Kinetics of Gas-Phase Unimolecular Isomerizations with Microwave Spectrometric Techniques S. N. Mathur and Marlin D. Harmony* Department of Chemistry, The University of Kansas, Lawrence, Kansas 66045
The kinetics of the unlmolecular gas-phase isomerization of exo-2,3-dideuteriobicyclo[2.l.O]pentane have been studied as a function of temperature with microwave spectrometry. In addition, a single measurement of the reaction rate constant ( k = k, k - , ) was performed for the isomerization of endo-2-methylbicyclo[2.1.0]pentane. A theoretical development shows that the kinetics can be obtained by measuring microwave relative intensities. The relatively simple and stralghtforward experimental methods have been described in detail.
+
Microwave spectrometry has found its greatest application as a tool for structural elucidation and for the investigation of various molecular electronic properties (1). However, its potential for a variety of analytical applications has been pointed out on several occasions (2-4). In this regard, the greatest hindrance in using microwave spectrometry for analytical purposes has been the inherent difficulty of relating quantitatively a molecular absorption intensity to molecular concentration. Some important studies have recently been made in this area ( 5 , 6 ) ,but the techniques require equipment of rather limited availability, and the measurements are still somewhat tedious and plagued by a variety of possible errors. I t is well-known that relative intensities of two different absorbing species can be related rather well to relative molecular concentrations under appropriate circumstances (7). In comparison to absolute intensity measurements, relative intensity measurements are comparatively easy to perform, but they too have possible pitfalls depending on the particular application. Perhaps the most common application is the determination of relative populations of a given molecular species in different vibrational states (8).From these data, it has then been possible to evaluate the vibrational energy spacings, i.e., more crudely, the fundamental vibrational frequency of the mode in question. In this work, we wish to illustrate the use of microwave relative intensity measurements for the determination of kinetic rate constants (and activation parameters) for gas-phase unimolecular isomerization reactions. The procedure to be described relies, in general, upon a calibration scheme but, for a certain class of isomerizations, which are essentially degenerate isomerizations, the calibration can be performed theoretically in a trivial fashion. The results to be presented will show that this method yields kinetic data whose quality is comparable to that obtained by other analytical procedures. Coupled with the usual high resolution capability of microwave spectrometry, for example, the ease of observation of molecules differing merely in the location of an isotopic label, the method provides a powerful means for investigating kinetic processes. T H E O R Y OF T H E METHOD Kinetic Scheme. We consider a reaction of the type
A*X
(1)
behaving unimolecularly. As shown by the standard works on kinetics (9), the integrated rate law for process 1 yields an expression for the sum of the forward ( k l ) and reverse ( k - 1 ) rate constants, 1 Xe k l + h-1 = -1nt x,-Ax
(2)
where x , is the concentration of X a t equilibrium ( t = QD) and Ax is the amount of X formed a t the time t in proceeding from an A-rich system a t t = 0 to an equilibrium mixture a t t = QD. If a0 and x o are the initial ( t = 0) concentrations of A and X, respectively, it can be shown that x, =
klao - k-lxo k l + k-1
(3)
+
In what follows, we shall define k hl k-1. Kinetic R a t e L a w i n T e r m s of Microwave Relative Intensities. In the last part of this section, we shall show that the relative intensity, R , of a particular pair of microwave absorption lines of species A and X, is related to the relative molecular concentrations of A and X by
(f)
R =P
(4)
where a and x are the concentrations of A and X a t any time t and p is a constant (for a given pair of absorption lines) t o be determined by a calibration procedure. Defining Ro and R , as the intensity ratios a t t = 0 and t = m, respectively, and using also the definition a X O / ( X , - X O ) , Equation 2 can be rewritten as
In the rather tedious but straightforward algebra leading from Equation 2 to Equation 5, it is useful to utilize three identities: a, = a0
