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Novel physically adapted STO##-3G basis sets. Efficiency for prediction of second-order electric and magnetic properties of aromatic hydrocarbons

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Abstract

Efficient scheme for construction of physically justified STO##-3Gel and STO##-3Gmag basis sets has been proposed. It is based upon the analysis of analytical form of the first-order correction functions to unperturbed STO basis sets under the perturbation by electric or magnetic fields. The test calculations of polarizability, magnetic susceptibility and chemical shifts performed for a series of aromatic compounds within the developed basis set in the framework of Hartree-Fock and Density Functional Theory (DFT) approaches show good agreement of the predicted properties with experiments. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

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... To account for the solvent effect, the SMD model with methanol (ε = 32.613) as a solvent was applied. The UV-vis calculations were performed at SMD/ PBE1PBE level using developed in our group STO ## -3G el basis set [37], which demonstrated high efficiency for electric and magnetic properties calculation [35]. The intensities of the calculated spectra were scaled using Gabedit program [38] to fit the most intensive band obtained experimentally. ...
... There is no correlation between the wavelength of the shortest band of UV-vis spectra of compounds 1-6 and the HOMO/LUMO energy gap (Tables 2 and 4), which is in consistence with involving several molecular orbitals in transition. The calculations, performed at SMD/PBE1PBE level using developed in our group STO ## -3G el basis set [37], demonstrate high efficiency for UV-vis spectra simulation and allow to determine the relative stability of different tautomers of 2-(3-hetaryl-1,2,4triazol-5-yl)anilines as well as 2-(3-(methoxyphenyl)-1,2,4triazole-5-yl)anilines that were shown in our previous work [35]. Table 3 contains calculated dipole moments for all compounds in the gas phase and methanolic solution. ...
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1,2,4-Triazole is a popular scaffold in drug design. According to chemical nature, the triazole ring tends to prototropic tautomerism. Tautomeric phenomena are important for studying the chemical reactivity and interaction of drugs based on triazole with biomolecules in the human body. Theoretical modeling was used to assign structures of newly synthesized 2-(3-hetaryl-1,2,4-triazol-5-yl)anilines. The procedure included quantum-chemical SMD/M06-2X/6–311++G(d,p) calculation of the relative stability for possible tautomers, simulation of UV/vis spectra for the most stable forms, and comparison of the resulting curves with the experimental spectral data taking into account the Boltzmann weighting. The influence of the substituents in triazole ring on tautomeric equilibrium was elucidated. NBO charge distribution, dipole moment, molecular electrostatic potential, and HOMO/LUMO gap for the most stable tautomers were analyzed.
... We have proposed in [20,21] a scheme for augmentation of basis sets which is based upon the expansion of the first-order correction functions on the unperturbed basis functions obtained applying perturbation theory for atomic orbitals (AO). Such approach for the construction of physically-adapted basis sets have been used by us in [22] where STO ## -3G mag and STO ## -3G el sets have been proposed for the calculation of magnetic or electric properties of molecules. These basis sets have been used in [23][24][25]. ...
... The values of energies for these atoms obtained using new coefficients show the opportunity to improve well-known STO-3G basis set. The STO(0)-3G set of atomic orbitals has been extended using the approach described in papers [22,33] by adding new basis AOs extracted from first-order correction functions to unperturbed Slater AOs in case of magnetic uniform field. ...
Article
The new approach for the determination of orbital exponents and contracted coefficients for STO-3G family basis sets has been proposed. Calculations of the necessary coefficients have been performed using Mathcad program package with Minerr solving block. This approach has been used to perform the approximation of the Slater-type orbital (STO) by three Gaussian-type orbitals (GTO). The performance of such modified basis sets has been tested for the calculations of atomic energies using STO(0)-3G basis set and for nuclear magnetic shielding tensors using STO(1M)-3G basis set. The obtained atomic energies are characterized by lower values than those calculated using old parameters. The results for ¹H and ¹³C chemical shifts calculations demonstrate better agreement with the experimental data compared to the data obtained using standard basis sets, such as 6-311G (2d, p), cc-pVDZ and pcS-1. Required time of calculations using the basic set suggested by us is less than the time spent on the calculation using standard basic sets with a similar number of basis functions. Physically adapted and at the same time small by size basic set STO(1M)-3G is perspective for the calculation of magnetic properties of big molecular systems. Proton and ¹³C chemical shifts have been calculated for molecules of adenosine monophosphate (AMP) and flavinadenine dinucleotide (FAD), that play an important role in various biological processes. For both molecules the results of the calculation have shown values close to the experimental data.
