4502
J . Phys. Chem. 1984, 88, 4502-4505
is observed, not simply a monotonic decrease of the IP toward the value of the work function as the cluster size increases. A comparison of these results with theory indicates that while the quantitative theoretical predictions are poor, qualitatively, the trends predicted theoretically are in reasonable agreement with the measured experimental trends. Even though the I P S of Nix clusters have been measured only within a fairly coarse grid of photon energies, we have been able to gain insights into possible alterations of electronic and structural characteristics of metal atom clusters and to demonstrate the limitations of the present theoretical calculations and experimental measurements. Further experimental work using tunable lasers will allow us to more
precisely define the metal cluster IPS.Such information represents a first step toward the characterization of the electronic and structural features of small metal clusters. Acknowledgment. We are particularly indebted to R. Smalley and D. J. Trevor for valuable discussions and R. Smalley and J. Hopkins for valuable technical assistance with the details of the pulsed solenoid valve as well as for providing some data acquisition software. We gratefully acknowledge the valuable technical assistance provided by Ken Reichmann during the course of these experiments. Registry No. Nickel, 7440-02-0.
Mass Spectral and Electric Deflection Study of Acetic Acid Clusters$ R. Sievert? I. Cadei,* J. Van Doren,I and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: January 31, 1983)
Acetic acid clusters, (CH3COOH),, up to n = 10, were produced in a supersonic beam expansion and analyzed in a molecular beam quadrupole mass spectrometer. A general mechanism for their mass spectral fragmentation was deduced. Polarity of the first four clusters was determined in a molecular beam electric deflection experiment, providing evidence for their vapor-phase structure.
Introduction Owing to the unusually large amount of dimer in the vapor phase,' studies of the vapor-phase polymers of acetic acid are of interest for testing theories of homogeneous nucleation: elucidating the nature of hydrogen bonding? and investigating the mechanism of cluster g r ~ w t h . Althoqgh ~ there has been extensive investigation of the acetic acid vapor p h a ~ eand ~ , ~the properties of the dimer are very well-known,' there is still some controversy about the amount of higher-order clusters in the gas phase's8 and their structures and proper tie^.^ The present study was undertaken to provide additional data on the polymers of acetic acid vapor, with attention to the problem of fragmentation mechanisms which inherently can influence the interpretation of the mass spectra of clusters. Evidence for the polarity of clusters containing the first four monomer subunits was obtained from the mass spectrometric experiments coupled with a quadrupole electric deflection technique. The first known evidence for the focusing of high-order polymers was obtained and are reported herein. These findings have provided the basis for making certain deductions concerning the structure of these clusters. Experimental Section The apparatus consists of four major parts: the stagnation chamber, the differential pumping chamber with nozzle exhaust, the focusing region, and the detection chamber (Figure 1). Clusters are produced by expanding the vapor of the stagnation chamber at about 1 atm through a sonic nozzle to form a free jet. The stagnation chamber consists of a heatable glass tube (0.d. = 16 mm) that is drawn down to form a nozzle with a throat diameter of 100 or 150 pm." The liquid acid (Baker, 99.9% pure) Preliminary phases of this research were undertaken at the University of Colorado. NATO Fellow, 1981-2. Present address: Institut fur Physikal Chemie, Tammann Str. 6, D-3400 G6ttingen, West Germany. 3 Visiting Fellow, University of Colorado, 1981-2 and Visiting Associate Professor, Pennsylvania State University, 1982. Present address: Institut za fiziku, P.O. Box 57, Studentski trg 12/Y, 11000 Beograd, Yugoslavia. Present address: Department of Chemistry, University of Colorado, Boulder. CO 80309.
