J. Phys. Chem. 1991, 95, 585-591
585
Structure of Protonated Solvation Complexes: Ammonia-Trimethylamine Cluster Ions and Thelr Metastable Decompositions S.Wei, W. B. Tzeng, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania Stare University, University Park, Pennsylvania 16802 (Received: February 14, 1990; In Final Form: July 30, 1990)
The stable structures and metastable decomposition processes of neat trimethylamine and mixed ammonia-trimethylamine cluster ions are studied by using a reflectron time-of-flight mass spectrometer in conjunction with multiphoton ionization. The kinetic energy release of neat trimethylamine cluster ions is measured, and the binding energies are determined by using Klots' and Engelking's models. Observations on the relative cluster ion intensity distributions of (NH3),( (CH3)3N),H+ under various experimental conditions suggest that NH3((CH3)3N)4H+, (NH3)z((CH3)3N)6H+,(NH3)3((CH3)3N)8H+, (NH3),and (NH3)s((CH3)3)N)lzH+ are particularly stable. The stable configurations of these ionic species have ((CH3)3N)IOH+, been proposed to have central species NH4+, NzH7+, N3Hlo+, N4H13+,and N5HI6+which provide 4, 6, 8, 10, and 12 hydrogen-bonding sites, respectively, for the surrounding (CH3)3Nmolecules. The pattern of magic numbers at m = 2(n + I ) begins to break down at n = 6. Experimental results of the metastable decomposition studies also support the proposed stable structures of mixed ammonia-trimethylamine cluster ions.
Introduction It has been known that the base strength of an ion can be greatly influenced by degrees of solvation.'-3 For example, in the gas phase the basicity of pyridine (213.1 kcal/mol) is greater than that of methylamine (205.7 kcal/mol) whereas pyridine has lower base strength toward the aqueous proton (pK, = 5.21) than methylamine (pK, = 10.66): In addition, the rate of a particular reaction is generally very different in the solution from that in the gas phasee5 The solvation effect on ion stability and reaction rate can be dramatic, especially for hydrogen-bonding cluster systems. One of the most commonly used methods for studying the stability of solvated cluster ions involves determining some quantitative thermodynamic properties by using a high-pressure mass spectrometry (HPMS) technique.612 For instance, currently available thermodynamic data of (CH30CH3)2H+,'3 (NH3)4NH4+,1C'7(CH30CH3)3H30+,18 and (CH3CN)3H30+l9 suggest that the stability in these hydrogen-bonded species is achieved by symmetric positioning of molecules around a central proton or protonated molecule. Since the determination of thermodynamic properties with the HPMS technique requires the ( I ) Trotman-Dickenson,A. F. J . Chem. Soc. 1949, 1293. (2) Pearson, R. G.; Vogelson, D. C. J . Am. Chem. Soc. 1958,80, 1038. (3) Condon, F. E. J . Am. Chem. Soc. 1965,87, 4481. (4) Taagepera, M.; Henderson, W. G.; Brownlee, R. T. C.; Beauchamp, J. L.; Holtz, D.; Taft, R. W. J . Am. Chem. SOC.1972, 94. 1369. (5) Han, C. C.; Dodd, J. A,; Brauman, J. I. J. Phys. Chem. 1986,90,471. (6) Kebarle, P. In Ion-Molecule Reactions; Franklin, J. L., Ed.; Plenum Press: New York, 1972. (7) Kebarle, P. Annu. Rev. Phys. Chem. 1977, 28, 445. (8) Kebarle, P.; Davidson, W. R.; French, M.; Cumming, J. B.; McMahon, T. B. Faraday Discuss. Chem. Soc. 1977, 64, 220. (9) Castleman, Jr., A. W. In Ion and Cluster Ion Spectroscopy and Structure; Maier, J., Ed.; Elsevier Science Publishers: Amsterdam, 1989. (IO) Castleman, Jr., A. W. In Kinetics of Ion-Molecule Reactions; AusIoos, P. W., Ed.; Plenum Press: New York, 1979. (11) Castleman, Jr., A. W.; Keesee, R. G. Chem. Reu. 1986, 86, 589. (12) Castleman, Jr., A. W.; Keesee, R.G. Acc. Chem. Res. 1986, 19,413. (13) Grimsrud, E. P.; Kebarle, P. J . Am. Chem. Soc. 1973, 95, 7939. (14) Hogg, A. M.; Haynes, R. M.; Kebarle, P. J . Am. Chem. SOC.1966, 88, 28. (15) Payzant, J. D.; Cunningham, A. J.; Kebarle, P. Can. J . Chem. 1973, 51, 3242. (16) Arshadi, M. R.; Futrell. J. H. J . Phys. Chem. 1974, 78, 1482. (17) Tang, I. N.; Castleman, Jr., A. W. J . Chem. Phys. 1975,62,4576. (18) Hiraoka, K.; Grimsrud, E. P.; Kebarle, P. J . Am. Chem. SOC.1974, 96. 3359. (19) Dcakyne, C. A.; Meot-Ner, M.; Campbell, C. L.; Hughes, M. G.; Murphy, S. P. J . Chem. Phys. 1986,84,4958.
