Nighttime tropospheric chemistry: Kinetics and product studies in the reaction of 4-Alkyl- and 4-Alkoxytoluenes with NO3 in gas phase

Article abstract of DOI:10.1021/es980584t

Alkylbenzenes are important constituents of gasoline and industrial solvents and contribute to the formation of tropospheric ozone. The removal of these compounds from the troposphere is due to reaction with OH during the day and with NO₃ during the night. Five para-substituted toluenes are reacted with the nitrate radical in gas phase. Samples to be used for product analysis were preconcentrated and analyzed by GC-MSD analysis with the use of a reference curve. The yields of methanol, ethanol, and formaldehyde were determined by FTIR analysis of the reaction mixture in the chamber. Carbonyl compounds, benzyl alcohols, and nitroderivatives were formed. In the case of the alkoxytoluenes, products were also an alkanol, benzyl alcohols, and nitrophenols. Indirect rate constants measurements were performed by comparing by FTIR measurements the decay of the aromatic compound under investigation to that of a reference compound, with a known rate constant for the reaction with NO₃ added to the gas mixture. Wall loss constants were evaluated by FTIR. A Hammett correlation with ρ = - 4.3 ± 0.6; r²-= 0.87 was obtained. This and the kinetic isotope effect of 1.5-1.8 suggest a reaction mechanism occurring via an addition-elimination pathway.

