THE SPECTROSCOPY AND PHOTOPHYSICS OF HYDROGEN-BONDED COMPLEXES : BENZENE-CHCIa

A vibronic level study of the spectroscopy and photophysics of the CtHt-CHC13 complex has been carded out using a combination of laser-induced fluorescence and resonant two-photon ionization (R2PI). In CtHt-CHC13, the S1-S0 origin remains forbidden while the 1610 transition is weakly induced. Neither 61o nor 1610 are split by the presence of the CHC13 molecule. On this basis, a C3v structure is deduced for the complex, placing CHC13 on the six-fold axis of benzene. The large blue-shift of the complex’s absorption relative to benzene (+178 cm-) and the efficient fragmentation of the complex following one-color R2PI reflect a hydrogen-bonded orientation for CHCla relative to benzene’ n cloud. Dispersed fluorescence scans place a firm upper bound on the ground state binding energy of the complex of 2,024 cm-. Both the 61 and 61 levels do not dissociate on the time-.scale of the S fluorescence and show evidence of extensive state mixing with van der Waals’ levels primarily built on the 0 level of benzene. The CtHt-(CHC13) cluster shows extensive intermolecular structure beginning at +84 cm-, a strong origin transition, and splitting of 61. A structure which places both CHC13 molecules on the same side of the benzene ring is suggested on this basis. The vibronic level scheme used to deduce the structure of CtH6---CHC13 is tested against previous data on other CtH6-X complexes. The scheme is found to be capable, in favorable cases, of deducing the structures of CtH6-X complexes based purely on vibronic level data. Finally, the results on CtHt--CHC13 are compared with those on CtI-It-HC1 and CtHt-H20 to evaluate the characteristics of the n hydrogen bond.


I. INTRODUCTION
Benzene's role as prototypical aromatic invites fundamental studies of its intermolecular interactions with various solvents. One approach to such studies focuses on the spectroscopy of the cold, gas phase benzene-Xn clusters formed in a supersonic expansion. These studies have in favorable cases provided information on the lowest energy benzene-solvent structures, bracketed their binding energies, made some headway in assigning intermolecular vibrations which characterize the intermolecular potential energy surface, and probed the nature and extent of One of the intriguing aspects of the studies of benzene-HC1, -H20, and -CH3OH is the insight they provide to the pseudo-hydrogen bonding of these protic solvents to benzene's electron-rich r cloud. By contrast to the conventional linear X...HY hydrogen bond, the benzene-..HY interactions involve delocalized electrons spread out over benzene's carbon framework. TM In this paper we extend our study of benzene complexes with hydrogen bonding solvents to include CHC13. Studies of benzene/chloroform solutions using n.m.r., 19 infrared, 2 and Raman 2 spectroscopies have pointed toward the formation of a hydrogen-bonded C6H6-CHC13 complex in solution. Here we characterize the gasphase C6H6-CHC13 complex by the perturbations imposed on the Sl-So spectrum of benzene by the CHC13 molecule. From the symmetries of the vibrational fundamentals which gain intensity upon complexation, a C3 geometry is deduced for the 1:1 complex. In the process of this study we have generalized a scheme for deducing vibrationally-averaged structures for C6H6-X complexes based solely on vibronic level symmetry arguments. We apply this scheme to the c6n6-x complexes studied by our group to date. A comparison of the structures and spectroscopy of C6H6--HC1, C6H6-H2 O, and c6n6-cnc13 provides some insight to the nature of the r hydrogen bond.

