J. Phys. Chem. 1993,97, 12282-12290
12282
Time-Dependent Mass Spectra and Breakdown Graphs. 17. Naphthalene and Phenanthrene Yehiel Gotkis, Maria Oleinikova, Mor Naor, and Chava Lifshitz'9t Department of Physical Chemistry and the Fritz Haber Research Center for Molecular Dynamics, Hebrew University of Jerusalem, Jerusalem 91 904, Israel Received: July 26, 1993; In Final Form: September 17, 1993"
Time-resolved photoionization efficiency (PIE) curves were measured for CIOH~",C I O H ~ and + , C&'+ from naphthalene (and for the analogous ions in naphthalene-dg) and for C I ~ H I O ' +CldHs+, , and C12Hs'+ from phenanthrene. PIE curves were modeled by k(E) dependences via RRKM/QET calculations. Loose transition states were adopted for the H losses and for C2H2 loss from naphthalene. A tight transition state was found for the CzH2 elimination from phenanthrene. The most stable daughter ion isomeric structures are formed, namely, phenylacetylene'+ and acenaphthylene'+, by CzH2 elimination from naphthalene and phenanthrene, respectively. Disagreement between the present activation parameters for naphthalene and the ones adopted by Ruhl et al. (ref 3) are discussed. Large kinetic shifts have been observed. The conventional shift for CzH2 loss from phenanthrene is 4.07 eV, while the intrinsic shift, due to radiative decay in the infrared, is 3.25 eV. The following heats of formation were deduced: AHroo(CgHa'+,phenylacetylene) = 280.6 f 1 kcal/mol, A H f O O (CIoH,+,naphthyl) = 275 f 3 kcal/mol, AHf0o(C~2H~'+,acenaphthylene)I260.2 f 1 kcal/mol, and A H f O O (C14Hs+,phenanthryl) = 28 1 f 3 kcal/mol. The C-H bond energies are - 0 . 3 4 4 eV higher in naphthalene'+ and phenanthrene'+ than in benzene'+. The resilience of polycyclic aromatic hydrocarbons (PAHs) toward dissociation is understood on the basis of their high C-H and C-C bond energies and their large numbers of degrees of freedom. It can be partly overcome by ion trapping-as has been done in the present study. Formation of acenaphthylene*+ from phenanthrene*+ constitutes a possible example of a chemical link between PAHs and pentagonal-containing motifs of fullerenes.
Introduction The mass spectrometric behavior,l gas-phase ion chemistry,2 and energetics and dynamics of ionic dissociations3of polycyclic aromatic hydrocarbons (PAHs) have been topics of interest in recent years, in view of the importance of PAHs in combustion and in interstellar chemistry. PAHs are considered to be the most abundant free interstellar organic molecules knowna4 Electron ionization of PAHs leads to a minimal amount of fragmentation, and isomers give almost identical fragmentation patterns.] The photofragmentation dynamics of the deuterated naphthalene cation CioD8'+ was studied by threshold photoelectron-photoion coincidence (TPEPICO).3 Dissociation rates for the C2D2 loss channel were measured between 16 and 17.7 eV photon energies and fitted by RRKMIQET calculations, resulting3 in a critical energy of activation EO = 3.50 eV and an activationentropyS'(1000K) =-14.1 J mol-' K-' (=-3.4eu). General schemes were developedsato rationalize low-energy and high-energy ionic decompositions in naphthalene and azulene. Deuterium isotope effects on CloDg'+ fragmentation thresholds were rationalized in terms of the RRKMIQET modeLsb The special resilience of PAHs toward decomposition can be understood; it is due to their rather high bond energies and their large numbers of degrees of freedom. These attributes make them attractive candidates for study by time-resolved photoionization mass spectrometry (TPIMS), a technique which we have developed in recent years.6 Ions can be trapped in a Paul-type cylindrical ion trap (CIT) for up to several tens of millisecond^.^ This allows enhanced fragmentations at low energies and enables the determination of 'kinetic shifts". The 'conventional" kinetic shift (CS) is defined as the excess energy required to observe detectable (1%) dissociation within 10 ps, appropriate for conventional mass spectrometer appearance energy measurem e n t ~ .The ~ ~"intrinsic" ~ kinetic shift (IS) is taken as the energy
* To whom correspondence should be addressed. f
e
Archie and Marjorie Sherman Professor of Chemistry. Abstract published in Aduance ACS Abstracts, October 15, 1993.
