Multiphoton ionization studies of C, H,-(CH, OH),, clusters. I. Comparisons with C, H,-(H, 0), clusters

Transcription

1 Multiphoton ionization studies of C, H,-(CH, OH),, clusters. I. Comparisons with C, H,-(H, 0), clusters Aaron W. Garrett, Daniel L. Severance, ) and Timothy S. Zwierb)vC) Department of Chemistry, Purdue University, West Lafayette, Indiana (Received 7 November 199 1; accepted 7 February 1992) Resonant two-photon ionization (R2PI) scans of the S,-S, spectra of C, H6-( CH, OH) n clusters with n = l-5 have been recorded. These scans provide an interesting comparison with earlier spectra from our laboratory on C, H, -( H, 0) n clusters. A variety of vibronic level arguments are used to constrain the geometries of the C!, H, -( CH, OH) n clusters. The 1: 1 and 1:2 clusters possess vibronic level features which are very similar to their aqueous counterparts. The 1: 1 cluster places the methanol molecule in a P hydrogen-bonded configuration on or near the sixfold axis of benzene. The spectral characteristics of the 1:2 cluster are consistent with both methanol molecules residing on the same side of the benzene ring as a methanol dimer. Higher C, H, -( CH, OH) n clusters show distinct differences from the corresponding C!, H, -( H, 0) n clusters. Vibronic level arguments lead to the following conclusions: the methanol molecules in the 1:3 cluster show the strongest hydrogen bonding to the?r cloud of any of the clusters and attach to benzene in such a way as to strongly break the sixfold symmetry of its r cloud. The 1:4 clusters are at most only very weakly hydrogen bonded to the P cloud, break benzene s sixfold symmetry moderately well, and posses strong activity in a very low frequency intermolecular mode. The methanol molecules in the 1:5 cluster show no hydrogen-bonding interaction with benzene s r cloud, induce remarkably little asymmetry in the P electron density, and produce very little van der Waals activity. Monte Carlo simulations using intermolecular potentials developed for liquid simulations serve as a guide to the possible minimum-energy structures for the clusters. The experimental results are used to distinguish between the possible structures. In all cases, the lowest energy structures produced by the calculations satisfactorily fit the vibronic level constraints placed on the structures by our data. I. INTRODUCTION Over the past several years, a number of groups,l-i9 including our own, have studied the spectroscopy and photophysics of various aromatic-x,, clusters. Many of these studies have focused on rare gas atoms as solvents -8 since they serve as useful model systems for the study of weak intermolecular forces. Recently, attention has increasingly turned to more chemically relevant solvents, -** but the increasing complexity of these solvents adds a new level of challenge to the detailed structural characterization of the clusters. We have recently taken a somewhat different approach to the study of larger clusters containing a single benzene molecule. In this case, the high symmetry of benzene and the forbidden nature of its S,,-S, origin produce vibronic level probes of the structures of the clusters.2g22 Admittedly these constraints are loose ones. Nevertheless, they provide a global view of the structures of these clusters for which the acquisition and analysis of fully rotationally resolved spectra are exceedingly formidable tasks. In a recent series of papers,** we used such a combination of rotational band contour analysis and vibronic level Present address: Department ofchemistry, Yale University, New Haven, CT ) Alfred P. Sloan Research Fellow. ) Author to whom correspondence should be addressed. arguments to study C!, H, -( H, 0) n clusters containing from one to five water molecules. These spectra point consistently to the clusters being composed of hydrogen-bonded water networks situated on one face of the benzene ring. The picture which is emerging is thus one in which the strong binding between water molecules is the major determinant of cluster structure, with the benzene molecule relegated (for clusters containing up to five water molecules at least) to the outskirts of the cluster due to its weaker rr hydrogen bonding to the water cluster. In this paper, we report similar spectroscopic measurements on C,H,-(CH, OH) n clusters with an eye toward comparison with those on C, H, -( H, 0) n. The substitution of CH, OH for H, 0 has several interesting consequences. First, despite the strong hydrogen bonding between metha- nol molecules, benzene is very soluble in methanol, in notable contrast to the immiscibility of benzene and water. Second, each CH,OH molecule is capable of acting only as a single hydrogen donor to its neighbors, whereas H,O can donate two hydrogens to hydrogen bonds. This is borne out by calculations on pure methanol clusters which predict the lowest energy structure for clusters containing up to 11 methanols to be a closed ring in which each methanol is involved in hydrogen bonding as single donor and accep- tor.23 By contrast, (H,O), clusters are calculated to form many similar-energy, compact, ice-like networks for cluster sizes containing six or more water molecules.24 Third, the J. Chem. Phys. 96 (lo), 15 May /92/l I 4$ American Institute of Physics 7245

2 7246 Garrett, Severance, and Zwier: CBH,-(CH,OH), clusters. I methyl groups are more bulky and polarizable than the hydrogen atoms of H, 0, adding steric constraints and dispersive forces to the interaction with other methanol molecules and with benzene which are not present in H,O. Finally, many of the tunneling mechanisms present in water clusters will be turned off in (CH, OH) R clusters, while others, involving methyl internal rotation, will be present. We will see that the smallest CgH6-(CH30H),, clusters have a vibronic spectroscopy quite similar to that of the corresponding C!, H, -( H, 0) n clusters. Larger C!, H,- (CH, OH) n clusters, on the other hand, show striking differences with their aqueous counterparts. Monte Carlo simulations using previously optimized intermolecular potentials25v26 predict minimum energy structures falling into one of two broad classes involving either chain or cyclic hydrogen-bonded CH,OH clusters attached to benzene. The vibronic level features of the clusters distinguish between the structural possibilities presented by the calculations, providing insight to the very different ways in which methanol and water molecules