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Intensity enhancement in the size distributions of acrylate cluster anions
Volume 197, number 4,5
CHEMICAL PHYSICS LETTERS
18 September 1992
Intensity enhancement in the size distributions of acrylate cluster anions Tatsuya Tsukuda and Tamotsu Kondow Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Received 15 February 1992; in final form 8 June I992
Cluster anions of methyl acrylate (MA) and methyl methacrylate (MMA) clusters were produced by electron transfer from high-Rydberg atoms of krypton and the mass spectra of the cluster anions were measured. The intensity of (MA), was particularly enhanced in the size distribution of MA); with an increase in the stagnation pressure of a sample gas seeded in helium, while no significant intensity enhancement was observed in the case of MMA); These results were interpreted as the occurrence of intracluster anionic polymerization in the formation of (MA);
1. Introduction Mass spectroscopic studies of various cluster ions reveal that there exist intensity anomalies (magic numbers) in otherwise smooth distributions of cluster ions [ 1- 14 1. Cluster ions with magic numbers are very often thermodynamically stable; for example, hydrogen-bonded cluster ions of ( CH30CHS)2H+ [l], (CH,OCHS),H,O+ [3] and (H20)4H+ [9] are thermodynamically stable and have particularly high intensities in the corresponding mass spectra. In the particular cases of cluster ions consisting of molecules which are readily polymerized, the magic numbers observed in the mass spectra have been interpreted as a consequence of intracluster polymerization. The intracluster polymer- ization results in the formation of chemically bonded molecular ions. Magic numbers of this kind have been reported in the clusters of ethylene [ 111, 1, l-difluoroethylene [ 11,121, acetone/ acetylene [ 13 1, isoprene [ 141, vinyl chloride [ 151 and acrylonitrile [ 161. These phenomena can be utilized for the investigation of the intracluster chemical reactions. In the present study, the intensity distributions of cluster anions of methyl acrylate (MA; CH*=CH-
CO&H,) and methyl methacrylate (MMA; CH2=C ( CH3 ) CO&H, ) were measured by means of mass spectrometry in the expectation that intensity enhancement of cluster anions with particular sizes results because of intracluster polymerization. These acrylates were studied because these molecules contain the highly electronegative carbonyl group and are polymerized by the anionic mechanism [ 17 1. In fact, the gas-phase anionic oligomerization of MA has been reported [ 18 1. Furthermore, it is expected that MMA is less reactive in the anionic polymerization because of the electron-releasing CH3 group in MMA [ 191. In mass spectroscopic studies, intracluster polymerization is recognizable through observation of magic numbers [ 1 l-l 41 or the loss of chemical species from intact cluster ions [ 15,161. It is essential, therefore, to use slow electrons so that the disturbance accompanied by the electron attachment is minimized and the cross section for the electron attachment is maximized [ 201. In order to meet with these requirements, a high-Rydberg krypton atom, IW*, was used as a slow electron source; the outermost electron of W* is regarded as a free electron having a kinetic energy equal to that of the outermost electron [ 2 11.
Correspondence to: T. Kondow, Department of Chemistry, Faculty of Science, The University ofTokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan.
438
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 197, number 4,5
I8 September 1992
CHEMICAL PHYSICS LETTERS
2. Experimental
The apparatus consists of a supersonic nozzle beam source, a triple-grid ion source, a quadrupole mass spectrometer (Extrel, 162-8) and a detector, as described in detail elsewhere [ 201. The liquid sample of MA (Tokyo Kasei, 99.01 pure) or MMA (Tokyo Kasei, 99.8O/bpure) without further purification was placed in a reservoir made of stainless steel and degassed three times by freeze-pump-thaw cycles immediately before use. Helium or argon gas of 0.6-3.2 atm was saturated with the sample vapor by flowing it over the liquid sample and expanded through an aperture of 0.05 mm diameter. A beam of acrylate clusters was skimmed and allowed to enter the ion source. The clusters were then ionized by impact of high-Rydberg krypton atoms in the central region of the ions source. In the highRydberg atom ionization @AI), 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. The principal quantum numbers of KP* were estimated to be in the range of 25-40 by the use of field ionization [ 20 1. The cluster anions thus produced were mass-analyzed and detected by a Ceratron (Murata, EMS108 1B ) after conversion to secondary positive ions on a conversion-dynode made of stainless steel. Signals from the detector were registered and processed by a multichannel analyzer based on a NEC 9801 microcomputer. The mass-to-charge ratios of the detected anions were calibrated with reference to those of (COZ); obtained in the same experimental conditions.
