Ind. Eng. Chem. Res. 1993,32, 3068-3071
3068
GENERALRESEARCH Vapor-Liquid Equilibria and UNIFAC Group Interaction Parameters for Thioamides Johann Gerdenitsch and Gerhard Gritzner’ Institut fur Chemische Technologie Anorganischer Stoffe, Johannes Kepler Universitat, A-4040 Linz, Austria
Adolf Schmidt Hoechst A . G., Abteilung fur Verfahrenstechnik,0-65929 Frankfurt, Germany
Vapor-liquid equilibria of binary mixtures of N,N-dimethylthioformamide and N-methyl-2thiopyrrolidinone, respectively, with benzene, toluene, ethylbenzene, ethanol, propanol-1, and butanol-1 were investigated experimentally in a modified Rock and Sieg recirculation still to obtain UNIFAC group interaction parameters for thioamides. The experimental data sets were checked for thermodynamic consistency. Large miscibility gaps between the thioamides and alkanes necessitated the simultaneous determination of the “CH2”, the “OH”,and the ”aromatic CH” group parameters from the complete data set. Several possibilities to represent the thioamide group were investigated. It was found that Nfl-dimethylthioformamideshould be a group in itself. N-Methyl2-thiopyrrolidinone can be divided into the “(CH~)(CHB)NCS(CH~)” main group and a “CH2”main group. These results allow the addition of thioamide groups to the UNIFAC parameter matrix and the calculation of activity coefficients for liquid mixtures containing such groups. Introduction
The UNIFAC group contribution method is a mathematical procedure to calculate activity coefficients for binary and multicomponent, nonpolymeric, nonelectrolyte liquid mixtures (Fredenslund et al., 1977; Macedo et al., 1983;Skjold-Jargenson et al., 1979). The calculations are based on separating the respective molecules into main groups and subgroups. Each group is considered to have a certain contribution to the activity coefficient. The contributions of each group are a function of the shape and size of the group and of the interactions of a given group with the other groups present in the liquid mixture. The shape and the size of the groups are obtained by evaluation of bond lengths and bond angles of the respective groups. The interaction parameters are derived from the evaluation of experimental vapor-liquid equilibrium data. Since data for group interaction parameters for thione groups are not available, we studied vaporliquid equilibria between N,N-dimethylthioformamide and N-methyl-2-thiopyrrolidinone, respectively, and aromatic hydrocarbons as well as with alcohols and estimated the group interaction parameters between thioamide groups and aliphatic and aromatic CH groups as well as OH groups. Experimental Section Preparation and Purification of Solvents. Prior to each measurement benzene, toluene, ethylbenzene, ethanol, propanol-1, and butanol-1 were rectified at ambient pressure under a N2 atmosphere over a 130-cm column filled with Raschig rings. Benzene (Merck p. A.) was rectified, and the middle fraction with a boiling point of 80.1 “C was used for the measurements. Toluene (Merck p. A.) was shaken with small portions of concentrated sulfuric acid until the acid phase became 0888-5885/93/2632-3068$04.00/0
