2143
J . Phys. Chem. 1991, 95, 2143-2145 The ratios of the quenching cross section for Hg(3PI)are the largest. The quenching cross sections for Hg(3P,) were obtained at 25 OC; therefore, the selectivity appears to be somewhat emphasized. As is shown in Table VII, the differences in the activation energy are larger for Cd(3P,) than for Hg(’P,). This shows that if the selectivity for Cd(’P,) is compared with that for Hg(3P,) at the same temperature, the former is larger than the latter. This tendency again fits in with the expectation from the excitation energies of the triplet cadmium and mercury. Difference in the activation energy decreases in the order Mg(’P,), Cd(3PJ), Hg(’P,), and Zn(’P,). It is difficult to explain from the excitation energies that the activation energy differences are smallest for ZII(~P,). The reversal of the order between Zn(’P,) and Hg(3PI) may come from (1) the involvement of the 3P2state for zinc, (2) the higher chemical reactivity of zinc atom as shown by the fact that the bond strength of ZnH (85.8 kJ/ mol-’) is larger than that of HgH (39.8 kJ mol-’),24 ( 3 ) the
involvement of the quenching to the 3P0state in ‘the quenching of Hg(’PI), and (4) the fact that the quenching of Hg(3PI) by alkanes appears to result in a direct cleavage of the C-H bonds without the intermediate formation of HgH as was pointed out by Breckenridge and Renlund.’* More experimental and theoretical work is needed to clarify this point.
Acknowledgment. The authors express their gratitude to Professor W. H. Breckenridge, University of Utah, for helpful discussion and suggestions. Registry No. Cd, 7440-43-9; ethane, 74-84-0; ethane-& 1632-99-1; propane, 74-98-6; butane, 106-97-8; isobutane, 75-28-5; hydrogen,
12385- 13-6. ~
~~~
~
~~~
(24) Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986-1987.
Specific C-C Bond Dissociation in Highly Excited Alkanes: An Example of Mode Selectivity in Large Molecules Joop Los, Jaap H. M. Beijersbergen,* Stefan Kornig, and Piet G. Kistemaker FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands (Received: July 24, 1990; In Final Form: September 25, 1990)
Highly excited normal alkanes (ethane, propane, butane, pentane, and hexane), formed after neutralization of their radical cations in a collision with cesium or sodium atoms, dissociate into two radical fragments. The dissociation takes place at the central C-C bond for these alkanes, except for hexane where each C-C bond can be broken. The kinetic energy release spectra for these dissociation reactions show that the dissociation proceeds suddenly via a repulsive state. The specific dissociation reactions of butane and pentane are clear examples of mode selectivity in large molecules.
Introduction In recent years many experiments and theories have been developed to study nonstatistical processes in unimolecular reactions. Mode selectivity defined as “special energy investment into reactants so that the reaction is directed to specific products”’ has been shown in dissociation reactions of small molecules. Triatomic molecules, HOD, for example,23 can be dissociated into specific fragments depending on the photon excitation scheme. In van der Waals complexes, nonstatistical product branching ratios can be obtained because the van der Waals bond serves as a dynamic bottleneck for energy transfer between the molecular constituents. This energy-transfer limitation is much more difficult to realize when only covalent bonds are present. For these systems bond-selective fission processes occur on potential energy surfaces of excited electronic states rather than in the electronic ground state. In particular, nonstatistical product formation can be obtained if the molecule is excited to a repulsive state from which dissociation takes place so fast that intramolecular vibrational energy redistribution cannot occur. These repulsive states can be populated by tailored multiphoton excitation schemes? Another way to populate repulsive states in neutral molecules is to neutralize the corresponding cation in a charge exchange process. The observed losses of H and D atoms from small organic molecules in neutral beam spectroscopy can be explained by invoking repulsive states. These dissociations form examples of mode selectivity in organic molecule^.^ (1) Manz, J.; Parmenter, C. S.Mode Selectivity in Unimolecular Reac-
rions. Chem. Phys. 1989, 139, 1-238 (special issue). (2) Imre, D. G.; Zhang, J. J . Chem. Phys. 1989, 139, 89. (3) Hartke, B.; Manz, J.; Mathis, J. Chem. Phys. 1989, 139, 123.