3. Results 3.1. Mass spectra Figs. 1A an 1B show typical mass spectra of the cluster anions of MA and MMA produced by RAI, respectively; MA and MMA clusters were produced at the stagnation pressure, PO,of 3.0 atm with helium as a seed gas. A salient series of peaks in the spectrum were identified as (MA); (n=2-12) and (MMA); (n=2-lo), respectively. Absence of their monomer anions shows that the vertical electron af-
A)
Pf Ah ‘f
4
3
5
7
6
8
9
10
1.I B)
1;
11
,
(MMAh +
3
4
200
’
460
S
6
7
8
9
IO
’
660
’
8bO
’
III00
Mass Number (m/z) Fig. 1. Mass spectra of cluster anions of methyl acrylate (MA) (panel (A)) and methyl methacrylate (MMA) (panel (B)) produced by impact of KFC at PO= 3.0 atm with helium as a seed gas. A trace amount of cluster anions containing one water molecule are also observed as a contaminant.
tinities of MA and MMA are negative as reported by Schafer et al. [22]. In the spectra of (MA); and (MMA); , the peaks at n = 2 were prominent in the entire range of the stagnation pressure studied. Furthermore, the peak at n= 5 in the (MA); spectrum was particularly enhanced with increasing stagnation pressure (see figs. 1A and 2A). On the other hand, its intensity enhancement was suppressed by introducing argon atoms into the parent neutral clusters (see fig. 3). 3.2. Cluster-ion distribution versus stagnation pressure 3.2.1. Helium seed
Figs. 2A and 2B show the intensity distributions of (MA); and (MMA); produced by RAI, respectively, where helium was used as a seed gas. The 439
(A)
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CHEMICAL PHYSICS LETTERS
Volume 197, number 4,5
:
h
3.0 atm
9
1.9 atm
+
1.3 atm
0
1.0 atm
-e
0.7 atm
-
3.0 atm
+
1.4 atm
9
1.0 atm
+
0.7 atm
200
Mass Ember Fig. 2. Intensity distributions of (MA); (panel (A)) and (MMA); (panel (B) ) produced by Kp impact at different stagnation pressures of helium.
error bars represent one standard deviation. As shown in fig. 2A, the intensity of (MA), increases most significantly, in comparison with those of (MA), and (MA);, as POincreases. This intensity enhancement at a high stagnation pressure should be associated with enrichment of large neutral clusters due to the increase in Pw The peak rise at n = 7 observed in the intensity distribution was not reproducible. In the (MMA); distribution, on the other hand, a broad peak in the n 3 3 range rises and shifts to a higher n as POincreases (see fig. 2B ). 3.2.2. Argon seed Fig. 3 shows the mass spectra of the cluster anions of MA produced at different stagnation pressures where argon was used as a seed gas. A large number of peaks assignable to binary clusters, [ (MA),(Ar),,] -, are observed in addition to those of (MA); at a high stagnation pressure. As shown in fig. 3, there is a tendency that the intensity distribution of (MA); becomes smooth as POincreases; 440
g/z)
Fig. 3. Mass spectra of the MA cluster anions produced at different stagnation pressures of argon: panel (A) 1.7 atm; panel (B) 2.6 atm; panel (C) 3.2 atm. The digit on each mass peak represents the cluster size, n, of (MA);. A large number of peaks appearing with those associated with (MA); are assigned to
l(MA),(Ar),,l-. intensities of the the at Po=3.2 atm, [ (MA),(Ar),,]peaks are comparable with those of the (MA); peaks (see fig. 3C).
4. Discussion
The intensity enhancement of (MA), is interpreted in such a manner that the neutral clusters larger than the pentamer are generated more efficiently at a high stagnation pressure and extensive evaporation after the electron attachment to the clusters results in enrichment of the stable anion, (MA), . The formation of (MA), is expressed as Kr’L (MA)m (m>,5)[WA),I* +(MA),+(m-5)MA.
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Volume 197, number 43
On the other hand, the result on the electron attachment to the binary cluster, [(Ma),(Ar),,], indicates that the evaporation of the constituent MA molecules from the nascent anion, [ (MA); ]* ( m > 5 ), is suppressed by introducing argon atoms in the MA cluster. Argon atoms weakly bound in [ (MA),( Ar ),, ] are first released when the excess energy is generated in the nascent cluster anion [ 23 1, namely,
[ (MAMAr),tlz -(MA)&
+m’Ar.