II. EXPERIMENTAL
The molecular beam time-of-flight mass spectrometer used in these studies has been described previously. 4 C6H6-(CHC13) clusters are formed by expanding a mixture containing C6H6 and CHC13 in helium from a pulsed valve of 0.8 mm diameter operating at 20 Hz. The concentrations of these vapors are controlled by metering flows of helium over the room temperature liquids using needle valves and mixing these flows with the main flow of helium. Typical expansion conditions employ-0.5% C6H6 and 0.1-1.0% CHC13 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 energies of 0.1-1.0 mJ/pulse are used. Mass-selected R2PI scans are recorded in the linear mode of the TOF mass spectrometer using a 100 MHz digital oscilloscope. Fluorescence excitation scans, dispersed fluorescence scans, and fluorescence lifetime measurements are recorded in a second apparatus which has also been described elsewhere. 2 C6H6-X complexes In several previous studies of C6H6-X complexes, we have made use of the forbidden nature of benzene's S(B2u) ---So(Ag) transition as a vibronic level indicator of the binding sites taken up by the complexing molecule. 2,[4][5][6][7] While the So-S transition is electric dipole forbidden in benzene, vibrational levels of e:g symmetry can induce intensity in the transition by coupling to the S3(Elu state whose transition from So is dipole-allowed and very intense (f = 0.88). 22 v6, an e2g in-plane ring elongation mode, is first-order allowed, and the 610 transition is one of the most intense vibronic transitions in the spectrum of free benzene. 23 Following complexation to a solvent molecule X, the reduced symmetry of the complex may induce intensity in vibronic transitions in benzene which are otherwise forbidden. , [5][6][7] In the course of this study, we have developed a more systematic procedure for using these transitions to deduce the symmetry of the complex. Table   1 presents a correlation of symmetry labels from benzene (D6h) to c6n6-x complexes of varying reduced symmetries. In the complexes, the solvent molecules induce intensity in new vibronic transitions in benzene by distorting the benzene's electron distribution away from its symmetry in free benzene. The new transitions induced by the solvent thus reflect the over-all symmetry of the complex.
aSymmetry types in bold-faced type have So(Ag)-SffB2.) Xo fundamentals vibronically-allowed in C6H6-X complexes via coupling to the E. state of benzene. bUnderlined entries will have Xo fundamentals in C6H6-X which are split by virtue of the loss of degeneracy in the vibrational modes of benzene induced by complexation with X. We will see that in the C6H6-CHC13 complex, the application of the scheme shown in Table 1 allows us to deduce the symmetry of the complex. However, information on the orientation of the solvent molecule within this symmetry type must be gained from other data. One such piece of evidence arises from the accumulated experience of workers investigating a large number of aromatic-X complexes. 2,[24][25][26] In general, complexes which are hydrogen-bonded to the benzene r cloud produce absorptions which are blue-shifted from those of the parent aromatic. For example, C6H6-HC1 and -n20 possess frequency shifts of +125, and /50 cm -, respectively.
Purely dispersive interactions, on the other hand, typically give rise to red-shifted transitions (e.g., C6H6-Ar and C6H6-CC14 have transitions shifted by-21 and -68 cm from C6H6).
A r hydrogen bonding interaction in the complex would also be indicated by efficient fragmentation to C6H6 -t-X following photoionization. As demonstrated clearly in C6H6-HC1, 2'26 the fragmentation of the ionized complex in R2PI results from vertical ionization from the hydrogen-bonded neutral structure to a repulsive part of the ionic potential energy surface resulting from the positive end of the dipole on X initially being oriented toward the newly-created positive charge on the benzene ring.
B. The C6H6-CHCl3 Complex Figure 1 presents a laser-induced fluorescence excitation scan in the region near the 60 transition of C6H6. Dominating the spectrum is a transition blue-shifted from benzene's by 178 cm-. A short progression with spacing 25 cm is also observed which scales in intensity with the main peak under all conditions. There is little interference from benzene transitions in the region, aside from a weak transition at +235 cm -(marked by a B). The narrow peaks marked by arrows in the figure are due to higher clusters whose assignment will be addressed later.
