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Photodissociation of methyl fluoride clusters
Journal of Electron Spectroscopy and Related Phenomena, 41 (19%) 411-418 Eleevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PHOTODISSOCIATION
OF METHYL FLUORIDE CLUSTERS*
E. RUHL, P. BISLING, B. BRUTSCHY and H. BAUMGARTEL Zn&itut fiir Phyaika~i8che und Theoretische Chemie, Freie Universitit Berlin, 33 (F.R.G.)
Berlin, Takuatr. 3. D-Zoo0
(Received 3 July 1936) ABSTRACT Photoionization efficiency curves of cations produced by photodissociation of methyl fluoride clusters ((CH,F),) have been measured between 1leV and 17eV using monochromatized synchrotron radiation. The appearance potentials (AP) of the most dominant cluster fragments are: AP(CH,F+) = 12.43eV, AP(C,H,F+) = 12.23eV, AP(C,&F+) = 12.47eV, AP(C,&F:) = 13.09eV, and AP(C,H,F,+ ) = 12.99eV. The energetica of the threshold processes are discussed in relation to an appropriate fragmentation mechanism. The absolute proton afllnity of CH,F (PA(CH,F) = 129 f 3 kcal mol-*) and the solvation energies (Ae) of CH,F+, CH: , and CH*F+ are reported. INTRODUCTION
The molecular beam photoionization technique is a useful tool for the measurement of reliable ionization potentials of weakly bound complexes [l-6]. According to the kind of intermolecular. interactions typical fragmentation reactions of the initially-formed clusters are observed: van der Waals (vdW) clusters of, for example, aromatics dissociate preferably by cleavage of the intermolecular vdW bond; hydrogen bond complexes are usually stabilized by proton transfer (e.g., hydrogen halides [Z], ammonia [3], alkylamines [4]). Recently, the cluster fragmentation of methyl fluoride has been studied by Garvey and Bernstein measuring the cluster electron impact mass spectrum [7, 81. The fragmentation products they observed have been discussed in terms of intramolecular ion-molecule reactions. From the pressure dependency of the mass spectrum it was deduced that the observed cluster fragments are formed via protonated clusters by ejection of neutral closed shell molecules. The role of larger clusters is assumed to be negligible because of the great reactivity of the molecular ions [S]. Ding et al. [9] have investigated ion-molecule reactions within methane clusters initiated by photoionization. Extensive chemical fragmentation patterns are found in the cluster mass regime. This observation is related to ion-molecule reactions where generally equivalent products are obtained. In this study we report the cluster fragmentation of methyl fluoride by means of the photoionization efficiency (PIE) curves of the most abundant cluster *Dedicated to Professor E. Heilbronner on the occasion of his 65th birthday.
o36R2048/86/$03.50
0 1939 Elsevier Science Publishers B.V.
412
fragments in the mass regime from m/z = 35 to m/z = 69. The measured APs are used to derive standard heats of formation (A#‘) of the observed cationic species at 0 K. This assumption is justified by the fact that the final temperature of the beam is approximately 15K according to temperature estimates [lo]. These data are used to provide thermochemical information on the major fragmentation processes. EXPERIMENTAL
The experimental arrangement used in this study has been described previously [5]. Briefly, the apparatus consists of a lm normal incidence monochromator (McPherson 225), which disperses the synchrotron radiation of the electron storage ring BESSY, a quadrupole mass spectrometer and a VUV light detector. The clusters are produced by expansion of pure CHSF gas through a 80 pm sonic nozzle and a 500 ,umCampargue type skimmer (pO = 3 bar, T, = 300K). Methyl fluoride was purchased from PCR Inc. (USA). No significant impurities have been observed in the photoionization mass spectrum. RESULTS
AND DISCUSSION
The cluster photoionization mass spectrum shows the same main features as the cluster electron impact mass spectrum reported in refs. 7 and 8. However the relative intensities of some fragmentation channels are different. The cluster photoionization mass spectrum has been measured at 300 K and p,, = 3 bar with undispersed synchrotron radiation. No corrections due to the transmission function of the mass spectrometer have been made. The dominant mass peaks in the regime between the molecular ion (CH,F+) and the protonated dimer (C,H,F:) are: CH,F+ (lOO%), CH,F+ (18%), C,H,F+ (8%), C21&F+ (32%), C,H,F: (3%), and C,H,F: (30%). The threshold energies of the fragmentation processes (Fig. 1) are listed in Table 1 showing that the cluster fragmentation starts in a narrow energy range (12.03-13.0eV). The APs of the fragment clusters are higher than that of a protonated dimer molecule. Therefore a small energy barrier for the rearrange-
TABLE 1 Appearance
potentials
(AP) of the (CH,F+ ). cluster fragments in eV
Cation
AP
CH,F+ C2H,F+ C,H,F+ C,H,F,+ C,H,F:
12.43 12.25 12.47 13.00 12.03
AP (cation) f f f f f
0.05 0.05 0.05 0.05 0.05
0.35 0.17 0.39 0.92 -
- AP (C,H,F,+ )
413
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13 ENERGY Fig. 1. The photoionization
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e5ciency (PIE) curves of the (CH,F+), cluster fragments.
