The prediction of (1)H chemical shifts in amines: a semiempirical and ab initio investigation

Twenty one conformationally fixed amines and their N,N-dimethyl derivatives were obtained commercially or synthesized. These included cis and trans 4-t-butyl cyclohexylamine, 2-exo and 2-endo norbornylamine, 2-adamantylamine, 4-phenylpiperidine, 1-napthylamine and tetrahydro-1-napthylamine. The (1)H NMR spectra of these amines were measured in CDCl(3) solution, assigned and the (1)H chemical shifts given. This data was used to investigate the effect of the amino group on the (1)H chemical shifts in these molecules. These effects were analyzed using the CHARGE model. This calculates the electric field and steric effects of the amino group for protons more than three bonds removed, together with functions for the calculation of two-bond and three-bond effects. The rotational isomerism about the C--N bond of the amino group was investigated by ab initio calculations of the potential energy surface (PES) about this bond at the HF/3-21G level. The resulting conformers were then minimized at the B3LYP/6-311 + + G (d,p) level. These geometries were then used to calculate the (1)H chemical shifts in the above compounds by CHARGE and the ab initio gauge-invariant atomic orbital (GIAO) method at the B3LYP/6-311 + + G(d,p) level and the shifts were compared with those observed. The compounds investigated gave 170 (1)H chemical shifts ranging from 0.60 to 8.2 ppm. The rms errors (obs.-calc.) were ca 0.1 ppm (CHARGE) and ca 0.2 ppm (GIAO). Large deviations of ca 1.0 ppm were observed for the NH protons in the GIAO calculations. The complex spectra of alkyl and aryl amines can thus be successfully predicted by both ab initio and semiempirical methods except for the NH protons, for which the ab initio calculations are not sufficiently accurate.     

1H chemical shifts in NMR: Part 22-Prediction of the 1H chemical shifts of alcohols, diols and inositols in solution, a conformational and solvation investigation

The (1)H NMR spectra of a number of alcohols, diols and inositols are reported and assigned in CDCl(3), D(2)O and DMSO-d(6) (henceforth DMSO) solutions. These data were used to investigate the effects of the OH group on the (1)H chemical shifts in these molecules and also the effect of changing the solvent. Inspection of the (1)H chemical shifts of those alcohols which were soluble in both CDCl(3) and D(2)O shows that there is no difference in the chemical shifts in the two solvents, provided that the molecules exist in the same conformation in the two solvents. In contrast, DMSO gives rise to significant and specific solvation shifts. The (1)H chemical shifts of these compounds in the three solvents were analysed using the CHARGE model. This model incorporates the electric field, magnetic anisotropy and steric effects of the functional group for long-range protons together with functions for the calculation of the two- and three-bond effects. The long-range effect of the OH group was quantitatively explained without the inclusion of either the C--O bond anisotropy or the C--OH electric field. Differential beta and gamma effects for the 1,2-diol group needed to be included to obtain accurate chemical shift predictions. For DMSO solution the differential solvent shifts were calculated in CHARGE on the basis of a similar model, incorporating two-bond, three-bond and long-range effects. The analyses of the (1)H spectra of the inositols and their derivatives in D(2)O and DMSO solution also gave the ring (1)H,(1)H coupling constants and for DMSO solution the CH--OH couplings and OH chemical shifts. The (1)H,(1)H coupling constants were calculated in the CHARGE program by an extension of the cos(2)phi equation to include the orientation effects of electronegative atoms and the CH--OH couplings by a simple cos(2)phi equation. Comparison of the observed and calculated couplings confirmed the proposed conformations of myo-inositol, chiro-inositol, quebrachitol and allo-inositol. The OH chemical shifts were also calculated in the CHARGE program. Comparison of the observed and calculated OH chemical shifts and CH.OH couplings suggested the existence of intramolecular hydrogen bonding in a myo-inositol derivative.     

