Weakly bound complexes of the water radical cation with argon (H2O+Arn, n = 1,2) were generated by an electrical discharge/supersonic expansion and probed with mid- and near-infrared photodissociation spectroscopy in the 2050–4550 and 4850–7350 cm−1 regions. To elucidate these spectra, these complexes were studied computationally at the CCSD(T) level including anharmonicity with the VPT2 method. The comparison between experiment and predicted spectra demonstrates that the VPT2 method is adequate to capture most of the vibrational band positions and their intensities. In addition to the fundamentals, overtones of the symmetric and the asymmetric water stretches and their combination were detected. Additional bands were assigned to combinations of the overtone of the bound O–H stretch with multiple excitation levels of the intermolecular Ar–H stretch. H2O+Ar2 could not be dissociated in the higher frequency region (4850–7350 cm−1).
I. INTRODUCTION
The water radical cation (H2O+) is an important species in Earth’s atmosphere, the tails of comet’s, and the interstellar medium.1–7 This simple triatomic has a 2B1 ground electronic state and has been widely studied with various spectroscopic8–22 and quantum chemical methods.23–26 Also, H2O+ was found to catalyze its own ortho to para conversion through the electron spin-nuclear spin interaction.27 Association of this ion with other small species (such as H2O or Ar)28–30 can serve as a simple paradigm for ion-neutral interactions. When combined with quantum chemical methods, infrared spectroscopy of such ions makes it possible to probe their structure and intermolecular interaction potentials.31–36 In this regard, complexes of H2O+ with argon, neon, and helium have been studied thoroughly in the mid-infrared and with computational methods.37–42 Such weakly bound complexes are recognized to exhibit strong anharmonicity and complex vibrational couplings, providing challenging tests for theory. Here we extend these studies on H2O+Arn to the near-infrared (NIR) region, where overtones and combination vibrations can be used to further assess the capability of anharmonic theory.
When argon associates with H2O+, binding can conceivably take place on one of the two hydrogens or on the oxygen. Quantum chemical methods predict that bonding to hydrogen is preferred and the H-bound structure (Ia, Fig. 1) is indeed the only isomer observed in the infrared spectrum of the H2O+Ar ion.38,39 This interaction is mainly facilitated by induction, as evident from a correlation of the dissociation energy with the square of the induced dipole moment of argon. Attachment of argon to the oxygen also leads to a minimum energy structure when the approach takes place over the oxygen p-orbital lobe (Ib, Fig. 2). This can be rationalized in terms of a partial charge-transfer from argon to the singly occupied molecular orbital (SOMO).38 Because of the higher ionization potentials of the lighter noble gases, such a p-bound structure is not a minimum on the potential energy surface (PES) for He or Ne.40,41 When a second argon comes into play, the binding to the available hydrogen is favored again according to theory (IIa, Fig. 2). However, an O-bound isomer has also been invoked to contribute to the IR spectrum of the H2O+Ar2 ion.38,39 Adding further ligands, two more argon atoms can bind over both sides of the atomic p-orbital of the water cation. The resulting H2O+Ar4 structure has been assigned in a matrix-isolation study in solid argon to account for the strong red-shifts observed.22 Progressive solvation of the water cation has been studied with photodissociation spectroscopy for complexes containing up to 14 argon atoms.39
Comparison of the experimental infrared spectrum of H2O+Ar with the computed VPT2 spectra of the isomers Ia and Ib at the CCSD(T)/cc-pVTZ level.
Comparison of the experimental infrared spectrum of H2O+Ar with the computed VPT2 spectra of the isomers Ia and Ib at the CCSD(T)/cc-pVTZ level.
Rotational simulation of the sub-band structure for the 2ν3 band using PGOPHER and the rotational constants A″ = 22.0 and A′ = 19.0 cm−1. The ground state A″ value is from Ref. 39, and the excited state A′ value is determined here. The ratio of parallel to perpendicular band types is 1.35:1. The band origin (B.O.) from the simulation is 6413 cm−1.
