1',3',3'-Trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-indoline]: Its thermal enantiomerization and the equilibration with its merocyanine

Article abstract of DOI:10.1016/S0957-4166(99)00538-8

The enantiomers of the title compound, the important photochromic material (RS)-1b, have been enriched semipreparatively by liquid chromatography. As a consequence, we were able to determine the barrier of the thermal interconversion (R)-1b?(S)-1b via time-dependent polarimetry, amounting to ΔG(≠)=85.9 kJ/mol at 22.0°C in d⁶-DMSO (Table 2). The thermal equilibration of the corresponding merocyanine 2b was monitored in d⁶-DMSO by time-dependent ¹H NMR, resulting in ΔG₁(≠)=102.8 and ΔG₂(≠)=92.0 kJ/mol at 22°C (Table 1). This means that, starting from (RS)-1b, the opened isomer 2b is attained by a slow reaction (ΔG₁(≠)=102.8 kJ/mol, Fig. 4). Therefore, the merocyanine 2b cannot be identified with the intermediate required for the fast process of C(sp³)-O bond cleavage (ΔG(≠)=85.9 kJ/mol) upon the above enantiomerization of (RS)-1b. Apparently, these two thermal isomerizations (Fig. 4) are independent of each other. The structure of the unknown intermediate of the interconversion (R)- 1b?(S)-1b must therefore differ from the known one of merocyanine 2b. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(99)00538-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 10 (1999) 4677–4687
10,30,30-Trimethyl-6-nitrospiro[2H-1-benzopyran-2,20-indoline]:
its thermal enantiomerization and the equilibration with its
merocyanine†
Roland Kießwetter, Nikola Pustet, Friedrich Brandl and Albrecht Mannschreck ∗
Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, D-93040 Regensburg, Germany
Received 6 October 1999; accepted 25 November 1999
Abstract
The enantiomers of the title compound, the important photochromic material (RS)-1b, have been enriched
semipreparatively by liquid chromatography. As a consequence, we were able to determine the barrier of the
thermal interconversion (R)-1bz(S)-1b via time-dependent polarimetry, amounting to ∆G==85.9 kJ/mol at 22.0°C
in d6-DMSO (Table 2). The thermal equilibration of the corresponding merocyanine 2b was monitored in d6-
1
DMSO by time-dependent H NMR, resulting in ∆G1==102.8 and ∆G2==92.0 kJ/mol at 22°C (Table 1). This
means that, starting from (RS)-1b, the opened isomer 2b is attained by a slow reaction (∆G1==102.8 kJ/mol,
Fig. 4). Therefore, the merocyanine 2b cannot be identified with the intermediate required for the fast process of
C(sp3)–O bond cleavage (∆G==85.9 kJ/mol) upon the above enantiomerization of (RS)-1b. Apparently, these two
thermal isomerizations (Fig. 4) are independent of each other. The structure of the unknown intermediate of the
interconversion (R)-1bz(S)-1b must therefore differ from the known one of merocyanine 2b. © 2000 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Many derivatives of 10,30,30-trimethylspiro[2H-1-benzopyran-2,20-indoline] 1a (Scheme 1) are of
interest because of their photochromism.2 The parent molecule 1a can be transformed photochemically
into the merocyanine 2a which isomerizes thermally with a very high rate back to 1a.3 Therefore,
unsubstituted 1a has no practical value with respect to photochromism. This situation changes upon
the introduction of a nitro group into the 6-position: the title compound 1b has probably been cited in the
literature most often among all photochromic materials. The corresponding merocyanine 2b is obtained
by irradiation and reverts to the equilibrium mixture (Scheme 1) consisting predominantly of the spiro
∗
†
Corresponding author. Fax: (+49) (941) 943 4617; e-mail: albrecht.mannschreck@chemie.uni-regensburg.de
Chiral 2H-pyrans, Part 10. Part 9: Ref. 1.
0957-4166/99/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00538-8
tetasy 3173