interact with the benzene ring. In the process of the experimental work, we have observed a rich, sizedependent intracluster ion-molecule chemistry following photoionization of the neutral clusters. This ion-molecule chemistry is the subject of the adjoining paper (Paper II). II. EXPERIMENT The molecular beam reflectron time-of-flight (TOF) mass spectrometer used in these studies has been described previously.20s27 C, H, -( CH, OH) n clusters are formed by expanding a mixture containing C6 H, and CH, OH in heli- I:1 um from a pulsed valve of 0.8 mm diam operating at 20 Hz. The concentrations of these vapors are controlled by metering flows of helium over the room temperature liquids using hl&/ needle valves and mixing these flows with the main flow of helium. Typical expansion conditions employ - 0.5% C, H, and 0.1%-0.4% CH, OH at a total pressure of 2-4 bar. The clusters are resonantly ionized by the unfocused output of an excimer-pumped dye laser operating on Coumarin 503, doubled in a /?-barium borate crystal. Typical per pulse energies of mj/pulse are used. Mass-selected resonant twophoton ionization (R2PI) scans are recorded in a linear TOF mass spectrometer using a 100 MHz digital oscilloscope. The ratio of intensities of the origin and 6; transitions are reported in this work for the C, H6-( CH, OH),, clusters. These ratios are recorded by integrating the intensity of the origin and 6: transitions of a given cluster in order to avoid nonresonant contributions to the ion intensity. Contributions from more than one mass channel are summed in cases where the resonant features are present in more than one mass channel by virtue of fragmentation. A single determination involves recording the spectra of all cluster sizes simultaneously in a single scan with origin and 6; regions scanned immediately after one another to minimize the effects of changes in expansion or laser conditions. In order to minimize saturation effects, all spectra are recorded under unfocused laser conditions at peak powers of about 3 X 10 W/cm2. Despite these modest laser powers, the reported 0:/6; intensity ratios must be taken as upper bounds on the true intensity ratios due to the potential for saturation effects. Ill. RESULTS AND DISCUSSION J. Chem. Phys., Vol. 96, No. IO,15 May 1992 A. C, H,-CH, OH and C, H,-(CH, OH), 1. Assignment of spectral features to a given cluster size Figure 1 (a) presents one-color R2PI scans of the regions just to the blue of the 6: transition of C6 H, monitoring the (C, H,-CH, OH) + mass in the TOF mass spectrum. The assignment of the observed transitions to a given cluster size is complicated by the efficient fragmentation these rr hydrogen-bonded complexes undergo following photoioni- I I:2 - -I I:2 5 I, I I I I, I I I I, I I, I,, Relative Frequency (cm- ) FIG. 1. Resonant two-photon ionization (R2PI) scans in the 6; region monitoring the (a) (C,H,-CH30H) + and (b) (C,H,-H,O)+ mass channels. The 1:l and 19 designations denote the numberofbenzene:methan01 or benzene:water molecules in the cluster responsible for the given absorption feature. The zero of the relative frequency scale is the 6: transition of free C, H,. (b)

3 Garrett, Severance, and Zwier: C,HB(CH,OH), clusters zation. High fragmentation efficiencies have been observed, even under extremely mild laser conditions, for C,H,-X complexes with other hydrogen-bonding solvents such as HCI, CHC13,27 and ( H20),.22 Furthermore, this fragmentation is difficult to remove in two-color scans due to poor Franck-Condon factors to regions of the ionic potential energy surface below the dissociation threshold. We would expect similar fragmentation to plague the present studies as well, necessitating careful attention in making assignments of features to a given neutral cluster size. As stressed in the Introduction, our recent studies of C, H,-( H, 0) n clusters are especially helpful as a backdrop to these studies. They will serve here as an aid in the assignments and later as an aid in the interpretation of the spectra. The R2PI scan in the 6: region monitoring the 1: 1 (C, H,-CH, OH) + channel [Fig. 1 (a) ] shows strong similarities to the spectrum of the (C,H,-H,O) + channel shown in Fig. 1 (b). Both spectra exhibit similar frequency shifts of the main features and similar low frequency structure built on the blue-shifted peaks. The analogous R2PI scans in the origin region monitoring the 1:l and 1:2 mass channels are shown in Figs. 2(a) and 2 (b), respectively. These spectra are also very reminiscent of corresponding spectra in C, H6-( H, 0) n. First, the transition + 44 cm - (relative to C,H, 6; ) in Fig. 1 (a) is not observable at the origin [Fig. 2(a) 1, while those beginning at + 80 cm - are readily observed. Second, the set of transitions near + 80 cm - is observed very weakly in the 1:2 mass channel [Fig. 2(b) 1, indicating that these transitions are actually due to the 1:2 cluster fragmenting with high efficiency ( - 80%) into the I:1 mass channel upon photoionization. This is precisely what is observed in C,H,-(H,O),, as shown in Fig. 2(c), where the spectrum in the origin region monitoring the 1:l H,O mass channel has been proven to be due entirely to the fragmenting 1:2 H, 0 cluster.22 The transition at + 44 cm - in Fig. 1 (a) is very different in its behavior than the peaks beginning at + 80 cm-, not only in its weaker origin intensity, but also in its much more efficient fragmentation into the (C, H, > + mass channel. Unfortunately, it was not possible to find expansion conditions where the 1:2 cluster was small by comparison to this peak. Nevertheless, we assign the + 44 cm- transition to the C, H,-CH, OH complex, in direct analogy to the assignment of the + 50 cm - r transition of Fig. 1 (b) to the C, H,-H, 0 complex. 