13C n.m.r. spectra of 2-substituted pyrimidines

¹³C n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH₂, OCH₃ and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and proton-proton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbon-proton coupling constants in 2(1H)-pyrimidinone and 2(1H)-pyrimidinethione which accompany anion and cation formation are reported.     

Carbon-13 nuclear magnetic resonance spectroscopy IX—monosubstituted ethylenes

Carbon-13 chemical shifts of sixteen monosubstituted ethylenes were obtained. In order to explain the chemical shifts, σ and π electron densities of these compounds are calculated by the σ-included ω-HMO method.

1 See Ref. 8.

A linear relationship exists between carbon-13 chemical shifts and the calculated electron densities, and also between substituent constants and electron densities. A slope of unity is obtained between the chemical shifts of α carbons of monosubstituted ethylenes and those of carbons adjacent to the substituents in monosubstituted benzenes. On the other hand, a plot of chemical shifts of C_ortho of benzene derivatives against that of the β carbon in ethylene derivatives gives a slope of 3. These slopes can be explained by the calculated electron densities. A slope of 4/3 is obtained between the direct coupling constant ¹J(C? H) of the α carbon in monosubstituted ethylenes and that in the corresponding substituted methanes.     

Struktur-Reaktivitätsuntersuchungen mit heterosubstituierten Nitrilen—VII: 13C-, 14N- und 19F-NMR-Spektroskopische Untersuchungen an Verbindungen vom Typ Ar?X?CN

The chemical shifts of aromatic nitriles of the general structure para-Y? C₆H₄? X? CN with X = O, S, Se and N(CH₃) have been investigated by the ¹³C NMR technique. For cyanates (X = O) the ¹⁴N shifts and for Y = F the ¹⁹F shifts were likewise measured. The chemical shifts and the corresponding ¹³C shift increments Δ_n have been found to correlate with the appropriate substituent constants σ_R⁰, σ_p⁰ and σ_I, as well as with the π-electron densities calculated in the PPP approximation.     

17O NMR investigation of p,π-interactions in α,β-unsaturated and aromatic ethers

The ¹⁷O chemical shifts have been measured for 51 α,β-unsaturated and aromatic ethers. A good linear relationship is found between the ¹⁷O chemical shifts in a series of dialkyl and the corresponding alkyl vinyl ethers. Hence, the extent of p,π-interaction, between the oxygen atom and the vinyl group in the latter series does not, apparently, depend upon branching at the α-carbon atom in the alkyl moiety of these ethers. The PhOBu^t ether, however, as compared to the other alkyl phenyl ethers, shows significantly weakened p,π-interaction, which is apparently related to the steric hindrance of this interaction. The effects of two unsaturated groups upon the ¹⁷O chemical shifts in the corresponding ethers are non-additive. This is undoubtedly a result of ‘rivalry’ between these groups for conjugation with the lone electron pairs on the ethereal oxygen. The ¹⁷O chemical shift ranges of substituted methyl and vinyl phenyl ethers are nearly equal (≈30 ppm). An analysis of the ¹⁷O shielding for cyclopropyl ethers shows no observable p,σ-conjugation in these compounds. Excellent correlation (r>0.99) between the values of ¹⁷O chemical shifts and the calculated (MO LCAO SCF, CNDO/2) π-electron charges on the corresponding oxygen atoms look promising for experimental estimations of π-electron densities on the ethereal oxygen.     

13C-NMR-spektren von di- und trinuclearen polymethincyanin-farbstoffen

¹³C NMR chemical shifts are reported and assigned for two vinylogous series of polymethinecyanine dyes with indolenine- and benzothiazole nuclei. The influence of the heterocyclic groups and the chain length upon the chemical shifts of the methine carbons and upon the corresponding ¹J_¹³C¹H-coupling constants has been studied. ¹³C chemical shifts of indolenine derivatives correlate with calculated π-electron densities. The combined application of ¹H- and ¹³C NMR spectroscopy turns out to be a useful method for detecting steric hindrance in polymethine dyes.     

