB experiment with DF: (a) after deposition at 10 K; (b) after warming to 24 K for 10 min. 4000
3900
3800
3700
3600
include a strong band at 3720 cm-' and a weaker band at 3588 cm-I. Two far-infrared experiments were conducted, and new bands were apparent at 376 and 247 cm-I. Upon annealing to 24 K, the 376-cm-l band grew over 2-fold and shifted to 385 an-', a new absorption appeared at 318 cm-' on top of a BB fundamental, the 247-cm-' band increased 2-fold, and (HF), was decreased as shown in Figure 4b. Similar experiments with BB and D F produced new bands at 2801 and 2759 cm-I in the D F stretching region plus D F dimer a t 2804 cm-', and the two bands present in the HF experiment, described above, at 746 and 462 cm-l. Warming the matrix provided significant growth in the 2801- and 2759-cm-I bands, and two new bands also appeared at 2732 and 2632 cm-I plus the two bands observed in the HF experiment at 3800 and 3755 cm-' (Table 111). Experiments performed in the far-infrared region produced distinct new bands at 290, 240, and 179 cm-' which increased during annealing and are also shown in Figure 4. Upon deuteration of the base submolecule (dBB), the bands observed with BB were shifted to 3797 and 3753 cm-I, respectively. The bromine-sensitive fundamental at 3 18 cm-' shifted to 3 12 cm-I in dBB which allows a u1 band to be observed as a shoulder at 316 cm-' on the high-energy side of the 3 12-cm-I fundamental. Discussion The products of the fluoro-, chloro-, and bromobenzene cocondensation reactions will be identified and the structures of the complexes will be discussed. Identification. The new product bands described above were not present when the C,H,X samples and H F or D F were deposited separately and will be assigned on the basis of their frequency, annealing behavior, and concentration dependence to 1:1 and 1:2 complexes. Fhorobenzene. Two distinctly different groups of product bands were produced upon cocondensation of FB and HF. The first group was present upon codeposition and grew in intensity upon annealing the matrix, while the second group was present
-
TABLE III: Bromobenzene and Hydrogen Fluoride Submolecule Fundamentals (cm-I) for 1:l and 1:2 (Base:HF) Complexes in Solid Argon reagents C6H5Br+ C6HSBr+ HF DF assienments (HF, DF) ~ ~ 1:l Complex 3919, 2877 3800" 2801" u,(HF) 3919, 2877 37556 2759' u,(HF) 738 746c 746c B, C-H wagd 459 462c 462c B2 €hensd 385' 290' ul(HF) 318* 240' v,(HF) 247" 179" ul(HF)
1:2 Complex 3720 2732 V,(HbF) 3588 2622 %(H,F) "Complex 6 . 'Complex 5, site splitting at 3768 cm-I for HF. CComplex5 and/or 6. dCounterparts in dBB experiment were observed at 619 and 410 cm-'. only in matrices with a relatively high H F concentration, and only then after annealing. The strong first group band at 3801 cm-I present before annealing is assigned to the HF stretch in the 1:l complex, u,(HF), and exhibits a v,(HF)/v,(DF) = 3801/2788 = 1.363 ratio, which is consistent with an H-F vibration in a hydrogen-bonded complex. The two absorptions a t 442 and 384 cm-' also present upon codeposition are assigned to the librational motions of H F in the 1:l complex, uI(HF), giving v,(HF)/v,(DF) ratios of 442/322 = 1.373 and 384/292 = 1.315, which are appropriate for H F complexes. Several perturbed fundamentals of the fluorobenzene base submolecule were observed upon complexation with HF and are assigned on the basis of proximity to the FB fundamental and their annealing behavior, i.e., whether they follow the 1:l or 1:2 complex bands. The band at 1483 cm-' is assigned to the B, C-C stretch in the complex, down from 1498 cm-I in FB, and the strong band
_
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2605
0 d
w
u
I-
rl
CI
x
'r
0
pI-
v,
py'
(bl
I
WAVENUMBERS
I
4 00
+
Figure 5. FTIR spectra of fluorobenzene complexes: (a) CdHSF HF from Figure lb; (b) C6H5F+ DF from Figure 2b; (c) matrix formed by deposition of 30 mmol of Ar/C6DSF= 200/1 and 34 mmol of Ar/HF = 200/1 at 10 K and after warming to 18 K for 10 min. Parent bands are denoted by P while complex bands are shown by arrows.
