J . Phys. Chem. 1985, 89, 4269-4272
n
. -
-40c
3 Lo
0
-"
0
4 - A L A -
-
20 .
in the selectivity among catalysts is brought about by the difference in the catalyst structure but not by the consecutive oxidation of co to coz. As shown in Figure 5, the oxidation state of the VzOs-U catalyst changes greatly with Po. Under excess oxygen conditions, the catalyst is kept in the highest oxidation state, Le. Vs+, while it is reduced as Po decreases. As shown in Figure 2, S ( C 0 ) does not change with Po. This means that the selectivity in butane oxidation is independent of the oxidation state of the catalyst at least under the present experimental conditions where conversion of butane is below 20% and consecutive oxidation of C O to COz is negligible and where the catalyst is in its steady state at a given level of Po. As shown in Table I, the selectivity changes greatly with the kind of unsupported V20s: The V205-Fis effective for the formation of COz, while the Vz05-Uand VZOs-ROproduct both CO and COz. As discussed above, the number of surface defects on
Effect of Cation Polarizing Power on Ai,Br,Nuclear Quadrupole Resonance
4269
the VzOS-Fis considered to be smaller than that on the Vz05-U and V205-R0. The result of the selectivity thus suggests that the surface V = O species at the surface defect produces both C O and C 0 2 , while that in the smooth (010) face leads to the selective formation of COz. As shown in Table 11, the selectivity also changes greatly with the kind of VzOs/TiO2 catalysts: VZO5/TiOzwith low Vz05 content forms COzselectively, while that with high Vz05content produces both C O and C 0 2 . Since a smooth VzOs surface is expected to be formed for VzOS/TiO2with low Vz05content, the result can also be explained by the above-mentioned idea that the surface V=O species at the surface defect produces both C O and COz, while that in the smooth (010) face leads to the selective formation of COz.zo In conclusion, catalytic behaviors of unsupported and supported Vz05for butane oxidation are explained in terms of the roughness of catalyst surface: Surface V=O at the surface defect exhibits high activity for the reaction to form both CO and COz, while that in the smooth (010) face is less active and produces COz selectively.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 59470097) and for Encouragement of Young Scientists (No. 59750655) from the Ministry of Education, Science, and Culture, Japan. Registry No. CH,(CH2)2CH3,106-97-8; V 2 0 5 , 13 14-62-1; rutile, 1317-80-2;anatase, 1317-70-0. (20) Although control of the roughness of the V 2 0 5 surface is the main factor of the catalytic behavior of V205/Ti02catalysts, the following points should also be noted as for the effect of the TiO, support from the results of turnover frequencies for the formation of CO [TF(CO) = R(CO)/L] and CO, [TF(C02)= R(C02)/L]: As shown in Table I, TF(C0) for V205-Fis ca. 100 times smaller than TF(C0) for V205-U, and TF(C0,) for V2O5-F is ca. 10 times smaller than TF(C02)for V,O,-U. TF(C0) for V205/TiOzwith low V2O5 content is also ca. 100 times smaller than TF(C0) for V~OS-U,a difference similar to that between V205-Fand V2O5-U. On the other hand, TF(C02)for V205/Ti02with low V,05 content is comparable to TF(C02) for V205-U,a behavior different from that for V,05-F and V205-U. This suggests that Ti02 support plays some role in the oxidation of butane to CO, on V205/Ti02with thin V,05 layers.
Anion Studied by means of ''Br and *,AI
Koji Yamada* and Tsutomu Okuda Department of Chemistry, Faculty of Science, Hiroshima University, Naka- ku, Hiroshima 730, Japan (Received: January 7 , 1985; In Final Form: May 13, 1985)
Bromine-81 and aluminum-27 nuclear quadrupole resonance (NQR) data have been obtained for a series of MAI2Br7compounds (M = NH4, (CH,),N, and (C2H5)4N).For the cation change from (C2HJ4N+to NH4+,the bridging 8'Br NQR frequency increases about 10 MHz, which is about 13% of that in the (C2H5),NAI2Br7compound and too large to arise from only crystal field effects. This large frequency shift is suggestive of the polarization of the anion induced by neighboring cations. That is, the negative charges of the anion are more localized at the terminal positions than at the bridging position with increasing polarizing power of the cation. The nuclear quadrupole coupling constants of 27Alincrease with increasing cation radius. This finding also supports the anion polarization model.