Full text of DOI:10.1021/es980584t

Environ. Sci. Technol. 1999, 33, 461-468
3
The NO -initiated oxidation of aromatic hydrocarbons
Nighttime Tropospheric Chemistry:
Kinetics and Product Studies in the
Reaction of 4-Alkyl- and
must be considered a minor sink for these species (3); however
the high concentration of aromatics observed in urban areas
demands a better knowledge of mechanisms and reaction
products, in particular in regard to the possible formation
of noxious compounds (e.g. nitroderivatives) (4).
4
-Alkoxytoluenes with NO in Gas
3
Difficulty in sampling and analysis explains the limited
number of product studies in atmospheric chemistry. The
ring-retaining products in the reaction of aromatic com-
pounds with OH have been extensively studied (5). Ring
cleavage products in this reaction have been very recently
elucidated (6). The reactions between the nitrate radical and
aromatics received less attention until recently. Benzyl
nitrates, aldehydes, benzyl alcohols, and nitroderivatives have
been found as reaction products of methylarenes (7). Here
we report on the kinetics and the product study concerning
other 4-substituted toluenes, e.g. 4-ethyl-, 4-isopropyltoluene
known to be constituents of gasoline, 4-tert-butyltoluene as
a probe of reaction mechanism, and 4-methoxy- and
4-ethoxytoluene known as biomass burning products.
Phase
E Z I O B O L Z A C C H I N I , S I M O N E M E I N A R D I ,
M A R C O O R L A N D I , A N D
B R U N O R I N D O N E *
Dipartimento di Scienze dell’Ambiente e del Territorio,
Universita’ di Milano, Via Emanueli, 15, I-20126 Milano, Italy
J E N S H J O R T H A N D
G I A N B A T T I S T A R E S T E L L I
Commission of the European Communities, Joint Research
Center, Environment Institute, I-21020 Ispra (VA), Italy
Experimental Section
Gas-Phase Reactions. The reactions were studied in purified
air at 760 Torr in a 480 L Teflon-coated 60 cm diameter
cylindrical chamber equipped with a 81.2 m total beam path
length White type mirror system connected to an FTIR
Alkylbenzenes are important constituents of gasoline and
industrial solvents and contribute to the formation of
tropospheric ozone. The removal of these compounds from
the troposphere is due to reaction with OH during the
day and with NO3 during the night. Five para-substituted
toluenes are reacted with the nitrate radical in gas phase.
Samples to be used for product analysis were precon-
centrated and analyzed by GC-MSD analysis with the use
of a reference curve. The yields of methanol, ethanol,
and formaldehyde were determined by FTIR analysis of
the reaction mixture in the chamber. Carbonyl compounds,
benzyl alcohols, and nitroderivatives were formed. In the
case of the alkoxytoluenes, products were also an alkanol,
benzyl alcohols, and nitrophenols. Indirect rate constants
measurements were performed by comparing by FTIR
measurements the decay of the aromatic compound under
investigation to that of a reference compound, with a
known rate constant for the reaction with NO3 added to
the gas mixture. Wall loss constants were evaluated by FTIR.
spectrometer. N
2
O
5
was synthesized in the chamber by mixing
. Subsequently the hydrocarbon
O
3
with an excess of NO
2
was added, and the reaction was allowed to proceed until
only insignificant amounts (<100 ppb) N O5 remained.
2
Typical initial concentrations were 20 ppmV N
ppmV NO , and 20 ppmV of the hydrocarbon.
2 5
O , a few
2
Samples to be used for product analysis were obtained
by pumping known volumes of the reaction mixture through
glass columns packed with 150 mg of coconut charcoal. The
desorption was performed by extraction with 0.5 mL of
dichloromethane under mechanical agitation for 1 h. The
products were identified by GC-MSD analysis of the dichlo-
romethane solution without further preconcentration, using
a HP 5890 gas chromatograph with a 30 m capillary column
(
(
SPB5, 0.25 mm I.D.) interfaced with a quadrupolar detector
HP 5970), operating in Electron Impact mode at 70 eV. The
yield was determined in triplicate with the use of a reference
curve obtained using known amounts of each compound.
2
A Hammett correlation with F ) - 4.3 ( 0.6; r ) 0.87
was obtained. This and the kinetic isotope effect of 1.5-
The yields of methanol, ethanol, and formaldehyde were
determined by FTIR analysis of the reaction mixture in the
chamber.
1
.8 suggest a reaction mechanism occurring via an addition-
elimination pathway.
Kinetic Measurem ents. Indirect rate constants measure-
ments were performed by comparing by FTIR measurements
the decay of the aromatic compound under investigation to
that of a reference compound, with a known rate constant
Introduction
The nitrate radical NO
the reaction of NO
2
in the equilibrium rapidly established with NO :
3
is generated in the atmosphere by
and temporarily stored as N
2
with O
3
2 5
O
for the reaction with NO
3
radicals, added to the gas mixture
(
8). Wall loss constants were evaluated by FTIR.
NO + O f NO + O
2
2
3
3
Results
NO + NO +M h N O +M
3
2
2
5
Figures 1 and 2 show the results of the kinetic measurements
of rate constants with the indirect method. The wall loss
constants and the rate constants thus obtained are shown
in Table 1. The rate constants of the reference compound
anisole (9) and 1,3,5-trimethylbenzene (10) were literature
data. The plot of log k for these 4-substituted toluenes and
other obtained from the literature (3) vs Hammett’s σ for the
Alkylbenzenes are important constituents of gasoline and
industrial solvents and contribute to the formation of
tropospheric ozone (1). The removal of these compounds
from the troposphere is due to reaction with OH during the
3
day and with NO during the night (2).
*
Corresponding author phone: +39 02 66474302; fax +39 02
gas-phase reaction with NO
3) F ) - 4.3 ( 0.6; r ) 0.87.
3
(nine compounds) gave (Figure
2
6
4474300; e-mail Bruno.Rindone@unimi.it.
1
0.1021/es980584t CCC: $18.00

1999 Am erican Chem ical Society
VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 6 1
Published on Web 12/23/1998

FIGURE 1. Kinetic measurements for 4-ethyltoluene 1, 4-isopropyltoluene 12, and 4-tert-butyltoluene 19.
FIGURE 2. Kinetic measurements for 4-methoxytoluene 24 and 4-ethoxytoluene 28.
Product Studies. 4-Ethyltoluene 1. 4-Ethyltoluene 1
reacted with NO giving products deriving from the reaction
at the methyl group such as 4-ethylbenzaldehyde 2, 4-eth-
ylbenzyl alcohol 6, and 4-ethyl benzyl nitrate 8 and products
deriving from the reaction at the methylene group such as
nitrate 1-(4- methylphenyl)ethan-1-nitrate 9, probably during
sampling. The two nitroderivatives 10 and 11 were also
observed. Table 2 shows the quantitative results.
4-Isopropyltoluene 12. 4-Isopropyltoluene 12 was more
3
reactive with NO than both 4-methyl- and 4-ethyltoluene.
3
4
-methylacetophenone 3 and 1-(4-methylphenyl)ethan-1-
ol 7. Also 4-methylstyrene 4 and its oxidation product
-methylbenzaldehyde 5 derived from the reaction at the
Two reaction products deriving from the reaction at the
benzylic primary position were 4-isopropylbenzaldehyde 13
and 4-isopropylbenzyl alcohol 14. 4-Isopropenyltoluene 15
derived from the abstraction of the tertiary hydrogen atom
in the isopropyl substituent and gave by further oxidation
4
methylene group. Compound 4 could be formed from the
elimination of nitric acid from the intermediate secondary
4
6 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 3, 1999

