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The ion–molecule reaction after multiphoton ionization in the binary cluster of ammonia and methanol
23 April 1999
Chemical Physics Letters 304 Ž1999. 60–68
The ion–molecule reaction after multiphoton ionization in the binary cluster of ammonia and methanol Lianbin Li ) , Xiuyan Wang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116 023, PR China Received 30 June 1998; in final form 25 February 1999
Abstract The binary cluster ŽCH 3 OH. nŽNH 3 . m was studied by using a multiphoton ionization time-of-flight mass spectrometer ŽMPI-TOFMS.. The measured two series of protonated cluster ions: ŽCH 3 OH. n Hq and ŽCH 3 OH. n NHq Ž . 4 1 F n F 14 were attributed to the ion–molecule reaction in the binary cluster ions. The mixed cluster of CH 3 OD with ammonia was also studied. The results implied that the proton transfer probability from the OD group was larger than that from CH 3 group. The ab initio calculation of the binary cluster was carried out at the HFrSTO-3G and MP2r6-31G ) ) levels of theory, and indicated that the latter process of the proton transfer must overcome a barrier of ; 29 kcalrmol. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Molecules with a protonic hydrogen and lone-pair acceptor site can form clusters containing hydrogen bonds whose aggregate number is quite large and the study of such cluster ions in the gas phase can often provide the interaction information between the proton and solvent molecules as a function of stepwise solvation w1–5x. Ammonia and methanol are two such kinds of molecule and they have been studied widely. They can form large clusters containing several dozen molecules.
The ion–molecule reactions in ammonia clusters have been investigated in detail by several laboratories w6–13x using electron impact ionization and multiphoton ionization. The ions they obtained were mainly protonated cluster ions ŽNH 3 . m Hq, and much weaker unprotonated cluster ions ŽNH 3 .q m . They concluded that the cluster ions were produced via the following paths of ionization, dissociation, ion– molecule reaction and molecular evaporation in the cluster: q
Ž NH 3 . m q nhn ™ Ž NH 3 . m q ey ,
Ž 1.
q
Ž NH 3 . m ™ Ž NH 3 . my py1 Hqq NH 2 q pNH 3 , Ž 2. q
)
Corresponding author. Fax: q86 411 467 5584
q
Ž NH 3 . m ™ Ž NH 3 . my q q qNH 3 .
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 2 8 8 - 2
Ž 3.
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
Morgan et al. w14–16x, Shukla and Stace w17x and Samy El-Shall and Mark w18x have studied the ion– molecule reaction mechanism of methanol clusters. The latter also researched the processes of proton transfer by using the method of isotope atom tracing. The predom inant protonated cluster ions ŽCH 3 OH. n Hq were obtained in their measurements. And the following processes of the ion–molecule reactions in the cluster ions were proposed: q
Ž CH 3 OH. n q nhn ™ Ž CH 3 OH. n q ey ,
Ž 4.
q
Ž CH 3 OH. n ™ Ž CH 3 OH. ny gy1 Hqq CH 3 O q g CH 3 OH , q n
Ž 5.
61
ter size, the loss of an ammonia molecule gradually became predominant. The mixed clusters of ammonia and methanol molecules have been theoretically studied w22x. In this Letter, we report results about proton transfer in binary clusters of ammonia and methanol, using the multiphoton ionization mass spectrometric method with different laser wavelength and energy from Xia et al. The formation mechanism of the protonated cluster ions was discussed by virtue of both ab initio calculations and isotope atom tracing results of deuterated methanol.
2. Experimental
q
Ž CH 3 OH. ™ Ž CH 3 OH. nyky1 H q CH 2 OH q kCH 3 OH .
Ž 6.
Using isotope atom tracing, Samy El-Shall and Mark showed that the transfer probability of H from CH 3 was 0.03–0.2 times as large as that from OH. Fu et al. w19x studied the vibrational mode effects on the ion–molecular reaction of NHq and 3 CD 3 ODŽCD 3 OH and CH 3 OD.. The product ions q they obtained were NH 3 Dq, NH 2 Dq 2 , CD 3 OD , q CD 3 OHD . They attributed the production of these ions to methyl and hydroxyl deuterium abstraction, deuterium abstraction with HrD exchange, charge transfer and proton transfer. Under their conditions the methyl and hydroxyl deuterium abstractions were approximately equal at low collision energies, but methyl deuterium abstraction occurred several dozen times more often than the hydroxyl deuterium abstraction at high collision energy. Karpas et al. w20x studied the collision-induced dissociation of the protonated binary cluster ions of methanol and ammonia and showed the structures of these clusters were a central NHq 4 core ion solvated by methanol molecules. Ping Xia w21x and Garvey studied the metastable decomposition of protonated binary cluster ions of methanol and ammonia using MPI-MS. They pointed out the procedure of protonation was that first a core NHq 4 ion formed via a proton combining with an ammonia molecule, and then the core ion was solvated by other molecules. When decomposing, the loss of a methanol molecule predominated in small clusters. With increasing clus-
Fig. 1 is the typical time-of-flight mass spectrometer used in the present experiment. It consists of a molecular beam, ionization area, free drift area and an ion detector. The ammonia gas mixed with helium Ž10% NH 3 and 90% He. flew through a reservoir containing pure methanol Žroom temperature. to form a mixed gas of methanol and ammonia. The neutral mixed clusters were formed through the supersonic expansion of the gas mixture from a pulsed nozzle valve Žthe diameter of the orifice of the pulse valve was 0.15 mm.. Then the cluster beam entered into the acceleration electrostatic field in the direction of TOFMS axis where the clusters were ionized by a pulse laser beam perpendicular to the cluster beam. The cluster ions produced were accelerated by the electrostatic field. After deflection electrostatic field adjacent to the acceleration electric field and a free drift tube which was ; 1.2 m long, the cluster ions were detected by the MCP detector. The signal was sampled by a 500 MHz transient recorder ŽModel 9846-500, EG & G. and dealt with by a computer. The pulse valve, pulse laser and transient recorder were synchronized by a four-channel digital delayrpulse generator ŽModel DG535, Stanford Research Systems.. The frequency during the operation was 5 Hz. In the TOFMS, the acceleration electric field is the typical Wiley–Mclaren two-field design w23x. During operation, the total voltage was 1000 V. The ratio of partition voltage was 1:9.6. The triple harmonic output of Nd:YAG laser Ž355 nm. was used to ionize the clusters.