process which initiates the fragmentation process has to be taken into account (Table 1). On the other hand, fragmentation of larger aggregates may cause kinetic shift effects as has been taken into account in the case of ethylene clusters [ll]. From a thermodynamic point of view fragmentation of large loosely-bonded complexes is expected to have no contribution at the ionization threshold of a smaller aggregate. In the case of unsaturated molecular complexes this assumption is not valid if stabilized isomers are formed in a rearrangement process [ll, 121.However this effect is excluded for methyl fluoride clusters. Therefore the APs measured in this study should not be affected by the
ment
414
of larger clusters. The pressure dependence indicates that the dominant fragments are mostly correlated with protonated aggregates ]3]. Therefore protonated clusters are assumed to be a common intermediate for the further fragmentation reactions. The dimer cation is not observed in the cluster mass spectra independent of the ionization process. Therefore a comparison with the AP of the dimer is not possible. The lack of (CH,F)E‘):may be explained by possible intramolecular fragmentation processes such as proton transfer reactions. This is a typical behaviour of hydrogen bonded complexes. However, ab initio calculations show that the H in intermolecular interactions of the neutral aggregates are not of hydrogen bond type [13]. From the AP of the protonated molecular ion the absolute proton aflinity (PA) is obtained according to the proton transfer mechanism occurrence
(CH,F)z + hv + (CH,F)H+ + CHzF + e-
(1)
The heat of formation of the neutral dimer molecule has been evaluated by the use of the calculated intermolecular potential (AE = - 2.5 kcalmol-‘) [14]. From these calculations the structure of the dimer in the ground state is predicted to be antiparallel staggered. Using the AP given in Table 1 and the thermochemical data (Table 2) A@(CH,F+) = 184 k 3 kcalmol-’ and PA (CH,F) = 127 + 3 kcalmol-‘. For a comparison with reference data the PA is
TABLE 2 Thermochemical data in kcal mol-‘. The data are corrected to 0 k with the corresponding heat capacities [2la,c]. The isoelectronic neutral molecules are used for those cations where no heat capacities are available Speciee
WVOW
CH,F (CH,F)e (CH,Fh (CH,F), CH,F+ C%H,F+ ‘G&F+ H Hf F FHF CH, CH,+ CH,F CH,F+ CH,
- 110.5 - 167.0 - 223.0 162 169 142 61.63 366.21 19.10 - 69.98 - 64.79 36.62 262 -6 200 - 15.95
- 54.0
Reference 21a
20 20 20 21c 21c 21c 21c 21c 21c 21c 21b-d 21b 21c
416
corrected for 298K using the known heat capacities [21cJ: PA(CH3F) = 129 kcalmol- l. From relative PA measurements using a bracketing technique results in PA (CHSF) = 151kcalmol-’ [15b]. As the observed AP corresponds to a vertical transition the absolute PA represents a lower limit to the adiabatic value. Therefore it is probable that CH,F+ is not formed in its ground state in the fragmentation reactions mentioned here but rather in a vibrational excited state. A similar observation has been made by Tiedemann and co-workers in the case of HF. A stable dimer ion was not found and the obtained PA was substantially lower than the reference value [2]. From the AP of the protonated dimer (C,H,F:) we obtain the absolute PA of the dimer by using data compiled in Table 2: PA(CH,F), = 137 f 3kcal mol-‘. As the PA is lower than the expected value for the isolated molecule we assume that the protonated dimer is also formed in a vibrational excited state. The red-shift of AP (C&H,F: ) in comparison to AP (CH, F +) is 0.35 eV (Table 1). With this red-shift and the intermolecular binding energy we obtain the solvation energy (de) of a protonated molecular ion A# = 10.5 f 2 kcal mall’ according to the reaction CH,F.CH,FH+
+ CH,F + CH,FH+
(2)
