The Journal of
Physical Chemistry
0 Copyright, 1992, by the American Chemical Society
VOLUME 96, NUMBER 14 JULY 9, 1992
LETTERS Anionic Polymerization in the Gas-Phase Cluster of 2-Chloroacrylonitriie Tatsuya Tsukuda and Tamotsu Kondow* Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 1 1 3, Japan (Received: December 23, 1991; In Final Form: April 16, 1992)
Intracluster reactions in the gas-phase cluster of 2-chloroacrylonitrile(CAN) following the collisional electron transfer from a high-Rydberg Kr atom (Kr**) were investigated by mass spectrometry. The cluster anions (CAN); with n 2 1 and [(CAN),-n'HCll- with 2 In I 4 and n ' l 1 were observed. The formation of [(CAN),-n'HCI]- indicates that intracluster anionic polymerization takes place and that excess energy associated with the polymerization is used for eliminating HCl molecules from the corresponding nascent anion in a small size range.
1. Introduction Ionization of clusters has attracted much attention, since evidence of intracluster chemical reactions is observed in the cluster ions produced.'-9 These reactions proceed in such a reaction environment that all the reactants bound weakly are arranged in a specific geometry and react with each other in the ionization. This specific environment may be utilized to explore characteristic chemical reactions which are scarcely encountered in ordinary reaction Given this prospect, we have investigated polymerization in an acrylonitrile (AN) cluster by taking advantage of a ring geometry made by the constituent AN molecuies.9 In the present report, electron attachment to a 2-chloroacrylonitrile (CAN) cluster was investigated by mass spectrometry. The cluster of CAN was used because a CAN molecule contains the highly electronegative cyano group and readily undergoes the anionic polymerization.I0 In addition, a CAN polymer is dehydrochlorinated by heat treatment into a polymer having a polyene s t r ~ c t u r e . ' ~ -The ' ~ easy release of HCl molecules by the heat treatment facilitates mass spectrometric recognition of the intracluster anionic polymerization through the loss of HCl molecules from the nascent polymer anion. Gentle and efficient electron attachment was performed by use of the collisional 0022-3654/92/2096-567 1$03.00/0
electron transfer of the outermost electron of a high-Rydberg Kr atom, Kr**.I5 2. Experimental Section The apparatus consists of a supersonic nozzle beam source, a triplagrid ion source, and a quadrupole mass spectrometer (Extrel, 162-8), as described in detail elsewhere.I6 In order to prepare the cluster of 2-chloroacrylonitrile(CAN), 5 atm of helium gas was passed Over its liquid sample in a stainless steel reservoir kept at 70 f 5 OC. On the other hand, the clusters with small sizes were prepared in a free jet expansion of the sample gas of 50 Torr seeded in helium gas at 6 atm; the sample was diluted with helium gas in a stainless steel reservoir of 7 L. A beam of the CAN cluster, after passing through a skimmer and a collimator, was allowed to enter the ion source consisting of filaments and three concentric cylindrical grids. The CAN cluster was then ionized either by high-Rydberg atom impact (RAI) or by electron impact (EI) in the central region of the ion source. In the RAI mode, krypton gas was admitted into the ion source at a pushing pressure of 0.2 Torr and then was excited by impact of 50-eV electrons. Charged species produced concurrently were repelled by appropriate potentials applied on each grid so as to admit only neutral species including Kr** into the central region. 0 1992 American Chemical Society
5672 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
-.-
Letters
I
v)
4-
5
p
-3HCI
-HCI
-2HCI
4
I
Number of HCI molecules ehmmnared fn Y
I
100
200
300
Mass Number (m/z) Figure 1. Mass spectrum of cluster anions produced in the collision of Kr** with 2-chloroacrylonitrile (CAN) clusters.
The principal quantum numbers of Kr** were estimated to be in the range 25-40 by field ionization.16 When the E1 mode was employed, the average energy of electrons, t, was varied in the range 2-10 eV. The energy spread (fwhm) was estimated to be 1 eV at t = 0 eV from the measured cross section curve for the electron attachment to SF6 in comparison with the reported one." The stagnation pressure dependences of anions were measured. In addition, the intensity of the cluster cation, (CAN),+ with n = 1-3, produced by impact of 14-eV electrons was measured as a function of the stagnation pressure. The cluster anions thus produced were mass-analyzed and detected by a Ceratron (Murata, EMS- 1081B) after conversion of the anions into secondary positive ions on a conversion dynode made of stainless steel. Signals from the detector were registered in and processed by a multichannel analyzer based on an NEC 9801 microcomputer.
