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Tautomerism and Microsolvation in 2-Hydroxypyridine/2-Pyridone Santiago Mata, Vanessa Cortijo, W. Caminati, Jose´ L. Alonso,* M. Eugenia Sanz, Juan C. Lo´pez, and Susana Blanco Grupo de Espectroscopía Molecular Edificio Quifima, Area de Química Física, Campus Miguel Delibes, UniVersidad de Valladolid, 47011 Valladolid, Spain ReceiVed: May 20, 2010; ReVised Manuscript ReceiVed: July 18, 2010

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H2O, 2HP-H2O, and 2PO-(H2O)2, have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the 14N quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -NdC(OH)- T -NH-C(dO)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms. Introduction The keto/enol tautomeric equilibrium is one of the most important and widespread equilibria in chemical and biological processes.1 It is indeed shown by the nucleic acid bases cytosine,2-5 thymine,6-9 and uracil,10-13 and it has been related to the appearance of DNA and RNA mutations induced by proton transfer reactions.14 One of the model systems of this type of equilibrium in heterocyclic compounds is the pair 2-hydroxypyridine/2-pyridone (2HP/2PO) represented in Scheme 1, where a hydrogen atom is transferred between the N and O sites of the molecule. The 2HP/2PO system is a widely studied model system chosen for its extreme ease of experimental manipulation and the simplification as compared to real nucleobase pairs. In this study we only consider the most likely hydration sites, far away from the real conditions encountered in biological systems.15 For the 2HP/2PO system, the tautomeric equilibrium is slightly shifted to the hydroxy form, the ground state of 2HP being more stable by 3.2 kJ/mol than that of 2PO.16 For this reason, both tautomers have an appreciable concentration at room temperature. In addition, both of them possess several spectroscopic properties, such as dipole moments, electronic fluorescence, etc., in such a way that it has been possible to investigate them by a variety of spectroscopic techniques, which supplied plenty of information on their relative abundance, rotational constants and structure, vibrational modes, dipole moments, 14N quadrupole coupling constants, etc.16-22 However, tautomeric equilibrium takes place, in nature, in the presence of water molecules. For this reason, several investigations have been dedicated to the hydrated forms of 2HP/ 2PO and to the effects of the hydration on the barrier to tautomerization of 2HP/2PO. It has been calculated that in the isolated molecule this barrier is 146-159 kJ/mol23,24 and that this value is reduced to about 54-63 kJ/mol in the presence of a single water molecule bridging the N-H and CdO sites of 2PO.24 A further reduction is calculated for the doubly hydrated form.25 The presence of water molecules not only affects the tautomerization barrier but it is also predicted to shift the tautomeric equilibrium from the hydroxy to the keto form (see * To whom correspondence should be addressed. Fax: +34 983186349. E-mail: [email protected].

Scheme 1) in a trend that accentuates as the number of water molecules increases.24-27 Several experimental investigations have been carried out on the water complexes of 2PO and 2HP with one and two water molecules. Nimlos et al.21 identified the S1 r S0 electronic origins of the monomers and some of the smaller water clusters of both 2PO and 2HP. Subsequently, Held et al.28 carried out a rotationally resolved LIF study of the S1 r S0 origin transitions of 2PO-H2O and 2PO-(H2O)2. Maris et al. measured the rotational spectrum of the 2PO-H2O complex.26 Matsuda et al.29 reported the infrared depletion spectra of the 2PO-(H2O)1,2 complexes. Florio et al.30 investigated 2PO and 2HP complexed with one and two water molecules in the NH/OH region by resonant ion-dip infrared spectroscopy and fluorescence-dip infrared spectroscopy. More recently, Brause et al.31 described the geometry change of 2PO-H2O upon electronic excitation to the ππ* state from the rotationally resolved fluorescence emission spectrum, and Sakota et al.32 measured the fluorescence excitation and dispersed fluorescence spectra of 2HP-(H2O)n (n ) 0, 1, 2) complexes to investigate excited-state proton transfer. Despite all this body of work, no experimental studies have been specifically directed to assess the effect of the presence of water on the tautomeric equilibrium. In this work we have investigated the hydrated complexes of the tautomers 2HP and 2PO with one and two water molecules by molecular beam Fourier transform microwave (MB-FTMW) spectroscopy in an attempt to prove experimentally whether there is a displacement of the 2HP/2PO equilibrium induced by water. We have observed the isolated monomers, their 1:1 water complexes, and the 2PO-(H2O)2 complex in their ground vibrational states. Careful measurements of the relative intensities of the rotational transitions have allowed us to estimate the abundance of these species in the molecular beam and determine that the tautomeric preferences of the 2HP/2PO pair change upon complexation. Experimental Section The rotational spectra of the species investigated were observed using a MB-FTMW spectrometer which operates in the 6-18.5 GHz frequency range and has already been described elsewhere.33 A sample of 2HP was purchased from Aldrich (97%, mp 105 °C) and used without further purification. The

