NMR Chemical Shift Calculations Research Articles

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XII. Calculation of the 13C NMR chemical shifts of fluoromethanes at the DFT level

Calculation was carried out of chemical shifts in 13C NMR spectra for a series of fluoromethanes CHnF4−n (n = 0–4) by the methods of the electron density functional theory GIAO-DFT taking in consideration the solvent effect in the framework of the polarizable continuum model Tomasi IEF-PCM. The best results were obtained at the use of Keal-Tozer KT3 functional combined with Pople standard basis sets 6-311G(d,p) and 6-311++G(d,p), and also with Jensen special set pcS-2 containing tight p-functions. The optimum reference in the calculation of chemical shifts in 13C NMR spectra for the fluoromethanes series is TMS.

Trapping acrylamide by a Michael addition: A computational study of the reaction between acrylamide and niacin

Neurotoxic and carcinogenic acrylamide (ACR) is present in many food products. This finding spurred numerous studies for ACR scavengers. Niacin is putatively one of them because it reacts via Michael addition with ACR to form 1‐propanamide‐3‐carboxy pyridinium. Here, we study the mechanism and energetics of this reaction in aqueous solution by density functional theory. The CAM‐B3LYP and M06‐2X functionals with the 6‐31+G(d,p) basis set and implicit solvent were used. Single point calculations at the MP2 level with the same basis set were performed on optimized structures obtained at the M06‐2X level. Solvent effects comprehended both polarizable continuum model and solvation model density solvation models. The calculated NMR chemical shifts of 1‐propanamide‐3‐carboxy pyridinium are in agreement with experimental results. The theoretical study favors thermodynamically the formation of the adduct while the calculated activation energies turn out not to be too dissimilar from the ones measured for the alkylation reaction between ACR and 4(p‐nitrobenzyl)pyridine. © 2014 Wiley Periodicals, Inc.

Quantum mechanical and spectroscopic (FT-IR, FT-Raman, 1H NMR and UV) investigations of 2-(phenoxymethyl)benzimidazole

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 2-(phenoxymethyl)benzimidazole have been investigated experimentally and theoretically using Gaussian09 software package. The energy and oscillator strength calculated by time dependent density functional theory results almost compliments with experimental findings. Gauge-including atomic orbital 1H NMR chemical shifts calculations were carried out and compared with experimental data. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. Molecular Electrostatic Potential was performed by the DFT method and the infrared intensities and Raman activities have also been reported. Mulliken’s net charges have been calculated and compared with the atomic natural charges. First hyperpolarizability is calculated in order to find its role in non-linear optics.

Spectroscopic (FT-IR, FT-Raman), first order hyperpolarizability, NBO analysis, HOMO and LUMO analysis of 1,7,8,9-tetrachloro-10,10-dimethoxy-4-[3-(4-phenylpiperazin-1-yl)propyl]-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione by density functional methods

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 1,7,8,9-tetrachloro-10,10-dimethoxy-4-[3-(4-phenylpiperazin-1-yl)propyl]-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione (TDPPAD) have been investigated experimentally and theoretically using Gaussian09 software package. Gauge-including atomic orbital 1H NMR chemical shifts calculations were carried out and compared with experimental data. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. Molecular Electrostatic Potential was performed by the DFT method and the infrared and Raman intensities have also been reported. Mulliken’s net charges have been calculated and compared with the atomic natural charges. Fist hyperpolarizability is calculated in order to find its role in non-liner optics. The calculated geometrical parameters (SDD) are in agreement with that of similar derivatives.

Vibrational spectroscopic and computational study of 1,7,8,9-Tetrachloro-4-(4-bromo-butyl)-10,10-dimethoxy-4-aza-tricyclo[5.2.1.02,6] dec-8-ene-3,5-dione

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 1,7,8,9-Tetrachloro-4-(4-bromo-butyl)-10,10-dimethoxy-4-aza-tricyclo[5.2.1.02,6] dec-8-ene-3,5-dione (TDAD) have been investigated experimentally and theoretically using Gaussian09 software package. Potential energy distribution of normal modes of vibrations was done using GAR2PED program. Gauge-including atomic orbital 1H NMR chemical shifts calculations were carried out and compared with experimental data. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. Molecular Electrostatic Potential was performed by the DFT method and infrared intensities and Raman activities are also reported. Mulliken’s net charges have been calculated and compared with the atomic natural charges. First hyperpolarizability is calculated in order to find its role in non-liner optics. The calculated geometrical parameters are in agreement with that of similar derivatives.