TABLE 1. Wall Loss Constants and Rate Constants of the Reaction of 4-Substituted Toluenes with NO
3
at 298 Ka
wall loss
constants
reference
compound (3)
ksubstrate/
kreference
k
-
1
(cm molecule s-1
3
-1
(
s
)
)
-
-
-
-
-
6
6
6
5
4
-16
-16
-16
-15
-15
4
4
4
4
4
-ethyltoluene 1
8.2 × 10
8.0 × 10
4.4 × 10
1.3 × 10
9.9 × 10
1,3,5-trim ethylbenzene
m ethoxybenzene
m ethoxybenzene
1,3,5-trim ethylbenzene
1,3,5-trim ethylbenzene
0.98
2.94
1.78
4.09
3.38
7.71 ( 1.10 × 10
9.98 ( 0.15 × 10
5.99 ( 0.45 × 10
3.12 ( 0.54 × 10
2.67 ( 0.53 × 10
-isopropyltoluene 12
-tert- butyltoluene 19
-m ethoxytoluene 24
-ethoxytoluene 28
a
Uncertainties are given as one standard deviation on experim ental data and thus express random experim ental error only.
3
TABLE 2. Percent Product Yields of the Gas Phase Reaction of NO with Excess Substituted Toluenes in Air in the Presence of
a
NO
2
reaction yield from the converted material (%)
substrate
convn
carbonyls
alcohols
nitroderivatives
others
4
4
-ethyltoluene 1
67 ( 2
2: 16 ( 2
6: 3 ( 1
7: 2 ( 1
10: 5 ( 1
11: 6 ( 1
4: 4 ( 1
8: 3 ( 1
3
5
: 17 ( 2
: 13 ( 2
-isopropyltoluene 12
50 ( 3
13: 15 ( 1
14: 2 ( 1
17 + 18: 24 ( 1
15: 1 ( 1
form aldehyde: 30 ( 1
1
6: 3 ( 1
4
4
-tert-butyltoluene 19
-m ethoxytoluene 24
54 ( 4
52 ( 2
20: 32 ( 1
25: 20 ( 1
21: 16 ( 4
22 + 23: 42 ( 3
26: 3 ( 1
m ethanol: 4 ( 2
ethanol: 30 ( 10
2
7: 7 ( 1
4
-ethoxytoluene 28
59 ( 2
29: 11 ( 1
1: 3 ( 1
30: 1 ( 1
3
aYields are calculated on the am ount of starting m aterial converted and result from the integration of the gas chrom atographic peaks in
com parison with concentration/gas chrom atographic plots for all the analytes.
1
7 and 18 were also found. Figure 4 shows the GLC-MS
analysis.
FIGURE 3. Hammett plot for the reaction of 4-substituted toluenes
with NO
3
.
4
-methylacetophenone 16 and formaldehyde. This latter
compound was quantified observing the FTIR spectrum of
the material in the reaction chamber. The nitroderivatives
4
-tert-Butyltoluene 19. 4-tert-Butyltoluene 19 was less
reactive than 4-isopropyltoluene but similar to 4-ethyl- and
4
-methyltoluene. Reaction products were 4-tert-butylben-
zaldehyde 20, 4-tert-butylbenzyl alcohol 21, and the two
nitroderivatives 22 and 23.
4
-Methoxytoluene 24. A different situation occurred with
4
-methoxytoluene 24. A major pathway involving the oxida-
tion of the methyl group gave 4-methoxybenzaldehyde 25.
Minor products were those arising from the O-demethylation
reaction, i.e. methanol and two nitrophenols 26 and 27.
4
-Ethoxytoluene 28. 4-Ethoxytoluene 28 gave ethanol
confirming the O-dealkylation reaction. Other reaction
products were the aldehyde 29 and the nitropenol 30. Also
NO
-methylphenyl acetate 31 was found as a reaction products.
Considerable amounts of the nitroethers 32 (R ) Me) and
3
attack on the ethyl substituent was observed since
4
3
3 (R ) Et) were also formed in the reactions of 4-methoxy-
and 4-ethoxytoluene.
VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 6 3