10.1021/jp104625z  2010 American Chemical Society Published on Web 10/06/2010

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SCHEME 1: Tautomeric Equilibria in 2PO/2HP, 2PO-H2O/2HP-H2O, and 2PO-(H2O)2/2HP-(H2O)2a

a

Fu et al, ref 27. Values in kJ mol-1.

TABLE 1: Experimental and Theoretical Spectroscopic Constants of the Monohydrated Forms of 2PO and 2HP

a Errors in parentheses are expressed in units of the last digit. b The remaining quartic centrifugal distortion constants were fixed to zero in the fit. c Number of quadrupole hyperfine components in the fit. d Rms deviation of the fit.

sample was vaporized in our heated pulsed nozzle34 at ca. 110 °C. Neon at stagnation pressures of about 6 bar was flowed through the nozzle, dragging the 2HP molecules into a Fabry-Pe´rot resonator placed in a vacuum chamber and creating a molecular beam. For the monohydrated and dihydrated species, a heatable water-filled reservoir was located in the gas inlet line to increase the water content of the carrier gas and favor complexation. The species in the molecular beam are then polarized by microwave radiation. Molecular pulses of ca. 0.90 ms duration followed by a MW polarization pulse of 40 mW at a pulse length of ca. 0.3 µs were found optimal. After the excitation radiation ceases, the molecular relaxation signals are collected in the time domain and Fourier transformed to the frequency domain. The pulsed molecular beam was introduced parallel to the axis of the Fabry-Pe´rot resonator, and consequently each observed transition appeared as a Doppler doublet. The measured frequency is the arithmetic mean of the frequencies of the two Doppler components. The accuracy of frequency measurements is estimated to be better than 3 kHz. Rotational Spectra Monohydrated Species. One monohydrated complex attributed to the keto tautomer, 2PO-H2O, had already been

investigated by millimeter wave spectroscopy.26 Therefore, its experimental rotational constants were used to predict the transitions of this complex in the frequency range of our spectrometer. Rotational transitions of µa- and µb-type were easily identified and measured. All observed transitions were split into several hyperfine components arising from the 14N quadrupole coupling interaction which was not analyzed in the previous study.26 Once the transitions of this monohydrate were identified, additional scans were conducted to detect other complexes in the rotational spectrum. A new set of R-branch µa-type transitions were identified close to those already assigned for the other monohydrated species. This new assignment was further confirmed with measurements of µb-type transitions. All detected transitions also showed a hyperfine pattern typical of a species with one 14N nucleus. The measured rotational transitions (available as Supporting Information) for both complexes were fitted using Pickett’s program35 to Watson’s Hamiltonian (A-reduction, Ir-representation)36 supplemented with a term to account for the nuclear quadrupole coupling interaction.37 The experimentally determined rotational and quadrupole coupling constants are shown in Table 1 and compared to the ab initio values predicted at the MP2/6311++G(d,p) level of theory (see Table 1) using the Gaussian

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Figure 1. Hyperfine structure patterns for the 2PO-H2O and 2HP-H2O complexes. Remarkable differences in the hyperfine structure of the rotational transition 30,3 r 20,2 for the 2PO-H2O and 2HP-H2O species act as fingerprints for their identification.