Analysis of Seven-Membered Lactones by Computational NMR Methods: Proton NMR Chemical Shift Data are More Discriminating than Carbon

We report an NMR chemical shift study of conformationally challenging seven-membered lactones (1-11); computed and experimental data sets are compared. The computations involved full conformational analysis of each lactone, Boltzmann-weighted averaging of the chemical shifts across all conformers, and linear correction of the computed chemical shifts. DFT geometry optimizations [M06-2X/6-31+G(d,p)] and GIAO NMR chemical shift calculations [B3LYP/6-311+G(2d,p)] provided the computed chemical shifts. The corrected mean absolute error (CMAE), the average of the differences between the computed and experimental chemical shifts for each of the 11 lactones, is encouragingly small (0.02-0.08 ppm for (1)H or 0.8-2.2 ppm for (13)C). Three pairs of cis versus trans diastereomeric lactones were used to assess the ability of the method to distinguish between stereoisomers. The experimental shifts were compared with the computed shifts for each of the two possible isomers. We introduce the use of a "match ratio"--the ratio of the larger CMAE (worse fit) to the smaller CMAE (better fit). A greater match ratio value indicates better distinguishing ability. The match ratios are larger for proton data [2.4-4.0 (av = 3.2)] than for carbon [1.1-2.3 (av = 1.6)], indicating that the former provide a better basis for discriminating these diastereomers.

A successful DFT calculation of carbon-13 NMR chemical shifts and carbon–fluorine spin–spin coupling constants in (η6-fluoroarene)tricarbonylchromium complexes

Carbon-13 isotropic shielding constants and C–F spin–spin coupling constants for fluorobenzene, 3- and 4-dimethylaminofluorobenzene as well as for their tricarbonylchromium complexes have been calculated using DFT methods.

ChemInform Abstract: 2‐Aryl(hetaryl)‐4H‐[1,2,4]triazolo[1,5‐a]benzimidazoles.

2-(4-Methylphenyl)-4H-[1,2,4]triazolo[1,5-a]benzimidazole and its previously unknown 2-(2-furyl)- and 2-(2-thienyl)-substituted analogs were synthesized by cyclization of benzimidazole-1,2-diamine with the corresponding carboxylic acid chlorides. The IR, 1H, 13C, and 15N NMR, and mass spectra of the cyclization products in combination with the results of quantum-chemical calculations of NMR chemical shifts showed radical differences of [1,2,4]triazolo[1,5-a]benzimidazoles having no substituent on N4 from the recently reported low-melting products of oxidation of 2-amino-1-arylmethylideneaminobenzimidazoles with (diacetoxy-λ3-iodanyl)benzene, which, as we believe, were erroneously assigned analogous structure.

Molecular structure, experimental and theoretical1H and13C NMR chemical shift assignment of cyclic and acyclic α,β-unsaturated esters

The experimental 1H and 13C NMR spectra of 13 phenyl cinnamates and four 4-methylcoumarins were investigated and their chemical shifts assigned on the basis of the two-dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6-311++G(d,p) level of theory. 1H and 13C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6-311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s-cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects. Copyright © 2013 John Wiley & Sons, Ltd.

Theoretical investigations on the molecular structure, vibrational spectra, HOMO–LUMO and NBO analysis of 5-chloro-2-((4-chlorophenoxy)methyl)benzimidazole

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 5-chloro-2-((4-chlorophenoxy)methyl)benzimidazole have been investigated experimentally and theoretically using Gaussian09 software package. The energy and oscillator strength calculated by time dependent density functional theory results almost compliments with experimental findings. Gauge-including atomic orbital 1H NMR chemical shifts calculations were carried out and compared with experimental data. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. Molecular electrostatic potential was performed by the DFT method and the infrared intensities and Raman activities are reported. Mulliken’s net charges have been calculated and compared with the atomic natural charges. Fist hyperpolarizability is calculated in order to find its role in non-linear optics.

Polymeric adducts of rhodium(II) tetraacetate with aliphatic diamines: natural abundance 13C and 15N CPMAS NMR investigations

Complexation properties of dimeric rhodium(II) tetracarboxylates have been utilised in chemistry, spectroscopy and organic synthesis. Particularly, the combination of these rhodium salts with multifunctional ligands results in the formation of coordination polymers, and these are of interest because of their gas-occlusion properties. In the present work, the polymeric adducts of rhodium(II) tetraacetate with flexible ligands exhibiting conformational variety, ethane-1,2-diamine, propane-1,3-diamine and their N,N'-dimethyl- and N,N,N',N'-tetramethyl derivatives, have been investigated by means of elemental analysis, (13)C CPMAS NMR, (15)N CPMAS NMR and density functional theory modelling. Elemental analysis and NMR spectra indicated the axial coordination mode and regular structures of (1 : 1)n oligomeric chains in the case of adducts of ethane-1,2-diamine, N,N'-dimethylethane-1,2-diamine N,N,N',N'-tetramethylethane-1,2-diamine and N,N,N',N'-tetramethylpropane-1,3-diamine. Propane-1,3-diamine and N,N'-dimethylpropane-1,3-diamine tended to form heterogeneous materials, composed of oligomeric (1 : 1)n chains and the additive of dirhodium units containing equatorially bonded ligands. Experimental findings have been supported by density functional theory modelling of some hypothetical structures and gauge-invariant atomic orbital calculations of NMR chemical shifts.

