6781
J. Phys. Chem. 1991, 95,6781-6783
Nonstatistlcal Charged Fragments Dlstrlbutlon In a Coulomb Explosion following a Site-Selective Ionizatlon I. Salman, J. Silberstein, and R. D. Levine* The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91 904, Israel (Received: January 14, 1991)
Computed fragmentation patterns of CH3CF3in the experiment of Habenicht et al. (preceding paper) are reported. Fully statistical computations, subject only to conservation of energy, matter, and charge, fail to reproduce the experimental results. Introducing a site-specific constraint that distinguishes the two carbon atoms leads to a better but not complete agreement with experiment. Results are also presented for a site-selective resonant excitation to a Rydberg level. The parent ion is then singly charged, and the observed fragmentation pattern is more nonstatisticalthan for the doubly charged ("Coulomb explosion") parent ion. The highest excess energy (-45 eV) of the parent ion (and the best agreement with experiment) is for the site-selective ionization from the Is orbital localized at the fluorine-carryingcarbon atom.
I. Introduction The novel experimentsof Habenicht et al.'J suggest that specific fragmentation of a molecule can be achieved for a site-selective e~citation.~In many ways the experimental results speak for themselves: Ionization from the 1s orbital localized on either one of the two distinguishable carbon atoms in CH3CF3 leads to distinct fragmentation patterns. Since such patterns are known to depend on the mean energy content of the parent ion, a more detailed analysis is also warranted. As a preliminary step in this dimtion, we present in this paper computational results using both a fully statistical formalism and one where sitespecific constraints are included. Our results support the experimental conclusion' that the fragmentation pattern reflects the selectivity of the initial excitation. The fully statistical approachc6 to mass spectral fragmentation patterns has been primarily applied to site nonselective ionization processes. These include multiphoton'J and electron impact i~nization.~Acceptable agreement with experiment was possible,SJOparticularly so for aromatic compounds. Exceptions could be traced to the presence of mechanistic constraints11J2or to narrow distributions of the excitation energy in the parent ion.9 The latter is typical of electron impact ionization and the former of alkyl and other saturated compounds. Either aspect could however be handled by a suitable modification of the fully statistical limit. The only example of siteselectiveexcitation processes which was previously studied13is the multiphoton ionization of van der Waals d i m e r ~ . ' ~ JFor ~ the case of the toluene dimer" including a site-specific constraint was sufficient to obtain an acceptable agreement. The present problem is more complicated (1) Habcnicht, W.; Baiter, H.; MOller-Dethlcfs,K.; Schlag, E. W. J. fhys. Chem., preceding paper in this issue. (2) (a) Habcnicht, W.; Chewter, L. A.; Sander, M.; MOller-Dethlcfs, K.; Schlag, E. W. J. fhys., Collq. 1987, 0 , 7 4 1 . (b) Habcnicht, W.; MOllerDethlefa, K.; Schlag, E. W. J . Electron Specrrosc. 1990, 52. 697. (3) MOller-Dethlefs, K.; Sander, M.; Chewter, L. A,; Schlag, E. W. J . fhys. Chem. 1984,88,6098. (4) Silbcrstein, J.; Levine, R. D. Chem. fhys. Lcrf. 1980, 74, 6. (5) S i l b t e i n , J.; Levine, R. D. J. Chem. Phys. 1981, 75, 5735. (6) Ohmichi, N.; Silbemtein, J.; Levine, R. D. fsr. J. Chem. 1984,24, 245. (7) Bernstein, R. B. J. fhys. Chem. 1982, 86, 1178. (8) Schlag, E. W.; Neusser, H. J. Acc. Chem. Res. 1983, 16, 355, (9) Silkratein, J.; Levine, R. D. J. Am. Chem. Soc. 1985, 107, 8283. (IO) Lichtin, D. A.; Bernstein, R. B.; Newton, K. R. J . Chem. fhys. 1981, 75. 5728. (1 1) Kuhlewind, H.; Neusser, H. J.; Schlag, E. W. J . fhys. Chem. 1985, 89, 5901. (12) Silbcntein, J.; Ohmichi, N.; Levine, R. D. J . fhys. Chem. 1985,89, 5606. (13) Silbentcin, J.; Ohmichi, N.; Levine, R. D. J. fhys. Chem, 1984,88,
4952. (14) quire, D. W.; Bernstein, R. 8.J . fhys. Chem. 1984, 88, 4948. (15) (a) Bbrren, K. 0.;Lin, S. H.; Selzle, H. L.; Schlag, E. W. J . Chem. fhys. 1% 90, 1299. (b) Kiermeicr, A.; Ernstbergcr, B.; Neuaacr, H. J.; Schlag, E. W. J . Phys. Chem. 1988, 92,3785.
