N. M. Sergeyev

High-resolution proton-coupled 13C NMR spectra of monosubstituted benzenes. Theoretical and empirical correlations of JCH

Abstract The high-resolution proton-coupled 13C NMR spectra of several monosubstituted benzenes, C6H5X (X = F, Cl, Br, I, NO2, NH2, CHO, CN, OH, OCH3, and Si(CH3)3), have been completely analyzed, and all the signs and magnitudes of the 13C-1H coupling constants have been determined. INDO MO calculations of JCH have been performed and a comparison with the experimental values is made. The calculations only reproduce the substituent trends for 1 J 44 , 2 J 12 , 3 J 13 , 3 J 26 , and 3J35, and the couplings of the ring carbons with the exocyclic protons in toluene. Empirical correlations of nJCH with nJHH are considered and discussed. Correlations with substituent electronegativity indicate that mesomeric and inductive effects of the substituents have an important influence. This leads to the consideration of correlations with σp and σ1.

Nuclear magnetic resonance spectroscopy of metal cyclopentadienyls V. Metallotropic rearrangement in silicon cyclopentadienyls: 1,2 shift and spectral regularities in σ-cyclopentadienyls

The saturation transfer technique, INDOR, and related computations have been used to provide a complete analysis of the PMR spectra of isomeric (trimethylsilyl)cyclopentadienes whilst the spectra of C5H5Si(CH3)2Cl and C5H5SiCl3 have been partially analysed. The main vinylic isomer in the series C5H5Si(CH3)nCl3-n (n is 0, 1, 2, 3) is the 1-substituted species while the 5-substituted isomers are subject to a metallotropic rearrangement. From the assumption that 3J(HH) > 0 and 4J(HH) < 0, it follows that the upfield olefinic signal of the 5-substituted isomers is related to the 1,4 protons. Unsymmetrical broadening shows that in all the compounds the migration proceeds predominantly through the 1,2 shift with a free energy of activation of 15 to 16 kcal/mole. As the electronegativity of the substituents attached to the silicon atom increases, the equilibrium associated with the prototropic rearrangement is shifted towards formation of the vinylic isomers while the corresponding metal migration rate decreases. A thorough analysis of the coupling constants, J(HH), and chemical shifts, δ(H), associated with the protons of σ-cyclopentadienyl compounds shows that the constant 3J15 is always positive and equal to 1.2±0.2 Hz while 4J25 is, as a rule, negative and variable within the range −1.6 to +0.2 Hz. In the absence of electron-withdrawing groups (e.g. as in carbonyls) at the metal, the condition δ14 < δ23 invariably applies.

The 1H2H, 17O1H coupling constants and the 16O/18O induced proton isotope shift in water

Abstract At very low concentration in nitromethane the coupling constants J (H,D) and J( 17 O,H ) of the water molecule at 353 K are found to be 1.091(±0.005) and 80.6(±0.1) Hz, respectively. The high accuracy was achieved by lineshape analysis using the QUADR program for the proton resonance and temperature variation for the 17 O-resonance. Measurements of the proton isotope shift due to 16 O/ 18 O substitution reveal to noticeable shift in contradiction to an early reported detection by others.

Nuclear magnetic resonance spectroscopy of metal cyclopentadienyls VI. Metallotropic rearrangement in the group IVB cyclopentadienyls

Abstract The 1 H NMR spectra of the fluxional C 5 H 5 Ge(CH 3 ) 3 and C 5 H 5 Sn(CH 3 ) 3 molecules have been studied at 100 MHz over a wide temperature range. The AA′BB′X spectrum has been analysed to show that the BB′ signal (δ A > δ B ) corresponds to the 1,4 protons. The metal in the cyclopentadienylgermane migrates through a 1,2 shift, as indicated by the unsymmetrical collapse of the olefin multiplets in such a way that a rapid broadening of the high-field side of the spectrum occurs. The AA′BB′X spectrum has been recorded for C 5 H 5 Sn(CH 3 ) 3 at −150° but its complete analysis proved to be impossible. The thermodynamic characteristics of metallotropic migration in the Group IVB C 5 H 5 M(CH 3 ) 3 derivatives have been found. The free energy of activation for migration, Δ G 300 , increases in the series Sn, Ge, Si, C. Essential features of the metallotropic rearrangement are discussed. The migration has been shown to proceed through a 1,2 shift in all cases (Si, Ge, Fe, Ru, H) where reliable data are available. An attempt is made to extend the concept of metallotropism to embrace both quasi-degenerate (indenyl) and non-degenerate (ring-substituted cyclopentadienyl) compounds.