2. Wibronic levelprobes of the structures of the L-1 and i:2 clusters The similar spectral characteristics of C, H,-CH, OH and C,H,-(CH,OH), clusters to those of C,H,-H,O and C, H6-( H, O), lead us to expect similar geometries for the corresponding clusters. In our recent work on the C, H,-H, 0 complex,22 we used a combination of rotational band contour analysis and vibronic level arguments to show that the water molecule is near the sixfold axis of C,H,, internally rotating about this axis. The hydrogen(s) of the water molecule are pointed in toward the benzene r cloud, l-l I 1:2 CH,CH I I,, I t I I 8 I, I I, I I:3 CH,OH 1:2 H,O (a) I I I Relative Frequency (cm ) FIG. 2. RZPI scans in the S,,-S, origin region monitoring (a) the (C,H,-CH,OH) + mass; (b) the [C,H,-(CH,OH), ] + mass; and (c) the (C,H,-H,O) + mass for comparison. Note that for both C,H,-(CH,OH), and C,H,-(H,O),, all the features present at the origin in the 1 :l mass channel are due to the 1:2 cluster fragmenting with high efficiency into the 1 :l mass channel. The zero of the relative frequency scale marks the position of the forbidden origin transition of free C, H,. giving rise to the efficient fragmentation and blue-shifted absorption observed.22v27 Due to the lack of signal present in the C&He-CH,OH cluster, its rotational band contour was not recorded. At the present experimental resolution, the rotational band contours of larger clusters also provided limited structural information. As a result, we must seek to constrain the geometries of the clusters using vibronic level arguments. Such arguments have been outlined in some detail in our study of C,H,-(H20),22 and C,Hg-CC142 clusters. We summarize them for the C, H,-( CH, OH) n clusters with n = 1,2 below. a. Frequency shifts. The accumulated experience of workers investigating a large number of aromatic-x complexes has led to the generalization that complexes which are hydrogen bonded to the benzene rr cloud produce absorp- tions which are blue shifted from those of the parent aroma- (c)

4 7248 Garrett, Severance, and Zwier: C,H,-(CH,OH), clusters. I tic. 8-22*27 For example, C,H,-HCl, -CHCl,, and -H,O possess frequency shifts of + 125, + 179, and + 50 cm -, respectively.27 Purely dispersive interactions, on the other hand, typically give rise to red-shifted transitions (e.g., C, H,-Ar and C, H,-Ccl, have transitions shifted by - 8 and - 68 cm - from that of C, H, ). Thus the blue-shifted absorptions of the 1:l and 1:2 clusters ( + 44 and + 80 cm - from the transitions of C,H, ) suggest a hydrogenbonding interaction of the methanol(s) with the benzene rr cloud. b. Ejicientfragmentation upon photoionization. Consistent with the frequency shift, the efficient fragmentation following photoionization also supports a rr hydrogen-bonding interaction in the clusters. In a manner analogous to C 6 H,-HC1,20*27 the fragmentation of the ionized complex results from vertical ionization to a repulsive part of the ionic potential energy surface in which the positive end of the methanol dipole is initially oriented toward the newly created positive charge on the benzene ring. c. van der Waals structure. The positions and intensities of van der Waals structure built on a given transition reflect changes in the geometry of the cluster which accompany electronic excitation on benzene. The spectrum of Fig. 1 shows little van der Waals structure in the 1:l cluster, though interference from the 1:2 features inhibits clear identification of such structure. By contrast, the 1:2 cluster possesses a number of very strong van der Waals transitions, indicating that the 1:2 cluster is more sensitive to changes in electronic excitation in benzene than is the 1: 1 complex. Table I lists the relative frequencies and intensities of the van der Waals structure of the C,H,-( CH, OH), cluster at the origin. Again, the 1: 1 and 1:2 C, H,-H, 0 clusters exhibit similar trends. While it is difficult to observe any van der Waals transitions in the spectra of the C!, H, -H, 0 complex (with a strongest peak - 10% of the AvVdw = 0 transition), the corresponding structure in C,H,-(H, O), is easily observed, with transitions 5.5, 17, and 22.5 cm- above the origin [Fig. 2(c)]. d. The 0 g/6 6 intensity ratio. In several previous studies, we have made use of the forbidden nature of benzene s TABLE I. Relative frequencies and intensities of van der Waals structure built on the origin transition of C, H,-( CH, OH),. Relative frequency (cm- ) The origin of C,H,(CH,OH), zene s origin. Relative intensity is 77 cm- blue shifted from free ben- S, ( B2U ) -So (A Ig ) transition as a vibronic level indicator of the binding sites taken up by the complexing molecule (s) While theso-+!?, transition is electric dipole forbidden in benzene, it can be vibronically induced by vibrations of ezg symmetry.28 v~, an ezg in-plane ring elongation mode, is first-order allowed, and the 6; transition is one of the most intense vibronic transitions in the spectrum of free benzene. In benzene-containing clusters, however, intensity can be induced in the SOS, origin of C,H, by the complexed molecule (s). So 4, origin intensity will be induced if (i) the vibrationally averaged positions of the complexing molecules relative to benzene reduce the effective symmetry of benzene to lower than threefold symmetric and (ii) the interactions of these molecules with benzene s v cloud are strong enough to distort the r electron density in this asymmetric fashion. The quantitative measure of this symmetry breaking is the 0:/6: intensity ratio. Table II lists these ratios for the C, H6-( CH, OH) n clusters together with the corresponding measurements in C, H, -( H, 0) n for comparison. As noted earlier, systematic errors introduced by saturation effects require the reported intensity ratios to be treated as upper bounds to the true intensity ratios. Nevertheless, it is the relative magnitude of this ratio in successive cluster sizes which is important for the arguments presented. Furthermore, since the observed trends in 0:/6: intensity ratio with changing cluster size will only be heightened in the fully unsaturated limit, the reported intensity ratios can reliably be used as a general structural guide in the way indicated. The origin transition of the C!, H,-CH, OH complex is not observable in our spectra, with an upper bound on its intensity of 2% of that at 6:. Difficulties in making large quantities of the 1:l complex have inhibited placing better TABLED. Cluster-inducedspectral featuresofthec,h,-(ch,oh), and C,H,-(H,O), clusters. CH, OH Hz0 Cluster (13) 1:l 1:2 1:3 1:4 1:5 1:l 1:2 1:3 1:4 1:5 Frequency 0: /6:, shift Intensity (cm- ) ratiod (% ) +44 < <O.l ; splitting (cm- )...E c...b =...=...c *Frequency shift to the blue of the 6; transition of C,H,. b 1.6 cm- splitting due to internal rotation ofthe water molecule. See Ref. 1 for details. No 6; splitting is observable at the present resolution. dthe intensity ratios presented must be considered as upper bounds due to the possible contribution from saturation effects. Despite this, the relative numbers from one cluster size to the next are reliable. See the text for further discussion.