0022-365419312097- 12282$04.00/0
needed for 10% fragmentation in competition with radiative relaxation of the excited ion.g The latter definition is appropriate to an ion trap appearance energy experiment unlimited by ion containment time. Very large kinetic shifts are predicted for the PAHs; however, to date appearance energies have only been determined on the microsecond time ~ c a l e . ~ * ~ J ~ J ~ We have recently reported on time-dependent mass spectra and breakdown graphs for methylnaphthalene^.^ Rather large kinetic shifts were observed for the H' loss reaction: CS = 2.05 eV and IS = 1.1 eV. The results confirmed the suggestion based on time-resolved photodissociation (TRPD)9that the cleavage of a methyl hydrogen from the methylnaphthalene ion does not require 2 eV more energy than the cleavage of a similar methyl hydrogen from the analogous toluene ion. The energetics and dynamics of the two dissociation reactions are rather similar-the differences lie in the extent of the kinetic shifts. Time-resolved dissociation of the bromonaphthalene ion was studied recentlyI2 by TPIMS and TRPD. While the methylnaphthalene H' loss demonstrates7a rather tight transition state, the Br' loss reactions demonstrated loose transition states characteristic of simple bond cleavages, yet again the kinetic shifts were much higher than for the analogous Br' loss reaction in bromobenzene.12 The heat of formation of the a-naphthyl cation was deduced to be A H C O O (C,oH7+) = 281 f 3 kcallmol, and the correlation between bond energies for benzene and naphthalene derivatives was discussed.I2 Part of that discussion was based on a preliminary report of TPIMS results for na~htha1ene.l~That preliminary work formed the basis for calculating time-dependent breakdown graphs for naphthalene14 and for comparing the resilience of naphthalene+ and C ~ Otoward + surface-induced decomposition (SID).I4J5 In this study we wish to report on the extension of the TPIMS work on naphthalene-hg, CioHg'+, naphthalene-& C & 7 ' + , and phenanthrene, C14Hio*+,emphasizing the energetics, dynamics, and mechanisms of the main fragmentation pathways. The preliminary study of naphthalenei3indicated a possible problem with the RRKMIQET analysis of the TPEPICO data3 which we wanted to resolve in a twofold manner: (a) to understand the 0 1993 American Chemical Society
Breakdown Graphs of Naphthalene and Phenanthrene critical energy for C2H2 (C2D2) elimination in terms of the thermochemistry of the reaction and (b) include parallel reactions in the analysis. The mass resolution and sensitivity ofour TPIMS system were improved recently, and storage times up to -0.5 s are now possible. Our recent detailed experiment on methylnaphthalenes' performed with a new ion source demonstrated the possibilities of the TPIMS technique in determining critical energies of activation Eo to within f0.5 kcal/mol, activation entropies AS*(lOOO K) to within f1.5 eu, and microcanonical rate constants k(E) to within 40-50%. All of these error limits reflect the accuracy of our experimental procedure and assume the adequacy of the RRKM formalism for the modeling of the experimental data. In addition to TPIMS results, we will report on collisionally activated dissociation (CAD) and mass-analyzed ion kinetic energy spectroscopy (MIKES) studies carried out on a ZAB-2F, to gain a better understanding of the reaction mechanisms. Experimental Section The experimental technique of TPIMS has been described in detail r e ~ e n t l y , ~ .and ~ , ~only ~ J ~a brief description will be given here. Photoionization is induced by a pulsed vacuum-UV light source, either the Hinteregger discharge in hydrogen, producing the many-line spectrum, or the Hopfield continuum in He. Photoions are trapped in a CIT. They are ejected into a quadrupole mass filter by a draw-out pulse, following a variable delay time. In this study ions were stored from -20 ps to -400 ms. The radio frequency (rf) of the potential applied to the cylindrical barrel electrode of the CIT is 0.5 MHz (w/2?r). The ion creation pulse is a train of short pulses applied to the light source. An ejection pulse is applied to the end-cap electrode of the CIT nearest the mass filter, and a detection pulse is gating the ion counter. The rf voltage is applied to the cylindrical electrode of the CIT throughout the whole cycle. The effective wavelength resolution employed is 5.0 A. This corresponds to an energy resolution of -0.025 eV near the ionization thresholds (-8 eV) and of -0.05 eV near the fragmentation onsets of naphthalene and phenanthrene. A simple Knudsen-type molecular beam source for lowvolatility compounds was constructed and was used to study the photoionization in the case of phenanthrene. MIKES and CAD measurements were carried out on a VG ZAB-2F double-focusing mass spectrometer of reversed geometry.I6 For MIKES, the magnetic field was set to select the ions of desired m / z value under investigation; ionic products of their decompositions in the second field-free region, between the magnetic and electrostatic analyzers, were detected by scanning the electric sector potential under conditions of good energy resolution with the energy-resolving 8-slit partially closed. Collisional activation spectral7were obtained by using air as the collision gas on 8-keV ions. The desired operating conditions have been described in the literature for metastable ion and CA spectra.I8J9 Naphthalene, phenanthrene, acenaphthylene, biphenylene, and phenylacetylene were commercial samples from Aldrich employed without further purification. Naphthalene-& (98+ atom % D) was a sample from Aldrich.
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12283 150
phenanthrene C,,H,o'
"
... - i .. m
100 .
" -!
d
. ..
m..
50 .
mm
" ,
7.7
,~,~..,m,m;,'
7.8
,
,
7.9
,
,
,
,
, , , ,
8.0
,
,
8.1
, , ,
,
, ,
8.2
,
,
,
, ,
,
,
8.4
8.3
Photon Energy, eV
Figure 1. Onset region of the photoionization efficiency (PIE)curve for phenanthrene. PIE, the C14Hlo'+ ion current divided by the light intensity (arbitrary units), is plotted versus the photon energy.
0 0
000
oo"
n=4 -1, 9.90
8
OO
0
& O
PHOTON ENERGY, eV
Figure 2. PIE curve for naphthalene parent ion over an extended energy range. Two of the maxima identified in the curve are Rydberg states of series converging to higher ionization energies (see text and ref 24).