Introduction In the Friedel-Crafts reaction AlzX7- (X = C1 or Br) anions have an important role to stabilize organic cations in a proper solvent.' Many salts containing A12Br7- anions have been confirmed in the solid state for alkali metal2 and alkylammonium (1) Mirda, D.; Rapp, D.; Kramer, G. M. J . Org. Chem. 1979,44, 2619. ( 2 ) Cronenberg, V. C. T. H. M.;Van Spronsen, J. W. Z . Anorg. Allg. Chem. 1967, 354, 103.
0022-3654/85/2089-4269$01 SO10
cations and studied by vibration3 and NQR spectra." According to X-ray analysis the AlZBr7- anion consists of two A1Br4 tetrahedra sharing one Br atom with a bridging bond angle close to regular tetrahedral angle.7s8 (3) Manteghetti, A.; Potier, A. Spectrochim. Acta, Parr A 1982, 38A. 141. (4) Merryman, D. J.; Edwards, P. A.; Corbett, J. D.; McCarley, R. Inorg. Chem. 1974, 13, 1471. ( 5 ) Deeg, T.; Weiss, Al. Ber. Bunsenges. Phys. Chem. 1975, 79, 497. (6) Yamada, K. J . Sci. Hiroshima Uniu., Ser. A : Phys. Chem. 1977,41, 71.
0 1985 American Chemical Society
Yamada and Okuda
4270 The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
In our previous N Q R and N M R experiments on MA12Br7the following points were discussed:6 (1) bending of the AI-Br (bridge) bond from a straight line, (2) reorientation of the AlBr, group below its melting points, and (3) dependence of 81Brand 27AlNQR parameters on cation radius. In this paper we will discuss the last effect choosing a series of tetraalkylammonium groups as cations. According to the Townes-Dailey theory9 the NQR frequency is very sensitive to the bond character of the terminal halogen, so that a little change in the bond character has a large effect on the NQR frequency. However, it is sometimes a troublesome problem to determine the origins of the small frequency shift observed for an anomalous temperature dependence and a cation dependence of N Q R frequencies. In a series of complex anions MX6" (X = C1, Br; n = 1-3; M = Sn, Te) NQR frequencies of the terminal halogens increase slightly with increasing cation radius.I0 Brown and Kent attributed the increase in N Q R frequency with cation radius to the change in the electron distribution, depending upon the cation polarizing power." On the other hand, Brill et al. studied SnC162-complexes by X-ray diffraction, IR, and NQR methods and concluded that the dominant factor is the variation bf the Sternheimer shielding factor.I2 In our previous paper we studied this point for a series of isomorphous complexes A21nX5.H20(A = K, NH4, Rb, and Cs; X = C1 and Br) and reported that the cation size effect is more prominent for the central indiuq.I3 These NQR data suggested that the cations affect the InX5.H202-anion by their electric field, whereby both polarization and bond character of the anion are affected. In this case the present compounds are interesting because these anions are not spherical with two chemically different halogen atoms and also because the cations are surrounded only by the terminal halogen, atoms. Therefore, if the polarization of the whole anions takes place, depending upon the cation polarizing power, their NQR frequencies may be expected to shift different directions from one another. The previous N Q R studies for Ga2C17- or AlzBr7- anions support this model,s.6 but the observed shifts are not so large probably because there is little change in the cation radius. In this study we have detected 81Brand 27AlNQR for complexes containing much larger cations in order to clarify this point.
TABLE I: *'Br NQR Frequencies for MAI,Br, (M = K, NH4, (CH3),N, and (C,H,),N) at 77 Ka terminal bridge compd u,MHz 3,'MHz u,MHz KA1zBr7b 90.925 86.406 86.334 86.947 86.167 85.619 85.179 83.599 90.397 87.065 86.330 87.070 87.070 86.91 1 86.193 84.747 91.561 88.176 80.223 88.662 88.61 1 87.898 86.684 85.639 9 1.968 88.436 76.500 90.901 90.056 87.813 85.230 84.650 Estimated error in Y, h0.005 MHz. quency of six terminal lines.