0022-3654/91/2095-0585$02.50/0
TABLE I: Molecular Proton Affinities (PA), Polarizabilities (a), and Dipole Moments (d molecule PA," kcal/mol a : A p.C D NH3 CHJCOCH, CH3CN CH3CHO (CH3)3N "Reference
204.0 196.7 189.2 186.6 225.1
2.22 6.39 4.48 4.59 7.76
1.47 2.88 3.92 2.69 0.6 1
49. bReferences 48, 53, and 54. 'Reference 55.
establishment of equilibrium in the source of the apparatus, it is often limited to clusters containing a few solvent molecules.20 Another approach on the studies of relative stability of mixed cluster ions has been done by investigating the abundances of cluster ions using single or double focusing mass spectrometryelectron impact i o n i z a t i ~ n . ~From ~ - ~ ~the observed metastable unimolecular dissociation and collision-induced dissociation, one is able to probe the configurations of stable cluster ions. Recently, it has been reported that the structures of stable cluster ions can dso be probed by using a multiphoton ionization time-of-flight (TOF) mass spectrometer equipped with a reflectron.ze26 This technique for studying the ion clusters utilizes preformed neutral clusters generated by a pulsed valve through supersonic expansion, and it provides a way for studying the stability of larger cluster i0ns.2~ Although the experiments do not directly yield information of the quantitative thermodynamic properties, they do provide information on the cluster ion stability from the appearance of relatively significant intensities in the cluster ion distribution^^"^ (magic numbers). Moreover, the relative binding strengths of the various interactions between an ion and the different species of a mixed cluster ion can be derived from study of the metastable decomposition in the TOF field-free region. The experimental (20) Keesee, R. G.; Castleman, Jr., A. W. J. Phys. Chem. Ref Dara 1986, I S , 1011. (21) Stace, A. J.; Moore, C. J . Phys. Chem. 1982,86, 3681. (22) Iraqi, M.; LIfshitz, C. Int. J . Mass Spectrom. Ion Processes 1986, 71, 245. (23) Iraqi, M.; Lifshitz, C. Int. J . Mass Specrrom. Ion Processes 1989.88, 45. (24) Tzeng, W. B.; Wei, S.; Neyer, D. W.; Keesee, R. G.; Castleman, Jr., A. W. J . Am. Chem. SOC.1990, 112, 4097. (25) Tzeng, W. B.; Wei, S.; Castleman, Jr., A. W. Chem. Phys. Lea. 1990, 166, 343. (26) Wei, S.; Tzeng, W. B.; Castleman, Jr., A. W. J . Chem. Phys. 1990. 93, 2506. (27) Wei, S.; Tzeng, W. B.; Castleman, Jr., A. W. J . Chem. Phys. 1990, 92, 332.
0 1991 American Chemical Society
586 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 results obtained from these methods are consistent with those obtained by HPMS.27 For small mixed cluster ions, it is well-known that the metastable decomposition channel with the lowest critical energy gives rise to the most intense metastable peak.21-28v29Therefore, the relative bonding strength between an ion and different components in the mixed cluster ion can be obtained from an investigation of unimolecular decomposition processes of mixed cluster ion systems. However, for cluster ions very much larger than those considered here, this might not be the case since this is a consequence of the size of the molecule and the fact that, at threshold, the rate constant for decomposition is commensurate with the detection time window associated with metastable detection.2g Recently, it has been that a local maximum occurs at n + m = 5 in the intensity distributions of mixed cluster ions (NH,),,(L),H+, where L = CH3COCH3,CH3CN,and CH3CH0. The results indicate that the proton is more strongly bonded to NH3 at all cluster sizes, which is consistent with its higher value of proton affinity (see Table I). The mixed cluster ions (NH3),(L),H+ can be pictured as a central species NH4+,bound to four constituents in any combination of NH3 and L molecules. In addition, the fact24Js*M that the NH, molecules are less strongly bonded to the central species NH4+ than the L molecules is due to the smaller dipole moment and polarizability. In this paper we present a study of stable structures of mixed ammonia-trimethylamine cluster ions (NH3),( (CH3)3N),H+. The trimethylamine has higher proton affinity and polarizability but lower dipole moment than ammonia. The results show how the stable structures and solvation shells of mixed cluster ions are influenced by the properties of the component molecules in the clusters. Experimental Section The apparatus used in these studies has been described in detail e l ~ e w h e r e . ~Briefly, ~ . ~ ~ neutral clusters are formed by coexpanding a gas mixture containing ammonia and trimethylamine (TMA) vapors at a selected mixing ratio through a pulsed nozzle (diameter = 150 pm). The base pressure of the source chamber is about 2X Torr. With the total stagnation pressure 2400 Torr, the pressure is raised to no more than 2 X 10" Torr. The relative intensities of the particular cluster ions appear to be independent of the nozzle expansion conditions. Varying the gas mixing ratio does affect the overall distributions of the neutral clusters in the beam. However, the general features (trends and magic numbers) in the ion distribution are invariant with the ammonia-TMA mixing ratio. These points are illustrated in the Results section. The neutral clusters are ionized by 355-nm light from a frequency-tripled Nd:YAG laser. Ions formed by the multiphoton ionization process are accelerated by a double electronic field to about 2.6 keV and thereafter travel through a 130-cm-long field-free region toward the reflectron. Ions are then reflected back at an overall angle of 3' and travel 80 cm through another field-free region toward a chevron microchannel plate (MCP) detector. The signal received by the MCP is fed into a 100-MHz transient recorder coupled to an IBM PC/AT microcomputer. The experiments operate at 10 Hz, and TOF spectra typically are accumulated for 3000 laser shots. The pressures in the drift field-free and detector regions are maintained at or below 2 X IO-? Torr during normal operation. Experiments have been conducted to ensure that collision-induced dissociation of the cluster ions is negligible." The parent ion birth potential, V,, established by the voltages applied to the TOF lens (28) Lifshitz, C.; Long, F. A. J. Chem. Phys. 1964, 41, 2468. Cooks, R. G.; Kruger, T. L. J. Am. Chrm. Soe. 1977, 99. 1279. (29) Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metsiusiuble Ions; Elsevier: Amsterdam, 1973.
(30)Tzeng, W. B.; Wei, S.;Castleman, Jr., A. W. Stability, Structure,and Unimolecular Dcwmposition of Mixed Ammonia-Acetonitrileand Ammo-
nia-Acetaldehyde Cluster Ions. To be submitted for publication. (31) Breen, J. J.; Kilgore, K.; Tzeng, W. B.; Wei, S.;Keesee, R. G.; Castleman. Jr.. A. W. J . Chem. Phvs. 1989. 90. 11. (32) Tzeng, W. B.; Wei, S.;Castleman, 'Jr.,'A. W. J. Am. Chem. SOC. 1989, 111, 6035.
Wei et al.