- xo/a
x, = XO(1
+ l/a)
P = a(Ro - R m )- R ,
(6) (7) (8)
From Equation 5 and Equation 8 it is clear that measurements of R as a function o f t (including t = 0 and t = QD) permit the determination of k ( = h l k-1) if the calibration constant P is experimentally known. Interestingly, Equation 5 could be used alone to determine both k and a by fitting the R ( t )data. We have found by model calculations, however, that the R ( t ) data must be of extremely high accuracy to permit direct evaluation of both k and a from R ( t )data only. In our applications, we shall fix P by calibration, and thus Equation 8 will yield a. Finally it should be stressed that Equation 5 is an exact equation following directly from Equation 2 using the special condition Equation 4 . Relationship between Line Intensities a n d Molecular Concentrations. The intensity of an observed microwave absorption line is a complicated function of numerous molecular and instrumental parameters. In the absence of power saturation and instrumental and other line broadening effects,
+
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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D
Table I. Data for Analytical Transitions Dideuteriobicyclopentane Exo
EX0 -
CH3 EX0 -
Figure 1. Reactions investigated in this study. Only the distinguishing substituents are shown, with hydrogen atoms being omitted. Each reaction is written in the form of Equation 1 in the text the peak signal intensity of a component i in a gas mixture may be written (IO)
s.= c,ni 1
1
A u;
(9)
where ni is the concentration, Aui is the line width, and C; contains all other molecular and instrumental parameters for the particular transition, but is independent of total pressure P and gas composition. For two such absorption lines of species i and j ,
(10) Subject to the condition that the terms in brackets remain constant under experimental conditions, Equation 10 becomes
which is of the form of Equation 4. Our interest lies in measuring R as a function of n, and n, using experimental conditions which guarantee that C, and C, remain constant as the reaction proceeds, that is, as nJn, varies smoothly with time. The necessary conditions are simple to achieve and are described in the Experimental section. The line-width parameters present a more serious problem, since they are potentially composition dependent, and will depend upon total pressure. However, if the species i and j have identical line-broadening parameters in t h e rotational states of interest (i.e., at the molecular level, the collisional relaxation times are identical for all bimolecular collisions), Au,/Au, will have the value unity regardless of the relative amounts of i and j present in the sample. Under this condition, and with constant C,/C,, Equation 11would apply with constant @ as desired. This condition would also cause @ to be independent of total pressure. The linewidth condition stated above will not generally be satisfied, but it should be satisfied to a very high accuracy for identical transitions when the species i and j differ only in the position of isotopic labels, such as deuterium or 13C. T h e condition may also be less precisely satisfied for isomeric molecules whose electronic structures and geometry are not very dissimilar. In situations where the linewidth condition is not satisfied, t h e method proposed here can be modified in such a way as to still lead to an equation of the form of Equation 4. What is necessary is that R must be defined as the ratio S,AuJS, Av,. 1510
Exo
Endo
Transition 2,, .-* 3,, 2,, 3 , , 2,, .-* 3,, 2,, -c 3,, Frequency 37 600.40a 31 825.28 39 990.90 34 459.83 Pb 1.05C 1.64d Transition l,, 2,, l,, 2,, 3,, 4,, 3 , , +. 4,, Frequency 30 478.44 29 695.35 35 777.23 33 770.35 1.33 2.00 P ... ... Transition l,, 2,, l,, 2,, Frequency 31 406.29 30 575.87 ..* *.. ... ... P 1.33 a Frequency units are MHz. b 0,defined by Equation 4, refers to exolendo ratios for the dideuterio case, and endo/ exo for the methyl case. C Values of P for the dideuterio case were obtained from a sample used for a 197.0 "C kinetic run. values varied more or less randomly by 3-4% over the period of several weeks. Short term fluctuations (periods of hours) were about half this amount. d Values of for the methyl case are the result of the single microwave determination o n the 206.6 "C sample. +
END0
CH3
Endo
Methyl bicyclopentane
+.
+
+.