at 1203 cm-' is assigned to the A, C6H5-F stretching fundamental in the 1:l complex down 20 cm-I from FB while the band at 802 is assigned to another A l F-sensitive complex band, red-shifted 9 cm-I from free FB; the 1203-mr' band involves substantial C-F stretching character and the large red shift indicates attachment to fluorine in the complex. The strong absorption at 765 cm-' is due to the B2 out-of-plane C-H bend in the complex while another B2 complex band is assigned to the F-sensitive mode at 495 cm-' and finally the 414-cm-' band is assigned to the Bl F-sensitive fundamental blue-shifted 8 cm-I from that of uncomplexed FB. The us and vI modes for the FB- -DF complex were assigned above and the perturbed FB fundamentals observed with HF were also seen with DF. However, with a D F / H F mixture two bands were present at 418 and 413 cm-I for the 1:l complex above the B, F-sensitive mode. The 418-cm-I absorption is assigned to the B, F-sensitive mode in the D F complex while the 4 13-cm-' band has been previously assigned to the HF complex. The explanation for this shift is the upper HF librational mode a t 442 cm-I is interacting with the Bl F-sensitive mode in the complex with the librational mode being blue-shifted and the B, band being red-shifted 5 cm-' relative to the D F complex, which is illustrated in Figure 5a,b. Other supporting evidences are the vI(HF)/~,(DF)ratios. For the upper band the ratio is 442/322 = 1.373, and for the lower band the ratio is 384/292 = 1.315. The upper librational isotopic ratio is slightly higher than would be expected relative to the lower librational isotopic ratio sup-
porting a blue shift in the 442-cm-' band. To aid in confirming this interaction dFB was codeposited with HF because the B, F-sensitive band shifts from 407 to 388 cm-I upon deuteration. This would allow the interaction to relax while a red shift in the 442-cm-I librational mode and an observable interaction with the upper D F librational mode was expected. Unfortunately a B2 F-sensitive band shifts from 498 to 430 cm-I upon deuteration covering the upper HF librational mode and it could not be resolved. However, the perturbed B, F-sensitive modes in the H F and D F complexes were observed and the HF band is at 393 cm-l while the D F band is a t 398 cm-I, blue-shifted 5 and 10 cm-l, respectively. Therefore, it appears that the interaction of the upper HF librational mode is relaxed and the upper D F librational mode is now interacting substantially causing a blue shift in the BI fundamental in that complex and a slight (1 cm-') red shift in the D F libration. Since only vibrations with the same symmetry can mix, the upper HF and D F librational modes must move in a plane parallel to the plane of the benzene ring. Several absorptions in the second group due to the 1:2 complex were observed upon annealing matrices with high HF concentrations. In the H F stretching region the two bands at 3748 and 3649 cm-' are assigned to the v,(HbF) and u,(H,F) stretches in the 1 :2 complex. The DF counterparts were present at 275 1 and 2679 cm-' giving u,(H,F)/u,(D,F) and u,(HbF)/u,(DbF) ratios of 1.362 for both sets. It is consistent with the basicities of the fluorine atoms for both v, modes in the 1:2 complex to be lower in frequency than the us mode in the 1:l complex. The H,F mode in the 1:2 complex is perturbed more than in the 1:l complex due to the fluoride ion affinity of HbF. The HbF submolecule is perturbed more than that of (HF), because of the increased basicity of the fluorine in H,F owing to the hydrogen bonding interaction between H,F and FB. The bands at 3736 and 2690 cm-' are assigned to the HbF and D,F stretches, respectively, in the mixed 1:2 complex while the 3657- and 2743-cm-' frequencies are assigned to the H,F and DbF stretches in the other mixed 1:2 complex. The stretching frequencies of the H,F and HbF submolecules in the natural isotopic 1:2 complex are separated by 100 cm-', which is close enough for an observable interaction between the two. When a D F is substituted for first one and then the other of the HF submolecules, this interaction is removed and