1
33.35
33 55
33.45
33 65
33 75
P l i i h t Time (mbcroaccond)
Figure 1. Experimental data points of the parent ion ((CH3)3N)2H+ (designated as 0) and daughter ion (designated as +) are fitted by the
Gaussian function (solid line).
elements, has been measured to be 2600 f 10 V in the present study. The hard reflection time-of-flight (TOF) ~pectrum'~ can be obtained when the voltage applied to the middle plate of the reflection unit (U,) is set higher than the initial parent ion energy (Vo).When a metastable decomposition process occurs in the field-free region, the daughter ion has an energy of (Md/Mp)&, where Md and M pare the daughter ion and parent ion mass. The daughter ion spectrum can be obtained when U,is set lower than Vobut higher than (h!td/hfp)uo. The ammonia (anhydrous, minimum purity of 99.99%) used in these experiments was obtained from Linde Specialty Gases whereas the trimethylamine (purity of 99.0%) was obtained from Scott Specialty Gases. Both gases were used without further purification. Results A. Neat Cluster Ions ((CH3),N),H+. The neat ammonia cluster ions (NH3)"H+ have been studied e x t e n s i ~ e l y . ~ ~ . ~ ~ . ~ ' However, very little information is known about the trimethylamine system. For example, the binding energy has been measured only up to the protonated dimer ion (22.5 kcal/mol By applying our recently developed method,27the kinetic energy release (KER) during metastable decomposition is obtained and the binding energy is determined up to ((CH,),N),H+. The values are reported below. ( 1 ) Measurements of KER of ((CH3)J9fl ( m = 2-7). The protonated cluster ions ((CH,),N),H+ ( m = 1-15) dominate the spectrum after multiphoton ionization of the neutral trimethylamine clusters. A small amount of ((CH3)3N),.(CH3)2NCH2+ ( m = 1-14) is also observed. Within the observed time window, all cluster ions undergo unimolecular decomposition by losing one TMA molecule:
((CH3)3N)mH+
-
( ( C H ~ ) ~ N ) ~ - I+H(CH3)3N +
(1) Dissociation of a metastable cluster ion is usually accompanied by some kinetic energy release resulting from the p r o c e s ~ . ~ ' * ~ ~ + ~ ~ By use of the method we developed recently, the average KER is measured based on a peak shape analysis of the TOF spectra of parent and daughter ions. A crucial requirement of this method is that the shape of both peaks of parent and daughter ions must be pure G a ~ s s i a n . ~As ~ , an ~ ~example, the experimental data points (10 ns apart from each other) of the parent (indicated as 0) and daughter ion (indicated as +) peaks of the cluster ion ((CH,),N),H+ are fitted to a pure Gaussian curve (solid line in Figure 1). The fit is seen to be very good. The broadening width, W,, resulting from kinetic energy release during the metastable (33) Tai, T. L.;El-Sayed, M.A. J. Phys. Chrm. 1986, 90, 4477. (34) Hwang, H. J.; Sensharma, D.K.: El-Sayed, M. A. Chem. Phys. Left. 1989, 160, 243. ( 3 5 ) Baldwin, M. A.; Derrick, P. J.; Morgan, R.P. Org. MassSpecfrom. 1976, 11, 440.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 587
Structure of Protonated Solvation Complexes TABLE II: Average Kinetic Energy Measurement of ((CH3)3N)2H+ WO . ns 62.9 56.4 62.8 56.5 62.9 56.6 90.1 a Measured
reported KER, WASa ns KER. meV meV 133.9
130.0 132.0 129.4
133.0 132.6 147.3
17.9 17.6 17.3 17.4 17.6
17.8
-
22
-
20
-
18
-
16
-
E
14
n''
12
-
"
10
-
r
0.5
2
,
09
-
080 7 0 6 -
a
xb;
19.0 17.4
050 ) -
03
at 22% peak height.
26 24
standard derivation, meV
1.2
1
o / 3
I
5
7
Number oI T U , n
Figure 3. A plot of the calculated binding energy of ((CH3),N),H+, m = 2-7, as a function of the cluster size, m. An abrupt decrease from m = 2 to m = 3 is evident. A,literature value: 0, deduced from Engelking's model; X, deduced from Klots' model.
*
8 -
Number of T U , m
Figure 2. A plot of the measured average kinetic energy release during the metastable decomposition of ((CH3),N),H+, m = 2-7, as a function of the cluster size, m.
dissociation can be obtained from the equation W: = Wd2- Wp', where W ,and Wp are the widths of the daughter and parent ion peak width a t 22% of the peak height. For a measured parent ion birth potential Uo,a daughter ion traveling a distance L in the field-free region, and undergoing a broadening width W,, the average kinetic energy release (E r ) is given2' by
( 4 )= ( ( ~2))-'(v0W,)2Md(2MpM)-I
(2)
where M p Md,and M a r e the masses of parent ion, daughter ion, and neutral fragment, respectively. The measured parent and daughter ion peak widths at 22% peak height are listed in Table 11, along with the derived KER and the standard derivation, for seven independent experiments involving the cluster ion ((CH,),N),H+. The K E R s for cluster ions ((CH,),N),H+ (m = 2-7) are presented in Figure 2. (2) Binding Energies of ((CH3)JV),H+ ( m = 2-7). Two independent approaches to the relationship of the binding energy and the KER for the cluster ion metastable decomposition are ( 1 ) Klots' evaporative ensemble model and (2) Engelking's modified RRK theory. In Klots' model, the binding energy is determined as follow^^^^^^ AE, = y(Er)(l - y/2Cm)-' (3) where AE,, ( Er),y,and C,,, are the binding energy, average KER, Gspann parameter, and the heat capacity, respectively. The Gspann parameter y is known to be about 25, independent of cluster systems and cluster sizes. In order to calculate the binding energy, the only parameter has to be determined is the heat capacity. As an approximation, the bulk liquid TMA heat capacity value C, = 27 cal/(mol deg)3* is used. Considering that the contributions to the heat capacity are from the cluster modes, which are proportional to (m- 1) [the maximum number is 6(m - I)]. and the internal molcculr modes, which are proportional to m, the heat capacity of the cluster ions is chosen to be C,,, = (36) Klots, C. E. 2.fhys. D 1987, 5. 83. (37) Klots, C. E. J . fhys. Chrm. 1988, 92, 5864. (38) Aston. J. G.; Sagenkahn, M.L.; Szasz. G.J.; Moessen, G. W.;Zuhr, H.F.J . Am. Chem. Soc. 1944,66, 1171.
