Frequency-correlation

The calculated C14—C13 DQ correlation frequency (Table 1) is 268.0 ppm correlations were expected to be observed at +93 ppm... 

Finally, the calculated C14-C15 DQ correlation frequency (Table 1) is — 291.6 ppm, with correlations expected at +58.4 ppm after folding in A single correlation is observed from the H20b proton at a DQ frequency of + 58 ppm the correlation pathway is shown by 41. 

Note (SCF frequencies scaled by 0.91 correlated frequencies scaled by 0.95). 

Correlation frequencies determined from T1 Tle and T2 relaxation times were plotted against reciprocal temperature and activation energies calculated. The methyl group had a comparatively high activation energy (4.7 kcal/mole) which was attributed to steric hindrance from the reorientation of the two methyls bound to the same carbon and steric hindrance arising from the two phenyl groups on the carbon atom. 

Fig. 73 Distribution of correlation frequencies for the phenyl ring yr-flips in BPA-c/4-PC and in a mixture with 25% AP, at the indicated temperatures, as obtained from 2H NMR spectra (from [17])...

Between the activation energy AE, the correlation frequency vc and the correlation frequency v00 for infinite temperatures a dependence corresponding to the Arrhenius equation is assumed ... 

Figure 5. Comparison of the correlation frequencies calculated according Equation 1 with the averaged tunneling frequencies vr calculated by Stejskal and Gutowsky for the methyl groups assumed as quantum-mechanical rotators as a function of the temperature for PC (x) and PMST (o). The potential height in units of kcal./mole is the parameter...

To confirm whether or not the results reported here can be described by a tunneling effect, an equation is derived for the correlation frequency vc from the Equations 3, 4, and 5 in the form ... 

T. Yet it must be remembered that the decision as to when a methyl group is in a frozen or a state of free rotation is made by the NMR experiment in such a way that the methyl group under consideration does or does not cross a specific rotational frequency. This deciding rotational frequency has been introduced in Equation 7 as the correlation frequency vc. According to Equation 7 vc is temperature dependent. Figure 5 shows the vc values calculated from Equation 7 as a function of the reciprocal temperature. 

Therefore, if it is possible to find a frequency distribution function corresponding to experimental results, it would be possible to find a relation using Equation 24 which would correspond to a Maxwell distribution. With this equation the variable 1/8T X Nme/Nm of Figure 11 should be transformed into the variable l/8v X Nme/Nm. If pairs of values using this performance corresponding to the intersection points of the correlation frequencies with the theoretically determined distribution (cf. Figure 5) are gained, the theoretically determined distribution function would be confirmed by experimental results. 

The first experimental evidences that electron transfer from QA to P+ and from QA to Qb in reaction centers are controlled by the protein conformational dynamics, was obtained in the late 1970 s (Berg 1978a,b Likhtenshtein et al., 1979 a, b) This conclusion was confirmed in subsequent experimental studies in which molecular dynamics of RC and the photsynthetic membrane were determined with a whole set of physical labels. (Kotelnikov et al., 1983, Kochetkov et al., 1984 Parak et al., 1983). It was shown that the electron transfer from reduced primary acceptor QA to secondary acceptor Qb takes place only under conditions in which the labels record the mobility of the protein moiety in the membrane with the correlation frequency u0 > 107 s-1 (Fig. 3.16). 

Figure 3.18. Data on the correlation frequency of the mobility of physical labels and their environment in bovine and human serum albumins and in the photosynthetic RC (I) and the rate constants of ET s primary donor (P) to the bacteriaophephytin acceptor (II) in the Arrhenius coordinates. (Likhtenshein, 1996). Reproduced with permission.

To investigate the effect of a protein on electron transfer and the energy conversion, the dual probes (R ) were incorporated to the hydrophobic pocket obovin serum albumin (Rubtsova et al., 1993 Vogel et al., 1994 Likhtenshtein, 1996 Lozinsky et al., 2001). Experimental temperature dependence on the rate constant of photoreduction kpr was found to be similar to that in the above-mentioned solvent. Values estimated from experiments of parameters of local molecular dynamics with the correlation frequency at... 

The minimum in the spin-lattice relaxation time is more difficult to account for. It cannot be attributed to the onset of the diffusional motion, because the jump frequency does not match the Larmor frequency at the temperature where diffusion becomes important. For this reason it is necessary to postulate an additional kind of motion in the lithium-vanadium bronze—a side-to-side jumping from one side of the channel to the other. In the structure there are sites on both sides of the channel roughly 2 A. apart which are equivalent but only one of which is occupied to fulfill stoichiometry. This kind of motion should start at a lower temperature than the above diffusion and lead to a correlation frequency that matches the Larmor frequency at the spin-lattice time minimum. Because of modulation of quadrupolar interaction, side-to-side motion could provide an effective spin-lattice relaxation mechanism. 

As a first example, let us consider the simplest possible H-bond between a pair of diatomics, as in (HFjj. Prior to formation of the complex, each molecule has a single vibrational mode, with a particular frequency and intensity. Table 3.4 collects data calculated over the years at various levels, for the harmonic frequencies in the monomer and the dimer - . The upper part of the table illustrates the expected overestimate of the vibrational frequency by SCF calculations. The correlated frequencies for the monomer in the first column of data better match the experimental quantity in the last row of the table, although there still remains some scatter from one method to the next. It does appear that a correlated treatment such as MP2, in conjunction with a large flexible basis set, can successfully reproduce the harmonic aspects of the experimental spectrum. 

The intermolecular frequencies are reported in Table 3.56 at various levels of theory, along with the calculated SCF intensities in the last column. With only one exception, the correlated frequencies are somewhat higher than the SCF values. Because of the bent nature of the H-bond in HjCO -HOH, the H-bond stretching frequency is not particularly pure. 

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