The relationship between 19F substituent chemical shifts and electron densities: meta- and para-substituted benzoyl fluorides

The ¹⁹F substituent chemical shifts (SCS) of meta- and para-benzoyl fluorides are found to correlate well with substituent parameters using the dual substituent parameter (DSP) equation, indicating that they reflect electronic perturbations induced by the substituent. The direction of the SCS values is such that donating substituents cause upfield shifts whilst acceptors cause downfield shifts. STO-3G calculations indicate that substituents induce only very small changes in π-electron density about the fluorine atom, but that these changes correlate reasonably well with the observed SCS values. For the para series, the slope of the relationship between δq and ¹⁹F SCS is 5000 ppm/electron, indicating the great sensitivity of the flourine atom to small changes in electron density.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

A 19F NMR study of the transmission of electronic effects in triarylantimony and triarylbismuth compounds. A comparison with the nitrogen- and carbon-containing analogues

A ¹⁹F NMR study of the transmission of electronic effects has been made for the systems Ar₂EC₆H₄F-4 (E = Sb, Bi, CH, N). The fluorine chemical shifts obtained are correlated with the polar constants (Σσ^o and Σσ) of the substituents, suggesting that electronic effects are transmitted through the SbC_ar, BiC_ar and CC_ar bonds predominantly by an inductive mechanism, whereas the transmission through the NC_ar bonds is contributed to significantly by classical resonance effects due to competitive conjugation of the lone pair with the aromatic rings, and the substituents therein. A dual parameter correlation of the fluorine chemical shifts with the inductive (σ_I) and resonance (σ^o_R and σ_R) parameters of the substituents in the aromatic rings has led to similar conclusions. The inductive transmission through the bridging Sb and Bi atoms has been assigned to the absence of conjugation of lone pair and vacant d-orbitals of the metals with π-electron systems of the aromatic rings. On the basis of the values of the ? coefficients for the correlation equations obtained it has been established that the transmitting ability of the BiC_ar bonds is close to that of the C_alC_ar bonds and considerably lower than the transmitting ability of the NC_ar bonds.     

Reaction of Nitrosonium Tetrafluoroborate with Nitroxyl Radicals

It was established by means of multinuclear magnetic resonance method (¹H, ^{1 3}C, ^{1 9}F and ^{1 4}N) that reaction of 2,2,6,6-tetramethyl-4-R-piperidin-1-oxyl radicals (R = H, OH, OMe, OCOPh, NHCOMe) with nitrosonium tetrafluoroborate gave rise to the corresponding 2,2,6,6-tetramethyl-1-oxo-4-R-piperidinium tetrafluoroborates. Linear correlations were found between the chemical shifts of atoms H⁴, C⁴ of cations and respectively ₁-constants of substituents R and chemical shifts of C⁴ atom calculated from increments of substitution. The conformational features of the generated nitrosonium cations are considered on the grounds of vicinal coupling constants J _{H H} and quantum-chemical calculations by AM1 method.     

Elektronenstruktur und NMR-Parameter von substituierten 5-Phenyl-2,4-pentadiensäuren

The ¹H and ¹³C NMR chemical shifts as well as vicinal HH coupling constants of substituted 5-phenyl-2,4-pentadienoic acids Ar? CH?CH? CR?CR? COOH are reported and discussed in connection with the molecular structure. The ¹³C chemical shift values show an alternation along the chain and can be linearly correlated to the π-electron charge densities as calculated by use of the PPP-method. The effect of para-substituents and solvents upon the ¹³C chemical shifts can be described in terms of the mutual atom-atom polarizabilities.     

19F n.m.r spectra of polyfluoroquinolines. Long range inter-ring 19F?19F coupling constants and 19F chemical shifts

The signs and magnitudes of every fluorine–fluorine coupling constant in perfluoroquinoline ( 1 ), 2,4-dichloropentafluoroquinoline ( 2 ) and 2-bromohexafluoroquinoline ( 3 ) have been determined by ¹⁹F n.m.r. These provide an unambiguous assignment of the spectrum of the first compound and its derivatives. Inter-ring fluorine–fluorine coupling constants were found to be positive over an odd number of bonds and negative over an even number of bonds, similar to that observed in proton–proton coupling constants in multicyclic systems. The ¹⁹F chemical shifts of perfluoroquinoline and its protonated salt are reported and directly correlated with SCF MO calculated π-electron densities at both fluorine and bonded carbon atoms.     