,
24
26
I=, ,
28
,
30
,.
32
,
34
j36
,
~
38
,
1 40
Fllght Tim. (mlcromeond)
Figure 4. Hard reflection TOF spectra at various gas mixing ratios taken at U,= 2800 V (V, = 2600 IO V): (a) ammonia/TMA = 50/1; (b) ammonia/TMA = lO/l; (c) ammonia/TMA = 3/1, A, = ( N H 3 ) X ,
Tm = ((CH,),N)mH+, B n = (NHdn((CH,),N)H+, Cm (NH,)n( ( C H ~ ) I N ) ~ H +X ,m NH~((CHI),N)~H+,Y m = (CH3)2((CH3)3N)mH+, Z m = (NH3)3((CH,)3N)mH+.
+
6(m - 1 ) 7.5m (in units of the Boltzmann constant ke). The derived binding energies are shown in Figure 3 (indicated as X along with the error bar resulting from the measurement of KER). The agreement with the thermal measurement for the dimer ion, where there is only 10%difference, indicates the parameters are properly chosen. The second approach is the modified RRK theory proposed by Engell~ing?~-~ Using that approach, one can calculate the binding energy using the following equation
where s is the effective modes within the cluster ions, C = l6m3pgS/I',, and A is the model scaling parameter (A = 0.5 given by Engelking40). In order to calculate the constant C,one needs to obtain information on Y (vibrational frequency of the cluster modes), p (reduced mass), g (channel degeneracy), S (geometrical cross section for forming cluster ion), and r,,, (unimolecular dissociation rate). Fortunately, the constant C is taken to a large root as seen in eq 4. Therefore, the calculated binding energy is not sensitive to the choices of these parameters which are simply chosen as following: v is 200 cm-' for m = 2 and 100 cm-' for m > 2, g is (m- l ) , S is 100 A*, rmis the inverse of the lifetime of the cluster ion. Two more parameters to be determined are s and A; the computed binding energies are quite sensitive to the choice of these values. The effective modes which (39) Engelking, P. C. J. Chrm. fhys. 1986, 85, 3103. (40) Engelking, P. C. J. Chem. Phys. 1987, 87, 936.
588 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
'
include the cluster modes and the low-frequency internal modes of TMA molecule are selcted as s = 6 ( m - 1) +7.5m, to be consistent with the values used in Klots' model. Since the binding energy of the dimer ion is known (22.5 kcal/mol), the scaling parameter A is determined to be 0.85 for this system by matching the calculated binding energy of the dimer ion to the known value. The complete results from Engelking's model (indicated as 0 ) are presented in Figure 3. It is seen that the agreement between two models is excellent. B. Mixed Cluster Ions (NH3),.((CH3)3ZV),H+.( 1 ) Hard Reflection Time-of-FlightSpectrum. The observed major cluster ions in the multiphoton of mixed neutral ammonia-trimethylamine neutral clusters are (NH,),H+, ((CH,),N),H+, and (NH,),((CH,),N),H+. It has been known that the component ratio of the gas mixture is a crucial factor in producing the mixed cluster ions.24*41v42The effect of the gas mixing ratio on the abundance distribution of mixed clusters is demonstrated in Figure 4a-c which displays the same portion of TOF spectra taken at various mixing ratios in the range of ammonia/TMA = 50/1 to 3/1. At ammonia/TMA = 50/1, the peaks of the protonated ammonia cluster ions (NH3),H+ dominate in the spectrum, and the mixed cluster ion series with only one TMA molecule, (NH,),((CH,),N)H+, are also observed in Figure 4a. When the ammonia/TMA decreases to lO/l, the intensities of mixed cluster ions increases, as seen in Figure 4b where the mixed cluster ions with two TMA molecules (NH,),((CH,),N),H+ are formed. The optimal mixed cluster ion signal is obtained at a gas mixing ratio ammonia/TMA = 3/1, as shown in Figure 4c where the neat ammonia cluster ions completely disappear and large mixed cluster ions are generated. In the present study, we are most interested in the relative stabilities of cluster ions, particularly the mixed cluster ions (NH,),((CH,),N),H+. Therefore, we concentrate on an examination of the general trends in the size distributions of cluster ions. Figure 5 displays the intensity distributions of cluster ions (NH3),((CH3),N),H+ as a function of number of TMA molecules, m, for n = 0-6, based on the conditions leading to the data in Figure 4c. In the case of neat TMA clusters, the maximum intensity of the protonated dimer ion ((CH,),N),H+ in the size distribution of ((CH,),N),H+ (as shown in Figure 5 ) indicates that ((CH,),N),H+ is a very stable ion. The observation of stable dwlvated proton species is consistent with findings for other cluster systems, such as dimethyl a c e t ~ n e ? ~and , ~ ~acetaldeh~de.4~9~~ Figure 5a displays the intensity distribution of NH,. ((CH3),N),H+ as a function of m, showing a local maximum at m = 4. Two local maxima occur at m = 3 and m = 6 in the ion intensity distribution of (NH3),,((CH3),N),H+, n = 2. In a similar fashion, two local maxima are observed at m = 2 and m = 8 in the ion intensity distribution of (NH3),((CH3),N),H+, n = 3. In Figure 5b, the magic numbers (n,m) at (4,lO) and (5,12) are consistent with the pattern m = 2(n + 1). However, this pattern begins to break down when n > 5, and no magic number at (6,14) is seen. These findings were further confirmed under various experimental conditions, e.g., gas component mixing ratios (ammonia/TMA = 50/1 to 1 / I ) , nozzle stagnation pressures (1000-5000 Torr), and laser powers (10-40 mJ/pulse). The results show that the feature of maximum intensity in the cluster ion distribution is independent of experimental conditions mentioned above. ( 2 ) Metastable Decomposition of ( N H 3 ) , ( ( C H 3 ) , N ) N( n = 1-3, m = 1-9). In general, there is a small difference in the flight times of the daughter ion and its parent (a few tenths of a microsecond) due to different ion flight trajectories in the reflec(41) Stace, A. J.; Shulkla, A. K. J . Am. Chem. Soe. 1982, 104, 5314. (42) Stace, A. J.; Moore, C. J. J . Am. Chem. Soc. 1983, 105, 1814. (43) Munson, M. S. B. J . Am. Chem. Soc. 1%5,87, 5313. (44) Yamabe, S.; Minato, T.; Hirao, K. Can. J . Chem. 1983, 61, 2827. (45) Tzeng. W. B.; Wei, S.;Castleman, Jr., A. W . Chem. Phys. Lrtr. 1990, 168, 30. (46) Echt, 0.;Dao, P. D.; Morgan, S.;Castleman, Jr., A. W . J . Chem. Phys. 1985.82, 4076.