a red shift of 12 cm-' is observed for H,F and a blue shift of 8 cm-I is seen for HbF. These assignments are supported by the relative intensities of the 1:2 HF and D F stretches. In a mixed H F / D F plus FB experiment the intensities of the two D F stretches in the two mixed 1:2 complexes are equal to those of the corresponding HF stretches, taking into account the isotopic intensity decrease when a D atom is substituted for H , which indicates approximately equal populations of the two mixed isotopic 1:2 species. A similar interaction was observed in the N2--(HF), complex where the H,F and HbF stretching frequencies were separated by only 29 cm-I. In the mixed 1:2 complexes the H,F mode red-shifted 9 cm-' while the HbF mode blue-shifted 14 cm-I.l4 Finally, the characterization of two mixed isotopic complexes verifies the stoichiometry and structure of the 1:2 complex. The two nondegenerate librational modes in the 1:2 complex were observed for H,F at 542 and at 483 cm-I; but the weaker HbF modes could not be detected due to the complexity of the spectrum in this region. Several perturbed fundamentals of FB were observed for the 1:2 complex with HF. The band at 1194 cm-I is assigned to the perturbed AI C6H5-F stretching fundamental, the band at 794 cm-l is assigned to an AI F-sensitive mode, and the absorption at 609 cm-' is assigned to the perturbed B, C-C-C ring deformation. These shifted further from the parent absorptions than the 1:l complex bands as expected owing to the stronger hydrogen bond in the 1:2 complex. With a D F / H F mixture these same complex bands were seen plus an extra one at 598 cm-I assigned to the perturbed BI C-C-C ring deformation in the 1:2 FB--(DF)* complex. An interaction between the upper H,F librational mode in the 1:2 complex and the B1 C-C-C ring (14) Andrews,
L.;Davis, S. R. J . Chem. Phys. 1985, 83, 4983.
2606
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
deformation is manifested in the spectra. In free FB the B, C - C - C ring deformation is a t 614 cm-' and in the HF complex it is seen at 608 cm-l but in the D F complex it is observed at 598 cm-'. The H,F librational mode at 542 cm-' is apparently interacting with the B, ring deformation causing a 10-cm-' blue shift relative to DF. In dFB the B, C-C-C fundamental is shifted to 589 cm-I and in the H F 1:2 complex with dFB the B1 C-C-C mode is seen at 61 1 cm-I while the upper H,F libration is redshifted 13 cm-' to 529 cm-' due to a stronger interaction since the libration and C-C-C deformation are closer in energy in dFB than in FB. This interaction dictates that the upper H,F libration in the 1:2 complex also be of in-plane symmetry. A C-C ring fundamental on the red side of the Bl C-C-C mode in dFB would preclude seeing the D F complex band. Deuteration of the base submolecule shifted the B2 F-sensitive fundamental to 430 cm-I and a B, F-sensitive parent band to 385 cm-' which hindered an accurate measurement of the two librational modes in the 1:l complex. The A, C-C stretching band in this complex is red-shifted to 1376 cm-l, the A I C6H5-F stretching fundamental in the complex is now at 1148 cm-' down 20 cm-I from dFB, the perturbed C-D out-of-plane wag is shifted to 631 cm-', and the lower BI F-sensitive fundamental is now at 394 cm-E for the H F complex and 398 cm-I for the D F complex. The reason for this 4-cm-I difference is due to the interaction of the upper librational modes (of the H F and D F complexes), with this Bl F-sensitive fundamental. This interaction was seen with HF and FB and was discussed earlier. The interaction with the HF librational mode forces the B, F-sensitive band down while interaction with the D F libration forces it up. Changes in the 1:2 complex bands with dFB include the absorption at 1140 cm-' assigned to the A, C6HS-F stretch while the band at 747 cm-' is due to the lower A, F-sensitive fundamental. The upper librational mode in the 1:2 complex is shifted from 542 to 528 cm-I due to the stronger interaction with the B, C-C-C ring deformation which is shifted from 614 cm-I in FB to 589 cm-' in dFB (see Figure 5c). The lower librational mode is