-+
-+
T h u s by measuring both linewidths and peak intensities, an experimental ratio proportional to the molecular concentration ratio is obtained. EXPERIMENTAL Sample Handling and Heating. In the present work we shall report on a complete kinetic study of the isomerization of exo-2,3-dideuteriobicyclo[2.1.O]pentaneto endo-2,3-dideuteriobicyclo[2.1.0] pentane as shown in Figure 1. This reaction is a unimolecular isotopic isomerization of the form of Equation 1. In addition, we shall report the results of a single-temperature determination of the rate constant h for the unimolecular isomerization of endo -2-methylbicyclo[2.1.0]pentane to exo- 2-methylbicyclo[2.1.O]pentaneas illustrated in Figure 1.Results for this latter reaction and mechanistic interpretations have previously been reported by Chesick (11). These isomerizations proceed at temperatures of about 200 OC. The gas sample to be studied was contained in a 100-ml sample bulb fitted with a Teflon vacuum stopcock. Heating of the sample was accomplished by immersing the sample bulb in an insulated constant temperature bath whose temperature was regulated to about 1 0 . 5 OC. Absolute temperature measurement was obtained to an accuracy of better than 10.5 "C using a calibrated thermometer. After heating for a specified period of time, the sample bulb was removed from the oil bath and quenched immediately to a temperature below room temperature. Timing errors due to this procedure are well under 0.5 min and are negligible for these kinetic runs which proceed over a period of hours. After quenching, a small amount of sample was withdrawn from the sample bulb for microwave spectrometric analysis at room temperature. After analysis, the sample was returned to the 100-mlbulb, which was then returned to the oil bath for a further increment of heating. High vacuum techniques were used for all gas-handling and gas-transfer operations, and care was taken to maintain clean systems, free of impurities. For the dideuterio isomerization runs, the sample bulb was filled to a pressure of 3.5 Torr at room temperature. Previous work of Chesick ( 1 1 ) indicates that under our experimental conditions, the observed kinetics are for the homogeneous gas-phase reaction in the high-pressure, first-order limit. For the single methyl compound run, the sample bulb was filled to a pressure of 3.0 Torr at room temperature, which is again in the high-pressure region. Microwave Relative Intensity Measurements. Relative intensity measurements were performed using a Hewlett-Packard 8460A microwave spectrometer with sample cell at room temperature (23 "C). For the dideuterio compound, three pairs of rotational transitions were selected for analysis purposes and, for the methyl compound, two pairs were used. These transitions, known from previous microwave studies in our laboratory ( 1 2 , 1 3 ) ,are listed in Table I. For each quenched sample, each pair of analysis lines was scanned over at least once, but in most cases two or three times, in order to determine the baseline and peak absorption positions. These data were displayed on the strip-chart recorder, the standard spectrometer output device. Line intensities were measured from the recorder traces
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
in arbitrary units, and the relative line intensities were computed from these data. For multiple scans of the same pair of analysis lines, average values were computed for the relative intensities. In all cases, amplifier and recorder gains were adjusted so as to minimize system nonlinearities. Using these procedures, relative intensities for any pair of analysis lines were obtained with a reproducibility of 2-3%. As described earlier, the relative intensity measurements must be carried out under conditions for which Equation 4 is satisfied. For each molecule of the two reaction systems of Figure 1,we have selected ground state rotational transitions having identical quantum numbers. Because of the electronic identity of the two dideuterio molecules, this choice essentially guarantees that the linewidth ratio contribution to (3 will be unity, and hence constant as desired. We have tested this conclusion by selected linewidth measurements for the dideuterio species, and have found it to be valid. For the methyl derivatives, the electronic properties are not identical, but they are certainly similar. We have not tested the linewidths in this case, but the results to be presented indicate that Equation 4 is satisfied to a good approximation. Although it turns out to be not critical, all microwave