DISCUSSION One of the principal objectives of this study was to evaluate the feasibility of using pattern recognition and cluster analysis techniques to correlate the chemical structure of the drugs (via the augmented atom fragments) to the pharmacological activity. In the case of sedatives and tranquilizers, the cluster analysis techniques (KNN, NLM, SSP, MST, and HAR) were able to classify 86 to 94% of the compounds correctly, while the discriminant functions (LMA and FRD) classified 84.9 to 8690 correctly. These results are very encouraging. This degree of success indicates that these techniques can be applied successfully to certain SAR problems. In addition, a n interesting result of using PR and CA techniques can be found by examining the compounds misclassified most frequently. For the cluster analysis techniques, these compounds were 27, 28, 29, 33, 35, 38, 42, 46, 63, 64, and 65, and for the discriminant functions 27, 33, 35, 38, 42, 46, 48, 63, 64, and 65. Although the compounds 33, 35, 38, 46, 48, 64, and 65 were classified by PR techniques as sedatives, they are used clinically as tranquilizers. However, Cutting indicates that these compounds gave many biological responses more similar to sedatives such as phenobarbital than to the classical tranquilizers-i. e. chlorpromazine or reserpine (20). (20) W. C. Cutting, "Handbook of Pharmacology," 5th ed., AppletonCentury-Crofts, New York, N.Y.. 1972,pp 549-562,570-589.
One of the advantages of using augmented atoms as features is that PR and CA techniques can detect the important structural differences between the classes of compounds. Of the 16 fragments in the files F16 and F16b, ten are most common to tranquilizers and six, to sedatives. The common substructural characteristics of the tranquilizers studied are the presence of aromatic sulfides and amines, tri-substituted aliphatic amines, methyl amines, and the C-C-N grouping, while the sedatives were characterized by amides, amidines, and urea linkages, isolated double bonds, and tetra-substituted carbon atoms. This structural information may be useful for further classifications of additional sedatives and tranquilizers as well as in developing new drugs. Further work is under way to examine the application of PR and CA techniques to other pharmacological activities as well as other SAR problems.
ACKNOWLEDGMENT The author would like to thank R. C. T. Lee, C. L. Chang, K. L. Ting, and M. Shapiro for the use of their programs and their helpful and informative discussions. Received for review October 31, 1973. Accepted March 29, 1974.
Investigation of Metallic Copper-Chloride Interaction in a Hydrogen-Air Flame David F. Tomkins and C. W. Frank Department of Chemistry, University of lowa, Iowa City, lowa 52242
A probable flame mechanism for the interaction of chlorine atoms with copper in the Tomkins burner assembly was developed. This mechanism proposed the direct chlorine atom interaction with metallic copper l o produce volatile copper(l) chloride at the tube surface. The copper(1) chloride was then decomposed to atomic copper in the hydrogen-air flame. The Inhibitive effect of sodium on the copper deflection was also studied as a function of various experimental parameters. This study revealed that sodium when present in the flame changed the flame equilibria by combining with chlorine atoms to form sodium chloride. The interaction of chlorine atoms with the copper tube surface is thus reduced.
A positive Beilstein test (green coloration of a flame) is produced when a trace of organic halide is introduced into a nonluminous flame on a copper wire ( I ) . Although it is a generally accepted fact that only a negative Beilstein test for the presence of halogen in an organic compound may be taken as being conclusive, only a few halogen-free compounds containing nitrogen, or nitrogen together with sulfur give a positive test (2-4). Previous investigators have drawn attention to the fact that pure volatile copper com(1)F. K. Beilstein. Ber., 5, 620 (1872). (2)M. Jurecek and F. Muzik, Chem. Listy, 44, 165 (1950). (3) J. Van Alphen. Red. Trav. Chim. Pays-Bas, 52, 567 (1933). (4)H. Milrath. Chem. Zfg., 33, 1249 (1935).