0022-3654/91/2095-2143%02.50/0
In this paper we explore specific bond dissociations in normal alkanes. The excited state of the neutral alkanes is populated from the ionic state rather than from the ground state. By this route a repulsive state can be reached which is much more difficult to populate from the ground state. The dissociation of the neutralized excited alkanes takes place only in the central C-C bond. However, hexane also shows other dissociation channels. The kinetic energy release spectra indicate that little energy ends up in the vibrational modes of the fragments and that the dissociation indeed proceeds via a repulsive state. The internal energy of the neutral molecules is determined by the resonant character of the charge exchange process. The released kinetic energy is smaller than the resonant energy due to the fact that the energy is distributed over the translational energy of the two fragments and the rotational energy of each fragment. Assuming an impulsive force and using the known bent geometry of the ion, the energy distribution can be derived, resulting in a high rotational energy of the fragments. Many other experiments have shown that by using neutralization gas with a higher ionization potential than cesium or sodium no specific bond dissociation occurs. In collisions of ethane, propane, and butane cations with their neutral parent molecules hardly any dissociation occurs.s In the neutralization reionization experiments no mode selectivity was observed for the neutralization of other organic cations, like C4Hs+, with cesium, sodium, and mercury and reionization with helium or (4) Hudgins, D. M.;Raksit, A. B.; Porter, R. F. Org. Mass Specrrom. 1988, 23, 375. Raksit, A. B.; Porter, R. F. Org. Muss Specrrom. 1987, 22, 410. ( 5 ) Shields, G. C.;Wennberg, L.; Wilcox, J. B.;Moran, T. F.Org. Mass Spectrom. 1986, 21, 137.
0 1991 American Chemical Society
Los et al.
2144 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
vor 4@ double detector Figure 1. Schematic diagram of the experimental geometry. The measured quantities RA, RB,and T are depicted together with the beam velocity vector oo and the drift length L. AB' is the mass-selected molecular ion, and M the alkali-metal atoms. The angle between the fragment velocity vectors is exaggerated;realistic values are between 1 O and 5 O . Experimental Section The experimental setup is tailored to measure dissociation reactions of neutralized cations yielding two neutral fragments. Earlier publication^^*^ and a coming paper9 give extensive descriptions of the apparatus; here we give the principle of operation. In an electron impact source the molecular radical cations are formed with 70-eV electrons. The ions are accelerated to 6330 eV. A sector mass spectrometer selects molecular ions. The ion beam enters the second stage which is a differential time of flight spectrometer. The ions are neutralized in a collision cell filled with an alkali metal vapor a t a pressure of about 0.1 Pa. A set of electrostatic plates deflects the charged particles leaving the collision cell, i.e., the molecular ions not neutralized in the cell and the ionic fragments resulting from collision-induced dissociations. The nondissociating neutralized beam is intercepted at the centre of the detector. In Figure 1 , the Newton diagram for a single dissociating molecule AB* A + B is shown in the experimental geometry. It is assumed that the dissociation takes place in the collision cell and that the centre of mass of AB is not deflected. This last assumption is reasonable for near-resonant charge transfer, e.g., for charge transfer to an electronically excited state with the same ionization potential as the alkali-metal atom. Conservation of momentum and energy leads to partitioning of the kinetic energy t d between the fragments A and B inversely proportional to their masses mA and mB. The dissociation process can be completely described if the fragment impact distances RA and RB and the arrival time difference ( T ) are measured. This measurement is achieved with a position- and time-sensitive two-particle coincidence d e t e c t ~ r . ~ For each detected fragment pair A,B, the values of t d and mAare calculated according to
-
'd
=
C C A B ~ O(RA ~ + RB)' + (w)* -
2
L2
0.50 0.65 0.80 0.50 0.65 0.80 relative mass of heavy fragment Figure 2. Neutral mass spectra of ethane, propane, butane, and pentane after neutralization of the molecular ions by cesium atoms. The identification of the reactions is given in the text. The mass axis is given in units of mA/mAB.