62
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
Fig. 1. Schematic diagram of the time-of-flight mass spectrometer.
The molecular beam source chamber and the TOFMS chamber were pumped by three turbomolecular pumps. During the operation, the pressure of the latter chamber was maintained at ; Ž1–2. = 10y5 Torr. The purity of the ammonia was 99.99%. That of methanol for HPLC produced by the Shandong Yuwang is 99.7% and was used without further purification. The purity of deuterated methanol Žproduced by Sigma Chemical. and helium Žproduced by Gas and Equipment. are 99.5% and 99.995%, respectively.
3. Computational details The ab initio calculations were carried out with the GAUSSIAN-94 package w24x. All of the stationary geometrical structures of the molecules, radicals and complexes were optimized first at the HFrSTO-3G level and further refined at the MP2r6-31G ) ) level. The results were confirmed by frequency analysis.
From the frequency analysis, the vibrational frequency and the energy can also be obtained.
4. Experimental result and discussion Fig. 2 shows a MPI mass spectrum obtained by using the laser wavelength of 355 nm with the mixed gas of He and 10% NH 3 carrying vapor of CH 3 OH in room temperature. In the spectrum, two series of cluster ion peaks were recognized. The first is a predominant series of protonated methanol cluster ions lacking any ammonia molecule, ŽCH 3 OH. n Hq Ž1 F n F 14.. The trend of the intensity abundance of the cluster ions seems analogous to the case of single component of CH 3 OH. The formation mechanism of these cluster ions is probably similar to the cases in homogenous methanol as shown in Eqs. Ž4. – Ž6. suggested by Morgan et al. w14–16x and Samy ElShall and Mark w18x. Whereas there is another way to form the cluster ions in the present mixture of NH 3 and CH 3 OH, in which all of the ammonia molecules
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
63
Fig. 2. MPI mass spectrum of 10% NH 3 and vapor of CH 3 OH Žstagnation pressure: 1.46 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
in the metastable neutral binary clusters were evaporated after photon absorption, then the cluster ions of single component of ŽCH 3 OH. n Hq formed. The other is a binary cluster ion series ŽCH 3Ž . OH. n NHq 4 1 F n F 14 , the peak intensities of which are weaker in comparison with the first one under present conditions of lower laser intensity. Referring to Karpas et al. w20x and Xia and Garvey w21x, this series of cluster ions come from the following intracluster reactions: q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. ny1 NHq4 q CH 3 O q Ž m y 1 . NH 3 , Ž 7. q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. ny1 NHq4 q CH 2 OH q Ž m y 1 . NH 3 ,
Ž 8.
q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. n NHq4 q NH 2 q Ž m y 2 . NH 3 .
Ž 9.
The intensities of the cluster ions ŽCH 3 OH. n NHq 4 decrease gradually with cluster size n. The trend of the intensities is similar to the case of a single component of NH 3 . But no obvious magic number structure can be observed. The fact that the only binary cluster ions we gained are the ŽCH 3 OH. n NHq 4
coincides with Karpas et al. w20x and Xia and Mark w21x. In the present measurement, the conventional ŽNH 3 . n NHq 4 cluster ions cannot be detected in the mass spectrum of the mixture. Whereas plenty of the ion fragments of ammonia can be observed. This implies that small quantities of single-component clusters of ammonia could be formed in the beam. We suggest that most of the ammonia exists in the form of both binary clusters and monomer in the supersonic beam. It is well known w25x that the vapour pressure of methanol at room temperature Ž258C. is 0.161 atm. The mass spectrum in Fig. 2 was measured at the stagnation pressure of 1.46 atm, i.e. the partial pressure of ammonia was 0.146 atm, which was almost the same as that for methanol. As mentioned above, in this measurement only the cluster ions Ž CH 3 OH . H q and the binary cluster ions ŽCH 3 OH. n NHq 4 were detected. The former with a larger intensity Ž; 2–23 times. than the latter. This indicates that small quantities of ammonia molecules were left in the cluster ions after the clusters were ionized. It is well accepted that in clusters the bonding energy between molecules is related to their polarizability and dipole moment. The polarizability and dipole moment of the ammonia molecule are 2.22 ˚ 3 and 1.47 D, respectively. Both values are 4 p´ 0 A
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
64
˚ 3 and smaller than those of methanol Ž3.23 4 p´ 0 A 1.7 D, respectively.. So the binding energies between methanol molecules are larger than those for other molecules. Therefore the ammonia molecule and radicals with weaker bonds may evaporate more efficiently from the neutral and ionic clusters than methanol molecules. This can explain well the phenomena mentioned above. It may be thought that the pure NH 3 cluster ions and the binary cluster ions of the ŽCH 3 OH. nŽNH 3 . m Hq with m G 2 may appear when the ammonia becomes richer. In order to understand the mechanism of the intracluster proton transfer, an analogous experiment was performed using CH 3 OD instead of CH 3 OH. Fig. 3 shows the MPI mass spectrum of CH 3 OD using He as carrier gas. The predominant deuterated cluster ion series ŽCH 3 OD. n Dq Ž1 F n F 14. was measured, where the Dq obviously came from OD group of another methanol molecule in the cluster: q n
q
Ž CH 3 OD. ™ Ž CH 3 OD. ny1 D q CH 3 O .
Ž 10 .
The second most intense ion series was the protonated cluster ions ŽCH 3 OD. n Hq, which is 1 mass number less than ŽCH 3 OD. n Dq. Similarly, the donor of Hq is the CH 3 group of another CH 3 OD in the cluster: q
Ž CH 3 OD. n ™ Ž CH 3 OD. ny1 Hqq CH 2 OD . Ž 11 . According to Samy El-Shall and Mark w18x, the probabilities of Eqs. Ž10. and Ž11. are approximately
equal. But subsequent isotopic scrambling and molecule evaporating are very efficient, resulting in the eventual loss of H incorporation at the protonic site. So the relative intensities of ŽCH 3 OD. n Hq ions decrease greatly:
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny1 Dqq CH 3 OH . Ž 12 . There are another series of ions in Fig. 3, which take the form of ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2.. It is supposed that they were produced by further replacement of D by H. The process may be schematically expressed as follows:
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny2 Ž CH 3 OH. Hq q CH 2 DOD ,
Ž 13 .