5 Garrett, Severance, and Zwier: C,H6(CH,0H), clusters. I 7249 bounds on the origin intensity. Nevertheless, the weakness of the origin is entirely analogous to the forbidden origin of the C, H,-H, 0 complex, indicating that the methanol molecule induces at most minor asymmetry in the benzene r cloud upon complexation. By contrast, the origin transition of the 1:2 cluster is readily observed with an 0:/6; intensity ratio of - 7%. Thus, addition of a second methanol induces significant asymmetry in the benzene r cloud. e. The 6 i splitting. The same symmetry reduction which results in induced So-& origin intensity can also break the degeneracy of the 6 vibrational level in the excited state. This splitting thus serves as a second measure of the degree of asymmetry imposed on benzene by the methanol molecule(s). In the case of the 6: splitting, the magnitude of the splitting may have both electronic (i.e., force constant) and kinematic (i.e., mass-related) contributions which are difficult to assess.2 *22 Nevertheless, the qualitative correlation is a good one-a large 6: splitting indicates that the methanol molecule(s) have significant interaction with the benzene ring from vibrationally averaged position(s) off the sixfold axis of benzene. As indicated in Table I, the 1: 1 complex has no observable 6; splitting, while the 1:2 complex has a splitting of 2.0 cm -, somewhat smaller than its C, H6-( H, O), counterpart. Taken as a whole, these vibronic level arguments show strong similarities between the C,H,-CH30H and C!, H,-H, 0 complexes. This close correspondence points to similar binding of methanol and water to the benzene ring. The methanol molecule induces only a very weak origin transition and does not observably split the degeneracy of ye, consistent with CH,OH taking up position on or near the sixfold axis. The efficient fragmentation and blue shift of the absorption of the complex suggest hydrogen-bonding interactions with the benzene ring, probably primarily through the O-H hydrogen. The determination of the exact orientation of the methanol molecule and its degree of nonrigidity will require rotationally resolved studies. However, as we will see in Sec. III C, the vibronic level constraints on the geometry are confirmed by Monte Carlo simulations based on intermolecular potentials developed by Jorgenson and co-workers The C,H,-( CH,OH), cluster also shows striking similarities with C, H6-( H, 0), in the general features of its spectroscopy: (i) a very similar frequency shift; (ii) the analogous presence of significant low-frequency van der Waals structure; (iii) a significant increase in 0:/6; intensity ratio relative to the 1: 1 complex; and (iv) an easily observable 6: splitting. We surmise from this that the two methanol molecules are interacting with the benzene ring in much the same way as two water molecules are in C, H,-( H, 0) 2. In C, H6-( H, O),, rotational band contour fitting provided center-of-mass positions of the two water molecules. Notably, the two water molecules are determined to be on the same side of the benzene ring at a water-water separation similar to that in the free water dimer. It is likely, then, that the C,H,-( CH,OH), complex is also characterized as a methanol dimer29 bound to one side of the ring with primary interaction with the benzene ring via the O-H s of the two methanols, much as in CbH6-(H20)2, The rotational band contour of the 1:2 cluster at the origin is shown in Fig. 3(a). No observable splittings are present in this band contour due, e.g., to methyl internal rotation. At the present resolution, only the shape of the band contour is resolved, making a structure determination untenable. In spite of this, the shape of the band contour does hold some confirming evidence to support the same side of the ring geometry for the two methanol molecules in the 1:2 cluster. Figure 3 (b) presents an asymmetric top band contour with methanol dimer attached to one side of the benzene ring at a geometry close to that provided by the calculations of Sec. III D. This contour is in qualitative accord with experiment. By contrast, the band contour of Fig. 3 (c), resulting from the two methanol molecules being placed on the Relative Frequency (cm ) FIG. 3. (a) Experimental rotational bandcontourofthe.s,,-s, origin ofthe C,H,-(CH,OH), cluster. (b) Calculated rotational band contour for the 1:2 cluster using a geometry close to the lowest energy structure predicted by the calculations of Sec. III D in which the two methanols are hydrogen bonded to one another on the same side of the benzene ring. (c) Calculated contour for the 1:2 cluster placing the two methanols near the sixfold axis on opposite sides of the ring. Note that the experimental band contour is much bettereproduced by (b) than (c).

