Accuracy of the Computational Methods

These benchmark studies demonstrate the importance of dispersion in computing structures in ruthenium-catalyzed olefin metathesis. Although B3LYP has been the method of choice for many years due to its great performance for small organic molecules, it has been shown to systematically underestimate the 

Nelson, D.J., Percy, J.M., and Vincent, M.A. (2013) ACS Catal, 3, 1929-1939. Nunez-Zarur, E, Solans-Monfort, X, Rodriguez-Santiago, L., and Sodupe, M. (2012) Organometallics, 31, 4203-4215. 

Garcia-Fandino, R., Castedo, L., Granja, J.R., and Cardenas, D.J, (2007) Dalton Trans., 2925-2934. 

Poater, A., Bahri-Lalehac, N., and Cavallo, L. (2011) Chem. Commun., 

---

The comparison between experimental frequencies and our calculated values is the only direct way in which we can obtain information about the accuracy of the computational method. Also, the unusually large deviations between calculated and observed frequencies for some vibrational modes suggest uncertainties in the empirical assignments. We begin our discussion with ferrocene. 

Determine the effect of basis set on the predicted chemical shifts for benzene. Compute the NMR properties for both compounds at the B3LYP/6-31G(d) geometries we computed previously. Use the HF method for your NMR calculations, with whatever form(s) of the 6-31G basis set you deem appropriate. Compare your results to those of the HF/6-311+G(2d,p) job we ran in the earlier exercise. How does the basis set effect the accuracy of the computed chemical shift for benzene ... 

Kansiz et al. has published a paper wherein they used MIR and sequential injections to monitor an acetone-butanol fermentation process.17 In this work, acetone, acetate, n-butanol, butyrate, and glucose were analyzed automatically, using computer-controlled sampling techniques. In this case, gas chromatography was the reference method. The SEPs for the components were acetone, 0.077 acetate, 0.063 butyrate, 0.058 -butanol, 0.301 and glucose, 0.493 g/1. The authors state that the precision and accuracy of the MIR methods were as good as the reference method. 

In the data collection, a literature search was conducted to identify data source publications (1-40). The publications were screened and copies of appropriate data were made. These data were then keyed into the computer to provide a data base of critical properties for compounds for which experimental data are available. The data base also served as a basis to check the accuracy of the estimation methods. 

A comparison of total hyperfine tensor component computed using the UBILYP method, with experimental values, was found to be disappointingly poor given the accuracy of the computed isotropic components and the apparent ease of calculation of the anisotropic component. One suggestion for the disparity between calculated and experimental values is the difficulty in accurate estimation of the values from an experimental spectrum, and thus it was concluded that currently it is not possible to comment on the suitability or otherwise of the UBILYP method for this type of calculation <2000CPL(327)409>. 

---