0.5 0.6 0.7
atom or molecule, cannot be detected due to the limited size of the detector. The kinetic energy scale was calibrated by using the low-energy peaks (up to td = 2 ev) from the dissociative charge and cesium.IO exchange reactions of 02+
Results and Discussion The neutral mass spectra, shown in Figure 2, are taken for the neutralization reaction of ethane, propane, butane, and pentane cations with cesium (ionization potential IPcs = 3.89 eV). The spectra show dissociations into two radical fragments where the centre C-C bond is broken. The neutral mass spectrum for hexane is depicted in Figure 3. For hexane, the other C-C bonds are also broken. No hydrogen shift during the fragmentation is observed in the mass spectra. Dissociation reactions leading to the loss of H' or H2 from the molecule cannot be detected. The mass peaks in the spectra correspond with the reactions A-E.
C2H6+ + CS C3&+
(6) Feng, R.;Wcsdemiotis, C.; Zhang, M.; Marchetti, M.; McLafferty, F. W. J. Am. Chem.Soc. 1989, 1 1 1 , 1986. (7) De Bruijn, D. P.;La, J. Rev. Sci. Instrum. 1982, 53, 1020. (8) Karnig, S.;bijersbergen, J. H.M.; Van der Zande, W. J.; Los, J. Int. J . Mass Spectrom. Ion Proc. 1989, 93, 49. ( 9 ) Beijersbergen, J. H. M.; KBrnig, S.;Kistemaker, P.G.; Los, J. To be published.-
0.9
Figure 3. Neutral mass spectrum of hexane after neutralization of the molecular ions by cesium atoms.
CS
+
C2H6* C&*
-+
(1)
where p A B = mA!t?gmAB-lis the reduced mass, uo is the velocity of the precursor ion, and L is the distance between collision cell and detector. Each dissociation event leading to two fragments is measured and the data are stored in such a way that the neutral mass value and the kinetic energy release value ate accumulated for each dissociation reaction. We define mA 1 mB. The mass resolution mA/AmA is about 30. The apparatus transmission function varies as a function of both mA and Ed. Fragmentation into two products with mA/mAB> 0.85, like the loss of a hydrogen
0.8
relative mass of heavy fragment
C4Hlo'
+ CS
C5HI2++ Cs C6H14+ + CS
+
-
CdHlo*
cS+
cS+
-
+
+ CS+
CH3'
-+ -
CH3'
C2H5.
C5HI2*+ Cs+ C3H7'
C2H5'
+
C4H9'
td
+ cS+
td
(B)
C2H5'
-+
cS+
(A)
C2H5*
C,jH14* + CS+ C3H7.
+
CH,'
C3H7.
+ cS++ t d ( c ) cS+
td
+ cS++ t d
+ C2H5' + cS++ ed
(D)
(El)
(Ea
The mass peak in the hexane spectrum around mA/mAB= 0.85 might involve also hydrogen transfer resulting loss of CH4. For such a large molecule unit fragment mass resolution is not achieved. The kinetic energy release spectra for the dissociation of the central C-C bond are shown in Figure 4. The spectra show peaks ~~
(10) Van der Zande, W. J.; Koot, W.; Peterson, J. R.; Los, J. Chem. Phys. Lett. 1987, 140, 175.
