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J. Phys. Chem. 1981, 85,2938-2941
Surface Electric Discharge during Sample Conqensation. A New Method for Producing and Trapping Molecular Ions Benuel J. Kelsall and Lester Andrews" Chemistry Department, University of Virginia, Chariottesville, Virginia 2290 1 (Received: February 25, I98 1; In Final Form: May 5, 198 1)
A surface electric discharge technique has been developed for proaucing and trapping molecular ions in solid argon. This new method is compared to recent matrix photoionization experiments with CH2F2,CHF3,and CFI. An unusually large yield of CF3+made possible identification of the v1 + v4 combination band of this carbocation.
Introduction block. This was used to support the anode and to reduce the amount of gas condensing on the cathode; the distance There has been considerable recent research activity on between the cathode and the anode was -1 cm. The the production and characterization of molecular ions by applied potential, which could be varied from 0 to 4000 matrix isolation techniques.l A variety of ions has been V, was generated by a dc power supply (Harrison Laboproduced by proton radiolysis and photoionization methods and trapped in solid argon for spectroscopic s t ~ d y . ~ - ~ratories, Model 6525A). Methylene fluoride (Du Pont), fluoroform (PCR Research Chemicals, Inc.), and tetraIn the photoionization experiments, argon is excited by a fluoromethane (Du Pont) were condensed at 77 K in a cold microwave discharge while flowing through a quartz disfinger and distilled to remove volatile impurities. CDF3 charge tube and then condensed on a suitable optical was synthesized from CDC13 as described previously.6 window with argon containing a reagent gasS4l5Although Argon (Air Products, 99.995%) was used without further this method is very convenient for producing ions, it is purification. limited to ions which can be formed with 11.6-11.8-eV Argon matrix gas/reagent gas mixtures of 400/1 were argon resonance radiation. The proton radiolysis technique deposited at 2.2 f 0.4 mmol/h with argon flowing through involves bombardment of a forming matrix with 2.0-keV a quartz discharge tube at 1.4 f 0.1 mmol/h. The gases protons to produce ions and radicals from precursor were condensed on a 10-20 f 1 K CsI substrate for 8-18 molecule^.^^^ The usefulness of the radiolysis method is h. During successful electric discharge experiments, the limited because it tends to preferentially produce radicals matrix emitted a yellowish green to bluish green glow. which have absorptions that can interfere with ion abInfrared spectra were recorded between 400 and 4000 cm-l sorptions. at 0.5-cm-l resolution with a Nicolet 7199 FT-IR specIn our continuing effort to explore the spectral properties trophotometer. Following sample preparation, the trapped of molecular ions, we have developed a new technique for species were subjected to filtered radiation of a highproducing ions involving an electric discharge on the pressure mercury arc (BH6-lB, 1000 W, Illumination Insurface of the forming matrix. This new technique has dustries, Inc.) or to thermal diffusion brought about by the been applied to CH2F2,CHF3, and CF4, which have all slow warming of the matrix substrate. been used as precursor molecules in argon matrix photoionization and proton radiolysis experiments. This paper Results contrasts the similarities and differences between the Several experiments were performed with methylene photoionization and electric discharge techniques, and it fluoride to determine the effect of a strong electric field also reports the identification of the heretofore undetected on the isolation of ions in an argon matrix. The first study combination band (vl + v4) for the 12CF3+ion. This finding showed that, with argon and sample condensing on the enables a reliable prediction of the v1 mode of CF3+to be window and electrodes and voltages between 2 kV and 30 made and, in conjunction with the v3 mode,6 provides a V, no current flowed between the two electrodes until the complete characterization of the C-F stretching modes of microwave-powered argon discharge in