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny3 Ž CH 3 OH. 2 Hq q 2CH 2 DOD .
Ž 14 .
The intensity ratios of the ŽwŽCH 3 OD. n Hqx q wŽCH 3 OD. nyg ŽCH 3 OH. g Hqx. rwŽCH 3 OD. n Dqx Žthe w x stands for the intensity of the ion. were estimated as 0.3 for n s 2–4 based on this measurement, which was larger than the result for CD 3 OH gained by Samy El-Shell and Mark w18x. The aforementioned ratio’s dependence on both the stagnation pressure and the laser intensity were
Fig. 3. MPI mass spectrum of CH 3 OD cluster Žstagnation pressure: 1.52 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
measured. Fig. 4 shows the results with n s 2–4. From Fig. 4a it can be seen that when the stagnation pressure was varied between 0.49 and 2.10 atm, the ratios could be considered to have no visible variation within the region of the pressure regarding the experiment uncertainty Žas shown, the ratios were all ; 0.3.. It is well known that in a beam of jet expansion the cluster distribution depends on the stagnation pressure. So the independence of the pressure in Fig. 4a could also support the protonation processes of intracluster reactions as shown in Eqs. Ž10. – Ž14.. However, the ratios decreased monotonically with the laser intensity increasing within the
65
region of 3.9–9.2 mJrpulse. The reason is probably that with increasing laser intensity more photons can be absorbed by a neutral cluster, and the ionized cluster will possess higher residual energy, so the evaporation process of the molecule as shown in Eq. Ž12. will be intense and will reduce the ratio efficiently. In the study by Samy El-Shall and Mark w18x, an electron energy of 70 eV was used to ionize the neutral cluster. This resulted in metastable cluster ions with a higher residual energy than could be produced by the photoionization technique used in this Letter. As a result of the higher residual energy, more evaporation could occur ŽEq. Ž12.. which ac-
Fig. 4. The dependence of the intensity ratio of ŽwŽCH 3 OD. n Hqx q wŽCH 3 OD. nyg ŽCH 3 OH. g Hqx. rwŽCH 3 OD. n Dqx on: Ža. the stagnation pressure Žlaser intensity: 9.2 mJrpulse; temperature: 258C. and Žb. the laser intensity Žstagnation pressure: 2.10 atm; temperature: 258C..
66
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
counts for the lower ratio obtained in Ref. w18x compared with this Letter. In the present experiment, no measure was taken to prevent the intramolecular or intermolecular H and D atom exchange before the clusters were ionized. Isotope exchange may have occurred when the complex flowed through the equipment tube and in the neutral clusters, however, based on measurements of CH 3 OH and CH 3 OD under the same conditions in our experiment, we conclude that the former exchange is so small as to be ignored in our results. Fig. 5 shows the mass spectrum of CH 3 OD vapor with mixed gas of 10% NH 3 and 90% He as carrier. Four series of methanol cluster ions can be measured: ŽCH 3 OD. n Dq, ŽCH 3 OD. n Hq, ŽCH 3 OD. ny1ŽCH 3 OH.Hq and ŽCH 3 OD. ny2 ŽCH 3 OH. 2 Hq. They may be produced via both processes as shown in Eqs. Ž10. – Ž14. and intracluster proton transfer reactions with an ammonia molecule as the proton donator. Comparing with the above CH 3 OD cluster system, it can be seen that because of the contribution of NH 3 , the intensities of ŽCH 3 OD. n Hq and ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2. become much larger than that in CH 3 OD system. When n G 2, the sum of the intensities of ŽCH 3 OD. n Hq and ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2. become larger than that of ŽCH 3 OD. n Dq. This indicates the contri-
bution of the H transfer from NH 3 is also efficient for producing the ion cores of CH 3 ODHq or CH 3 OHq 2 in the cluster. The second series of ions in Fig. 5 are ŽCH 3q Ž q Ž . . OD. n NHq 4 , CH 3 OD n NH 3 D , CH 3 OD n NH 2 D 2 , q q ŽCH 3 OD. n NHD 3 , ŽCH 3 OD. n ND4 Ž1 F n F 13.. As in the case for the binary cluster ions of the ŽCH 3 OH. n NHq 4 in Fig. 2, there is an ion core of deuterated ammonia in the clusters of the series. The exclusive donor of deuterium is CH 3 OD in the cluster. The cluster ions containing a deuterated ammonia were produced via the processes of the intracluster ion–molecule reactions and evaporation of molecules which are similar to the case for CH 3 OHrNH 3 binary cluster as shown in Fig. 2. Because of the lower probability of the H transfer from the CH 3 group compared to that from the OD group and the large probability of evaporation of the NH 3 in the neutral and ionic clusters, the intensities q of the ion series ŽCH 3 OD. n NHq 4 with a NH 4 ion core were much lower than that of the others in this series as shown in the insert of Fig. 5. The theoretical calculation results on the binary cluster CH 3 OH– NH 3 discussed later imply that after ionization it should dissociate into NHq 4 . In the more complex binary cluster of ŽCH 3 OH. nŽNH 3 . m , the fragments of protonated ammonia may also be efficiently produced. As mentioned above, in the binary clusters
Fig. 5. MPI mass spectra of 10% NH 3 and vapor of CH 3 OD Žstagnation pressure: 1.54 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
the ammonia molecule evaporates more easily from the cluster than the methanol molecule. q
Ž CH 3 OD. n Ž NH 3 . m ™ Ž CH 3 OD. ny1 NH 3 Dq q CH 3 O q Ž m y 1 . NH 3 . Ž 15 . An intracluster reaction of isotopic replacement among the NHq 4 ion core and adjacent CH 3 OD molecules can deuterate the NHq 4 ion core. So the cluster ions containing the deuturated ammonia ion core were detected in this binary system.
67
level. The following processes can be inferred from the data. CH 3 OH–NH 3 ™ CH 3 OH q NH 3 y 6.8 kcalrmol , Ž 16 . q CH 3 OH–NHq 3 Ž ver . ™ CH 3 OH q NH 3
y 30.6 kcalrmol , CH 3 OH–NHq 3
™
y 19.7 kcalrmol , CH 3 OH–NHq 4
™
Ž 17 .