FIGURE 4. GLC-MS analysis of the reaction mixture from 4-isopropyltoluene 12.
Discussion
The Rate-Determ ining Step. Scheme 1 shows a general
description of the reactivity of substituted toluenes with OH
X ) H) and NO (X ) NO ). A competition between the
addition channel (ADD) represented by the equilibrium
constant K ) k / k-1 , the H-abstraction reaction (H-ABS)
represented by the rate constant k , and the electron transfer
ET) represented by the rate constant k is likely to occur in
(
3
2
1
5
(
7
both cases. However, the ET mechanism has been never
suggested in gas phase. The intermediate benzyl radical 38
is formed directly (i) by H-ABS, (ii) by elimination of HOX
from the adduct 34 in the ADD mechanism, and (iii) by
deprotonation of the cation radical 40 in the ET mechanism.
Hence, products deriving from the functionalization of the
methyl group such as aldehydes, and benzyl alcohols, benzyl
nitrates are not diagnostic of the occurrence of one particular
mechanism. A sharp predominance (≈90%) of the ADD
mechanism has been shown in the reaction of OH with
toluene and substituted toluenes (11), and this explains the
formation of phenols 37 and ring fission products via the
reversible reaction with dioxygen 35 (X ) H) and of
The occurrence of mononitroderivatives in the reactions
of 4-alkyltoluenes with NO is of particular interest. Hence,
3
some control experiments were performed in order to
ascertain whether these compounds were formed in the
reaction chamber or during the trapping procedure. The
results showed that nitroderivatives of aromatic hydrocarbons
form essentially in the gas phase in the reaction chamber
and not mainly in the carbon trap. On the contrary,
mononitro-4-alkoxytoluenes 32 and 33 were formed mainly
during the sampling procedure.
nitroderivatives 39 via the adduct with NO 36 (X ) H) (12).
2
The phenol/ ring fission ratio is probably related to the
stereochemistry of the adduct 35, having two stereogenic
carbons.
4
6 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 3, 1999

