Gauge-Including-Atomic-Orbitals-mPW1PW91/6-31G(d) Scaling Factor as a Satisfactory Cost-Effectiveness Ratio for H-1 Nuclear Magnetic Resonance Chemical Shift Calculations

Recently, several users of nuclear magnetic resonance spectroscopy have considered the employing of quantum chemical methods in the spectra predictions, since these methods are now well developed and implemented in popular program packages. Based on the experience of our group, the purpose of this article is to test the feasibility in the generating of a scaling factor for hydrogen-1 chemical shifts calculations at the GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level of theory in gas phase to assist in the determination of organic molecule structures. It is important to highlight that our level of theory requires low computational time, consequently it can be used even in personal computers. We used 80 organic molecules to yield a scaling factor equation: scaled chemical is equal to 0.98 (calculated chemical shift)+0.09, (calculated tetramethylsilane value of 32.26 parts per million). The test molecule is oleana-12(13), 15(16)-dienoic acid, a triterpene with a complex structure, and with various biological and pharmacological applications. The error values of root mean square were slightly higher for the triterpene molecule compared to the 80 molecules (1.40 percent and 1.53 percent, respectively. We believe that this was due to the greater flexibility of the triterpene molecule. Thus, taking into consideration the cost-effectiveness ratio, the 1H NMR calculations at the GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level of theory have produced promissory results.