CH 2 OH q NHq 4
Ž 18 .
CH 3 OH q NHq 4 y 24.5 kcalrmol ,
5. Theoretical calculation and discussion Because of the limited computer resources only the binary cluster which contains one methanol molecule and one ammonia molecule was calculated. For the binary molecular cluster, one stationary structure for the neutral cluster and three stationary structures for the ionic cluster have been gained. All the calculated results correspond basically with Ref. w22x. Table 1 lists parts of the energies of the molecules, radicals and complexes obtained at MP2r6-31G ) ) Table 1 Energies of the molecules, radicals and complexes at MP2r631G ) ) level
NH 2 NH 3 NHq 3 NHq 4 CH 3 O CH 2 OH CH 3 OH CH 3 OHq CH 3 OHq 2 CH 3 OH–NH 3 Ž .a CH 3 OH–NHq 3 ver qŽ . CH 3 OH–NH 3 1 Ž . CH 3 OH–NHq 3 2 Ž . CH 3 OH–NHq 3 3 Ž . CH 3 OH–NHq 3 TS CH 3 OH–NHq 4 a
E
DE
Žhartree.
Žhartree.
Žkcalrmol.
y55.690224 y56.347769 y55.994953 y56.682788 y144.671367 y114.685435 y115.329047 y114.938117 y115.623325 y171.687691 y171.334612 y171.382584 y171.371329 y171.399576 y171.336549 y172.050850
0.0 0.353079 0.305107 0.316362 0.288115 0.351142 0.363159
0.0 221.560603 191.457694 198.520319 180.795044 220.345116 227.885904
This is the ionic structure which maintains the structure of the ground state neutral cluster.
Ž 19a. ™ CH 3 OHq 2 q NH 3 y 50.0 kcalrmol . Ž 19b . From Table 1, it can be seen that the energy of q Ž . the CH 3 OH–NHq 3 ver is higher than that of NH 4 q q CH 3 O and NH 4 q CH 2 OH, whereas it is lower than q that of CH 3 OHq 2 q NH 2 and CH 3 OH q NH 3 . So it can be expected that for CH 3 OH–NH 3 or larger cluster, the preferential products after ionization are a series of ions containing an NHq 4 core. The dissociation process of the cluster ion to produce NHq 4q CH 2 OH is more exothermic Ž; 9 kcalrmol. than that of NHq 4 q CH 3 O, so it seems that the channel which corresponds to the production of NHq 4q CH 2 OH is easier than that producing NHq 4 q CH 3 O. According to Ref. w22x, it is apparent the channel producing NHq 4 q CH 2 OH must accompany a process in which the H atom combining with a C atom transfers to O atom. It is clear that the energy of the transition state is high, so the channel producing NHq 4 q CH 2 OH must overcome a barrier of ; 29 kcalrmol, so it is more difficult. From Table 1, it also can be seen that the energy of the CH 3 OH– Ž . NHq 3 ver is ; 0.7 kcalrmol higher than that of the q Ž . CH 3 OH–NHq 3 TS , so the channel producing NH 4 q CH 2 OH is also possible. As a result, the productivity of NHq 4 q CH 2 OH is much less than that of NHq 4 q CH 3 O. This obviously corresponds well to the experimental results.
6. Summary The binary clusters of ŽCH 3 OH. nŽNH 3 . m were formed with supersonic expansion and a series of protonated cluster ions ŽCH 3 OH. n Hq Ž1 F n F 14.,
68
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
ŽCH 3 OH. n NHq Ž . 4 1 F n F 14 were measured. Under the lower laser intensity, the intensities of the cluster ions containing an NH 3 molecule are weaker in comparison with those containing a sole methanol molecule because of the efficient evaporation of ammonia molecules from neutral or ionic clusters. The formation mechanism of protonated cluster ions was attributed to the ion–molecule reactions in the binary cluster. The experimental results imply that the proton transfer probability from the OH group is larger than that from the CH 3 group in CH 3 OH. The ab initio calculation of the binary cluster were carried out at the HFrSTO-3G and MP2r6-31G ) ) levels. The results sustain that the proton transfers from the OH group more efficiently than from the CH 3 group in methanol. The latter process must overcome a barrier of ; 29 kcalrmol.
Acknowledgements This work has been supported by the Natural Science Foundation of China.
References w1x E.P. Grimsrud, P. Kebarle, J. Am. Chem. Soc. 95 Ž1973. 7939. w2x V. Hermann, B.D. Kay, A.W. Castleman Jr., Chem. Soc. 89 Ž1967. 5753. w3x A.J. Stace, A.K. Shukla, J. Am. Chem. Soc. 104 Ž1982. 5314. w4x A.J. Stace, C. Moore, J. Am. Chem. Soc. 105 Ž1983. 1814. w5x M. Meot-Ner, J. Am. Chem. Soc. 108 Ž1986. 6189. w6x J.A. Odutola, T.R. Dyke, B.J. Howard, J.S. Muenter, J. Chem. Phys. 70 Ž1979. 4884.