suite of programs.38 One can note that the MP2/6-311++G(d,p) level of theory works better for enol than for keto aromatic species. The particular monohydrated species observed in the supersonic expansion cannot be conclusively identified considering only the rotational constants. The predicted and experimental values (see Table 1) for both 2PO-H2O and 2HP-H2O monohydrates are very close due to their similar mass distribution. Fortunately, the values of the quadrupole coupling constants provide an independent approach to identify the complexes. Specifically χcc corresponds to one of the principal quadrupole coupling tensor elements and provides information on the electric field gradient along the direction of an axis perpendicular to the heterocyclic ring. For pyrrolic nitrogens χcc is negative.16,39,40 In contrast, pyridinic nitrogens have positive values of χcc.16,40,41 The experimental values of -2.18 and 1.993 MHz in Table 1 allow us to establish unambiguously the presence of the keto and enol tautomers, respectively, and the conclusive identification of the 2PO-H2O and 2HP-H2O monohydrates in our supersonic expansion. The difference in the nature of the 14N nucleus is reflected in the different hyperfine structure pattern for the 2PO-H2O and 2HP-H2O complexes (see Figure 1). The quadrupole hyperfine structure can thus be taken as a unique fingerprint to distinguish between different tautomers of a molecule when their rotational constants are alike. The excellent match between the observed and predicted spectroscopic constants in Table 1 corroborates the identification. Dihydrated Species. After the signals belonging to the monomers and monohydrates were discarded from the spectrum, additional searches were performed to characterize the dihydrated forms. Based on structural models predicted by ab initio calculations, a set of R-branch a-type transitions showing the expected 14N quadrupole hyperfine structure was identified. The measured transitions (see Supporting Information) were fitted in the same manner as described for the monohydrated species, and the determined rotational constants are reported in Table 2, along with the ab initio predictions for comparison. Despite the good agreement between the experimental and 2PO-(H2O)2 predicted values of the rotational constants, once again the values of the χaa, χbb, and χcc quadrupole coupling constants determine unambiguously the dihydrated species observed. Notice the excellent matching between experimental values and those predicted at the MP2/6-311++G(d,p) level of theory for the 2PO-(H2O)2 complex. Deep scans of wide

frequency intervals were conducted for a-type transitions of the 2HP-(H2O)2 species. No signals were observed in the rotational spectrum attributable to this dihydrated complex. Discussion The rotational constants of the monohydrated complexes are very similar to those predicted ab initio and consistent with a relative arrangement of the monomers where the water molecule binds to the keto (enol) and amino (amide) groups, acting as both proton donor and acceptor and closing a six-membered cycle. For 2PO-H2O, the water unit acts as a proton donor establishing an O-H · · · O hydrogen bond with the keto group and as an acceptor involved in the hydrogen bond N-H · · · O with the amino group. In 2HP-H2O the situation is reversed, and the water molecule acts as an acceptor of the H of the enol group (O-H · · · O bond) and as a donor with the pyridinic nitrogen (O-H · · · N bond). This arrangement is similar to that already observed in other 1:1 water complexes, like the most stable formamide-water heterodimer42 or glycine-water,43 where the water binds to the carboxylic group also forming a cycle. More information can be extracted from the experimental values of the inertial defects ∆c (see Tables 1 and 2), which give an indication of the planarity of the complexes. For both 1:1 complexes the ∆c values are close to zero, which indicates that all heavy atoms lie on the molecular plane. The ∆c values are similar to those calculated ab initio, where the hydrogen atom of the water molecule not involved in the intermolecular hydrogen bond is predicted to be slightly out of the molecular plane, a disposition observed in other complexes42,43 and suggested for 2PO-H2O by Held and Pratt.28 All this indicates that both 2HP-H2O and 2PO-H2O are fundamentally planar except most likely for the hydrogen atom mentioned above. In the case of the dihydrated species, both water molecules play the double role of proton donor and proton acceptor and form three hydrogen bonds (N-H · · · O, Ow-H · · · Ow, and Ow-H · · · O) spanning the functional groups in the tautomer. The experimental ∆c value for the 2PO-(H2O)2 complex corresponds to that expected for a species with two hydrogen atoms out of the molecular plane.42 This is partially consistent with the ab initio calculations that predict the water hydrogens not involved in the intermolecular interactions to lie on opposite sides of the molecular plane in an up-down configuration,