is heated in a glass flask connected to the tube. Its temperature is held constant as monitored by a thermocouple, and the partial pressure of the acid is determined by interpolating from known vapor pressure curves. A regulated flow of carrier gas is bubbled through the liquid. The total pressure of the stagnation chamber is measured with a Bourdon type gauge. The whole glass tube is heated resistively; its temperature, as well as that of the nozzle, is measured by a thermocouple mounted at each location. The nozzle temperature has to be set higher than that of the bulb temperature in order to prevent condensation. The nozzle exhaust is a stainless steel chamber (id. = 30 mm) pumped by a freon baffled oil diffusion pump. It is separated from the differential pumping chamber by a skimmer having a I-mm aperture. The pressure during operation with the gas load is approximately 5 X 104-10-3 torr in the nozzle exhaust and IO" in the differential pumping region. The movable nozzle is aligned with the skimmer and the axis of the apparatus by optimizing the mass spectrometric signal of the cluster beam. The molecular beam enters the focusing region through a hole (i.d. = 5 mm). The region is a stainless steel cyclinder pumped by a liquid nitrogen baffled oil diffusion pump. The typical pressure under a gas load is lo-' torr. This chamber houses the electric deflection field. It is comprised of a set of stainless steel quadrupole rods (length 57.15 cm, diameter 0.476 cm), which are mounted in a support of machinable glass and stainless steel that (1) Chao, J., Zwolinski, B. J . Phys. Chem. Ref. Data 1978, F l , 363. (2) Heist, H. R.; Colling, K. M.; DuPuis, C. S. J . Chem. Phys. 1976, 65, 5147. (3) Joesten, M. D.; Schaad, L. J. "Hydrogen Bonding", Marcel Dekker: .~ New York, 1974. (4) Castleman, A. W., Jr.; Kay, B. D.; Sievert, R.; Stephan, K.; Van Doren, J.: Miirk, T. D. Presented at the Seventh International Symposium on Gas . . Kinetics, Gottingen, 1982. ( 5 ) Karle, J.; Brockway, L. 0. J. Am. Chem. SOC.1944,66, 574. (6) Clague, A. D. H.; Bernstein, J. H. Spectrochim. Acta, Part A 1969, 25, 593. (7) Frurip, D. J.; Curtiss, L. A.; Blander, M. J. Am. Chem. SOC.1980, 102, 2610. (8) Ritter, H. L.; Simons, J. H. J. Am. Chem. SOC.1945, 67, 757. (9) Johnson, E. W.; Nahs, L. K. J . Am. Chem. SOC.1950, 72, 547. (10) Kay, B. D. Ph.D. Thesis, University of Colorado, Boulder, 1981. (11) Kay, B. D.; Lindeman, T. G.; Castleman, A. W., Jr. Reu. Sci. Instrum. 1982, 53, 473.
0022-365418412088-4502$01.50/0 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4503
Acetic Acid Clusters NEUTRAL BEAM APPARATUS
40 eV
0TN :4OoC
BEAM OBSTACLE
ELECTROSTATIC
hz9 TN =50°C
QUAORUPOLE FOCUS-
IONIZATION
SUPERSONIC
REGION TURBOMOLECULAR PUMP
DIFFUSION PUMP
IS^^^^^?^^ I
DIFFERENTIAL PUMPING CHAMBER
I
FOCUSING REGION
I
DEAETh;g
I
Figure 1. Experimental apparatus.1°
can be aligned so that field axis and beam axis are collinear. The beam obstacle is a ceramic rod (1.8 mm), which can be moved in and out of the beam axis; its purpose is to block on-axis beam molecules. This provision is necessary since the electric field and field gradient vanish along the field axis. The quadrupole rods are connected to two high-voltage power supplies of maximum 10-kV output. A hole of 2-mm aperature separates the detection region from the focusing region. The latter is pumped by a 100 L/s turbomolecule pump. Typical operating pressures are 5 x 10-9-10-8 torr. The detection chamber contains the electron-impact ionization quadrupole mass spectrometer aligned perpendicular to the beam axis. Standard pulse counting circuitry is employed, with a multichannel analyzer used for data storage. A ramp generator and sweep counter enable multiple scans of an entire mass range. In the deflection experiments the mass spectrometer is set to a single mass peak of interest, and the signal is averaged.