colorless. The organic phase was then neutralized with a 10% aqueous sodium carbonate solution and washed three times with water. After drying with MgC12, toluene was rectified twice over sodium wire. The fraction with a boiling point of 109.5 “C was used for measurements. The purification of ethylbenzene was carried out as described for toluene. The boiling point of the middle fraction was 134.5 “C. Ethanol was dried over freshly activated molecular sieves (4 A) and recitified twice. The middle fraction had a boiling point of 78.3 “C. Propanol-1 was rectified twice over sodium wire. The boiling point of the middle fraction was 97.1 OC. The purification of butanol-1 was carried out as for propanol-1, resulting in a product with a boiling point of 117.7 “c. N-Methyl-2-thiopyrrolidinonewas produced by the reaction of N-methyl-2-pyrrolidinone with diphosphorus pentasulfide as described by Eiblingsfeld et al. (1963). N-Methyl-2-thiopyrrolidinone was rectified twice under reduced pressure and under a N2 atmosphere over 130cm-long Vigreux column to remove unreacted N-methyl2-pyrrolidinone. The middle fraction with a boiling point of 110.9 “C at 0.5 kPa was used for the measurements. NJV-Dimethylthioformamidewas prepared and purified following a procedure given by Willstlitter and Wirth (1909) by reacting N,N-dimethylformamide with diphosphorus pentasulfide. NJV-Dimethylthioformamide was purified by a double rectification. The fraction with a boiling point of 84.4“C at 0.6 kPa was employed in the measurements. Apparatus and Method. The measurements were carried out in amodified Rock and Sieg (1955)recirculation still. Pt-100 thermistors in combination with Keithley 197 multimeters were employed for temperature measurements. The thermistors were calibrated at the triple points of water and benzoic acid. Calibration between these two points was carried out by means of officially calibrated 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3069 Table I. Compilation of Data for the Calculation of Second Virial Coefficients. Antoine constantsb(T/K, P/kPa) solvent benzene butanol-1 DMTFe ethanol ethylbenzene NMTPj propanol-1 toluene
A
B
C
6.030 533 6.601 717 5.940 084/ 7.338 237 6.082 084 7.635 961f 6.744 122 6.079 565
1211.03 1362.39 1554.5of 1652.05 1424.26 3029.07f 1375.14 1344.80
-52.36 -94.43 -105.311 -48.68 -59.95 -2.42f -80.15 -53.67
crit press./MPa crit temp/K 4.796 4.303 4.4778 6.218 3.642 4.3238 5.033 4.106
crit compr factor
562.1 562.9 808.08 516.3 617.1 855.08 536.7 592.0
0.274 0.259 0.1898 0.248 0.272 0.184 0.252 0.271
RD~/A d 3.004 3.225 3.213h 2.250 3.821 3.053h 2.736 3.443
dipole moment/D
0.0 2.2 0.0 1.4 0.0 0.0 1.4 0.0
0.00 1.66 4.44' 1.69 0.58 4.86k 1.68 0.36
a According to Hayden and O'Connel (1975). log p = A - ( B / ( C + r)). Mean radius of gyration. Association parameter. e N,NDimethylthioformamide.f Our measurements. 8 Evaluated by the Lydersen group contribution method (Reid et al., 1977). h Evaluated with a group contribution method using the Parachor (Reid et al.. 1977: Minkin et al., 1970). Diggle and Bogsanyi (1974). j N-Methyl-2-thiopyrrolidinone. Gritzner et al. (1977).
Table 11. Vapor Pressures of N,iV-Dimethylthioformamide and N-Methyl-2-thiopyrrolidinone temp./"C press./kPa temp./"C press./kPa
N,N-Dimethylthioformamide 84.39 97.42 107.80 116.72 128.95
0.60 1.20 1.99 2.98 5.05
139.12 146.96 158.39 167.11 180.57
7.50 10.00 15.00 20.00 30.00
N-Methyl-2-thiopyrrolidinone 110.85 125.96 135.15 146.70 154.50
0.50 1.00 1.51 2.46 3.26
164.69 177.29 186.54 194.32
4.79 7.50 10.30 13.30
Table 111. Vapor-Liquid Equilibrium Data of Binary Mixtures of Benzene, Ethylbenzene, and Toluene with NJY-Dimethylthioformamide(DMTF) Dress./kPa %la .YI~ Benzene/DMTF: 70.00 OC
XI"
vlb
0.2000 0.2657 0.3534 0.4953 0.5670
0.9905 0.9926 0.9947 0.9979 1.0000