\ i
v
I
1.50
Experimental Section
I
-
(7) Rytter, E.; Rytter, B. E. D.; 0ye, H. A.; Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1973, B29, 1541. (8) Rytter, E.; Rytter, B. E. D.; 0ye, H. A.; Krogh-Moe, J. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1975, 831, 2177. (9) Townes, C. H.; Dailey, B. P. J . Chem. Phys. 1949, 17, 782. (IO) Ramakrishnan, L.; Soundarajan, S.; Sastry, V. S.S.; Ramakrishna, J. Coord. Chem. Rev. 1977, 22, 123. (11) Brown, T. L.:Kent, L. G. J . Phvs. Chem. 1970, 74, 3572. (12) Brill, T. B. J . Chem. Phys. 1974; 61,424. Brill, T. B.; Gearhart, R. C.; Welsh, W. A. J . Magn. Reson. 1974, 13, 27. ( 1 3) Yamada, K.; Weiss, AI. Ber. Bunsenzes. Phys. Chem. 1983,87, 932. (14) Emshwiller, M.; Hahn, E. L.; Kaplan, D. Phys. Rev. 1960, 118, 414. (15) Wciden, N.; Weiss, AI. J . Magn. Reson. 1975, 20, 334. (16) Weiden, N.; Weiss, AI. J. Magn. Reson. 1978, 30, 403.
I
I
I .55
I
3.00 Frequency
MAL2Br7( M = NH4, (CH3)4N,and (C2H5),N) were obtained by slowly crystallizing the molten mixture of AlBr, and the relevant halide in a sealed tube. In this case a little excess of AlBr, above the stoichiometric ratio (1 :2.05) was sealed in a glass tube. 8'Br N Q R was detected by a superregenerative spectrometer and a Matec pulse spectrometer (Model 5100 + 525 and receiver Model 625). For the detection of 27Al N Q R a spin-echo double-resonance technique (SEDOR) was applied for the 81Br 27Al ~ y s t e m . ' ~ - ' ~The double-resonance spectrometer in this experiment consisted of a Matec pulse spectrometer, Matec gated amplifier (Model 515) for lower frequency channel, and surrounding homemade circuits. The echo amplitude of 81BrNQR was monitored on an oscilloscope trace or recorder through the gating circuit, changing the frequency of the 27Alradio-frequency irradiation. In most cases a weak static magnetic field was applied
Reference 6. 'Average fre-
/
I
3. I O
MHz
Figure 1. Typical 27AI SEDOR for NH4AI2Br7at 77 K. A spin-echo signal at 86.91 1 MHz, which could be assigned to the A1(2)Br3 group, was monitored to record 27A1NQR.
in order to extend T2, because the sensitivity of this method depends upon the pulse separation T between 90' and 180' pulses. Typical experimental conditions were 100-300 11s for T and 5, 10, and 50 ~s for 81Br 90°, 81Br 180°, and 27Al 180' pulses, respectively.
Results 81BrNQR. Table I shows the 81BrNQR frequencies at 77 K for (C2H5),NAlzBr7together with the reported frequencies for KA12Br7,NH4A17Br,, and (CH3)4NA12Br7.6For KA12Br7and NH4A12Br7all seven lines appeared on the same frequency range of about 83-90 MHz, so that the assignment of the bridging Br atom could be done on the basis of the Zeeman effect or SEDOR experiments.6s16 As is shown in Table I, on the other hand, the N Q R spectra for (CH3),NA12Br, and (C2H5),NA12Br7 show different features compared with KA12Br7and NH4A12Br7. That is, only one resonance line appears at a lower frequency than the other six lines. This lowest frequency line was assigned to the bridging Br atom for (CH3),NA12Br7on the basis of the Zeeman effect? In analogy with (CH3),NA12Br7the lowest frequency line of (C2H5)4NA12Br7could be assigned to the bridging atom. This assignment was also confirmed by the double-resonance method as will be described later. Table I lists mean frequencies for terminal atoms. The mean frequency increases slightly with increasing cation radius, whereas the bridging Br frequency de-
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4271
Effect of Cation Polarizing Power on A12Br7-
TABLE II: 27AI NQR Parameters for MAIIBr7 (M = K, NH,, (CH3),N, and (C2H5),N) at Liquid Nitrogen Temperature' v1(1/2c+3/2),
compd
assignt
I
I
e2Qdh
u2(3/2-J/2),
MHz 1.304 1.628
MHz 2.562 3.068
1.507 1.553 1.726 1.819
2.984
1.820 1.909
3.428 3.538
3.083 3.203 3.636
I)
0.1 18 0.219
0.088
10.289
0.076 0.248
10.806
0.021 0.221
12.121 1 1.537 11.937
I '
I
I
MHz 8.563 10.323 9.963
I
I
0.250
1
I
l
- 8 - 109.5'
I
I
I
- 0.3 L
I
"*
I
I
70
I
I 1
90 Frequency / M H r
80
Figure 3. Relationship between negative charge at the bridging position NQR frequency according to eq 1-4.
pB and its
Figure 2. Cation dependence of 81Brand 27AlNQR parameters: 0, 0 , 81BrNQR frequencies for the terminal and bridging Br atoms, respectively; 0,quadrupole coupling constant for 27Al.
