Anharmonicity. hydrogen bonds

Our second example in Sect. 5 illustrates how theoretical dynamical spectroscopy is successful at capturing strong anharmonic hydrogen bonds of the type M - -solvent and the associated subtly enhanced solvent—solvent H-bonds of varying strengths in irniic clusters (where M is an imi and the solvent can be water or methanol). Strong anharmcmicities of 700-900 cm have been correctly captured by the dynamical spectra, in relation to IR-PD experiments conducted at 100 K temperature. 

We give in Fig. 1 a schematic illustration of the main anharmonicities that are assumed to take place in hydrogen bonds. 

The cornerstone of the strong anharmonic coupling theory relies on the assumption of a modulation of the fast mode frequency by the intermonomer distance. This behavior is correlated by many experimental observations, and it is undoubtly one of the main mechanisms that take place in a hydrogen bond. Because the intermonomer distance is, in the quantum model, represented by the dimensionless position coordinate Q of the slow mode, the effective angular frequency of the fast mode may be written [52,53]... 

Now, recall that for weak hydrogen bonds the high-frequency mode is much faster than the slow mode because 0 m 20 00. As a consequence, the quantum adiabatic approximation may be assumed to be verified when the anharmonic coupling parameter aG is not too strong. Thus, neglecting the diabatic part of the Hamiltonian (22) and using Eqs. (18) to (20), one obtains... 

Bratos and Hadzi have developed another origin of the anharmonicity of the fast mode X-H -Y, the Fermi resonance, which is supported by several experimental studies [1,3,63-70], Widely admitted for strong hydrogen bonds [67], the important perturbation brought to the infrared lineshape by Fermi resonances has also been pointed out in the case of weaker hydrogen bonds [53,71-73]. 

When neglecting the strong anharmonic coupling—that is, in the situation of a pure Fermi coupling (no hydrogen bond)... 

Figure 2. Extraction of anharmonicity constants ojexe from the comparison of fundamental OH stretching spectra (center) with overtone spectra (top) and OD spectra (bottom) for the case of jet cooled trifluoroethanol (M) and its most stable dimer conformation, which features a hydrogen bond donor stretching band (Dd) and an acceptor stretching band (Da). The deuteration analysis yields slightly different constants than the overtone approach and underestimates the hydrogen bond effect on donor stretching modes [89].

T. Di Paolo, C. Bourderon, and C. Sandorfy, Model calculations on the influence of mechanical and electrical anharmonicity on infrared intensities Relation to hydrogen bonding. Can. 

J. P. Perchard and Z. Mielke, Anharmonicity and hydrogen bonding I. A near infrared study of methanol trapped in nitrogen and argon matrices. Chem. Phys. 264, 221 234 (2001). 

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