This solvation energy is comparable with those of other fragment ions as discussed below. From the pressure dependence of the cluster electron impact mass spectrum a three-step mechanism for the formation of C2HsF+ is deduced [7, 81. This cation is also produced in the ion-molecule reaction CHSF + CH,F+ + &.H,F+ + HF [15]. The structure assumed is a dimethylfluoronium ion (CH,),F’. We discuss below the formation of &H,F+ from an energetical point of view by means of the measured AP. First the monomolecular decomposition of the dimer is discussed. According to eqn. (3) and using the thermochemial data given in Table 2, A@(C,H,F+) = 158 f 3kcalmoll’. (CH,F), + hv + C2H,Ff + F + e-
(3)
From PA measurements the heat of formation of a protonated fluoroethane is obtained: A@(CH,CH2FH +) = 142kcal mall’ (Table 2). If a protonated fluoroethane is formed in the fragmentation process (eqn. 3) an excess energy of 16 kcalmol-’ is calculated. CH,CH,FH+ is the most stable isomer, whereas a methylcation solvated with methyl fluoride is less stable as discussed below. At the threshold C&H,F+may be formed in an ion pair formation reaction starting from a dimer (CH,F), + hv + C,H,F+ + F-
(4)
From this reaction we calculate A@‘(C,H,F+) = 237 f 3 kcalmol-‘. The excess energy for this process is approximately 44 f 4 kcalmol-’ if a (CH,+ - CH,F) complex is formed (see below). The ion pair formation process of the isolated molecule occurs with an excess energy of 25 kcal mol-‘. This excess energy is attributed to internal energy of CH,+and translational energy carried by both fragments CH: and F- [16].
416
Another reaction channel ending up with the same cationic fragmentation product may start from a trimer (CH,F), + hv + C,H,F: C,H,F;
+ CH,F + e-
(5a)
--, C$H,F+ + HF
(5b)
The intermediately formed protonated dimer (eqn. 5a) decomposes in a second step into the observed C2HsF+ (eqn. 5b). The activation barrier is 0.39 eV (Table 1). From the AP we get A@(C,H,F+) = 193 + 3 kcalmol-‘. In comparison to A@‘(CH,CH,FH+) this reaction predicts an excess energy of 51 kcalmolll. However, if a solvated methylcation is formed in eqn. (5b) the corresponding Al$ is given by A@(CH:
*CH,F) = A@(CH:)
+ A@‘(CH,F)
+ D(CH,+-CH,F)
If we use A@‘(C,H,F+) = 193 + 3kcalmol-’ a lower limit of the intermolecular binding energy D(CH,+-CH,F) is 15 + 3 kcal mol-‘. This agrees with the expected value, which should be larger than in the neutral dimer molecule. In the PIE curve a second onset in the relative photofragmentation crosssection is observed at 15.85eV (Fig. 1). In this energy range the &‘A,) and 8(2E) states of the monomer are accessible [17]. Krauss et al. report a rise of the CH,’ photoion efficiency curve at 16.25eV which they interpreted as the threshold of the following fragmentation process [18] CH,F + hv + CH,+ + F + e-
(6)
At this energy Locht and Momigny investigated the kinetic energy of the CH:
ions [16]. They found in addition to a thermal energy distribution a broad kinetic energy distribution. These features are attributed to kinetic and internal energy of the fragments. In photoion photoelectron coincidence measurements up to 2 eV kinetic energy release was found as well [19]. The onset in the PIE curve of C2H,F+ is similar to that of CH: but is red-shifted from 16.25eV to 15.9eV. From this shift and the ground state intermolecular potential a solvation energy of 10.5 f 2 kcal mol-’ is calculated. This agrees well with the solvation energy of the protonated cluster fragments. Therefore the opening of anadditional fragmentation channel of the solvated methyl ion is concluded to start at 15.9eV. In accordance with Garvey and Bernstein we favour the fragmentation via eqn. (5). The occurrence of a solvated methylcation is deduced from the shape of the PIE curve and the energetics discussed above. Fragmentation channel (4) may be ruled out since the ion pair formation cross-section is expected to be much smaller. On the other hand the structure of a solvated methylcation agrees with the dimethylfluoronium ion proposed in ion-molecule reaction experiments [15b, c]. For the photofragmentation product C,H,F: starting from the dimer by abstraction of a hydrogen atom we calculate A@‘(C,H,F,+) = 138 f 3 kcal mol-‘. The decay of a trimer via a protonated dimer followed by an elimination
417
of molecular hydrogen gives AI$‘(C,H,F:) reaction may start from the tetramer (CH9F)4 + hv + C,H,F:
= 141 f 3kcalmol-‘.