21.60 28.30 35.10 45.20 50.70
0.6568 0.7473 0.8764 0.9523
1.oooO 1.oooO 1.oooO 1.oooO
uress./kPa 54.90 59.90 66.60 70.90
0.3952 0.5697 0.7114 0.8014 0.8533 0.9046
2.46 3.37 4.87 6.85 9.20 12.82
0.2207 0.2752 0.3298 0.4316 0.7333 0.8625
0.9251 0.9398 0.9472 0.9606 0.9801 0.9936
15.62 17.80 19.79 22.70 29.20 31.60
Benzene/DMTF: 110.30 "C 0.0223 0.0414 0.0525 0.0683 0.0714
0.8069 0.8834 0.9070 0.9214 0.9281
0.0078 0.0141 0.0217 0.0431 0.0858 0.1047 0.1324 0.1663
0.4859 0.6368 0.7161 0.8343 0.8946 0.9110 0.9286 0.9436
11.40 18.19 23.35 27.70 29.80
0.0809 0.0966 0.1138 0.1324 0.2105
0.9346 0.9456 0.9532 0.9596 0.9757
33.50 38.50 43.70 49.80 74.00
Toluene/DMTF 100.00 "C
a
3.08 4.21 5.44 8.75 13.88 16.50 19.80 23.50
Xla
ylb
0.0146 0.0498 0.1089 0.1509 0.2250 0.3341
0.8842 0.9588 0.9779 0.9800 0.9899 0.9958
0.0182 0.0605 0.1424 0.2238 0.3284 0.3857
0.9320 0.9776 0.9833 0.9899 0.9943 0.9949
0.0548 0.1075 0.1555 0.2200 0.3210 0.3868
0.9884 0.9896 0.9910 0.9928 0.9952 0.9969
0.0093 0.0138 0.0395 0.0829 0.1057 0.1862
0.3745 0.4501 0.7054 0.8290 0.8630 0.9169
0.0105 0.0346 0.0506 0.0799 0.1023 0.1324 0.1686
0.7125 0.8788 0.9152 0.9386 0.9533 0.9635 0.9679
press./kPa xla ylb EthanoUDMTF: 75.12 OC 3.52 10.50 22.10 29.10 40.50 51.60
0.4714 0.5450 0.6646 0.8432 0.9320
0.9975 0.9969 0.9981 0.9993 1.oooO
press./kPa 63.15 67.60 73.90 82.40 87.10
Ethanol/DMTF 70.60 O C 3.87 10.84 22.50 31.75 40.90 46.10
0.4765 0.5432 0.6634 0.7588 0.8432 0.9276
0.9981 0.9978 0.9978 0.9984 0.9993 0.9993
50.50 53.90 58.60 61.90 65.00 68.40
Ethanol/DMTF 60.58 "C 5.95 11.25 15.59 21.30 27.30 30.00
0.4770 0.5412 0.6579 0.8394 0.9289
0.9972 0.9978 0.9984 0.9993 0.9996
33.80 35.60 38.50 42.30 44.50
Butanol-l/DMTF: 112.00 "C
Ethylbenzene/DMTF: 100.00 OC 0.0092 0.0195 0.0381 0.0653 0.0992 0.1619
Table IV. Vapor-Liquid Equilibrium Data of Binary Mixtures of Ethanol, Propanol-1, and Butanol-1 with NJY-Dimethylthioformamide(DMTF)
0.2041 0.2594 0.3274 0.4079 0.5098 0.6595 0.8959
0.9562 0.9650 0.9713 0.9763 0.9813 0.9876 0.9964
27.50 32.80 38.20 44.50 50.60 58.30 68.90
Liquid-phase mole fraction. * Vapor-phase mole fraction.
mercurythermometers (f0.015 K). The calibration curve was fitted by a second-order polynomial. The pressure was measured by a piezoresistivepressure meter (Balzers APG 010). The respective pure liquid N,N-dimethylthioformamide or N-methyl-2-thiopyrrolidinonewas first placed in the
3.87 4.41 8.35 14.13 17.33 26.40
0.2980 0.4034 0.5490 0.6668 0.8602 0.8584
0.9466 0.9565 0.9729 0.9752 0.9892 0.9968
37.00 44.60 53.50 59.60 69.00 75.80
Propanol-l/DMTF: 82.00 OC
a
2.31 4.96 6.93 10.08 12.58 15.48 18.90
0.2050 0.2420 0.3002 0.3798 0.5530 0.7184 0.8554
0.9737 0.9785 0.9833 0.9873 0.9912 0.9948 0.9956
21.60 24.50 28.40 33.10 40.40 45.70 48.10
Liquid-phase mole fraction. b Vapor-phase mole fraction.
apparatus, and the Antoine constants for the two solvents were determined. Increasing amounts of the second component were then added. The mixtures were refluxed until equilibrium was reached. Temperatureand pressure were recorded,and the chemical compositionsof the liquid phase and the condensed vapor phase were determined from the refractive indexes. The calibration curves for the concentrations as a function of the refractive indexes were established from 10 mixtures for each binary system employing third-order polynomials. Throughout this work an isothermic mode of operation was chosen, because in this mode the Gibbs-Duhem equation can be applied without additional enthalpy data.