measurements for the dideuterio species were performed at a constant total pressure. Using a Hastings vacuum gauge, the selected pressure was 80 mTorr. Since this gauge is not an absolute pressure gauge, and was not calibrated for these particular molecules, the gauge reading is not correct. Nevertheless, it serves as a convenient means of obtaining a reproducible constant pressure for all measurements. The pressure dependence of the intensity ratios was tested for the dideuterio case, and it was found that the ratios were independent of pressure over the range 50-100 mTorr (Hastings gauge reading). Aside from obvious instrumental adjustments such as gain, etc., the microwave power level is the only other factor needing proper control for the satisfaction of Equation 4. The first necessary requirement is that the measurements be made a t low powers so that the absorption lines are unsaturated, or at least nearly so. Since the spectral transitions were observed at moderately high pressure, it was relatively easy to achieve the necessary low power condition and stili have sufficiently intense absorption lines for analysis purposes. An additional factor making unsaturated conditions readily available is the small electric dipole moment (-0.30 D) for each of the molecules under investigation ( 1 3 , 1 4 ) .For our purposes a suitable criterion for eliminating saturation effects was to simply decrease the power until the intensity ratios became constant. After suitable conditions were found, we simply chose a single crystal current value which could be used for all absorption lines. With the choice of a constant value of the crystal current, and assuming no time variation of crystal diode characteristics, standing wave patterns, etc., it can be safely assumed that the microwave power level for each absorption line is constant during each kinetic run. This method does not, of course, guarantee that the microwave power level is identical for all absorption lines. Indeed, it surely is not, since the transmission characteristics of the microwave system are rather strongly frequency dependent. Nevertheless, the power constancy at each analytical frequency does provide the necessary guarantee that the CJC, factor of Equation 10 will be constant. Thus all the conditions necessary for the validity of Equation 4 are satisfied. Samples. The preparation of the dideuterio sample has been reported previously (12).No significant impurities were present (small amounts of undeuteriated molecules or other inert species would have no effect anyway) so the sample was used without additional purification. This dideuterio mixture was rich in the exo species, and consequently the isomerization reaction proceeded exo endo as indicated in Figure 1. The relative intensity and calibration constant data indicate the molecular concentration ratio to be exolendo = 1.91 =! 0.06. The methyl compound used in this study was a remnant of an investigation performed two years ago in our laboratory (13).Barely 1 mg was available for this kinetic study. We used microwave spectrometry as an analytical tool to check its purity, and found it to be free of any polar molecules. Consequently, this small sample was used without further treatment, also. The available amount was just sufficient to make the single kinetic run reported later. Our relative intensity and calibration data indicated this sample had a molecular concentration ratio of endolexo = 2.23 =! 0.10, and consequently the exo as shown in isomerization proceeded in the direction endo Figure 1. Calibration. As shown by Equation 4, a microwave measurement of R for each pair of analysis lines for any given mixture, and a single independent measurement of a/x for the same mixture, would serve to fix the constant p for each pair of lines. For the methyl compound, Chesick (11) reports a/x = 0.60 at 203 "C for the equilibrium mixture.
-
-
Table 11. Kinetic Data for Dideuterio Isomerization 1 7 7 . 4 'C
187.2 OC
t, min
R
t, min
R
0 30 60 120 180 240 420
2.077b 1.857 1.694 1.454 1.335 1.249 1.142 1.085
0 15 30 45 60 120
2.609 2.312 2.038 1.896
W
p = 1.085
k = 1.02 X 10-4 ( 3 ) ~
1.522 1.337
W
p = 1.337 k
=
2.23 x 10-4 (11) 202.4 O C
197.0 OC
t, min
R
0 10 20 30 40 60 120
2.056 1.697 1.482 1.334 1.191 1.124 1.057 1.042
W
1.810
t, min
0 10
20 30 40 50 00
R
2.011 1.550 1.283 1.183 1.100 1.082 1.053
