Single site spectra of Zn porphin in triphenylene

65,90101

JOURNALOPMOLECULARSPECTROSCOPY

Single

Site Spectra

(1977)

of Zn Porphin

in Triphenylenel

BORIS F. KIM AND JOSEPH BOHANDY Tlae Johns

Hopkins

University Applied

Physics

Laboratory,

Laurel, Maryland

20810

Single site spectra of Zn porphin in triphenylene have been recorded at low temperature. There were three principal site species, each of which had slightly different fluorescence and absorption spectra. Because the spectra of each site were separately recorded, the confusion which resulted from the spectra of multiple site species in previous studies was eliminated. Vibrational assignments in the ground and excited electronic states were made, and relative intensities of the spectral lines recorded. The general pattern of intensity of the spectra was in qualitative agreement with the theory of vibronic borrowing in the cyclic polyene model.

INTRODUCTION

Optical spectroscopic studies of a number of metalloporphins have been made in this laboratory using triphenylene as a host for the guest porphin molecules (1-3). The use of this host material results in sharp line optical spectra when observed at low temperature. In single crystal specimens, the guest molecules are spatially oriented, allowing polarization of the spectra to be observed. The potential advantages of these specimens were not fully exploited in previous studies, however, because of limitations imposed by the occurrence of multiple inequivalent metalloporphin site species in the host matrix. Since the spectra from all the inequivalent site species were recorded simultaneously, the spectra were exceedingly complex and, in some cases, resulted in overlapping lines from different site species. Hence, definitive spectral assignments were difficult or impossible in many cases. Similar problems also occur in the use of Shpol’skii matrices to study spectra. In this study, narrow line selective excitation of fluorescence was used as a technique to obtain single site absorption and fluorescence spectra of Zn porphin in triphenylene. This allowed the spectra intrinsic to each site species to be recorded separately, thereby eliminating interference from spectra of different sites. The spectra observed with this technique were generally in agreement with previous work in which the spectra of multiple site species were recorded. The simplification of the spectra in the present study, however, provided an improved basis for spectral assignments and interpretation of the spectra of Zn porphin in triphenylene. EXPERIMENTAL

DETAILS

A scanning dye laser with approximately 1 A bandwidth was used as a light source for selective excitation of fluorescence. A pulsed nitrogen laser with 10 nsec pulse width * Work supported Sciences.

by Public

Health

Services

Grant

GM21897,

National

Institute

of General

Medical

90 Copyright 0 1977 by Academic Press. Inc. All rights of reproduction in any form reserved.

ISSN 0022-2852

Zn PORPHIN

SPECTR.\

01

and 100 k1V peak power was used to pump the dye laser at a rate of 50 pps. Single site fluorescence spectra were recorded bp exciting a single absorption line, while the corresponding absorption spectra were obtained by recording the excitation spectra of ;I single fluorescence line. The excitation spectra in the region 5100~5730 A were observecl in three separate recordings because three different dye solutions were required in the dye laser to cover this region. All spectral observations were made with the sample at -1.2 I\;. Detection of fluorescence was made with an ERII W&J photomultiplier. .4 1 m spectrograph with linear dispersion of 5 &mm was used in the fluorescence observations. A boxcar integrator (PAR Model 162) with 1 nsec aperture was used to acquire the average peak intensity of the pulsed fluorescence signal. A measure of the intensit! of the dye laser was obtained using a silicon photodiode detector and boxcar integrator (10 nsec aperture) to measure the average peak intensity. .4n internal option on the dual channel boxcar integrator allowed direct recording of the ratio of fluorescence signal to laser source signal. Time dependences of the 0-O transitions were obtained 1)) the time scanning feature of the boxcar integrator. The samples used in this study have been described in a previous report (I). In order to obtain relative intensities of the spectral lines, single crystals were crushed into a powder to eliminate polarization effects of both the dye laser and spectrograph in the excitation and fluorescence spectral observations. RESI’LTS

