Toward the Complete Prediction of the ¹H and ¹³C NMR Spectra of Complex Organic Molecules by DFT Methods: Application to Natural Substances

Abstract

The NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, in the case of boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the Integral Equation-Formalism Polarizable Continuum Model (IEF-PCM) continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but also by visual comparison of line spectra. The origin of the remaining discrepancies is attributed to the incomplete modeling of conformational and specific solvent effects.

FT-IR, NMR spectroscopic and quantum mechanical investigations of two ferrocene derivatives

“…For the NMR calculations, molecular structures of optimized at B3LYP/6-31G(d) level in chloroform ( After optimization, 1 H and 13 GIAO method [23][24][25][26][27][28][29][30] in chloroform at the B3LYP/6 Relative chemical shifts were then estimated by using the corresponding TMS shieldings calculated in advance at the same theoretical levels isotropic chemical shieldings for TMS at the B3LYP/6 using the IEFPCM were 31.87 ppm and 183.76 ppm, respectively. Experimental chemical shieldings for TMS are given as pectroscopic and quantum mechanical investigations of two ferrocene derivatives Bull.…”

“…The standard 1D and 2D hetero and homonuclear NMR experiments are enough to afford complete assignment of compounds and very useful to clarify molecular structures [20][21][22]. For the theoretical NMR investigations, the gauge including atomic orbitals/density functional theory (GIAO/DFT) approach is widely used for various types of compounds [23][24][25][26][27][28][29][30][31]. The DFT/B3LYP method exhibits good performance on electron affinities, excellent performance on bond energies and reasonably good performance on vibrational frequencies and geometries of inorganic or ionic compounds [32,33] as well as organic and neutral compounds [34][35][36][37][38][39][40].…”

“…After the optimization, in order to confirm the convergence to minima on the potential energies, harmonic vibrational frequencies of the present compounds were determined using analytic second derivatives with the ) and 0.967 (under 1800 absence of imaginary frequencies confirmed that the optimized PED calculations were carried out by vibrational 3pica were first fully = 4.9) using the IEFPCM method [27][28][29][30].…”

Differentiation between 6- and 7-Membered Rings Based on Theoretical Calculation of NMR Parameters

“…Firstly, it is highly important that a systematic search of potential contributing conformational participants be undertaken. Secondly, that sufficient geometry optimization using a reasonable level of theory [4,5] is utilized. In the present day with the computational power available to most practitioners, this can be a quite modest requirement.…”

“…For example, the chemical shifts of nuclei such as 1 H (δ H ), 13 C (δ C ), and 15 N (δ N ); various spin-spin (scalar) coupling constants between nuclei such as 1 H and 1 H (J H,H ), 1 H and 13 C (J H,C ), and 1 H and 15 N (J H,N ); NOEs between nuclei such as 1 H and 1 H (η H,H ) and 1 H and 13 C (η H,C ). The prediction of NMR parameters [2,3] such as chemical shifts, but especially coupling constants, has come of age recently in terms of development and practical application [4][5][6], and especially with respect to conformational analysis [7][8][9].…”