3070 Ind. Eng.Chem. Res., Vol. 32, No. 12, 1993 Table V. Vapor-Liquid Equilibrium Data of Binary Mixtures of Benzene, Toluene, Ethylbenzene, Ethanol, and Butanol-1 with N-Methyl-2-thiopyrrolidinone(NMTP) X1’ ylb press./kPa ylb press./kPa BenzenelNMTP: 99.77 O C 61.80 0.2686 0.9931 1.48 0.0037 0.7974 72.40 9.90 0.3196 0.9945 0.0329 0.9717 0.3798 0.9959 83.90 22.80 0.0857 0.9868 92.80 25.60 0.4287 0.9959 0.0986 0.9882 101.50 0.4792 0.9973 31.50 0.1257 0.9903 108.20 0.5186 0.9973 47.90 0.2002 0.9924 0.0025 0.0639 0.1484 0.2074 0.2712
0.9019 0.9624 0.9818 0.9862 0.9923
Toluene/NMTP: 109.90 O C 0.3786 0.9949 3.91 10.78 0.4989 0.9949 23.50 0.6810 0.9984 0.8027 0.9975 31.90 0.9571 Loo00 40.00
52.00 64.00 78.30 86.40 96.80
0.0209 0.0565 0.1002 0.2078 0.3525 0.4428
Ethylbenzene/NMTP: 119.17 O C 0.5009 0.9893 0.7907 3.13 0.9227 7.55 0.5662 0.9893 0.9478 11.81 0.6680 0.9903 0.7500 0.9945 0.9634 22.40 0.8438 0.9965 0.9800 33.10 38.70 0.9331 0.9965 0.9872
41.70 45.10 49.30 52.60 56.10 59.80
0.0138 0.0460 0.0820 0.1299 0.2086 0.2760
0.9678 0.9874 0.9921 0.9952 0.9973 0.9977
EthanoUNMTP: 79.71 O C 3.36 0.3845 0.9979 9.45 0.5174 0.9998 16.20 0.6146 1.oooO 25.40 0.7730 1.oo00 38.20 0.8580 Loo00 48.10
61.90 73.80 81.20 91.20 97.30
0.0213 0.0622 0.1010 0.1546 0.2053 0.3004
0.8362 0.9448 0.9643 0.9732 0.9789 0.9876
ButanOl-l/NMTP: 109.90 OC 2.76 0.4181 0.9924 0.5127 0.9933 8.33 13.02 0.6624 0.9959 0.7688 0.9963 19.15 0.8461 0.9972 24.40 0.9394 0.9980 33.00
42.20 48.70 56.50 63.00 67.20 73.10
a
Liquid-phase mole fraction. b Vapor-phase mole fraction.
Estimation of Experimental Errors. Prior to the measurements the apparatus was checked with benzene and with water to estimate the experimental errors for temperature and pressure. Accurate data are available from Golding and Machin (1987) for benzene and from
Ambrose (1977)for water. The following precisions were found: *0.02 kPa in the range of 0-20 kPa, f0.05 kPa in the range of 20-150 kPa, and f0.02 K in the temperature range of 20-150 OC. The precision of the refractive index was f0.0001 over the whole concentration range. The uncertainties in the refractive index correspond to deviations of less than fO.OO1 in the mole fractions. The precision of the overall measurements including errors due to the working principle of the still was checked with the binary system acetonebenzene. Deviations from the values published by Brown and Smith (1957) were below 3%. Consistency Test. Activity coefficients were obtained from the P-T-xi data set by fitting the experimental data with Legendre polynomials. The composition of the vapor phase was calculated from these activity coefficients and compared with the experimental values. Only data for which the average deviation was below 0.01 were accepted. Results Vapor-Liquid Equilibrium Data. The Antoine constants in Table I are for the following form: logp = A (B/(C+ r ) ) ,where p is given in kPa and T i n K. The pure component parameters necessary to convert the P-T-xiyi data sets into T-xtri data sets are summarized in Table I. The data were taken from Reid et al. (1977) unless stated otherwise. The literature data for the Antoine constants were converted for the form of the Antoine equation used by us. Second virial coefficients were calculated from these data according to Hayden and O’Connell(1975). The results from our own vapor-liquid equilibrium measurements are given in Tables 11-V. Hildebrand Parameter. The Hildebrand (1970) parameters at 25 “C were calculated from the data in Table 11 and were found to be 29.0 MPa112for NJV-dimethylthioformamide and 25.8 MPa1I2for N-methyl-2-thiopyrrolidinone. Group Assignment. The structural groups considered as UNIFAC main groups are given in Table VI together with the van der Waals group volume and surface parameters. The van der Waals group volume and surface parameters were calculated according to a procedure published by Bondi (1968).
