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 CH _n F_4?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.     

High-Pressure 19F NMR Measurements of a Series of Fluorinated Benzenes in Supercritical Carbon Dioxide

The high-pressure and high-resolution NMR cell method has been developed for precise measurements of supercritical carbon dioxide solutions. ¹⁹F NMR chemical shifts of a series of fluorinated benzenes, C₆H _n F _m (n = 6 ? m and m = 1 ～ 6) in CO₂ at dilute concentrations were measured over a wide pressure range up to 35 MPa at 314.3 K. The density dependence of the corrected chemical shift, where the bulk magnetic susceptibility contribution was subtracted, was well represented by a cubic function of CO₂ density for any fluorinated benzene. The linear coefficients, arising from pairwise intermolecular interactions, were found to be dependent on the numbers and positions of fluorine atoms in the fluorinated benzenes. The solute–solvent interaction between fluorine and CO₂ was discussed.     

Ligand effects in the substitution chemistry of cis-bis(piperidine)tetracarbonylmolybdenum(O). A molybdenum-95 NMR study

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₄(pip)_2−nL_n(n = 1,2; L = substituted pyridine ligands). The σ(⁹⁵Mo) values correlate well with the pK_a values for the substituted pyridines; for the n = 1 series, σ (⁹⁵Mo) ranges from − 1053 ppm (pK_a = 1.86 for 4-CN) to − 1120 ppm (pK_a = 9.61 for 4-NMe₂). The effects of solvent polarity and some in situ reactivity studies are described and the nature of the MoL bond compared to that with piperidine and some other ligands is discussed.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

A proton NMR investigation of metalmetal bonding in trimethyltin-aluminium, -gallium, -indium and -thallium organometallic compounds

Proton NMR data for the Group III methyl derivatives, MMe₃ and LiMMe₄ are compared with NMR data for the novel tin—Group III-metal bonded species, Li[Me₃SnMMe₃] (M  Al, Ga, In and Tl) and for Li[(Me₃Sn)_n-TlMe_4?n] (n = 0 to 4), reported here for the first time.The presence of tinmetal bonding in these derivatives is established by the observed tin-across-metal coupling constants and for the thallium derivatives by the additional observation of thallium-across-tin coupling.The variation in the magnitudes of ²J(SnCH), ²J(TlCH), ³J(SnMCH) and ³J(TlSnCH) are reported as a function of M and as a function of the number of Me₃Sn groups bond to thallium in the [(Me₃Sn)_nTIME₄?_n]^?anions. Proposals concerning the factors governing the changes in these coupling constants and the chemical shifts are presented.     

Organolead chemistry : III. 207Pb chemical shifts in some organolead compounds

²⁰⁷Pb chemical shifts are reported for the compounds (CH₃)_4?nPb X_n, where n = 1 · 4, X = 4-FC₆H₄; n = 1, 2, 4, X = CH₃ CC; n = 1, 4, X = CH₂CH; n = 1, X = Cl, CH₃O, CH₃CO₂. A correlation between δ(²⁰⁷Pb) and δ(¹⁹F) for the 4-fluorophenyl derivatives is discussed, and solvent effects on δ(²⁰⁷Pb) for the propynyl derivatives are interpreted in terms of complex formation.     

Prediction of 195Pt NMR chemical shifts of dissolution products of H2[Pt(OH)6] in nitric acid solutions by DFT methods: how important are the counter‐ion effects?