SCHEME 1
The occurrence of either the ADD or the H-ABS mech-
anism in the gas-phase reaction of NO with substituted
toluenes is under discussion. Three experimental facts can
be used for distinguishing ADD, H-ABS, ET in the reaction
compete with ET in a recent study on the reaction of several
aromatics with NO in water (22).
The negative F value obtained in the kinetic experiments
3
3
2
with NO reported in this paper (F ) - 4.3 ( 0.6; r ) 0.87)
3
of NO
ie), and (iii) the Hammett correlation.
1) H-ABS. The H-ABS mechanism involves the rate-
3
: (i) positional selectivity, (ii) kinetic isotope effects
indicates an electrophilic attack involving a polar transition
state (23). The analogous reaction of aromatic compounds
with OH in gas phase has been reported to follow the general
description by the Hammett equation (24). In our hands a
similar calculation with the available rate constants of the
reaction of 4-substituted toluenes with OH shows a F ) -
(
k
(
determining cleavage of a carbon-hydrogen bond in the
methyl group. A low Hammett F value and a primary (kie) is
expected from this reaction, in analogy with other reactions
occurring via the formation of the benzyl radical via a rate-
determining H-ABS. In fact, with electrophilic radicals such
as bromine atoms from N-bromosuccinimide a F of -1.46
was obtained in liquid phase. Using molecular bromine in
solution a F ) -1.07 resulted (13). Here the kinetic isotope
effect was respectively kie ) 4.86-5.11 and 4.59-5.15 (14).
The gas-and the liquid-phase radical chlorination of sub-
stituted toluenes had lower kie (1.99-2.09) and F values
2
2.3 ( 0.2; r ) 0.96 (25). This negative F value is then the
result of the predominant rate-determining addition of OH
involving a polar transition state (11). Hence, the similarity
of the F values in the reactions of OH and NO
3
suggests the
to 4-sub-
is the
predominant rate-determining addition of NO
3
stituted toluenes. The more negative F value with NO
result of its major electrophilicity over OH. The kinetic isotope
3
effect value which was found (26) to be 1.5-1.8 in the reaction
of toluene and xylenes with NO , slightly higher than that of
3
0.98-1.13 found for the reaction of toluene with OH (2) could
be the result of the concerted loss of nitric acid from the
adduct 34 to form the benzyl radical 38.
(
-0.76) as expected in an exothermic reaction with very low
-1
activation energy (≈1 kcal mol ). The endothermic bromi-
-
1
nation had 7 kcal mol activation energy (15) in accord with
the Hammond postulate (16) which states that exothermic
radical reactions are likely to have reactant-like transition
The Product-Determining Steps. Concerning the product-
determining steps, the reversible addition reaction of NO to
3
states. Hence, they will have low values of primary kie
.
However, the H-ABS mechanism in the reaction of NO with
3
the aromatic has four different regiochemistries 41-44 in
the case of 4-substituted toluenes (Scheme 2). The position
of the equilibrium in the formation of adducts in the reaction
of OH with toluene has been recently calculated using an ab
initio approach and the ortho adduct has been shown to be
the most stable (27). This explains the predominance of the
formation of o- over p-cresol in this reaction (28). Preliminary
semiempirical calculations in the formations of the adducts
alkylaromatics had to be checked by comparison with the
radical bromination. In both reactions only benzylic hydro-
gens are abstracted, whereas in the radical chlorination also
hydrogens other than benzylic are abstracted (17). Moreover,
the reaction of NO with toluene has been calculated to be
3
exothermic (18).
(
2) ET. The liquid-phase oxidation of toluene with Co(III)
acetate occurring via ET concerted with deprotonation has
3
in the reaction of NO with toluene show a similar stability
the higher F value of ) -2.4 and the primary kie of 3.7 (19).
of the ortho-, meta-, and para-adducts but a lower stability
for the ipso-adduct (29). A refinement of this calculation
using an ab initio method shows that this exothermic reaction
gives the ortho adduct from toluene as the most stable adduct
(
3) ADD. A different behavior was observed in the liquid-
phase reaction of NO with several toluenes substituted with
3
electron withdrawing groups. Here, the ADD mechanism in
acetonitrile was suggested by a F value of - 3.2 (20). More
recently, a reinvestigation of this reaction led to the conclu-
sion that an inner-sphere ET mechanism was always oc-
(
18).
Alkyl-substituted nitrotoluenes may have their origin in
the reaction chamber in the reaction of the NO -adducts
(41-44, shown in Scheme 2 for toluene) with NO to form
3
curring via an intermediate NO
3
-aromatic complex. This gave
2
-
NO
3
and the cation radical 40 which underwent fast
diastereoisomeric dihydrobenzenes 45-51 that eliminate
nitric acid to give the nitroderivatives 52-54. This reaction
has been shown to be fast (30). The finding of high amounts
deprotonation to the benzyl radical 38. This mechanism
resulted in kie ) 1.6 (21). The ADD channel was shown to
VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 6 5

