SUMMARY These carbon film OTEs are the first such films to be produced with conductivity high enough to be employed as electrodes, while still retaining sufficient infrared transparency for internal reflection spectrometry. Other infraredtransparent materials with the diamond crystal structure should also make good carbon OTE substrates. Cadmium telluride, with its longer infrared window, is a particularly attractive candidate, although one attempt to coat CdTe has failed. Getting carbon to adhere to CdTe will require proper selection of coating temperature, and perhaps a modified cleaning procedure. The procedure for putting metal films on diamond was described in the introduction, and diamond is a very attractive candidate as an OTE substrate. LITERATURE CITED (1)W. N. Hansen, R. A. Osteryoung, and T. Kuwana. J. Am. Chem. Soc., 88, 1062 (1966). (2) W. N. Hansen. T. Kuwana. and R . A. Osteryoung, Anal. Chem., 38, 1910 (1966). (3)H. 8. Mark, Jr. and 5. S.Pons, Anal. Chem., 38, 119 (1966). (4)D. Laser and M. Ariel, Nectroanal. Chem. Interfacial Hectrochem., 35, 405 (1972). (5)D. R. Tallant and D. H. Evans, Anal. Chem., 41, 835 (1969).
(6)A. Trifonov, B. Jordanov, and M. Poneva, Comm. Dept. Chem., Bulg. Acad. Sci., 4, 131 (1971). (7)A. Z.Trifonov and I. D. Schopov, Nectroanal. Chem. Interfacial Necfrochem., 35, 415 (1972). (8) A. 2 . Trifonov. T. Popov, and 6. Jordanov. J. Mol. Struct., 15, 257 (1973). (9)A. H. Reed, Diss. Abstr. Int. B., 29,4561 (1969). (IO)A. H. Reed and E. Yeager, flectrochim. Acta, 15, 1345 (1970). (11) D. Evans, Univ. of Wisconsin, private communication, January 1973. (12)D. Laser and M. Ariel, J. Nectroanal. Chem., 41, 381 (1973). (13)R . W. Rice, Ph.D. Thesis, Yale University, 1972. (14)J. V. Sanders, in "Chemisorption and Reactions on Metallic Films", Vol. 1, J. R. Anderson, Ed., Academic Press, London, 1971,pp 1-39. (15)H. Schmidt-Brucken and W. Schlapp, Z. Angew. fhysik., 32, 307 (1971). (16)B. D. Epstein. D. Daiie-Molle. and J. S.Mattson. Carbon, 9,609 (1971). (17)J. S.Manson, Anal. Chem., 45, 1473 (1973). (18)J. S. Mattson and C. A. Smith, Chapter 2 in "Computers in Chemistry and Instrumentation", Vol. 7,J. S. Mattson, H. B. Mark, Jr., and H. C. MacDonald. Jr.. Ed. Marcel Dekker, Inc., New York (in press). Preprints will be available from the authors until volume is published.
RECEIVEDfor review December 23, 1974. Accepted February 18, 1975. This research was supported by the National Institutes of Health, National Heart and Lung Institute Grant No. HL-15919-01A1, and a Cottrell research grant from Research Corporation, Atlanta, GA. Contribution from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL.
New Ambient Temperature Molten Salt Solvent for Electrochemistry: Triethyl- n-hexylammonium Triethyl- nhexylboride Warren 7 . Ford Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, lL 6 180 7
Preparation of a series of tetraalkylammonium tetraalkylborides which are liquid a t room temperature makes available a new class of solvents. ( I ) . The least viscous of these molten salts, triethyl-n-hexylammonium triethyl-nhexylboride (N222&2226), has been used as a solvent for kinetics (2, 3) and NMR spectroscopy ( 4 ) . N2226B2226 is not appreciably soluble in alkanes or water, but it is completely miscible with many organic solvents ranging from benzene to methanol. Experiments reported here demonstrate its promise as a solvent for electrochemistry. Only one other liquid salt, tetra-n-hexylammonium benzoate hemihydrate, has been reported as suitable for room temperature electrochemical experiments (5).
EXPERIMENTAL All experiments were performed in a nitrogen atmosphere. Conductance of liquid N 2 2 2 6 B 2 2 2 6 was measured in a 4-ml cell with platinum black plates a n d a constant of 0.686 cm-'. Temperatures were accurate t o f O . O l O , a n d conductances were identical a t 1- a n d 3-kHz oscillator frequencies. Cyclic voltammograms were obtained in a 2-ml cell. T h e working electrode was a cross-section of 18 gauge platinum wire sealed in glass. A platinum wire served as t h e anode, and a n aqueous Ag/ AgC1, saturated KC1 electrode placed about 5 m m from t h e working electrode served as t h e reference. Because of an appreciable iR drop, t h e cathodic and anodic peak potentials usually varied with scan rate over a 0.05-V range, a n d their separation was larger t h a n t h e theoretical 58 mV for a reversible one-electron process a t 22'. Peak potentials a t a single scan rate in repeated scans all fell with-
in a 0.02-V range. Slight leakage from t h e reference electrode left 0.1 wt % water in t h e N 2 2 2 6 B 2 2 2 6 after a typical series of experiments.