... where AB = H 2 , N 2 , O 2 , CO, NO and E(SWCNT· · ·AB), E(SWCNT) and E(AB) are the corresponding energies of the optimized adduct, nanotube and the covalently adsorbed molecule, respectively. The GIAO NMR [17,[27][28][29][30][31][32] calculations on previously optimized structures were performed using the CAM-B3LYP/STO-3G mag level of theory [43,44]. The CAM-B3LYP [43] density functional was recently used for accurate prediction of 13 C NMR shieldings in fullerenes [45] and van der Waals dimers [46]. ...
... The CAM-B3LYP [43] density functional was recently used for accurate prediction of 13 C NMR shieldings in fullerenes [45] and van der Waals dimers [46]. The modified STO-3G basis set was recently introduced [44] as an efficient alternative for accurate calculations of carbon nuclear magnetic shieldings. Its excellent performance was recently confirmed [47][48][49]. ...
... where AB = H 2 , N 2 , O 2 , CO, NO and E(SWCNT· · ·AB), E(SWCNT) and E(AB) are the corresponding energies of the optimized adduct, nanotube and the covalently adsorbed molecule, respectively. The GIAO NMR [17,[27][28][29][30][31][32] calculations on previously optimized structures were performed using the CAM-B3LYP/STO-3G mag level of theory [43,44]. The CAM-B3LYP [43] density functional was recently used for accurate prediction of 13 C NMR shieldings in fullerenes [45] and van der Waals dimers [46]. ...
... The CAM-B3LYP [43] density functional was recently used for accurate prediction of 13 C NMR shieldings in fullerenes [45] and van der Waals dimers [46]. The modified STO-3G basis set was recently introduced [44] as an efficient alternative for accurate calculations of carbon nuclear magnetic shieldings. Its excellent performance was recently confirmed [47][48][49]. ...
... This basis set was downloaded from Environmental Molecular Sciences Laboratory (EMSL) exchange basis set library [71,72]. Finally, we selected somehow smaller and more compact STO-3G mag basis set, designed by Leszczyński and coworkers [73] for efficient prediction of carbon shieldings in larger molecular systems. The latter basis sets was taken directly from their article [73]. ...
... Finally, we selected somehow smaller and more compact STO-3G mag basis set, designed by Leszczyński and coworkers [73] for efficient prediction of carbon shieldings in larger molecular systems. The latter basis sets was taken directly from their article [73]. ...
Article
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The applicability of popular and efficient B3LYP hybrid density functional and medium-size Pople-type basis set in combination with computationally expensive anharmonic model to obtain more accurate theoretical structure, vibrational frequencies and GIAO NMR parameters of cytosine was tested. We report on prediction of cytosine equilibrium (R e ) and rovibrationally averaged (R v ) structures and vibrational frequencies in the gas phase and DMSO solution using density functional theory combined with 6-311++G** basis set. The harmonic and anharmonic vibrational frequencies (using second-order vibrational perturbation theory, VPT2) were critically discussed. In comparison with initial harmonic data, a significantly better agreement between scaled and anharmonic frequencies and experiment was observed. Proton and carbon nuclear magnetic shieldings were calculated at R e and R v structures of cytosine in the gas phase and DMSO solution using BHandH and B3LYP density functionals combined with 6-311++G**, aug-cc-pVTZ-J and STO-3Gmag basis sets. The obtained NMR results were compared with available experimental data and discussed at length.
... The gauge including atomic orbital [51,52] (GIAO) calculations of nuclear isotropic shieldings for optimized UB3LYP/6-31G* geometries were performed using the BHandH [43,44] functional and the STO-3G mag basis set [53]. The BHandH [44] density functional and a modified basis set were successful in calculating carbon nuclear shieldings (and chemical shifts) in earlier studies on hydrocarbons, small molecules or nanocarbons [15,36,50,[54][55][56][57]. ...
... The STO-3G mag basis set was developed for accurate description of magnetic properties with a reasonable effort. Leszczyński and coworkers' idea of STO-3G mag basis set arises from an extension of standard STO-3G basis set by functions obtained from analytical first-order corrections using the Green's function [53]. This modified STO-3G basis set shows a very good accuracy, comparable to Jensen's pcS-2 basis set [58], dedicated to accurate prediction of nuclear shieldings, while being half the size of the latter [50]. ...