Rotational simulation of the sub-band structure for the 2ν3 band using PGOPHER and the rotational constants A″ = 22.0 and A′ = 19.0 cm−1. The ground state A″ value is from Ref. 39, and the excited state A′ value is determined here. The ratio of parallel to perpendicular band types is 1.35:1. The band origin (B.O.) from the simulation is 6413 cm−1.
Mid-infrared spectroscopy at high resolution by Dopfer and co-workers has already provided detailed structural information for the noble gas complexes of the water cation.39–41 Although for argon only rotational resolution and spin-rotation splittings were observed,39 the neon and helium complexes also exhibited tunneling splittings due to interconversion of the two equivalent H-bound structures.40,41 Whereas the ν3 fundamental of the water cation is slightly blue-shifted (25 cm−1) upon argon binding, a pronounced red shift of the bound O–H stretch (ν1) of 541 cm−1 was observed indicative of a fairly strong interaction. Also, a relatively intense combination vibration of the latter mode with the intermolecular Ar–H stretch (νs) was observed throughout all cluster sizes studied. It was found that the νs band undergoes a blue shift when the stretch of the bound OH group is excited.39 A two-dimensional ab initio and quantum dynamical study of the system was able to confirm this finding and predicted energies of even higher excited states of this type that are located in the NIR.42 However, photodissociation spectroscopy on ions is not usually carried out in this region of the spectrum because of the much weaker transition intensities and the limited ion densities available. Exceptions to this are, for example, the CH3+ carbenium ion43 and protonated water clusters, [H+(H2O)n], where a small range of the spectrum has been recorded for limited cluster sizes.44 Here, we investigate the water cation complexes H2O+Ar and H2O+Ar2 in the mid and near-infrared regions and compare the experimental spectra to the predictions of high level anharmonic vibrational computations. In doing so, we also re-assess the issue of potential isomeric structures for these ions.
II. METHODS
A. Experimental
The water radical cation H2O+ was produced and cooled in a pulsed electrical discharge/supersonic expansion of argon doped with water vapor at ambient temperature. Complexes of this ion with one and two argon atoms were mass-selected in a reflectron time-of-flight mass-spectrometer and studied with laser photodissociation spectroscopy, as described previously.45 Mid- and near-infrared transitions were probed in the regions from 2050 to 4550 and 4850 to 7350 cm−1 with a Nd:YAG-pumped infrared OPO/OPA laser system (LaserVision). Spectra were collected as the fragment ion yield as a function of the laser frequency. To obtain sufficient fragmentation in the 4850–7350 cm−1 region, where much weaker absorptions are expected, a 20 cm quartz lens was used to optimize the overlap of the laser beam and ion cloud.
B. Computational
Stationary points on the H2O+Ar potential energy hypersurface were optimized at the frozen core CCSD(T)/cc-pVTZ level of theory. However, for complexes having two argon atoms, we were only able to obtain structures employing a cc-pVDZ basis set. Restricted open-shell Hartree-Fock (ROHF) wave functions were utilized throughout as they do not suffer from spin contamination.46–48 Focal point analyses were carried out on top of the optimized structures targeting the valence electron CCSD(T) complete basis set (CBS) limit.49–52 Thus, single point electronic energies were computed employing Dunning’s correlation-consistent cc-pVXZ basis sets [with X equal to 2(D), 3(T), 4(Q), and 5]; these basis sets were constructed to systematically converge to the CBS limit.53 The ROHF energy was extrapolated with an exponential function54 and the two highest quality correlation energy points of each structure were fit to a power law.55 Anharmonic frequencies were computed within second order vibrational perturbation theory (VPT2) using finite differences of analytic second derivatives. The reported final energies include anharmonic zero-point vibrational energy (ZPVE) corrections. All computations were carried out with the CFOUR program package56 unless stated otherwise.
III. RESULTS AND DISCUSSION
A. H2O+Ar
The experimental infrared photodissociation spectrum measured for the H2O+Ar ion is presented in the upper black trace of Fig. 1. The mid-IR region of this spectrum was reported previously by Dopfer and co-workers,39 whereas the higher frequency near-IR region has not been measured before. The mid-IR region contains prominent bands at 2670, 2762, and 2904 cm−1 and a multiplet centered near 3282 cm−1. Our recorded photodissociation spectrum here agrees well with that previously reported by Dopfer, although the resolution of our laser system is somewhat lower (about 2 cm−1 vs 0.02 cm−1).39 A comparison of the observed band positions with those measured previously is given in Table I. We follow the nomenclature introduced earlier, i.e., ν1 and ν3 are the symmetric and asymmetric stretches of the water IR chromophore and ν2 corresponds to the H–O–H bending mode. Intermolecular modes due to argon coordination are named separately according to their nature and symmetry; νs is used to label the Ar–H stretch.
