Correlation between calculated and

The computational chemists were able to form collaborations with their fellow physical chemists. Some of the research questions dealt with molecular conformation and spectroscopy. The 1970s were full of small successes such as finding correlations between calculated and experimental properties. Some of these correlations were published. Even something so grand as the de novo design of a pharmaceutical was attempted but was somewhat beyond reach. 

The excellent correlation between calculated and experimental log P values was obtained by vast investigations of the partitioning of simple chemicals in different mutually immiscible two-phase liquid systems by means of universal model based on the MOD theory [23] ... 

A possible method for predicting absorption bandwidths of chromogenic molecules or FBAs using PPP-MO theory (section 1.5) has been devised. It is based on the empirical linear relationship stated by the Pestemer rule. Thus theoretical Stokes shifts are computed by the PPP-MO method and related to bandwidths. The requisite MO parameters for various typical absorption bands have been developed for use in these calculations. Reasonable correlation between calculated and experimental half-bandwidth data was found, suggesting that this approach has practical potential in predicting colour tone and brightness intensity [ 19]. 

The results presented in Table 6.21 show that the good correlation between calculated and experimental barriers observed for the addition of the individual radicals (Tables 6.15 - 6.20) deteriorates significantly when the radicals are examined together. Further work is in progress to try to understand this variation in performance. The largest deviations from experiment are rather larger than desirable (LD ranging from -11.0 to +16.6 kJ/mol). 

Fig. 12. Correlation between calculated and measured free energies of hydration for different cations (92)...

The reported block function was adopted to improve the overall correlation between calculated and experimental pICso at the two different concentrations. As reported previously (25, 36), in the lower range of inhibition, below 5%, a stronger correlation between plCgjj computed at 10 xM concentration and experimental pICgQlc was found, when compared to 1 xM. An opposite trend was present in the upper range of inhibition (above 99%), where pICgQlc computed at 1 xM concentration tended to correlate better with experiment than that at 10 xM. For inhibition values between the previously defined cut-offs, we used the average pICso. 

Here also, there is excellent correlation between calculated and experimental values. 

FIGURE 21. Linear correlation between calculated and experimental proton affinities. The computed data were obtained at the MP2/6-31+G(d,p)//6-31G level. The correlation equation is PA (exp) = (0.932 0.037)PA(calc) + (16.1 16.1) kcalmol-1, r = 0.996, sd = 0.85 kcalmoU1. (The numbers in this Figure refer to the compound numbers in Table 28)... 

The base Amax value for the former is taken as 214 nm and for the latter as 253 nm. To these values are added increments according to the nature of the substituent present and other structural features as shown in the table. In those cases where homoannular and heteroannular chromophores coexist in conjugation, the base value for the homoannular contribution is used, to which is added the appropriate increment for extended conjugation. It should also be noted that homoannular dienes in ring systems other than the six-membered ring do not give a satisfactory correlation between calculated and experimentally determined Amax values. The rules however are adequate for acyclic or heteroannular... 

At first the calculations were performed on eleven simple molecules ethane, propane, isobutane, neopenthane, ethene, 2-butene, methanol, dimethyl ether, dimethoxymethane, trimethoxymethane, and tetramethoxymethane. The correlation between calculated and experimental principal values is shown in Figure 1. The correlation coefficient is R2 = 0.97 and the root mean square deviation of the calculated from the experimental values is 13 ppm. 

Figure 1. Correlation between calculated and experimental principal values of 13C chemical shift tensors...

More details of the fits are given in Table 4, which shows the number of protons of each type, the observed and computed mean structural shifts, and the linear correlation coefficient r between calculated and observed values for each type of proton. It is clear that base protons are better accounted for than are sugar protons, both in terms of the mean shifts and in terms of the correlation between calculated and observed results. Probable reasons for this behavior are discussed below. 

Combariza et al.122 calculated the 33S shielding tensor by the ab initio LORG (localized orbital local origin) method in the following oxothiomolybdates MoSO, Mot+O, MoS2 and M0S4-. An excellent correlation between calculated and experimental chemical shifts has been found with all the basis sets used ... 

O. Methods of various degrees of sophistication, for the V electron density at the nitrogen atom. Taking the ft population as constant, there is a good correlation between calculated and observed a values 70>. To extend 71> 72> the calculation to asymmetric molecules, like pyrimidine or pyridazine, the new orientation of the electric field gradient tensor must be considered. In pyridazine the two nitrogen Oz axes are rotated in a way that brings them nearer parallel and, from microwave measurements, this rotation was estimated at 9° 73 ... 

With the very encouraging correlation between calculated and real values for cloud points involving nonyl phenol ethoxy-lates and octyl phenol ethoxylates, it would seem that there is justification for proceeding with this mathematical model. 

We have utilized a chemical model in this investigation to interpret the equilibrium behavior of iron in acid mine waters. A successful correlation between calculated and measured Eh values has been found, using WATEQ2, the computerized ion association model. This correlation supports the basic assumption of homogeneous solution equilibrium in these waters and simultaneously corroborates both the validity of the aqueous model and the quantitative interpretation of Eh measurements in these waters. This interpretation makes it possible to calculate the distribution of iron... 

Figure 5.14. (a) Correlation between calculated and experimental NMR chemical shifts of carbocations (see also Scheme 5.16). (b) Correlation between the experimental C NMR chemical shifts of carbocations and the calculated NMR chemical shifts of the corresponding isoelectronic boron analogs (see also Scheme 5.16). 

On the other hand, approximate calculations of ti can also be used [85].In this way, cumbersome solutions of the differential equations are not required if one merely wishes to obtain a general idea of substrate-related diffusional restrictions. A profound insight into effectiveness factors is given by Kasche [86] where a good correlation between calculated and experimental data is demonstrated. 

Figure 2. Correlation between calculations and experiments for lattice constants of vmious metals.

FIGURE 6.16 Correlation between calculated and predicted molecular polarizability for 50 benzene derivatives encoded with one-stage filtered D2Q transformed Cartesian RDF (128 components). The standard deviation of the prediction error is 0.6 A. ... 

FIGURE 6.34 Correlation between calculated and predicted effective concentrations (ECjq) for 44 progestagen derivatives with the leave-one-out technique. The standard deviation of the error of prediction is 0.1 uM/L. 

Figure 5 Correlation between calculated and experimental Fe chemical shifts (ppm relative to Fe(C0)5> for two different functionals. The regression lines do not include the FeCp2 results. (From Ref. 92.)...

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