... The aug-cc-pVTZ-J basis set was downloaded from EMSL. 43 Recently, Leszczynski and coworkers 44 proposed modified basis sets STO-3G mag for efficient calculations of NMR shieldings. So, in this work, we want to test the performance of their basis sets 44 in nuclear shieldings calculation for model peptides. ...
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Effective peptidomimetics should posses structural rigidity and appropriate interaction pattern leading to potential spatial and electronic matching to the target receptor site. Rational design of such small bioactive molecules could push chemical synthesis and molecular modeling toward faster progress in medicinal chemistry. Conformational properties of N-t-butoxycarbonyl-glycine-(E/Z)-dehydrophenylalanine N',N'-dimethylamides (Boc-Gly-(E/Z)-ΔPhe-NMe2 ) in chloroform were studied by NMR and IR spectroscopy. The experimental findings were supported by extensive calculations at DFT(B3LYP, M06-2X) and MP2 levels of theory and the β-turn tendency for both isomers of the studied dipeptide were determined in vacuum and in solution. The theoretical data and experimental IR results were used as an additional information for the NMR-based determination of the detailed solution conformations of the peptides. The obtained results reveals that N-methylation of C-terminal amide group changes dramatically the conformational properties of studied dehydropeptides. Theoretical conformational analysis reveals that the tendency to adopt β-turn conformations is much weaker for the N-methylated Z isomer (Boc-Gly-(Z)-ΔPhe-NMe2 ), both in vacuum and in polar environment. On the contrary, N-methylated E isomer (Boc-Gly-(E)-ΔPhe-NMe2 ) can easier adopt β-turn conformation, but the backbone torsion angles (ϕ1 , ψ1 , ϕ2 , ψ2 ) are off the limits for common β-turn types.
... The latter functional was recently introduced for 1 H and 13 C chemical shift calculations. For the calculation of nuclear shieldings, we used 6−311 + G(2d,p), aug-cc-pVQZ basis sets and a relatively small and efficient STO(1M)−3G one, designed by Leszczynski et al. [73,74], for the accurate prediction of 13 C NMR chemical shifts in large hydrocarbons. ...
Article
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We report on the density functional theory (DFT) modelling of structural, energetic and NMR parameters of uracil and its derivatives (5-halogenouracil (5XU), X = F, Cl, Br and I) in vacuum and in water using the polarizable continuum model (PCM) and the solvent model density (SMD) approach. On the basis of the obtained results, we conclude that the intramolecular electrostatic interactions are the main factors governing the stability of the six tautomeric forms of uracil and 5XU. Two indices of aromaticity, the harmonic oscillator model of aromaticity (HOMA), satisfying the geometric criterion, and the nuclear independent chemical shift (NICS), were applied to evaluate the aromaticity of uracil and its derivatives in the gas phase and water. The values of these parameters showed that the most stable tautomer is the least aromatic. A good performance of newly designed xOPBE density functional in combination with both large aug-cc-pVQZ and small STO(1M)−3G basis sets for predicting chemical shifts of uracil and 5-fluorouracil in vacuum and water was observed. As a practical alternative for calculating the chemical shifts of challenging heterocyclic compounds, we also propose B3LYP calculations with small STO(1M)−3G basis set. The indirect spin–spin coupling constants predicted by B3LYP/aug-cc-pVQZ(mixed) method reproduce the experimental data for uracil and 5-fluorouracil well.
... Chemical shifts were obtained using benzene and TMS as a reference. In addition, for efficient calculation of nuclear shieldings, we also used a newly modified STO-3G mag basis set [46]. Thus, we decided to test a performance of this basis set in case of NMR parameters of model peptides. ...
Article
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Conformational propensities of N-t-butoxycarbonyl-glycine-(E/Z)-dehydrophenylalanine N′-methylamides (Boc-Gly-(E/Z)-ΔPhe-NHMe) in chloroform were investigated by NMR and IR techniques. The low-temperature crystal structure of the E isomer was determined by single crystal X-ray diffraction and the experimental data were elaborated by theoretical calculations using DFT (B3LYP, M06-2X) and MP2 approaches. The β-turn tendencies for both isomers were determined in the gas phase and in the presence of solvent. The obtained results reveal that the configuration of ΔPhe residue significantly affects the conformations of the studied dehydropeptides. The tendency to adopt β-turn conformations is significantly lower for the E isomer (Boc-Gly-(E)-ΔPhe-NHMe), both in gas phase and in chloroform solution.