A one-color R2PI spectrum over the same 60 region monitoring the [C6H6-CHC13] / and [C6H6] mass channels is shown in Figure 2. The +178 cm peak and the progression built on it dominate the spectra from both mass channels with no corresponding feature in the 1 2 mass channel. The spectrum in the 1 1 mass channel shows some interference to the red. Concentration studies indicate that these transitions are likely due to the 1 2 cluster, as discussed in the next section. We thus assign the +178, 205, and 231 cm transitions to the C6H6-CHC13 complex. The blue shift of these transitions relative to benzene is greater than any other C6H6-X complex so far Relative Frequency (cm) Figure 1 Laser-induced fluorescence excitation scan in the region of the 61o transition of C6H6-CHCI3. The frequency scale is relative to the 610 transition of free benzene. The peaks marked by an asterisk have been assigned to C6H6-CHC13, a weak benzene transition is marked by a 'B', and two transitions due to higher clusters are marked with arrows. Expansion conditions employ 0.5% C6H6, 0.5% CHC13 at a total pressure of 3 bar.
studied. In addition, the complex undergoes fragmentation with 90-95% efficiency to C6H6 + -I-CHC13 following one-color photoionization through 6o, even under unfocussed laser conditions following extrapolation to zero laser power.
The large intensity of the 610 transition of C6H6-CHC13 makes it possible to search with good sensitivity for the corresponding 00 transition. This transition is not observed, with an upper bound on its intensity of 0.1% of the 610 transition. Etalon scans of the 61o transition show no evidence of splitting of the 610 transition.
Despite not being able to observe the origin transition, the 1610 transition of the complex is observed ( Figure 3) with an intensity about 4% of the 610 transition, v16 is an e2u vibration in benzene, and its 160 fundamental is forbidden in the isolated The scan in a) is at a 15 times higher sensitivity, indicating extensive fragmentation following photoionization in one-color R2PI. Transitions tentatively assigned to C6H6-(CHC13) are marked in the figure. Scan b) was taken at a lower spectral resolution. molecule. The S frequency of V16 in the complex is unchanged from its value in the free molecule (+238 cm-). No splitting can be resolved in the transition.
Based on this vibronic level data, which is summarized in Table 2, a C6v or C3v structure is deduced for the C6H6--CHC13 complex from Table 1. Given the symmetry of the CHC13 molecule, the only allowable choice for a rigid complex is C3v, placing the CHC13 on the six-fold axis of benzene. Both the large blue shift of its absorption relative to benzene and the efficient fragmentation of the complex following photoionization point to a hydrogen-bonded orientation for the CHC13 molecule in which the hydrogen points in toward the benzene ring.
The vibronic level data of Table 2 cannot rule out non-rigidity in the complex. For instance, the lack of an origin transition could also be produced by an off-axis structure which is capable of free internal rotation about, the six-fold axis with an internally-rotationally averaged structure which retains benzene's six-fold symmetry. Scans a) and b) were recorded at 15 times higher sensitivity than those in c) and d). The 160 transition observed in the complex is [orbidden in CH. Alternatively, even if the CHC13 molecule is on the six-fold axis, it may still be capable of internal rotation of the chlorine atoms about the C-H bond axis. However, the small internal rotation constant of the CHC13 molecule (c.f. that in H20, for 14 instance) means that barriers to internal rotation of otdy a few wavenumbers would 194 effectively localize the chlorine atoms. The splitings induced in the spectrum by internal rotation are too small to be observed in the present study, justifying treatment as a C3v hydrogen-bonded structure at our resolution.
Dispersed fluorescence (DFL) scans from the 6 , 6 1 , and 6 12 levels are shown in Figures 4-6, respectively. In Figure 4a, the DFL scan of 6 in free benzene is shown for comparison to 6 in the complex. At 6 , the fluorescence reflects the same vibronic state character as that carrying the oscillator strength in absorption, indicating that state mixing at this level of excitation (521 cm-) is modest. No evidence of dissociation is present in the spectrum. A "shelf" of intensity just to the red of the 621, transitions may signal the beginnings of state mixing with intermolecular levels. The fluorescence lifetime of the 61 level of the complex is 32 + 5 nsec com- of the benzene ring. The predicted spectral characteristics for such a cluster structure would include (i) a strong blue-shifted absorption nearly twice the +178 cm observed for the 1 1 cluster, (ii) a forbidden origin, and (iii) no splitting of the 610 transition. However, under the range of expansion conditions sampled in the present study, no transitions due to higher clusters were observed to the blue of the 1 1 absorptions. Instead, as Figure 7a highlights, a set of transitions is observed in the 1 1 mass channel to the red of the 1 1 peaks which grow at higher CHC13 flow relative to the 1" 1 features. The transitions begin at +84 cm and show extensive intermolecular vibrational structure. The 610 transition and all combination bands built on it are split by 1.5 cm-. The origin is clearly observed with an intensity 9% that at 610 We tentatively assign these transitions to the 1:2 cluster fragmenting with high efficiency into the 1:1 mass channel following photoionization. The vibronic level data is summarized in Table 2 .:20 230 Relative Frequency (cm) Figure 8 A section of the LIF scan blue of the 6to transition (d) compared to R2PI scans monitoring a) fhe 2" 3*, b) 1/, and c) 2" 2 mass channels, indicating that these peaks are due to higher clusters, probably of composition 2"3 and 2" 4.