The Journal of Physical Chemistry, Vol. 95, NO. 6,1991 2145
C-C Bond Dissociation in Excited Alkanes
u
0
0
1
3
9
4
5
kinetic energy release [eV]
Figure 4. Kinetic energy release spectra of the dissociative charge exchange reactions A through E l . The neutralization gas is cesium. The arrows indicate the excess energy for resonant charge exchange Era.
at a kinetic energy release value ranging from 3.5 eV for ethane to 0.9 eV for hexane. The arrows indicate the kinetic energy release E, for a resonant charge exchange process where all the excess energy is transferred to translational energy of the fragments where 1PAB is the ionization potential of the molecule AB and Do is the bond dissociation energy, defined as AH,-(A) + AH,-(B) AH,-(AB). The spectra obtained after neutralization with sodium have similar shapes but the peak positions are shifted 0.4-0.9eV toward lower energy. The shift is expected as can be seen from eq 3 since sodium has a higher ionization potential (IPN, = 5.14 eV). The kinetic energy release spectra show no intensity a t zero kinetic energy. This means that they cannot be described by statistical distributions, but they can be explained with the assumption that the dissociation reaction proceeds via a repulsive potential energy surface. The width of the kinetic energy release distribution is determined by the charge exchange process, the Franck-Condon overlap factors, and the internal energy distribution of the ion. The shift of the kinetic energy release peak in going from cesium to sodium can be understood by knowing that the charge exchange reaction with sodium takes place to a lower energy on the repulsive potential. The average energy released in the translational motion of the fragments is less than the raonance energy E, due to the dynamics of the dissociation process. In the dissociation the central carbon atoms repel each other. This repulsion is transferred to a translational motion of the two fragments and to a rotational motion of each of the fragments. The result is that the available energy is divided
between translational and rotational energy. Indeed, in the case of ethane where no rotational motion can be excited, the distribution peaks closely at the resonant value. The measured kinetic energy release for ethane cation with sodium equals the kinetic energy release for this reaction measured by Hop et al." Theoretical calculations of the geometry of ethane, propane, and butane cations show that in the minimal potential energy configuration one C-C bond is extensively lengthened.I2 In the case of butane, the central bond length of 1.54 A in the neutral molecule is lengthened to 2.02 A in the cation. Other geometries for the butane cation, which correspond to structures with one terminal C-C bond lengthened, both terminal C-C bonds lengthened, the central and one terminal C-C bond (constrained to be equal) lengthened, and finally all C-C bonds (constrained to be equal) lengthened, have higher potential energies (ranging from 0.3 to 0.1 eV). The authors state that these geometries are equally likely within the accuracy of their calculations. However, the measured neutral mass spectrum of butane shows a specific C-C bond dissociation in the center. We conclude that the butane cations which enter our collision cell have the minimum potential energy configuration; i.e., the central C-C bond is extensively lengthened. For pentane, similar reasoning can be applied, but as far we know no calculations have been made for the structure of the ion. In the case of hexane the other C-C dissociation channels are also observed. We have two suggestions for the geometry of the hexane ion: not only the central C-C bond is lengthened but all C-C bonds are lengthened or the position of the longer C-C bond is not localized. The mass spectra of butane and pentane show that only the central C-C bond dissociates and no terminal C-C bond is broken. This specific bond dissociation in butane and pentane forms a clear example of mode selectivity in large molecules. In a coming article9 we will discuss the charge exchange process to the repulsive state for normal alkanes and the energy partitioning over the various degrees of freedom in more detail.
Acknowledgment. The discussions with Dr. W. J. van der Zande and with Prof. N. M. M. Nibbering from the Institute for Mass Spectrometry a t the University of Amsterdam are acknowledged. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie FOM (Foundation for Fundamental Research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek N W O (Netherlands Organization for Advancement of Research). Registry No. H3CCH3,74-84-0; H3CCH2CH3,74-98-6; H3C(CH2)2CH3, 106-97-8; H,C(CH2)&H,, 109-66-0; HpC(CH2)SCHj, 1 1054-3. (11) Hop, C. E. C. A.; Holmes, J. L.; Wong, M. W.; Radom, L. Chem. Phys. Lett. 1989, 159, 580. (12) Bouma, W. J.; Poppinger, D.; Radom, L. Isr. J . Chem. 1983,23,21.