Higher ionization energies for naphthalene and phenanthrene are known from photoelectron ~ p e c t r o s c o p y . ~Several ~ - ~ ~ Rydberg series converging to higher ionization energies in naphthalene have been identified.3q24 Figure 2 represents the naphthalene PIE curve over the energy range from 8 to 13 eV. Some structure is discernible, with steps around 8.9 eV (close to 12),2210.1 eV (close to 13),22923 and 11 eV (close to Superimposed on the steps are some sharp resonances which can be attributed to autoionizing Rydberg states, particularly a t 9.1 eV, the n = 4 member of a series converging to the third IE, and at 9.9 eV, the n = 4 member of a series converging to the fourth IE.24 We conclude that the main features of the naphthalene PIE fine structure are in good agreement with those observed in the photoabsorption spectrum a t corresponding photon energies.24 We have studied two major primary reaction channels in naphthalene-ha, naphthalene-& and phenanthrene, namely, H' (DO)loss and C2H2 (C2D2) loss, reactions 1-3
Results and Discussion Time-Resolved Photoionization Efficiency Curves. a. Experiment. The threshold regions of the parent PIE curves served to determine the ionization energies (IEs) for naphthalene-ha, naphthalene-&, and phenanthrene as 8.12 f 0.02, 8.12 f 0.02, and 7.87 f 0.02 eV, respectively. The curve for phenanthrene is presented in Figure 1. The IEs are in excellent agreement with available literature data, 8.1442 f 0.0009 eV for naphthalene,20 8.12 f 0.01 eV for naphthalene-&,3 and 7.86 f 0.01 eV for phenanthrene.21
There are four well-established reaction channels which the benzene ion undergoes: H', H2, C2H2, and C3H3' losses.25 In
Gotkis et al.
12284 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993
/
naphthalene-d,
14.0
14.5
15.0
15.5
16.0
14.5
15.0
15.5
16.0
16.5
"
17.0
Photon Energy, eV
Photon Energy, eV
Figure 3. Time-resolved experimental [ ( O ) 24 p, (V) 400 ms] and calculated (lines) daughter ion (CaH6") PIE curves for naphthalene. The relative intensities of the experimental PIE curves of the parent served to scale the experimental daughter ion PIEs. The PIEs are in arbitrary units, to scale.
R p r e 5 . Experimental [(A) C&'+, (0)CloD7+]andcalculated [(- -) C&,*+,(-) CloD7+] PIE curves for the microsecond range in naph-
-
thalene-&.
naphthalene 24 ps
30
CIA'. ( p he n a nt h re ne)
A A A A l
/
A A
1".
A A A A
20
C,H,+
10 ms
I
I 1
A
10
14.0
0 13.5
14.5
15.5
16.5
Photon Energy, eV
Figure 4. Time-resolved experimental [(0) 24 ps; (V) 10 ms] and
calculated (lines) daughter ion (C12H8'+) PIE curves for phenanthrene. The relative intensities of the experimental PIE curves of the parent served to scale the experimental daughter ion PIEs. naphthalene and phenanthrene the two major unimolecular reactions are H' lass and acetylene loss, as judged from metastable ion spectra.10,26There are other minor low-energy reactions in na~hthalene?,~ e.g., C4H2 and Hz lasses. While ionization energies were determined for the source molecules from PIE curves without ion trapping (see previous two paragraphs), the appearance energies (AEs) of daughter ions were measured from PIE curves with and without ion trapping. Time-resolved PIE curves for C8H6*+from naphthalene at 24 ja and 400 ms are presented in Figure 3, while curves for C12H8'+ from phenanthrene at 24 ps and 10 ms are given in Figure 4. Fairly large kinetic shifts are clearly observed. The H' (D') loss channels were much more difficult to measure at long ion trapping times than were the C2H2 (C2D2) loss channels because of mass resolution problems. Contributions from the parent ions to the mass positions of (M-H)+ or (M-D)+ had to be subtracted. PIE curves for CloD7+ and C&'+ from naphthalene-& are presented in Figure 5 for t 24 ps (without ion trapping) and for CsH6" and C.&'+ from naphthalene48 in Figure 6. We have observed similar isotope effects on AEs as the ones published by Jochims et in other words, AEs were a few tenths of an electronvolthigher for CloD8 than for CloH8, but all of our values are considerably lower, indicating a shorter time scale in the previous experiments3qSthan theoneweestimate (-24ps) without ion trapping. Pronounced AEs are summarized in Table I. The
-
15.0
17.5
16.0
17.0
Photon Energy, eV
Figure 6. Experimental [(A) CaHa'+, (0) C6H6'+] PIE curves for the
microsecond range in naphthalene-hg. TABLE I: Time-ResolvedAppearance Energies (AEs) for Ion8 from Naphthalene and Phenanthrene ion (source molecule) AE, eV (time) CloH7+ (naphthalene-h8) 15.1 f 0.1 (24 IS), 14.0 f 0.2 (7 ms), C&'+ (naphthalene-h8) C&&'+ (naphthalene-ha) Cl&+ (naphthalene-&) C&'+ (naphthalene-&,) C14H9+ (phenanthrene) C12Ha'+ (phenanthrene)
13.9 f 0.2 (400 ms) 15.1 f 0.1 (24 ps); 14.3 f 0.1 (7 ms); 14.3 f 0.1 (400 ms) 15.3 & 0.1 (24 ps) 15.5 f 0.1 (24 ps) 15.5 f 0.1 (24 ps), 14.8 f 0.1 (400 ms) 15.8 f 0.2 (24 ps) 15.2 f 0.1 (24 as), 14.3 i 0.2 (10 ms)
AEs did not change upon extending the ion trapping time beyond 7-10 m ~ . ThePIEcurves for parent (C14Hlo'+) and daughter (C12Hs'+) ions from phenanthrene are shown for two storage times over similar energy ranges in Figure 7a,b. Several clear changes are observed on extension of the ion storage time: (1) the rising onset for C12H8*+shifts to lower energies (as noted before, Figure 4), and (ii) the relative abundance of C12Ha'+ versus C14Hlo'+ increases. Similar observations were made for all the fragmentations of this study. b. RRKMIQE T Calculations. Time-resolved PIE curves were modeled by RRKMIQET calculation^.^^^^^^^ The microcanonical rate coefficients k ( E ) were calculated as a function of energy by an RRKM program.29 This was done for the major parallel reactions C-H (C-D) cleavage and C-C cleavage leading to acetylene loss. All the minor parallel reactions were neglected.