not shifted, in support of the assignment to an out-of-plane motion. Chlorobenzene. The spectra for experiments with CB and H F indicate that two different types of 1:l complexes are formed upon deposition. The two bands at 3796 and 3771 cm-' are assigned to the H F stretching modes while the three bands at 375, 315, and 251 are assigned to the HF librational modes of these two different 1:l complexes. D F counterparts were observed for the v, and vl modes with the same band pattern of two v, modes and three vI modes giving v,(HF)/v,(DF) ratios of 3796/2788 = 1.362 and 3771/2771 = 1.361, and vl(HF)/vl(DF) ratios of 375/284 = 1.320, 315/242 = 1.302, and 251/184 = 1.364, all consistent with HF hydrogen-bonded complexes. These results compare favorably to the benzene- -HF and CH3CI- -HF complexes previously r e p ~ r t e d . ~The . ~ HF complex with benzene has a v, mode at 3798 cm-I and one doubly degenerate vI mode at 253 cm-I while the CH3CIcomplex exhibits a vs at 3726 cm-' and two librational modes at 436 and 378 cm-'. Several CB fundamentals were perturbed slightly upon complexation with HF or DF, and these include an A, C1-sensitive band a t 1100 cm-l, the absorption at 751 cm-I assigned to the B2 C-H bend, and another A, C1-sensitive band at 701 cm-'. Evidence for a 1:2 complex was found in bands at 3740 and 3600 cm-I, which are assigned to the HF stretches, with DF counterparts at 2741 and 2642 cm-', while the absorption at 755 cm-' is assigned to the B2 C-H bend. A similar interaction between the two HF stretches in the 1:2 complex, as seen in FB, was anticipated but the bands were not as sharp and were also split, making an accurate measurement difficult. Bromobenzene. The results obtained with BB and H F are very similar to the C B observations discussed above. The two bands at 3800 and 3755 cm-' are assigned to the H F stretches in two 1:l complexes giving v,(HF)/v,(DF) ratios of 3800/2801 = 1.357 and 3755/2759 = 1.361. Three HF librational motions were observed at 385, 318, and 247 cm-l with D F counterparts at 290, 240, and 179 cm-'. Again, comparison with benzene- -HF and .~ CH,Br- -HF spectra identifies these two 1:l c ~ m p l e x e s . ~Two
Davis and Andrews perturbed fundamentals were seen for the 1:1 complexes and the band at 746 cm-I is assigned to the B2 C-H wag shifted from 738 cm-I in free BB, and the absorption at 462 cm-I is assigned to a B2 Br-sensitive band in the complex. The two bands centered at 3720 and 3588 cm-I are assigned to the H F stretches in the 1:2 complex with D F counterparts at 2732 and 2632 cm-'. Upon deuteration of bromobenzene, the perturbed C-D out-of-plane wag is shifted to 619 cm-l in the 1:l complex while the B2 Brsensitive complex band is shifted to 410 cm-'; other product bands were not shifted. Structure of the Complexes. The halobenzenes FB, CB, and BB can be classified as weak Lewis bases with two different sites capable of electron donation to HF. One site is the halogen atom with its three pairs of nonbonded electrons, and the other is the benzene ring moiety with its six delocalized a electrons. HF complexes with a hydrogen bond to both methyl halides and benzene have been observed in argon mat rice^^-^ giving H F stretching modes between 3700 and 3800 cm-'. The structures of complexes between HF and the three halobenzenes studied will be discussed. The proposed structures for the 1:l and 1:2 complexes of FB and H F are shown in structures 1 and 2. A hydrogen bond to @\\
H
@F\,
\F
1
H
\F - -H-F
2
the fluorine atom of FB is supported by the number of perturbed F-sensitive fundamentals observed and the frequencies of the HF-stretching and librational modes. The C6HS-F stretching mode in FB and dFB were both red-shifted 20 cm-' upon complexation with HF following that found for the C-F stretch in the H,CF- -HF complex3 which red-shifted 37 cm-' upon complexation. The benzene- -HF complex exhibited an HF stretching frequency of 3795 cm-' and a single librational mode at 253 cm-' in contrast to the two nondegenerate librational modes at 442 and 384 cm-l for FB- -HF. The vs and vI modes also compare well with 3774,455, and 435 