the quartz disthe CF3+carbocation. charge tube4was initiated. After current flow started, it continued giving the matrix a greenish glow, even after the Experimental Section argon discharge was turned off. With voltages greater than The cyrogenic apparatus, the vacuum system, and the 2 kV, current flow could be initiated without the assistance microwave discharge photoionization sources have been of an argon discharge. Applied potentials below 30 V described previously.H The electric discharge apparatus produced no measurable current under any conditions. consisted of a stainless-steel anode (20 X 3 X 0.6 mm), Figure 1 shows infrared spectra in the 1450-1200-~m-~ which was placed in contact with the front face of the CsI region obtained for three different matrix samples conmatrix support, and an indium and copper cathode which taining methylene fluoride. Trace a illustrates the specalso served to support the CsI plate. A Teflon mask with trum of a sample prepared in the presence of argon resoa 20 X 25 mm rectangular opening was bolted to the copper nance radiation and no electric field, trace b is the spectrum of a different sample prepared with a 100-V electric (1) Andrews, L. Annu. Rev. Phys. Chem. 1979, 30, 79. discharge between the electrodes, and trace c is the (2) Andrews, L.; Grzybowski, J. M.; Allen, R. 0. J. Phys. Chem. 1975, spectrum of another sample prepared with a 500-V electric 79,904. (3) Ault, B. S.; Andrews, L. J. Chem. Phys. 1975,63, 1411. discharge. The samples for the latter two spectra were (4) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1977, 67, 1091. prepared in the absence of argon resonance radiation from (5) Andrews, L.; Tevault, D. E.; Smardzewski,R. R. Appl. Spectrosc. the microwave discharge. The absorbances of the principal 1978, 32, 157. absorptions shown in Figure 1 are summarized in Table (6) Prochaska, F. T.;Andrews, L. J. Am. Chem. SOC.1978,100,2101. 0022-3654/81/2085-2938$01.25/0
@ 1981 American Chemical Society
Surface Electric Discharge and Sample Condensation
The Journal of Physical Chemistry, Vol. 85, No. 20, 198 1 2939
TABLE 11: Product Absorptions Observed from an Argon Matrix Sample Containing Fluoroform (Ar/CHF, = 700/1) Prepared at 1 5 K with 350-V Electric Discharge v,
cm-'
absorbance
510 562 567 603 700 768 776 803 954 967 984 1040 1085 1102 1134 1146 1166 1173
0.15 0.10 0.12 0.13 0.40 0.12 0.10 0.02 0.08 0.05 0.05 0.22 0.7 1.6 1.4 >2 1.4 1.3
assigna
v,
cm-l
absorbance assigna
CHF, A A
1211 1221 1250 A 1260 1274 CHF, OCF, 1280 OCF, 1316 ? 1330 ? 1377 OCF, 1389 ? 1587 1594 0, CF, 1608 CF, 1611 CHF, 1624 CHF, 1665 A,CHF, 3555 A,CHF, 3596
0.6 1.0 >2 1.0 >2 0.5' 0.15 0.15 1.5 0.14 0.08 0.10 0.03 0.02 0.35' 0.9 0.3 0.5
CHF, CF, CF, CF, CF, A CHF, C,F, CHF, HO, CF,+ H,O CHF; H,O CF,+ CF,+ A A
a A = intramolecular, hydrogen-bonded, electron-capture product anion FH-CF, - ;see ref 9. Unresolved shoulder. ' Similar experiments with other precursors predict that ca. A = 0.1 of this band is probably due to H,O based on the 1594-and 1611-cm-' band absorbances observed in this experiment.
'
F4
1300
1400
1200
WAV ENUMBE R s (crn -1)
Flgure 1. Infrared spectra in the region between 1450 and 1200 cm-' for three different argon matrix samples containing methylene fluoride: (a) Ar/CH,F, = 600/1 prepared at 21 f 1 K with microwave-induced discharge; (b) Ar/CH,F, = 600/1 prepared at 11 f 1 K with 100-V electric arc discharge; (c) Ar/CH,F, = 75011 prepared at 21 f 1 K with 5004 electric arc discharge.
TABLE I: Product Absorptions in Photoionization and Electric Discharge Argon Matrix Experiments with Methylene Fluoride absorbance cm-'
a'
bb
CC
assign
1222 1246 1253 1255 1260 1274 1280 1317 1330 1368 1408 1437
0.26 0.29 0.32 0.24
0.79 0.29 0.27 0.23
0.08
0.02 0.00 0.18 0.05 0.02 0.06 0.34
0.05 0.19 0.42 0.21 0.02 0.08 0.40
CF, CH,FZt CH,F,+ CH,F,+ CF, CF, CF CHF, C,F, CHF' CH,F,+ CH,F,
V,
0.12 1.45 0.03 0.10 0.04
0.01 From Figure l a , Ar/CH,F, = 600/1prepared at 21 K with argon resonance photoionization. From Figure 1 b, Ar/CH,F, = 600/1prepared at 11 K with 100-Velectric arc discharge. From Figure IC,Ar/CH,F, = 750/1prepared at 21 K with 500-Velectric discharge.