Vibrational assignments for the observed infrared transitions (cm−1) of the H2O+Ar ion compared to VPT2 computational predictions at the CCSD(T)/cc-pVTZ level.
Experimental . | Assignment . | ω . | ν (I)a . | Reference 39 . |
---|---|---|---|---|
2670 | ν1 | 2905 | 2646 (862) | 2672.22b |
2714 | ν1 + νs νs | … | … | 2705 |
2762 | 2ν2 | 2956 | 2794 (676) | 2767 |
2904 | ν1 + νs | 3122 | 2883 (58) | 2904.5 |
3282 | ν3 | 3463 | 3292 (255) | 3283.91b |
n.o. | ν3 + δOH–Ar | 3826 | 3639 (21) | |
5015 | 2ν1 | 5810 | 4973 (3.9) | |
5092 | 2ν1 + νs νs | … | … | |
5307 | 2ν1 + νs | 6027 | 5239 (0.0) | |
5384 | 2ν1 + 2νs νs | … | … | |
5566 | 2ν1 + 2νs | … | … | |
5963 | ν1 + ν3 | 6368 | 5952 (0.5) | |
6413 | 2ν3 | 6926 | 6423 (4.4) |
Experimental . | Assignment . | ω . | ν (I)a . | Reference 39 . |
---|---|---|---|---|
2670 | ν1 | 2905 | 2646 (862) | 2672.22b |
2714 | ν1 + νs νs | … | … | 2705 |
2762 | 2ν2 | 2956 | 2794 (676) | 2767 |
2904 | ν1 + νs | 3122 | 2883 (58) | 2904.5 |
3282 | ν3 | 3463 | 3292 (255) | 3283.91b |
n.o. | ν3 + δOH–Ar | 3826 | 3639 (21) | |
5015 | 2ν1 | 5810 | 4973 (3.9) | |
5092 | 2ν1 + νs νs | … | … | |
5307 | 2ν1 + νs | 6027 | 5239 (0.0) | |
5384 | 2ν1 + 2νs νs | … | … | |
5566 | 2ν1 + 2νs | … | … | |
5963 | ν1 + ν3 | 6368 | 5952 (0.5) | |
6413 | 2ν3 | 6926 | 6423 (4.4) |
Intensities in km mol−1.
Vibrational band origin.
As already assigned by Dopfer,39 the four bands in the mid-IR region are the ν1 (2670 cm−1), 2ν2 (2762 cm−1), ν1 + νs (2904 cm−1), and ν3 (3282 cm−1 multiplet) vibrations. We use our band positions here, which deviate slightly from those reported by Dopfer either because of different laser calibrations or linewidth uncertainties. The ν1 mode corresponds to the bound O–H stretch with the transition dipole moment mainly along the main molecular a-axis, thus producing a parallel-type transition.57 The overtone of the H–O–H bending mode 2ν2 is assigned to the band at 2762 cm−1. The 2904 cm−1 band is a ν1 + νs combination of the bound O–H with the Ar–H stretch, indicating a strong coupling of the two modes. A weak transition in the high frequency tail of ν1 at 2705 cm−1, which probably corresponds to a band at 2714 cm−1 in our experiment, was tentatively attributed to a ν1 + νs νs transition in the previous work assuming a vibrationally hot expansion.39 Based on that assignment, the intermolecular νs stretch is estimated to be 199 and 234 cm−1 in the ground vibrational and ν1 states, respectively.58 This is in good agreement with the predictions of a sophisticated vibrational treatment and indicates a stronger Ar–H interaction once ν1 is excited and the O–H bond distance elongates.42 The ν3 band of the free water cation becomes the stretching mode of the free O–H group with parallel and perpendicular type transition moments thus displaying a set of sub-bands with ΔKa = ±1,0. Using the previously reported rotational constants, we simulate our spectrum containing this sub-band structure and estimate our rotational temperature to be about 30 K (see Fig. S2 in the supplementary material).57
To obtain photodissociation in the near-infrared, a quartz lens was used to maximize the overlap of the normally diverging infrared laser beam with the ion cloud in the center of the reflectron. With this setup, it is possible to measure the near-infrared spectrum with a signal-to-noise level that is comparable to that in the mid-infrared, although NIR intensities are roughly a hundred times weaker according to theory. The most prominent features measured here are the intense band at 5015 and a multiplet centered around 6400 cm−1. Several weak to very weak features are located in between these two. It is immediately evident that the 5015 cm−1 band occurs at a position roughly twice the energy of the 2670 cm−1 band in the mid-IR. Likewise, the 6400 cm−1 multiplet lies at an energy roughly twice that of the 3282 cm−1 multiplet.