Breakdown Graphs of Naphthalene and Phenanthrene
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12285
200
100 0.25
-
0 14.50
15.50
16.50
17.50
phenanthrene 10 ms
. .
13
14
IS
16
,
,
,
, 17
, ,
, 18
times for naphthalene.
C,,H,'XlO
o lo 13.50 0=
14.50
15.50
16.50
Photon Energy, eV
F i p r e 7 . (A) Parent C14Hlo'+ and (0) daughter C I ~ H ~ion ' + PIE curves for phenanthrene. The PIE (arbitrary units, to scale) is plotted versus the photon energy (in eV) at the storage times of (a) 24 1 s and (b) 10 ms. A, 0, experimental results; lines, calculated.
The vibrational frequencies of the reactant ions were originally taken equal to those of the corresponding neutrals, which following Leach and c o - w o r k e r ~ ~are , ~ known for naphthalene-hs, naphthalene-& and anthracene from Sverdlov et al.30 Those for phenanthrene'+ were adopted from the known anthracene frequencies. Since then theoretically calculated vibrations of ionized naphthalene have become known.3' However, RRKM calculations carried out with theionic frequenciesled tonegligible differences when compared to the results for the neutral frequencies. Vibrational frequenciesof the transition states were varied to get the best agreement with experiment. Reaction coordinates were chosen, and the corresponding frequencies were removed in the transition states as follows: C-H cleavage, 3061 cm-l for naphthalene and 3026 cm-I for phenanthrene; C-D cleavage, 2227 cm-I for naphthalene-ds; C-C cleavage, 780 cm-I for naphthalene, 682 cm-1 for naphthalene-ds, and 523 cm-I for phenanthrene. Several other modes were varied in the transition state. It is widely accepted that the details of the frequency changes in the transition state are not very important;32 the important factor is the degree of tightness or looseness of the transition state, which is characterized by a single parameter-the activation entropy at 1000 K, M'IOOO K. Time-resolved parent and daughter ion breakdown curves were calculated from the rate-energy dependences (k(E)'s) at 0 K, these give the internal energy dependence of the fractional abundanceof the ions. They can be converted into time-resolved breakdown graphs of the molecule as a function of photon energy by adding the ionization energy of the molecule to the internal energy. Some typical curves are presented in Figures 8 and 9. In these calculated breakdown curves, an additional parallel energy-independent rate process representing radiative decay in the infrared and/or collisional cooling was employed, together
Photon Energy, eV Figure 9. Calculated time-resolved 0 K breakdown curves for parent and daughter ions (CI~HIO'+, C I ~ H ~and + , C I ~ H ~ *at+ )the two indicated reaction times for phenanthrene.
with the dissociative rates. A similar approach was applied p r e v i o ~ s l y . ~ JThe ~ J ~radiative ~ ~ ~ decay rate of the naphthalene ion is of the order of lo2s-1.3c36 The two-photon relaxation rate for naphthalene is quite fast in comparison with other molecules, and the rate of emission of the first IR photon from naphthalene ion excited to 2-3 eV internal energy has been estimated to be -400 s-1.12,37The radiative decay constants which gave best agreement with experiments may, as was noted b e f ~ r e reflect ,~ contributions from collisional relaxation under the higher pressures ( ~ ( 2 - 5 )X 10-6 mbar) which prevail in our ion source compared with an ICR spectrometer.3k36 The 0 K breakdown curves were convoluted with the instrumental slit function, with the calculated thermal energy distribution at the temperature of the experiment and with the energy deposition function. Rotational constants were taken equal to those for the neutral species. The naphthalene threshold photoelectron spectrum (TPES),3 in combination with the naphthalene photoelectron spectrum (PES):' and the phenanthrene PES22were employed as energy deposition functions for naphthalene and phenanthrene, respectively. The resultant curves represent the calculated first derivatives of the PIE curves of the ions, provided that the threshold law for photoionization is a step f u n c t i 0 n . 3 ~These ~ ~ ~ curves were integrated to compare them with the experimental time-resolved PIE curves, and they are included in Figures 3, 4, 5 , and 7a,b. The activation parameters-the critical energies Eo and activation entropies A S * l K, ~ which best fit the whole set of experimental PIE curves-are summarized in Table I1 together with the optimal values of the radiative (and collisional) decay
Gotkis et al.