Quantum chemical calculations of NMR chemical shifts of organic molecules: XI. Conformational and relativistic effects on the 31P and 77Se chemical shifts of phosphine selenides

Conformational and relativistic effects on the 31P and 77Se chemical shifts of phosphine selenides were analyzed in terms of the ZORA-GIAO-B1PW91/TZP approach. The effect of conformation of phosphine selenides related to internal rotation about the single P-C bonds was found to be insignificant, while the contribution of relativistic spin-orbit interaction to the calculated values of 77Se chemical shifts did not exceed 10 ppm. On the other hand, relativistic effects arising from magnetic shielding of the phosphorus nucleus in the P=Se fragment by selenium are fairly strong (25–30 ppm), which indicates the necessity of including the contribution of relativistic spin-orbit interaction in the calculation of 31P chemical shifts in phosphine selenides.

Structural Characterization of (C5H5)Co(PPh3)(η2-alkyne) and (C5H5)Co(η2-alkyne) Complexes of Highly Polarized Alkynes

The solid state structures of the cobalt–alkyne complexes (η5-C5H5)(PPh3)Co{η2-(R3Si)C≡C(SO2Ar)} (3-TMS, R = Me, Ar = C6H5; 3-TIPS, R = CH(CH3)2, Ar = p-C6H4CH3) and the noncoordinated alkyne (Me3Si)C≡C(SO2Ph) (6-TMS) have been characterized by X-ray crystallography and, in the case of 3-TMS, 6-TMS, and (η5-C5H5)Co{η2-(Me3Si)C≡C(SO2Ph)} (5-TMS-calc), by B97D/Def2-TZVPP computational analysis. The phosphine-dissociated complex 5-TMS-calc is determined to be a ground state singlet. Analysis of bond angle and distance metrics, calculated NMR chemical shift data, and molecular orbital analysis provide strong evidence for a four-electron-donor alkyne ligand in 5-TMS-calc. The degree of asymmetry in metal–alkyne bonding, as defined by the Gladysz alkyne-slippage parameter, is dramatically reduced in 5-TMS-calc relative to that in the precursor complex 3-TMS-calc.

Quantum mechanical calculations on cellulose–water interactions: structures, energetics, vibrational frequencies and NMR chemical shifts for surfaces of Iα and Iβ cellulose

Periodic and molecular cluster density functional theory calculations were performed on the Iα (001), Iα (021), Iβ (100), and Iβ (110) surfaces of cellulose with and without explicit H2O molecules of hydration. The energy-minimized H-bonding structures, water adsorption energies, vibrational spectra, and 13C NMR chemical shifts are discussed. The H-bonded structures and water adsorption energies (ΔEads) are used to distinguish hydrophobic and hydrophilic cellulose–water interactions. O–H stretching vibrational modes are assigned for hydrated and dry cellulose surfaces. Calculations of the 13C NMR chemical shifts for the C4 and C6 surface atoms demonstrate that these δ13C4 and δ13C6 values can be upfield shifted from the bulk values as observed without rotation of the hydroxymethyl groups from the bulk tg conformation to the gt conformation as previously assumed.

Structure reassignment of laurefurenynes A and B by computation and total synthesis.

The originally assigned stereostructures of laurefurenynes A and B have been reassigned on the basis of DFT calculations of NMR chemical shifts, synthesis of model compounds and total synthesis of laurefurenyne B, demonstrating the power of this combined approach for stereostructure elucidation/confirmation.

13C NMR chemical shift calculations of charged surfactants in water — A combined density functional theory (DFT) and molecular dynamics (MD) methodological study

Structural and magnetic properties of one anionic and one cationic amphiphile molecule (sodium octanoate and hexadecyltrimethylammonium chloride, respectively) in water are studied comparing different methods to account for the presence of the solvent. Calculated 13C NMR chemical shifts are used as the probe for accuracy of the theoretical electronic structures obtained with different descriptions of the surfactants in water solution. The best agreement with the experimental data are obtained by averaging 13C NMR isotropic chemical shifts over a large number of conformational structures of sodium octanoate while considering the electronic structure of the solvent molecules. The 13C chemical shift values of the hexadecyltrimethylammonium alkane chain are systematically overestimated by 10–15 ppm even if an extensive conformational sampling and water as the polarized continuum medium have been taken into consideration. The role of the basis set quality has been studied and discussed as well.