w7x K.D. Cook, J.W. Taylor, Int. J. Mass. Spectrom. Ion Phys. 30 Ž1979. 345. w8x K. Stephan, J.H. Futrell, K.I. Peterson, A.W. Castleman Jr., H.E. Wagner, N. Djuric, T.D. Mark, J. Mass. Spectrom. Ion Phys. 44 Ž1982. 167. w9x J.H. Futrell, K. Stephen, T.D. Mark, J. Chem. Phys. 76 Ž1982. 5893. w10x S.T. Ceyer, P.W. Tiedemann, B.H. Maham, Y.T. Lee, J. Chem. Phys. 70 Ž1979. 14. w11x H. Shinohara, N. Nishi, Chem. Phys. Lett. 87 Ž1982. 561. w12x H. Shinohara, N. Nishi, Chem. Phys. Lett. 106 Ž1984. 302. w13x O. Echt, P.D. Dao, S. Morgan, A.W. Castleman, J. Chem. Phys. 82 Ž1985. 4076. w14x S. Morgan, A.W. Castleman Jr., J. Am. Chem. Soc. 109 Ž1987. 2867. w15x S. Morgan, A.W. Castleman Jr., J. Am. Chem. Soc. 93 Ž1989. 4544. w16x S. Morgan, R.G. Keesee, A.W. Castleman Jr., J. Am. Chem. Soc. 111 Ž1989. 3841. w17x A.K. Shukla, A.J. Stace, J. Phys. Chem. 92 Ž1988. 2579. w18x M. Samy El-Shall, C. Mark, J. Phys. Chem. 96 Ž1992. 2045. w19x H. Fu, J. Qian, R.J. Green, S.L. Anderson, J. Chem. Phys. 108 Ž1998. 2395. w20x Z. Karpas, G.A. Eiceman, C.S. Harden, R.G. Ewing, J. Am. Soc. Mass Spectrometry 4 Ž1993. 507. w21x P. Xia, J.J. Garvey, J. Phys. Chem. 99 Ž1995. 3448. w22x Y. Li, X. Liu, X. Wang, N. Lou, Chem. Phys. Lett. 276 Ž1997. 339. w23x W.C. Wiley, I.H. Mclaren, Rev. Sci. Instrum. 26 Ž1955. 12. w24x M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheesman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Allaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzalez, J.A. Pople, GAUSSIAN-94W, Gaussian, Inc., Pittsburgh, PA, 1995. w25x R.C. Weast, M.J. Astle ŽEds.., CRC Handbook of Chemistry and Physics, 63rd ed., CRC Press, Boca Raton, FL, 1982– 1983.
Chemical Physics Letters 304 Ž1999. 60–68
The ion–molecule reaction after multiphoton ionization in the binary cluster of ammonia and methanol Lianbin Li ) , Xiuyan Wang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116 023, PR China Received 30 June 1998; in final form 25 February 1999
Abstract The binary cluster ŽCH 3 OH. nŽNH 3 . m was studied by using a multiphoton ionization time-of-flight mass spectrometer ŽMPI-TOFMS.. The measured two series of protonated cluster ions: ŽCH 3 OH. n Hq and ŽCH 3 OH. n NHq Ž . 4 1 F n F 14 were attributed to the ion–molecule reaction in the binary cluster ions. The mixed cluster of CH 3 OD with ammonia was also studied. The results implied that the proton transfer probability from the OD group was larger than that from CH 3 group. The ab initio calculation of the binary cluster was carried out at the HFrSTO-3G and MP2r6-31G ) ) levels of theory, and indicated that the latter process of the proton transfer must overcome a barrier of ; 29 kcalrmol. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Molecules with a protonic hydrogen and lone-pair acceptor site can form clusters containing hydrogen bonds whose aggregate number is quite large and the study of such cluster ions in the gas phase can often provide the interaction information between the proton and solvent molecules as a function of stepwise solvation w1–5x. Ammonia and methanol are two such kinds of molecule and they have been studied widely. They can form large clusters containing several dozen molecules.
The ion–molecule reactions in ammonia clusters have been investigated in detail by several laboratories w6–13x using electron impact ionization and multiphoton ionization. The ions they obtained were mainly protonated cluster ions ŽNH 3 . m Hq, and much weaker unprotonated cluster ions ŽNH 3 .q m . They concluded that the cluster ions were produced via the following paths of ionization, dissociation, ion– molecule reaction and molecular evaporation in the cluster: q
Ž NH 3 . m q nhn ™ Ž NH 3 . m q ey ,
Ž 1.
q
Ž NH 3 . m ™ Ž NH 3 . my py1 Hqq NH 2 q pNH 3 , Ž 2. q
)
Corresponding author. Fax: q86 411 467 5584
q
Ž NH 3 . m ™ Ž NH 3 . my q q qNH 3 .
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 2 8 8 - 2
Ž 3.
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
Morgan et al. w14–16x, Shukla and Stace w17x and Samy El-Shall and Mark w18x have studied the ion– molecule reaction mechanism of methanol clusters. The latter also researched the processes of proton transfer by using the method of isotope atom tracing. The predom inant protonated cluster ions ŽCH 3 OH. n Hq were obtained in their measurements. And the following processes of the ion–molecule reactions in the cluster ions were proposed: q
Ž CH 3 OH. n q nhn ™ Ž CH 3 OH. n q ey ,
Ž 4.
q
Ž CH 3 OH. n ™ Ž CH 3 OH. ny gy1 Hqq CH 3 O q g CH 3 OH , q n
Ž 5.
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ter size, the loss of an ammonia molecule gradually became predominant. The mixed clusters of ammonia and methanol molecules have been theoretically studied w22x. In this Letter, we report results about proton transfer in binary clusters of ammonia and methanol, using the multiphoton ionization mass spectrometric method with different laser wavelength and energy from Xia et al. The formation mechanism of the protonated cluster ions was discussed by virtue of both ab initio calculations and isotope atom tracing results of deuterated methanol.
2. Experimental
q
Ž CH 3 OH. ™ Ž CH 3 OH. nyky1 H q CH 2 OH q kCH 3 OH .
Ž 6.