+ CH, + HF + CHaF + e-
Another (7)
From this reaction we obtain A@(C,H,F:) = 188 + 3 kcalmoll’. This value is considerably higher than the other values. We therefore favour the processes starting from a trimer or a dimer. A#‘(C,H,F2+) is not reported in the literature. However, the formation of a solvated CH,F’ ion may be taken into account. From both fragmentation processes a binding energy of the ion-dipole complex may be obtained if the reaction occurs under adiabatic fragmentation conditions: D(CH,F+-CH,F) = 8 kcal mol-’ or 5 kcal mol-‘, respectively. The isolated fragment ion (CH,F+) is formed at 13.37eV [Ml. From the corresponding red-shift of AP (CH,F+ - CH,F) (0.37 eV) and the intermolecular potential [14] AZ = 11 + 2 kcalmol-’ is obtained. This agrees fairly well with the solvation energies and binding energies discussed above. For the C,H,F+ fragment the formation of a solvated methine cation can be excluded, as the heat of formation for this cation exceeds the value calculated from the measured threshold energy. Therefore, a protonated fluoroethene is proposed as a fragment ion (A@ = 159kcalmoll’). From the reaction pathways discussed here (CH,F), + hv + C,H,F’
+ HF + H + e-
(CH,F), + hv + C,H,F+ + HF + Hz + CHzF + e-
(8) (9)
we obtain A@(C,H,F+) = 185 ? 3 kcalmol-’ (eqn. 8) and A@(C,H,F+) = 188 f 3 kcal mall’ (eqn. 9). Therefore this cation is not formed in the ground state during both fragmentation reactions. The observed excess energies are nearly identical. In the investigations of ion-molecule reactions of CH,F+ an analogous reaction to eqn. (8) was proposed [15]. The process which produces H, instead of HF can be ruled out from energetic arguments. CONCLUSION
As a result of energetic considerations the photofragmentation pa&ternof CH,F+ clusters produced in a supersonic beam is explained by intramolecular ion-molecule reactions. In agreement with recent investigations C&H,F+ should be produced by fragmentation of a trimer. C2HsF: and C,H,F+ may be formed by fragmentation of a ,dimer as well. From threshold energies, structures of the cluster fragment ions are deduced. Solvated CHIF+, CH,F+, and CH,’ are observed occurring with a solvation energy of approximately 10 + 2 kcal mall’. ACKNOWLEDGEMENT
Financial support of the Bundesministerium fur Forschung und Technologie is gratefully acknowledged.
418 REFERENCES 1 2 3 4 6 6 7 8 9 10 11 12 13 14 16
16 17 18 19 26 21
C.Y. Ng, Adv. Chem. Phys., 52 (1983) 263, and references therein. P.W. Tiedemann, S.L. Anderson, S.T. Ceyer, T. Hirooka, C.Y. Ng, B.H. Mahan and Y.T. Lee, J. Chem. Phys., 71 (1979) 606. ST. Ceyer, P.W. Tiedemann, B.H. Mahan and Y.T. Lee, J. Chem. Phys., 70 (1979) 14. P. Bisling, E. Riihl, B. Bnrtechy and H. Baumgiirtel, J. Phys. Chem., submitted. E. Riihl, P.G.F. Bisling, B. Brutechy and H. Baumgiirtel, Chem. Phys. Lett., 126 (1986) 232. N. Gonohe, H. Abe, N. Mikami and M. Ito, J. Phys. Chem., 89 (1985) 3642. J.F. Garvey and R.B. Bernstein, Chem. Phys. Lett., 126 (1986) 394. J.F. Garvey and R.B. Bernstein, J. Phys. Chem., 90 (1986) 3677. A. Ding, R.A. Cassidy, J.H. Futrell and L. Cordis, J. Phys. Chem., submitted. D.H. Levy, Ann. Rev. Phys. Chem., 31(198O) 197. W.B. Tzeng, Y. Ono, S.H. Linn and C.Y. Ng, J. Chem. Phys., 83 (1985) 2813. Y. Ono, S.H. Linn, W.