The 1H NMR spectra of quinoline,quinoline N-oxide,the quinolinium ion and of their monomethyl derivatives

¹H chemical shifts of quinoline, quinoline N-oxide and the quinolinium ion were obtained by complete analysis of their NMR spectra and interpreted critically in an attempt to quantify the possible different effects acting on the shielding constant of protons in these systems. Semi-empirical SCF calculations of electron charge densities and ring current contributions were also performed. It was found that the same effects that act on the shielding of protons directly bonded to the heterocyclic system also act, in an attenuated form, on the proton chemical shift of methyl groups in monomethyl derivatives. Vicinal coupling constants were rationalised in terms of π-electron bond order and electro-negativity of neighbouring atoms.     

NMR study of E/Z isomerism in N‐alkoxybenzoimidic acid derivatives

Well defined E/Z isomers of N‐methoxy‐p‐nitrobenzimidoyl chloride, N‐methoxybenzimidoyl chloride, methyl N‐methylbenzohydroximate and ethyl N‐hydroxybenzimidate were prepared in order to provide model data for studies of benzhydroximic acid derivatives and related compounds. NMR parameters [¹H, ¹³C and ¹⁵N chemical shifts and ¹J(¹³C, ¹³C) coupling constants] were determined. The results show that stereochemically most significant are the values of ¹J(¹³C, ¹³C) couplings between aromatic C_ipso and C?N carbons and that the relationship, |J_cis| > |J_trans|, known for this coupling from oximes, is not affected by electronegative substituents at the C?N carbon atom, but the values are. Copyright © 2002 John Wiley & Sons, Ltd.     

Direct and indirect substituent effects on 1J(CH) in vinyl derivatives and related compounds

The variation in the one–bond couplings ¹J(CH) in vinyl derivatives with substituent has been examined. For the geminal proton ¹J correlates very badly with substituent electronegativity but extremely well with σ_I, if conjugating substituents are excluded. In the case of halogen substituents the marked stereospecificity of ¹J(CH) for the cis and trans protons can be rationalised in terms of an intrinsic dependence of π_CH on the dihedral angle between the coupling atoms and the perturbing substituent, with an additional positive increment to the cis coupling due to direct interaction of the substituent non-bonding electrons or to orbital circulation of the substituent electrons. The intrinsic specificity of β-substituent effects on ¹J(CH) is also found in analogous compounds containing C?N and C?O bonds.     

Group IV organometal derivatives of pyridine. I—A 1H and 13C NMR investigation of trialkylmetal derivatives of pyridine

The proton and carbon-13 NMR spectra of thirteen trialkylmetal derivatives of pyridine, several of which were previously unknown, have been recorded and analysed. The proton NMR spectra show variations in proton chemical shifts but not in proton-proton coupling constants when the metal substituent is changed; the ring proton-metal coupling constants ⁿJ(M? H) in the tin and lead derivatives correspond closely with the corresponding proton-proton couplings ⁿJ(H? H) in pyridine. The carbon-13 chemical shifts of the carbons bound to the metal can apparently be correlated with the electron-donating ability of the trialkylmetal group. In the trimethylstannylpyridines the value of ¹J(Sn? C_ring) varies greatly with the position of the Me₃Sn group.     

NMR-spektroskopische Untersuchungen an 15N-markierten geminal disubstituierten Cyclotriphosphazenen

NMR Spectroscopic Studies of ¹⁵N Labelled Geminally Disubstituted Cyclotriphosphazenes It is demonstrated by means of some selected ¹⁵N labelled geminally disubstituted cyclotriphosphazenes, ¹⁵N₃P₃X₄Y₂ (X = Cl; Y = F, NH₂, or SEt), as an example, that the coupling constants ¹J_PN may be of different signs. The absolute value of ¹J_PN is significantly influenced only by those substituents, which are bonded to the phosphorus nucleus directly concerned in the coupling. Also the ¹⁵N chemical shifts are only changed by substituents on directly bonded phosphorus atoms.     

Analysis of 1H NMR spectra of fluorene and azafluorenes: Long-range coupling constants

Analyses of the ¹H NMR spectra of fluorene, 2-nitrofluorene, 1-azafluorene, 2-azafluorene, 4-azafluorene, 7-nitro-4-azafluorene and 1,4-diazafuorene have been carried out using computer calculations and homo double resonance techniques. The relative signs and magnitude of the ⁴J, ⁵J and ⁶J long-range coupling constants of the 9-CH₂ group protons with the pyridine and phenylene ring protons have been measured by five- and six-spin calculations. It was established that the presence of substituents and the N heteroatom has only a very weak effect on the values and signs of these coupling constants. The usual alternating signs of long-range coupling constants involving π-electron systems were confirmed, giving negative signs for even and positive signs for odd numbers of bonds separating the coupling protons.     