A comparison with C6H6-(CH3OH)2 and C6H6--(H20)2 is given in Figure 7b, c. Note the strong similarities between the spectral characteristics of these 1 2 clusters with their intense van der Waals' structure, clear 60 splitting, strong origin and similar frequency shifts. While the present data provides insufficient evidence for a firm structural determination, the observed transitions are clearly not due to a structure in which the second CHC13 attaches along the six-fold axis on the opposite side of benzene. Furthermore, the similarities with C6H6-(CH3OH)2 and C6H6-(H20)2 suggest a similar "same side of the ring" structure for the C6H6-(CHC13) cluster. It seems likely that this structure is the lowest-energy structure for the cluster by virtue of our inability to observe absorptions due to other structural types. However, a strong kinetic preference for formation of this conformer over the C13CH-C6H6-HCC13 conformer cannot be ruled out categorically. SPECTROSCOPY AND PHOTOPHYSICS OF 199 2. The 2 n Clusters: Figure 8d reproduces a portion of the fluorescence excitation scan from Figure 1 in the 610 region of the C6H6-CHC13 complex. As pointed out earlier, this spectrum is complicated by the presence of transitions due to larger clusters. In order to identify the carders of these transitions, R2PI spectra have been recorded in this region under a wide range of expansion conditions while monitoring an array of mass channels for larger clusters. As Figure 8a, c reveal, the transitions in question appear in the 2:2 + and 2:3 + mass channels. Given the efficient fragmentation of even the 1 1 cluster by loss of a CHC13 molecule, it seems likely that these absorptions are due to the 2:3 and 2:4 clusters, respectively.
It is noteworthy that these transitions are associated, not with 60, but with 10, with red-shifts of-197 and-204cm from the parent transition. This has been confirmed by scans such as those shown in Figure 9 which identify the corresponding red-shifted transitions associated with the origin, 61o, and 110. The transitions built on 6o10 and 120 have also been identified. As shown in Figure 9b, the -197 and -204 cm transitions are split at 610 by 2.2 and 2.4 cm-, respectively, and are redshifted slightly from their positions relative to free benzene. Given the uncertainty in the assignment of cluster size and the presence of (at least) two C6H6 molecules in the cluster, the vibronic level data at hand provide no trustworthy structural deductions. Nevertheless, that any (C6H6)m-(CHC13) clusters would have such a large red shift given the large blue shift of the 1:1 complex points to significant structural differences in the interactions of benzene and CHC13 in these clusters, providing fertile ground for further study.

IV. DISCUSSION
The major deductions of this work center on the C6H6--CHC13 complex. In this case, the vibronic level data indicate a structure for the complex in which CHC13 is on the six-fold axis of benzene, "hydrogen-bonded" to benzene's r cloud. It is not often that the structure of a complex can be deduced with good certainty based solely on vibronic level data. However, the simple, benzene-specific scheme developed in Table 1 and applied to C6H6-CHC13 makes use of symmetry arguments to deduce its point group with little ambiguity. Given the number of c6n6-x complexes which have been studied in some detail, it is instructive to review their spectral features using these methods. To that end, Table 3 and Figure 10 summarize the spectral characteristics of several C6H6-X complexes with the polar solvents CHC13, HC1, and H20, and with the non-polar solvents C2H2 and CC14. As we have seen, in C6H6--CHC13, the lack of an S0-S origin, the presence of a 1610 transition, and no splitting of the degenerate 61 or 161 levels predicts a C3v structure for the complex, as shown schematically in Figure 10.