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compound 1b. The rate of isomerization of 2b is much lower than that of the 2a→1a reversal.3–8 Although
analogs have now been found which are more stable to light than 1b, the latter has been significant for
the development of practical applications of photochromism and continues to be significant for basic
research,2,9,10 e.g. with respect to 1b chemically bonded to another molecule. A further nitro group in
the 8-position again changes the properties: only a very small amount of the spiro compound 1c appears
in the thermal equilibrium11,12 (Scheme 1) in dipolar aprotic solvents, which means that the observed
photochromism is a reversible one with limited applicability.
Scheme 1. Thermal equilibria between spiro compounds and merocyanines. The positional numbers given in the formula of the
merocyanines 2 do not correspond to their systematic names but to the ones of the spiro compounds 1
In addition to the above transformations between constitutional isomers (e.g. Scheme 1), recently
a second thermal process was investigated: the enantiomerization,13 i.e. the interconversion of the
enantiomers (R) and (S) of the spiro compounds 1 with the rate constant ke according to Scheme 2. The
methods used in this case were temperature-dependence measurements of 1H NMR signals13 of suitable
diastereotopic groups or time-dependent polarimetry at constant temperature.13 The latter technique is
much more generally applicable; however, it requires an enrichment of one of the enantiomers which
was accomplished recently by liquid chromatography13–15 on nonracemic sorbents. In the case of (RS)-
1
1c (Scheme 2), H NMR showed the interconversion of the enantiomers to be very fast.11,16,17 No
information about the enantiomerization of (RS)-1a was available; our corresponding results12 will
1
be briefly mentioned below. (RS)-1b was investigated16,18 via the coalescence of H NMR signals at
138°C (80 MHz; d6-DMSO) and at 205°C (no transmitter frequency given; biphenyl). However, our
intention was to compare the rate of the (R)-1bz(S)-1b process (Scheme 2) with that of the 2b→(RS)-1b
isomerization (Scheme 1) which cannot be monitored at such high temperatures. Therefore, we needed
to enrich the unknown enantiomers of 1b and to measure their interconversion at lower temperatures, e.g.
by time-dependence polarimetry.

R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
4679
Scheme 2. Thermal enantiomerizations of spiro compounds
2. Equilibration of the merocyanine 2b with the spiro compound (RS)-1b
The preparation19–23 of the merocyanine results in a mixture of 2b and 1b, in our case in a ratio of
75:25, estimated from 1H NMR in CD2Cl2 at −90°C. Apparently, the pure solid 2b is not known. The
stereoisomer depicted is assumed17,24–26 to be the preferred one in solution and is consistent with our 1H
NMR results. Formula 2b is one of several resonance structures.
Rates of the thermal 2b→1b transformation (Scheme 1) have been measured several times3–8 by
monitoring the UV band around 560 nm after irradiation of the spiro compound 1b, without isolation
of 2b. In order to have conditions comparable with the ones of our measurement of enantiomerization of
(R)- and (S)-1b (see Section 4 below), we had to monitor the 2b→1b isomerization in the same solvent at
the same temperature. For this purpose, we chose 1H NMR with its narrow linewidths, a method which
can compare the decrease of signals of 2b quantitatively with the increase of signals of 1b (Fig. 1).
Isolation and characterization of enriched merocyanine 2b (Fig. 1; 0 min) offers a more reliable basis
for kinetics than use of a less characterized solution. The presence of a very small amount of 2b in the
equilibrium mixtures has been stated.3,5,26 No signals of the aromatic protons of 2b are visible at lower
concentration (e.g. Fig. 1; ∼24 h). The solution of the spiro compound 1b in d6-DMSO is slightly yellow
for a very short time, whereas the equilibrated one looks violet. At a concentration of 0.09 mol/l, an
equilibrated solution of 1b in d6-DMSO showed signals of 1.2% of 2b (cf. Experimental), in addition to
the absorptions of 1b. The CMe2 peaks at δ=1.10 and 1.20 for 1b and the CMe2 signal at δ=1.73 for 2b
were chosen to determine the equilibrium constant K=[2b]:[1b]=0.012 and the free enthalpy difference
∆G=+10.8 kJ/mol at 22.1 °C (Table 1). The equilibrium was attained from both sides. The experimental
data (cf. Fig. 1) were treated as a reversible first-order reaction40 with no intermediates, resulting in
∆G1==102.8 and ∆G2==92.0 kJ/mol. Our data are in approximate agreement with the ones calculated
from literature results3–8 which were obtained by UV photometry in solvents other than d6-DMSO.
3. Preparative separation and characterization of the enantiomers of the spiro compound (RS)-1b
The enantiomers were enriched semipreparatively by liquid chromatography (LC) in heptane:2-
propanol, 9:1, on tris(3,5-dimethylphenylcarbamoyl)cellulose/SiO2 at 3 bar. The samples obtained were
characterized by analytical LC (Fig. 2) under conditions similar to the ones given above. A novel, highly
sensitive UV–circular dichroism (CD) detector27–30 was used for LC measurements. The enantiomeric
purity (P) obtained28,31–33 served to correct the experimental CD spectra for P=100% (Fig. 3). In