SCHEME 2
of m-nitrotoluene 54 from toluene using sampling procedures
which should not enrich samples in one particular isomer
SCHEME 3
(
7) is an indication of a radical nitration (31). This again
suggests the importance of the ADD mechanism and is
explained by the prevailing equilibrium amount of the ortho
adduct 43. In fact should these nitrotoluenes derive exclu-
sively from the chain-terminating reaction of benzyl radical
3
8 (Scheme 1) only the para 52 and the ortho isomer 53
should be formed.
Also the O-dealkylation reaction observed with 4-meth-
oxy- 24 and 4-ethoxytoluene 28 is an indication of the
occurrence of the ADD mechanism. In particular, the
oxidative O-dealkylation reaction leading to nitrophenols and
nitrophenyl ethers may be explained to occur via the “ipso”
adduct 55 formed by initial addition of NO
which has been suggested also in the case of the reaction of
phenol with NO (32), will then react with NO to give a
diastereoisomeric intermediate 56 that may react further via
elimination reactions to form (i) the nitrophenol 57 and (ii)
a nitrophenyl ether 58 (Scheme 3).
Alcohols and carbonyls formed in this reaction derive
from the intermediate benzyl radical 59 formed in all the
mechanistic alternatives (Scheme 4). This will be transformed
into a peroxy radical 60 which will disproportionate to
carbonyls 61 and alcohols 62. Alternatively, peroxy radicals
3
. This adduct,
6
3
0 may react with NO to form alkoxy radicals 63 and then
carbonyl compounds 61. The observed predominant forma-
tion of carbonyls over benzyl alcohols with 4-alkyltoluenes
suggests a major role for this latter pathway.
3
2
The nature of the carbonyl compounds from 4-alkyltolu-
enes is likely to be controlled by two factors: (i) the stability
of the corresponding benzyl radical and (ii) the regiochemistry
of the attack of NO . The physicochemical characteristics of
3
the carbonyls obtained are so similar that one could expect
similar trapping and elution efficiency. This allows to compare
4
6 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 3, 1999

SCHEME 4
In the reaction of 4-isopropyltoluene 12, compounds 15
16 derive from the fragmentation of the intermediate
+
tertiary radical 69, whereas the aldehyde 13 and the alcohol
4 derive from the primary radical 68. Hence in this case the
1
ratio between the products formed by the tertiary 69 vs the
primary radical 68 is 1:3.7. This implies that the pathway
leading to the primary radical 68 is more important than
that leading to the tertiary radical 69 via the adduct 67. This
is probably due to the fact that the equilibrium concentration
of the adducts 66 and 67 is strongly influenced by steric
3
factors. In fact, attack of NO could occur either ortho to the
bulky isopropyl group to generate the adduct 66 or ortho to
the methyl group to generate the adduct 67. The equilibrium
concentration of the latter, more stable, should be higher
that of the former, less stable. Consequently, a higher amount
of the primary radical 68 over the tertiary radical 69 is formed.
A preliminary calculation of the difference in heat of
formation of adducts 66 and 67 using a semiempirical AM1
-
1
approach showed that adduct 66 is 0.71 kcal mol more
stable that adduct 67 (18).
3
In conclusion, the reactions of NO radicals with aromatic
hydrocarbons are relatively slow and thus believed to be of
little importance as a sink for these in ambient air. Carbonyls
and nitroderivatives are main reaction products. However,
the finding of nitroderivatives and nitrophenols in rainwater
in polluted areas (34) is of particular interest because of the
toxicity of these compounds, and it needs to be established
if their formation is taking place at a significant rate in the
atmospheric gas phase or in the aqueous atmospheric phase.
Some preliminary observations (35) suggest that inspection
of the isomeric distribution could throw light on this point
because of the radical nature of the gas-phase process and
the electrophilic mechanism of the aqueous reaction. These
two pathways lead to different isomer distributions.
reaction yields in order to obtain information about branch-
ing ratios. In fact, in the reaction of 4-ethyltoluene 1, the
ratio between the yields of compounds 3 + 4 + 5 (arising
from the reaction at the methylene group) and 2 + 6 + 8
(
arising from the reaction at the methyl group) is 1.5:1. This
Acknowledgments
is also due to the different stability of the corresponding
benzyl radicals 64 and 65 (Scheme 5) obtained in the loss of
one hydrogen atom from the initial adduct. However, this is
not the major factor, since the benzylic hydrogen abstraction
has the following order of reactivity (33): tertiary ) 6.8-260
We thank Mr. G. Ottobrini and our students A. Gambini, P.
Rossi, and M. Lusardi for technical help. This work was
supported by the EC grant ENV4-CT97-0411. We thank also
the Italian CNR and the Italian-German VIGONI program
for financial support.
>
secondary ) 3.2-50 > primary ) 1.
SCHEME 5
VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 6 7

(
(
(
(
(
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Received for review April 23, 1998. Revised manuscript re-
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