¹⁹⁵Pt NMR chemical shifts of octahedral Pt(IV) complexes with general formula [Pt(NO₃)_n(OH)_{6 ? n}]^2?, [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} (n = 1–6), and [Pt(NO₃)_{6 ? n ? m}(OH)_m(OH₂)_n]^{?2 + n ? m} formed by dissolution of platinic acid, H₂[Pt(OH)₆], in aqueous nitric acid solutions are calculated employing density functional theory methods. Particularly, the gauge‐including atomic orbitals (GIAO)‐PBE0/segmented all‐electron relativistically contracted–zeroth‐order regular approximation (SARC–ZORA)(Pt) ∪ 6–31G(d,p)(E)/Polarizable Continuum Model computational protocol performs the best. Excellent second‐order polynomial plots of δ_calcd(¹⁹⁵Pt) versus δ_exptl(¹⁹⁵Pt) chemical shifts and δ_calcd(¹⁹⁵Pt) versus the natural atomic charge Q_Pt are obtained. Despite of neglecting relativistic and spin orbit effects the good agreement of the calculated δ ¹⁹⁵Pt chemical shifts with experimental values is probably because of the fact that the contribution of relativistic and spin orbit effects to computed σ^{iso 195}Pt magnetic shielding of Pt(IV) coordination compounds is effectively cancelled in the computed δ ¹⁹⁵Pt chemical shifts, because the relativistic corrections are expected to be similar in the complexes and the proper reference standard used. To probe the counter‐ion effects on the ¹⁹⁵Pt NMR chemical shifts of the anionic [Pt(NO₃)_n(OH)_{6 ? n}]^2? and cationic [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} (n = 0–3) complexes we calculated the ¹⁹⁵Pt NMR chemical shifts of the neutral (PyH)₂[Pt(NO₃)_n(OH)_{6 ? n}] (n = 1–6; PyH = pyridinium cation, C₅H₅NH⁺) and [Pt(NO₃)_n(H₂O)_{6 ? n}](NO₃)_{4 ? n} (n = 0–3) complexes. Counter‐anion effects are very important for the accurate prediction of the ¹⁹⁵Pt NMR chemical shifts of the cationic [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} complexes, while counter‐cation effects are less important for the anionic [Pt(NO₃)_n(OH)_{6 ? n}]^2? complexes. The simple computational protocol is easily implemented even by synthetic chemists in platinum coordination chemistry that dispose limited software availability, or locally existing routines and knowhow. Copyright © 2016 John Wiley & Sons, Ltd.     

Metalation in hydrocarbon solvents: the mechanistic aspects of substrate-promoted ortho-metalations

The methoxy-substituted aromatic reagents 1,2- and 1,3-dimethoxybenzene (1,2-DMB and 1,3-DMB) and 1,2,4-trimethoxybenzene (1,2,4-TMB) each undergo directed ortho-metalation in high yield in n-BuLi/hydrocarbon media without the aid of a catalyst. These reactions, coined ‘substrate-promoted ortho-metalations’, proceed with the methoxy aromatic substrate functioning as both the directing metalation group (DMG) and as the deoligomerization agent. Evidence that the substrates themselves serve to deoligomerize n-BuLi comes from ¹³C NMR. The relative extent of metalated product formed as a function of time for each of the three aromatics directly correlates with the substrate's time-dependent ability to coordinate to n-BuLi as measured by ¹³C NMR. The interpretation of NMR results from experiments involving 1,2,4-TMB is consistent with the metalation proceeding via the activated complex [(1,2,4-TMB)₂·(n-BuLi)₂]. Finally, conclusions from solubility experiments are that for every substrate-promoted metalation investigated, a precipitate forms in the hydrocarbon solvent, and this precipitate mostly contains the ortho-lithiated aryl intermediate.     

Solvent effects on the electronic spectrum of the hexathiocyanatochromate (III) anions. A revised octahedral crystal field splitting parameter for the thiocyanate ion

An early report on the solvent dependence of the electronic spectra of complex ions considered the electrostatic effect of solvent molecules in the second coordination sphere, on the position of the absorption bands.¹ Solvent effects on the ligand field bands for solutions of K[Cr(NH₃₂(NCS)₄] have been discussed by Adamson² who emphasised a correlation between the solvent shifts in the spin-allowed bands and the shifts in the quartet-doublet bands. This work describes the effects of various solvents on the electronic absorption spectra of [Cr(CNS)₆]^3?.     

Thermochemistry of the Reaction of Solvated Sodium Ion Clusters with Thymine in the Gas Phase: An Example of the Reaction in Microcosmic Environment