cm-I values for the CH3F--HF complex in solid argon.3 We conclude that the HF is in the plane of the benzene ring owing to interaction of the 442 cm-' libration with the in-plane F-sensitive fundamental at 413 cm-'. There are two in-plane lone electron pairs on the fluorine of FB suitable for interaction with HF; the "a-type" lone pair along the C6H5-F bonding axis and the pseudo-a lone pair arising from a 2p atomic orbital of the fluorine atom. The a lone pair is lower in energy; therefore the interaction is expected with the higher energy inplane pseudo-n lone pair giving the complex a bent structure such as that found in the (HF), c o m p l e ~ . ' ~ The 1:2 complex is predicted to have structure 2 as ascertained by the frequency shifts of the HF stretching modes in the complex and the further perturbation of the C6H5-F stretching fundamental which is 20 cm-I for 1 and 30 cm-' for 2. The H,F and HbF stretches are red-shifted 151 and 53 cm-I relative to the HF stretch in 1, respectively, and such red shifts would not be expected if the second HF interacted with the benzene ring or the fluorine atom of FB, each being very similar in basicity. Also, the mixing of the H,F and HbF modes as derived from the mixed HF,DF species confirms that HbFis bonded to the H,F submolecule in 2. The H,F submolecule is in the plane of the ring in 2 also as evidenced by the interaction of the H,F libration at 542 cm-' with the in-plane C-C-C ring deformation a t 608 cm-I. It is not possible to determine if HbF is in the plane of the ring. As mentioned earlier, two HF stretching modes were observed upon codeposition of CB and HF suggesting the two 1 :1 complexes depicted in structures 3 and 4. The HF stretch at 3771 cm-l and the two HF librations at 375 and 315 cm-I are assigned to complex (15) Dyke, T.R.; Howard, B. J.; Klemperer, W. J . Chem. Phys. 1972,56, 2442. (16) Andrews, L.; Johnson, G. L.; unpublished results.
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2607
FTIR Spectra of Halobenzene Complexes
0-
(O>-Y
-H-F
\
TABLE I V HF Stretching and Librational Fundamentals (cm-I) for Methyl, Vinyl, and Phenyl Halide Complexes in Solid Arnon base V, VI PA C6HSF
C~H,F~ 'F
3
3 while the HF stretch a t 3798 cm-' and the librational mode at 25 1 cm-' are assigned to complex 4, since the latter modes are near the 3795- and 253-cm-I values for the benzene- -HF complex. Support for a structure like 3 is provided by the methyl chloride- -HF complex4 which produced a HF stretch a t 3726 cm-' and two H F librational modes at 436 and 378 an-'.The increased perturbation in the HF stretch going from FB to C B is expected due to the higher basicity of C1 and is similar to that observed for alkyl halides with HF.4 For p-chlorotoluene the band at 3767 cm-I is probably due to a complex like 3 while the band at 3791 cm-' is due to a complex like 4. Bands observed for the 1:2 complex cannot be assigned conclusively to complexes based on 3 or 4. The basicities of the benzene ring and the chlorine atom are very similar as evidenced by the similar HF stretches for 3 and 4, and so a second HF could presumably bind to the first HF of either complex. The H F fluorine of complex 3 would be expected to have a slightly higher proton affinity than that of 4 because of the greater HF stretching perturbation, and so on this basis alone a 1:2 complex like that found for FB is expected. The red shift in the 1:2 v, modes of C B relative to those in FB follows a comparison of the HF stretches in the 1:l complexes 1 and 3 giving evidence for the 1:2 C B complex being similar to 2 with the (HF), chain bonded to the chlorine atom. The results found for BB and HF mimic those of CB and HF and the structures of the two 1:l complexes are shown below. The bands at 3755,385, and 318 cm-' are assigned to complex 5 while Br
'\ F
6
5