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Fig. 1. 1H NMR of protons H-8 of 2b and H-3 of (RS)-1b in d6-DMSO at 400 MHz. Equilibration of the merocyanine 2b with
the spiro compound (RS)-1b at 22.1°C in the absence of light. Times of measurement of enriched 2b are given. See text for the
presence of 2b after the end of the reaction
agreement with the CD spectra of other 6-substituted [2H-1-benzopyran-2,20-indolines]15,34 and with
the 6-unsubstituted 1a,12 the enantiomers of 1b show an intense band around 250 nm and a weaker one
around 290 nm with unequal signs.
4. Enantiomerization of the spiro compounds (R)- and (S)-1b
At 22.0°C, (+)436- and (−)436-1b racemize in d6-DMSO. The plot of ln α versus t (α being the
polarimetric angle of rotation at 436 nm) is linear during four half-life periods. We treated our kinetic
data as a reversible first-order reaction.35 For the enantiomerization via an unknown intermediate (or

R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
4681
Table 1
Equilibration between spiro compounds (RS)-1 and merocyanines 2 at 22°C, measured by time-
1
dependent UV absorptions3 for (RS)-1a
2a and by time-dependent H NMR intensities for
the other compounds
unknown intermediates), a statistical factor13 of 2 was applied, i.e. k=2ke. We obtained t0.5=2.9 min,
k=3.94·10−3 s−1 and the ∆G= value in Table 2 for the enantiomerization‡ of the widely used (RS)-1b.
The polarimeter cell was protected from external light. Under our usual experimental conditions, a
significant photochemical contribution of the light of the polarimeter lamp to the decrease of the rotation
angle had been excluded earlier.13 A similar experiment for (−)436-1b in heptane:2-propanol, 9:1, at 436
nm with the usual continuous irradiation resulted in ∆G==97.1±0.3 kJ/mol at 34.1°C. The reading of the
transmission through the cell was strongly reduced by a shutter and the angle was measured periodically
in 20 second intervals. This reduced irradiation resulted in ∆G==97.2±0.3 kJ/mol at 33.8°C. The spiro
compound 1b shows very weak absorption at λ=405 nm but no significant ones at 436 and 546 nm
(cf. Fig. 3). Racemizations at these wavelengths with the usual continuous irradiation did not show any
dependence upon λ which might have occurred by eventual photoreactions. From the above experiments,
we conclude that the decrease of the rotation angle is effected thermally.
‡
In the above section on equilibration of the merocyanine 2b the very small amount of 2b in the equilibrium with 1b has
been mentioned. This means that the spiro compounds (R)- and (S)-1b react mainly by enantiomerization but, in addition, do
so by formation of the merocyanine 2b, with a very small probability. This formation of 2b contributes to the decrease of the
angle of rotation, but this decrease is mainly caused by the interconversion of (R)- and (S)-1b. Our calculations,36 including
equilibrium constants between merocyanines and spiro compounds, showed that, for all molecules in Table 2, the contributions
of the formation of merocyanines to the decrease of the angles of rotation are within the experimental errors. Therefore, the data
given in Table 2 represent the true enantiomerization processes of the spiro compounds via an intermediate.