,
I
I
Figure 2. Size distributions of cluster anions, (CAN), (ion a) and [(CAN),-nlHCl]- (ion b), produced in the collision of Kr** with CAN clusters. The intensities of ion b are given by shaded histograms.
1
1o4
i
4'
t
3. Results 3.1. Observed Ions and Size Distributions. Figure 1 shows a typical mass spectrum of cluster anions produced by RAI from the cluster of 2-chloroacrylonitrile, (CAN),,,. The mass spectrum of each cluster anion exhibits several isotope peaks with the relative intensities expected from the isotope abundances of 35Cland 37Cl. The mass-techarge ratios of the peaks were determined by taking advantage of the ratio of j5CI and 37Cl. The following cluster anions were observed
(a) (CAN); (b)
with n L 1
[(CAN),-n'HCl]-
(c) Cl-(CAN),
with n 1 2 and n' 2 1
with n 1 1
where, for example, [(CAN),-HCl]- denotes the cluster anion produced by abstraction of one HCI molecule from (CAN),-. The intensity of a given cluster anion was obtained from the sum of the intensities of the isotope peaks. Figure 2 shows the intensities of ions a and b thus obtained as functions of the cluster size, n, and the number of released HCl molecules, n'. As shown in Figure 2, the intensities of (CAN),- (ion a) decrease monotonically with n, with a slight enhancement at n = 3. The intensity of ion b is prominent at (n,n? = (2,1), (3,2), and (3,3); ion b with n 1 4 is scarcely populated. In addition, [(CAN)-HCl]- was not produced by either RAI or E1 ionizations. 3.2 Depemlence of Ion Intessities 011 Stagnatioa Pressure. The intensities of [(CAN)2-n'HCl]- with n' = 1 and 2 produced by RAI are plotted against the stagnation pressure, Po, in Figure 3. Also shown are the intensities of (CAN),' with n = 1-3 produced by impact of 14-eV electrons as a function of Po. The ion intensities are proportional to Pou(")in the onset pressure region. The values of a(n) calculated by the least-squares method for (CAN),+ are a(1) = 0.81 i 0.01, 4 2 ) = 3.5 i 0.2, and a(3) = 4.3 f 0.4, while those for [(CAN),-HCl]- and [(CAN),-
-c0
4 100' 1'02
1'03
!
Po i Torr Figure 3. Intensities of [ (CAN)*-HCl]- (closed squares) and [(CAN)2-2HCI]- (open squares) plotted against the stagnation pressure, together with those of (CAN),' (closed circles) produced by impact of 14-eV electrons. A digit on each curve represents the cluster size, n, of (CAN),'.
2HClI- are 3.7 i 0.4 and 3.6 f 0.6, respectively. Evidently, the a values for [(CAN),-n'HCl]- with n = 1 and 2 agree with that of (CAN),' within the experimental uncertainties. This agreement indicates that ((CAN),-nHCl]- with n r = 1 and 2 are produced from the same neutral precursor with that of (CAN),'. In the formation of (CAN),' by impact of electrons having a nearthreshold ionization energy, the excess energy associated is approximated by the difFerence between the vertical and the adiabatic ionization potentials of CAN, if the positive charge in (CAN),' is localized in one component molecule. As no vertical and adiabatic ionization potentials of a CAN molecule are available, the data of vinyl chloride'* and ethylene'* were used instead for the estimate of the excess energy. The excess energy thus estimated was 0.18 eV if vinyl chloride was used, while 0.00 eV for ethylene. Hence, no extensive evaporation is conceivable in the formation of (CAN)2+,because the excess energy thus estimated is much less than the bond dissociation energy of C2H4+C2H4
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5673
Letters (0.69 eV).I9 This argument leads us to conclude that (CAN)2 is the dominant neutral precursor of (CAN)2+ and hence of [(CAN)2-n'HCl]-. 4. Discussion As stated in section 3.2, the neutral precursor of [(CAN),nWCl]- with n' = 1 and 2 is (CAN), and the overall reaction proceeds as
-