creases about 10 M H z from NH4A12Br7 to (C2H5)4NA12Br7. 27AINQR. Typical SEDOR spectra for NH4A12Br7are shown in Figure 1. In this case a spin-echo signal at 86.91 1 MHz, which was assigned to the A1(2)Br3 group, was monitored to detect 27A1 NQR. Then, the Al(2) site gave rise to stronger signals v I and v2 than those from the Al( 1) site, because the sensitivity of this method depends upon dipole-dipole interactions between 81Brand 27A1.14*16 Therefore, in this experiment 81BrNQR for the bridging pair could Br atom and also the 27Al~1(~/2++~/2) and be assigned uniquely. In Table I1 27AlNQR parameters at liquid nitrogen temperature are summarized for MA12Br7.
Discussion Figure 2 shows the cation dependence of the SIBrand 27AlNQR parameters, where mean values are plotted for the terminal Br and A1 sites. The NQR spectrum of (C2H5)4NA12Br7 is regarded as an extreme example with weak electrostatic interactions between cations and anions. It is interesting that the cation size dependence of the slBr NQR frequency is very large for the bridging Br atom and its direction is reversed compared with that of the terminal frequency. According to the NQR Zeeman effect using single crystals for KA12Br7and NH4A12Br7,the z axis of the electric field gradient for the bridging Br atom was determined to be perpendicular to the AI-Br-AI plane.6 This finding suggests that the signs of the quadrupole coupling constant for the terminal and bridging Br atoms are reversed with respect to each other, so that the electric field gradient from the crystal lattice contributes differently to the N Q R frequency for the terminal and bridging Br atoms, respectively. However, the observed frequency shift for the bridging Br atom is too large as to be a result of only crystal field effects, and accordingly some changes in bonding property may be expected for the bridging atom probably due to the po-
larization of the anion as a whole. We will now discuss the relationship between the bridging Br NQR frequency and the negative charge on it, because the NQR frequency for the bridging atom depends not only upon negative charge but also upon its bond angle. The bridging bond angle LAl-Br-A1 is almost equal to the tetrahedral angle (109.3' and 107.7' for KA12Br7and NH4A12Br7,respectively), so that sp3 hybrid orbitals with C,, symmetry are assumed for the bridging bond. Two orbitals are occupied by lone pair electrons, and the remaining two orbitals are used for the bonding with Al. According to the Townes-Dailey theory the quadrupole coupling constant e2Qq/h (81Br)for the bridging Br atom is described a d 7
( e 2 Q q / h ) / ( e 2 Q q p / h=) (2 - B)(1 + cot2 (0/2))/2
(1)
where e2Qqp/his the quadrupole coupling constant due to one 4p electron (gQqp/h= -643.03 MHz) and g Q q / h is the observed coupling constant. B is the occupation number for the two hybrid orbitals which point along each Br-A1 bond. Then we can express the negative charge on the bridging Br atom pB using B as pB=2B-3
(2)
In most experiments only resonance frequencies are observed. For a nuclear spin Z = 3 / 2 system eZQq/his calculated by
e*Qq/h = 2v/(l
+ v2/3)-'l2
(3)
where 9 is the asymmetry parameter of the electric field gradient, and in the case of the bridging atom we cannot ignore it because of its large value. For the sp3 hybrid orbital model with two lone pair electrons, the 7 value depends upon bridging bond angle 0 as follows: 7 =
-3
COS
0, 0
< 109.5'
(4)
The e2Qq/hand 7 , which we could not determine only from the N Q R frequency, are eliminated from eq 1-4, and the relation (!7) Lucken, E. A. C. "Nuclear Quadrupole Coupling Constants"; Academic Press: New York, 1969.