The excitation and fluorescence spectra observed with narrow line excitation are consistent with the hypothesis that there are three principal site species of Zn porphin in these triphenylene host crystals. The three strong lines associated with the 0-H) transitions (5678, 5687, and 5701 A) were each separately recorded in both excitation and fluorescence spectral observations, using the methods described in the previous section. The fluorescence and excitation spectra which were recorded with each of the OLO lines were distinct from one another except for several lines from different sites which coincided. The three site species will hereafter be designated by the wavelengths of the lowest O-O transitions, i.e., 5678, 5687, and 5701. I:igure 1 shows typical recordings of fluorescence spectra corresponding to the three site species. These particular recordings were made with wavelength excitation in the 0-O transitions at 3678, 5687, and 5701 8. Similar spectra were observed with excitation of other wavelengths (5463, 5598, and 5610 A) but with somewhat reduced signal-tonoise ratio. The O-O transitions were observed in the Huorescence spectra with excitation at these lower wavelengths. The OH) transitions were characterized by a strong, sharp line (zero phonon line) which is associated with the pure electronic transitions of the molecule, and a phonon wing which is associated with vibronic transitions involving vibrations of the near neighbor environment of the guest molecule. In the excitation (absorption) spectra, the phonon wing occurs on the high-energy side of the zero phonon line, and in fluorescence, on the low-energy side. The width and strength of the phonon wing are rather sensitive to temperature. These line shape characteristics which are associated with the spectra of species in host materials have been observed and discussed by other workers. .4 recent review which includes this subject has been written by Rebane (4).

92

KIM AND BOHANDY

FIG. 1. Fluorescence spectra at 4.2 K of Zn porphin in triphenylene for site species (top to bottom) 5701, 5687, and 5678.

Table I lists the stronger fluorescence spectral lines and their relative intensities, and the corresponding ground-state vibrational energy levels for the three sites. The ability to observe the line shapes in the present study has resulted in some wavelength measurements which differ slightly from those in our previous study. Since fluorescence is from the lowest excited singlet electronic state, the fluorescence spectra yield directly groundstate vibrational energy levels which have allowed transitions to the excited state. The linewidths of these spectral lines were of the order of 4 cm+. These spectra appear to

5678 5738 57425 5746 5801 5 6806 5913 59235 5925 5948 5 60205 6022 60235 6028 5 60405 6044 5 60655 6094 614, 61615 6198 6204 62385 62565 62865 6316

0

186 199 210 375 396 701 131 736 802 1003 1008 ,012 1026 1058 ,069 ,181 1204 1329 ,384 ,480 1495 ,584 1630 1706 1781

5657 57435 5746 5809 56205 5821 5 5932 5934 5 5957 5 6021 60295 6049 5 6095 61505 6161 6214 6222 6247 6264 6293 6321 6338 ,1329+315 ,I584 + 199

100 2 16 14 2 :, z 4 12 8 10 4, 4 11 4 35 30 : 2

0 173 187 369 403 406 726 133 798 992 999 1054 ,177 ,325 1353 ,491 1512 1576 ,620 1693 1164 ,806

1,325 f 187, 11324 t 3691 1,576 + 184, ,lFPcl+ 187,

5701 5758 5762 5 51645 58235 59465 5949 5 5973 5980 60425 60455 60625 6067 60995 61105 6166 6176 61855 6187 62225 6231 62385 6263 62805 6310 6337 5

L

0 114 167 193 369 724 733 799 818 991 1000 1046 1058 1146 1176 1323 1349 ,374 1378 1470 1492 1611 1574 ,618 1693 1762

113231187, ,1324+c%91 11574* 1871

Zn POKPHIS SI’ECTk2 be generally

consistent

with recent work reported

03

on Shpol’skii spectra

of %n porphin

(5). The lifetimes of the O-O transitions were less than 10 nsec for each of the site species. The ground-state vibrational energy level structure of the 5701 site species was characterized by three energy levels with very strong transitions at 1323, 1574, and 1018 cm-‘. Other strong but somewhat less intense transitions were observed at 187, 360, 724, 733, 1000, 1058, 1176, and 1492 cm-‘. Some of the vibrational levels with weaker vibrational transitions were classified as due to vibrations in combination. These were located at 1511 (1323 + 187), 1693 (1323 + 369), and 1769 cm-’ (1575 + 187). The ground vibrational structure of the 5687 site species was very similar to that of the 5701 site species. The strong lines for both sites were in agreement within 2 cm-l, with the exception of one case where the difference was 4 cm -I. Lines attributed to combinations of vibrations were observed at 1764 (1576 + 187), 1512 (1325 + 187), 1806 (1620 + 187), and 1693 cm-’ (1325 + 369). The structure of the ground electronic state vibrational manifold for the 5678 site species exhibited somewhat greater differences in detail from that of the other two sites. The three strongest lines occurred with energy levels which differed on the order of 10 cm--l from the corresponding lines of the other two site species. The energies of the other strong lines were similarly shifted. The structure in the region of 200 cm-l was noticeably different, the dominant line occurring at 199 cm-‘, although the 186 cm-’ line was still present. Vibrational combination lines were observed at li00 (1329 + 375) and 1781 cm-’ (1584 + 199). Figures 224 show typical recordings of excitation spectra for the three sites in the wavelength region 5580-5730 A. These spectra were observed with wavelength detection at 6141, 6247, and 6280 A. The stronger spectral lines of the excitation spectra are >