1H chemical shifts in NMR. Part 21--prediction of the 1H chemical shifts of molecules containing the ester group: a modelling and ab initio investigation

The 1H NMR spectra of 24 compounds containing the ester group are given and assigned. These data were used to investigate the effect of the ester group on the 1H chemical shifts in these molecules. These effects were analysed using the CHARGE model, which incorporates the electric field, magnetic anisotropy and steric effects of the functional group for long-range protons together with functions for the calculation of the two- and three-bond effects. The effect of the ester electric field was given by considering the partial atomic charges on the three atoms of the ester group. The anisotropy of the carbonyl group was reproduced with an asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond with values of Deltachi(parl) and Deltachi(perp) of 10.1 x 10(-30) and -17.1 x 10(-30) cm3 molecule(-1). An aromatic ring current (=0.3 times the benzene ring current) was found to be necessary for pyrone but none for maleic anhydride. This result was confirmed by GIAO calculations. The observed 1H chemical shifts in the above compounds were compared with those calculated by CHARGE and the ab initio GIAO method (B3LYP/6-31G**). For the 24 compounds investigated with 150 1H chemical shifts spanning a range of ca 10 ppm, the CHARGE model gave an excellent r.m.s. error (obs - calc) of <0.1 ppm. The GIAO calculations gave a very reasonable r.m.s. error of ca 0.2 ppm although larger deviations of ca 0.5 ppm were observed for protons near to the electronegative atoms. The accurate predictions of the 1H chemical shifts given by the CHARGE model were used in the conformational analysis of the vinyl esters methyl acrylate and methyl crotonate. An illustration of the use of the CHARGE model in the prediction of the 1H spectrum of a complex organic molecule (benzochromen-6-one) is also given.     

Understanding sterol-membrane interactions part I: Hartree-Fock versus DFT calculations of 13C and 1H NMR isotropic chemical shifts of sterols in solution and analysis of hydrogen-bonding effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.     

Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis

A model based on classical concepts is derived to describe the effect of the nitro group on proton chemical shifts. The calculated chemical shifts are then compared to ab initio (GIAO) calculated chemical shifts. The accuracy of the two models is assessed using proton chemical shifts of a set of rigid organic nitro compounds that are fully assigned in CDCl3 at 700 MHz. The two methods are then used to evaluate the accuracy of different popular post-SCF methods (B3LYP and MP2) and molecular mechanics methods (MMX and MMFF94) in calculating the molecular structure of a set of sterically crowded nitro aromatic compounds. Both models perform well on the rigid molecules used as a test set, although when using the GIAO method a general overestimation of the deshielding of protons near the nitro group is observed. The analysis of the sterically crowded molecules shows that the very popular B3LYP/6-31G(d,p) method produces very poor twist angles for these, and that using a larger basis set [6-311++G(2d,p)] gives much more reasonable results. The MP2 calculations, on the other hand, overestimate the twist angles, which for these compounds compensates for the deshielding effect generally observed for protons near electronegative atoms when using the GIAO method at the B3LYP/6-311++G(2d,p) level. The most accurate results are found when the structures are calculated using B3LYP/6-311++G(2d,p) level of theory, and the chemical shifts are calculated using the CHARGE program based on classical models.     

1H chemical shifts in NMR: Part 23, the effect of dimethyl sulphoxide versus chloroform solvent on 1H chemical shifts

The 1H chemical shifts of 124 compounds containing a variety of functional groups have been recorded in CDCl3 and DMSO-d6 (henceforth DMSO) solvents. The 1H solvent shift Delta delta = delta(DMSO) - delta(CDCl3) varies from -0.3 to +4.6 ppm. This solvent shift can be accurately predicted (rms error 0.05 ppm) using the charge model of alpha, beta, gamma and long-range contributions. The labile protons of alcohols, acids, amines and amides give both, the largest solvent shifts and the largest errors. The contributions for the various groups are tabulated and it is shown that for H.C.C.X gamma-effects (X = OH, NH, =O, NH.CO) there is a dihedral angle dependence of the gamma-effect. The group contributions are discussed in terms of the possible solvent-solute interactions. For protic hydrogens, hydrogen bonding is the dominant interaction, but for the remaining protons solvent anisotropy and electric field effects appear to be the major factors.     