J . Phys. Chem. 1985,89, 4272-4271
4212
TABLE I11 Net Charges on the Bridging Br, Terminal Br, and Central AI Atoms MALBr,
PR
PT
PM
M = NH4
-0.380
M = (CH3)dN M = (C2HS)4N
-0.424 -0.45 1
-0.68 1 -0.677 -0.676
1.733 1.743 1.754
between v and pB is drawn in Figure 3. It is very convenient that the N Q R frequency for the bridging atom depends mostly upon the negative charge on it and only slightly upon its bond angle in the range 100' < 0 < 110'. Then the decrease of NQR frequency from the K or N H 4 salt to the (C2H5),N salt corresponds to an increase of negative charge of about 0.07, which is about 15% of ps(NH4). According to the N Q R study for charge-transfer complexes, C13Ga-donor, 69Ga N Q R frequencies decrease drastically with increasing donor strength but those of 35Cldecrease slightly.18 If
we consider the bridging Br atom as a donor, the relationship between terminal Br and A1 NQR parameters which is shown in Figure 2 could be similarly understandable. Table 111 shows the net charges on each position by using NQR parameters of Tables I and I1 on the basis of the Townes-Dailey model applied for the Al,Br,These parameters show that the increased negative charges for each terminal Br atom, ApT = pT((C*H5)4N) - pT(NH4),are quite small, but as a whole six terminal atoms affect appreciably the charges on the central A1 and bridging Br sites. Our NQR data presented here suggest that the polarization and the bond character of the large anion are affected slightly by the cation polarizing power. Registry No. NH4A1,Br7, 56959-50-3; (CH3),NAI2Br7, 64164-18-7; (C2H5),NA12Br,, 79738-85-5; *'Br, 14380-59-7; AI, 7429-90-5.
(18) Tong, D. A. Chem. Commun. 1969, 790.
Optical Properties of Pyrromethene Derivatives. Possible Excited-State Deactivation through Proton Tunneling J. A. Pardoen, J. Lugtenburg, and G. W. Canters* Gorlaeus Laboratories, State University, 2300 R A Leiden, The Netherlands (Received: January 22, 1985;
In Final Form: April 4 , 1985)
The optical spectra observed at 1.2 and 4.2 K of dilute solid solutions of tetramethylpyrromethene (TMPM), its HBr and HPOzC12salts, and its BF2complex are reported. The spectra of TMPM-BF2 show a well-resolved vibronic structure. Neither TMPM nor its deuterated analogue exhibits fluorescence at 1.2 K. Deactivation of the SI state through proton or deuterium tunneling is not incompatible with available theoretical evidence, from which tunnel rates of the order of 10s-lO'o s-l are estimated.
Introduction The interest in the spectroscopy of pyrromethenes (PM's) ( 1 , see Scheme I) and pyrromethenones derives from their Occurrence in nature as building blocks of linear tetrapyrrole pigments like biliverdin (BV) and bilirubin (BR). Both compounds consist of two PM moieties linked together by a methine (BV) or a methylene (BR) bridge, respectively. The growing interest in the structure and mode of action of the plant growth regulating phytochromes and light-harvesting plant proteins like phycocyanins and the phycoerythrins, all of which carry BV- or BR-like chromophores, has focused attention to the spectroscopic properties of the biliverdins and bilirubins.'" Their conformational mobility has been studied in detail as well as their ability to form interand intramolecular hydrogen bonds in connection with the various possible pathways of radiationless deactivation of electronically excited states. A study of the optical characteristics of the simpler PM's seems warranted, therefore. The gross optical features of the PM's have been described in the literat~re.'*~.~ The parent PM (2) exhibits a strong absorption (1) For a recent review of the literature see: Braslavsky, S.E.;Holzwarth, A. R.;Schaffner, K. Angew. Chem., Int. Ed. Engl. 1983, 22, 656-67'4. "Chemistry and Spectroscopy of Bile Figments" Isr. J . Chem. 1983, 23. (2) Fasternak, R.;Wagnite, G. J . Am. Chem. Soc. 1979, 101, 1662. (3) Friedrich, J.; Scheer, H.; Zickendraht-Wendelstadt, B.; Haarer, D. J. Am. Chem. Soc. 1981, 103, 1030. (4) Scheer, H.; Formanek, H.; Schneider, S. Photochem. Photobiol. 1982, 36, 259. (5) Cha, T.-A.; Maki, A. H.; Lagarias, J. C. Biochemistry 1983.2.7.2846. (6) Friedrich, J.; Haarer, D. Angew. Chem., Int. Ed. Engl. 1984, 23, 113. (7) Falk, H.; Hofer, 0. Monatsh. Chem. 1975, 106, 91.
0022-3654/85/2089-4272$01.50/0
SCHEME I R
R
R
M3
CH3
F\"B\
CH3
CH3
( L )
in the visible region but only a very faint fluorescence, if any at all, a t 77 K.7*8 The suggestion frequently encountered in the literature to explain this lack of emission is that quenching of the SI state is promoted by the motion of the imino proton between the two pyrrole rings of the PM. In the 2 syn34structure this motion takes place inside the double well potential of the intramolecular hydrogen bond between the pyrrole nitrogen atoms. The (8) Falk, H.; Grubmayr, K.; Neufingerl, F. Monatsh. Chem. 1977, 108, 1185.
0 1985 American Chemical Society