12286 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993
TABLE II: RRKM/QET Parameters for Naphthalene, Naphthalene-&, and Phenanthrene Cation Fragmentations at Near-Threshold Energies H'(D') loss C2H2 (C2D2) IOSS hs'lo00
reactant k d , s-I 600 CloH8" CloDs" 100 C14Hlo" 800
K.
hs'lo00
io9
I 0' 10'
IO'
K,
EO,eV
eu
u
Eo,eV
eu
u
4.23 4.33 4.10
+3.0 +2.7 +4.4
8 8
4.60 4.60 3.34
+8.5
8 8 2
10
+5.6
-5.1
T
10'
v1
or"
IO'
c)
d
IO' IO'
constants as well as the u values, which represent the number of equivalent reaction pathways. The rate energy dependences are givenin Figures 10-12. Figure 10 representsour calculated k(E) dependences for the two parallel reactions in naphthalene-hs. As expected from the models, the k(E) curves cross each other; the energies at which the k(E)'s are lo5 s-I are equal for the two reactions (7.83 eV). A clear isotope effect is observed in the H versus D loss (Figure 11) from naphthalene-hs and naphthalenedg, respectively. In phenanthrene (Figure 12), contrary to naphthalene (Figure lo), C2Hz loss is the prevalent reaction at low energies and H' loss a t high ones. c. Comparison of Calculated and Experimental KineticShifts. The kinetic shifts C S and IS were calculated from the k ( E ) dependences for naphthalene and phenanthrene, respectively. With these calculated shifts, the AEs expected for the 10-1s range and for an ion trap experiment with unlimited time storage were calculated as well. The results are presented in Table I11 and compare favorably well with experimental AEs for short and long storage times, respectively (Table I). The breakdown curves allow one tocalculatecrossover energies (the energies a t which the parent has dropped to 50% abundance) and crossover shifts (the shifts of these values between the microsecond range and the long storage time chosen). The crossover energies for naphthalene and phenanthrene a t 24 1 s are 15.42 and 16.34 eV, respectively, and the crossover shifts are 0.83 and 1.245 eV, respectively, for the 10-100 ms time range. Reaction Mechanisms, Thermochemical Information, and Comparison with Previous Data. The models we have chosen (Table 11) to fit our data may be questioned. We chose several guidelines: (a) we only chose a model which could fit for a certain PAH cation simultaneously both of the reaction channels studied; (b) a starting point was the very well-known reaction scheme for b e n ~ e n e In . ~C6H6'+, ~ ~ ~the ~ ~C-H ~ ~cleavage is characterized by the lowest critical energy (EO= 3.65 eV) out of all the four parallel reactions and a somewhat loose transition state (AS*= +2.69 eu) while the C-C cleavage leading to CzH2 has a higher critical energy (EO= 4.13 eV) and a very loose transition state (AS*= +10.57 eu). Inspection of Table I1 demonstrates a somewhat similar situation in the case of naphthalene. This approach may need refinement in the future for two reasons: (i) The treatment we have chosen in the modeling is strictly RRKM with fixed transition states. Variational RRKM calculations have recently been carried out for C6H6'+,4' and a higher EO = 3.88 eV was obtained for the C-H cleavage reaction as well as a looser transition state, AS*= +5.7 eu, than in the study of Neusser and C O - W O ~ ~ ~ (ii) ~ S We . ~ have ~ + ~neglected ~ parallel reaction channels other than H' and C2Hz loss. No other significant reactions were observed for phenanthrene. The C4H2 loss and the HZloss reactions in naphthalene were of too low abundances for accurate experimental time-resolved measurements to be made. (c) We tried to obtain critical energies EOin agreement with available thermochemical data. The structures of the product ions are unknown, and we have studied this aspect separately (see next section). As will be seen shortly, our results agree with formation of the most stable ion structures possible in each case. Our model for C2D2 loss from naphthalene-d8 (reaction 2b) is in disagreement with the results of Ruhl et a1.3~5While these authors propose an activation energy EO= 3.50 eV and a rather
-
IO"
5
4
7
6
8
9
1 0 1 1 I Z
Internal Energy, eV
Figure 10. Calculated rate energy, &E), dependencies for reactions la and l b in naphthalene, for the activation parameters given in Table 11.