Using isotope atom tracing, Samy El-Shall and Mark showed that the transfer probability of H from CH 3 was 0.03–0.2 times as large as that from OH. Fu et al. w19x studied the vibrational mode effects on the ion–molecular reaction of NHq and 3 CD 3 ODŽCD 3 OH and CH 3 OD.. The product ions q they obtained were NH 3 Dq, NH 2 Dq 2 , CD 3 OD , q CD 3 OHD . They attributed the production of these ions to methyl and hydroxyl deuterium abstraction, deuterium abstraction with HrD exchange, charge transfer and proton transfer. Under their conditions the methyl and hydroxyl deuterium abstractions were approximately equal at low collision energies, but methyl deuterium abstraction occurred several dozen times more often than the hydroxyl deuterium abstraction at high collision energy. Karpas et al. w20x studied the collision-induced dissociation of the protonated binary cluster ions of methanol and ammonia and showed the structures of these clusters were a central NHq 4 core ion solvated by methanol molecules. Ping Xia w21x and Garvey studied the metastable decomposition of protonated binary cluster ions of methanol and ammonia using MPI-MS. They pointed out the procedure of protonation was that first a core NHq 4 ion formed via a proton combining with an ammonia molecule, and then the core ion was solvated by other molecules. When decomposing, the loss of a methanol molecule predominated in small clusters. With increasing clus-
Fig. 1 is the typical time-of-flight mass spectrometer used in the present experiment. It consists of a molecular beam, ionization area, free drift area and an ion detector. The ammonia gas mixed with helium Ž10% NH 3 and 90% He. flew through a reservoir containing pure methanol Žroom temperature. to form a mixed gas of methanol and ammonia. The neutral mixed clusters were formed through the supersonic expansion of the gas mixture from a pulsed nozzle valve Žthe diameter of the orifice of the pulse valve was 0.15 mm.. Then the cluster beam entered into the acceleration electrostatic field in the direction of TOFMS axis where the clusters were ionized by a pulse laser beam perpendicular to the cluster beam. The cluster ions produced were accelerated by the electrostatic field. After deflection electrostatic field adjacent to the acceleration electric field and a free drift tube which was ; 1.2 m long, the cluster ions were detected by the MCP detector. The signal was sampled by a 500 MHz transient recorder ŽModel 9846-500, EG & G. and dealt with by a computer. The pulse valve, pulse laser and transient recorder were synchronized by a four-channel digital delayrpulse generator ŽModel DG535, Stanford Research Systems.. The frequency during the operation was 5 Hz. In the TOFMS, the acceleration electric field is the typical Wiley–Mclaren two-field design w23x. During operation, the total voltage was 1000 V. The ratio of partition voltage was 1:9.6. The triple harmonic output of Nd:YAG laser Ž355 nm. was used to ionize the clusters.
62
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
Fig. 1. Schematic diagram of the time-of-flight mass spectrometer.
The molecular beam source chamber and the TOFMS chamber were pumped by three turbomolecular pumps. During the operation, the pressure of the latter chamber was maintained at ; Ž1–2. = 10y5 Torr. The purity of the ammonia was 99.99%. That of methanol for HPLC produced by the Shandong Yuwang is 99.7% and was used without further purification. The purity of deuterated methanol Žproduced by Sigma Chemical. and helium Žproduced by Gas and Equipment. are 99.5% and 99.995%, respectively.
3. Computational details The ab initio calculations were carried out with the GAUSSIAN-94 package w24x. All of the stationary geometrical structures of the molecules, radicals and complexes were optimized first at the HFrSTO-3G level and further refined at the MP2r6-31G ) ) level. The results were confirmed by frequency analysis.
From the frequency analysis, the vibrational frequency and the energy can also be obtained.
4. Experimental result and discussion Fig. 2 shows a MPI mass spectrum obtained by using the laser wavelength of 355 nm with the mixed gas of He and 10% NH 3 carrying vapor of CH 3 OH in room temperature. In the spectrum, two series of cluster ion peaks were recognized. The first is a predominant series of protonated methanol cluster ions lacking any ammonia molecule, ŽCH 3 OH. n Hq Ž1 F n F 14.. The trend of the intensity abundance of the cluster ions seems analogous to the case of single component of CH 3 OH. The formation mechanism of these cluster ions is probably similar to the cases in homogenous methanol as shown in Eqs. Ž4. – Ž6. suggested by Morgan et al. w14–16x and Samy ElShall and Mark w18x. Whereas there is another way to form the cluster ions in the present mixture of NH 3 and CH 3 OH, in which all of the ammonia molecules
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
63
Fig. 2. MPI mass spectrum of 10% NH 3 and vapor of CH 3 OH Žstagnation pressure: 1.46 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
in the metastable neutral binary clusters were evaporated after photon absorption, then the cluster ions of single component of ŽCH 3 OH. n Hq formed. The other is a binary cluster ion series ŽCH 3Ž . OH. n NHq 4 1 F n F 14 , the peak intensities of which are weaker in comparison with the first one under present conditions of lower laser intensity. Referring to Karpas et al. w20x and Xia and Garvey w21x, this series of cluster ions come from the following intracluster reactions: q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. ny1 NHq4 q CH 3 O q Ž m y 1 . NH 3 , Ž 7. q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. ny1 NHq4 q CH 2 OH q Ž m y 1 . NH 3 ,
Ž 8.
q
Ž CH 3 OH. n Ž NH 3 . m ™ Ž CH 3 OH. n NHq4 q NH 2 q Ž m y 2 . NH 3 .
Ž 9.
The intensities of the cluster ions ŽCH 3 OH. n NHq 4 decrease gradually with cluster size n. The trend of the intensities is similar to the case of a single component of NH 3 . But no obvious magic number structure can be observed. The fact that the only binary cluster ions we gained are the ŽCH 3 OH. n NHq 4
coincides with Karpas et al. w20x and Xia and Mark w21x. In the present measurement, the conventional ŽNH 3 . n NHq 4 cluster ions cannot be detected in the mass spectrum of the mixture. Whereas plenty of the ion fragments of ammonia can be observed. This implies that small quantities of single-component clusters of ammonia could be formed in the beam. We suggest that most of the ammonia exists in the form of both binary clusters and monomer in the supersonic beam. It is well known w25x that the vapour pressure of methanol at room temperature Ž258C. is 0.161 atm. The mass spectrum in Fig. 2 was measured at the stagnation pressure of 1.46 atm, i.e. the partial pressure of ammonia was 0.146 atm, which was almost the same as that for methanol. As mentioned above, in this measurement only the cluster ions Ž CH 3 OH . H q and the binary cluster ions ŽCH 3 OH. n NHq 4 were detected. The former with a larger intensity Ž; 2–23 times. than the latter. This indicates that small quantities of ammonia molecules were left in the cluster ions after the clusters were ionized. It is well accepted that in clusters the bonding energy between molecules is related to their polarizability and dipole moment. The polarizability and dipole moment of the ammonia molecule are 2.22 ˚ 3 and 1.47 D, respectively. Both values are 4 p´ 0 A
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
64
˚ 3 and smaller than those of methanol Ž3.23 4 p´ 0 A 1.7 D, respectively.. So the binding energies between methanol molecules are larger than those for other molecules. Therefore the ammonia molecule and radicals with weaker bonds may evaporate more efficiently from the neutral and ionic clusters than methanol molecules. This can explain well the phenomena mentioned above. It may be thought that the pure NH 3 cluster ions and the binary cluster ions of the ŽCH 3 OH. nŽNH 3 . m Hq with m G 2 may appear when the ammonia becomes richer. In order to understand the mechanism of the intracluster proton transfer, an analogous experiment was performed using CH 3 OD instead of CH 3 OH. Fig. 3 shows the MPI mass spectrum of CH 3 OD using He as carrier gas. The predominant deuterated cluster ion series ŽCH 3 OD. n Dq Ž1 F n F 14. was measured, where the Dq obviously came from OD group of another methanol molecule in the cluster: q n
q
Ž CH 3 OD. ™ Ž CH 3 OD. ny1 D q CH 3 O .