The Balle et al. 3b to be C6v symmetry with HC1 hydrogen-bonded to benzene. R2PI spectra by our group have confirmed this structure and extended it to the S state by the rotational band contour fitting of the 6o transition. 2 At the same time, the vibronic level data for the complex (Table 3) are uniquely consistent with this structure. As Figure 10 shows in schematic form, no So-St origin is observed for the complex. Neither is the 6o transition of C6H6 measurably split by HC1. A search was never carded out for the 16o transition, but it is predicted to be weakly allowed and unsplit, as it was in C6H6--CHC13. From Table 3, a C6v or C3 structure is deduced, with the C6v structure being consistent with a diatomic's attachment to benzene.

Cs(Oz)
A'lx,zI+A"ly) A'lx,z) A"ly) El+IX,y) El/(x,y) is not clear from the vibronic level alone whether the acetylene molecule lies perpendicular to the benzene ring, or freely internally rotates in a parallel orientation.
The spectral features of the c6n6-cc14 complex (Table 3) have been detailed in a recent paper by Gotch et al. In this case, the origin is strongly induced by complexation, with intensity 18% of that at 60 The 160 and 60 transitions are both observed and are split by about 2.5 cm-. The members of the 160 and 60 doublets are of unequal intensity. As Figure 10 shows, the complex is deduced on this basis to have at most a C2v(Z) structure. 3 For a complex of this symmetry, the S0-S transition is dipole allowed with transition moment along the x axis, producing an origin transition. The e2g and eu vibrations split to a + a, with the 'a' component being vibronically induced while the 'a' level has both dipole allowed and vibronically induced intensity contributions. It should be noted that the deduction of References 11 that the complex must be at most Cs in symmetry is incorrect due to an error in the symmetry of v6.
As a final example, the C6H6-H20 complex addresses the effect of non-rigidity on the vibronic level data. 4 As shown in Table 3, the C6H6-H20 complex possesses no origin (with intensity <0.1% of 60), but its 60 transition is split to an unequal intensity doublet with 1.6 cm splitting. Taken as a rigid structure, these two features are incompatible with one another. However, the doubling at 60 is removed when HDO is substituted for HzO. A partial fit of the rotational structure for the complex is accomplished after allowing for non-rigidity of the HzO molecule involving internal rotation about the six-fold axis and exchange of water's hydrogens which produce rn 0, rn +1 levels with different nuclear spin symmetry (where rn is the internal rotation quantum number). TM If nuclear spin statistics alone dictate intensities, a 3 1 ratio for rn 0/m +1 is predicted. Am 0 transitions out of rn 0 and rn +1 lead to a doubling in the 60 transition, as observed. The high resolution studies of Suzuki, et al. 3 have confirmed and sharpened this picture. In addition, they have shown unambiguously that the water molecule has the hydrogen(s) pointing in to the ring, consistent with the blue-shift and efficient fragmentation observed in our R2PI studies. Thus, the presence of non-rigidity in the complex can lead to anomalous behavior even at the vibronic level which is a clue to its presence. This same nonrigidity significantly complicates the rotational structure and its analysis in a high resolution scan. Some final comments on the n hydrogen bond are in order. Table 3 and Figure  10 include three structurally well-characterized rt hydrogen-bonded complexes: C6H6-HC1, C6H6-CHC13, and C6H6-H20. CHC13 and HC1 have a single hydrogenbonding hydrogen while water has two such hydrogens. In all three complexes, the vibrationally-averaged structure for the complex retains the six-fold symmetry of benzene. In the case of C6H6-HC1, the microwave studies 2 indicate that while the chlorine atom is on the six-fold axis, the hydrogen takes up a vibrationally-averaged structure which is 20 degrees off the six-fold axis. The microwave data is not able to distinguish between a global minimum in the potential energy surface (PES) for hydrogen on the six-fold axis and a surface with a small barrier at 0 0. However, molecular mechanics on clusters (MMC) calculations on C6H6-HC1 show a single minimum with the HC1 hydrogen on the axis, indicating a strong orientation preference for H down. TM Even in this case, a vibrationally-averaged off-axis orientation results from the heavier weighting of off-axis structures in the degenerate torsional wave functions.