13C-, 14N- und 1H-kernresonanzspektroskopische Untersuchungen an m-/p-substuierten N-Methylpyridiniumjodiden

In a series of 21 m-/p-substituted N-methylpyridinium salts, N-methyl proton, carbon-13 and pyridinium nitrogen-14 chemical shifts were determined by heteronuclear double resonance, and partly in the case of the ¹⁴N nucleus and the methyl protons by direct measurement. In a few compounds the quadrupole relaxation times proved to be too short for the ¹⁴N coupling to be detected. This problem was overcome by adequately rising the sample temperature. For all three nuclei a marked dependence of the chemical shifts on the nature of the substituent could be established. Increased nitrogen π-electron density shifts the resonances towards higher fields. In the case of the ¹⁴N shieldings, this tendency is attributed to changes in the paramagnetic screening term, whereas for ¹³C and ¹H an interpretation is given in terms of a neighbour anisotropy contribution. The latter explanation is based on the observation that the calculated carbon and hydrogen charge densities show no significant variations throughout the series. Excellent shift correlations were obtained with Hammett substituent constants when σ⁺-values were used for donor substituents. A similar substituents, dependence could be observed for the direct ¹³CH methyl coupling constants, whose magnitude increases with decreasing nitrogen charge density.     

NMR Spectra and Structure of Phosphorus-Containing Cycles with Acyclic P=Se and Cyclic P-C Bonds

Abstract

Using ¹H, ¹³C, ³¹P, ⁷⁷Se NMR it has been established that six-membered rings of 5-seleno-5-phenyl-2,2-dimetiiyl-l,3,5-dioxaphosphorinanes, 5-seleno-1,3,5-diazaphosphorinanes and 5-seleno-5-phenyl (alkyl)-2,2-dimethyl-1,3,2,5-dioxa-silaphosphorinanes in the solution are in chair conformation with the equatorial PSe group, whereas the eight-membered ring of 6-seleno-2,2,6-trimethyl-1,3,2,6-dioxa-silaphosphocane is mainly in the chair-chair conformation with the symmetry plane passing through the silicon atom and the axially oriented P=Se group. The selenium nuclear chemical shifts δ77_se -266–416 ppm in these compounds depend on the size of the cycle, and on the effect of the substituent of the cycle. The phosphorus nuclear chemical shifts δ31_pO-50 ppm depend on the phosphorus valence angle at the cycle and the electron infulence of the closest phosphorus substituents. For the stereoisomer pairs of the six-membered cycles the ³¹P nuclei in the isomer with the equatorial P=Se group have been shown to be magnetically screened to a greater degree. The spin-spin coupling of the directly bonded nuclei ¹J_PSe?708–752, ¹J_PC45–48, ¹J_CH128–152 Hz depends both on the size of the cycle and the character of substituents in it. The values of ¹J_PSeare affected by the position of substituents in the bicycles. The overall values of ¹J_PSe are greater for the axial orientation of the P=Se group than for the equatorial.     

NMR study of organosilicon compounds. III—Silicon-29, nitrogen-14, carbon-13 and proton NMR spectra of silylakylamines

²⁹Si, ¹⁴N ¹³C and ¹H NMR data are presented for a series of homologous (methylethoxysilyl)alkylamines of the type (CH₃)_3?n(C₂H₅O)_nSi(CH₂)_mNH₂(n=o to 3; m = 1 to 4). The measured ¹³C and ¹H chemical shifts correlate with the total net charges Q_A on the corressponding atoms, estimated by the Del Re method. ¹⁴N and ²⁹Si chemical shifts which do not show simple linear relationships to the charges are found to correlate with the relative basicities of the compounds. The influence of the remote substituent (? NH₂ and others) on the ₂₉Si chemical shifts is shown to depend on the number and nature of substituents directly on the silicon atom. Argyments for d-orbital participation in the Si? O bounds are given. The chemical shifts of ²⁹Si, ¹⁴N and ₁₃C nuclei are not consistent with the fromation of intramolecular ‘long bonds’ between the solicon and nitrogen atoms in aliphatic silymethylamines.