6 7250 Garrett, Severance, and Zwier: C,H&H,OH), clusters. I sixfold axis on opposite sides of the benzene ring, is not consistent with experiment. Clearly, higher resolution spectra are needed before the detailed structure and degree of nonrigidity of this cluster can be determined. B. C,H,-(CH,OH),, n= Assignment of spectral features to a given cluster size Figures 4( a)-4( c) present R2PI scans in the 6: region monitoring the 1:2-1:4 mass channels, respectively. The broad background present in the spectra of the larger clusters is especially sensitive to the amount of benzene present in the expansion, indicating that the background arises from fragmentation of higher clusters containing two or more benzene molecules. The scans in the figure were taken at concentrations of C, H, of less than 0.2%. By extrapolation from what we have learned already about the efficient frag- mentation of the 1: 1 and 1:2 clusters following photoionization, one would expect that the major features in mass channel 1:2-1:4 can be assigned to the 1:3-15 clusters, respectively. This is precisely the assignment that was made in the R2PI spectra 22 of C, H6-( H, 0) n. It is remarkable, however, that no remnant of the absorption features is observable in the next higher mass channel, even though little interference exists in that channel at the resonant wavelengths of interest. One must postulate either extremely efficient fragmentation ( >99%) or the presence of reactive processes which occur alongside fragmentation to further reduce the ion intensity in the parent channel. Both these mechanisms appear to be in operation in the C, H6-( CH, OH) n clusters. The identification of product ions resulting from intracluster ion-molecule chemistry following photoionization leads to an unambiguous assignment of the neutral cluster responsible for a given absorption feature. Paper II focuses on this intracluster ion chemistry. Here, the assignment afforded by the ion-molecule chemistry is illustrated using the 1:3 cluster as example. The spectrum of Fig. 5 (a) is recorded (a) Unassigned Higher Clusters (b) Relative Frequency (cm- ) FIG. 4. R2PI scans ofthe 6; region monitoring the (a) 1:2; (b) 1:3; and (c) 1:4 mass channels. The assignments for the major features are given in the figure and discussed in the text. The zero of the relative frequency scale is the 6h transition of free C, H, Relative Frequency (cm- ) FIG. 5. R2PI scans of the 6; region monitoring the (a) 1:2 and (b) [ (CH,OW, ] + mass channels. The fact that the same resonant features are present in both these channels confirms our assignment of these features to the I:3 cluster. The zero of the relative frequency scale is the 6; transition of free C, H,. J. Chem. Phys., Vol. 96, No. IO,15 May 1992

7 Garrett, Severance, and Zwier: C,H,-(CH,OH), clusters. I 7251 monitoring the 1:2 mass channel. The spectrum is dominated by a strong, sharp absorption feature shifted 147 cm- * blue of the C!, H, 6: transition. However, the mass spectrum recorded with the laser tuned to this absorption feature also shows significant ion intensity at mass 96, corresponding to M,+ (M = methanol) arising from an intracluster dissociative electron transfer reaction. The presence of this ion in resonance with the absorption feature in the I:2 channel [Fig. 5(b) ] ensures that the neutral precursor contains three methanol molecules, strongly supporting the assignment of the peak at cm- as being due to the C, H6-( CH, OH) 3 cluster. Similar processes are observed in higher clusters. In each case, the major feature(s) present in the 1:n channel also appear in the M,++, channel following dissociative electron transfer and we assign these absorptions to the 1: (n + 1) neutral cluster. There are several other features present in the R2PI scans of the 1:3 and 1:4 mass channels [Figs. 4(b) and 4(c) ] besides the main peaks we have described. The band at cm - in Fig. 4(b) appears to be due to the 2:3 cluster, while the red-shifted doublet at - 35 cm - in Fig. 4(b) is not assigned, but is most likely due to a larger cluster as well. Based on the present data, we cannot exclude the possibility that the - 35 cm - peak arises from a second 1:4 conformer. However, differences in the intracluster ion chemistry of these peaks (Paper II) confirm their carrier as a chemically distinct species from the main peaks at + 19 cm- I. 2. Spectral characteristics of the i:3-1.5 clusters As with the 1: 1 and 1:2 clusters, the major point of comparison of the C, H6-( CH, OH) n clusters is with the corresponding C, He-( H, O), spectra which are reproduced in Fig. 6. The important vibronic level characteristics of both types of clusters are summarized in Table II. a. The I:3 cluster. The overall appearance of the 6; spectral region of the C,H,-(CH,OH), [Fig. 4(a)] and C,H6-(H,O), [Fig. 6(a)] clusters is similar. Both spectra are dominated by a single, sharp absorption well blue shifted from that of smaller clusters. In C, H6-( CH, OH) 3, a short, weak progression in a single van der Waals mode of 47 cm - is built on the 6: transition. Closer scrutiny reveals significant differences between the spectra of the two types of 1:3 clusters, however. First, the frequency shift of C, H6-( CH, OH) 3 ( cm - ) is 50% larger than that in C,H,-(H,O), ( +98 cm- ). Second, whereas the C,H,-(H,O), cluster has a very weak origin, the C, H6-( CH, OH) 3 cluster has a strong origin about 14% as intense as the vibronically induced 6: transition. Third, the 6: splitting is readily observed in C,H,-(CH,0H)3, but unresolved in C,H,-(H,O),. Thus the vibronic level constraints on the structure of the C,H,-( CH, OH) 3 cluster are these: (i) the cluster exhibits by far the strongest hydrogen bonding to the Pcloud of any of the C, H, -( CH, OH) n clusters and significantly greater than that in C,H,-(H,0)3; (ii) the three methanols bind to benzene in such a way as to strongly break the sixfold symmetry of benzene s rr cloud, qualitatively different than C, H6-( H, 0) 3 ; Relative Frequency (cm- ) FIG. 6. RZPI scans of the 6: region monitoring the (a) 1:2; (b) 1:3; and (c) 1:4 mass channels of C, H6-( H, 0) n clusters for comparison with Fig. 4. The zero of the relative frequency scale is the 6; transition of free C, H,. (iii) the small amount of van der Waals structure suggests a fairly rigid structure for those methanols which are most strongly interacting with benzene. b. The I:4 cluster. The C,H,-( CH,OH), spectrum [Fig. 4(b) ] shows even less similarity to that of C6 H, -( H, 0 ) 4. The 1:4 methanol cluster absorption shifts dramatically to the red by 128 cm - relative to the 1:3 absorption, while C,H,-( H,O), exhibits virtually no shift from C,H,-(H,O),. The C,H6-(CH,OH), 0:/6: intensity ratio is seven times larger than that in C,H,-(H,O),. C, H6-( CH, OH), also exhibits a highly congested spec- trum composed of seven or eight transitions spaced from one another by no more than 2-3 cm -, indicating significant Franck-Condon activity along a very low frequency intermolecular mode. A close-up scan of this congested spectrum at the origin of the 1:4 cluster is shown in Fig. 7. The structural consequences of these features are the following: 200 la)