0 = 1.053 5.64 X 10-4 (44) k = 9.75 x 10-4 ( 8 2 ) a For the 177 "C, 197 "C, and 202 "C runs, the data have been reduced and smoothed as described in the text, using the 2,, + 3,, transition as the reference. For the 187 "C. run, the data are the raw values for the l,, + 2,' transition. Measurements from the other two analysis pairs had to be discarded because of an instrument malfunction, namely, the BWO was not locking a t these frequencies. 0 Experimental uncertainty of ratios is i: 3%. C k has units of s-' . Values in parentheses represent the uncertainties in the final digits. fl = 1.042 k
=
Although there is a slight temperature dependence of (a/x),, we have used this value for our run performed at 206.6 "C. Coupled with our measurements of R,, the p values listed in Table I were obtained. Our kinetic analysis shows that an error of 10%in ( a l x ) , would lead to an error of only 1%in the rate constant and, consequently the slight uncertainty (no more than a few percent) in the 0.60 value has a negligible effect on our results. For the dideuterio case, no independent concentration ratio data were available for any mixture. Although it might have been possible to analyze the initial sample mixture via NMR, a simpler expedient was available. Namely, the value of a l x at equilibrium must be essentially unity for these species differing only in the location of isotopic labels. This follows directly from thermodynamic ideas. AS" for the reaction must be very close to zero, as must AH', and consequently AGO. AH", for example, differs from zero only by an amount equal to the difference in zero point vibrational energies of the two isotopic species. This could amount to no more than a few cm-', since the endo and ero hydrogen force constants must be nearly identical. We conclude that AGO must be within a few calories of zero, and consequently the equilibrium constant, K , is within a few percent of unity. Thus we take a/x = 1.0 at t = -, and p = R , from Equation 4. Table I gives the computed values of (3 for the dideuterio run taken at a temperature of 197 "C. It should be noted that ( a / x ) =should be essentially temperature independent, since A H o is nearly zero. Consequently, (3 should be independent of the temperature of the equilibrium mixture, which agrees with our observations to within experimental errors. Finally, it should be mentioned again that the values of the rate constant k obtained from Equation 5 are quite insensitive to the precise value of p. For example, if p is changed by f5%, the computed rate constants for the dideuterio case change by only f0.3%.
RESULTS Dideuteriobicyclopentane Isomerization. Kinetic runs were performed at four temperatures covering the easily ac- ' cessible range. In Figure 2, we show a typical set of raw data
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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2.2
.
i.8r
Table 111. Kinetic Data for Isomerization of Methyl Compound at 206.6 "C t , min 0 15 30 45 105 135 R 3.475a 1.852 1 . 5 5 0 1.353 1.026 0.986 0.939 0= 1.565 k = 5.07 X lo-" (85)b aExperimenta1 uncertainty of ratios is ~ 3 %b. Units of k are s-', and number in parentheses is the uncertainty in final digits.
I
I I I I I
graphically in Figure 3. The best least squares line is given by 0
20
40
t (HOURS)
6 0
-
Figure 2. Relative intensity ratio vs. time for dideuterio isomerization at 177.4 O C . Ratios are for endo over ex0 using 202 312transitions
(12)
Methylbicyclopentane Isomerization. A single run was made a t a temperature of 206.6 "C. The data from the two analysis line pairs were reduced and averaged in the same manner as described in the preceding section. These results are given in Table I11 along with the best value of k obtained from the data using Equations 8 and 5. DISCUSSION The principal results of our study are embodied in the kinetic relationship of Equation 12 for the dideuteriobicyclopentane isomerization. Before discussing this, however, it is worthwhile comparing the methyl compound rate constant of Table I11 to the value derived from Chesick's (11) more complete study. For this case, Chesick reports k = 1014.45 exp ((-38.9 f 0.8)lRT)
1000 T
Figure 3. Arrhenius plot of dideuterio isomerization. Straight line is the least squares fit of the data points
-