pounds and products such as lard, butter, and suet even without copper give a green coloration in the flame (2, 3 ) . Generally, it may be stated that substances which give a positive test form volatile compounds with copper by either direct sample combination or the combination of sample pyrolysis products with copper(I1) oxide. A positive Beilstein test for the presence of organic halides was conclusive when the sample was introduced into the flame on a strong platinum wire underneath a hot copper gauze (2). A modification of the Beilstein test for the halogens has been utilized by Van der Smissen for the determination of halogen-containing compounds in air (5, 6). Although the burner emitted the spectra of Cu, CuO, CuH, and CuOH, the spectrum of CuCl was not observed (7). Gilbert used an indium-coated copper tube burner for the determination of chloride ion uia the InCl emission band (7, 8). A proposed flame mechanism included the conversion of sample chloride to HCl in the fuel rich flame, HCl contact with indium metal, and the formation of InCl which was excited in a secondary flame. In a previous paper (9), it was shown that chloride in contact with hot metallic copper increased the atomic copper content of the flame above the burner assembly in pro(5) Dragerwerk, German Patent, 1,095,551 (December 1960). (6)C. E. Van der Smissen, US. Patent 3,025,141 (March, 1962). (7) P. T. Gilbert, Anal. Chem.,38, 1920 (1966). (8)P. T. Gilbert. US. Patent 3,504,976 (April, 1970). (9)D. F. Tomkins and C. W. Frank, Anal. Chem., 44, 1451 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9. AUGUST 1974
1187
CuoH CuCl Cu H
0
I
i'
A
5000
4000
6000
WAVELENGTH ( % )
Figure 1. Emission spectrum of Beilstein flame at suggested condiitions cu
CuOH
Table I. Suggested Operating Conditions
\
Photomultiplier voltage, V Hydrogen flow rate, 1.) min Air flow rate, 1.1min Burner height, mm Horizontal Slit width, pm
EMISSION SIGNAL
Figure 2. Vertical flame emission profiles for species when one part per thousand ammonium chloride was nebulized at suggested conditions Species
Scale sensitivity
cu x1 CuOH x 10 CuCl x 20 CuH x 20 portion to the chloride concentration. T h e present work was undertaken to study solid copper-chloride vapor interaction in a hydrogen-air flame. A probable flame mechanism consistent with experimental data was developed which proposed the direct chlorine atom interaction with metallic copper to produce volatile copper(1) chloride which decomposed to atomic copper in the flame.
EXPERIMENTAL The burner assembly and support equipment were identical to those described previously ( 9 ) . A quartz sheathed burner was constructed to simulate the brass 1188
900 6.70 1.60 15
center 200
burner chimney (described previously) from a quartz (%-in.i.d.) tube with six vent holes positioned one inch above the base. The base was a steel rod (%-in.high) fashioned to hold both the Beckman burner and quartz sheath in proper position. The burner assembly was mounted on a rack and pinion which allowed both vertical and horizontal adjustment. Flame temperatures were measured using the sodium line reversal method. The support equipment (a tungsten strip lamp and optical pyrometer) was operated as described in the current literature. Tube temperatures were determined using a calibrated chromelalumel thermocouple (Hoskins Manufacturing Co., Detroit, Mich.) The temperature of the tungsten strip lamp filament (GE No. T10) was measured using a calibrated Leeds and Northrup optical pyrometer (No. 709379). Solutions were made from reagent grade ammonium chloride dissolved in distilled, deionized water. The stock solution was one part per thousand and dilutions were made from the same stock solution.