8 7252 Garrett, Severance, and Zwier: CBH,(CH,OH), clusters. I 2 0. I * I * -. Relative Frequenyy (cm- ) FIG. 7. R2PI scan of the origin region of the 1:4 cluster (monitoring the 1:3 mass channel). Note the closely spaced set of transitions present in this cluster. The zero of the relative frequency scale marks the forbidden origin transition of free C,H,. (i) the large red shift in the absorption of the 1:4 cluster relative to 1:3 indicates a significant reduction in the strength of rr hydrogen bonding of the methanols to benzene; (ii) the sixfold symmetry of benzene s r cloud is broken moderately strongly, comparable to that in the 1:2 cluster, but less than that in the 1:3 cluster; (iii) electronic excitation of benzene produces a deformation of the methanol cluster along one or more very low frequency vibrations. c. The I:5 Cluster. The C, H, -( CH, OH) 3 R2PI scan of Fig. 4(c) (monitoring the 1:4 mass channel) is once again dominated by a single transition (i.e., Au = 0 Franck-Condon factors), but is shifted even further red to a position - 13 cm- from C, H, 6;. The C, H6-( H, 0) 5 absorption, by comparison, remains at + 97 cm-. Despite this difference, the 0:/6: intensity ratio is comparable to that in C,H,-(H,O), and is very weak (- 1%). The structural constraints placed on the 1:5 cluster are as follows: (i) the further red shift to a position red of that of free C,H, indicates that the 7r hydrogen-bonding interaction of the methanol molecules has been replaced almost entirely by dispersive interactions; (ii) intriguingly, the methanol molecules now produce a very small asymmetry in the benzene 7r cloud; (iii) the lack of van der Waals structure suggests a fairly rigid structure for the methanol molecules which is relatively insensitive to benzene electronic excitation. C. A qualitative comparison with C,H,-(H,O), These striking differences between the C,H,- (CH,OH), and C,H,-(H,O), spectra with n>3 must reflect different interactions of the C,H, molecule with methanol than with water clusters. In C,H,-( H,O),, n = 3-5, the nearly unchanged spectral characteristics of the successively larger clusters argue for cluster structures in 6 0 which the fourth and fifth water molecules are added to the C,H,-( H,O), cluster in positions far from the benzene ring, thereby disturbing neither the positions nor strengths of the interactions of the other water molecules with the benzene ring.22 Guided by calculations on water clusters which predict cyclic, hydrogen-bonded structures for the trimer, tetramer, and pentamer, our data pointed consistently to C,H,-( H,O), structures which are composed of cyclic hydrogen-bonded networks of water molecules oriented above the plane of the benzene ring (Fig. 8).24 The primary interaction of the water cluster with the benzene ring seems to occur via a single water molecule hydrogen bonded to the ring near benzene s sixfold axis. As we will see, these general features of the C, H6-( H, 0) n clusters are confirmed by calculations on the clusters discussed in Sec. III D. It seems likely that the C, Hs-( CH, OH) n clusters will retain the general features of the corresponding C, H6-( H, 0) n cluster geometries in being composed of hydrogen-bonded methanol clusters bound to one face of the benzene ring. Structures such as this are consistent with the presence of (CH, OH), product ions following photoionization (Paper II). This dissociative electron transfer reaction would seem unlikely in a neutral cluster structure in which the methanol molecules were distributed on both sides of the benzene ring. (al FIG. 8. Lowest energy structurescalculated for thec,h,-(h,o), clusters containing from n = 2-5 water molecules using the methods of Sec. III D. The calculated structures are in excellent accord with the experimental data on these clusters from Refs. 1 and 2. (b) (d). P

9 Garrett, Severance, and Zwier: C,H,(CH,OH), clusters. I 7253 The structure of pure methanol clusters larger than the dimer is not known experimentally. Electric deflection measurements on (CH,OH), with n = 3-5 show them all to have negligible dipole moments, consistent with cyclic structures in which all methanol molecules are equivalent or nearly equivalent. This general picture is confirmed by the semiempirical calculations (Sec. III D and Ref. 23) which predict minimum-energy structures which are cyclic, hydrogen-bonded networks in which each methanol acts singly as donor and acceptor with neighbors on either side. In C,H,-(CH,OH),, clusters, the possibility thus exists that, beginning with the 1:3 cluster, the OH hydrogens may be bound up in hydrogen bonding with other methanol molecules and would thereby not be free to hydrogen bond with the C, H, P cloud. The lack of dangling OH groups would mean that the methanol cluster must interact with C,H, via the methyl groups and/or oxygen atoms of the methanol cluster. Thus, while the 1: 1 and 1:2 GH,-(CH,OW,, clusters are quite analogous to C, H,-( H, O),, it is not surprising that the larger clusters show substantial differences. D. C,H,-(CH,OH), and C&H,-(H,O), structure calculations The indirect, vibronic level arguments given in the previous section are capable of providing only qualitative tests of the intermolecular potentials governing the interactions of C, H, and methanol molecules. However, the large size of the clusters of interest here require calculations using simple, computationally efficient intermolecular potentials, so that the match between the present levels of experiment and theory is not a bad one. In this section, we present results of Monte Carlo molecular dynamics simulations using a set of intermolecular potentials developed by Jorgenson and coworkers.25*26 Given the very different task for which these potentials have been optimized (namely, bulk solution phase behavior), the potentials are not necessarily an accurate predictor ofevery detail of the positions and orientations of each of the molecules in the cluster, nor will our results require them to be such. The calculations will be used to focus our attention on the broad classes of stable cluster structures which may be present. The experimental data can then be used to select between these possible structure types. The optimized intermolecular potential functions for both benzene and methanol have been developed previous- 1y.25*26 The functional form of the intermolecular potential incorporates both Lennard-Jones and electrostatic contributions. Both molecules are represented as collections of interaction sites centered on the atoms. Charges on the atoms and Lennard-Jones parameters were optimized for (i) reasonable geometric and energetic results for the gas phase benzene and methanol dimers and (ii) good agreement with the properties of the bulk liquids. The lack of explicit account of dispersive forces and three-body effects is crudely corrected for by a choice of charge distributions which compensate for their absence.26 While more sophisticated intermolecular potentials may need to be developed eventually to account for the detailed structures and intermolecular vibrations of these clusters, we will see that the present intermolecular potentials possess lowest energy structures very much in keeping with experiment. Table III collects the results of these calculations for the C,H,-(CH,OH). and C,H,-(H,O), clusters for n = l- 5. In the table, the entries in bold type are those which are consistent with the experimental constraints on their structures. The lowest energy structures calculated for the C, H6-( H, 0) n clusters are presented in Fig. 8 for comparison with those for the C, H6-( CH, OH) n clusters. 