18 September 1992
CHEMICAL PHYSICS LETTERS
1(MA),(ArLf- I’ (2)
The intensity distribution of (MA); thus produced should be smooth as the parent cluster, [ (MA),(Ar),, 1, has a smooth distribution with respect to the size, m. These results lead us to conclude that the intensity of (MA), is enhanced because the stable (MA) I anion is enriched favorably after the extensive evaporation of the nascent cluster anions larger than the pentamer. The energetics of the evaporation process indicates that exothermic intracluster reactions occur in the formation of the stable (MA), anion, as given below in the electron attachment to the hexamer, ( MA)6, as an example. The excess energy, E,,( 6), is estimated to be x0.79 eV if the electron is captured in a single MA molecule of ( MA)6 without any further intracluster reactions (see Appendix). The number of the internal degrees of freedom in (MA) m is considered to be 6m-6, among which 3m-6 modes are attributed to the intermolecular vibrational modes and 3m to the molecular rotational modes. If E,,(m) is distributed statistically among the 3m-6 intermolecular vibrational and 3m molecular rotational modes, [ 1/(6m-6)]&,(m) is available for dissociation of the intermolecular bonds. Therefore, the excess energy transmitted to each intermolecular vibrational mode in [ (MA); ]* turns out to be 0.026 eV. This excess energy is not sufficient to liberate one MA molecule from (MA) a , because the energy required for the liberation of MA is estimated to be at least 0.24 eV by assuming that only the charge-dipole interaction between MA and MA- plays an important role in the bonding of (MA);. In summary, the extensive evaporation cannot be explained by such a mere electron-capture process in the cluster. It is rather likely that exo-
thermic intracluster reactions such as anionic polymerization are induced by the electron capture. In the case of (MMA); , no particular peak enhancement in the smooth intensity distribution was observed with the increase of PO up to 3.0 atm (see fig. 2B). Exothermic intracluster reactions do not seem to be involved in the formation of (MMA); . The difference in the reactivities of (MA); suggests that the intracluster reaction is anionic polymerization since the reactivity of the anionic polymerization of MA is greatly suppressed by the introduction of an electron-releasing methyl group (MMA) [ 191. At the present stage, no clear picture on the structure of (MA) F can be provided. In order to gain a further insight into the structure, a measurement of collision-induced dissociation will be useful.
Acknowledgement The authors are indebted to Dr. T. Nagata for valuable discussion. Thanks are also due to Professor R. Okazaki for discussion on the anionic polymerization of MA and MMA. 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.
Appendix: Estimation of the excess energy in the RAI of (MA),,,
The excess energy, E,,(m),generated in the collisional electron transfer from Kpc to (MA) mis given by the difference in the adiabatic electron affinity of (MA),,,, EA,( m), and the ionization potential of Kr** on the assumption that the electron is localized in a single component molecule as [ 24,25 ] (3)
&,(m)=E&(m),
where the ionization potential of Kr** ( 1O-20 meV) is negligible. The EA, (m) value is given by [ 25,261 BA,(m)=EA,(l)-&o(m) +&,(m-
1)
(m>2)
,
(4)
where Ebo(m) is the binding energy of (MA),,, and E,,(m - 1) is the solvation energy of MA- provided 441
Volume 197, number 4,5
CHEMICAL PHYSICS LETTERS
by m - 1 constituent molecules. The binding energy of (MA); can be approximated as E,,i(m-l)=(m-l)E,,,
(5)
E = -- ’
(6)
cd
pe 47~~ r2 ’
where p is the permanent dipole moment of MA and r is the distance between MA- and a solvating MA. The dipole moment of MA is approximated by that of methacrylic acid ( 1.65 D [ 271) and the r value is taken from the intermolecular distance between the nearest neighbors in the MA crystal (4.55 8, [ 281). In the calculation of the binding energy of (MA),, the MA cluster is postulated to have a geometry identical with the MA crystal. The binding energy for a pair of MA molecules is estimated from the heat of vaporization, AH= 0.37 eV [ 29 1, and the number of the molecules in the first shell, nNN= 14, as E,,(2) z AH/nNN ~0.03 eV .
