Experimental Gas Phase ¹H NMR Spectra and Basis Set Dependence of ab initio GIAOMO Calculations of ¹H and ¹³C NMR Absolute Shieldings and Chemical Shifts of Small Hydrocarbons

Abstract

High-resolution gas phase measurements of 1H NMR spectra at 400 MHz and atmospheric pressure of seven small hydrocarbons are presented. The developed new method and the experimental set-up are described. Ab initio GIAO MO calculations of 1H and 13C NMR absolute shieldings on the HF, MP2 and B3LYP levels using 25 standard gaussian basis sets are reported for these hydrocarbons, based on experimental re distances. The measured gas phase 1H chemical shifts have been converted to an absolute σ0 shielding scale by use of the literature shielding of methane. These and gas phase 13C literature values have been transferred with literature ZPV data to estimated σeexp shieldings which are used to evaluate the basis set dependence of the calculated σe shieldings utilizing linear least squares regressions. Exponential extrapolations of Dunning basis set calculations allow the determination of basis set limits for 1H and 13C shieldings. 1H and 13C chemical shifts have been derived from the HF calculated shieldings with shieldings of TMS which has been geometry optimized and GIAO calculated in each basis. Standard deviations (esd) as low as 0.09 ppm for 1H and 0.76 ppm for 13C calculations have been obtained.The statistically best basis set for simultaneous calculation of 1H and 13C absolute shieldings or relative shifts is 6-311G* within the HF and B3LYP methods. Aiming for highest accuracy and precision, 1H and 13C have to be treated separately. In this case, best results are obtained using MP2/6-311G** or higher for 1H shieldings and MP2/cc-pVTZ for 13C shieldings.

Isolation of the Pyrrolizidine Alkaloid Intermedine-N-oxide from Cerinthe glabra and ab initio Calculation of its 13C NMR Shifts

“…[18] This work Ref. [18] This work In an initial step several optimization cycles for the full geometry optimization of 1 had to be performed [25,26]. The calculated average energies of each series of optimizations steps of each used level of theory and the employed basis set are presented in Table 2.…”

“…Subsequently, the 13 C NMR shift values relative to tetramethylsilane (TMS) were calculated for each optimized structure by the GIAO method [25,26]. The results are shown in Table 3 Table 1).…”

Validation of Computational Methods Applied in Molecular Modeling of Caffeine With Epithelial Anticancer Activity: Theoretical Study of Geometric, Thermochemical and Spectrometric Data

“…18 Studies suggest that most of calculations using basis sets without polarization function are inadequate choices to reproduce experimental values. 19,20 Thus, theoretical data regarding geometric parameters, heat capacity (at constant pressure), IR, Raman and NMR spectra were here calculated for the caffeine molecule.…”

On the influence of vibrational modes and intramolecular isomerization processes on the NMR parameters of bullvalene: A Feynman path integral – ab initio investigation

“…We believe that the present data are a manifestation of an error compensation which is not very systematic. This behaviour has been discussed in several publications [52,53]. In other words, when using standard II there is no a priori guarantee that an improved theoretical approach leads to NMR quantities of better quality.…”

“…A reliable reproduction of the 13 C and 1 H chemical shifts of bullvalene is possible when combining reference results v(R 0 ) in a large basis with ensemble corrections in the smaller basis set (prerequisite: standard I). In the framework of standard II we are confronted with error compensations that cannot be predicted from first principles; again we refer to [52,53].…”