Ž 10 .
The second most intense ion series was the protonated cluster ions ŽCH 3 OD. n Hq, which is 1 mass number less than ŽCH 3 OD. n Dq. Similarly, the donor of Hq is the CH 3 group of another CH 3 OD in the cluster: q
Ž CH 3 OD. n ™ Ž CH 3 OD. ny1 Hqq CH 2 OD . Ž 11 . According to Samy El-Shall and Mark w18x, the probabilities of Eqs. Ž10. and Ž11. are approximately
equal. But subsequent isotopic scrambling and molecule evaporating are very efficient, resulting in the eventual loss of H incorporation at the protonic site. So the relative intensities of ŽCH 3 OD. n Hq ions decrease greatly:
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny1 Dqq CH 3 OH . Ž 12 . There are another series of ions in Fig. 3, which take the form of ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2.. It is supposed that they were produced by further replacement of D by H. The process may be schematically expressed as follows:
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny2 Ž CH 3 OH. Hq q CH 2 DOD ,
Ž 13 .
Ž CH 3 OD. n Hq ™ Ž CH 3 OD. ny3 Ž CH 3 OH. 2 Hq q 2CH 2 DOD .
Ž 14 .
The intensity ratios of the ŽwŽCH 3 OD. n Hqx q wŽCH 3 OD. nyg ŽCH 3 OH. g Hqx. rwŽCH 3 OD. n Dqx Žthe w x stands for the intensity of the ion. were estimated as 0.3 for n s 2–4 based on this measurement, which was larger than the result for CD 3 OH gained by Samy El-Shell and Mark w18x. The aforementioned ratio’s dependence on both the stagnation pressure and the laser intensity were
Fig. 3. MPI mass spectrum of CH 3 OD cluster Žstagnation pressure: 1.52 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
measured. Fig. 4 shows the results with n s 2–4. From Fig. 4a it can be seen that when the stagnation pressure was varied between 0.49 and 2.10 atm, the ratios could be considered to have no visible variation within the region of the pressure regarding the experiment uncertainty Žas shown, the ratios were all ; 0.3.. It is well known that in a beam of jet expansion the cluster distribution depends on the stagnation pressure. So the independence of the pressure in Fig. 4a could also support the protonation processes of intracluster reactions as shown in Eqs. Ž10. – Ž14.. However, the ratios decreased monotonically with the laser intensity increasing within the
65
region of 3.9–9.2 mJrpulse. The reason is probably that with increasing laser intensity more photons can be absorbed by a neutral cluster, and the ionized cluster will possess higher residual energy, so the evaporation process of the molecule as shown in Eq. Ž12. will be intense and will reduce the ratio efficiently. In the study by Samy El-Shall and Mark w18x, an electron energy of 70 eV was used to ionize the neutral cluster. This resulted in metastable cluster ions with a higher residual energy than could be produced by the photoionization technique used in this Letter. As a result of the higher residual energy, more evaporation could occur ŽEq. Ž12.. which ac-
Fig. 4. The dependence of the intensity ratio of ŽwŽCH 3 OD. n Hqx q wŽCH 3 OD. nyg ŽCH 3 OH. g Hqx. rwŽCH 3 OD. n Dqx on: Ža. the stagnation pressure Žlaser intensity: 9.2 mJrpulse; temperature: 258C. and Žb. the laser intensity Žstagnation pressure: 2.10 atm; temperature: 258C..
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L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
counts for the lower ratio obtained in Ref. w18x compared with this Letter. In the present experiment, no measure was taken to prevent the intramolecular or intermolecular H and D atom exchange before the clusters were ionized. Isotope exchange may have occurred when the complex flowed through the equipment tube and in the neutral clusters, however, based on measurements of CH 3 OH and CH 3 OD under the same conditions in our experiment, we conclude that the former exchange is so small as to be ignored in our results. Fig. 5 shows the mass spectrum of CH 3 OD vapor with mixed gas of 10% NH 3 and 90% He as carrier. Four series of methanol cluster ions can be measured: ŽCH 3 OD. n Dq, ŽCH 3 OD. n Hq, ŽCH 3 OD. ny1ŽCH 3 OH.Hq and ŽCH 3 OD. ny2 ŽCH 3 OH. 2 Hq. They may be produced via both processes as shown in Eqs. Ž10. – Ž14. and intracluster proton transfer reactions with an ammonia molecule as the proton donator. Comparing with the above CH 3 OD cluster system, it can be seen that because of the contribution of NH 3 , the intensities of ŽCH 3 OD. n Hq and ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2. become much larger than that in CH 3 OD system. When n G 2, the sum of the intensities of ŽCH 3 OD. n Hq and ŽCH 3 OD. nyg ŽCH 3 OH. g Hq Ž g s 1, 2. become larger than that of ŽCH 3 OD. n Dq. This indicates the contri-
bution of the H transfer from NH 3 is also efficient for producing the ion cores of CH 3 ODHq or CH 3 OHq 2 in the cluster. The second series of ions in Fig. 5 are ŽCH 3q Ž q Ž . . OD. n NHq 4 , CH 3 OD n NH 3 D , CH 3 OD n NH 2 D 2 , q q ŽCH 3 OD. n NHD 3 , ŽCH 3 OD. n ND4 Ž1 F n F 13.. As in the case for the binary cluster ions of the ŽCH 3 OH. n NHq 4 in Fig. 2, there is an ion core of deuterated ammonia in the clusters of the series. The exclusive donor of deuterium is CH 3 OD in the cluster. The cluster ions containing a deuterated ammonia were produced via the processes of the intracluster ion–molecule reactions and evaporation of molecules which are similar to the case for CH 3 OHrNH 3 binary cluster as shown in Fig. 2. Because of the lower probability of the H transfer from the CH 3 group compared to that from the OD group and the large probability of evaporation of the NH 3 in the neutral and ionic clusters, the intensities q of the ion series ŽCH 3 OD. n NHq 4 with a NH 4 ion core were much lower than that of the others in this series as shown in the insert of Fig. 5. The theoretical calculation results on the binary cluster CH 3 OH– NH 3 discussed later imply that after ionization it should dissociate into NHq 4 . In the more complex binary cluster of ŽCH 3 OH. nŽNH 3 . m , the fragments of protonated ammonia may also be efficiently produced. As mentioned above, in the binary clusters
Fig. 5. MPI mass spectra of 10% NH 3 and vapor of CH 3 OD Žstagnation pressure: 1.54 atm; laser intensity: 9.2 mJrpulse; temperature: 258C..