Results and Discussion Cluster Formation. There has been only indirect evidence for higher polymers in the normal vapor phase of acetic acid8s9and there is no verification of these results in later experiments.' The presence of a trimer having an enthalpy of formation of -22.7 kcal mol-' was indicated as being a component of the vapor in equilibrium with liquid acetic acid based on the vapor density study by Johnson and NashSg In contrast, the work of Ritter and Simons8 was found to be compatible with the presence of a small fraction of the tetrameric species in normal acetic acid vapor; an enthalpy of -6.75 kcal mol-' was deduced from the measurements. Recent data of Frurip et a1.'* do not exclude the presence of trimers and tetramers in the equilibrium vapor, but the results also do not provide evidence for substantial amounts of either of these. Their data for the dimerization of acetic acid yield an enthalpy value of -14.64 kcal mol-'. In the present studies, expansion of pure acetic acid vapor in the pressure region of 40-740 torr (43-1 16 "C) was found to give less than a few percent trimer signal, with no indication of higher clusters at all. Further, it is possible that even this small trimer signal might arise from clusters formed in the supersonic expansion. Therefore, excluding the problem of fragmentation, this small signal was believed to represent an upper limit for the amount of preexisting higher clusters. A typical spectrum (relative to m / e 240) of expanded acetic acid seeded in about 1 atm of Ar, which leads to appreciably lower temperature^,'^ is shown in Figure 2. The spectrum is taken at relatively low mass resolution (Am = 3, at m = 600 amu) and an ionization energy of 40 eV. Minor peaks are omitted for clarity. Raising the nozzle temperature to 50 O C with all other conditions staying the same was found to shift the most abundant peak to (12) Frurip, D. J.; Curtiss, L. A,; Blander, M. J . Am. Chem. SOC.1980, 102, 2610. (13) Smalley,R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977, 10, 139.
I
9902 Ma55
Figure 2. Spectrum at 40 eV, stagnation chamber pressure 660 torr, acetic acid pressure 77.6 torr. Two different nozzle temperatures. x1.5
I
It
PT x 680 torr
xz.5
0 T, = 90-C hzD TN=120'C PT =450 torr
103
105
121
I60
Ma55
Figure 3. Spectrum at higher resolution and two nozzle temperatures and pressures. Acetic acid pressure in both cases 23 torr.
lower masses and reduce the amount of higher clusters. This is in agreement with findings showing the importance of concomitant unimolecular decomposition during cluster g r o ~ t h . ~ J No ~J~ evidence is found for different mechanisms of growth of systems composed of associated vapors compared to unassociated vapors. The cluster distributions were smooth, displaying a maximum in the distribution which was dependent upon nozzle temperature and expansion pressure. The spectra show a sequence of mass peaks separated by Am = 60 amu, which is equivalent to one acetic acid molecular mass. Mass peaks up to m / e 660 indicate a cluster size of at least 11 molecules produced during the course of the expansions. In those experiments made with argon, no evidence for clusters containing rare gas atoms was obtained. Mass Spectral Fragmentation. Lowering the energy of the ionizing electrons to 20 eV did not change the principal shape of the spectrum but did shift the largest peak of the distribution to higher masses. The shift is due to the fact that fragmentation becomes less important at lower ionization energies. To investigate the fragmentation of lower clusters in detail, we performed experiments in a smaller mass range but with higher resolution (Am = 1, m / e 300) and at nozzle temperatures high enough to prevent formation of large amounts of higher clusters. These conditions ensured that mass spectral fragmentation of those at smaller (14) Kay, B. D.; Castleman, A. W., Jr. J . Chem. Phys., in press. (15) Kay, B. D.; Hermann, V.; Castleman, A. W., Jr. Chem. Phys. Lett. 1981, 80, 469.