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R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
Fig. 2. Liquid chromatography of ∼4 µg of enriched (+)436-1b in heptane:2-propanol, 9:1, on tris(3,5-dimethyl-
phenylcarbamoyl)cellulose/SiO2. Absorbance A and differential absorbance ∆A at 250 nm. Top left: experimental chromato-
grams A(V) and ∆A(V). Top right: experimental ∆A(A) diagram, resulting in a slope ∆A/A. Bottom: computer deconvolution28
of the experimental chromatogram ∆A(V) by using the ∆A/A value obtained. Electronic integration results in the retention
factors k0=0.8 and 1.0 as well as an enantiomeric purity of 71±2%, determined in spite of the peak overlap
5. Discussion of the two different isomerizations investigated
The cleavage of the C(sp3)–O bond of the spiro compound is the essential prerequisite of both the
enantiomerization and the formation of the merocyanine(s). We have represented the transition states
of the enantiomerization by the structures13,37 (R)- and (S)-(Z)-TS in Fig. 4, right side, on the basis of
ab-initio calculations38 and several other theoretical approaches cited in a review.13 Very little is known
about the central area of the reaction profile (Fig. 4, right side). At least one intermediate, lower in
energy than (R)- and (S)-(Z)-TS, is postulated for symmetry reasons. With reference to the necessary
calculations concerning this intermediate (or these intermediates), we prefer to postpone the discussion
of detailed structure(s).
The merocyanine 2b cannot be identified with this intermediate (or these intermediates) because 2b
is attained by a slow isomerization of (RS)-1b (∆G1==102.8 kJ/mol, Table 1; Fig. 4, left side) which
apparently is kinetically independent of the fast enantiomerization of (RS)-1b (∆G==85.9 kJ/mol, Table

R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
4683
Fig. 3. Circular dichrograms ∆ε(λ) of (+)436- and (−)436-1b and ultraviolet spectrum ε(λ) of (±)-1b in heptane:2-propanol, 9:1.
∆ε and ε in l mol−1 cm−1, corrected for an enantiomeric purity of 100%. The signs (+)436 and (−)436 refer to polarimetry
2), a fact which we had already mentioned in a review article.13 The unknown intermediate(s) of the latter
process must therefore have a structure different from 2b. The same conclusion can be drawn for the
analogs (RS)-1c/2c, bearing two nitro groups,11,17 as well as for (RS)-1a/2a, bearing no nitro group.3,12
The corresponding ∆G=-values have been included in Tables 1 and 2.
The barriers ∆G1= in Table 1 do not depend significantly upon the substituent pattern. Apparently, the
ground states of the merocyanines 2 are stabilized by nitro groups (∆G-values in Table 1). Therefore,
=
∆G2 increases appreciably in the order 2a→2b→2c reflecting the different thermodynamic stabilities
of the opened isomers 2. It appears that the experimental overall barriers ∆G1=, e.g. 102.8 kJ/mol for
(RS)-1b→2b (Fig. 4, left side), comprise the cleavage process for the C(sp3)–O bond in 1 and, in addition,
subsequent E/Z-isomerizations around the partial double bonds, required for attaining the merocyanine
2.
The barrier between (RS)-1b and 2b has been symbolized by a simple curve in Fig. 4, left side, although
this interconversion should consist of several steps. This assumption has been confirmed by calculations
of Abe, Nakao, Horii, Okada and Irie.24 A corresponding calculation39 for the interconversion of (R)-
and (S)-1b could contribute to answer the question of the structure of the unknown intermediate(s). In
addition, calculations may solve another remaining problem: at which molecular state, e.g. at which
point of the reaction coordinates (horizontal lines) in both parts of Fig. 4 do the two isomerizations, e.g.
(R)-1→2b and (R)-1→(S)-1b, start to use different pathways?
Further work should show whether the different isomerizations described in Fig. 4 are a general phe-
nomenon of 2-donorsubstituted 2H-1-benzopyrans and also of the corresponding benzo-1,4-oxazines.