-B. Taeng and C.Y. Ng, J. Chem. Phys., 80 (1984) 1482. T. Oi, E. Sekreta and T. Ishida, J. Phys. Chem., 87 (1983) 2323. H.J. B&m, R. Ahlrichs, P. Scharf and H. Schiffer, J. Chem. Phys., 81(1984) 1389. (a) N.A. McAskill, Austr. J. Chem., 23 (1970) 2301. (b) J.L. Beauchamp, D. Holta, S. Woodgate and S.L. Patt, J. Am. Chem. Sot., 94 (1972) 2798. (c) R.J. Blint, T.B. McMahon and J.L. Beauchamp, J. Am. Chem. Sot., 96 (1974) 1269. (d) M.J.K. Pabst, H.S. Tan and J.L. Franklin, Int. J. Mass Spectrom. Ion Phys., 20 (1976) 191. R. Locht and J. Momigny, Int. J. Mass Spectrom. Ion Proc., 71 (1986) 141. G. Bieri, L. Asbrink and W. v. Niessen, J. Electron Spectrosc. Relat. Phenom., 23 (1981) 281. M. Krauss, J.A. Walker and V.H. Dibeler, J. Res. NBS, 72a (1968) 281. J.H.D. Eland, R. Frey, A. Kuestler, H. Schulze and B. Brehm, Int. J. Mass Spectrom. Ion Phys., 22 (1976) 166. S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 696. (a) D.R. Stull and H. Prophet (Eds.), JANAF Thermochemical Tables NSRDS-NBS 37, Washington DC, 1971. (b) H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 Suppl. 1 (1977). (c) D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nuttall, J. Phys. Chem. Ref. Data, 11 Suppl. 2 (1982). (d) D.F. McMillen and D.M. Golden, Ann. Rev. Phys. Chem., 33 (1982) 493.
PHOTODISSOCIATION
OF METHYL FLUORIDE CLUSTERS*
E. RUHL, P. BISLING, B. BRUTSCHY and H. BAUMGARTEL Zn&itut fiir Phyaika~i8che und Theoretische Chemie, Freie Universitit Berlin, 33 (F.R.G.)
Berlin, Takuatr. 3. D-Zoo0
(Received 3 July 1936) ABSTRACT Photoionization efficiency curves of cations produced by photodissociation of methyl fluoride clusters ((CH,F),) have been measured between 1leV and 17eV using monochromatized synchrotron radiation. The appearance potentials (AP) of the most dominant cluster fragments are: AP(CH,F+) = 12.43eV, AP(C,H,F+) = 12.23eV, AP(C,&F+) = 12.47eV, AP(C,&F:) = 13.09eV, and AP(C,H,F,+ ) = 12.99eV. The energetica of the threshold processes are discussed in relation to an appropriate fragmentation mechanism. The absolute proton afllnity of CH,F (PA(CH,F) = 129 f 3 kcal mol-*) and the solvation energies (Ae) of CH,F+, CH: , and CH*F+ are reported. INTRODUCTION
The molecular beam photoionization technique is a useful tool for the measurement of reliable ionization potentials of weakly bound complexes [l-6]. According to the kind of intermolecular. interactions typical fragmentation reactions of the initially-formed clusters are observed: van der Waals (vdW) clusters of, for example, aromatics dissociate preferably by cleavage of the intermolecular vdW bond; hydrogen bond complexes are usually stabilized by proton transfer (e.g., hydrogen halides [Z], ammonia [3], alkylamines [4]). Recently, the cluster fragmentation of methyl fluoride has been studied by Garvey and Bernstein measuring the cluster electron impact mass spectrum [7, 81. The fragmentation products they observed have been discussed in terms of intramolecular ion-molecule reactions. From the pressure dependency of the mass spectrum it was deduced that the observed cluster fragments are formed via protonated clusters by ejection of neutral closed shell molecules. The role of larger clusters is assumed to be negligible because of the great reactivity of the molecular ions [S]. Ding et al. [9] have investigated ion-molecule reactions within methane clusters initiated by photoionization. Extensive chemical fragmentation patterns are found in the cluster mass regime. This observation is related to ion-molecule reactions where generally equivalent products are obtained. In this study we report the cluster fragmentation of methyl fluoride by means of the photoionization efficiency (PIE) curves of the most abundant cluster *Dedicated to Professor E. Heilbronner on the occasion of his 65th birthday.
o36R2048/86/$03.50
0 1939 Elsevier Science Publishers B.V.