(7)
The value of EA,( 1) may be negative since in the present measurement MA- is not observed in the mass spectrum for the cluster anions of (MA)m by EI. In order to estimate the upper limit of EA,( m), EA, is set to be zero. In the case of m = 6, E,, ( 5 ) and EbO(6) are calculated to be 1.19 and 0.40 eV, respectively. Taking the values altogether, one obtains the adiabatic electron affinity of (MA), to be 0.79 eV.
References [ 1] E.P. Grimsrud and P. Kebarle, J. Am. Chem. Sot. 95 ( 1973) 7939. [2] S. Lin, Rev. Sci. Instr. 44 (1973) 516. [ 31 K. Hiraoka, E.P. Grimsrud and P. Kebarle, J. Am. Chem. Sot. 96 (1974) 3359. [4] J.Q. Searcy and J.B. Fenn, J. Chem. Phys. 61 (1974) 5282.
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[ 51 J.L. Kassner and D.E. Hagen, J. Chem. Phys. 64 (1976) 1860. [6] G.M. Lancaster, F. Honda, Y. Fukuda and J.W. Rabalais, J. Am. Chem. Sot. 101 (1979) 1951. [ 7) Y.K. Lau, P.P.S. Saluja and P. Kebarle, J. Am. Chem. Sot. 102 (1980) 7429. [8] P.M. Holland an A.W. Castleman Jr., J. Chem. Phys. 72 (1980) 5984. [9] A.J. State and C. Moore, J. Phys. Chem. 86 (1982) 3681. [lo] N. Nishi, K. Yamamoto, H. Shinohara, U. Nagashima and T. Okuyama, Chem. Phys. Letters 122 (1985) 599. [ 111 J.F. Garvey and R.B. Bernstein, Chem. Phys. Letters 126 (1986) 394. [ 121 M.T. Coolbaugh, G. Vaidynathan, W.R. Peiferx and J.F. Garvey, J. Phys. Chem. 95 (1991) 8337. [ 131 S.G. Whitney, M.T. Coolbaugh, G. Vaidynathan and J.F. Garvey, J. Phys. Chem. 95 ( 1991) 9625. [ 141 MS. El-Shall and C. Marks, J. Phys. Chem. 95 ( 1991) 4932. [ 151 MS. El-Shall and K.E. Schriver, J. Chem. Phys. 95 ( 1991) 3001. [ 161 T. Tsukuda and T. Kondow, J. Chem. Phys. 95 (1991) 6989. [ 171 M. Okamoto, C. Aoki and 0. Ishizuka, Nippon Kagaku Kaishi (J. Chem. Sot. Japan Chem. Indust. Chem.) (1977) 103. [ 181 R.N. McDonald and A.K. Chowdhury, J. Am. Chem. Sot. 105 (1983) 2194. [ 191 H.R. Allcock and F.W. Lampe, in: Contemporary polymer chemistry, 2nd Ed. (Prentice Hall, Englewood Cliffs, 1990). [20] T. Kondow, J. Phys. Chem. 91 (1987) 1307. [ 211 B.G. Zollars, C. Higgins, F. Lu, C.W. Walter, L.G. Gray, K.A. Smith, F.B. Dunning and R.F. Stebbings, Phys. Rev. A 32 (1985) 3330. [22] 0. Schafer, M. Allan, E. Haselbach and R.S. Davidson, Photochem. Photobiol. 50 ( 1989) 7 17. [ 231 H. Shinohara, H. Sato and N. Washida, J. Phys. Chem. 94 (1990) 6718. [24] F. Misaizu, K. Mitsuke, T. Kondow and K. Kuchitsu, J. Phys. Chem. 93 (1989) 4263. [25] F. Misaizu, T. Kondow and K. Kuchitsu, Chem. Phys. Letters 178 (1991) 369. [26] K. Mitsuke, T. Kondow and K. Kuchitsu, J. Phys. Chem. 90 (1986) 1552. [27] A. Kotera, J. Chem. Sot. Japan 63 (1942) 364. [28] T. Gillbro, P.O. Kinell and A. Lund, J. Polym. Sci. A-2 9 (1971) 1495. [ 291 R.C. Weast, in: Handbook of Chemistry and Physics, 59th Ed. (CRC press, Boca Raton, 1978/79).