To further investigate these spectra, we have performed computations on the different isomers of this ion. Our geometry optimizations at the CCSD(T)/cc-pVTZ level of theory confirm the previously localized minimum energy structures on the H2O+Ar potential energy surface. According to our focal point analysis (Table II), the H-bound structure (indicated as isomer Ia here) is more stable than the p-bound (indicated as isomer Ib) by 0.6 kcal mol−1. The D0 argon binding energies are 6.7 and 6.1 kcal mol−1 for the isomers Ia and Ib, respectively. The new CCSD(T) computations resolve a scatter in computed energetics previously found with lower level methods. While the p-bound structure is not a minimum at all at the Hartree-Fock level, B3LYP overemphasizes this interaction and predicts an argon binding energy of 14.3 kcal mol−1. MP2 theory finds a shallow minimum with a small binding energy of 3.7 kcal mol−1.38 As can be seen from the focal point table (Table II), the highest levels of theory in combination with extremely large basis sets are needed to properly describe this interaction. The p-bound structure is actually more stable in electronic energy only at the CCSD(T)/cc-pVQZ level of theory and above. Inclusion of the zero-point vibrational energy (ZPVE) restores the previously found energetic ordering, making the hydrogen-bound structure the global minimum. Additionally, we report a transition state TS1 (Table S1 of the supplementary material) not previously considered connecting the Ia and Ib structures; TS1 is associated with a barrier of only 1.6 kcal mol−1 (including harmonic ZPVE) relative to Ib, consistent with a very shallow intermolecular potential. The validity of this transition structure has been confirmed by computing the intrinsic reaction path at the uMP2/cc-pVTZ level with Gaussian09.59 The imaginary mode of TS1 mainly corresponds to a motion of the light hydrogen while the heavy atoms largely stay in place, suggesting the involvement of rapid tunneling given the low height of the barrier. Thus, the p-bound structure might not be kinetically stable at all despite the cold temperatures and at the short time scales of our experiment (300 μs). The vibrational modes for the minimum energy structures determined here are depicted in the supplementary material in Figs. S4 and S5.
Focal point analysis of the Ia → Ib isomerization in kcal mol−1. ΔZPVE (anharmonic) = +1.31. ΔH0 = −0.75 + 1.31 = +0.56 kcal mol−1.