412
fragments in the mass regime from m/z = 35 to m/z = 69. The measured APs are used to derive standard heats of formation (A#‘) of the observed cationic species at 0 K. This assumption is justified by the fact that the final temperature of the beam is approximately 15K according to temperature estimates [lo]. These data are used to provide thermochemical information on the major fragmentation processes. EXPERIMENTAL
The experimental arrangement used in this study has been described previously [5]. Briefly, the apparatus consists of a lm normal incidence monochromator (McPherson 225), which disperses the synchrotron radiation of the electron storage ring BESSY, a quadrupole mass spectrometer and a VUV light detector. The clusters are produced by expansion of pure CHSF gas through a 80 pm sonic nozzle and a 500 ,umCampargue type skimmer (pO = 3 bar, T, = 300K). Methyl fluoride was purchased from PCR Inc. (USA). No significant impurities have been observed in the photoionization mass spectrum. RESULTS
AND DISCUSSION
The cluster photoionization mass spectrum shows the same main features as the cluster electron impact mass spectrum reported in refs. 7 and 8. However the relative intensities of some fragmentation channels are different. The cluster photoionization mass spectrum has been measured at 300 K and p,, = 3 bar with undispersed synchrotron radiation. No corrections due to the transmission function of the mass spectrometer have been made. The dominant mass peaks in the regime between the molecular ion (CH,F+) and the protonated dimer (C,H,F:) are: CH,F+ (lOO%), CH,F+ (18%), C,H,F+ (8%), C21&F+ (32%), C,H,F: (3%), and C,H,F: (30%). The threshold energies of the fragmentation processes (Fig. 1) are listed in Table 1 showing that the cluster fragmentation starts in a narrow energy range (12.03-13.0eV). The APs of the fragment clusters are higher than that of a protonated dimer molecule. Therefore a small energy barrier for the rearrange-
TABLE 1 Appearance
potentials
(AP) of the (CH,F+ ). cluster fragments in eV
Cation
AP
CH,F+ C2H,F+ C,H,F+ C,H,F,+ C,H,F:
12.43 12.25 12.47 13.00 12.03
AP (cation) f f f f f
0.05 0.05 0.05 0.05 0.05
0.35 0.17 0.39 0.92 -
- AP (C,H,F,+ )
413
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13 ENERGY Fig. 1. The photoionization
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e5ciency (PIE) curves of the (CH,F+), cluster fragments.
process which initiates the fragmentation process has to be taken into account (Table 1). On the other hand, fragmentation of larger aggregates may cause kinetic shift effects as has been taken into account in the case of ethylene clusters [ll]. From a thermodynamic point of view fragmentation of large loosely-bonded complexes is expected to have no contribution at the ionization threshold of a smaller aggregate. In the case of unsaturated molecular complexes this assumption is not valid if stabilized isomers are formed in a rearrangement process [ll, 121.However this effect is excluded for methyl fluoride clusters. Therefore the APs measured in this study should not be affected by the
ment
414
of larger clusters. The pressure dependence indicates that the dominant fragments are mostly correlated with protonated aggregates ]3]. Therefore protonated clusters are assumed to be a common intermediate for the further fragmentation reactions. The dimer cation is not observed in the cluster mass spectra independent of the ionization process. Therefore a comparison with the AP of the dimer is not possible. The lack of (CH,F)E‘):may be explained by possible intramolecular fragmentation processes such as proton transfer reactions. This is a typical behaviour of hydrogen bonded complexes. However, ab initio calculations show that the H in intermolecular interactions of the neutral aggregates are not of hydrogen bond type [13]. From the AP of the protonated molecular ion the absolute proton aflinity (PA) is obtained according to the proton transfer mechanism occurrence