those at 3800 and 247 cm-' are assigned to 6. The basicity trend F < C1 < Br accounts for the HF stretching frequencies at 3801, 3771, and 3755 cm-' for structures 1,3, and 5, respectively. The HF stretches in complexes 4 and 6, 3798 and 3800 cm-I, respectively, show that the basicity of the ring is not affected appreciably compared to that of benzene at 3795 cm-I. Even in p-chlorobenzene, the ring complex exhibits an HF stretch within 9 cm-' of that for C B and BB and 4 cm-' of that for benzene. However, a methyl substituents alone does affect the basicity of the aromatic ring; the toluene- -HF complex exhibits a split us mode at 3781 and 3763 cm-', 15-33 cm-' from benzene.16 Again, the structure of the 1:2 complex could not be determined but the higher basicity of the bromine suggests that a complex like 2 is formed. Again, the 1:2 us modes are red-shifted compared to those of FB and C B and follow the us modes in complexes 1, 3, and 5 suggesting the 1:2 complex for BB has a structure similar to that of 2 with the (HF), chain bonded to the bromine atom. Bonding Trends. It is interesting that for FB only one type of 1:l complex was formed while for CB and BB two types were produced. The fluorine atom is the weakest base of the three halogens studied and, according to the us a t 3801 cm-I, should be equivalent with the benzene ring. One explanation is that the high electronegativity of the fluorine atom causes electron withdrawal through the C6H6-F a-bond lowering the basicity of the ring. This ring deactivation process is not as substantial for C B and BB, due to the lower electronegativities of the Cl and Br atoms, and the ring complex is then competitive in stability.
CH3FC C&Cl C2H3Clb CH3Cld C6HSBr C,H,Brb CH3Brd
3801 3805 3774 3771 3774 3726 3755 3759 3721
442, 384 421, 384 455,435 375, 315
183' 183"
436, 378 385, 318
160'
413, 358
163c
"Lau, Y. K.; Kebarle, P. .I. Am. Chem. SOC.1976, 98, 7452. bReference 18. cReference 3. dReference 4. 'Beauchamp, J. L. Znteractions Between Ions and Molecules; Ausloos, P., Ed.; Plenum: New York. 1975.
A measure of the relative stabilities of the two complexes observed with CB and BB can be determined by the relative growth of the bands assigned to these two complexes. For CB, when the matrix is annealed the us for 3 grew slightly more than that for 4. When a matrix of BB and H F was annealed, the us of 5 grew by a factor 4 while the u, of 6 grew 3-fold. Since the complex with the HF bonded to the halogen atom is favored, and intuitively there should not be a kinetic barrier to formation of the ring complex, the complexes 3 and 5 appear to be slightly more stable than 4 and 6. Similar behavior has been observed for phenylalkyne- -HF complexes where the alkyne attachment site gives the more stable complex than the ring ~ o m p l e x . ' ~ Another interesting trend is that the frequencies of the librational modes do not follow the Au, trends for complexes 1,3, and 5. The librational modes provide a measure of the angular dependence of the potential surface of the hydrogen bond, and as the hydrogen-bond strength increases the potential surface usually becomes sharper with respect to HF libration. However, the trend observed here is that the potential surface becomes broader as the hydrogen-bond strength increases. This type of behavior has also been observed with alkyl halides4 and is attributed to the increase in size of the lone electron pair and increased polarizability of the halogens as the atomic number increases. The halogen atoms studied are slightly less basic when bonded to the phenyl ring than to an alkyl group as witnessed by higher v, and lower vI modes for the halobenzene complexes (Table IV). The halogen atoms withdraw electron density from the ring through the C6HS-X a-bond but can donate electron density to the ring through the A lone pair on the halogen atom. The same type of electron withdrawal and donation is possible for HzC= CHX- -HF complexes. As can be seen in Table IV the u, modes of the halobenzenes are very similar to those of the haloalkenes.'* The electron donation from the halogen atom to the A orbitals of the ring, in C6HS-X--HF complexes, could increase the effective nuclear charge felt by the other lone pair electrons shrinking their MO's slightly. This could also happen for the haloalkenes. The more dense lone pairs are less basic, explaining the higher V, modes (smaller displacements) in halobenzene complexes.