L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
the ammonia molecule evaporates more easily from the cluster than the methanol molecule. q
Ž CH 3 OD. n Ž NH 3 . m ™ Ž CH 3 OD. ny1 NH 3 Dq q CH 3 O q Ž m y 1 . NH 3 . Ž 15 . An intracluster reaction of isotopic replacement among the NHq 4 ion core and adjacent CH 3 OD molecules can deuterate the NHq 4 ion core. So the cluster ions containing the deuturated ammonia ion core were detected in this binary system.
67
level. The following processes can be inferred from the data. CH 3 OH–NH 3 ™ CH 3 OH q NH 3 y 6.8 kcalrmol , Ž 16 . q CH 3 OH–NHq 3 Ž ver . ™ CH 3 OH q NH 3
y 30.6 kcalrmol , CH 3 OH–NHq 3
™
y 19.7 kcalrmol , CH 3 OH–NHq 4
™
Ž 17 .
CH 2 OH q NHq 4
Ž 18 .
CH 3 OH q NHq 4 y 24.5 kcalrmol ,
5. Theoretical calculation and discussion Because of the limited computer resources only the binary cluster which contains one methanol molecule and one ammonia molecule was calculated. For the binary molecular cluster, one stationary structure for the neutral cluster and three stationary structures for the ionic cluster have been gained. All the calculated results correspond basically with Ref. w22x. Table 1 lists parts of the energies of the molecules, radicals and complexes obtained at MP2r6-31G ) ) Table 1 Energies of the molecules, radicals and complexes at MP2r631G ) ) level
NH 2 NH 3 NHq 3 NHq 4 CH 3 O CH 2 OH CH 3 OH CH 3 OHq CH 3 OHq 2 CH 3 OH–NH 3 Ž .a CH 3 OH–NHq 3 ver qŽ . CH 3 OH–NH 3 1 Ž . CH 3 OH–NHq 3 2 Ž . CH 3 OH–NHq 3 3 Ž . CH 3 OH–NHq 3 TS CH 3 OH–NHq 4 a
E
DE
Žhartree.
Žhartree.
Žkcalrmol.
y55.690224 y56.347769 y55.994953 y56.682788 y144.671367 y114.685435 y115.329047 y114.938117 y115.623325 y171.687691 y171.334612 y171.382584 y171.371329 y171.399576 y171.336549 y172.050850
0.0 0.353079 0.305107 0.316362 0.288115 0.351142 0.363159
0.0 221.560603 191.457694 198.520319 180.795044 220.345116 227.885904
This is the ionic structure which maintains the structure of the ground state neutral cluster.
Ž 19a. ™ CH 3 OHq 2 q NH 3 y 50.0 kcalrmol . Ž 19b . From Table 1, it can be seen that the energy of q Ž . the CH 3 OH–NHq 3 ver is higher than that of NH 4 q q CH 3 O and NH 4 q CH 2 OH, whereas it is lower than q that of CH 3 OHq 2 q NH 2 and CH 3 OH q NH 3 . So it can be expected that for CH 3 OH–NH 3 or larger cluster, the preferential products after ionization are a series of ions containing an NHq 4 core. The dissociation process of the cluster ion to produce NHq 4q CH 2 OH is more exothermic Ž; 9 kcalrmol. than that of NHq 4 q CH 3 O, so it seems that the channel which corresponds to the production of NHq 4q CH 2 OH is easier than that producing NHq 4 q CH 3 O. According to Ref. w22x, it is apparent the channel producing NHq 4 q CH 2 OH must accompany a process in which the H atom combining with a C atom transfers to O atom. It is clear that the energy of the transition state is high, so the channel producing NHq 4 q CH 2 OH must overcome a barrier of ; 29 kcalrmol, so it is more difficult. From Table 1, it also can be seen that the energy of the CH 3 OH– Ž . NHq 3 ver is ; 0.7 kcalrmol higher than that of the q Ž . CH 3 OH–NHq 3 TS , so the channel producing NH 4 q CH 2 OH is also possible. As a result, the productivity of NHq 4 q CH 2 OH is much less than that of NHq 4 q CH 3 O. This obviously corresponds well to the experimental results.
6. Summary The binary clusters of ŽCH 3 OH. nŽNH 3 . m were formed with supersonic expansion and a series of protonated cluster ions ŽCH 3 OH. n Hq Ž1 F n F 14.,
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L. Li, X. Wang r Chemical Physics Letters 304 (1999) 60–68
ŽCH 3 OH. n NHq Ž . 4 1 F n F 14 were measured. Under the lower laser intensity, the intensities of the cluster ions containing an NH 3 molecule are weaker in comparison with those containing a sole methanol molecule because of the efficient evaporation of ammonia molecules from neutral or ionic clusters. The formation mechanism of protonated cluster ions was attributed to the ion–molecule reactions in the binary cluster. The experimental results imply that the proton transfer probability from the OH group is larger than that from the CH 3 group in CH 3 OH. The ab initio calculation of the binary cluster were carried out at the HFrSTO-3G and MP2r6-31G ) ) levels. The results sustain that the proton transfers from the OH group more efficiently than from the CH 3 group in methanol. The latter process must overcome a barrier of ; 29 kcalrmol.
Acknowledgements This work has been supported by the Natural Science Foundation of China.
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