Table VI. Structural Groups Considered as UNIFAC Main Groups
RK~
P U P
//”
,
W,
QK*
group name n
3.508
3.036
‘DMTF”f
1.706
1.340
“N(A1k)CHS”
2.380
1.876
“N(A1k)CS-1”
\ 4’ / N-c\
1.479
1.028
“N(A1k)CS-2”
hs
3.728
2.956
‘N(A1k)CSd”f
N-C
YC
\H
\ 4’ / N-c\H 4’
bC\
/ N-c\
hc,
.N-C
/
-I
\
*
group name mc
am- d
an,md
“CH2” “ACH” ‘ACCHa” ‘OH” ‘CH2” “ACH“ ’ACCHz” ‘OH” “CH2” ‘ACH” ‘ACCHz” ‘OH” “CH2” “ACH” “ACCHz” “OH” “CH2” “ACH” “ACCHz” “OH”
-4.67 -193.19 54.24 250.10 -426.63 -204.06 458.31 -243.99 290.58 -150.86 4680.90 -274.60 -6019.01 -6031.60 5868.31 -6389.21 85.76 -224.88 -66.12 -216.98
115.68 413.72 242.66 671.46 152.80 904.39 208.15 616.42 90.26 21483.00 64.50 808.48 168.59 4998.00 600.78 678.37 40.04 575.44 377.71 823.00
quality of predictionel%
-
A y = 4.3,
r.=
-
-
-
-
-
-
-
-
20.9
A y = 5.4, Aym = 31.4
A y = 2.6, Aym = 9.2
AT = 6.9, Ay- = 19.1
AT = 2.3,. AT. - = 10.3
a van der W aals group volume. van der Waals group surface area. UNIFAC group names accordingto Gmehling et al. (1982). The indices m and n refer to the columns of group names. UNIFAC group interaction parameters. e Average mean deviations between experimental and calculated activity coefficients over all data points (Ay) and at infinite dilution (AT.). f Recommended parameters.
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3071 Group Parameter Estimation. The computer programs necessary for the group parameter estimation were based on programs published by Fredenslund et al. (1977). These programs allow only the estimation of one parameter pair in one step. Large miscibility gaps between the thioamide solvents and the alkanes prevented the direct estimation of the “CH2”-“DMTF” (UCH~,DMTF;UDMTF,CH~) and “CH2”-“NMTP” (WH~,NMTP; ~ N M T P , C H ~ group ) parameters. Therefore all pairs of UNIFAC parameters of N,N-dimethylthioformamideand of N-methyl-2-thiopyrrolidinone, respectively, with the aliphatic CH3, CH2, CH, and C groups (“CH2”),with aromatic CH groups (“ACH”), with aliphatic CH3, CH2, and CH groups attached to an aromatic carbon atom (“ACCH2”)and with the OH group (“OH”)had to be determined simultaneously. It was thus necessary to modify the original programs by Fredenslund et al. (1977) to allow the simultaneous estimation of five parameter pairs following a procedure outlined by Gmehling et al. (1982). The group parameters obtained from our calculations are summarized in Table VI. The average values of the deviations between experimental and calculated activity coefficients as well as the average deviations between the experimental and calculated activity coefficients at infinite dilution were calculated according to the following formula:
(F)
(G,)
A? =
100 -E 2N
2N lyyF*C
F1
- ?EXPI
y yxp
The comparison of the activity coefficients at infinite dilution represents the worst case of mismatch and is thus a measure of the maximum deviations which occurred. Discussion
The analysis of the results obtained for different possible group assignments given in Table VI suggests that N,Ndimethylthioformamide should be a group in itself. Higher homologues of the alkyl groups on the N atom of thioformamides, however, can be represented by the “DMTF” group and “CH2” groups. From the three possible group assignments for N-methyl-2-thiopyrrolidinone,the “N(Alk)CS-l”and the “N(A1k)CS-3” groups show comparable deviations between the calculated and experimental data. We recommend that the (CHa)(CH2)NCS(CH2)group (“N(Alk)CS-3” group) be used, since erroneous results may occur if the CH2 groups adjacent to the thioamide group are replaced by strongly interacting groups such as OH or halogens. The Nfl-dimethylthioformamideand N-methyl-2-thiopyrrolidinone molecules, however, must be treated separately, since N,N-dimethylthioformamidehas a hydrogen atom attached to the thione carbon. Similar distinctions have been made previously for the assignment of group parameters for methanol on one hand and ethanol and higher
homologues on the other by Fredenslund et al. (1977) and for N,N-dimethylformamide versus Nfl-diethylformamide and higher homologues by Gmehling et al. (1982). The “N(Alk)CS-3” group should be applicable to estimateactivity coefficientsof N,N-diethylthioacetamideand higher homologues as well as other cyclic thioamides. Literature Cited Ambrose, D. Recommended reference materials for realization of physicochemical properties. Pure Appl. Chem. 1977,49, 1437. Bondi, A. Physical Properties of Molecular Crystals, Liquids and Glasses; Wiley: New York, 1968; p 450. Brown, I.; Smith, F. Liquid-vapor equilibriums. VIII. The systems acetone + benzene and acetone + carbon tetrachloride. A u t . J. Chem. 1957,10,423. Diggle, J. W.; Bogsanyi, D. Physical properties and electrochemical stability of the thio solvents dimethylthioformamide and hexamethylphosphorothioic triamide. J. Phys. Chem. 1974, 78, 1018. DIN IEC 751. Beuth Verlag: Berlin October 1985. Eiblingsfeld, H.; Seefelder, M.; Weidinger, H. Reaktionen an der funktionellen Gruppe NJV-disubstituierter Carbonsiureamidchloride. Chem. Ber. 1963,96, 2671. Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria Using UNZFAC; Elsevier: Amsterdam, 1977; p 198. Gmehling,J.;Rasmussen, P.; Fredenslund, A. Vapor-liquidequilibria by UNIFAC group contribution. Revision and extension. 2. Znd. Eng. Chem. Process Des. Dev. 1982,21, 118. Golding, P. D.; Machin, W. D. The vapour pressure of benzene. J. Chem. SOC.,Faraday Trans. 1987,83,2719. Gritzner, G.; Rechberger, P.; Gutmann, V. Polarographic and voltametric investigations in Nmethylpyrrolidinone(2) and Nmethylthiopyrrolidinone(2).J. Electroanal. Chem. 1977,75,739. Hayden, G.; O’Connel, J. P. A generalized method for predicting secondvirial coefficients.Znd.Eng. Chem. Process Des. Dev. 1975, 14, 209. Hildebrand, J. H.: Prausnitz, J. M.: Scott, R. L. Regular and Related Solutions; Van Nostrand-Reinhold Princeton, 1970. Macedo, E. A.; Weidlich, U.; Gmehling, J.; Rasmussen, P. Vaporliquid equilibria by UNIFAC group contribution. Revision and extension. 3. Znd. Eng. Chem. Process. Des. Dev. 1983,22, 676. Minkin, V. I.; Osipov, 0. A.; Zhdanov, Y. A. Dipole Moments in Organic Chemistry; Plenum: New York,1970. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed. McGraw-Hilk New York, 1977. Rikk, H.; Sieg,L. Messungenvon Verdampfungsgleichgewichtenmit einer modernisierten Umlaufapparatur. Z. Phys. Chem. (Frankfurt) 1955, 3, 355. Skjold-Jargeneen, S.; Kolbe, B.; Gmehling,J.; Rasmussen, P. Vaporliquid equilibria by UNIFAC group contribution. Revision and extension. Znd. Eng. Chem. Process Des. Dev. 1979, 18, 714. Willstiitter, R.; Wirth, T. Ober Thioformamid. Chem. Ber. 1909,42, 1908.
Received for review January 25, 1993 Revised manuscript received August 9, 1993 Accepted August 25, 1993. Abstract published in Advance ACS Abstracts, October 15,
1993.