RESULTS AND DISCUSSION The specific conductance of N2226B2226 increases by a factor of 7.8 when it is warmed from 25' to 75' (Table I). Its specific conductance at 25' is the same as that of 0.0035M aqueous KCl. The marked temperature dependence is a consequence of its high viscosity and the nearly equal sizes of its cation and anion. The activation energies for conductance and shear viscosity of N2226B2226 both lie in the 8-10 kcal/mol range at 25-75'. Coetzee and Cunningham (6) have recommended tetraisoamylammonium tetraisoamylboride as a reference electrolyte for the evaluation of single ion conductivities because its anion and cation would have nearly equal mobilities. Carbon-13 spin-lattice relaxation times of neat N2226B2226 indicate that internal rotational motion in the anion is slightly faster than in the cation, perhaps because the B-C bonds are longer than the N-C bonds ( 7 ) . If the B2226 anion is slightly larger than the N2226 cation, the anion should have a slightly lower mobility in the neat liquid. Typical cyclic voltammograms of anthracene and benzophenone in N2226B2226 a t 2 2 O are shown in Figure l. The reduction of anthracene (assumed to be a one-electron process by analogy to many previous investigations of anthraANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1125
Table 1. Specific Conductance of Liquid N2226B2226 T ,“C
25.00 35.00 45.06 55.00 65 .OO 75.06
lo4 y , ohm-!cm-l
I
5.120 8.665 13.69 20.43 29.13 39.81
/
I
/
M
1.2
cene in aprotic solvents) (8) appears to be completely reversible because, after correction for background current, the cathodic and anodic peak currents are equal within experimental error. Observed peak potentials, however, a t scan rates of 50-800 mV/sec varied from -1.99 to -2.03 V, and cathodic and anodic peak separations were about 0.10 V, because of uncompensated resistance in the solution. Extrapolated to zero scan rate, the anthracene E,, = -1.95 V. By the cathodic and anodic peak current criterion, the first reduction step of benzophenone is only partly reversible, perhaps because a trace of water in the N2226B2226 reacts with the benzophenone radical anion. Extrapolated to zero scan rate, the benzophenone E,, = -1.71 V. The irreversible second-reduction step appears a t about -2.1 V. Cyclic voltammograms of anthracene and benzophenone in N2226B2226 at 51’ appear qualitatively the same as at 22’ except that peak currents are 1.7 to 2.0 times larger. The useful working range of N222&226 is about -0.5 to -2.5 V. At more anodic potentials, oxidation of the B2226 ion proceeds readily (9) and, at more cathodic potentials, the background current is unacceptably high. Its residual current appears qualitatively to be smaller than that of tetra-n- hexylammonium benzoate hemihydrate (5), probably because N2226B2226 contains 10.1 wt % water. Two features of N2226B2226 should make it attractive for further electrochemical investigation. It is a single component conducting fluid a t room temperature, and its high viscosity makes diffusion processes much slower than in common
/
I6
I
24
20
- E (v) Flgure 1. Cyclic voltammograms in N222&226
at 22’
( a ) 2.7rnM anthracene, 100 rnVlsec. (b) 2.6rnM benzophenone, 200 rnV/ sec. E is referenced to aqueous Ag/AgCI. saturated KCI
aprotic solvents. Moreover, its viscosity and conductance can be varied markedly by relatively small temperature changes.
ACKNOWLEDGMENT I thank L. Faulkner and S. G. Smith for helpful discussions and use of their instruments.
LITERATURE CITED (1) W. T. Ford, R. J. Hauri, and D. J. Hart, J. Org. Chem., 38, 3916 (1973). (2) W. T. Ford and R. J. Hauri, J. Am. Chem. SOC.,95, 7381 (1973). (3) W. T. Ford, R. J. Hauri. and S. G. Smith, J. Am. Chem. SOC.,96, 4316 (1974). (4) W. 1.Ford and D. J. Hart, J. Am. Chem. SOC.,98, 3261 (1974). (5) C. G. Swain, A. Ohno, D . K. Roe, R. Brown, and T. Maugh 11, J. Am. Chern. SOC.,89, 2648 (1967). (6) J. F. Coetzee and G. P. Cunningham, J. Am. Chem. SOC., 88, 3403 (1964). (7) W. T. Ford, unpublishedresults, 1974. ( 8 ) C. K. Mann and K. K. Barnes, “Electrochemical Reactions in Nonaqueous Solvents”. Marcel Dekker, Inc.. New York. NY. 1970, p 59. (9) K. Ziegler and 0.-W. Steudel. Justus Liebigs’ Ann. Chem., 652, 1 (1962).
RECEIVEDfor review January 7, 1975. Accepted March 3, 1975. This research was supported by National Science Foundation grant GP 38493.
Probability Discriminant Functions for Classifying Binary Infrared Spectral Data S. R. Lowry, H. B. Woodruff, G. L. Ritter, and T. L. lsenhour Department of Chemistry, University of North Carolina, Chapel Hill, NC 275 14
In computer classification of chemical data, each compound is represented by a feature vector whose components correspond to physical measurements made on the compound (melting point, % light transmittance, mass spectrum, etc.). Although the actual measuring process may produce a continuum of values (analog output), for the data to be computer compatible, it must be digitized into a discrete form. Depending on the resolving capabilities of the measuring device and the resolution required by the classification algorithm, the digitized data may either approximate the continuous state or, in the opposite extreme, may be limited to the binary state. A number of pattern recognition investigations have dealt with “continuous” data (1). Less work, however, has involved chemical pat1126
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
tern recognition of binary data ( 2 , 3 ) .This work reports the use of probability methods for classifying compounds into similar structural groups on the basis of their binary infrared spectra ( 4 ) . Two main reasons for investigating infrared spectral data are: 1) access to the ASTM file of over 90,000 infrared spectra stored in binary form (made accessible a t the Triangle Universities Computation Center by the R. J. Reynolds Tobacco Company); and 2) widespread use throughout industry. To represent the infrared spectrum of a compound by a binary vector, each element of the vector must correspond to some interval of the spectral region of interest. For the present work, all intervals are 0.1 pm. If a peak maximum is present in a 0.1-pm interval, the corresponding element