FIG. 2. Excitation The arrow indicates

spectra (SW--573Ob) at 4.2 h of Zn porphin the Qz (0, 0) phonon wing.

in triphenylcne

for 5701 site specks.

94

KIhl

FIG. 3. Excitation The arrow

indicates

spectra

(558~5730

the Qz (0,O) phonon

AND

BOHANDY

A) at 4.2 K of Zn porphin

in triphenylene

for 5687 site species.

wing.

listed in Table II. These lines represent transitions from the ground state to the lowest excited singlet vibronic energy levels. The symmetry species, where indicated, were based upon polarization of the line and selection rules. The lines at 5610 (5701 site), 5600 (5687 site), and 5642 A (5678 site) were classified as O-O transitions to the Q1, electronic state. Qz and QV are degenerate in a molecule with Dqhsymmetry, which is the nominal symmetry for metalloporphins, but these levels have usually been observed to be split in experimental studies where these levels could be resolved (2, 6). The QZ-Q2/ splitting for the 5678, 5687, and 5701 site species were 111, 273, and 283 cm-‘, respectively. The corresponding splitting of these levels for Zn porphin in n-octane was reported to be 109 cm+ (6). The removal of the QI, Q, degeneracy is attributed to a lowering of the symmetry (probably to Dzh)of the molecule by crystal field interactions (6). Assignment of the Qy level in each case was made by noting that this line should not exhibit mirror symmetry in the absorption and fluorescence spectra with respect to Qz as would be the case for the Q5 vibronic lines. The previously observed polarization characteristics of the absorption spectra were also helpful in confirming the assignments for the 5687 and 5701 site species. The two components of the lowest excited singlet electronic states have orthogonal transition moments in the plane of the porphin molecule. In the cases of the 5687 and 5701 site species, the QZ(O-O) transition was strongly polarized (u) with respect to the crystal c axis. One might then expect the

0.5

ZII PORPI-IIN SPEC’IXZ

(J1(O-O) transition to be polarized (rj with respect to the crystal c axis. This was in fact observed for the assigned Q1,levels. The orientations of the guest porphin molecules in the host lattice are not completely known, although the orientations of the normals to the porphin planes for some site species have been obtained from ESR data (7). Therefore, quantitative measurements of the polarization intensities would not provide the orientations of the transition moments with respect to the porphin molecules. The qualitative observations, however, are consistent with our assignments. The Q1, transition was broader than the corresponding Qz transition and had lower peak intensity. The lower peak intensity of the QUlevel in each case aided the assignment of vibronic levels since the intensities of the Q, vibronic levels would have correspond ingly lower peak intensity. Because of the lower peak intensities of the Qy vibronic levels, most of the vibronic lines observed belonged to the ()= electronic state. Librational assignments were made by comparing the Chronic energies and intensities of the two Q states with the energies and intensities of the ground-state vibrational levels obtained in the fluorescence spectra. Generally there was fairly good agreement between the ground state and excited sttte vibrational spectra at energies up to the region of 1000 cm-‘. This is shown in Figs. 5-7 for each of the three site species. The departure from agreement above 1000 cm+ was most striking since the three strongest transitions Q&O1

I:rc. 4. Excitation The arrow

indicates

spectra

(55804730

the QJ (0, 0) phonon

i)

at 4.2 K of Zn porphin

wing.

in triphenylene

for 5678 site species.