4504
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
masses were negligible. Figure 3 shows the spectra, relative to the monomer signal at m / e 60, obtained from experiments made at two different stagnation chamber conditions. Independent mass calibration with CC14 proved that there was no signal at mass 120 (corresponding to the parent ion of the dimer) but a signal at 121 corresponding to a fragment of the trimer. The low pressure in the detector excludes the possibility that any ion-molecule reactions of the dimer lead to the formation of mass 121. Subtraction of the pure monomer and the monomer and dimer spectrum, which can be obtained at very low pressures and very high nozzle temperatures (values that were found to be in good agreement with the literat~re'~,''),enabled a determination of the mass spectral fragmentation of the trimer. The main mass peaks of the acetic acid dimers are m / e 43, 61, and 105 (ratio of abundance (3-4):lO:l) and of the trimer m / e 43, 61, 103, and 121 (ratio of 103:121 = 1:(2-3)). It was found that none of the clusters has a parent ion corresponding to its molecular mass peak. The higher polymers had a prominent peak at their molecular mass minus 59, and a smaller peak of a few percent of the signal of that prominent peak at molecular mass minus 77 up to the hexamer. Other fragments were found at molecular mass minus 119 and minus 103. Quantitative assignments were not possible because of mass interferences of higher clusters with smaller ones. A mechanism compatible with these results, which explains the appearance of the detected ions (underlined), is given below. The formulae of the ions at certain masses are given to clarify similarities in the spectra of the clusters and do not intend to be representative of the real ion structures. The dimer decomposes according to
-
(CH3COOH)2+
-
(CH3COOH)H+ m / e 61
+ CH3CO0 m = 59
+
CH3CO+ (CH,COOH)OH 43 77 A minor channel (13%) is the loss of a methyl (CH3COOH)COOH' + CH3 105 15 The fragmentation of the trimer follows basically the same pattern, the first channel being the most abundant. (CH3COOH)3+
-
-
(CH,COOH)2H+ + CH3COO m / e 121 m = 59
(CH3COOH)H' 61
-
+ (CH3COOH)CH3C00 129
CH3CO+ + (CH3COOH)20H 43 137
+
(CH3COOH)CH3CO+ (CH,COOH)OH 103 17
The concentration of the trimer was not large enough to see the methyl loss of a few percent if it exists. This general scheme is true for higher clusters as well. The main fragmentation channel is the formation of a protonated acetic acid ion-cluster and loss of a CH3CO0 moiety. The minor channel is the formation of CH3CO+ clustered to ( n - 2) acetic acid molecules, where n denotes the number of molecules in the original cluster. Starting at n = 7 the molecular ion loses a (CH3COOH ) C H 3 C 0 radical, leaving an ion (CH3COOH),20H+. This generates a sequence of mass peaks found every 60 units from m / e 311 to 497. The general scheme is (CH,COOH),+
-
-
(CH3COOH),-,H+
(CH3COOH),2H+
+ CH3CO0 59
+ (CH3COOH)CH3C00 119
(16) Cook, K D.; Taylor, J. W. Int. J. Mass Spectrom. Ion Phys. 1980,
35. (17) Cook, K. D.; Taylor, J. W. Int. J . Mass Spectrom. Ion Phys. 1979, 30, 93.