-
-
109
I 0' I 0'
IO' 101
" y,
or"
2
c)
101
103
IO' IO"
lb"l: 10'3
IO"
3
I/, 4
5
,
,
,
,
,
,
6
7
8
9
IO
II
, I2
tight transition state with AS* = -14.1 J/(mol K) (-3.37 eu),3 we suggest Eo = 4.60 eV and A S = +5.6 eu (Table 11). The currently known most stable C8H6*+isomer is phenyla~etylene~~ (see Table IV), and this was the ion structure proposed by Leach and c o - w o r k e r ~as~the ~ ~product ion of acetylene elimination from naphthalenehs and naphthalene-& However, the endothermicity of reactions 1 b and 2b is much higher than their proposed EO= 3.5 eV value and is in fact in very close agreement with our suggested Eo = 4.6 eV. This can be seen from the following calculation based on the 298 K data of Table IV for reaction 1b; the calculated thermochemical AE for CsH6" from naphthalene is 276 54.5 - 35.9 = 294.6 kcal/mol = 12.78 eV. Subtracting the naphthalene ionization energy gives EO = 4.64 eV. We can
+
The Journal of Physical Chemistry, Vol. 97,No. 47, 1993 12281
Breakdown Graphs of Naphthalene and Phenanthrene
TABLE 111: Kinetic Shifts for Naphthalene and Phenanthrene Dissociations (eV)
cs
IS
AE (10 ps, calculated)
AE (t
-
-, calculated)
molecule
H' IOSS
C2H2 IOSS
H' loss
CzH2 loss
H' loss
CzHz loss
H' loss
C2H2 loss
naphthalene phenanthrene
2.42 3.53
2.19 4.07
1.9 2.9
1.71 3.25
14.77 15.5
14.91 15.28
14.24 14.88
14.43 14.46
TABLE I V Thermochemical Data. M r o o (neutral), kcal/mol
species Clo& (naphthalene) C14HIO (phenanthrene) CsH6 (phenylacetylene) C&7'
OK 41.4b 56.4d
298 K 35.9 0.3 49 f 0.2 73 0.5 73.3 0.Q
*
*
M r o o (ion), kcal/mol
IE, eV 8.1442 0.0009c 7.86 A O.0lc 8.81 0.04
OK 229.2b 237.7b 280.6 f l b
298 K 223.6 230.3 276 260.3h
* 0.04
281 f 38 275 f 3b 260.2 l b 281 f 3b
*
*
(a-naphthyl) 62 f 0.2
C12H8 (acenaphthylene) C14H9. (phenanthryl) C2H2 (acetylene)
54.7
H'
5 1.63
8.22
*
**
252
54.5 0.25 54.51 0.17f 52.10
a All the values are from ref 42 unless otherwise noted. Present results. Reference 20. Reference 43. Reference 2 1. f Reference 44. g Reference 12. Reference Sa.
Id
(naphthalene C8D6" - d8)
/
104
4-
Id
VI
IO4
Riihl. Rice &Lush Model 0 Riihl, Rice &Leach Experiment
15.0
0
16.0
1 0
Photon Energy, eV Figure 14. Experimental [(0)24 p s , (0)400 ms] and calculated (lines) time-resolved PIE curves for CeD6" from naphthalene-d8. The calculations are based on the model by Riihl et al. (see ref 3 and Figure 13).
\
1 1
i/.
C,,H,* Figure 15. Schematic potential energy profile based on the Ruhl et al. model (refs 3 and 5) for reactions 1band 2b. Thevalue4.6 eV corresponds to the endothermicity of the reaction and to Eo suggested in the present paper. The value 3.5 eV is the critical energy of the Riihl et al. model (see text).
clear which step is rate determining a n d possesses a tight transition state (TTS) a t 3.5 eV above t h e naphthalene cation radical. However, a schematic presentation is given by us in Figure 15. (Clearly more maxima a n d minima are called for.) Our results suggest a rather loose, orbiting transition state (OTS) with a critical energy equal t o t h e endothermicity, while Leach and coworkers suggest a TTS a s t h e rate-determining step a t least at t h e high internal energies a t which measurements are made. Transition-state switching between t h e two is possible, but more
Gotkis et al.
12288 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993
experimental data are needed to verify this notion. Our data do not require such an assumption and are in agreement with many previous experimentswhich indicate that for a multiwell potential energy surface, in which isomerization maxima lie below the dissociation limits, the rate is determined by the density of states of the deepest well and the transition state is due to the last step of the dissociative exit channel.45 The surface given in Figure 15 is clearly speculative; all it suggests is a way to account for our own data and those of refs 3 and 5! The heat of formation of the naphthyl cation deduced in the present study, AZfroo(CloH7+)= 275 f 3 kcal/mol, is included in Table IV. It is less certain than the value for C&'+, since mass resolution limitations prevented accurate measurements a t long trapping times. The value is lower than but, within the sum of experimental errors, equal to the one deduced from the bromonaphthalene reaction.I2 It is higher than thevalue deduced by Jochims et who assumed similar C-H bond energies in benzene and naphthalene. A recent analysis which we havecarried out12 demonstrated in several cases a higher bond energy in naphthalene derivatives than in benzene derivatives, and the assumption that the C-H bond energiesareequal is not warranted. There are no previous data available for comparison with the suggested phenanthrene activation parameters. The results could not be fit, as in the case of naphthalene, with a loose transition state and a high (-4.5 eV) critical energy for the acetylene elimination. The C1&" isomer formed at threshold cannot be naphthylacetylene*+nor can it be biphenylene'+, on the basis of the known thermochemistry of these ions.42 The most stable C12Hs'+ ion is acenaphthylene'+ (ref 42 and Table IV), and there is an excellent fit between the experimental Eo = 3.34 eV and the one calculated from the known thermochemical data at 298 K (Table IV). The thermochemically calculated AE for C12Hs'+ from phenanthrene is 252 54.5 - 49 = 257.5 kcal/mol = 11.17 eV. Subtracting the phenanthrene ionization energy gives = 3.3 1 eV. We can use our EO = 3.34 eV value and thermochemical data a t 0 K from Table IV to calculate the heat of formation of C12Hs" a t 0 K, AZffOo(C12Hs'') = 260.2 f 1 kcal/mol. We ascribe the derived value to acenaphthylene cation radical. The reaction degeneracy factor u = 2, which fits reaction 3b, is in line with the suggested mechanism (Scheme I) for the formation of acenaphthylene cation radical from phenanthrene. This reaction is a possible example of a chemical connection between PAHs and fullerenes. Acenaphthylene and p y r a ~ y l e n e ~ ~ are structural motifs in the fullerenes. Neutral PAHs related to fullerenes can undergo many interesting high-temperature skeletal rearrangement^.^' A thermal reaction which is of particular interest in connection with the production of acenaphthylene'+ from phenanthrene'+ (Scheme I) is the synthesis of pyracylene at 1000 K from 1,5-diethynylnaphthalene.48 While acetylene elimination from naphthalene occurs via a loose transition state, the analogous reaction for phenanthrene is via a tight transition state. This is entirely plausible for the reaction mechanisms proposed. Even the mechanism proposed by Jochims et is consistent with a loose transition state for naphthalene, namely, two consecutive C-C cleavages forming phenylacetylene. The prior hydrogen shift which they propose to form their intermediate 18 (ref Sa) cannot be responsible for the tight transition state in acetylene loss, since the same intermediate is also operative in the H' loss reaction. The reaction in phenanthrene has to be as follows: cleavage of the C-C bond between carbons 1 and 10a (Scheme I) followed by a concerted ring closure and C-C cleavages between carbons 2 and 3. Intermediate formation of naphthylacetylene*+is not possible on the basis of thermochemistry, since it is 27 kcal/mol less stable than acenaphthylene'+ 42 and the EOagrees with the thermochemistry of acenaphthylene'+ plus acetylene as noted before. The concerted ring closure requires a tight transition state as is observed experimentally (Table 11).