... Density functionals (B3LYP [28], PBE0 [29], BHandHLYP [30] and VSXC [31]) were used recently for predicting geometrical structure and energy parameters of nanotubes and fullerenes [32÷35]. NMR parameters for carbon nanotubes (geometrical structures were optimized at level B3LYP/6-31G*) were calculated using density functional VSXC and specifically modified basis set STO-3G mag [36]. Both functional VXSC and modified "Leszczynski's" basis set (not to be confused with original and very inaccurate set STO-3G) were used successfully for prediction of chemical shifts 13 C NMR in a number of medium-and macromolecular compounds [32]. ...
... У роботі [54] було продемонстровано, що спектри поглинання азосполук отриманих на основі α-та β-похідних фенілнафтиламінів, суттєво відрізняються. За допомогою функціоналу PBE1PBE з використанням базисних функцій STO##-3Gel [55], який показав високу ефективність для розрахунків електронних спектрів [56], розраховані спектри поглинання для трьох форм сполуки 4а. Вплив розчинника (діоксану), в якому було записано експериментальний спектр, враховувався за сольватаційною моделлю SMD. ...
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Досліджено реакцію азосполучення ряду ізомерних N-толіл-1-нафтиламінів та N-толіл-2-нафтиламінів. Представлені результати теоретичного дослідження таутомерних, конформаційних властивостей і величин хімічних зсувів ядер в спектрах 1Н ЯМР для 1- (4-нітрофенілазо) -N-фенілнафтил-2-аміну; 1- (4-нітрофенілазо) -N-п-толілнафтил-2-аміну та 4-(4-нітрофенілазо)-N-фенілнафтил-1-аміну. Розрахунки проведені методом теорії функціонала густини B3LYP з використанням фізично адаптованого для розрахунку магнітних властивостей набору базисних функцій 6-31G_JSKE методами CSGT і GIAO. Вплив розчинника (CDCl3), враховувалося методом SMD. Стабільність можливих конформерів і таутомерних форм встановлена на основі розрахованих в наближенні M06-2X / 6-311 ++ G ** величин вільної енергії Гіббса. Заселеність кожної конформації була розрахована за методом Больцмана. Наведено дані теоретичних розрахунків спектрів поглинання азокбарвника 1-(4-нітрофенілазо)-N-фенілнафтил-2-аміну (4а) методом PBE1PBE / STO ## - 3Gel. Дані теоретичного дослідження спектральних характеристик досліджуваних азобарвників добре корелюють з отриманими експериментальними даними і підтверджують переважне існування азосполук ряду N-арил-2-нафтиламіну у вигляді азо-таутомерів з внутрішньомолекулярним водневим зв'язком.
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The interaction of phenylalanine diamide (Ac-Phe-NHMe) with egg yolk lecithin (EYL) in chloroform was studied by (1) H and (13) C NMR. Six complexes EYL-Ac-Phe-NHMe, stabilized by N-H···O or/and C-H···O hydrogen bonds, were optimized at M06-2X/6-31G(d,p) level. The assignment of EYL and Ac-Phe-NHMe NMR signals was supported using GIAO (gauge including atomic orbital) NMR calculations at VSXC and B3LYP level of theory combined with STO-3Gmag basis set. Results of our study indicate that the interaction of peptides with lecithin occurs mainly in the polar 'head' of the lecithin. Additionally, the most probable lecithin site of H-bond interaction with Ac-Phe-NHMe is the negatively charged oxygen in phosphate group that acts as proton acceptor. Copyright © 2014 John Wiley & Sons, Ltd.
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Rapid growth of nanoscience and nanotechnology requires new and more powerful modeling tools. Efficient theoretical modeling of large molecular systems at the ab initio and Density Functional Theory (DFT) levels of theory depends critically on the size and completeness of the basis set used. The recently designed variants of STO-3G basis set (STO-3Gel, STO-3Gmag), modified to correctly predict electronic and magnetic properties were tested on simple models of pristine and functionalized carbon nanotube (CNT) systems and fullerenes using the B3LYP and VSXC density functionals. Predicted geometries, vibrational properties, and HOMO/LUMO gaps of the model systems, calculated with typical 6-31G* and modified STO-3G basis sets, were comparable. The 13C nuclear isotropic shieldings, calculated with STO-3Gmag and Jensen’s polarization consistent pcS-2 basis sets, were also identical. The STO-3Gmag basis sets, being half the size of the latter one, are promising alternative for studying 13C nuclear magnetic shieldings in larger size CNTs and fullerenes.