KIM AND BOHANDY

96

ne 7.” P

II 5678 5642 5 5637 5 5623.5 5622 56155 5613.5 5662 5560.5 5658.5 5561.5 55266 :z5 5453 5436.5 6425 5403.5 5376.5 5376 53705 53665 6361.5 5359 5346.5 5340 %5 5329 5323.5 5307.5 5294 5281.6 6242

-7 -

:8”

6 6 9

:: 6b 16 9 ii, 9 1,' 34 :: 20 1, 6 : 23 1,bb 7 10 1; 6th ,,bb lcbb 15bb 6th

hl”

tr

567,

rpecler

kbr,>,,,

--

T 1 I’“” ,C”i ’ /

0

727’

17, 175 196 203

;LY’ 1

I.

0 92 169 164 ,273, 279 368 398

0

1921 194

373 379 402

463 372

662 71, 72, 762 18221 I8951 980 990 1008 1021 1039 1046 1,066, 11123, ,151 1154 ,114 1230 1::; 1464

III 722 783 819 967 984

711 784

,034 1120 1165 I*,, 974 1003 1012

!

1326 1343 ,422 ,499

I’OX ,

cm

, 5701 5670 5 56465 56415 5610.5 56635 5553 5552 55275 5496 546,5 5476 5 54745 54565 544, 5403 5396 539, 5 5363 5369 5345 5332 5299 52945 52715 62605 5171 5162

100 7

:9 28

0

24

6” 6b :: 7 :: :4 15 6 2, 14 22b 24b 9 &b IObbk 6bbb Sbbb

165 469 683 712 726 765 816 966 965 1036 ,119 1166 1215 133, 134, 1429 1505 ,796 1669

266 365

125

1792

-

-

in the ground-state vibrational spectra were not easily identified in the excited-state vibrational spectra. There also appeared to be some significant differences in the region from &400 cm+ for the 5678 and 5701 site species. Two of the strong features in the fluorescence spectra (187 and 369 cm-l) in the 5687 and 5701 site species appeared to be harmonically related. This observation was confirmed in the excitation spectra but with additional components at 92 cm-l (95 cm+) 279, and 463 cm-l (471 cm-‘) in the 5687 (5701) site species. These results suggest that these spectral components may be a vibrational sequence with fundamental at 92 cm+ (95 cm-l). The “odd harmonics” were observed with weak intensity in the absorption spectra (with the exception of 279 cm-’ in the 5687 site species) but were not observed in the fluorescence spectra. The 279 cm-’ line in the 5687 site species was nearly degenerate with the Qy line and may have interacted significantly with this level and thus increased in intensity. The even harmonics, 184 and 369 cm+ were polarized in the same sense as QZ with respect to the crystal c axis, and therefore were probably x polarized. The fundamental vibronic line at 92 cm-l (95 cm-l) appeared to be unpolarized (from photographic recordings) indicating that it was probably not x or y polarized. Spectral components which could be identified with a 95 cm-l vibrational series with respect to the QU levels were also observed. The relative intensities of the odd harmonic components were not weak relative to the even harmonic components in this case, however, possibly because of mixing of the QU levels with vibronic components of the QZ levels. DISCUSSION

The data in this study provide fairly detailed information on the ground-state vibrational energy levels and the lowest excited singlet vibronic levels with observable

Zn PORPHI?;

SPECTR.1

oi

transitions between these electronic states. The fact that there exist three inequivalent site species in the samples studied here may possibly be regarded as an advantage with single site recording techniques since it allows, in a qualitative manner, some measure of the effect of the host lattice on the energy structure of the free porphin molecule. Two of the site species studied, 5687 and 5701, appear to be very similar to one another in the structure of the excited and ground states. A measure of this is shown in the zero field splittings of the excited Q states which, in these two cases, differ by 4%. In contrast, the corresponding splitting for the 56i8 site species differs from the other two site species by approximately 600/,. These observations are consistent with the previously observed polarization properties of the Qr(O 0) transitions in the absorption spectra. The 5687 and 5701 A lines were observed to be strongly polarized with respect to the crystal c axis while, in contrast, the 5678 A line was not polarized. (The polarization was not strong enough to be noticeable in the photographic recordings of the spectra.) From this, one would conclude that the 5687 and 5701 sites are oriented similarly in the host lattice while the 5678 site species is oriented differently. Recently, jansen and Noort (5) reported results of work on %n porphin in a single crvstal of jl-octane which produced evidence that inequivalent site species in that host

