Moderate H-bonds

Crystals with the quasi-symmetric fragment and moderate H-bonds 

Here A is the energy asymmetry of the potential energy minima and J is the tunneling splitting of the ground vibrational state (in the considered doublewell potential when A equals zero). In benzoic acid crystals A A , because J 0.2 cm-i [58] and AE 65 [48]. 

Asymmetry was introduced to the model 2D PES, see Eq. (9.6), by adding a new term A(s), describing the energy difference between the depths of the two wells. The solution of the 2D Schrodinger equation was found by the method of adiabatic separation, see Section 9.2.1. In so doing, only the symmetric coordinate s of a two-proton system was taken into account 

Here S is the coordinate of the ith (i = 1, 2) proton, measured from the midpoint of the hydrogen bond bridge. The antisymmetric coordinate a was set to be zero  

the double-proton transfer was assumed to proceed via a concerted mechanism, with the one-dimensional kinetic-energy matrix appropriate to one particle of twice the proton mass. A similar model is widely used to analyze the vibrations of cyclic hydrogen-bonded dimers (for example, see Ref. [59]). The O -O coordinate in this case symbolizes the intermolecular stretching of a doubly H-bonded system. The problem of the double-proton transfer for the benzoic acid dimer is thereby reduced to a problem conceptually similar to that for a single O -H -O entity except for the denoted light atom mass and consequences of the asymmetric 2D PES. 

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Properties Strong H-bonds Moderate H-bonds Weak H-bonds... 

This chapter focuses on results from INS for the stronger and moderate H-bonds where, as explained below, the dynamics of hydrogen transfer should be accessible. Since most known examples of moderate H-bonds occur in oxygen-oxygen systems they will assume a particular prominence in this chapter. H-bonds also determine, to a great extent, the tertiary and quaternary structure essential to achieve biological activity ( 10.3). 

In this chapter we will consider molecular crystals with normal hydrogen bonds in which the donor A H interacts with an acceptor B. The so-called bifurcated and trifurcated H-bonds [1] as well as the new multiform unconventional H-bonds [2] are beyond the scope of the present chapter. We will focus on the proton dynamics in molecular crystals with strong and moderate H-bonds [3] in the ground electronic state. Attention will be focused on the interpretation of the structural and spectroscopic manifestations of the dynamics of the bridging proton as established in X-ray, neutron diffraction, infrared, and inelastic neutron scattering (INS) studies of H-bonded crystals. 

According to Refs. [35, 56] the mechanism of proton tunneling assisted by the excitation of low-frequency modes in molecular crystals with moderate H-bonds differs strongly from that in the gas phase [62]. In the isolated formic acid dimer the tunneling splitting increases monotonically with the 0---0 excitation, while in crystals the dependence of the probability of proton tunneling on the excitation low-frequency modes is non-monotonic. 

IT. Moderately H-bonded liquids (ketones, esters, ethers and ether-alcohols). 

Typical epoxide adhesive Dispersion plus hydrogen bond 45 8-13 (moderate H bonding) ... 

In [(bnpapa)Zn-0H]Cl04, two H-bonding interactions involving the zinc-bound hydroxide are present. Both of these are moderate H-bonds [54], with heteroatom N "0 distances of around 2.73 A. 

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