Sievert et al. More fragmentation of this kind could not be detected because of mass interference. Minor channels were (CH,COOH),+
(for n=7-10)
(for n=2-6)
(CH,COOH),,CH,CO+
(CH3COOH),_20H+
+ (CH,COOH)OH 77
+ (CH3COOH)CH3CO
103 These observations of mass spectral fragmentation compare very well with species formed in ion-molecule reactions occurring in pure acetic acid at a few tenths of a torr.'* The most prominent products were solvated proton species H+(CH,COOH), with n up to n = 9, CH,CO+, and its solvates with acetic acid. Deflection Experiments. Electric deflection experiments were made to help further elucidate the fragmentation of the clusters and to give hints concerning their structures. Molecular beam electric deflection in a quadrupole field has been a useful technique for studying molecular geometrie~,'~ as polar molecules refocus in a quadrupole field. The force exerted on a molecule having a permanent dipole moment as it passes through an inhomogeneous electric field is proportional to the negative gradient of the field energy. Therefore, the deflections give information about the sign and magnitude of the molecular Stark effect. In the case of the electric deflection experiments performed during the course of this work, a beam obstacle was positioned to block any straight-through trajectories from directly entering the mass spectrometer detection chamber. Deflections and refocusing were accomplished with the application of voltage of the quadrupole rods. In the case of polar molecules which have a positive Stark effect, their energy increases with increasing electric field. In general, polar molecules will have rotational states with both positive and negative Stark effects in approximately equal numbers, but the deflection of those with negative effects are not detected in the case where the beam obstacle is positioned between the source and detection chamber. Therefore, an increase in beam signal is a measure of the polar nature of the molecule. Generally, nonpolar molecules have negative Star_keffects resulting from the electronic polarizability interaction -E&E. Therefore, nonpolar molecules will be deflected, leading to the observation of defoc~sing.'~-~~ Carboxylic acid dimers in general, and acetic acid isomers in particular, are known to have a head-to-tail configurations and to be Under conditions where very little higher polymers were present, masses 61 and 105 were due primarily to fragmentation of the dimer and displayed the expected defocusing behavi0r.2~ At lower nozzle temperatures when trimer was present and detected at mass 121 and 103, mass 61 showed focusing behavior, although always at a relatively lower percentage than mass 121 and 103. This finding suggests that acetic acid trimer is polar and has a fragment ion a t mass 61. Mass 105, which corresponds to a methyl loss of the dimer, did not focus under the same conditions. It is free of mass contributions from higher clusters and could be taken as a standard for the dimer. In those experiments made in which the trimer dominated the higher cluster spectrum, it could be readily detected at masses 103 and 121. Under these conditions the trimer was found to refocus by 2-5% in comparison to a much larger refocusing (approximately 40-45%) in the case of the pure monomer. Although the percent refocusing is not directly related to the dipole moment, the difference in the magnitudes is indicative of a somewhat larger dipole moment for the monomer compared to the trimer. Several structures are compatible with the polarity (18) Luczynski, Z.; Wloclek, S.;Wincel, H. Adu. Mass Spectrom. 1978, 7A. (19) Berg, R. A,; Wharton, L.; Klemperer, W.; Biichler, A.; Stauffer, J. L. J . Chem. Phys. 1965, 43, 2416. (20) Bennewitz, H. G.; Paul, W.; Schlier, Ch. 2.Phys. 1955, 141, 6 . (21) Odutola, J. A.; Viswanathan, R.; Dyke, T. R. J . Am. Chem. SOC. 1979, 101, 4187. (22) Almenningen, A,; Bastiansen, 0.; Motzfeldt, T. Acta. Chem. Scand. 1969, 23, 2848. (23) Derissen, J. L. J . Mol. Struct. 1971, 7, 67.
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4505
Acetic Acid Clusters
/
s-8:n Figure 4. Proposed trimer structure showing the third molecule bridged across the dimer ring. The 1”-2” axis dose not lie directly along the x axis and the 3”-1”-4” plane is slightly tilted counterclockwise from the xz plane. The l ” 4 ” distance is assumed to be the same as 1 4 and 1’4, being equal to 1.9 A. The dipole moment of this structure would be essentially equivalent to that of a monomer acetic acid molecule.
