ABSTRACT
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.
ABSTRACT
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 â¼3300 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.
ABSTRACT
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.