1. The 1:l and 1.2 clusters Figure 9 presents the lowest energy structures for the 1: 1 and 1:2 clusters. These structures confirm the qualitative interpretation given to the vibronic level characteristics of the clusters. The methanol molecule in the 1:l cluster has a center-of-mass position near the sixfold axis, consistent with the low 0:/6: intensity ratio observed and the lack of an observable 6; splitting. The methanol molecule interacts with benzene s r cloud via a hydrogen bond involving O-H, in keeping with the blue shift of its absorption and the cluster s efficient fragmentation following photoionization. The similar frequency shift to that in C, H,-H, 0 is also consistent with similar positions and strengths of interaction in the C,H,-H,O and C,H,-CH30H. The binding energy predicted for the C,H,-CH,OH cluster (ignoring zero point corrections) is 4.8 kcal/mol (Table III). The Monte Carlo simulations likewise predict a structure for the 1:2 cluster consistent with the qualitative structure suggested by experiment (Sec. IV A 2). The calculated structure is very analogous to the C, He--( H, O), structure [Figs. 9(b) and 9(c) (cf. Fig. 8) ] in incorporating two methanol molecules on the same side of the benzene ring as a slightly distorted methanol dimer. The structure of Fig. 10 is 3.9 kcal/mol more stable than the most likely alternative in which the second methanol attaches to the other side of the benzene ring. The positions of the methyl groups in these structures are not well tested by the present experiments. 2. The 1~3 cluster Figures IO(a) and IO(b) present the lowest energy structure calculated for the 1:3 cluster. Interestingly, this structure does not involve a cyclic methanol cluster as we might expect based on the calculated structures for the pure methanol clusters, but rather a hydrogen-bonded chain of methanols. A secondary minimum involving a cyclic methanol trimer is shown in Fig. lo( c). This structure is computed to be 1.1 kcal/mol less stable than that in Figs. 10(a) and 10(b). The I:3 clusters formed in the supersonic expansion are clearly more consistent with the chain structure than the cyclic structure. The chain structure has a dangling O-H free to hydrogen bond to the rr cloud, while the cyclic trimer interacts with the Z- cloud primarily via a methyl group. Thus the former is capable of producing a large So-S, blue shift while the latter is not. Furthermore, the strength of r hydrogen bonding present in the 1:3 chain structure would be predicted to be significantly greater than that for the 1:2

10 7254 Garrett, Severance, and Zwier: C,H,(CH,OH), clusters. I TABLE III. Calculated structures and binding energies for C,H,-(CHsOH). and C,H,-(H,O), clusters. Cluster AE(C,H,..*M,) (M = CH,OH, AE AEW, or W,P (Cd-h..*W,) W = H,O) (kcal/mol) (kcal/mol) (kcal/mol) Structure C,H,-M On the sirfold axis GH,-M, (7.6) 6.6 The same side, methanol dimer Opposite sides of the benzene ring G&.-M, (I) Methanol chain (19.0) 3.9 (II) Methanol cycle 3%-W (I) Methanol chain (32.9) 4.8 (II) Methanol cycle C&-M, (I) T-shaped, M, cycle as donor (44.3) 5.7 (II) T-shaped, C, H, as donor C,H,-W On the sixfold axis Cd-b-W, (6.8) 5.2 The same side, water dimer Opposite sides of the benzene ring GH,-Wa (I) Water chain G He-W, (18.3) 4.2 (II) Water cycle -3od (I) Water chain (31.2) 4.0 (II) Water cycle C&.-W, (41.5) 3.7 (I) T-shaped, W, cycle as donor (II) T-shaped, C, H, as donor Calculated total binding energy for the cluster. Zero point energy corrections are not included. %alculated binding energy of the methanol or water cluster in the C,H,-W, and C,H,-M, clusters. The difference of columns 1 and 3. The numbers in parentheses are the calculated binding energies of the pure methanol and water clusters after the clusters are allowed to relax to their minimum energy configuration. Calculated binding energy of C,H, to the M, or W, cluster in the specified configuration. In determining these energies, the M, and W, clusters are held llxed in the structures, while C,H, is pulled away from the cluster. d Not an energy minimum. estructures in bold face type are those which are most consistent with the present experimental data. cluster by virtue of the third methanol molecule further polarizing the hydrogens on the other methanols which interact directly with the benzene rr cloud. This is also in accord with the additional blue shift of the transitions of the 1:3 cluster from those of the 1:2 cluster. Finally, the chain structure is also consistent with the strong breaking of the sixfold symmetry of benzene s r cloud observed for the 1:3 cluster. The strong polarization of the two methanol molecules by hydrogen bonding to the third methanol should distort the benzene QT cloud along a single direction, inducing a strong origin transition. The comparison with C, H, -( H, 0) 3 is striking. As Table III indicates, the C, H,-( H, 0) 3 cluster is also calculated to have two nearly equally stable conformers composed of either a cyclic or a chain-like hydrogen-bonded water trimer attached to benzene s 7r cloud. The experimental results suggest that, of these two structures, the cyclic (H, 0) 3 structure [Fig. 8(b) ] is the one preferentially formed by C,H,-( H,O), clusters in the expansion, even though the chain structure is more consistent with the experimental datainc,h,-(ch,oh),. 