since the excitation is site-selective for a molecule whose fragmentation pattern is non fully statistical even for a nonselective excitation. To ensure a good match with the observed pattern, we would therefore need to include both mechanistic and sitespecific constraints. We chose not to do so, in the interest of obtaining sharper conclusions but at the price of failing to obtain an accurate reproduction of the observed pattern. What we show is primarily that (i) the observed' fragmentation is not fully statistical, (ii) inclusion of a single site-specific constraint significantly improves the agreement with experiment, and (iii) doubly charged parent ions have a more statistical fragmentation pattern. We report results for CH3CF3. Two distinct site-selective experiments are discussed. One (the "continuum excitation") is where a localized carbon 1s electron is ionized. An Auger process then leads to a doubly charged parent ion. The other ("resonant excitation") is where a localized carbon 1s electron is excited to a Rydberg level. The Auger process then leads to a singly charged, Rydberg state, parent ion. Altogether there are four possible such experiments using CH3CF3(i.e., either type. of excitation on either one of the two distinguishable carbon atoms). For each one of the four experiments we carried out two types of computations. One is the fully statistical one, and the other includes a single sitespecific constraint. Each one of such eight computations was carried out either in a predictive way, using no experimental input, or as a fit to the experimental fragmentation pattern (the mcalled surprisal analysis). All in all, we have four computed patterns for each experiment on CH3CF3or a grand total of 16 computations. We shall report only a brief summary of the results. A table giving the detailed results for all 16 computations is available from us upon request. Section I1 provides the essential points of the method and explains the technical difference between a predictive computation and a fit to experiment. Section I11 gives the main results. A discussion and summary conclude the paper. 11. Computational Methods
In a fully statistical theory the fragmentation pattern is given by4" Here XIis the number of fragments of chemical type j . j enumerates both charged and neutral fragments. X
x = 9x1 is the total number of fragments. X i s not known a priori and is computed by the theory. Q, is the usual canonical partition function of species j , referred to as a zero of energy common to all species present. The "temperature" T at which all the Q 's are to be computed is determined by the theory in terms of the
0022-3654/91/2095-6781 $02.50/0 Q 199 1 American Chemical Society
6782 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
fragments. Neither X nor aojcan be so determined. But, by H minimi~ing’.~ H = w j c ” P In v;cXP/fj) (6)
TABLE I: fhnuce& of Tkrmoekmicrl Data ion CJHJ3f C2H3F2 CZHJp+
heat of formation 16 26 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17
C2H2F3
CZHZF2’ CZHZF+ C2HF2” C2HP CZF3’ CZF2’ CHZF2’ CHIP CHF,’ CHF2’ CHF’ CF3’ CF2’ CP
vibrational frequencies 23, 28 24, 28 22 24, 28 27 25 21
moment of inertia 23 24 19 24 19 b 17
d
C
17 17
17 17
d
c
17
17 b 17 17 17
b 17 17 17 17
J
where fi””’ is the experimental fraction, one can determine the yk’s (k = 1, ..., K ) and therebyh. Note that for CH3CF3three Lagrange multipliers are fitted to the observed fragmentation pattern, in order to reproduce all of it. Since H has a unique global minimum,20the fitting procedure is guaranteed to be at least as good as, if not better than, the predictive procedure. If there is a “memory”’ of the initial site-selective excitation, the chemical potential of the two carbon atoms will be different. Our site-specific constraint is thus to assign to the carbon atom that is the site of the excitation its own, yC., chemical potential. Equation 3 is thus of the form
C
17
K+ I
= ca]krk
ffj
17
OOther input not shown is from refs 17 and 29. bThe input for CzHF2+,C2P, CHF2+,and C P was computed by MINDO version 5. rCalculated from structure. fEstimated.
mean energy, (E), of the parent ion. To compute the partition function, we require the heat of formation and the relevant spectroscopic information for each and every possible ionic and neutral fragment. For many of the fragments of CH3CF3this information is not available from the standard sources.1619 A list of sources for different ions is given in Table I. In addition, for several ions we performed quantum chemical calculations at the MINDO version 5 level, with empirical scaling. aj is the Planck chemical potential of species j
(3) Here a]!, j = 1, ..., K,is the number of atoms of type k in the species]. K = 3 for CH3CF3in the fully statistical computation. a is the charge on speciesj (zero, one, or two). The so-called “fagrange multipliers”, yk(s, are the chemical potentials of the elements. These are computed’ from the conservation of elements and of charge via the implicit equations c k
Fakjxj,
k
0, 1,
..., K
Salman et al.