taken a t 177.4 "C using the 202 312 analysis transitions. Rather than presenting all the raw data, Table I1 presents the data for each temperature in a reduced and averaged form, except for the 187 OC run where we present a single raw data set. The reduced data have been obtained ffom the raw data points in the following way. From Equation 4, it is clear that for any sample to be analyzed, the intensity ratio obtained from any pair of analysis lines is related to that for either of the other two pairs by a constant factor. Consequently we have reduced each intensity ratio data point from the 1 2 transition pairs by a constant factor which causes the raw data to superimpose (in an average sense) upon the 2 3 intensity ratio data. The two reduced data sets were then averaged point by point with the 2 3 data set to obtain the values of Table 11. In this way, a single smoothed data set is bbtained a t each temperature, and the procedure tends to smooth out minor instrumental fluctuations which might affect the calibration constant p during the course of a run. For each of the smoothed data sets, we have computed the best average p value using the relationship p = R , as described in the calibration section. Note that the average p values a s listed in Table I1 differ by a few percent from the value given in Table I for the 202 -+ 312 transition, which was obtained from unsmoothed data. Using Equations 8 and 5, each data set was then analyzed to obtain the best value of the rate constant k . These results are presented in Table I1 also. ' The rate constants of Table I1 have been subjected to a least squares fit to the standard Arrhenius equation as shown
-
1512
-
+
which leads to k = 5.28 X s-l a t a temperature of 206.6 "C. Thus, our value of 5.07 X lo-* s-l, while of much lower precision, is in good agreement with Chesick's results. This serves as a good indication of the validity of the general method we have used, and encourages us to believe that it contains no major flaws. Of course, as mentioned earlier, we were working with an extremely small quantity of sample and were able to perform only one run. A more careful microwave study of this isomerization would require that linewidth measurements be made in order to check the validity of Equation 4. Turning to our dideuterio results of Equation 12, it is seen that they are very similar t o those of Chesick for the methylbicyclopentane isomerization. Our precision for the activation energy is only modestly poorer than that reported by Chesick, who used standard VPC methqds of analysis. The near identity of the Arrhenius activation parameters for the two systems is not surprising, since the methyl group should have little influence upon the activation process, which involves primarily a breaking (partial or complete) of the bridgehead bond (11).In fact, out results may be taken to imply that the methyl group plays no electronic or steric role in the process. The principal aim of this work has been to show that relatively slow gas-phase isomerizations (involving reactants and products differing only in location of isotopic labels) are ideally suited for study by microwave techniques. The high spectroscopic resolution makes it possible to study isolated spectral lines of each species with trivial ease. More standard analytical methods such as VPC, NMR, IR, etc., would require extreme sophistication, or would fail entirely for many systems of this type. The chief problem with the microwave method, viz., measurement of molecular concentrations, has been shown to be a tractable one without resorting to extreme instrumental
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
sophistication. We believe that the relative intensity measurements, and consequently the rate constants, can be improved upon in certain cases. One factor of great importance is the intensity of the microwave absorption lines being used as the composition probes. In the present case, the spectral lines are all very weak because of the small electric dipole moments (-0.3 D in all cases) (14).Consequently, the signal-to-noise ( S I N )ratio is intrinsically rather poor, and thus the precision of the peak intensity ratios is not as high as it might be. For molecular systems having larger dipole moments, it should be possible t o obtain a considerable improvement in this key factor.
(4) C. W. Gillies and R. L. Kuczkowski, J. Am. Chem. Soc., 94,6337 (1972). (5) H. W. Harrington, J. Chem. fhys., 46, 3698 (1967). (6) H. W. Harrington, J. Chem. fhys., 49, 3023 (1968). (7) A. S. Esbitt and E. B. Wilson, Rev. Sci. Instrum., 34, 901 (1963). (8) H. W. Harrington, J. Ch8m. fhys., 44, 3481 (1966). (9) See, for example, K. J. Laidler, "Chemical Kinetics", McGraw-Hill, New York, 1965, pp 19-20. (10) See, for example, Ref. ( I ) , Chapter 3. (11) J. P. Chesick, J. Am. Chem. SOC., 84, 3250 (1962). (12) S. N. Mathur, M. D.Harmony, and R. D.Suenram, J. Chem. fhys., 64,4340 (1976). (13) M. D.Harmony, C. S. Wang, K. B. Wiberg, and K. C. Bishop Ill, J. Chem. fhys., 63, 3312 (1975). (14) R. D. Suenram and M.D. Harmony, J. Chem. fhys., 56,3837 (1972).