I along with assignments given in an earlier study; the 1000-1200-cm-1 region contained strong precursor absorptions and the stronger CHF2radical bands at 1175 and (7) Andrews, L.; Prochaska, F. T. J. Chem. Phys. 1979, 70, 4714.
1165 cm-'. The absorptions in traces a and b correspond well with each other. The intensities of the parent ion CH2F2+were comparable; however, several neutral product intensities were increased 2-4-fold and the 1280-cm-' CF radical8 band was markedly increased. A higher voltage discharge produced primarily the stable CF4 product, as shown in trace c. Destruction of the 1246-, 1253-, 1 2 5 , and 1408-cm-l bands by Pyrex-filtered mercury arc radiation (290-1000 nm) in both the microwave and electric discharge experiments confirms the assignment of these bands to the same species, which has been identified as CH2F2+.' In another experiment, ions produced by argon resonance radiation were allowed to diffuse in the presence of a l-kV electric potential. In this experiment the electric field did not appear to accelerate or decelerate the rate at which the ion absorptions decreased with diffusion, compared to neutral fragments. Six electric arc experiments with CHF3diluted by matrix gas conducted with applied potentials between 100 and 800 V produced isolated species analogous to those formed by argon resonance r a d i a t i ~ n . ~The ~ ~ infrared absorption frequencies, absorbances, and assignments for these species are given in Table I1 for the 350-V experiment; in general, the lowest voltage used produced fewest products, the highest voltage favored stable products like CF4, and intermediate voltages gave the maximum yield of desired radical and cation products. The table shows that both cations and anions were produced and trapped, although the relative intensities of the two molecular anion FH-CFf matrix sitesg depended upon matrix preparation. The major electron-trapping species in these experiments, believed to be the fluoride anion, could not be observed. In the electric discharge experiments, CF3+was produced in much greater yield than in the photoionization experiments. Figure 2 shows the infrared spectrum between 1680 and 1560 cm-l for the best experiment using the electric arc. The very intense band at 1665.2 cm-l and the ac(8) Jacox, M. E.; Milligan, D. E. J. Chen. Phys. 1969, 50, 3252. (9) Andrews, L.; Prochaska, F. T. J. Phys. Chern. 1979,83,824.
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The Journal of Physical Chemistry, Vol. 85, No. 20, 198 1
0
Ln
0
l6bO
'
1640 ' 16b0 WQVENUMBERS
'
1560
Flgure 2. Infrared spectrum In the region between 1680 and 1560 cm-' for an argon matrlx sample containing CHF, (Ar/CHF, = 700/1) prepared at 15 K with 3 5 0 4 electric arc discharge.
companying shoulder at 1662 cm-l have been assigned to CF3+.s The moderately intense band with splittings at 1625,1624, and 1622 cm-' decreased at the same rate as the 1665-cm-' band (20% in 2 h) when the sample was exposed to the full radiation of a mercury arc (220-1000 nm). The very weak band at 1643 cm-' is due to CHF3. The weak bands near 1608 and 1606 cm-l have been assigned to CHF2+,and the two bands near 1587 and 1584 cm-l have been tentatively assigned to CF2+.677The weak absorptions detected near 1611,1600,1594, and 1590 cm-' are due to water; a small part of the 1624-cm-' product absorption is probably due to the well-known 1624-cm-' HzO absorption in solid argon. The remaining very weak absorptions showed no change in intensity upon exposure to the full radiation of a mercury arc and could not be associated with any other identified species detected in this experiment. Fluoroform-d was used as the reagent molecule in an electric discharge experiment. Despite very careful monitoring and controlliig of the experimental conditions, very little CF3+or other fragmentation products were detected in the infrared spectrum. This failure may be due to a kinetic isotope effect for dissociating the C-D bond. When CF4 was used as the reagent gas in electric arc discharge experiments with argon, no fragmentation products or ions were detected from the parent reagent. A weak band ( A = 0.07) was, however, detected at 954 cm-l. This band probably belongs to the same species as the weak unidentified 954-cm-' band detected in the CHF3 experiments, since in all experiments the 954-cm-' band readily decreased with mercury arc photolysis. Discussion The electric arc discharge under conditons of moderate voltages can produce products which are analogous to argon resonance photolysis products while electric arc discharges under high voltage conditions tend to give only stable products formed from molecular fragments. Before attempting to deduce the mechanism of the electric discharge experiments, it is important to note that the electric arc experiments were unable to produce a significant yield
Kelsall and Andrews