Wei et al.
0
2
4
6
0
Number o i TMA,
10
12
I4
b
1 (4.10)
4 0
~ 2
~
~ 6
4
16
m
0
l 10
~ IO
I4
l 16
Number o i T U . m
Figure 5. (a) lon intensity distribution of (NHJn((CH3),N),H+, n = 0-3. Magic numbers (n,m) at (0,2),(1,4), (2,3), (2,6), (3,2), and (3,E) are seen. (b) Ion intensity distribution of (NH,),((CH3)3N),H+, n =
Magic number (n,m) at (4,lO) and ($12) are observed. However, no magic number at (6,14) is seen.
4-6.
t r ~ n . ~ ~This > ' * allows one to assign the metastable ion peaks by simply comparing the arrival times of daughter and parent ions in TOF spectra. In order to complete the metastable decomposition dynamics studies, we apply a daughter ion cutoff potential method. This method has been introduced previously for studies of cluster ion dissociation dynamics using our reflectron timeof-flight mass ~ p e c t r o m e t e r . ~Briefly, ~ , ~ ~ the technique involves observing the presence/absence of daughter ion peaks upon changing the voltage settings on the middle plate of the reflectron unit ( V I ) . In the general case, there are two possible metastable decomposition channels for the mixed cluster ion (NH3)n((CH3)3N),H+: (NH,)n((CHJ,N)mH+
-
(NH3)n-i((CH3)3N)mH+ + NH3 (5)
For a parent cluster ion of mass Mp,the corresponding daughter ion masses are (M,- 17) for process 5 and (M,- 59) for process 6 . As described in the Experimental Section, the daughter ions have energies of [(M, - 17)/M ]VOand [(M,-59)/M,]U0. Here, we apply the daughter ion cutok potential method for investigation of the three possible metastable decomposition mechanisms: (a) both channels coexist, (b) only channel 5 is open, and (c) only channel 6 is open. First of all, if two channels open simultaneously, two daughter ion peaks will appear in the daughter ion spectrum for a corresponding parent ion. The separation of these two peaks in the daughter ion TOF spectrum is determined by the following equation
~
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 589
Structure of Protonated Solvation Complexes
1
I Lo N
r
x " E L
(d)
60
40
80
39
41
Flight Time (microsecond)
I
I.
I I
20
40
43
45
47
49
51
Fllght Time (microBecond)
Figure 6. A typical hard reflection TOF spectrum, gas mixing into radio ammonia/TMA = 3/1, taken at Ul = 2800 V.
-I
i
A
I
I
20
60
I 80
Flight Time (mlCrOsacond)
Figure 7. A daughter ion TOF spectrum, Ul = 2550 V. The asterisk
indicates that one TMA molecule is lost in the metastable decomposition process, and corresponding parent ions are labeled according to those in Figure 1, The triangle series indicate that one ammonia molecule is lost,
and the corresponding parent ions are also labeled.
where h f d l and h f d 2 are the masses of the two daughter ions from the same parent ion as indicated in processes 5 and 6. The constant Cis equal to 1.02(2L)g'/2 (t= 1.5 cm is the distance of the first grid and the middle grid of the reflectron unit) with At in microseconds, M hfdl, and h f d 2 in amu, uoand u, in volts, and q the unit of cKarge. As an example, for parent ion NH3((CH3)3N)4H+(Mp= 254 amu) with an energy of 2600 f 10 eV, the daughter ion energies are 2426 and 1996 eV for channels 5 and 6, respectively. If the two channels are open simultaneously, two daughter ion peaks for parent ion NH3((CH3)3N)4H+will separated by 0.16 ps at U, = 2550 V. This can be easily identified with our apparatus, which has a TOF resolution better than 0.05 ps (mass resolution m / A m = 1200) in the mass range of 250. Secondly, if only one of the two dissociation channels is open, only one daughter ion peak will be seen in the daughter ion spectrum for a corresponding parent ion. If the metastable decomposition only follows channel 5, the daughter ion peak will disappear when (II is set between 1996 and 2426 V. However, if the daughter ion peak still exists at 1996 C U, C 2426, it may be considered that channel 6 is the only possible metastable decomposition process. Whether channel 6 is the only possible metastable decomposition process can be further confirmed by observing the disappearance of the daughter ion peak when U, is lowered to a value less than 1996 V. Typical hard reflection and daughter ion TOF spectra are shown in Figures 6 and 7,respectively. The experimental conditions and peak assignments are included in the figure captions. Figure 8 displays a portion of TOF spectra at various U, settings to demonstrate the cutoff potential method. Only five peaks of parent ions (T, = (CH3)3N)3H+,X3 = NH3((CH3)3N)3H+,Y, = (NH3)2((CH3)3N)3H+,Z3 = ( N H M ( C H M W I + , and X4 =
Figure 8. A daughter ion cutoff potential study. Detailed discussion is in the text. (a) A parent ion TOF spectrum taken at Ul = 2800 V. Five peaks are T , = (CH,),N),H+, X, = NH,((CH,),N),H+, Y , =