CHEMICAL PHYSICS LETTERS
18 September 1992
Intensity enhancement in the size distributions of acrylate cluster anions Tatsuya Tsukuda and Tamotsu Kondow Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Received 15 February 1992; in final form 8 June I992
Cluster anions of methyl acrylate (MA) and methyl methacrylate (MMA) clusters were produced by electron transfer from high-Rydberg atoms of krypton and the mass spectra of the cluster anions were measured. The intensity of (MA), was particularly enhanced in the size distribution of MA); with an increase in the stagnation pressure of a sample gas seeded in helium, while no significant intensity enhancement was observed in the case of MMA); These results were interpreted as the occurrence of intracluster anionic polymerization in the formation of (MA);
1. Introduction Mass spectroscopic studies of various cluster ions reveal that there exist intensity anomalies (magic numbers) in otherwise smooth distributions of cluster ions [ 1- 14 1. Cluster ions with magic numbers are very often thermodynamically stable; for example, hydrogen-bonded cluster ions of ( CH30CHS)2H+ [l], (CH,OCHS),H,O+ [3] and (H20)4H+ [9] are thermodynamically stable and have particularly high intensities in the corresponding mass spectra. In the particular cases of cluster ions consisting of molecules which are readily polymerized, the magic numbers observed in the mass spectra have been interpreted as a consequence of intracluster polymerization. The intracluster polymer- ization results in the formation of chemically bonded molecular ions. Magic numbers of this kind have been reported in the clusters of ethylene [ 111, 1, l-difluoroethylene [ 11,121, acetone/ acetylene [ 13 1, isoprene [ 141, vinyl chloride [ 151 and acrylonitrile [ 161. These phenomena can be utilized for the investigation of the intracluster chemical reactions. In the present study, the intensity distributions of cluster anions of methyl acrylate (MA; CH*=CH-
CO&H,) and methyl methacrylate (MMA; CH2=C ( CH3 ) CO&H, ) were measured by means of mass spectrometry in the expectation that intensity enhancement of cluster anions with particular sizes results because of intracluster polymerization. These acrylates were studied because these molecules contain the highly electronegative carbonyl group and are polymerized by the anionic mechanism [ 17 1. In fact, the gas-phase anionic oligomerization of MA has been reported [ 18 1. Furthermore, it is expected that MMA is less reactive in the anionic polymerization because of the electron-releasing CH3 group in MMA [ 191. In mass spectroscopic studies, intracluster polymerization is recognizable through observation of magic numbers [ 1 l-l 41 or the loss of chemical species from intact cluster ions [ 15,161. It is essential, therefore, to use slow electrons so that the disturbance accompanied by the electron attachment is minimized and the cross section for the electron attachment is maximized [ 201. In order to meet with these requirements, a high-Rydberg krypton atom, IW*, was used as a slow electron source; the outermost electron of W* is regarded as a free electron having a kinetic energy equal to that of the outermost electron [ 2 11.
Correspondence to: T. Kondow, Department of Chemistry, Faculty of Science, The University ofTokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan.
438
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 197, number 4,5
I8 September 1992
CHEMICAL PHYSICS LETTERS
2. Experimental
The apparatus consists of a supersonic nozzle beam source, a triple-grid ion source, a quadrupole mass spectrometer (Extrel, 162-8) and a detector, as described in detail elsewhere [ 201. The liquid sample of MA (Tokyo Kasei, 99.01 pure) or MMA (Tokyo Kasei, 99.8O/bpure) without further purification was placed in a reservoir made of stainless steel and degassed three times by freeze-pump-thaw cycles immediately before use. Helium or argon gas of 0.6-3.2 atm was saturated with the sample vapor by flowing it over the liquid sample and expanded through an aperture of 0.05 mm diameter. A beam of acrylate clusters was skimmed and allowed to enter the ion source. The clusters were then ionized by impact of high-Rydberg krypton atoms in the central region of the ions source. In the highRydberg atom ionization @AI), 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. The principal quantum numbers of KP* were estimated to be in the range of 25-40 by the use of field ionization [ 20 1. The cluster anions thus produced were mass-analyzed and detected by a Ceratron (Murata, EMS108 1B ) after conversion to secondary positive ions on a conversion-dynode made of stainless steel. Signals from the detector were registered and processed by a multichannel analyzer based on a NEC 9801 microcomputer. The mass-to-charge ratios of the detected anions were calibrated with reference to those of (COZ); obtained in the same experimental conditions.