+
SCHEME I: Suggested Mechanism for C a z Elimination from Phenanthrene'+ Leading to Acenaphthyleae'+ * r
0
t
10
Twoaltemativeroutesareshown: (1)cleavageofaC-Cbondbetwecn carbons 1 and 10a or (2) cleavage of a C-C bond between carbons 8 and 8a. These cleavages are followed by secondary C-C cleavages between
carbons 2 and 3 or 6 and 7, respectively, which occur in a concerted fashion with closure of the five-membered ring. The carbon numbering in the acenaphthylene has been done in a consistent manner with phenanthrene. 1w w-
70 80
w50
-
Figure 16. Daughter spectrum produced by collisionally activating the molecular ion ( m / z 152) of acenaphthylene at 8 keV. The sharp peak at m / r = 76 is due to 152". The peak at m / z 126is also observed under unimolecular MIKES. Identical spectra were obtained for the molecular ion of biphenylene and for the acetyleneloss daughter ion of phenanthrene.
MILES and CAD Spectra. The CAD spectra of m / z = 102 ions from phenylacetylene and from naphthalene were identical in all details, including the Occurrence of a charge stripping peak at m / z = 5 1. This gives some support to the identification of the m / z = 102 ion from naphthalene as phenylacetylene'+ although previous with CAD of PAHs demonstrates that characteristic ions are formed from all PAHs, regardless of their structures. Abundant daughter ions in the CAD spectra of m/z = 102arem/z = 101,100,99,98,75,74,63,62,61,51,50. Ions at m / z = 76 and 52 have unimolecular origin. The CAD spectra of m / z = 152 ions from acenaphthylene and phenanthrene were identical in all aspects, including the charge stripping peak at m / z = 76. The CAD spectrum of acenaphthylene parent ion is presented in Figure 16. The peak at m / z = 126 is due to unimolecular C2H2 elimination. While this might have been taken as conclusive evidence that the m / z = 152 ion from acenaphthylene has the acenaphthylene structure, this is not the case. We ran the CAD spectrum of biphenylene and it gave a spectrum identical with the one shown in Figure 16. It seems clear that this technique is not very well suited for structure elucidations, and our previous evidence based on critical energy data and thermochemical calculations is much more conclusive.
Breakdown Graphs of Naphthalene and Phenanthrene
t
640
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12289
I
I
645
I
650
ESA
I
655
I 660
I
665
3
I
670
695
ESA
I
voltage. V
Figure 17. Metastable ion peak shape (second field-free region, ZAB-2F dissociation) for acetylene loss from naphthalene. The normal parent ion beam passed at an energy of 7822 eV. The metastable peak is obtained by scanning the electrostatic energy analyzer (ESA) voltage.