A molecular mechanics and ab initio prediction of the 1H chemical shifts of pinanes

Molecular mechanics calculations plus the application of a refined Karplus equation gave the conformations of 19 pinanes. These range from a Y‐shaped geometry in the apopinene and α‐pinene series to a pseudo chair conformation in β‐pinene, nopinone and verbanone, a flattened chair in pinocarvone and the pinocarveols and a distorted Y shape for iso‐verbanone. These structures were then used as input to predict the ¹H chemical shifts of these compounds by semi‐empirical (¹H‐NMR spectra (HSPEC)) and ab initio gauge‐invariant atomic orbital (GIAO) calculations, the latter at the B3LYP hybrid density functional theory level using 6‐31++G** basis set. The two methods gave generally good agreement with the 184 observed shifts with root mean square (RMS) errors 0.07 ppm (HSPEC) and 0.10 ppm (GIAO), but the GIAO calculations gave several significant (>0.25 ppm) errors. One was for the H₃ proton in apopinenone and other α,β unsaturated ketones; the others occurred for protons in close proximity to hydroxyl groups. To provide more information, smaller analogues of known geometry and chemical shifts were subject to the same analysis. In cyclopentenone, the Gaussian geometry gave good agreement with the observed shifts, but the MMFF94, MMX and MM3 geometries all gave errors for different protons. These results show clearly that the molecular geometries of the α,β unsaturated ketones are responsible for the errors. The errors for the alcohols were examined using ethanol as model and were shown to be due to the different possible conformations of the OH group. Similar GIAO calculations on substituted methanes gave good agreement for the methyl compounds but poor agreement for di and tri halosubstituted methanes. The aforementioned method of molecular mechanics plus GIAO calculations is shown to be a very useful tool for the investigation of molecular geometries and conformations. However, multihalogen compounds may require different basis sets for accurate calculations. Copyright © 2017 John Wiley & Sons, Ltd.     

Stereoelectronic effects on 1H nuclear magnetic resonance chemical shifts in methoxybenzenes

Investigation of all O-methyl ethers of 1,2,3-benzenetriol and 4-methyl-1,2,3-benzenetriol (3-16) by 1H NMR spectroscopy and density-functional calculations disclosed practically useful conformational effects on 1H NMR chemical shifts in the aromatic ring. While the conversion of phenol (2) to anisole (1) causes only small positive changes of 1H NMR chemical shifts (Delta delta < 0.08 ppm) that decrease in the order Hortho > Hmeta > Hpara, the experimental O-methylation induced shifts in ortho-disubstituted phenols are largest for Hpara, Delta delta equals; 0.19 +/- 0.02 ppm (n = 11). The differences are due to different conformational behavior of the OH and OCH3 groups; while the ortho-disubstituted OH group remains planar in polyphenols due to hydrogen bonding and conjugative stabilization, the steric congestion in ortho-disubstituted anisoles outweighs the conjugative effects and forces the Ar-OCH3 torsion out of the ring plane, resulting in large stereoelectronic effects on the chemical shift of Hpara. Conformational searches and geometry optimizations for 3-16 at the B3LYP/6-31G** level, followed by B3LYP/6-311++G(2d,2p) calculations for all low-energy conformers, gave excellent correlation between computed and observed 1H NMR chemical shifts, including agreement between computed and observed chemical shift changes caused by O-methylation. The observed regularities can aid structure elucidation of partly O-methylated polyphenols, including many natural products and drugs, and are useful in connection with chemical shift predictions by desktop computer programs.     

Experimental and theoretical NMR study of selected oxocarboxylic acid oximes

1H and 13C NMR spectra of the oxocarboxylic acid oximes 2-hydroxyiminopropanoic acid (1), 2-(4-methylthiazol-2-yl)-2-(hydroxyimino)acetic acid (2) and 2-cyano-2-(hydroxyimino)acetic acid (3) were measured in DMSO-d6, D2O and acetone-d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron-withdrawing effect due to the cyano group in 3. The effect of intra- and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen-bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge-including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the alpha-carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character.     

15N NMR chemical shifts of ring substituted benzonitriles

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon.     