(4)
Here c&is the number of atoms of type k (or of the charge) in the parent ion. Note that the Xis are extensive variables. If we multiply the Cis by N (e.g., Avogadro’s number), all the Xis scale by the same factor N. So far we have described the fully statistical procedure. Given the chemical identity of the parent (e.g., CZH3F3+,Co = 1, CC = 2, C, = 3, C, = 3) and its mean excess energy, (E), one can compute the fragmentation pattern. This route is fully predictive albeit the predictions may fail to agree with experiment. Rather than compute the 72s one can determine them by a fit to experiment. In a typical mass spectrometric setup one does not detect the neutrals nor is the number of parent ions known. What is measured is the relative fractionh of the ionic fragments
Here the prime on the sum denotes inclusion only of the charged (16) Lias, S.G.; Bartmers, J. E.; Liebman, J. F.; Holmes, J. L.; Levine, R. D.; Mallard, W. G. Gas Phase Ion and Neutral Thermochemistry; American Chemical Society: Washington, DC, 1988. (1 7 ) JANAF Thermochemisrry Tables; NSRDS-NBS-37; US.Department of Commerce. National Bureau of Standarb: Washington, DC, 1970. (1 8) JANAF Thermochemistry Tables; NSRDS-NBS-Sh US.Department of Commerce, National Bureau of Standardi: Washington. DC. 1982. (19) Bornstein, L. Molecular Corurants, B11/1415; Hellwege, K. H., Hellweg, A. M., Eds.; Springer: Berlin, 1982.
k-0
(7)
where we distinguish between two types of carbon atoms. Note that the magnitude of yc. can be predicted since Cc. is known (e.g., 1 for CH3CF3). Alternatively, yc. can be fitted to experiment. Note that either way each fragment j can now have two types of carbon atoms, one which was the site of the initial excitation and one which was not. While for some fragments (e.g., CH3+,CF3+)this can be decided in a fairly unambiguous manner, for others (e.g., C H P ) this is far less clear. 111. Results We present the results for all four possible site-selective excitations together, in the order of increasing computational sophistication. The predicted fully statistical fragmentation pattern is not in good agreement with the observed one. In quantitative terms, the value of H (eq 6) for the experimental vs the computed results is about 2 for resonance excitation and 1.5 for ionization to the continuum. This is comparable to the value of H for one type of experimental pattern vs another. In words, the predicted fully statistical pattern is as different from the measured ones as these differ from one another (cf. Table I1 of ref 1). The result that the doubly charged parent ions dissociate (or “Coulomb explode”) somewhat more statistically appears to us to be significant and is consistent with the rest of our computations. Other noteworthy predictions of the fully statistical computation include the following: (i) The number of fragments, X,in the Coulomb explosion (nearly four per one parent molecule) is significantly higher than for a singly ionized parent. (ii) The mean energy per parent molecule is significantly higher (over 40 eV) for the Coulomb explosion than for a singly charged parent (about 30 eV). (iii) The nearest to statistical experiment is ionization at the carbon attached to the fluorine site. A predictive computation with a site-specific constraint leaves these general conclusions unchanged except that the agreement with experiment is necessarily better. For the Coulomb explosion following excitationat the fluorine-carrying carbon, the agreement is nearly but not quite acceptable (Hvalue of 1; to see what this means, either type of excitation at the CH3 end, shown in Figure 1, has an H value of about 1). It is of interest to the theorist that (20) Levine, R. D. In Theory of Reactive Collisions;Baer, M., Ed.;CRC
Pres: Boca Raton, FL, 1984.
(21) Mann, D. E.; Acquista, N.; Plylev,E. K. J. Chem. Phys. 1954,22, 1586. (22) Torkington, B.;Tbowpon, H. W. Tram.Forodoy Soc. 1945,41,236. (23) Cowan, R. D.; Herzbcrg, G.; Sinaha, S.P.J . Chem. Phys. 1950,18, 1538. (24) Rodgcrs, A. S. In Fluorine Containin Free Radicals, Ktnetics and Dynamics; Root, J. W., Ed.; ACS Sympceium 66; American Chemical Society: Washington, DC, 1978. (25) Jacor, M. E. Chem. Phys. 19%0,53, 307. (26) Heinis, T.; Bgr, R.; Bbrilin, K.; Jungen, M. Chem. Phys. Lett. 1984, 105, 327. (27) Kagel, R. 0.;Powell, D. H.; Overend, J.; Ramos, M.N.; h i , A. B. M.S.;Runs, R. E. J . Chem. Phys. 1982, 77, 1099. (28) Chen, S.S.;Rodgen, A. S.;Chao. J.; Wilhot, R. C.; Zwolinski. 9. J. J . Chem. Phys. Ref Data 1975, 4, 169. (29) Rarenatack, H. M.; Draxl, K.; Steiw, B. W.; Hmon,J. T. Enrgetfw of Gaseous Ions; American Chemistry Society: Washington, DC, 1977.
Fragmentation Patterns of CH3CF3
The Journal of Physical Chemistry, Vol. 95, No. 18. 1991 6783
0.40
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1 2 1 4 1 5 1920 2 4 2 5 2 8 2 1 3 1 3 2 3 3 44415 5051 6 2 6 3 6 5 89
1 2 1 3 1 4 1 5 2 4 2 5 2 9 2 1 31Zt334 4446 5051 6 2 6 3 W I So
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