Can MIKE spectra aid us in the analysis of reaction mechanisms? The metastable peak shapes for the two acetylene loss reactions in naphthalene and phenanthrene are given in Figures 17 and 18, respectively. The peak for phenanthrene is characteristic of a reaction with a TTS and reverse activation energy, since it is clearly dish-topped. The value T,,,i,, the minimum kinetic energy release calculated from the peak horns, is 0.24 eV. This indicates that the AHf"o(ClzHg+)quoted in Table IV is an upper limit. Theactualvaluemust beat least 5.5 kcal/mollower, i.e., 1254.7 kcal/mol, since these results definitely indicate at least this much reverse activation energy. The metastable peak shape for acetylene loss from naphthalene (Figure 17) is not dish-topped and indicatesno TTS and no reverse activationenergy, as is expected from our suggested loose transition state. It is quite broad, in view of the excess energy required for the dissociation to occur in the microsecond time range, characteristic of the second field-free region of our ZAB-2F instrument. The kinetic energy release, T I / *calculated , from the width at halfheight is 0.47 eV, in excellent agreement with the number quoted by Jochims et al.5a as the kinetic energy release determined by Tobita (0.49 eV). Conclusion We have demonstrated the capabilities of combining good energy resolution (through vacuum-UV photoionization) with ion trapping in gaining understanding, as well as quantitiative information, concerning the energetics and dynamics of PAH radical cation dissociations. Several analytical methods have been developed to study PAHs, e.g., CADI and more recently surface-induced dissociations ( S I D ) . ' V ~High ~ energies are required to bring about dissociation, and the fragmentations are usually not structure specific. The present study demonstrates that fragmentation can be enhanced by ion trapping. The high energies required for fragmentation to occur stem from large kinetic shifts. The maximum kinetic shifts required to observe dissociation without time limitation have been determined for naphthalene and phenanthrene. Heats of formation of some of the key ions have been determined, and characteristics of the transition states were deduced. Since rearrangements are abundant below the high dissociation thresholds, the decomposition products are the ones which are most stable thermodynamically. Thus, acetylene loss from phenanthrene forms acenaphthylene. This is a compound containing a five-membered ring and a key motif in fullerene structures. The occurrence of other, less stable isomeric product ion structures at higher energies above threshold cannot be ruled out.
1
I
79a
705
0
voltage, V
Figure 18. Metastable ion peak shape for acetylene loss from phenanthrene. See caption to Figure 17. The normal parent ion beam passed at an energy of -7814 eV.
Clearly, more work is needed to refine the models. One important route would be to determine k ( E ) dependencesdirectly in the 102-104 s-I range to decide between the model presented here for acetylene elimination from naphthalene and the one developed by Leach and c o - w ~ r k e r s(see ~ ~ ~Figure 14). Furthermore, microcanonical variational transition-state theory should be applied to pin down more accurately the C-H bond energies in PAHs. Extension of the modeling to include more parallel reactions is a future development. Acknowledgment. This research was supported by the Basic Research Foundation administered by the Israel Academy of Sciences and Humanities. Y .G. thanks the Israeli Ministry of Absorption and Ministry of Science and Development for their support. We thank Professor R. C. Dunbar for valuable discussions. Mr. M. Rejwan was involved in some of the early measurements, and Mrs. J. Laskin helped with some of the calculations. References and Notes (1) Pachuta, S.J.; Kenttimaa, H. I.; Sack, T. M.; Cerny, R. L.; Tomer, K. B.; Gross, M. L.; Pachuta, R. R.; Cooks, R. G. J. Am. Chem. Soc. 1988, 110, 657 and references therein. (2) Bohme, D. K. Chem. ReJ. 1992,92, 1487. (3) Riihl, E.; Price, S.D.; Leach, S.J . Phys. Chem. 1989,93, 6312. (4) Salama, F.; Allamandola, L. J. J . Chem. Phys. 1991, 94, 6964. ( 5 ) (a) Jochims, H. W.; Rasekh, H.; Riihl, E.; Baumgirtel, H.; Leach, S . Chem. Phys. 1992,168,159. (b) Jochims, H. W.; Rasekh, H.; Riihl, E.; Baumgirtel, H.; Leach, S.J . Phys. Chem. 1993,97,1312. (6) Lifshitz, C. Int. . I . Mass Spectrom Ion Processes 1991, 106, 159. (7) Gotkis, I.; Lifshitz, C. Org. Mass Spectrom. 1993,28, 372. (8) Chupka, W. A. J. Chem. Phys. 1959,30, 191. (9) Huang, F.-S.; Dunbar, R. C. J. Am, Chem. SOC.1990, 212, 8167. (10) Van Brunt, R. J.; Wacks, M. E. J . Chem. Phys. 1964,41, 3195. (11) Natalis, P.; Franklin, J. L. J . Phys. Chem. 1965,69,2935. (12) Gotkis, I.; Naor, M.; Laskin, J.; Lifshitz, C.; Faulk, J. D.; Dunbar, R. C. J. Am. Chem. SOC.1993,115, 7402. (13) Gotkis, I.; Rejwan, M.; Naor, M.; Lifshitz, C. 12th International Mass Spectrometry Conference, Amsterdam, 1991. In A h . Mass Spectromefry;Kistemaker,P.G.,Nibbering,N. M. M.,Eds.;Elsevier: Amsterdam, 1992; Vol. 12, p 877. (14) Lifshitz, C.; Gotkis, I.; Sandler, P.; Laskin, J. Chem. Phys. Leu. 1992,200, 476. (15) Ohmichi, N.; Gotkis, I.; Steens, L.; Lifshitz, C. Org. Mass Spectrom. 1992,27, 383. (16) Morgan, R. P.; Beynon, J. H.; Bateman, R. H.; Green, B. M. Inr. J . Mass Spectrom Ion Phys. 1978,28, 171. (1 7) McLafferty, F. W. Interpretation of Mass Spectra; Benjamin: New York. 1973. (18) Terlouw, J. K.; Burgers, P. C.; Hommes, H. Org. Mass Spectrom. 1974,14, 387. (19) Burgers, P. C.; Holmes, J. L.; Szulejko, J. E.; Mommers, A. A.; Terlouw, J. K. Org. Mass Spectrom. 1983,18, 254. (20) Cockett, M. C. R.; Ozeki, H.; Okuyama, K.; Kimura, K. J . Chem. Phys. 1993, 98, 7763.
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