Basis set . | ΔEe[ROHF] . | ΔEe[MP2] . | ΔEe[CCSD] . | ΔEe[CCSD(T)] . |
---|---|---|---|---|
cc-pVDZ | +5.54 | +3.00 | +1.00 | +0.50 |
cc-pVTZ | +6.22 | +3.05 | +1.16 | +0.39 |
cc-pVQZ | +6.05 | +2.42 | +0.60 | −0.28 |
cc-pV5Z | +6.05 | +2.22 | +0.40 | −0.52 |
CBS limit | [+6.06] | [+2.03] | [+0.22] | [−0.75] |
Basis set . | ΔEe[ROHF] . | ΔEe[MP2] . | ΔEe[CCSD] . | ΔEe[CCSD(T)] . |
---|---|---|---|---|
cc-pVDZ | +5.54 | +3.00 | +1.00 | +0.50 |
cc-pVTZ | +6.22 | +3.05 | +1.16 | +0.39 |
cc-pVQZ | +6.05 | +2.42 | +0.60 | −0.28 |
cc-pV5Z | +6.05 | +2.22 | +0.40 | −0.52 |
CBS limit | [+6.06] | [+2.03] | [+0.22] | [−0.75] |
As shown in Fig. 1 (middle trace; blue), the vibrational spectrum predicted with VPT2 computations at the CCSD(T)/cc-pVTZ level in the mid-infrared region agree well with the experimental spectrum reproducing the observed spectral pattern. However, the computed intensity of the 2ν2 overtone of the water bend appears to be overemphasized. The predicted very weak ν3 + δOH–Ar combination was not observed. In agreement with the previous analysis, all bands can be attributed to the single Ia isomer and there is no evidence for structure Ib (lower trace; red), probably due to the ease of its interconversion to the more stable isomer Ia.
For the higher energy region of our experiment (4850–7350 cm−1), theory predicts three observable infrared transitions: the intense overtones of the ν1 and ν3 O–H stretches as well as their combination, which has a much weaker intensity. Indeed, these bands can be clearly identified in the experimental spectrum. We assign 2ν1 to the most intense of the observed NIR bands at 5015 cm−1. Like its fundamental, which is also the most intense transition in the mid-infrared, the band is not rotationally resolved. This is expected for the parallel-type band at the low resolution of our laser. The weaker overtone of the free O–H stretch 2ν3 is observed as four sub-bands at 6392, 6412, 6432, and 6468 cm−1. The spacings between these bands are on the order of the A rotational constant of the H2O+Ar molecule, suggesting again a rotational sub-band multiplet. We simulated this sub-band structure, as shown in Fig. 2, using the spectroscopic simulation program PGOPHER60 and the previously determined rotational constants for the ground state.39 We obtain an A′ rotational constant of 19.0 cm−1, which is somewhat smaller than the values for the ground state (A″ = 21.98 cm−1) or the fundamental band (A′ = 20.98 cm−1).57 Although we should not attach too much significance to the value of this constant because of the low resolution, a lower A value is consistent with a structure that is more extended from vibrational averaging. The sub-band structure is consistent with a hybrid band type, having parallel and perpendicular components with about the same weighting (1.35:1) as the fundamental band (1:1 in our simulation). This is understandable as there should be both parallel and perpendicular transition dipole moments for the first overtone as already observed for the fundamental of ν3. The ν1 + ν3 combination appears as a weak multiplet centered at about 5985 cm−1. Although the signal level is very weak here, this structure can also be recognized as rotational sub-bands, but with contribution primarily from a perpendicular band type. The fit to this band is shown in Fig. S3 of the supplementary material. The resulting band origin is 5963 cm−1 and the A′ value for this level is 22.6 cm−1.
There are further weak bands to the blue of the strong 2ν1 overtone that are not picked up by the VPT2 computation. The enlarged view in Fig. 3 indicates a regular, recurring pattern of a stronger band that is accompanied by a weaker one with a spacing of 77 cm−1. Inspired by the observation of the combination of ν1 with νs in the mid-IR, and following Dopfer’s previous assignment of hot bands next to ν1,39 we assign these peaks tentatively to be two series of bands of the type 2ν1 + mνs at 5307 and 5566 cm−1 and 2ν1 + mνs νs at 5092 and 5384 cm−1 with m = 1,2. From the combination of the bound O–H stretch overtone with the intermolecular Ar–H stretch 2ν1 + νs at 5307 cm−1, the value of νs in the 2ν1 state can be estimated to 292 cm−1. This observation is in a qualitative agreement with the trend of an Ar–H bond strengthening and blue-shifting of νs upon ν1 excitation (vide supra) and in excellent quantitative agreement with a theoretically predicted value of 299 cm−1.42 Assigning the band at 5566 cm−1 to 2ν1 + 2νs leads to a value of 551 cm−1 for 2νs in the 2ν1 state. This number is in reasonable agreement with the predicted 584 cm−1 interval.42 From the 2ν1 + νs νs band at 5092 cm−1, a second value of νs in the ground vibrational state can be derived to be 215 cm−1. Both estimated values (199 and 215 cm−1) are in good agreement with the VPT2 value of 209 cm−1.