RESULTS AND DISCUSSION The spectrum (3000 8, to 6000 8,) of the green Beilstein flame revealed the presence of OH, Cu, CuH, CuCl, and CuOH. This spectrum (Figure 1) was obtained by nebulizing a one part per thousand ammonium chloride solution through the copper tube burner a t suggested conditions. (The suggested conditions for the emission studies on the copper tube burner are presented in Table I.) To illustrate the CuCl and CuH band systems more clearly, the sensitivity was increased by a factor of five a t approximately 350 nm. T h e spectrum was calibrated by measuring the band heads in relation to the copper 3247-8, line and the sodium 5890-8, line. The identification of the various band systems was made after relating their positions to those previously identified (10). The spectrum (Figure 1) indicated that the green coloration was due to the diffuse CuOH band system. (10) E. M. Bulewicz and T.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
(1956).
M. Sugden, Trans.
faraday Soc., 52, 1475
t
18
19
20
21
22
HYDROGEN FLOW (ARBITRARY UNITS) L
I
I
Figure 4. Flame stoichiometry vs. copper absorption at 3247 a constant air flow of 1.6 I./min
I
A
with
The numbers above the lines indicate the chloride concentration in ppm
Table 11. Copper T u b e T e m p e r a t u r e s a t Various Hydrogen Flow Rates"
I. niin
Tuhe temperature, 'C
6.39 7.31 8.77 9.50
670 675 680 670
Hydrogen flow rate,
"
H, + C l - H C l +H gave an overwhelming excess of HCl over any other substances containing chlorine (including free atoms) in the gases because of the great excess of molecular hydrogen. However, the flames used in this study were lean in hydrogen and Figure 4 indicated that an increase in hydrogen flow rate decreased copper absorption. This could not be attributed to any significant temperature change (Table (11) T. G. Cowley, V. A. Fassel, and R. N. Kniseley, Spectrocbim. Acta, 238, 771 (1968). (12) E. M. Bulewicz, C. G. James, and T. M. Sugden, Roc. Roy. SOC.(London), A235, 94 (1958).
Air flow rate was constant a t 1.60 1. min.
11).A similar enhancement of atomic copper occurred in an acetylene-air flame (11). The flame was sufficient to reduce copper(I1) oxide (wire form) to metallic copper a t suggested conditions. This study in which copper(I1) oxide was suspended in the flame with a stainless steel screen indicated that the copper tube surface was not oxidized by the lean flame; therefore, the interaction of chlorine was assumed to be with metallic copper. The formation of copper(1) chloride as an intermediate was postulated on the basis of its presence in the flame (indicated by spectral observations) and the higher probability of a three-body collision over collisions of greater number. The third body for the formation was the copper tube surface. Also the vapor pressure of copper(1) chloride was sufficient for its volatilization from the copper surface. The vapor pressure (Table 111) exceeded the maximum copper(1) chloride concentration (calculated assuming a flame dilution factor of 2.5 X lo5 (13) and a nebulization rate of 2 ml/min for a one part per thousand NH&l solution) by a calculated factor of lo5. The copper tubes were coated with copper(1) chloride by passing dry hydrogen chloride over the copper tube surface at 180 "C (14). The coated copper tubes were placed in the burner assembly and exposed to the hydrogen-air flame a t suggested conditions. The copper(1) chloride was completely removed from the tube surface in less than two minutes and a very intense Beilstein flame was observed. The identity of the white powder (CuCl) (mp 430 "C) was confirmed by an X-ray powder pattern. (13) Aztec Instruments Circular, "The Elements," Bulletin No. 9, Westport, Conn., 1965. (14) 0. Honigschmid and T. Johannsen, Z.Anorg. Cbem., 252,364 (1944).