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TABLE 2: Experimental and Theoretical Spectroscopic Constants of the Dihydrated Forms of 2PO and 2HP

a Errors in parentheses are expressed in units of the last digit. b The remaining quartic centrifugal distortion constants were fixed to zero in the fit. c Number of quadrupole hyperfine components in the fit. d Rms deviation of the fit.

TABLE 3: Ab Initio Dipole Moment Components for the 2PO/2HP, 2PO-H2O/2HP-H2O, and 2PO-(H2O)2/ 2HP-(H2O)2 Pairs 2PO/2HP µa/D µb/D µc/D

2PO-W1/2HP-W1

2PO-W2/2HP-W2

2PO

2HP

2PO-W1/

2HP-W1

2PO-W2/

2HP-W2

4.1 1.4 0.0

-1.5 -0.5 0.0

-2.1 2.1 -1.0

1.8 1.6 -1.2

1.9 -1.9 0.5

-1.7 -1.6 -0.1

similar to that found in other 1:2 water complexes.42,44-46 It should be noted, however, that the theoretical ∆c value for 2PO-(H2O)2 is considerably larger than the experimental one since the ab initio calculations also predict the water oxygens to lie out of the molecular plane. Effect of Hydration on the Tautomeric Equilibrium. The tautomeric equilibrium of 2HP/2PO is shifted to 2HP for the isolated species but predicted to favor the keto form when the molecules interact with water.24-27 Now that the hydrated complexes of both tautomers have been experimentally observed, we can ascertain whether the predicted reversal in stability takes place. We examined first the 2HP/2PO equilibrium and estimated the relative abundances of the tautomers from relative intensity measurements. In the supersonic jet, assuming that all molecules are brought in the supersonic expansion to their ground vibrational states, intensity is proportional to the number density Ni of the tautomer or conformer i and the dipole moment component µi involved in the selection rules of the examined rotational transition. Taking the values of the dipole moment components predicted by ab initio calculation (see Table 3), we have estimated a ratio N2HP/N2PO ) 3.7 ( 0.4 that can be approximately related to the stability of the tautomers. This ratio can be taken as the population ratio of tautomers in the pre-expansion mixture at the stagnation temperature T from which the relative Gibbs energies can be

obtained: ∆GT ) GT(2PO) - GT(2HP) ) RT ln(N2HP/N2PO) ) 4.5 ( 1 kJ/mol. The electronic energies, zero-point corrected energies, and Gibbs energies are predicted to have close values, so it can be concluded that 2HP is therefore more stable than 2PO. The population ratio value obtained is in agreement, within the quoted errors, with the previously reported data from a rotational investigation,16 so we can argue that our relative intensity measurements are also reliable for the 2PO-H2O/ 2HP-H2O pair. Following the same procedure as for the monomers, we estimated the relative abundance of the complexes as N2HP-H2O/N2PO-H2O ) 0.6 ( 0.2. However, it is not said that this ratio is exactly related to the relative energies of 2PO-H2O and 2HP-H2O, as we do not know whether thermodynamical equilibrium has been achieved when we observe the complexes. The complex begins its formation at the point where the supersonic expansion starts. There, the ratio of the 2HP/2PO molecules is about 3-4/1 as corresponds to the pre-expansion temperature, and thus a water molecule does have a much higher probability to encounter a 2HP rather than a 2PO molecule. Once the complexes are formed, they will experience subsequent collisions with the carrier gas atoms and other molecules which favor the lowest energy form until the number of collisions eventually drops to zero at a certain distance from the nozzle. For a local thermodynamical equilibrium to occur for 2PO-H2O and 2HP-H2O, the collision rate should be high enough for a certain period of time. In our experiment we have no way of knowing whether this occurs or not. In any case, it is clear though that the abundance ratio of the two tautomers is inverted after their complexation with water, the 2PO-H2O species being the most abundant form in the supersonic jet. In a similar case, for N-methylformamide-water,47 the most stable species of the complex, formed from the less abundant species of the monomer (5%),