(CH,F)z + hv + (CH,F)H+ + CHzF + e-
(1)
The heat of formation of the neutral dimer molecule has been evaluated by the use of the calculated intermolecular potential (AE = - 2.5 kcalmol-‘) [14]. From these calculations the structure of the dimer in the ground state is predicted to be antiparallel staggered. Using the AP given in Table 1 and the thermochemical data (Table 2) A@(CH,F+) = 184 k 3 kcalmol-’ and PA (CH,F) = 127 + 3 kcalmol-‘. For a comparison with reference data the PA is
TABLE 2 Thermochemical data in kcal mol-‘. The data are corrected to 0 k with the corresponding heat capacities [2la,c]. The isoelectronic neutral molecules are used for those cations where no heat capacities are available Speciee
WVOW
CH,F (CH,F)e (CH,Fh (CH,F), CH,F+ C%H,F+ ‘G&F+ H Hf F FHF CH, CH,+ CH,F CH,F+ CH,
- 110.5 - 167.0 - 223.0 162 169 142 61.63 366.21 19.10 - 69.98 - 64.79 36.62 262 -6 200 - 15.95
- 54.0
Reference 21a
20 20 20 21c 21c 21c 21c 21c 21c 21c 21b-d 21b 21c
416
corrected for 298K using the known heat capacities [21cJ: PA(CH3F) = 129 kcalmol- l. From relative PA measurements using a bracketing technique results in PA (CHSF) = 151kcalmol-’ [15b]. As the observed AP corresponds to a vertical transition the absolute PA represents a lower limit to the adiabatic value. Therefore it is probable that CH,F+ is not formed in its ground state in the fragmentation reactions mentioned here but rather in a vibrational excited state. A similar observation has been made by Tiedemann and co-workers in the case of HF. A stable dimer ion was not found and the obtained PA was substantially lower than the reference value [2]. From the AP of the protonated dimer (C,H,F:) we obtain the absolute PA of the dimer by using data compiled in Table 2: PA(CH,F), = 137 f 3kcal mol-‘. As the PA is lower than the expected value for the isolated molecule we assume that the protonated dimer is also formed in a vibrational excited state. The red-shift of AP (C&H,F: ) in comparison to AP (CH, F +) is 0.35 eV (Table 1). With this red-shift and the intermolecular binding energy we obtain the solvation energy (de) of a protonated molecular ion A# = 10.5 f 2 kcal mall’ according to the reaction CH,F.CH,FH+
+ CH,F + CH,FH+
(2)
This solvation energy is comparable with those of other fragment ions as discussed below. From the pressure dependence of the cluster electron impact mass spectrum a three-step mechanism for the formation of C2HsF+ is deduced [7, 81. This cation is also produced in the ion-molecule reaction CHSF + CH,F+ + &.H,F+ + HF [15]. The structure assumed is a dimethylfluoronium ion (CH,),F’. We discuss below the formation of &H,F+ from an energetical point of view by means of the measured AP. First the monomolecular decomposition of the dimer is discussed. According to eqn. (3) and using the thermochemial data given in Table 2, A@(C,H,F+) = 158 f 3kcalmoll’. (CH,F), + hv + C2H,Ff + F + e-
(3)
From PA measurements the heat of formation of a protonated fluoroethane is obtained: A@(CH,CH2FH +) = 142kcal mall’ (Table 2). If a protonated fluoroethane is formed in the fragmentation process (eqn. 3) an excess energy of 16 kcalmol-’ is calculated. CH,CH,FH+ is the most stable isomer, whereas a methylcation solvated with methyl fluoride is less stable as discussed below. At the threshold C&H,F+may be formed in an ion pair formation reaction starting from a dimer (CH,F), + hv + C,H,F+ + F-
(4)
From this reaction we calculate A@‘(C,H,F+) = 237 f 3 kcalmol-‘. The excess energy for this process is approximately 44 f 4 kcalmol-’ if a (CH,+ - CH,F) complex is formed (see below). The ion pair formation process of the isolated molecule occurs with an excess energy of 25 kcal mol-‘. This excess energy is attributed to internal energy of CH,+and translational energy carried by both fragments CH: and F- [16].