LITERATURE CITED
RECEIVEDfor review April 16,1976. Accepted June 8,1976. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation (Grant MPS 74-22178) for support of this research.
(1) W. Gordy and R . L. Cook, "Microwave Molecular Spectra", Wiley-lnterscience, New York, 1970. (2) L. H. Scharpen and V. W. Laurie, Anal. Chem., 44, 378R (1972). (3) L. H. Scharpen, R. F. Rauskalb, and C. A. Tolman, Anal. Chem., 44,2010 (1972).
Direct Optical Encoding of Recorded Spectra with a Computer Interfaced Vidicon Television Camera J. A. de Haseth, W. S. Woodward, and 1.L. Isenhour" Department of Chemistry, Univeristy of North Carolina, Chapel Hill, N.C. 275 14
A vidicon tube television camera interfaced to a Raytheon 704 minicomputer enables the direct optical encoding of any spectral curve, provided the curve be a single-valued function of the abscissa value. The appllcation presented is the digitization of Infrared spectra. The algorithm is sufflciently flexible that it Is not iimlted by spectral range or physical size of the hardcopy medium being encoded. The encoder is accurate and able to digitize approximately 35 spectra per hour when operating in batch mode.
For the studies relating to the use of large chemical data bases, it has become clear that the acquisition of these data files has not always been easy. One such series of compilations is infrared spectra. Various research ideas have depended on large infrared data files such as studies involving pattern recognition (1-7), and search systems (8-27). Originally collections of these data bases were kept on coded index cards, but this form of data processing proved unwieldy. Emphasis then shifted t o converting the files into a computer manageable format, but the conversion of a spectrum from an analog representation to a computer readable format has thus far been a tedious and expensive process. Spectra are available in digitized formats such as the binary file of 135 000 spectra assembled by the American Society for Testing and Materials. However, this file is available only in a peak-no-peak format and may therefore be unsuitable for some applications. Other files may be expensive or difficult to obtain. Therefore, many workers compile their own files. There are several methods of obtaining digitized spectra. One commonly employed method is t o directly interface the spectrometer to a computer. Such interfacing can now be done routinely and is neither difficult nor expensive. Often a disadvantage to this approach is the extent to which the computer is devoted t o the spectrometer and thus unavailable during the recording of a spectrum. This can lead to serious computer-time losses if a high-resolution spectrum of several
hours run-time is being recorded. The computer-time loss problem can be resolved to some degree if an intermediate device such as a paper tape punch with an analog-to-digital converter is used. This gives a computer readable format for the spectrum, but the digitization may still be time consuming. Of course Fourier transform infrared (FT/IR) spectrometers are available with their own processors but few give output in any format other than the absorption curve of the sample or its Fourier transform. A major drawback with these methods is that the spectrum must be obtained in its digitized form while being recorded. Thus, to obtain the digitized form of a spectrum that was recorded a t some time in the past requires that the experiment be rerun and digitized. This approach may be difficult if the sample is unstable, in short supply, unavailable, or expensive. Another method that is commonly used is to digitize the spectrum manually. This involves reading the intensity a t each wavelength position and transcribing its value onto some form of input medium such as paper tape or computer cards. This is expensive and tedious to say the least. This paper presents an alternate method of digitizing spectra that is fast, efficient, and can be obtained from graphical representations of the spectra. A computer interfaced vidicon tube television camera is employed, a peripheral which permits the direct optical encoding and computer digitizing of curves. The apparatus has been described previously (28);however, some modification have been made to the computer such as the addition of 24K words of core. The only modification made to the equipment expressly for this project was the introduction of an inexpensive electronic bandpass filter to the TV camera-Raytheon 704 interface to enhance fine spectral lines. Vidicon tubes have recently been applied to several other chemical problems, mainly as spectrometric detectors (29-49) and as image encoders for bubble chambers (50-52). The camera is a relatively inexpensive piece of apparatus. The Panasonic WV-2007 vidicon tube television camera used for the interface cost only $220.
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