In the same way, the CHC13 molecule appears to have a clear orientational preference for hydrogen down, both from its large blue-shift and efficient fragmentation of the photoionized complex. The present data cannot determine whether the global minimum in the PES is on the six-fold axis. Energy minimization calculations on the complex using an intermolecular potential developed by Severance and Jorgenson predict a minimum for H on the six-fold axis. 32 Once again, heavier offaxis weighting of the degenerate intermolecular torsional modes will lead to a vibrationally-averaged structure which is somewhat off the six-fold axis. Thus, in both C6H6-HC1 and C6H6-CHC13 the n hydrogen bond differs structurally from a conventional bond primarily in providing a highly symmetric potential for motion of H off the bond axis which leads to a vibrationally-averaged structure for the hydrogen bond with a slight tilt away from 0 0. By contrast, in C6H6-H20 the two hydrogen-bonding hydrogens on H20 interact with the delocalized n cloud to produce significant floppiness in the complex. Internal rotation of the water molecule about the six-fold axis and exchange of hydrogens is allowed even at the zero point level. MMC calculations TM predict a barrier to internal rotation of the water molecule about the six-fold axis of benzene of less than 2 cm-1.
They show further that the water molecule can tumble about 1.6/ across the face of the benzene ring by swapping the hydrogen which is bonded to the ring with almost no change in the total binding energy to the ring. Thus, the net effect of water's two hydrogen bonding hydrogens on its interactions with benzene's delocalized rt cloud is to produce an intermolecular potential energy surface which supports large-amplitude motions of water about a nominally hydrogen-bonded configuration. This ability of the water molecule to re-orient on the benzene n cloud with little cost in energy is thus quite different than a traditional X...HY hydrogen bond in which the hydrogen bond is localized between two electronegative atoms X and Y.
The strengths of the n hydrogen-bonds in C6H6-HC1,-CHC13, and-H20 show a considerable range, but appear to be about one-third to two-thirds the strength of an X...HY hydrogen bond. The upper bounds on the binding energies listed in Table 3 are firm since they are based on detection of emission from free benzene following vibrational predissociation of the S complexes. Tentative lower bounds are given in parentheses, where the assumption is made that a lack of predissociation from an $1 vibronic level on the time-scale of the fluorescence indicates that the complex has been excited below the dissociation threshold. If an ordering of the upper bounds for the complex can serve as a rough guide to binding strengths, it seems that Do(C6H6-CHC13) > Do(C6H6-HC1) > Do(C6H6-H20). This ordering reflects, not the dipole moments of the molecules (1.01 D, 1.08 D, and 1.85 D, respectively), but rather their polarizabilities (8.50, 2.63, and 1.48 x 10 -24 cm3, respectively). The ordering also is in keeping with the ordering of blue-shifts of the S0-S absorptions relative to benzene (+178 > +125 > +55 cm-) which we have taken as a rough indicator of the strength of n hydrogen bonding. The magnitude of the binding in C6H6-CHC13 (4.6 < Do" < 5.8 kcal/mol) is somewhat larger than the binding calculated by the intermolecular OPLS potential of Jorgenson and co-workers (De" 4.3 kcal/mol). 32 Whether this difference reflects a kinetic, shift in the experimental data or a deficiency in the intermolecular potential is still an open question. Despite this, a binding energy of this magnitude provides additional support for solution phase studies whose interpretation hinges on the formation of a welldefined 1:1 c6n6-cnc13 complex in solution. 19,2 Finally, the strikingly different spectral characteristics of 1 1 (large blue shift, no origin) and 2" n (large red shift, intense origin) clusters suggests that even in solution it may be possible to choose ultraviolet excitation wavelengths which select certain solvent orientations around solute molecule(s). 33