Expanded view of the experimental infrared spectrum of H2O+Ar in the region from 4850 to 5700 cm−1 with tentative assignments of the observed transitions.
Expanded view of the experimental infrared spectrum of H2O+Ar in the region from 4850 to 5700 cm−1 with tentative assignments of the observed transitions.
B. H2O+Ar2
Our spectrum for the H2O+Ar2 ion is shown in Fig. 4. We were able to obtain the photodissociation signal in the region of 2050–4200 cm−1, but not at any frequencies higher than this. The spectrum of this ion in the 2700–3400 cm−1 region was reported previously by Dopfer and co-workers.39 Except for a band reported at 2700 cm−1, which we do not observe, and a new band at 2787 cm−1, our spectrum in this region agrees substantially with that reported by Dopfer. The comparison of our bands and those reported previously is given in Table III. We also extend the spectral range to somewhat higher frequencies and obtain two new bands at 3537 and 4193 cm−1.
Comparison of the experimental infrared spectrum of H2O+Ar2 with the computed VPT2 spectra of the isomers IIa, IIb, and IIc at the CCSD(T)/cc-pVDZ level.
Comparison of the experimental infrared spectrum of H2O+Ar2 with the computed VPT2 spectra of the isomers IIa, IIb, and IIc at the CCSD(T)/cc-pVDZ level.
Vibrational assignments for the observed infrared transitions (cm−1) of the H2O+Ar2 ion compared to VPT2 computational predictions at the CCSD(T)/cc-pVDZ level.
Experimental . | Assignmenta . | ω . | ν (I) . | Reference 39 . |
---|---|---|---|---|
n.o. | … | 2700 | ||
2787 | 2ν2 | 2960 | 2776 (34) | |
2821 | ν3 | 3099 | 2860 (1471) | 2821 |
2875 | ν1 | 3123 | 2887 (342) | 2875 |
2989 | ν1 + νss (a1) | 3233 | 2999 (118) | 2990 |
3074 | ν1 + νsas (b2) | 3275 | 3029 (200) | 3077 |
3324 | ν1 + δOH–Ars (o.o.p., b1) | 3524 | 3251 (92) | 3324 |
ν3 + δOH–Aras (o.o.p., a2) | 3518 | 3262 (78) | ||
3537 | … | … | … | |
4193 | ν2 + ν3 | 4579 | 4237 (2.2) |
Experimental . | Assignmenta . | ω . | ν (I) . | Reference 39 . |
---|---|---|---|---|
n.o. | … | 2700 | ||
2787 | 2ν2 | 2960 | 2776 (34) | |
2821 | ν3 | 3099 | 2860 (1471) | 2821 |
2875 | ν1 | 3123 | 2887 (342) | 2875 |
2989 | ν1 + νss (a1) | 3233 | 2999 (118) | 2990 |
3074 | ν1 + νsas (b2) | 3275 | 3029 (200) | 3077 |
3324 | ν1 + δOH–Ars (o.o.p., b1) | 3524 | 3251 (92) | 3324 |
ν3 + δOH–Aras (o.o.p., a2) | 3518 | 3262 (78) | ||
3537 | … | … | … | |
4193 | ν2 + ν3 | 4579 | 4237 (2.2) |
Intensities in km mol−1.
Our geometry optimizations employing the CCSD(T)/cc-pVDZ level also confirm the previously reported minimum energy structures for this ion.38 The global minimum is the doubly hydrogen-bound structure IIa, closely followed by a mixed H- and O-bound isomer IIb that is +0.8 kcal mol−1 less stable. Structure IIc in which both argon atoms are attached to the oxygen is notably higher in energy (+2.4 kcal mol−1). The D0 argon binding energies are 5.7, 4.9, and 4.0 kcal mol−1 for structures IIa, IIb, and IIc, respectively. We surmise that for the H2O+Ar2 system, an energetically feasible transition state exists in analogy to the singly tagged molecule that allows for a quick relaxation of isomer IIb into the global minimum.