A 1H NMR and theoretical investigation of the conformations of some monosubstituted cyclobutanes

The complete analysis of the complex (1)H NMR spectra of some monosubstituted cyclobutanes was achieved to give all the (1)H chemical shifts and (n)J(HH) (n = 2, 3 and 4) coupling constants in these molecules. The substituent chemical shifts of the substituents in the cyclobutane ring differ significantly from those in acyclic systems. For example, the OH and the NH(2) groups in cyclobutanol and cyclobutylamine produce a large shielding of the hydrogens of the opposite CH(2) group of the ring compared with little effect on the comparable methylene protons of butane. These effects and the other (1)H shifts in the cyclobutanes were modelled successfully in the CHARGE program. The RMS error (calculated vs observed shifts) for the 34 (1)H shifts recorded was 0.053 ppm. The conformational equilibrium in these compounds between the axial and the equatorial conformers was obtained by comparing the observed and the calculated (4)J(HH) couplings. These couplings in cyclobutanes, in contrast to the corresponding (3)J(HH) couplings, show a pronounced orientation dependence; (4)J(eq-eq) is ca 5 Hz and (4)J(ax-ax) ca 0 Hz. The couplings in the individual conformers were calculated at the B3LYP/EPR-III level. The conformer energy differences ΔG(ax-eq) vary from 1.1 kcal mol(-1) for OH to 0.2 kcal mol(-1) for the CH(2)OH substituent. The values of the conformer energy differences are compared with the previous IR data and the corresponding theoretical values from molecular mechanics (MM) and DFT theory. Generally, good agreement is observed although both the MM and the DFT calculations deviate significantly from the observed values for some substituents.     

1H and 13C NMR spectra, structure and physicochemical features of phenyl acridine-9-carboxylates and 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulphonates--alkyl substituted in the phenyl fragment

The 1H and 13C NMR spectra of twelve phenyl acridine-9-carboxylates--alkyl-substituted in the phenyl fragment--and their 10-methyl-9-(phenoxycarbonyl)acridinium salts dissolved in CD3CN, CD3OD, CDCl3 and DMSO-d6 were recorded in order to examine the influence of the structure of these compounds and the properties of the solvents on chemical shifts and 1H-(1)H coupling constants. Experimental data were compared with 1H and 13C chemical shifts predicted at the GIAO/DFT level of theory for DFT(B3LYP)/6-31G** optimised geometries of molecules, as well as with values of 1H chemical shifts and 1H-(1)H coupling constants, estimated using ACD/HNMR database software to ensure that the assignment was correct. To investigate the relations between chemical shifts and selected structural or physicochemical characteristics of the target compounds, the values of several of these parameters were determined at the DFT or HF levels of theory. The HOMO and LUMO energies obtained at the HF level yielded the ionisation potentials and electron affinities of molecules. The DFT method provided atomic partial charges, dipole moments, LCAO coefficients of pz LUMO of selected C atoms, and angles reflecting characteristic structural features of the compounds. It was found that the experimentally determined 1H and 13C chemical shifts of certain atoms relate to the predicted dipole moments, the angles between the acridine and phenyl moieties, and the LCAO coefficients of the pz LUMO of the C atoms believed to participate in the initial step of the oxidation of the target compounds. The spectral and physicochemical characteristics of the target compounds were investigated in the context of their chemiluminogenic ability.     

1H NMR spectra part 31: 1H chemical shifts of amides in DMSO solvent

The ¹H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts Δδ (DMSO‐CDCl₃) are large (1–2 ppm) for the NH protons but smaller and negative (?0.1 to ?0.2 ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as Δδ(CHO) are not well reproduced by the GIAO calculations. The ¹H chemical shifts of the amides in DMSO were analysed using a functional approach for near ( ≤ 3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the π density on the αN and βC atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl₃. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl₃. There is no steric effect of the C=O oxygen on the NH proton in an NH…O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106 ppm for the data set of 257 entries). Copyright © 2014 John Wiley & Sons, Ltd.     

NMR property calculations and experimental study of the 1,6-epoxycarvone and α-epoxypinene: a comparison of models

This work aims at using theoretical calculations of shielding tensors (σ) through different methods [gauge-independent atomic orbital (GIAO), continuous set of gauge transformations (CSGT) and individual gauges for atoms in molecules (IGAIM)] and spin-spin coupling constants J using GIAO method to compare these methods and to corroborate the data obtained with the assignment of all of (1)H and (13)C NMR signals and the relative stereochemistry of the 1,6-epoxycarvone and the α-epoxypinene. All the (1)H and (13)C NMR signals were assigned unequivocally. The stereochemistry for the epoxides is trans and the B3LYP theory level with CSGT and IGAIM methods is the best choice to evaluate theoretical chemical shifts for compounds studied.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

Supramolecular H-bonded assemblies of redox-active metallodendrimers and positive and unusual dendritic effects on the recognition of H2PO4-