The thermochemistry of the reaction of the microsolvated Na⁺ such as [Na(H₂O) _n ; n?=?1?6]⁺, [Na(NH₃) _n ; n?=?1?6]⁺ and [Na(H₂O) _n (NH₃) _m ; n?+?m?=?2?6]⁺ with thymine (Thy), as an example of a reaction in the microcosmic environment, have been studied in this work, theoretically. It was found that the increase of the number of solvent molecules in the structure of microsolvated Na⁺ is accompanied by the decrease of the standard enthalpy (\(\Delta H_{r}^{^\circ }\)) and Gibbs free (\(\Delta G_{r}^{^\circ }\)) energies of the reaction (Thy?+?[Na(X) _n ]⁺→Thy-Na(X) _n ⁺ ; X?=?solvent molecule). Also, the calculations showed that the electronic intermolecular interaction (?E_int) between the Thy and microslovated Na⁺ decreased with the increase of solvent molecules. For the interaction of the [Na(H₂O) _n ; n?=?4, 5 and 6]⁺ ions with the Thy, there was the probability of forming of the hydrogen bond between water molecules in the structure of solvated Na⁺ and the Thy. The gas phase infrared (IR) spectra of the complexes of the microsolvated Na⁺ with the Thy for different values of n were calculated and compared with each other to follow the change in the frequency of the stretching vibration of the interaction path between the C=O group of the Thy and Na (O…Na) with n. Using the calculated values of \(\Delta G_{r}^{^\circ }\) of the reactions, the mole fractions of the complexes of microsolvated Na⁺ ions with the Thy were calculated at different humidity.     

17O and 183W NMR studies of the paratungstate anions in aqueous solutions

¹⁷O (40.7 MHz) and ¹⁸³W (12.5 MHz) NMR spectra of aqueous Na₁₀[H₂W₁₂O₄₂]·27H₂O (1), Na₆[W₇O₂₄]·14H₂O (2) and (NH₄)₆[Mo₇O₂₄]·nH₂O solutions, as well as of 2, 1 and 0.1 M Na₂WO₄ and 2 M Li₂WO₄ solutions acidified up to P = 0.5, 1 and 1.14 have been measured. The composition of the W₇O₂₄^6? anion remains unchanged (2), its structure being similar to that of Mo₇O₂₄^6?183W NMR spectrum shows three resonances with the chemical shifts + 269.2, ?98.8 and ?178.9 ppm relative to WO₄^2? and intensity ratio 1:4:2. “Paratungstate A” produced during polycondensation of WO₄^2? at P ? 1.17 is identical with heptatungstate W₇O₂₄^6?. The [H₂W₁₂O₄₂]^10?183W NMR spectrum in the acidified 2 M Li₂WO₄ solution has four resonances with the chemical shifts in the range - 105–145 ppm and intensity ratio 1:2:1:2. As suggested by NMR data, the H₂W₁₂O₄₂^10? ? W₇O₂₄^6? transformations occur, which depend upon concentration and temperature.     

Influence de la temperature et du gradient de vitesse sur la viscosite des solutions diluees de xanthane

The dependence of the relative viscosity of a dilute xanthan solution (0.4 g·l^?1) has been studied as a function of temperature (20–70°) and of the ionic strength of the solvent. When the rate of shear exceeds 100 sec^?1, the viscosity can be expressed as γ̊ the exponent (n ? 1) varies with the polymer conformation. In aqueous solution in the absence or in the presence of added salt at a temperature above the melting temperature T_M (depending on the ionic strength of the solvent), the exponent (n ? 1) is ?0.285 and corresponds to the unordered conformation; at temperatures below T_M, the local helical conformation is rigid and (n ? 1) is ～ ?0.44 almost independent of the temperature.     

Calculations and assignments of endohedral helium-3 chemical shifts of open-cage fullerenes and higher fullerenes

The endohedral ³He NMR chemical shifts of open-cage fullerene compounds and higher fullerenes ³He@C _n (n = 82, 84, 86) have been calculated at the GIAO-B3LYP/3-21G//AM1 level. The predicted ³He NMR chemical shifts of open-cage fullerene compounds agree well with the experimental data. More importantly, the challenging peak assignments in the two ³He NMR spectra of higher fullerenes have been successfully achieved by our computed endohedral ³He chemical shifts in combination with experimental results.     

New methods of preparation of hydroxy-closo-decaborates [B10H10 − n (OH) n ]2− (n = 1, 2)

New methods of preparation of hydroxy-closo-decaborates [B₁₀H_{10 ? n} (OH) _n ]^2? (n = 1, 2) that are based on the reaction of anions [B₁₀H_{10 ? n} (OAc) _n ]^2? and alkoxyethylidenoxonio-closo-decaborates [2-B₁₀H₉OC(OR)CH₃]^? with aqueous solution of hydrazine are proposed. The obtained compounds were characterized by IR, ESI/MS, and NMR (¹H, ¹¹B, ¹³C) spectroscopy.     