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R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
Table 2
Enantiomerization of spiro compounds (R)- and (S)-1, measured by time-dependent polarimetric
rotation angles and, for 1c, by temperature-dependent 1H NMR spectra11
6. Experimental
6.1. General methods
Melting points were determined on a Büchi SMP-530 apparatus and are not corrected. 1H NMR spectra
were recorded on a Bruker ARX-400 (400 MHz) spectrometer using tetramethylsilane (δ=0 ppm) as stan-
dard. The temperature of 22°C was determined by a sample of methanol. Compound 2b was equilibrated
in the NMR probe in d6-DMSO in the absence of light (Fig. 1). Our unpublished program DIASTER was
used for evaluation of the data on the basis of a reversible first-order reaction.40 All 1H NMR analyses
included intensities; no decomposition products were detected. IR spectra were measured on a Beckman
Acculab 1, the mass spectra on a Varian MAT 331A (70 eV) spectrometer. A Hitachi U 2000 instrument
was used for UV spectroscopy. CD spectra were recorded on a Jasco J-710 dichrograph. A mixture of
heptane:2-propanol, 9:1, was used as solvent because the barrier to racemization is strongly solvent de-
pendent (d6-DMSO, ∆G==85.9 kJ/mol at 22.0°C, t0.5=3 min; heptane:2-propanol, 9:1, ∆G==97.2±0.3
kJ/mol at 34.1°C, t0.5=74 min). Differential absorption coefficients ∆ε of the maxima were given after
correction for enantiomeric purities (P) of 100%. Our computer program CDUV F. WINDOWS served
for plotting and smoothing the spectra (e.g. Fig. 3). The enrichments and analyses of enantiomers were
performed37,41 by liquid chromatography (LC) on a tris(3,5-dimethylphenylcarbamoyl)cellulose/SiO2
(Chiralcel OD) analytical column, length 25 cm, diameter 4.6 mm. The novel Jasco CD-995 instrument42
was used for combined UV/CD detection of LC and for determination28 of P. Our computer program
SEPP F. WINDOWS served to record the chromatograms and to process the LC data (e.g. Fig. 2).28 For
the thermal racemizations of the enriched enantiomers, the cell of a Perkin–Elmer 241 polarimeter was
thermostated, e.g. to 22.0°C, and the decrease of the angle of rotation, e.g. at 436 nm, was monitored. On
this occasion, (+)436- and (−)436-1b were distinguished, (+) and (−) standing for the signs in polarimetry,
[+] and [−] for the signs in CD. The contents of a fresh phial of d6-DMSO (99.8%; Deutero GmbH,

R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
4685
Fig. 4. Schemes of free enthalpy changes (kJ/mol) in d6-DMSO at 22°C. Left side: equilibration of spiro compound 1b
and merocyanine 2b; a simple overall curve has been drawn which does not show individual reaction steps. Right side:
enantiomerization of spiro compounds (R)- and (S)-1b (see Section 4). (R)- and (S)-(Z)-TS are the transition states for the
cleavage process of the C(sp3)–O bonds. These structures are symbolized by only one of the possible resonance formulae
Kastellaun, Germany) was used as a solvent for polarimetric and 1H NMR kinetics. The racemizations
were measured during two to three half-life periods. The data were treated by first-order kinetics and
corrected for an intermediate, as described in Section 4 on enantiomerization of the spiro compounds.
The results (Table 2) for the two enantiomers agreed within error limits.
6.2. (±)-6-Nitro-10,30,30-trimethylspiro[2H-1-benzopyran-2,20-indoline] 1b43
A solution of 0.54 g (3.23 mmol) of 2-hydroxy-5-nitrobenzaldehyde and 0.52 g (3.00 mmol, d=0.98,
0.53 ml) of 2-methylene-1,3,3-trimethylindoline in 10.0 ml of ethanol (99%) was heated at reflux for 2
h. Upon cooling, yellow crystals precipitated from the deep coloured reaction mixture and were filtered
off. Recrystallization from ethanol afforded (±)-1b as yellow needles (0.75 g, 2.33 mmol, 78%), mp
1
172–174°C. UV, see Fig. 3. IR (KBr) cm−1: 2970, 1650, 945; H NMR (d6-DMSO): δ=1.12/1.22
3
3
(3H/3H, s/s, C(CH3)2), 2.68 (3H, s, NCH3), 6.00 (1H, d, J=10.4 Hz, H-3), 6.62 (1H, d, J=7.3 Hz,
H-70), 6.81 (1H, m, H-50), 6.89 (1H, d, J=8.9 Hz, H-8), 7.14 (2H, m, H-40 and H-60), 7.23 (1H, d,
3
3J=10.4, H-4), 8.00 (1H, dd, J=8.9 Hz, J=2.8 Hz, H-7), 8.22 (1H, d, J=2.8 Hz, H-5). MS, m/z 322
(M+), 159. Anal. calcd for C19H18N2O3 (322.4): C, 70.79; H, 5.63; N, 8.69. Found: C, 70.76; H, 5.62;
N, 8.69.
3
4
4