Conclusions Fluoro-, chloro-, and bromobenzene each formed complexes with HF upon d e p o s i t i o n at 10 K. The FB- -HF complex was shown to be planar with a hydrogen bond between the fluorine atom and HF owing to interactions between the acid ligand and in-plane base submolecule vibrations. In the 1:2 FB- -(HF), complex, the ring and H,F submolecules were also found to be coplanar with the HbFbonded to H,F. Two types of 1:1 complexes were formed for CB and BB with hydrogen bonds to the halogen and to the aromatic ring based on comparisons with benzene and alkyl halide complexes. The 1:2 complexes are believed to be of the type C6H5-X--(HF), with one hydrogen bond between H,F (17) Davis, S. R.; Andrews, L., submitted for publication in J. Mol. Srmcr. (18) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Am. Chem. SOC.1982, 104, 6180.
J . Phys. Chem. 1986, 90, 2608-2615
2608
and the halogen and the other between H,F and HbF. The basicities of the halogen atoms increase with size but are less than the alkyl halides presumably due to electron donation from the *-lone pair of the halogen to the aromatic ring. Acknowledgment. We gratefully acknowledge financial support
from N.S.F. Grant C H E 82-17749 and helpful discussions with Carl Trindle. Registry No. C6H5F, 462-06-6; C2H3F,75-02-5; CH3F, 593-53-3; C6H5C1, 108-90-7;C Z H ~ C75-01-4; I, CH3C1, 74-87-3; C6HSBr, 108-86-1; C2H3Br,593-60-2; CH,Bre, 74-83-9; HF, 7664-39-3; Ar, 7440-37-1.
Magnetic Circular Dichroism and the Jahn-Teller Effect in the 2S Sodium and Lithium Atoms Isolated in Xenon Matrices
-
*P Transition of
J. Rose, D. Smith, B. E. Williamson,+ P. N. Schatz,* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
and M. C. M. O'Brien* Department of Theoretical Physics, University of Oxford, Oxford OX1 3NP, England (Received: December 3, 1985)
-
-
The magnetic circular dichroism (MCD) and absorption spectra of the 2S 2P transition of Na and Li atoms in Xe matrices have been studied from 1.4 to 40 and 35 K, respectively, by a unique matrix injection technique. Earlier results reported for Na/Xe by other workers are shown to be in error, and our previously reported value of the excited-state spin-orbit coupling in Li/Xe is shown to be about 25% low. Jahn-Teller-active (noncubic) modes completely dominate the bandwidths in both systems, and the temperature dependence of these widths and theoretical analysis show conclusively that the characteristic triplet structure in these systems is a consequence of a strong Jahn-Teller effect. The absorption and MCD contours are fit well by a cluster model Jahn-Teller calculation assuming a single effective frequency and equal coupling to eg and t2, modes [T @ (e + t2)]. A rationale is offered for the applicability of the equal coupling case. The effective spin-orbit coupling constants of Na/Xe and Li/Xe are -196 and -213 cm-I, respectively, compared to the Li and Na free-atom values of 0.23 and 11.5 cm-I. The Na/Xe and Li/Xe Jahn-Teller energies are calculated to be 1100 and 1000 cm-', respectively.