of the trimer, including that of a nonplanar 12-membered ring suggested by Johnson and Nash9 on the grounds of thermodynamic considerations. The dipole moment of such a structure could approach that of a monomer acetic acid molecule. If the bonding of a third acetic acid molecule to the dimer is as large as indicated by Johnson and N a ~ hit, ~must contain at least one more hydrogen bond than the dimer structure. The formation of the trimer from the dimer would require distruption of the head-to-tail configuration of the dimer. In order to be consistent with the reported enthalpy of formation for the trimer, at least two new hydrogen bonds must be formed thereafter. This requirement is incompatible with a catamer structure25since the formation of the trimer would yield no new hydrogen bonds. Hence, its formation from the equilibrium dimer structure would be almost thermoneutral. Other than a nonplanar ring structure, another plausible possibility would be a structure comprised of the third molecule hydrogen bonded to one of the carbonyl groups on the dimer. However, the rotational constant for such a species would be relatively small, and this would be expected to lead to a species which would refocus more appreciably than has been observed for the trimeric acetic acid cluster in the present experiments. Additionally, hydrogen bonding at the carbonyl location of an already cyclic dimer would be relatively weak and probably could not account for the relatively strongly bound species indicated by the earlier work? Therefore, among these possibilities the most likely structure is one having a bridging molecule across the dimer ring, Le., the carbonyl group of the additional acetic (24) Odutola, J. A.; Dyke, T. R. J. Chem. Phys. 1978, 68, 4663. (25) (a) Lifson, S.; Hagler, A. T.; Duaber, P. J. Am. Chem. SOC.1979, 101. (b) Hagler, A. T.; Duaber, P.; and Lifson, S. Ibid. 1979, 101, 5131.
acid monomer is proposed to be bridged to one of the hydrogens of the cyclic dimer, while the hydrogen from the OH of the monomer is bridged to a carboriyl group in the dimer structure (see Figure 4). Such a species would have a small dipole moment (based on geometric considerations) and would provide a stronger bonding than a simple hydrogen bond to the preexisting dimer. This structure would also be compatible with the observations for the case of the tetramer discussed in what follows. In the case of higher clusters, mass interference became very strong so that only for the tetramer could unambiguous results be obtained. At conditions that did not produce appreciable concentrations of clusters larger than the tetramer, the primary fragmentation product of the tetramer (mass 181) defccused. The deflection experiments thereby showed that it is nonpolar. With clusters larger than the tetramer present, mass 18 1 showed weak focusing. These results indicate that there are still higher-order clusters which have polar structures. It is doubtful that the tetramer has the structure suggested by Ritter and Simons8 since the work of Johnson and Nash9 led to the observation that trimethylacetic acid formed higher polymers even more readily than does acetic acid under comparable conditions. It would be expected that the proposed layered dimer structure would be sterically hindered, especially in the case of the trimethylacetic acid, and would not be able to form the proposed stacked resonant structure. It is difficult to rationalize the thermodynamic data published by others in terms of a catamer structure for the tetramer. It is interesting to note that recent studies by Bertagnolli show that even the liquid does not have a catamer-like structure.26 Furthermore, such a structure would also not be compatible with the present focusing results which show the tetramer to be nonpolar. In terms of the trimer alone it is difficult to decide between a structure comprised of a nonplanar ring or the one shown in Figure 4. However, in view of the observations for both the trimer and tetramer, a plausible structure is one in which the second acetic acid adds to the dimer (or the first one to the trimer) by another bridging structure across the opposite face of the cyclic dimer. This is in accord with the present observations and enables building of the structure from preexisting dimer and trimer. Such a structure would also be expected to have a thermodynamic stability comparable in magnitude with that suggested by the thermodynamic measurements. Acknowledgment. This work was made possible due to a Scholarship awarded to Dr. Rita Sievert by the Committee for Sciences of NATO, through the DAAD. Support of the US. Army Research Office (Grant No. DAAG29-79-C-0133) is gratefully acknowledged. Registry No. Acetic acid, 64-19-7. (26) Bertagnolli, H. Chem. Phys. Lett. 1982, 93, 287.