of products which could not be produced by argon resonance radiation. Consequently, any mechanism must account for this apparent limitation. If the electric arc discharge involved electron impact to ionize the reagent gas, as in a mass spectrometer, the limiting factor would be the output voltage of the power supply. This is not the case, however. An alternative mechanism can involve an electron beam to excite and/or ionize the excess argon which then transfers this excess energy either by collision, such as Penning ionization, or by radiative pathway to the reagent. Since the argon serves as a temporary storage for energy, the reagent molecule can never receive more energy than the argon can store. As a result, the electric arc discharge has the same energy limitations as the microwave-induced discharge and cannot be expected to produce any new species. The electric arc discharge method does provide an advantage over the microwave discharge method in that it generates reactive argon species in the vicinity of the reagent molecules. This can result in a higher yield of products, but it can also result in a different partition ratio of products. The latter occurs if the reactive argon species are produced in sufficient numbers for a primary product to undergo a second reaction to form another product. The result of continued reaction eventually leads to stable products; evidence for this is given by the high-voltage electric arc discharge experiments involving CH2Fzwhich produce almost exclusively CF4, a species stable to argon resonance radiation. The diffusion experiment involving matrix warming in the presence of an applied electric field failed to give new evidence to support the assignment of the ion bands detected in these experiments. This may be due to two factors. The first is that the local electric field at the ion sites in the matrix is too weak to enhance ion migration, and the second is that ion reaction involves an uncharged atom or small molecule which can diffuse through the matrix at a lower temperature than a bulky ion but would not be affected by the electric field. In the CHF3 electric arc discharge experiment, the 1624-cm-' band and its satellites are of particular interest since these bands exhibited the same photolysis behavior as the 1665-cm-', CF3+band. On the basis of the Fermi resonance splitting of the v3 and v1 + v4 bands for 13CF3+, the vl + u4 combination band for WF3+was predictede to absorb radiation near 1625 cm-l. Since the electric arc discharge produced CF3+in higher yield than the microwave discharge, the 1624-cm-' combination band was observed without significant interference from water absorptions. No evidence was found for the vl, uz, or u4 absorptions of CF3+. Unfortunately, very intense absorptions in these regions prevent any possible observation of the weaker CF3+bands; the v1 mode is, of course, not infrared active for this presumably trigonal planar species. The v4 fundamental may be predicted at 500 f 30 cm-l. With this reasonable estimate of v4, the 1624-cm-' combination band reveals v1 = 1124 f 30 cm-l. The CF3+ carbocation exhibits substantially higher bond-stretching fundamentals (ul = 1124 f 30 cm-l, u3 = 1665 cm-l) than the pyramidal CF3 radical (ul = 1087 cm-l, v3 = 1251 cm-', solid argon)'O and the planar "BF3 molecule (vl = 888 an-', v3 = 1454 cm-l, gas phase)." The high stretching fundamentals for CF3+attest to substantial T-T bonding in this cation. It would be of considerable interest to compare the 1665,1662-cm-' argon matrix fundamental for CF3+to the (10) Milligan, D. E.; Jacox, M. E. J. Chern. Phys. 1968, 48, 2265. (11) Vanderryn, J. J. Chern. Phys. 1959, 30, 331. Dows, D.A. Ibid. 1959, 31, 1637.
J. Phys. Chem. 1981, 85,2941-2946
gas-phase value in order to ascertain the effect of matrix solvation on the vibrational potential function. Gas-phase data, of course, are not presently available. Observation of possible krypton matrix counterparts in a krypton resonance photoionization experiment at 1660 and 1655 cm-’ (A = 0.01) does, however, provide some information. For the carbocations CF3+and CHBr2+,the argon-krypton matrix differences of 5-10 cm-l are only slightly larger than the argon-krypton matrix differences of 3-4 cm-I for the CF3 and CHBr2free radicals.lJ2J3 This suggests that the gas-argon difference should be on the order of 10 cm-’ for ions like CF3+and CHBr2+. Some support for this argument is found in the gas-phase measurement of vl and v3 for the CF3 radical at 1090 and 1259 f 2 cm-l, respectively,14only 3-8 cm-l above the argon matrix values and 6-11 cm-I above the krypton matrix values,12and in the molecule isoelectronic with CF3+,namely, BF3, which exhibits boron isotopic v3 fundamentals in the gas phase only (12)Andrews, L.;Keelan, B. W., unpublished results, 1979. v1 and v3 of CF, in solid krypton at 1084 and 1248 f 1 cm-’. (13)Andrew, L.;Prochaska, F. T.; Ault, B. S. J. Am. Chern. SOC.1979, 101,s. (14)Carlson, G.L.;Pimentel, G. C. J. Chern. Phys. 1966,44, 4053.