Frc. 5. Stick diagram showing vibrational energy levels in the ground and excited species: (A) Q, vibronic energy levels with respect to Qz enerm Icvel, E - /:‘(Q=) (Q, (B) Qz vibrational energy levels, fi - I?(Q=); (C) ground-state vibrational enerm The lower case letters indicate corresponding vibronic lines in the ground state and stales.

states for 5701 site level at 283 cm-‘) ; levels, I:‘(Qz) - I<. QJ and Q, excited

98

KIM

I

”

FIG. 6. Stick diagram

showing

BOHANDY

c

J 200

AND

400

600

vibrational

800 ,000 Energ" /ctn '1

energy

1200

-

levels in the ground

400

and excited

states

for 5687 site

species : (A) QY vibronic energy levels with respect to Qz energy level, E - E(Q,) (Q., level at 273 cm-i) ; (B) Qz vibrational energy levels, E - E(Q,) ; (C) ground-state vibrational energy levels, E(QJ - E. The lower case letters indicate corresponding vibronic lines in the ground state and Qz and QU excited states.

were the result of solvent ligands in the host lattice. They also suggested that such solvent effects in previous work on Zn porphin in triphenylene reported by the present authors may have been responsible for inequivalent sites in these samples. Although we do not as yet have evidence which addresses this question, their suggestion does not appear to be unreasonable, particularly with regard to the 5687 and 5701 sites. The rather small difference in the energy structures of these two site species and the indication that they are probably oriented the same in the host lattice could be explained by a solvent ligand present on one of the two site species. An axial ligand would be expected to have the least effect on the zero field splitting of the Q levels and on the in-plane transition moments of the electronic excited states. There was fairly good agreement in the general structures of the ground-state and excited-state energy structures. This agreement gives rise to the phenomenon referred to as “mirror symmetry” between emission and absorption spectra when the 0-O transition occurs in both emission and absorption. Figures 5-7 show this for the three site species. In all cases where a direct comparison between excited-state and groundstate vibrations was possible, the excited-state vibrations were shifted somewhat to lower energies. The intensities of the stronger vibrational lines (with the exception of

Zn PORPHIN

SPECTR.I

09

the three strongest ground-state vibrational lines) were, on the average, two to three times more intense in the excited vibronic spectra than in the emission spectra. Shifts in intensity and energy such as observed here have been predicted for vibronic transitions which are allowed by vibronic interaction between the Q and S states (8). The S states are responsible for the strong Soret band in absorption spectra. The general pattern of the intensity of the spectra can perhaps be understood within the vibronic borrowing model of Perrin, Gouterman, and Perrin (8). In this model, metalloporphin is represented as a 16 member cyclic polyene with 18 a electrons. l’he intensities of transitions between the ground vibronic states and the vibronic states in the Q band are enhanced in this treatment by vibronic interaction of the Q band with the S band. According to Perrin et al., there are three normal coordinates corresponding to normal modes of species es0 a nd b?, (Z&h symmetry) which can couple the excited (1 and S states. In DL’hsymmetry, the es0 becomes (I, and blQ, while the b,, species becomes h,,,. The vibronic states which are singly excited in these modes have allowed transitions between the ground states and the excited Q states as a result of this interaction. This can be seen from the form of the v:ibronic wavefunctions which were obtained by I’errin

_.

1

1 1400

h$O”

FIG. 7. Stick diagram showing vibrational energy levels in the ground and excited states for 5678 site species : (A) QY vibronic energy levels with respect to Qz energy level, E - E(Q2) (Qy level at 111 cm-* 1; (D) Qz vibrational energy levels, E - E(QZ) ; (C) gr ound-state vibrational energy levels, E(Q,) - I(. The lower case letters indicate corresponding vibronic lines in the ground state and Qz and Qy excited states.

KIM

100

AND BOHANDY

et al. (8) for the cyclic polyene model (D& and are shown below recast in a form to reflect the perturbation to lower symmetry (&J appropriate to Zn porphin : Iti ground ; r> = #0L4 &O(Y)

;

IQ=;0)

= +(Qz)+ wWM4

IQ=;4

= J/(Q&b,>+ PdGz) + ...;

IQz:h,)

= ~(QzMh,) +PdGJ

IQz;ho')

= $(Q&h')

+P3vGJ

+ wi4%h%J

+

~3W&@1~‘);