416
Another reaction channel ending up with the same cationic fragmentation product may start from a trimer (CH,F), + hv + C,H,F: C,H,F;
+ CH,F + e-
(5a)
--, C$H,F+ + HF
(5b)
The intermediately formed protonated dimer (eqn. 5a) decomposes in a second step into the observed C2HsF+ (eqn. 5b). The activation barrier is 0.39 eV (Table 1). From the AP we get A@(C,H,F+) = 193 + 3 kcalmol-‘. In comparison to A@‘(CH,CH,FH+) this reaction predicts an excess energy of 51 kcalmolll. However, if a solvated methylcation is formed in eqn. (5b) the corresponding Al$ is given by A@(CH:
*CH,F) = A@(CH:)
+ A@‘(CH,F)
+ D(CH,+-CH,F)
If we use A@‘(C,H,F+) = 193 + 3kcalmol-’ a lower limit of the intermolecular binding energy D(CH,+-CH,F) is 15 + 3 kcal mol-‘. This agrees with the expected value, which should be larger than in the neutral dimer molecule. In the PIE curve a second onset in the relative photofragmentation crosssection is observed at 15.85eV (Fig. 1). In this energy range the &‘A,) and 8(2E) states of the monomer are accessible [17]. Krauss et al. report a rise of the CH,’ photoion efficiency curve at 16.25eV which they interpreted as the threshold of the following fragmentation process [18] CH,F + hv + CH,+ + F + e-
(6)
At this energy Locht and Momigny investigated the kinetic energy of the CH:
ions [16]. They found in addition to a thermal energy distribution a broad kinetic energy distribution. These features are attributed to kinetic and internal energy of the fragments. In photoion photoelectron coincidence measurements up to 2 eV kinetic energy release was found as well [19]. The onset in the PIE curve of C2H,F+ is similar to that of CH: but is red-shifted from 16.25eV to 15.9eV. From this shift and the ground state intermolecular potential a solvation energy of 10.5 f 2 kcal mol-’ is calculated. This agrees well with the solvation energy of the protonated cluster fragments. Therefore the opening of anadditional fragmentation channel of the solvated methyl ion is concluded to start at 15.9eV. In accordance with Garvey and Bernstein we favour the fragmentation via eqn. (5). The occurrence of a solvated methylcation is deduced from the shape of the PIE curve and the energetics discussed above. Fragmentation channel (4) may be ruled out since the ion pair formation cross-section is expected to be much smaller. On the other hand the structure of a solvated methylcation agrees with the dimethylfluoronium ion proposed in ion-molecule reaction experiments [15b, c]. For the photofragmentation product C,H,F: starting from the dimer by abstraction of a hydrogen atom we calculate A@‘(C,H,F,+) = 138 f 3 kcal mol-‘. The decay of a trimer via a protonated dimer followed by an elimination
417
of molecular hydrogen gives AI$‘(C,H,F:) reaction may start from the tetramer (CH9F)4 + hv + C,H,F:
= 141 f 3kcalmol-‘.
+ CH, + HF + CHaF + e-
Another (7)
From this reaction we obtain A@(C,H,F:) = 188 + 3 kcalmoll’. This value is considerably higher than the other values. We therefore favour the processes starting from a trimer or a dimer. A#‘(C,H,F2+) is not reported in the literature. However, the formation of a solvated CH,F’ ion may be taken into account. From both fragmentation processes a binding energy of the ion-dipole complex may be obtained if the reaction occurs under adiabatic fragmentation conditions: D(CH,F+-CH,F) = 8 kcal mol-’ or 5 kcal mol-‘, respectively. The isolated fragment ion (CH,F+) is formed at 13.37eV [Ml. From the corresponding red-shift of AP (CH,F+ - CH,F) (0.37 eV) and the intermolecular potential [14] AZ = 11 + 2 kcalmol-’ is obtained. This agrees fairly well with the solvation energies and binding energies discussed above. For the C,H,F+ fragment the formation of a solvated methine cation can be excluded, as the heat of formation for this cation exceeds the value calculated from the measured threshold energy. Therefore, a protonated fluoroethene is proposed as a fragment ion (A@ = 159kcalmoll’). From the reaction pathways discussed here (CH,F), + hv + C,H,F’
+ HF + H + e-
(CH,F), + hv + C,H,F+ + HF + Hz + CHzF + e-
(8) (9)
we obtain A@(C,H,F+) = 185 ? 3 kcalmol-’ (eqn. 8) and A@(C,H,F+) = 188 f 3 kcal mall’ (eqn. 9). Therefore this cation is not formed in the ground state during both fragmentation reactions. The observed excess energies are nearly identical. In the investigations of ion-molecule reactions of CH,F+ an analogous reaction to eqn. (8) was proposed [15]. The process which produces H, instead of HF can be ruled out from energetic arguments. CONCLUSION
As a result of energetic considerations the photofragmentation pa&ternof CH,F+ clusters produced in a supersonic beam is explained by intramolecular ion-molecule reactions. In agreement with recent investigations C&H,F+ should be produced by fragmentation of a trimer. C2HsF: and C,H,F+ may be formed by fragmentation of a ,dimer as well. From threshold energies, structures of the cluster fragment ions are deduced. Solvated CHIF+, CH,F+, and CH,’ are observed occurring with a solvation energy of approximately 10 + 2 kcal mall’. ACKNOWLEDGEMENT
Financial support of the Bundesministerium fur Forschung und Technologie is gratefully acknowledged.
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