(NH3)A(CH3),N),Ht, 2, = (NHM(CHM%H+, and X4 = NH3((CH3)3N)4Ht. (b) A daughter ion TOF spectrum taken at U,= 2550 V. (c) A daughter ion TOF spectrum at U,= 2300 V. (d) A daughter ion TOF spectrum at Ul = 1900 V. NH3((CH3)3N)4H+)are seen in Figure 8a. Figure 8b is the daughter ion spectrum taken a t VI = 2550 V, showing the five corresponding peaks. By comparing parts a and b of Figure 8, it is evident that peaks T3 and X4 (corresponding to the loss of TMA moiety) are shifted more in the ion arrival time than peaks X3, Y3, and Z3 (corresponding to the loss of ammonia). This is due to the fact that the daughter ions corresponding to the loss of a heavier neutral moiety carry less energy. As a result, they do not penetrate as deep into the reflective field of the reflectron unit and hence arrive earlier at the ion detector. As shown in Figure 8c, when (I,is lowered to 2300 V, peaks X3, Y3, and Z3disappear; this is due to the fact that the daughter ion energies are 2373, 2393, and 2407 V, which are larger than Ut. The daughter ion peak X4, which corresponds to the loss of one TMA molecule in the cluster ion, has an energy of 1996 eV and disappears at VI = 1900 V; see Figure 8d. Since the daughter ion peak a, corresponding to the loss of one TMA molecule, has an energy of 1738 V, it is still seen in Figure 8d. The complete studies of the metastable decompositions of the mixed cluster ions, (NH,),((CH,),N),H+, show the following results: (NH3)n((CH3)3N)mH+ (NH3),I((CH3)3N),H+ -+
(NH3)n((cH3)3N)mH+
-
+ NH,;
m C 2(n
+ I), n = 1-5 (8)
(NH3)n((cH3)3N)m-lH+ + (CH3),N; m 1 2 ( n + l), n = 1-5 (9)
An attempt was made to measure the KER of each metastable decomposition channel. Unfortunately, it failed for the reason that some daughter ion peaks partially overlap which results in the peaks shape not being Gaussian.
Discussion In mixed cluster ion systems (NH3),(L),H+ (L = acetone, acetaldehyde, and acetonitrile), the local maximum at n + m = 5 in the ion intensity distributions has been observed in our previous s t ~ d i e s . In ~ ~addition, , ~ ~ investigations on the metastable decompositions of these cluster ions led us to conclude that four constituents in any combination of ammonia and the above-mentioned L molecules form a first complete solvation shell around the central species NH4+. In the case of mixed ammonia-trimethylamine cluster ions, as shown as Figure 5, the local maxima in the intensity distributions of (NH3)n((CH3)3N)mH+ are seen at n + m = 5 for n = 1-3 and m = 2(n + 1) for n = 1-5. In addition, investigation of metastable decomposition processes of (NH,),((CH,),N),H+ leads to further information on relative bonding strength in the
590 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
cluster ions and stable cluster ion structures. ( A ) (NH3)n((CH3)aN),H*, n = 1. Studies of metastable decomposition processes contribute insights into the relative bonding strength in the mixed cluster i0ns.25*47f@' Our experimental results show that cluster ions NH3((CH3),N),H+, m = 1-3, lose an ammonia moiety in the metastable decomposition processes as shown in reaction 8. Since the proton affinity of TMA is higher than that of ammonia (225.1 and 204.0 kcal/mol, r e ~ p t i v e l y 4 ~ ) , in the gas phase the proton is clearly bonded more strongly to the TMA than to the ammonia. The present experimental observation of the metastable decomposition process of NH3((CH3),N),H+ (losing an ammonia moiety) implies that the TMA molecule is more strongly bonded to ((CH,),N)H+ than the ammonia molecule. This finding is in agreement with the thermochemical data where the bond dissociation energies of (CH3),NH+.NH3and (CH3),NH+.(CH3),N have been reported to be 17.3 and 22.5 kcal/mol, r e ~ p e c t i v e l y . ~Although ~ * ~ ~ the bond dissociation energies of ((CH3),N),H+-NH3and (NH3(CH3),N)H+.(CH3),N have not been reported, our result in the metastable decomposition process of NH3((CH3),N),H+ indicates that the TMA molecule is more strongly bonded in the cluster ion than the ammonia molecule. This observation can account for the fact that the polarizability of TMA (7.76 A3) is much larger than that of ammonia (2.22 A3) (larger ion-induced dipole interaction for TMA).48*52 Interestingly, the dissociation pattern is changed for the larger protonated cluster ions containing only one ammonia molecule. It is found that the metastable decomposition processes of cluster ions NH3((CH3),N),H+, m 2 4, involves loss of a TMA molecule as expressed in reaction 9. First, we consider cluster ion NH3((CH,),N),H+. The experimental results imply that the NH, is more tightly bonded in the cluster ion system that the (fourth) (CH3),N. In other words, the dissociation channel that leads to the formation of NH3((CH3),N),H+ is energetically more favorable than that leading to the formation of ((CH3),N),H+. The previous argument based on ion-induced dipole interaction within the cluster ion does not provide a satisfactory explanation for this case. Instead, it is evident that a proton together with the ammonia molecule is at the central position in the cluster and that both ammonia and TMA molecules can form a N-H-N bond to the four available hydrogen-bonding sites provided. Whether this should be strictly viewed as a NH4+core ion is not definitive since the affinity of TMA is larger than that of ammonia, but the central position is clear. Recall that a local maximum occurs at n + m = 5 in this hydrogen-bonded mixed cluster ion system (as well as other systems containing a m m ~ n i a ~ ~This . , ~ .finding implies that the observed ionic species NH3((CH3)3N)4H+is stabilized by the structural change of forming maximum hydrogen bonding. Namely, the maximum stability of cluster ion is achieved when every four hydrogen-bonding sites of the NH4+ entity are being fully occupied. Thus, the proposed stable structure of the hydrogen-bonded cluster ion NH3((CH3)3N)4H+is expressed as follows:
Wei et al.