+ a..; +

..-a

In these expressions, # is the electronic wavefunction, and 4(-r) the vibrational wavefunction where y represents the vibrational symmetry species. A subscript 0 indicates a ground-state wavefunction. Similar expressions can be written for the Qz, vibronic wavefunctions. In the singly excited Qz vibronic wavefunctions, the terms which are not shown do not contribute to the transition moments. Hence, within the approximations of this model, there are nonzero fluorescence transition moments which have the form (9)

and nonzero matrix elements

for the absorption

spectral

transitions

The three strong fluorescence lines which correspond to ground-state frequencies, 1323, 1574, and 1618 cm-‘, would be associated with the vibrational species bl,‘, bl,, and a,, discussed above which couple the Q and S states in the cyclic polyene model. Following a suggestion of Perrin et al., on the probable assignments of similar frequencies in the spectra of free base porphin (Shpol’skii spectra) (IO), we would assign bl,’ to 1329 cm+, and a, and bl, to the 1574 and 1618 cm-’ lines (not necessarily in that order, for the latter two vibrations). The closeness of the latter two lines suggests their common origin es8 in the cyclic polyene model. The cyclic polyene model, which is a crude approximation to the structure of metalloporphin, thus appears to yield a satisfactory rationale for the occurrence of three predominant lines in the fluorescence spectra of Zn porphin. Perturbations on this model which would be required to describe the metalloporphin structure with D4h or D2h symmetry would presumably yield additional vibrational coordinates which would interact between the Q and S states. This could conceivably account for the remainder of the strong lines in the fluorescence spectra. The absorption spectra should have six predominant transitions corresponding to the three singly excited vibrations bIgI, a,, and bl, with Q5 and Qy. The fact that these transitions were not easily identified in the excitation spectra in this study is attributed to the broad, diffuse character of the Soret band. The singly excited vibronic Q states, which are more strongly coupled to the S states than are the unexcited Q states, would be expected to be broadened by these interactions. We then attribute the broad features above 1000 cm-’ to these levels, although detailed assignments were not made. The interpretation of the excita-

Zn PORPHIN

SPE(‘TR.1

101

tion spectra in terms of the cyclic polyene model could be made in better detail with a metalloporphin in which the Soret band is not broad.

Site selection techniques have been used to obtain the spectra of %n porphin in triphenylene in greater detail than has been possible with conventional techniques. Difficulties associated with multiple site spectra, such as overlapping lines from different sites, etc.? have been largely eliminated and this has allowed more definitive assignments and interpretation of the spectra. In this regard, it appears that some aspects of the intensity structure of the spectra can be understood in terms of the vibronic borrowing model of Perrin, Gouterman, and Perrin. ‘The site selective techniques used in this study are currently being used to study free base and other metalloporphins. KECEI~I’I~: October

12, 1976 REI:ERENCES

13. V. KIM, J. UUH.NDY, AND C. K. JEN, J. C/rem. Phys. 59, 213 (1973). J. Bomnuu, B. F. KIM, AND C. IL JEN, J. Mol. Spedrosc.49, 365 (1974). J. BOHMDY AND B. 1'.KIM, .Spectroc/zit~z. A& A 32, 1083 (1976). K. IL RIGMNE, “Optical Properties of Ions in Solids” (B. IX Bartolo and I). Pachero, Eds.). I’lcnu~n, New ‘k’orli, 1975. 5. G. J.ANSES AND M. NOORT, S’pectrochirrc. .lcltt .i 32, 747 (lY76). 6. G. W. CZSTERS, J. V.ANF,CKWD, T. J. SCHAI~FSMA,ANIJ J. H. VAN VBR W.\ar.s, .Ifol. P//ys.24, 1203 (1972). 7. J. BOHAND~, B. I;. %3r1,.4h?) C. 6. JEN, .I. Jiqp. Reson 1.5, 420 (1974). A’. M. H. PISRRIN,M. GOUTERMAN, AND C. L. PERKIN,J. Chem. Phys. 50, 4137 (lY69). Y. G. HEXZIXRC, “Molecular Vol. III, i-an Nostrand, Princeton, Spectra and Molecular Structure,” N. J.. 1066. /o. .I. N. SEW11Wi0, E;. N. .%LOV'W, s. F. SflKIRM.\N, .\SV ?rl.u. %VZHI~:\.S1;.\yA, P,'Oc. .I<&. .)'ci. C’SSR, Sect. Phys. Chew. 153, 1151 (1963). 1. ,7. .?. -I.