3. Results 3.1. Mass spectra Figs. 1A an 1B show typical mass spectra of the cluster anions of MA and MMA produced by RAI, respectively; MA and MMA clusters were produced at the stagnation pressure, PO,of 3.0 atm with helium as a seed gas. A salient series of peaks in the spectrum were identified as (MA); (n=2-12) and (MMA); (n=2-lo), respectively. Absence of their monomer anions shows that the vertical electron af-
A)
Pf Ah ‘f
4
3
5
7
6
8
9
10
1.I B)
1;
11
,
(MMAh +
3
4
200
’
460
S
6
7
8
9
IO
’
660
’
8bO
’
III00
Mass Number (m/z) Fig. 1. Mass spectra of cluster anions of methyl acrylate (MA) (panel (A)) and methyl methacrylate (MMA) (panel (B)) produced by impact of KFC at PO= 3.0 atm with helium as a seed gas. A trace amount of cluster anions containing one water molecule are also observed as a contaminant.
tinities of MA and MMA are negative as reported by Schafer et al. [22]. In the spectra of (MA); and (MMA); , the peaks at n = 2 were prominent in the entire range of the stagnation pressure studied. Furthermore, the peak at n= 5 in the (MA); spectrum was particularly enhanced with increasing stagnation pressure (see figs. 1A and 2A). On the other hand, its intensity enhancement was suppressed by introducing argon atoms into the parent neutral clusters (see fig. 3). 3.2. Cluster-ion distribution versus stagnation pressure 3.2.1. Helium seed
Figs. 2A and 2B show the intensity distributions of (MA); and (MMA); produced by RAI, respectively, where helium was used as a seed gas. The 439
(A)
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CHEMICAL PHYSICS LETTERS
Volume 197, number 4,5
:
h
3.0 atm
9
1.9 atm
+
1.3 atm
0
1.0 atm
-e
0.7 atm
-
3.0 atm
+
1.4 atm
9
1.0 atm
+
0.7 atm
200
Mass Ember Fig. 2. Intensity distributions of (MA); (panel (A)) and (MMA); (panel (B) ) produced by Kp impact at different stagnation pressures of helium.
error bars represent one standard deviation. As shown in fig. 2A, the intensity of (MA), increases most significantly, in comparison with those of (MA), and (MA);, as POincreases. This intensity enhancement at a high stagnation pressure should be associated with enrichment of large neutral clusters due to the increase in Pw The peak rise at n = 7 observed in the intensity distribution was not reproducible. In the (MMA); distribution, on the other hand, a broad peak in the n 3 3 range rises and shifts to a higher n as POincreases (see fig. 2B ). 3.2.2. Argon seed Fig. 3 shows the mass spectra of the cluster anions of MA produced at different stagnation pressures where argon was used as a seed gas. A large number of peaks assignable to binary clusters, [ (MA),(Ar),,] -, are observed in addition to those of (MA); at a high stagnation pressure. As shown in fig. 3, there is a tendency that the intensity distribution of (MA); becomes smooth as POincreases; 440
g/z)
Fig. 3. Mass spectra of the MA cluster anions produced at different stagnation pressures of argon: panel (A) 1.7 atm; panel (B) 2.6 atm; panel (C) 3.2 atm. The digit on each mass peak represents the cluster size, n, of (MA);. A large number of peaks appearing with those associated with (MA); are assigned to
l(MA),(Ar),,l-. intensities of the the at Po=3.2 atm, [ (MA),(Ar),,]peaks are comparable with those of the (MA); peaks (see fig. 3C).
4. Discussion
The intensity enhancement of (MA), is interpreted in such a manner that the neutral clusters larger than the pentamer are generated more efficiently at a high stagnation pressure and extensive evaporation after the electron attachment to the clusters results in enrichment of the stable anion, (MA), . The formation of (MA), is expressed as Kr’L (MA)m (m>,5)[WA),I* +(MA),+(m-5)MA.
(1)
Volume 197, number 43
On the other hand, the result on the electron attachment to the binary cluster, [(Ma),(Ar),,], indicates that the evaporation of the constituent MA molecules from the nascent anion, [ (MA); ]* ( m > 5 ), is suppressed by introducing argon atoms in the MA cluster. Argon atoms weakly bound in [ (MA),( Ar ),, ] are first released when the excess energy is generated in the nascent cluster anion [ 23 1, namely,
[ (MAMAr),tlz -(MA)&
+m’Ar.