Based on this structure, the magic number (n,m) at (2,3) and (3,2) for (NH3),(TMA),H+ can be explained when the outside TMA molecules are replaced by one or two ammonia molecules. The completely solvated ion NH3((CH3),N),H+ is then the building block of the larger cluster ions NH3((CH3),N),H+, m > 4. Similar findings have been reported in other hydrogenbonded mixed cluster ion systems containing water, such as (CH3COCH3)3.H30+ 21 and (CH3CN)3*H30+.19~22 ( B ) (NH,),,((CH,),N),,,W, n = 2-5. The same argument can be applied to the mixed cluster ion systems containing two and three ammonia molecules. For cluster ions (NH3),,((CH3),N),,,H+, n = 2-5 and m C 2(n + l), the metastable decomposition processes involve a loss of an ammonia molecule as expressed in reaction 8. The metastable dissociation pattern can be explained by a weaker ion-induced dipole interaction of the ammonia molecule in the cluster ion. The uswitchover" in the dissociation pattern occurs at m = 2(n + 1) for (NH,),((CH,),N),H+, as expressed in process 9. As shown in Figure 5 , local maxima occur at m = 2(n 1) in the ion intensity distributions of (NH,),((CH,),N),H+, which implies that the observed ionic species (NH3),((CH3),N),H+ and (NH,),((CH,),N),H+ reach stable structures that contain a maximum number of hydrogen-bonding sites. Structures I1 and 111 are the proposed stable structures for the hydrogen-bonded cluster ions (NH3)2((CH3)3N)6H+and (NHJ3((CH3),N),H+. It is evident that there are six and eight
+
+,
H
I
H
I
I
(CHd3N-H-N--H-N-H-N(CH3)3
I
I
H
H
I
(CH3)aN -H-N-H-N-H-N
I
H
H
H
H
I
I
I
H
I
-H-N(CH&
H
111 (47) Wei, S.; Tzeng, W. B.; Kccscc, R. G.;Castleman, Jr., A. W. Metastable Unimolccular and Collision-Induced Dissociation of Hydrogen-Bonded Clusters: Evidence for Intracluster Molecular Rearrangement and the Structure of Solvated Protonated Complexes. J . Am. Chem. Soc., in press. (48) Stace, A. J. J . Am. Chem. Soc. 1984. 106, 2306. (49) Lias, S.G.; Liebman. J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1904, 13, 695. (50) Meot-Ner, M.J . Am. Chem. SOC.1984, 106, 1257. (51) Keesee, R. G.;Castleman, Jr., A. W. J . Phys. Chem. Ref: Data 1986, I S , 1011. (52) Castellan, G. W. Physlcal Chemistry: Addison-Wesley: London, 1964. (53) Rad& A. A.; S m i m . B. M. Reference Data on Atoms, Molecules, and Ions; Springer-Verlag: New York, 1985. (54) Applquist, J.; Carl, J . R.; Fung, K. K.J . Am. Chem. Soc. 1972, 94, 2952. (55) Nelson, Jr., R. D.; Lide. Jr., D. R.; Maryott, A. A. Selected Values of Electric Dipole Moments for Molecules in the Gas Phase, NSRDS-NBS 10; U S . Government Printing OFfice: Washington, DC, 1967.
hydrogen-bonding sites provided by the central ions N2H7+and N3H,O+,respectively, for solvated species (NH3)2((CH3)3N)6H+ and (NH,),((CH,),N),H+. Similarly, the magic numbers at (4,lO) and (5,12) can be pictured as the solvation core ions with N4H13+and N5Hlp+which provide a maximum of 10 and 12 hydrogen-bonding sites, respectively, for the surrounding (CH,),N molecules. Both experimental observations on the relative abundances of cluster ions and the metastable decomposition patterns support the above proposed stable structures. However, this pattern with magic numbers at m = 2(n + 1 ) begins to break down at n = 6. Interestingly, the studies of (H,O),(A),H+ by Stace et al.21*48 show that the magic numbers are seen a t m = n + 2 for m C 6 and this pattern breaks down at m = 6 . The coincidence of these two systems could suggest that the stable core ion with the chainlike hydrogen-bonding network can only be extended to a certain cluster size. For the ammonia system, the
591
J. Phys. Chem. 1991, 95, 591-598 magic number a t n = 5 and m = 12 can be pictured as follows: the central NH4+provides a maximum of four available hydrogen-bonding sites for four ammonia molecules to which the 12 (CHJ3N molecules are hydrogen-bonded, As for the intermediate cluster ions before the shell is fully closed, the structures and the stabilities are investigated and discussed in detail else~here.4~ For the larger cluster size with n > 5 , the most stable ion structure cannot be achieved by simply positioning TMA molecules to form
the chainlike hydrogen-bonding network. This suggests that a ring structure may form at n = 6 which leads to a breakdown in the observed pattern corresponding to m = 2(n 1).
+
Acknowledgment. Financial support by the US.Department of Energy, Grant No. DE-FG02-88-ER60648, is gratefully acknowledged. A.W.C. Jr. thanks Prof. J. Michl for helpful discussions concerning the structures of the cluster ions.