The DSM polyamine dendrimers dend-DAB-(NH2)x of generations 1 (x = 4) to 4 (x = 32) form H-bonded dendritic assemblies with the phenol AB3 units p-HOC6H4C(CH2CHCH2)3 and p-HOC6H4C{(CH2)3SiCH2NHCOFc}3 (Fc = ferrocenyl), as shown by the shifts of the NH2 and OH signals giving a concentration-dependent common signal between 2.4 and 4.1 ppm in CDCl3. The supramolecular dendrimers efficiently recognize H2PO4- anions with positive and unusual dendritic effects upon electrochemical titration involving half-stoichiometry for G1, a sudden cyclovoltammetry wave change at the equivalent point, and a dramatic intensity decrease of the new wave.     

A computational and experimental study of the structures and Raman and 77Se NMR spectra of SeX3+ and SeX2 (X = Cl, Br, I): FT-Raman spectrum of (SeI3)[AsF6

The ability of MP2, B3PW91 and PBE0 methods to produce reliable predictions in structural and spectroscopic properties of small selenium-halogen molecules and cations has been demonstrated by using 6-311G(d) and cc-pVTZ basis sets. Optimized structures and vibrational frequencies agree closely with the experimental information, where available. Raman intensities are also well reproduced at all levels of theory. Calculated GIAO isotropic shielding tensors yield a reasonable linear correlation with the experimental chemical shift data at each level of theory. The largest deviations between calculated and experimental chemical shifts are found for selenium-iodine species. The agreement between observed and calculated chemical shifts for selenium-iodine species can be improved by inclusion of relativistic effects using the ZORA method. The best results are achieved by adding spin-orbit correction terms from ZORA calculations to nonrelativistic GIAO isotropic shielding tensors. The calculated isotropic shielding tensors can be utilized in the spectroscopic assignment of the 77Se chemical shifts of novel selenium-halogen molecules and cations. The experimental FT-Raman spectra of (SeI3)[AsF6] in the solid state and in SO2(l) solution are also reported.     

NMR spectra of free-base porphine, porphyrazine, phthalocyanine and naphthalocyanine as well as their metal complexes: density functional calculations

Nuclear magnetic shielding tensors of porphine have been calculated at density functional B3LYP and PBE level using the gauge independent atomic orbital (GIAO) method. The geometries used were optimized using the 6-31G(d) basis set and the NMR calculations were performed using 6-31G(d) and 6-311G(d,p) basis sets, respectively. The calculated NMR shielding tensors and chemical shifts of porphine are compared with previous calculations as well as experimental data and satisfying results are obtained. Further NMR calculations are extended to metal-free and metallo-porphyrazine, -phthalocyanine, and -naphthalocyanine for the first time and the results are compared with experimental data available. The chemical shifts of the atoms in these compounds are assigned according to the experimental data available.     

1H chemical shifts in NMR. Part 27: proton chemical shifts in sulfoxides and sulfones and the magnetic anisotropy, electric field and steric effects of the SO bond

The (1)H chemical shifts of a series of sulfoxide and sulfone compounds in CDCl(3) solvent were obtained from experiment and the literature. These included dialkyl sulfoxides and sulfones (R(2)SO/R(2)SO(2), R = Me, Et, Pr, n-Bu), the cyclic compounds tetramethylene sulfoxide/sulfone, pentamethylene sulfoxide/sulfone and the aromatic compounds p-tolylmethylsulfoxide, dibenzothiopheneoxide/dioxide, E-9-phenanthrylmethylsulfoxide and (E) (Z)-1-methylsulfinyl-2-methylnaphthalene. The spectra of the pentamethylene SO and SO(2) compounds were obtained at -70 degrees C to obtain the spectra from the separate conformers (SO) and from the noninverting ring (SO(2)). This allowed the determination of the substituent chemical shifts (SCS) of the SO and SO(2) functional groups, which were analyzed in terms of the SO bond electric field, magnetic anisotropy and steric effect for long-range protons together with a model (CHARGE8d) for the calculation of the two and three bond effects. After parameterization, the overall root mean square (RMS) error (observed-calculated) for a dataset of 354 (1)H chemical shifts was 0.11 ppm. The anisotropy of the SO bond was found to be very small, supporting the dominant single bond S(+)--O(-) character of this bond.