2-Hydroxypyridine/2-Pyridone was not observed since the system could not reach the thermodynamic equilibrium abundance in the supersonic expansion. No transitions were detected for the 2HP-(H2O)2 complex. Considering the intensity of the rotational transitions observed for the 2PO-(H2O)2 complex and the similarity of the theoretical µa dipole moment components for 2PO-(H2O)2 and 2HP-(H2O)2, we should have been able to observe the 2HP-(H2O)2 spectrum for an abundance ratio 2HP-(H2O)2/2PO-(H2O)2 of about 0.5. This seems to indicate that the preference for the ketonic rather than the enolic tautomer is even more pronounced in the case of the complex involving two water molecules. This is consistent with ab initio calculations on the binding energies of the hydrated species performed at the MP2/6-311++G(d,p) level of theory. The binding energy including basis set superposition error (BSSE)48 and fragment relaxation49 is 40.2 kJ/mol for 2PO-H2O and 33.5 kJ/mol for 2HP-H2O (this work and ref 26). This provides a binding energy difference for the monohydrated species of almost 7 kJ/mol, indicating that water prefers to form N-H · · · Ow and CdO · · · H bonds rather than O-H · · · Ow and Ow-H · · · N hydrogen bonds. For the dihydrated species, the predicted binding energies are 81.3 kJ/mol for 2PO-(H2O)2 and 68.3 kJ/mol for the 2HP-(H2O)2, a difference of 13 kJ/mol in favor of the keto complex. Dihydration widens the gap in the binding energies of the complexes further shifting the equilibrium toward 2PO-(H2O)2. Conclusions In this paper, we have characterized three hydrated forms of the tautomeric system 2PO/2HP through the investigation of their pure rotational spectra. The nuclear quadrupole hyperfine structure has proved to be a fundamental tool to achieve an unambiguous assignment of the different species in the molecular beam as it is highly dependent on the nature of the 14N nucleus, and therefore substantially different for the pyrrolic or pyridinic nitrogens in 2PO and 2HP, respectively. The values of the quadrupole constants clearly differentiate between the 2PO-H2O/2HP-H2O pair and provide a confident identification of the 2PO-(H2O)2 complex. The keto/enol equilibrium is shifted progressively from the enol to the keto form as the number of water molecules linked to the aromatic molecule increases. This effect has been observed in other molecular systems.50 The ratio N2PO-(H2O)n/ N2HP-(H2O)n is about 1/3 for the monomers (n ) 0), becomes larger than 1 for n ) 1, and becomes plausibly even larger for n ) 2, when the rotational spectrum of 2PO-(H2O)2 but not that of the 2HP-(H2O)2 species is observed. Acknowledgment. This work has been supported by the Ministerio de Educacio´n y Ciencia (MEC, grants CTQ200605981/BQU and Consolider Ingenio 2010 CSD2009-00038) and the Junta de Castilla y Leo´n (grant VA070A08). V.C. thanks the Ministerio de Educacio´n y Ciencia for an FPI predoctoral fellowship. W.C. thanks the group of molecular spectroscopy of the University of Valladolid for their kind hospitality, and the Minister of Education of the Spanish Government for a grant for a position as Invited Professor. Supporting Information Available: Complete ref 38; Table 1s, frequencies (ν, MHz) and discrepancies between experimental and calculated values (∆ν, MHz) of the measured transitions of 2PO-H2O and 2H-H2O; Table 2s, frequencies (ν, MHz) and discrepancies between experimental and calculated values (∆ν, MHz) of the measured transitions of 2PO-(H2O)2.

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