(CAN)2
+ Kr**
+ HCl + Kr' + 2HCl + Kr+
[(CAN)rHCl]-
[(CAN)2-2HCl]-
(1)
(2) In reaction 2, all the constituent CAN molecules in the cluster participate in the abstraction reaction of HCl. In other words, an intracluster reaction such as intracluster polymerization takes place in the production of [(CAN)2-2HCl]-. It seems that one HCl molecule is liberated from at least two CAN molecules in the ionization, since [(CAN),-HCl]- with n 1 2 is observed but [(CAN)-HCl]- is not by mass spectrometry (see Figure 2). This finding lends further support for the intracluster polymerization. The consideration of the energetics of reaction 1 leads us to conclude that intracluster polymerization occurs in the formation of [(CAN)rHCl]- as discussed below: Let us assume here that the electron is dissociatively captured by one of the component molecules without any intracluster polymerization, namely
+ Kr**
-
+ HC1+ Kr+
[(CAN)3-3HCl]- is a radical anion of 1,3,5-tricyanobemene whose electron affinity is positi~e.2~ In fact, formation of the six-membered ring is the most favorable in the cyclization as explained from statistical and energetic considerations.26Evidently, there is a tendency that the intensity of [(CAN),-n'HCl]- decreases with n; in particular,the tetramer anion (ion b with n = 4) is much less abundant than the trimer anion. Also observed is a slight enhancement of (CAN)3-. These behaviors indicate, in the framework of the cyclic polymerization model, that chain prop agation from the trimer to the tetramer is suppressed by the cyclization of the trimer itself and hence is bottlenecked by it. It is concluded that in the first place the cyclic trimer anion, (CAN)3-', is formed in the nascent cluster anion and the cyclic trimers of (CAN)3- and [(CAN)3-n'HCl]- emerge as a result of the release of CAN and HCl molecules. No condensation polymerization is likely to occur since HC1 molecules are not removed from the nascent cluster anion, (CAN);*, in a stoichiometric manner. Acknowledgmenr. We are indebted to Dr. T. Nagata for valuable discussion. The present work has been supported by a Grant-in-Aid for Scientific Research on Priority Areas by the Ministry of Education, Science and Culture of Japan.
References and Notea
(1) Garvey, J. F.; Bcmstein, R. B. J . Phys. Chem. 1986, 90,3577. (2) Garvey, J. F.; Bemstein, R. B. J. Am. Chem. Soc. 1987, 109, 1921. (3) Coolbaugh,M. T.; Peifer, W. R.; Garvey, J. F. Chem. Phys. Le??.1989, The heat of reaction for this process is given by 156, 19. (4) Coolbaugh, M. T.; Pcifer, W. R.; Garvey, J. F. Chem. Phys. Let?. 1990, AH D(CH2CCN-Cl) D(H-CHC(C1)CN) - D(H-Cl) 168, 337. { D ( C H e C N ) - D(HC'4'CN)J E A , ( C H e C N ) (4) (5) Coolbaugh, M. T.; Peifer, W. R.; Garvey, J. F. J . Phys. Chem. 1990, 94, 1619. where the bond dissociation energy, D(H-Cl), is reported to be (6) Morita, H.; Freitas, J. E.; El-Sayed, M. A. J . Phys. Chem. 1991,95, 4.4 eV.20 The bond dissociation energies D(CH,CCN-Cl), D1664. (7) El-Shall, M. S.;Marks, C. J . Phys. Chem. 1991, 95, 4932. (H-CHC(Cl)CN), D ( C H a C N ) , and D ( H C ' 4 ' C N ) are (8) El-Shall, M. S.; Schriver, K. E. J. Chem. Phys. 1991, 95, 3001. approximated from those of CH,CH-Cl (3.9 eV)?' H-CHCH2 (9) Tsukuda, T.; Kondow, T. J . Chem. Phys. 1991.95, 6989. (4.7 eV),22C H e H (10.0 eV),23and C H 2 4 H z (7.5 eV)?3 (10) Mark, H.; Tobolsky, A. V. Physical Chemistry of High Polymeric respectively. The term ( D ( C H 4 C N ) D ( H C ' 4 ' C N ) J r e p Systems; 1950. (11) Grassie, N.; Grant, E. M. J . Polym. Sci. C 1967, 16, 591. resents an energy gain by the formation of the triple bond from (12) Okamoto, M.; Aoki, C.; Ishizuka, 0.Nippon Kagaku Kaishi 1977, the double bond. The adiabatic electron affinity of C H W C N , 103. EA,(CH