New hydrofluorocarbon (HFC) solvents for antimony pentafluoride: Generation and characterization of 1-alkoxypentafluoroallyl cations

Certain hydrofluorocarbons (HFC) stable towards the strong Lewis acid, antimony pentafluoride, were found to function as a solvent for this aggressive reagent. CF₃CF₂CH₂F (HFC-236cb) was demonstrated to be an excellent solvent for SbF₅ and was used for the generation of stable polyfluorinated benzyl and allyl cations. Using this solvent 1-methoxypentafluoroallyl cation and R_FOCFCFCF₂⁺ (R_F = n-C₃F₇ and n-C₄F₉), were generated and characterized by NMR spectroscopy.     

Structure-based predictions of H NMR chemical shifts of sesquiterpene lactones using neural networks

In this work the prediction of ¹H NMR chemical shifts of CH_n protons of sesquiterpene lactones by means of neural networks is described. This method is based on the incorporation of experimental chemical shifts of protons of sesquiterpene lactones as additional memory of an associative neural network system previously trained with chemical shifts of other organic compounds. One advantage of this method is its ability to distinguish between CH₂ diastereotopic protons belonging to rigid substructures since stereochemistry is considered. This is achieved via the automatic conversion of the 2D structure diagram into a 3D molecular structure. The predicted ¹H NMR chemical shifts of the sesquiterpene lactones showed a high level of accuracy. This is the first report on a fully automatic proton assignment of structures of sesquiterpene lactones of an accuracy that allows its use in structure elucidation.     

CNDO/2 study of 29Si NMR chemical shifts

A CNDO-2 study of ²⁹Si NMR chemical shifts for compounds of the type (CH₃)_nSiX_4-n (X = H, F, Cl) is reported. The paramagnetic screening constants σ_p are given. The general variation in σ_p with n agrees fairly well with the variation of the observed chemical shifts for X = H and F, but the correlation is not so good for X = Cl.     

Proton-transfer effect in hydrogen-bonded complexes of phospho-diester groups with amines in non-aqueous solvents

The proton transfer in 1 : 1 mixtures of phosphoric acid-di-n-butylester with various aromatic amines has been studied in deutero-chloroform and DMSO-d₆ by ¹H NMR spectroscopy. The 50% equilibrium was found for both solvents at ΔpK_a(50%)-values from 4.4 to 5.2, i.e. the strength of the base is 4.4 to 5.2 higher than that of the acid. The titration curves are steeper in DMSO than in chloroform, indicating a higher potential barrier in POH … N ? PO^? … H⁺N hydrogen bonds in the more polar solvent.     

NMR assignments and the acid–base characterization of the pomegranate ellagitannin punicalagin in the acidic pH-range

In exploring the capability of nuclear magnetic resonance (NMR) spectroscopy for pomegranate juice analysis, the eight aromatic singlet resonances of α- and β-punicalagin were clearly identified in the ¹H NMR spectra of juice samples. The four downfield resonances were found to be sensitive to small pH changes around pH 3.50 where the NMR spectra of the juice samples were recorded. To understand this unusual behavior, the ¹H and ¹³C resonance assignments of the punicalagin anomers were determined in aqueous solution and pH titrations with UV and ¹H NMR detection carried out to characterize the acid–base properties of punicalagin over the pH range 2–8. Simultaneous fitting of all of the pH-sensitive ¹H NMR signals produced similar but significantly different pK _a values for the first two deprotonation equilibria of the gallagic acid moiety of the punicalagin α- (pK _a1?=?4.57?±?0.02, pK _a2?=?5.63?±?0.03) and β- (pK _a1?=?4.36?±?0.01, pK _a2?=?5.47?±?0.02) anomers. Equivalent pK _a values, (α?:?6.64?±?0.01, β?:?6.63±?0.01) were measured for the third deprotonation step involving the ellagic acid group, in good agreement with a prior literature report. The punicalagin anomer equilibrium readjusts in parallel with the proton dissociation steps as the pH is raised such that β-punicalagin becomes the most abundant anomer at neutral pH. The unusual upfield shifts observed for the glucose H3 and H5 resonances with increasing pH along with the shift in the α/β anomer equilibrium are likely the consequence of a conformational rearrangement.