3. The i-4 cluster The lowest energy structure calculated for the I:4 cluster is shown in Figs. 11 (a) and 11 (b) (0, = 37.5 kcal/ mol), while a secondary minimum structure (D, = 34.8 kcal/mol) is shown in Fig. 11 (c). Interestingly, by contrast to the 1:3 cluster, the calculations predict that in the 1:4 cluster, the methanols prefer a cyclic structure over a hydrogen-bonded chain structure. The present experiments support this predicted preference for the cyclic structure. The most striking feature of the 1:4 cluster is the dramatic shift to the red of its absorption relative to the 1:3 cluster. The interpretation we have given this shift is that it represents a large reduction in the strength of the hydrogen-bonding interaction with the 7r cloud. This is precisely what one would expect for the cyclic C, H, -( CH, OH) 4 cluster of Figs. 11 (a) and 11 (b) in which the primary interaction with the IT cloud is now via the methyl group near the sixfold axis. The O-H hydrogen bonding in the chain structure of Fig. 11 (c) would likely produce a strong blue shift comparable to that in the 1:3 cluster. The modest 0:/6: intensity ratio (5% ) and 6: splitting (2 cm - ) of the cluster is also consistent with the cyclic structure in that the calculated strength of interaction of the cyclic methanol cluster with the benzene P cloud is reduced significantly. Table III lists the binding energy of the benzene molecule to the methanol cluster [ AE(C, H, * * *M, ) ] in each of the cluster structures of interest. In the 1:3 cluster calculation, the chain type methanol trimer, which is hydrogen bonded to the P cloud, has a binding energy of some 7.9 kcal/mol to benzene. By contrast, in the 1:4 cluster, the cy-

11 Garrett, Severance, and Zwier: C,H,(CH,OH), clusters. I 7255 (81,a (a) (b) lb) FIG. 9. Lowest energy structurescalculated for (a) 1:l and (b) and (c) I:2 C,H,-(CH,OH), clusters. Note the similarity of these structures to the experimentally determined structures for C,H,-H, 0 and C,H,-(H,O), clusters. FIG. 10. (a) and (b) Lowest energy structure calculated for the 1:3 cluster, involving a chain-type hydrogen-bonded methanol trimer. (c) A secondary minimum structure for the 1:3 cluster involving a cyclic hydrogen bonded methanol cluster. This structure is calculated to be 1.1 kcal/mol less stable than that in (a) and (b). clic methanol tetramer, which lacks this hydrogen bonding, is calculated to bind to benzene by only 4.8 kcal/mol. The possibility is thus raised that even though the location of the methanols is still highly asymmetric in the 1:4 cluster, the induced origin intensity may be weakened (relative to the 1:3 cluster) by the weaker interaction of the benzene P cloud with the cyclic methanol tetramer. The drop in C,H,**- (CH, OH) n binding energy is more than compensated for in the cyclic 1:4 cluster by the extra methanolmethanol hydrogen bond so produced [ AE(M, > 1. That just the reverse is true in the 1:3 cluster is a testimony to the subtle balance of intermolecular interactions which is at play in determining the global minimum energy structure in these clusters. Thus, based on the present data, the C,H,-( CH, OH)., [Figs. 11(a) and 11(b)] and C,H,-(H,O), [Fig. 8(c)] clusters formed in the expansion are both consistent with structures composed of cyclic, hydrogen-bonded solvent clusters bound to benzene. The calculated structures point to the Z- hydrogen-bonding interaction present in C,H,- (H,O), orienting the water cluster off away from the benzene plane. However, the calculations suggest that the much less directional bonding of the methyl group in C, H, -(CH, OH), allows the methanol cluster to lie down on the benzene ring and even to begin to wrap around the ring. 4. The 1:5cluster Figure 12 presents the lowest energy structure calculated for the 15 cluster. It is not surprising that a cyclic methanol cluster, is predicted to be the most stable configuration once again. What is surprising at first is the orientation of the cyclic methanol pentamer with respect to the benzene ring. The methanol pentamer takes up a T-shaped configuration in which the primary interaction is with the hydrogen(s) on the benzene ring rather than with the benzene r cloud. The structure is similar in type to that of the benzene dimer in which the cyclic methanol cluster plays the role of one of the J. Chem. Phys., Vol. 96, No. IO, 15 May 1992

12 (a) *1- +v c jg, c 7256 Garrett, Severance, and Zwier: C,H&CH,OH), clusters. I FIG. 12. Lowest energy structure calculated for the I:5 cluster. FIG. 11. (a) and (b) Lowest energy structure calculated for the 1:4 cluster involving a cyclic hydrogen-bonded methanol cluster. (c) A secondary minimum structure for the 1:4 cluster involving a chain-type hydrogenbonded methanol tetramer. This structure is calculated to be 2.8 kcal/mol less stable than that in (a) and (b). benzene molecules. The beginnings of this tipping of the methanol cluster onto the side of the benzene ring are already evident in the calculated structure for the 1:4 cluster [Figs. 11 (a) and 11 (b) 1, but here the reorientation is complete. As with the smaller clusters, the intermolecular potential used in the Monte Carlo calculations reproduces the major characteristics of the 15 clusters present in the molecular beam. For instance, in the calculated structure, the interaction of the methanol cycle with the benzene ring is completely lacking in 7r hydrogen-bonding character, supporting the further shift of its absorption to a position red of that of free benzene ( - 14 cm - ). Second, the very small 0:/6; intensity ratio and negligible 6: splitting of the cluster requires that the methanol molecules induce only small asymmetry in the benzene rr cloud. If the 15 clusters in the expansion take on astructure similar to that of Fig. 12, they might be expected to induce little asymmetry as a simple consequence of the very weak interaction of the methanol pentamer with the benzene P cloud. The asymmetric orientation predicted for the methanol pentamer relative to benzene s sixfold axis may not be enough to induce the origin transition since it is the rr cloud which is the sensitive probe of benzene s surroundings in the ~-+r* electronic transition. The calculated structure is even consistent with the sharp reduction observed in the number and strength of the van der Waals transitions in the spectrum (by comparison to 1:4) by the weak interaction of the methanol cluster with the v cloud involved in the electronic excitation. The comparison of the calculated structures of C!, H,- (CH, OH), and C,H,-( H,O) 5 is once again revealing. Qualitatively speaking, both calculated lowest-energy structures are T-shaped structures composed of two rings. However, in C, H, -( H, 0) 5, the dangling hydrogens present orient the water cluster as a proton donor into the benzene R cloud. The cyclic methanol cluster, which possesses no such dangling O-H groups, shows a calculated preference toward orienting itself as proton acceptor for the hydrogen of benzene. In this case, the electron-rich region presented by the five oxygen atoms of the methanol cluster serves as the binding site for one of benzene s hydrogen atoms. Additional experimental constraints on the structures of these clusters will need to be devised to test these general conclusions further. IV. CONCLUSIONS In this paper, we have used a series of vibronic level arguments to constrain the geometries of several small