Measurement and Model Calculations of the Vibrational Clrcular Dichroism Spectrum of 6,8-Dloxablcycio[ 3.2.1]octane Thomas Eggimann, R. Anthony Shaw,and Hal Wieser* Department of Chemistry. University of Calgary, Calgary, Alberta, Canada T2N 1 N4 (Received: February 26, 1990; In Final Form: August 10, 1990)
The vibrational circular dichroism (VCD) and absorption spectra of 6,8-dioxabicycl0[3.2.1]0~tane between 800 and 1500 cm-' are reported. Also presented here for the same region are the complete assignment of the vibrational bands, and the calculated absorption and VCD intensities, based on a previously obtained harmonic ab initio 3-21G force field which was scaled and refined to fit 195 observed frequencies of the title compound and five analogues. Relative absorption intensities are predicted successfully by the atomic polar tensor expression ( A m ) , but not with the fixed partial charge model (FPC). APT also gives correct signs and reasonable absolute VCD intensities for most bands. FPC reproduces most VCD signs but underestimates the intensities considerably, particularly for modes that involve C-0 stretching. The low FPC absorption and VCD intensities are substantially improved by adding one parameter, transferred from 2-methyloxetane, that introduces electronic charge flow along the C-0 bonds. These model calculations suggest that significant contributions to the vibrational dipole moment derivatives are generated by electronic charge redistributions which can be adequately modeled by a transferred charge flow parameter.
Introduction Vibrational circular dichroism (VCD) holds the promise of becoming a valuable spectroscopic tool for elucidating conformations and configurations of molecules in solution and for studying vibrational dynamics.' The VCD intensities arise from vibrational transitions and are governed by the magnitudes and the relative orientation of the electric and magnetic dipole transition moments which originate from moving nuclei and electron densities. While the electric dipole transition moment can be calculated in a straightforward manner within the Born-Oppenheimer approximation, the evaluation of the magnetic dipole transition moment is difficult. For the important electronic contributions to the magnetic moment to be nonzero, the expressions must include mechanisms accounting for the coupling between nuclear vibrations and electronic events, necessitating treatments beyond the Bom-Oppenheimer approximation for exact theoretical formulations.1a Such vibronic coupling VCD expressions have been developed and applied successfully by using different approaches" but are (1) For rcccnt reviews see: (a) Stephens, P. J.; Lowe, M. A. Annu. Reo. Phys. Chem. 1985,36,213-241. (b) Freedman, T.B.; Nafie, L. A. Topics in Stereochemistry; Eliel, E. L., Wilen, S.,Eds.; Wiley: New York, 1987; Vol. 17, pp 1 13-206. (c) Nafie, L. A. Advances in Infrared and Raman Spectroscopy; Clark, R. J. M., Hater, R. E., Eds.; Wiley-Hayden: Chichester, 1984; Vol. I I, pp 49-93. (2) (a) Stephens, P. J. J . Phys. Chem. 1985,89,748-752. (b) Lowe, M. A.; Stephens, P. J.; Segal, G. A. Chrm. Phys. Lerr. 1986,123, 108-116. (c) Stephens, P. J. J . Phys. Chem. 1987,91, 1712-1715. (d) Dothe, H.; Lowe, M. A.; Alper, J. S. J . Phys. Chem. 1988, 92, 6246-6249. (e) Kawiecki, R. W.; Devlin, F.; Stephens, P. J.; Amos, R. D.; Handy, N . C. Chem. Phys. Lett. 1988.145,411-417. (0 Lowe, M.A.; Alper, J. S. J . Phys. Chem. 1988.92, 4035-4040. (g) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.; Handy, N . C. 1. Phys. Chrm. 1988, 92, 1781-1785. (3) (a) Nafie, L.A.; Freedman, T. B. J. Chem. Phys. 1983,78,7108-7116. (b) Nafie, L. A. J . Chem. Phys. 1983, 79, 4950-4957. (c) Nafie, L. A.; Freedman, T. B. Chem. Phys. Letr. 1987. 134.225-232. (d) Freedman, T. B.; Nafie, L. A. 1. Chem. Phys. 1988,89, 374-384.
0022-3654/91/2095-0591$02.50/0
presently tractable only for relatively small molecules. The computations required may often be prohibitively costly or may preclude altogether the evaluation of VCD intensities of large molecules. In such cases, spectral interpretations are still possible with predictions of simpler empirical models' which generate electric and magnetic dipole transition moments by relating the motion of electronic charge density with nuclear displacements in certain approximate ways. Furthermore, such models may give a readily understood physical picture of the dominant electronic events accompanying molecular vibrations by partitioning the overall effect into simple mechanisms such as charge following the nuclei perfectly, local charge redistributions, and more extended charge flows, provided of course that such descriptions properly reproduce observed VCD band signs and intensities. Obviously these simplified models cannot be complete, and only qualitative agreement with experiment may be expected realistically. Recent studies have demonstrated that this degree of accuracy is often sufficient for detailed studies of conformation and absolute c~nfiguration.~-' Our approach has been to test the applicability of some of the conceptually simple methods with small molecules, using a b initio M O calculations where appropriate. For the assignment of the vibrational spectra we have relied on ab initio force fields? which have proven to be dependable and which, after scaling with ap(4) Dutler, R.;Rauk A. J . Am. Chem. Soc. 1989, 1 1 1 , 69576966, (5) (a) Su, C. N.; Keiderling, T. A. J . Am. Chem. Soc. 1980, 102, 51 1-515. (b) Annamalai, A.; Keiderling, T. A. J . Am. Chem. Soc. 1987,109. 3125-3132. (c) Narayanan, U.;Keiderling, T. A. J . Am. Chem. Soc. 1988, 110,4139-4144. (d) Freedman, T.B.; Kallmerten, J.; Lipp, E. D.; Young, D. A.; Nafie, L. A. J . Am. Chem. Soc. 1988.110,689-698. (e) Zuk, W. M.; Freedman, T. B.; Nafie, L. A. J . Phys. Chem. 1989, 93, 1771-1779. (6) Shaw, R. A.; Ibrahim, N.; Wieser, H. J . Phys. Chem. 1990. 94, 125-133. (7) Shaw, R. A.; Ibrahim, N.; Wieser, H. Can. J . Chem., in press. (8) (a) Fogarasi, G.;Pulay, P. VibrarionulSpectra and Srructure; Durig, J. R., Ed.;Elsevier: Amsterdam, 1984; Vol. 14, pp 125-219. (b) Fogarasi, G.; Pulay, P. Annu. Rev. Phys. Chem. 1984, 35, 191-243.
0 1991 American Chemical Society
