Infrared Spectroscopy and Anharmonic Vibrational Analysis of (H2O-Krn)+ (n = 1-3): Hemibond Formation of the Water Radical Cation.

The hemibond is a nonclassical covalent bond formed between a radical (cation) and a closed shell molecule. The hemibond formation ability of water has attracted great interest, concerning its role in ionization of water. While many computational studies on the water hemibond have been performed, clear experimental evidence has been hardly reported because the hydrogen bond formation overwhelms the hemibond formation. In the present study, infrared photodissociation spectroscopy is applied to (H2O-Krn)+ (n = 1-3) radical cation clusters. The observed spectra of (H2O-Krn)+ are well reproduced by the anharmonic vibrational simulations based on the hemibonded isomer structures. The firm evidence of the hemibond formation ability of water is revealed.

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol-(Acetylene)n (∼16 ≤ n ≤ ∼30) and (Acetylene)n+ (10 ≤ n ≤ 70) Clusters.

The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with π-electrons, which shows a clear hydrogen bond property, is called activated CH/π interaction. In this study, cooperative enhancement of the activated CH/π interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)n (∼16 ≤ n ≤ ∼30) and (acetylene)n+ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/π interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20-30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/π interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger.

Vibrational Coupling in Solvated H3O+: Interplay between Fermi Resonance and Combination Band.

Complex vibrational features of solvated hydronium ion, H3O+, in 3 µm enable us to look into the vibrational coupling among O-H stretching modes and other degrees of freedom. Two anharmonic coupling schemes have often been engaged to explain observed spectra: coupling with the OH bending overtone, known as Fermi resonance (FR), has been proposed to account for the splitting of the OH stretch band at ∼3300 cm-1 in H3O+···Ar3, but an additional peak in H3O+···(N2)3 at the similar frequency region has been assigned to a combination band (CB) with the low-frequency intermolecular stretches. While even stronger vibrational coupling is expected in H3O+···(H2O)3, such pronounced peaks are absent. In the present study, vibrational spectra of H3O+···Kr3 and H3O+···(CO)3 are measured to complement the existing spectra. Using ab initio anharmonic algorithms, we are able to assign the observed complex spectral features, to resolve seemingly contradictory notions in the interpretations, and to reveal simple pictures of the interplay between FR and CB.

Infrared Spectroscopy of Protonated Phenol-Water Clusters.

To explore the microhydration structures of protonated phenol, size-selective infrared spectroscopy of protonated phenol-(water) n clusters ( n = 1-5) was performed. The protonation of phenol can occur either at the phenyl ring or at the hydroxy group. The coexistence of the two isomer types separated by the high isomerization barrier was reconfirmed for bare protonated phenol. Preferential hydration of the hydroxy group initially occurs in both the two isomer types of protonated phenol. Development of the water hydrogen-bond network is localized around the hydroxy group up to n = 2. Intracluster proton transfer from the phenol moiety to the water moiety was observed in n ≥ 3-4. The water moiety with the H3O+ ion core resides on the phenyl ring, and the water moiety is bound to the phenyl ring with a π-hydrogen bond. Such a structure is in striking contrast to those of phenol+-(water) n radical cation clusters, in which the water moiety is located away from the phenyl ring even when intracluster proton transfer occurs.

Fermi resonance in solvated H3O+: a counter-intuitive trend confirmed via a joint experimental and theoretical investigation.

The spectral features of H3O+ between 3000 and 3800 cm-1 are known to be dominated by coupling between the fundamentals of stretching modes and the overtones of bending modes. A strong Fermi resonance (FR) pattern has been observed in Ar-tagged H3O+, and the sensitive dependence of the FR pattern on the number of Ar tags has been analyzed by Li et al. [J. Phys. Chem. A, 2015, 119(44), 10887]. Based on ab initio anharmonic calculations with MP2/aug-cc-pvDZ, Tan et al. investigated the influence of different types of rare gas and found a counter-intuitive trend that the strength of the coupling between the overtones of bending modes and the fundamentals of stretching modes decreases as the strength of solvation increases [Phys. Chem. Chem. Phys., 2016, 18(44), 30721]. In the present work, we combine both experimental and theoretical tools to gain a better understanding of the FR in H3O+. Experimentally, spectra of H3O+ with light and much more weakly-bound Ne tags were measured for the first time and spectra of Ar-tagged H3O+ were re-measured for comparison. Theoretically, we have implemented several computational schemes to improve both the accuracy and efficiency of the anharmonic treatments with higher-level ab initio methods (up to CCSD/aug-cc-pVTZ). With the good agreement between the experimental and theoretical spectra, we are confident about the prediction of the modulation of coupling strength by the solvation environments.