18 September 1992
CHEMICAL PHYSICS LETTERS
1(MA),(ArLf- I’ (2)
The intensity distribution of (MA); thus produced should be smooth as the parent cluster, [ (MA),(Ar),, 1, has a smooth distribution with respect to the size, m. These results lead us to conclude that the intensity of (MA), is enhanced because the stable (MA) I anion is enriched favorably after the extensive evaporation of the nascent cluster anions larger than the pentamer. The energetics of the evaporation process indicates that exothermic intracluster reactions occur in the formation of the stable (MA), anion, as given below in the electron attachment to the hexamer, ( MA)6, as an example. The excess energy, E,,( 6), is estimated to be x0.79 eV if the electron is captured in a single MA molecule of ( MA)6 without any further intracluster reactions (see Appendix). The number of the internal degrees of freedom in (MA) m is considered to be 6m-6, among which 3m-6 modes are attributed to the intermolecular vibrational modes and 3m to the molecular rotational modes. If E,,(m) is distributed statistically among the 3m-6 intermolecular vibrational and 3m molecular rotational modes, [ 1/(6m-6)]&,(m) is available for dissociation of the intermolecular bonds. Therefore, the excess energy transmitted to each intermolecular vibrational mode in [ (MA); ]* turns out to be 0.026 eV. This excess energy is not sufficient to liberate one MA molecule from (MA) a , because the energy required for the liberation of MA is estimated to be at least 0.24 eV by assuming that only the charge-dipole interaction between MA and MA- plays an important role in the bonding of (MA);. In summary, the extensive evaporation cannot be explained by such a mere electron-capture process in the cluster. It is rather likely that exo-
thermic intracluster reactions such as anionic polymerization are induced by the electron capture. In the case of (MMA); , no particular peak enhancement in the smooth intensity distribution was observed with the increase of PO up to 3.0 atm (see fig. 2B). Exothermic intracluster reactions do not seem to be involved in the formation of (MMA); . The difference in the reactivities of (MA); suggests that the intracluster reaction is anionic polymerization since the reactivity of the anionic polymerization of MA is greatly suppressed by the introduction of an electron-releasing methyl group (MMA) [ 191. At the present stage, no clear picture on the structure of (MA) F can be provided. In order to gain a further insight into the structure, a measurement of collision-induced dissociation will be useful.
Acknowledgement The authors are indebted to Dr. T. Nagata for valuable discussion. Thanks are also due to Professor R. Okazaki for discussion on the anionic polymerization of MA and MMA. 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.
Appendix: Estimation of the excess energy in the RAI of (MA),,,
The excess energy, E,,(m),generated in the collisional electron transfer from Kpc to (MA) mis given by the difference in the adiabatic electron affinity of (MA),,,, EA,( m), and the ionization potential of Kr** on the assumption that the electron is localized in a single component molecule as [ 24,25 ] (3)
&,(m)=E&(m),
where the ionization potential of Kr** ( 1O-20 meV) is negligible. The EA, (m) value is given by [ 25,261 BA,(m)=EA,(l)-&o(m) +&,(m-
1)
(m>2)
,
(4)
where Ebo(m) is the binding energy of (MA),,, and E,,(m - 1) is the solvation energy of MA- provided 441
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CHEMICAL PHYSICS LETTERS
by m - 1 constituent molecules. The binding energy of (MA); can be approximated as E,,i(m-l)=(m-l)E,,,
(5)
E = -- ’
(6)
cd
pe 47~~ r2 ’
where p is the permanent dipole moment of MA and r is the distance between MA- and a solvating MA. The dipole moment of MA is approximated by that of methacrylic acid ( 1.65 D [ 271) and the r value is taken from the intermolecular distance between the nearest neighbors in the MA crystal (4.55 8, [ 281). In the calculation of the binding energy of (MA),, the MA cluster is postulated to have a geometry identical with the MA crystal. The binding energy for a pair of MA molecules is estimated from the heat of vaporization, AH= 0.37 eV [ 29 1, and the number of the molecules in the first shell, nNN= 14, as E,,(2) z AH/nNN ~0.03 eV .
(7)
The value of EA,( 1) may be negative since in the present measurement MA- is not observed in the mass spectrum for the cluster anions of (MA)m by EI. In order to estimate the upper limit of EA,( m), EA, is set to be zero. In the case of m = 6, E,, ( 5 ) and EbO(6) are calculated to be 1.19 and 0.40 eV, respectively. Taking the values altogether, one obtains the adiabatic electron affinity of (MA), to be 0.79 eV.
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