Relative migratory abilities of metals for cyclopentadienyl and indenyl ligands

Abstract The 1 H and 13 C{ 1 H} NMR spectra of the fluxional molecules C 9 H 7 Si-(CH 3 ) 3 , C 9 H 7 Ge(CH 3 ) 3 , and C 9 H 7 Sn(CH 3 ) 3 have been studied over a wide range of temperatures. The metal atom in the indenylgermane migrates through a 1,2 shift. This is shown by isolation of the Diels—Alder cycloaddition adducts of this compound with the dienophiles tetracyanoethylene, maleic anhydride and acetylene dicarboxylate. The thermodynamic characteristics of metallotropic migration in the Group IVB indenyl derivatives show that the difference between the free energies of activation of indenyl and the corresponding cyclopentadienyl compounds in each case is close to 8.5 kcal/mole. New parameters, μ and r a , characteristic of the fluxional behaviour of some organometallic compounds have been introduced, and some predictions based on these parameters have been made.

Nuclear magnetic resonance spectroscopy of metal cyclopentadienyls IV.13C NMR spectra of σ-cyclopentadienyl compounds of silicon, germanium and tin

Abstract 13 C NMR spectra of silicon, germanium and tin cyclopentadienyl compounds have been studied. 13 C chemical shifts and J ( 13 C H) constants verify the σ-structure of the compounds. The variation of the 13 C NMR spectrum of C 5 H 5 Ge(CH 3 ) 3 with temperature shows that a fast metallotropic rearrangement occurs in this compound at as low a temperature as 20° 13 C NMR data are discussed with reference to the structure of metal cyclopentadienyls.

Dynamic 13C N.M.R. studies

The 13C nuclear magnetic resonance technique has been applied to dynamical processes occurring in a number of compounds for which a 1H study would be difficult, owing to complications in the proton spectra. Simple 13C signals in the proton-decoupled spectra are very convenient. The 13C line shapes at various temperatures have been analysed, to give the activation barriers to rotation around the C-N bond in stereoisomeric 2-phenylcyclopropanecarboxylic acid N,N-diethylamides (ΔH ‡ = 16·7 ± 0·5 kcal/mole, ΔS ‡ = -0·6 ± 1·6 e.u. for the trans isomer; ΔH ‡ = 18·4 ± 0·6 kcal/mole, ΔS ‡ = 2·8 ± 1·9 e.u. for the cis isomer), around the Ar-N(O) bond in p-nitroso-N,N-dimethylaniline (ΔH ‡ = 14·5 ± 0·6 kcal/mole, ΔS ‡ = 4·0 ± 2·0 e.u.), around the C-C bond in 9-chloromethyltriptycene (ΔH ‡ = 13·5 ± 0·7 kcal/mole, ΔS ‡ = -1·8 ± 2·5 e.u.) and to the nitrogen inversion in N-(1,2-dicarbomethoxyethyl)-aziridine (ΔH ‡ = 21·8 ± 0·9 kcal/mole, ΔS ‡ = 10·6 ± 3·6 e.u.).

Heteronuclear double magnetic resonance studies of 199Hg nuclear shielding constants in organomercury compounds

Abstract Results obtained by the heteronuclear double magnetic resonance technique for 199 Hg chemical shifts, using a spectrometer locked on the proton signal of benzene or dichloromethane, are reported for a number of organomercury compounds. Deviations from previously reported values are attributed, in part, to medium effects.

13C isotope effects on the parameters of proton magnetic resonance spectra of benzene

Abstract The proton-coupled 13C NMR spectra and the 13C satellites in the 1H NMR spectra of commercial benzene containing about 6.6% of benzene- l-13C were measured and analyzed using the iterative LACX program; 271 lines were assigned and used in the calculations (105 lines from the proton-coupled 13C NMR spectra, 118 lines from outer satellites, and 48 lines from inner satellites in the 1H NMR spectra). The exact values of the 13Cz.sbnd;1H coupling constants were obtained with the standard deviations not exceeding 0.004 Hz. The 12C/13C-induced proton isotope chemical shifts were found to be 1 Δ = 2.44; 2 Δ = 1.34; 3 Δ = 0.51; and 4 Δ = 0.33 (in parts per billion). The 12C/13C-induced isotope effects on the proton-proton coupling constants do not exceed 0.005 Hz.

Nuclear magnetic resonance spectroscopy of metal cyclopentadienyls : III. Analysis of the spectrum of 5-methyldichlorosilylcyclopentadiene (AA′BB′X system)

The PMR spectrum of 5-(methyldichlorosilyl)cyclopentadiene is analysed at various temperatures. At −10°, no dynamic process occurs and the spectrum is described as an AA′BB′X system. The parameters of the system have been analysed completely. The tickling experiments show that the downfield signal belongs to the 1,4-protons. The assignment, as well as the nature of the unsymmetric collapse, show that the metal migrates predominantly through a 1,3-shift. The methyne proton line width has been measured vs. temperature at 0−50° and the activation energy of the metallotropic rearrangement is shown to be 9±1 kcal·mole−1. The prototropic rearrangement proceeds, ceteris paribus, by six to seven orders slower.