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 9. AUGUST 1974
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Table 111. Vapor Pressure of Cuprous Chloride a t
Various T e m p e r a t u r e s Vapor pressure, atm
Temperature,
3K
3.064 X 4.051 x 1.094 X 1.122 X 6.134 X 2.182 X 7.949 x 1.204 x 2.144 x 3.364 X 4.783 X 7.781 X 1.034
500 600 700 800
900 1000 1100 1200 1300 1400 1500 1700 1900
10-3
lo-* 10-2 10-l 10-1
lo-' lo-' lo-'
A flame mechanism is now proposed: RCl
-
R(,)
Cl,,, + CU,,) 1.0
.50
.25
Figure 5. Effect of sodium on the copper absorption when chloride was nebulized through the tube burner assembly The numbers over the lines indicate the chloride concentration in ppm
Possible reactions were chosen on the basis of thermodynamic calculations and the higher probability of a threebody collision over one of a higher order in the flame. Three reactions were consistent with these considerations:
+
+x
Cl,,)
-
CUCl,,)
+x
A F (at 1000 "K)= -37.566 Cu,,)
+
HCl,,,
+
X-
CuCl,,,
+ l/zHz(,) +
I X
AF (at 1000 "K) = 4.224
I1
+ 1/202(,)
CUO,,) + Cl,,) + X + C U C l ( , ) AF (at 1000
OK)
= -21.635
I11
where X is the third body. Reaction I11 was eliminated because CuO was reduced to metallic copper in the hydrogenair flame at suggested conditions. Reaction I1 has a large temperature dependence with an unfavorable equilibrium above 700 O K while reaction I has no such limitation. From the thermodynamic calculations (15) and flame stoichiometric considerations, reaction I is favored. This reaction indicates that free chlorine atoms release their excess energy a t the surface of metallic copper to form copper(1) chloride. The copper(1) chloride was then volatilized from the tube surface and subsequently decomposed to atomic copper in the hydrogen-air flame. The formation of CuH and CuOH from C q g ) has been investigated by Sugden e t al. (16)and follows reactions:
c u w + H,,) CU,,)
+
OH,,)
+
X
-
+
CuHw CuOH,,,
+
X
Thus, these species can be directly correlated to the atomic copper content of the flame. (15)JANAF Thermochemical Tables, Dow Chem. Co. P. 6. 168370 (1965). (16)E. M. Bulewicz and T. M. Sugden, Trans. Faraday SOC., 52, 1481 (1956). 1190
-
C1,g)
+x
cuc1,,,
Na/CI RATIO
CU,,)
+
(in lower f l a m e )
CUCl,,, + x ( w h e r e X is t h e tube s u r f a c e ) CU,,) + C 4 , )
Since it had been shown that sodibm limited the analyticai potential of the method (9),the inhibitive effect of sodium on the copper absorption was investigated in the hydrogenair flame at suggested conditions. A study in which sodium was also monitored at 5890 8, after exposure of the copper tubes to a one part per thousand sodium hydroxide solution in the hydrogen-air flame revealed that sodium was released into the flame for several minutes after nebulization was terminated. To determine the character of the interference a t other copper lines, absorption measurements were made a t the 3274.5-A and 2225.7-A copper lines. Since the sodium continuum extends from 310 nm to 600 nm (9),the 2225.7-A line was selected to monitor copper absorption with sodium present. Sodium had a large inhibitive effect at the 2225.78, copper line indicating that the sodium interference was not spectral. The effect of various concentrations of sodium (Figure 5 ) on the absorption due to 100, 500, and 1000 ppm ammonium chloride solutions was studied at suggested conditions. The interference was dependent upon the sodium to chloride ratio and not upon the concentration of sodium. Prolonged nebulization of a 200 ppm ammonium chloride solution containing 50 ppm sodium indicated that no decrease in absorption was observed for long nebulization periods. The presence of sodium in the flame would alter the flame equilibria by reducing the amount of chlorine atoms available for interaction with the tube surface. A thermodynamic calculation indicated that sod,ium chloride is formed preferentially over copper(1) chloride at 2000 O K by a factor of approximately lo3. Sodium when present in the flame limits the amount of free chlorine atoms available for interaction with the tube surface by forming NaCl. The amount of CuCl formed at the tube surface was reduced; therefore, the copper absorption was decreased. Potassium had a similar inhibitive effect on the copper absorption in hydrogen-air and acetylene-air flames a t suggested conditions when water was the solvent. Received for review July 13,1973. Accepted April 18, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