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The need for accurate calculations on atoms and diatomic molecules is motivated by the opportunities and challenges of such studies. The most commonly-used approach for all-electron electronic structure calculations in general – the linear combination of atomic orbitals (LCAO) method – is discussed in combination with Gaussian, Slater a.k.a. exponential, and numerical radial functions. Even though LCAO calculations have major benefits, their shortcomings motivate the need for fully numerical approaches based on, e.g. finite differences, finite elements, or the discrete variable representation, which are also briefly introduced. Applications of fully numerical approaches for general molecules are briefly reviewed, and their challenges are discussed. It is pointed out that the high level of symmetry present in atoms and diatomic molecules can be exploited to fashion more efficient fully numerical approaches for these special cases, after which it is possible to routinely perform all-electron Hartree–Fock and density functional calculations directly at the basis set limit on such systems. Applications of fully numerical approaches to calculations on atoms as well as diatomic molecules are reviewed. Finally, a summary and outlook is given.
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Fifteen procedures based on hybrid density functional theory were used to calculate magnetic properties for the carbon-bound hydrogen nuclei of 80 small to modest-sized organic molecules. The predicted isotropic shieldings derived from the various methods were compared with each other and also with solution experimental data. The computational methods investigated included the IGAIM and GIAO procedures, the 6-311++G(d,p), 6-311++G(2df,p), and 6-311++G(3df,2p) basis sets, the B3LYP, B3P86, and B3PW91 hybrid density functionals, and molecular geometries optimized using both MP2 and B3LYP methods. Although agreement with experiment consistently improved as the basis set was enlarged, the improvement upon going from 6-311++G(2df,p) to 6-311++G(3df,2p) was extremely small, and even the difference between 6-311++G(d,p) and 6-311++G(2df,p) was of a modest size. The GIAO and IGAIM procedures yielded very similar results in conjunction with the largest basis set, but GIAO suffered considerably less degradation than did IGAIM as the basis set size was decreased. The three functionals B3LYP, B3P86, and B3PW91 performed in an extremely similar fashion, although B3LYP proved marginally superior to the others. The method of geometry optimization also was found to make little difference. Of the computational methods investigated, the GIAO/B3LYP/6-311++G(d,p)//B3LYP/6-31+G(d) procedure probably represents the best compromise between accuracy and expense and yielded proton chemical shifts having a root-mean-square error of 0.15 ppm in comparison with solution experimental values after empirical linear scaling. The more expensive GIAO/B3LYP/6-311++G(2df,p)//B3LYP/6-31+G(d) method provided only a slightly lower root-mean-square error of 0.14 ppm.
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Diamagnetic susceptibility exaltation, defined as the difference between the measured molar susceptibility and the susceptibility estimated neglecting the contribution of ring current, is found to be a property solely of aromatic compounds by calculating the exaltations of a wide variety of aromatic and nonaromatic hydrocarbons. Nonbenzenoid aromatic compounds exhibit exaltation, pseudoaromatic compounds do not. Exaltation is proportional to the theoretically calculated "London diamagnetism" of aromatic compounds.
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A new gaussian basis set, explicity dependent on the external electric field perturbation, is proposed. The analytic form of the electric field dependence of gaussian atomic follows from a simple physical and model and the harmonic oscillator theory. These so-called electric-field-variant (EFV) orbitals are shown to form a very effective basis set for the calculation of molecular polarizabilities. The present proposal appears to circumvent serious basis set problems encountered in the molecular polarizability calculations.