The vibrational band assignments obtained from computed harmonic and anharmonic infrared absorptions are provided in Table III. For those bands measured in common with Dopfer, we obtain essentially the same assignments. The energetic ordering of the symmetric and asymmetric O–H stretches is reversed compared to the free molecule; the asymmetric ν3 stretch occurs at 2821 cm−1 while the symmetric ν1 is at 2875 cm−1. The features at 2989 and 3074 cm−1 are assigned to combinations of the ν1 mode with the symmetric and asymmetric Ar–H stretches. Further weak bands in this region are probably also resulting from combinations of the O–H stretches with other intermolecular modes. A weak band to the red of ν3 at 2787 cm−1, that has not been reported before, is assigned to the overtone of the water bending mode 2ν2 based on the good agreement with the computed VPT2 infrared spectrum. This band was detected at 2762 cm−1 for the single-argon complex. Another band not seen previously at 4193 cm−1 can be assigned to the ν2 + ν3 stretch-bend combination based on excellent theoretical agreement. All in all, the performance of the VPT2 method for this system is compelling. This gives us the confidence to reassign the band at 3324 cm−1 that was tentatively attributed to the free O–H stretch of the IIb isomer before.39 However, based on the VPT2 computations, this feature is more likely to be the two combinations of the ν1 and ν3 O–H stretching modes with symmetric and asymmetric out of plane OH–Ar deformation modes, respectively (see Fig. 4 and Table III). Furthermore, we do not observe the band at 2700 cm−1 that was previously assigned to the bound O–H stretch of structure IIb. According to our frequency computations, this band should be close to the ν3 of the experimentally observed isomer IIa. The position of an additional band at 3537 cm−1 likely corresponds to a combination of an O–H stretch with two quanta in δOH–Ar type deformation modes. The interval for this is about right, although the predicted intensity for this band from our anharmonic theory is essentially zero.
Unfortunately, we were unable to fragment the molecule in the high energy region (4850–7350 cm−1) of our experiment. With two argon atoms bound to the water radical cation, a large number of low frequency modes are present that can dissipate energy, leading to a slower predissociation process. Although fragmentation is rapid enough to facilitate fragmentation in the mid-IR, the oscillator strengths of the NIR transitions in H2O+Ar2 are probably too low for a sufficient number of molecules to undergo photodissociation.
IV. CONCLUSION AND OUTLOOK
In this study, we report the first near-infrared photodissociation spectroscopy experiments for the H2O+Ar1,2 ion complexes. This system provides an interesting test for anharmonic theory because the ions are small enough to have limited vibrational degrees of freedom, but the argon binding provides the possibility for low frequencies and strong couplings to the water vibrations. As we have shown, VPT2 computations are in convincing agreement with the experimental spectra. This is quite remarkable given the similarity to proton bridged dimer complexes that often display complex vibrational spectra and require more sophisticated anharmonic treatment.61–68 Bands that were not predicted by theory could be tentatively assigned to combinations of the ν1 bound O–H stretch with the intermolecular νs Ar–H stretch. Most interestingly, the νs vibration blue shifts upon the excitation of the bound O–H stretch indicating a stronger intermolecular interaction. We hope that the data provided here can be utilized to further improve the theoretical treatment of anharmonicity outside the region of the fundamentals in this prototype system.
SUPPLEMENTARY MATERIAL
See supplementary material for a complete description of the computational studies and for additional rotational simulations of selected vibrational bands.
ACKNOWLEDGMENTS
We gratefully acknowledge support for this work by the National Science Foundation (MAD Grant No. CHE-1464708). J.P.W. is thankful to the Alexander von Humboldt Foundation for a Feodor Lynen Postdoctoral Fellowship.
REFERENCES
As already noted by Dopfer et al., the H-bound structure Ia closely resembles a prolate symmetrical top. The A rotational constant is most sensitive to the position of the hydrogen not attached to the argon, and because of its light mass this constant is relatively large (near 20 cm−1). At our low resolution, we are sensitive to this A value, but cannot resolve structure corresponding to the B or C, rotational contants.
We use the value reported by Dopfer for the ν1 + νs νs transition (2705 cm−1) as our spectrum is not so well resolved in the tail of the ν1 band.