-
-
I. Introduction The MCD of S P transitions in noble gas matrices provides three important measures of the interaction between guest atoms and the host. First, a partial quenching of the free-atom angular momentum of the excited P states is observed.'g2 Second, and more striking, in the zS 2P case, the excited-state spin-orbit coupling is radically different from its free-atom v a l ~ e . ' . ~ -Third, ~ moment analyses in conjunction with the absorption spectrum permit a quantitative assessment of the relative contributions of totally symmetric ("cubic") and Jahn-Teller-active ('noncubic") modes to the bandwidth thus permitting a detailed analysis of guest-host vibronic coupling. In a recent paper' the magnetic circular dichroism (MCD) of the 2S 2P transition of Li atoms in Ar, Kr, and Xe matrices was discussed. The results and analysis strongly supported the P triplet structure, at least in view that the characteristic S the case of Li/Xe and Li/Kr, is a consequence of a Jahn-Teller (JT) effect. It was suggested' that the case of equal t2 and e mode coupling [T @ (e t2)] offered promise for a detailed interpretation of Li/Xe whose spectrum is very clean with remarkably well-resolved triplet structure notable for its distinctive, sharp, high-energy component.' Subsequent calculations by O'Brien6 for the T @ (e + t2) JT case produced absorption and MCD contours both of which agreed well with experiment. We report in this paper MCD measurements on both Na/Xe and Li/Xe along with a JT analysis of both systems. In obtaining the experimental data, we use an "injection" technique described below which permits a high-precision study of the absorption and MCD temperature dependence over essentially the entire range of stability of both systems. We show that a recent MCD studys of Na/Xe is in error in several very important respects and that the spin-orbit coupling constant of Li/Xe previously reported by
-
-
-+
+
some of us' was appreciably low because of systematic errors in the temperature monitoring over a very limited range. The new Li/Xe data also show that the contribution of cubic modes is small, as required by the O'Brien calculation,6 thus removing a serious discrepancy between theory and the earlier data.' The JT analysis is done in the low-temperature limit and produces contours that fit the experimental absorption and MCD data well for both Li/Xe and Na/Xe. A rationale is also offered to explain why the T @ (e + t2) JT case is operative for a Li or Na atom at a substitutional site in a noble gas host. We conclude that the case for a JT interpretation of the triplet structure in Li/Xe and Na/Xe is compelling. A similar interzP transition of Cu/Ar, Cu/Kr, Cu/Xe, pretation of the ?S and Ag/Ar3s4 is also strongly supported by further calculations of O'Brien' again on the basis of a T @ (e + tz) JT effect. The case for a J T interpretation of the triplet structure in the 'S IP transition of Mg isolated in Ar, Kr, and Xe also seems strong.2
-
-
11. Experimental Section We use a top-loading split-coil superconducting solenoid magnet system (Oxford Instruments modified SM4) in conjunction with a continuous flow cryostat (Oxford Instruments CF 204) to inject ( 1 ) Lund, P. A.; Smith, D.; Jacobs, S. M.; Schatz, P. N. J . Phys. Chem. 1984, 88, 31-42.
( 2 ) Mowery, R. L.; Miller, J. C. Krausz, E. R.; Schatz, P. N.; Jacobs, S. M.; Andrews, L. J . Chem. Phys. 1979, 70, 3920-3926. (3) Zeringue, K.; ShakhsEmampour, J.; Rivoal, J.-C.; Vala, M. J . Chem. Phys. 1983, 78, 2231-2239. (4) Vala, M.; Zeringue, K.; ShakhsEmampour, J.; Rivoal, J.-C.; Pyzalski, R. J . Chem. Phys. 1984, 80, 2401-2406. ( 5 ) Armstrong, S.;Grinter, R.; McCombie, J. J . Chem. SOC.,Faraday Trans. 2 1981. 77. 123-133. ~~(6) O'Brien, M. C. M. J . Chem. Phys. 1985, 82, 3870-3871. (7) O'Brien, M. C. M. J . Phys. C 1985, 18, 4963. (8) Homes, J.; Schiller, J. Chem. Phys. 1983, 7 4 , 433-439. 1
'Present address: Chemistry Department, University of Canterbury, Christchurch 1, New Zealand.
0022-3654/86/2090-2608$01.50/0
1
~~~
0 1986 American Chemical Society