Temperature and Size Dependence of Characteristic Hydrogen-Bonded Network Structures with Ion Core Switching in Protonated (Methanol)6-10-(Water)1 Mixed Clusters: A Revisit.

Hydrogen-bonded network structures and preferential ion core in the protonated methanol-water mixed clusters, H+(methanol)n-(water)1 (n = 6-10), were explored by a combination of infrared spectroscopy and theoretical calculations. Infrared spectra of the OH stretch region of the clusters were measured at the two different temperature ranges by using Ar-tagging. Stable isomer structures of the clusters were searched by the multiscale modeling approach and temperature dependent infrared spectra were simulated based on the statistical populations of the isomers. The combined experimental and theoretical studies revealed that the characteristic multiring structures begin to form at n = 7 under the low temperature condition and they are preferential at the wide temperature range in n ≥ 8. It was also demonstrated that the preferential ion core type changes from methanol (MeOH2+) to water (H3O+) with increasing cluster size. In n ≤ 8, the observed infrared spectral features partly depend on the monitoring vibrational predissociation channel, and weak correlations between the hydrogen-bonded network structure and preferential dissociation channels were suggested. However, the ion core type does not necessarily correlate to the preferential dissociation channel. This implies that large rearrangement of the hydrogen-bonded network structure occurs prior to the dissociation.

An ab initio anharmonic approach to study vibrational spectra of small ammonia clusters.

Fermi resonance between the N-H stretching (ν1 and ν3) and the overtone of N-H bending (2ν4) in ammonia has hindered the interpretation and assignments of experimental spectra of small ammonia clusters. In this work, we carried out anharmonic vibrational calculations using MP2/aug-cc-pVDZ to examine the vibrational spectra of (NH3)n=1-5 with a focus on the size evolution. The enhancement of hydrogen bond strength due to cooperative effects will cause ν1 and ν4 to red-shift and blue-shift, respectively, when the size of the cluster increases. Our calculations show that the energy order of fundamental of ν1 and overtone of ν4 is reversed between n = 3 and n = 4. Therefore, while the resultant mixed levels do not show remarkable shifts in their peak positions, the main identity of these mixed levels changes and this causes significant re-distribution of their intensities. Furthermore, our ab initio anharmonic calculation scheme can directly evaluate the coupling strength between different N-H stretching and overtone of N-H bending without any experimental parameters, thus leading us to a simpler picture to understand the Fermi resonance in (NH3)n.

Infrared spectroscopy of large-sized neutral and protonated ammonia clusters.

Size-selective infrared spectroscopy was applied to neutral and protonated ammonia clusters, (NH3)n (n = ∼5-∼80) and H(+)(NH3)n (n = 8-100), to observe their NH stretching vibrations. The moderate size selection was achieved for the neutral clusters by the infrared-ultraviolet double resonance scheme combined with mass spectrometry. The size dependence of the observed spectra of (NH3)n is similar to that of the average size-controlled clusters doped in He droplets. The ν1 (NH sym stretch)/ν3 (NH asym stretch) band intensity ratio shows a rapid decrease in the size range n ≤ ∼20. This demonstrates that ammonia begins to form crystalline like hydrogen bond networks at the much smaller size region than water. The precise size selection was achieved for H(+)(NH3)n by infrared photodissociation spectroscopy combined with a tandem type quadrupole mass spectrometer. The spectra of the protonated clusters become almost identical with those of the corresponding neutral clusters at n ≥ ∼40, demonstrating that the radial chain structures, which are characteristic of the small-sized protonated clusters, develop into the crystalline like structures seen in the neutral clusters up to n = ∼40.