2941
7 cm-’ above the argon matrix values.”Js This leads to a prediction that v3 of CF3+will absorb near 1675 f 10 cm-’ in the gas phase.
Conclusions This study has developed an electric arc discharge technique for generating unstable ions and radicals for matrix isolation investigations. A comparison between the electric arc technique and argon resonance photolysis shows that, although the two methods are similar in most respects, the former has a greater range of intermediary products which it can produce. As a result of the ability of this new technique to produce higher yields of certain products under suitable conditons, the v1 + v4 combination band for 12CF3+at 1624 cm-l was identified for the first time. Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation under Grant CHE 79-10966. (15)Miller, J. H.; Andrews, L., unpublished results, 1978. v3 of ‘”BFs = 1498 cm-’ and uS of “BF3 = 1447 cm-’ in solid argon from expanded
scale spectra.
Better Estimates of Exponential Decay Parameters Peter Hall*+ and Ben Selinger’ The Facukies, Australian National Universw, Canberra, Australia (Received April 2, 198 1)
For simple exponential decay counting data collected in a finite number of channels, the maximum likelihood estimator (MLE) provides the simplest and, in an asymptotic sense, equal best (with least squares) estimator of the lifetime parameter. Both methods are more efficient, in a statistical sense, than the method of moments. The MLE algorithm is so simple it can be performed graphically or on a hand calculator. It could be hard-wired (ROM) into data collection units. A minimization procedure is unnecessary. We provide an expression for obtaining the optimal channel width (TAC range) for data collection.
Introduction While chemists quickly adopt the most recent instrumental innovations for producing chemical data, the methods of analysis of that data have remained traditional and lack new statistical input.lV2 Standard parametric techniques were adopted and have remained unchallenged. As these methods become hardwired into our calculators, there will be even less chance of having something better adopted. It is not generally realized that all of the standard methods of estimation (e.g., maximum likelihood and least squares) are ad There are statistical theories which justify their use for large samples, but for small samples there are very few theoretical criteria for assessing estimators. The reason for this is that in small samples the distribution of the estimators is usually not normal and then there may not be an agreed method of comparing them. In this paper we shall look at one particular model common to several areas of chemistry and the various methods of fitting data to it. The model is for a first-order process which follows an exponential decay law, be it in kinetics or in the area that we are particularly concerned with, Department of Statistics. Department of Chemistry. 0022-365418112085-2941$01.25/0
single photon decay spectroscopy. It is perhaps not fully appreciated that, where an experimenter wishes to estimate a parameter 0 for a model for_which_he has data, he has in fact a choice. Suppose 01, d2, ...,On are different estimators which yield different numerical values. Which one is “best”? Sometimes the only way to give anything like a definite answer is to examine the asymptotic (i-e.,large-sample) properties of the estimators. Thus, if n denotes the sample size and n1I2(ai - 0 ) has a bmiting normal N(p,a;) distribution for 1 Ii 5 n , then Bi is “better” than 0, if ai2 < a?. However, an estimator which performs “well” for large samples can behave erratically with more realistic small samples. Moreover, two estimators which appear to be computed in very different ways from the sample values can be asymptotically equivalent. We shall give an example of this phenomenon. Again, for realistic small samples, their behavior need not be equivalent. There is a very large statistical literature on choosing the “best” e ~ t i m a t o r . The ~ various estimators presently (1)A. E. W. Knight and B. K. Selinger, Spectrochirn. Acta, Part A , 27, 1223 (1971). (2)A. E. W.Knight and B. K. Selinger, Aust. J. Chern., 26,l (1973). (3)P. R. Bevington, “Data Reduction and Error Analysis for the Physical Sciences”, McGraw Hill, New York, 1969.
0 1981 American Chemical Society