2-aryl(hetaryl)-4H-[1,2,4]triazolo[1,5-a]benzimidazoles

2-(4-Methylphenyl)-4H-[1,2,4]triazolo[1,5-a]benzimidazole and its previously unknown 2-(2-furyl)- and 2-(2-thienyl)-substituted analogs were synthesized by cyclization of benzimidazole-1,2-diamine with the corresponding carboxylic acid chlorides. The IR, 1H, 13C, and 15N NMR, and mass spectra of the cyclization products in combination with the results of quantum-chemical calculations of NMR chemical shifts showed radical differences of [1,2,4]triazolo[1,5-a]benzimidazoles having no substituent on N4 from the recently reported low-melting products of oxidation of 2-amino-1-arylmethylideneaminobenzimidazoles with (diacetoxy-λ3-iodanyl)benzene, which, as we believe, were erroneously assigned analogous structure.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: X. Dynamic equilibrium effects in the 29Si NMR spectra of silacycloalkanes

Dynamic equilibria related to change of the coordination number of the silicon atom in bis[N-(dimethylamino) imidato-N′,O]silacycloalkanes in solution were studied by theoretical calculations and experimental measurement of the 29Si NMR chemical shifts. Silacyclopentane derivatives were found to exist in solution as mixtures of species with six- and five-coordinate silicon atoms, and silacyclohexane derivatives, as mixtures of five- and four-coordinate silicon compounds.

The Syntheses, Molecular Structures, Spectroscopic Properties (IR, Micro– Raman, NMR and UV–vis) and DFT Calculations of Antioxidant 3– alkyl–4–[3–methoxy–4–(4–methylbenzoxy)benzylidenamino]–4,5–dihydro– 1H–1,2,4–triazol–5–one Molecules

The syntheses, antioxidant activities, acidity properties, experimental and theoretical investigations of vibrational spectra (FT–IR and micro–Raman), 13C and 1H NMR chemical shifts and electronic properties of 3–alkyl–4–[3– methoxy–4–(4–methylbenzoxy)benzylidenamino]–4,5–dihydro–1H–1,2,4–triazol–5–one (Me, Et and n–Pr) molecules have been presented for the first time. The new compounds were analyzed for their potential antioxidant activities in three different methods.The calculations of molecular structures, vibrational frequencies, 13C and 1H NMR chemical shifts and electronic absorption wavelengths of the title molecules were computed by using the DFT/B3LYP method with 6– 31G(d) basis set which was used to have the structural and spectroscopic data about the mentioned molecules in the ground state and the results calculated were compared with experimental values. Furthermore, gauge invariant atomic orbital (GIAO) 1H and 13C NMR chemical shifts in different solvents (gas phase, DMSO and cholorofom), UV–vis. TD– DFT calculations in ethanol solvent, the highest occupied molecular orbital (HOMO–1, HOMO), lowest unoccupied molecular orbital (LUMO, LUMO+1), molecular electrostatic potential map (MEP), atomic charges and thermodynamic properties of the title compounds have theoretically verified and simulated at the mentioned level. In addition, the calculated infrared intensities and Raman activities of the compounds under study have also been reported. Keywords: 1, 2, 4–triazol derivatives, Antioxidant activity, Vibrational spectroscopy, UV–vis spectroscopy, DFT/B3LYP method, 1H and 13C NMR chemical shifts.

Quantum mechanical and spectroscopic (FT-IR, FT-Raman, 1H NMR and UV) investigations of 2-(p-nitrobenzyl) benzoxazole

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 2-(p-nitrobenzyl)benzoxazole have been investigated experimentally and theoretically using Gaussian09 software package. Potential energy distribution of the normal modes of vibrations are done using GAR2PED program. The optimized geometrical parameters are in agreement with that of similar derivatives. The energy and oscillator strength calculated by Time Dependent Density Functional Theory results almost compliments with experimental findings. Gauge-including atomic orbital 1H NMR chemical shifts calculations were carried out by using B3LYP functional with 6-31G* basis sets. The HOMO and LUMO analysis is used to determine the charge transfer with in the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. MEP was performed by the DFT method and the infrared and Raman intensities have also been reported. Mulliken’s net charges have been calculated and compared with the atomic natural charges. The calculated first hyperpolarizability of the title compound is 82.31 time that of the standard NLO material urea and hence is an attractive object for future studies of nonlinear optical properties.