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Forty-five proton chemical shifts in 14 aromatic molecules have been calculated at several levels of theory: Hartree–Fock and density functional theory with several different basis sets, and also second-order Møller–Plesset (MP2) theory. To obtain consistent experimental data, the NMR spectra were remeasured on a 500 MHz spectrometer in CDCl3 solution. A set of 10 molecules without strong electron correlation effects was selected as the parametrization set. The calculated chemical shifts (relative to benzene) of 29 different protons in this set correlate very well with the experiment, and even better after linear regression. For this set, all methods perform roughly equally. The best agreement without linear regression is given by the B3LYP/TZVP method (rms deviation 0.060 ppm), although the best linear fit of the calculated shifts to experimental values is obtained for B3LYP/6-311++G**, with an rms deviation of only 0.037 ppm. Somewhat larger deviations were obtained for the second test set of 4 more difficult molecules: nitrobenzene, azulene, salicylaldehyde, and o-nitroaniline, characterized by strong electron correlation or resonance-assisted intramolecular hydrogen bonding. The results show that it is possible, at a reasonable cost, to calculate relative proton shieldings in a similar chemical environment to high accuracy. Our ultimate goal is to use calculated proton shifts to obtain constraints for local conformations in proteins; this requires a predictive accuracy of 0.1–0.2 ppm. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1887–1895, 2001
Article
Reliability of calculated (1)H and (13)C NMR chemical shifts for various classes of organic compounds obtained with gauge-invariant atomic orbital (GIAO) approach has been studied at the PBE/3ζ level (as implemented in PRIRODA code) using linear regression analysis with experimental data. Empirical corrections for the calculated chemical shifts δ(H,calc) = δ(PBE/3ζ) - 0.08 ppm (RMS 0.18 ppm, MAD 0.66 ppm) and δ(C,calc) = δ(PBE/) (3) (ζ) - 6.35 ppm (RMS 3.09 ppm, MAD 9.42 ppm) have been developed using the sets of 263 and 308 experimental values for (1)H and (13)C chemical shifts, respectively. The confidence intervals of NMR chemical shifts at 95% confidence probability are δ(H,calc) ± 0.35 ppm for (1)H and δC,calc) ± 6.05 ppm for (13)C.
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The influence of the reference standard employed in the calculation of (13)C NMR chemical shifts was investigated over a large variety of known organic compounds, using different quantum chemistry methods and basis sets. After detailed analysis of the collected data, we found that methanol and benzene are excellent reference standards for computing NMR shifts of sp(3)- and sp-sp(2)-hybridized carbon atoms, respectively. This multi-standard approach (MSTD) performs better than TMS in terms of accuracy and precision and also displays much lower dependence on the level of theory employed. The use of mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level is recommended for accurate (13)C NMR chemical shift prediction at low computational cost.
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The (13)C chemical shifts of six tertiary amines of unambiguous conformational structure are compared to predicted (13)C NMR chemical shifts obtained via empirically scaled GIAO shieldings for geometries from MM3 molecular mechanics calculations. An average deviation, absolute value of Deltadelta(av), of 0.8 ppm and a maximum deviation, absolute value of Deltadelta(max), of 2.8 ppm between predicted and experimental (13)C shifts of the six tertiary amines of unambiguous structure are found. In several cases of tertiary amines subject to rapid exchange, where experimental (13)C shifts at room temperature are weighted averages of multiple conformers, a comparison of calculated (13)C shifts of all reasonable MM3 predicted conformers with experimental (13)C shifts via a multiple independent variable regression analysis provides an efficient method of determining the major and minor conformers. The examples presented are 2-methyl-2-azabicyclo[2.2.1]heptane and 1,6-diazabicyclo[4.3.1]decane, which each have two expected contributing structures, and 2-(diethylamino)propane and 1,8-diazabicyclo[6.3.1]dodecane, where ten and seven low-energy conformers, respectively, are predicted by MM3 calculations.
Article
Proton chemical shifts of eight cyclic amide molecules were measured in DMSO and D2O solutions. The magnetic shieldings of the corresponding aliphatic, aromatic, and amide protons were calculated by Hartree-Fock and DFT, using the 6-311G**, 6-311++G**, and TZVP basis sets. For aliphatic protons, all of these methods reproduce the experimental values in DMSO solutions excellently after linear regression. The Hartree-Fock method tends to give slightly better agreement than DFT. The best performance is given by the HF/6-311G** method, with an rms deviation of 0.068 ppm. The deviations from experimental chemical shifts in D2O solutions are only slightly larger than those in DMSO solutions. This suggests that we can use the calculated gas phase proton chemical shifts directly to predict experimental data in various solvents, including water. For amide protons, which exchange with water and form hydrogen bonds with DMSO, only modest agreement is obtained, as expected. The present studies confirm that the GIAO approach can reach high accuracy for the relative chemical shifts of aliphatic and aromatic protons at a low cost. Such calculations may provide constraints for the conformational analysis of proteins and other macromolecules.
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