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R. Kießwetter et al. / Tetrahedron: Asymmetry 10 (1999) 4677–4687
6.3. (+)436-6-Nitro-10,30,30-trimethylspiro[2H-1-benzopyran-2,20-indoline] 1b
Semipreparative enrichment was performed by LC on Chiralcel OD, heptane:2-propanol, 9:1, flow
rate 0.5 ml/min, 3 bar. A total amount of 10 mg of (±)-1b was injected in several portions, yielding two
fractions. The one at k0=1.0 represented a pale yellow solid, mp 175–176°C. P=71±2% (Fig. 2). CD
(heptane:2-propanol, 9:1, 21°C) λmax/nm (∆ε): 220 (−10), 253 (+33), 298 (−6.6), ∼332 (+4.7).
6.4. (−)436-6-Nitro-10,30,30-trimethylspiro[2H-1-benzopyran-2,20-indoline] 1b
The semipreparative enrichment described for (+)436-1b yielded a fraction at k0=0.8, representing a
pale yellow solid, mp 172–174°C. P=57±2%. CD (heptane:2-propanol, 9:1, 21°C) λmax/nm (∆ε): 220
(+17), 253 (−41), 298 (+9.6), ∼332 (−5.7).
6.5. 4-Nitro-2-[(E)-20-(100,300,300-trimethyl-3H000-200-indoliumyl)-10-ethenyl]-1-phenolate 2b19
Compound (±)-1b (0.35 g, 1.09 mmol) in 100 ml of n-hexane was irradiated with UV-light (30 min,
Heraeus TQ 150). The pale yellow solution turned blue and a brownish solid precipitated which was
filtered off and vacuum dried. 0.21 g (0.65 mmol, 59%) of a mixture of 2b and (±)-1b was obtained as a
red brown powder, mp 134–165°C (lit.19 150–175°C). IR (KBr) cm−1: 2970, 1617; 1H NMR (d6-DMSO,
75% 2b, 25% 1b): The positional numbers in the following assignments do not correspond to the above
systematic name of 2b but to the numbers given in its structural formula in Scheme 1. δ=1.74 (6H, s,
C(CH3)2), 3.85 (3H, s, N–CH3), 6.26 (1H, d, 3J=9.8 Hz, H-8), 7.46/7.53 (1H/1H, ddd, 3J=7.9 Hz, 3J=7.5
4
3
Hz, J=2.6 Hz, H-50 and H-60), 7.70/7.76 (1H/1H, d, J=7.9 Hz/3J=7.5 Hz, H-40 and H-70), 7.81 (1H,
dd, 3J=9.8 Hz, 4J=3.1 Hz, H-7), 8.33–8.45 (2H, m, H-3 and H-4), 8.68 (1H, d, 4J=3.1 Hz, H-5). MS, m/z
323.3 (MH+). Anal. calcd for C19H18N2O3 (322.4): C, 70.79; H, 5.63; N, 8.69. Found: C, 70.95; H, 5.66;
N, 8.71.
Acknowledgements
We are grateful to Dr. T. Burgemeister, Dipl.-Chem. M. Hofweber and Mr. F. Kastner for careful spec-
tral measurements. Dr. Y. Abe, Sakai, Japan and Dr. M. Frieser, Regensburg, contributed considerably
to the discussion of mechanisms. Mr. R. Würdinger, Jasco Deutschland GmbH, Groß-Umstadt, kindly
gave access to the CD-995 detector. Support of the present work by Fonds der Chemischen Industrie is
acknowledged.
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