George B. Savitsky

Effect of cis–trans Isomerism on the Carbon‐13 Chemical Shifts of Some Unsymmetrically Substituted Ethylenes

The difference in shielding between the 13C nuclei of cis and trans isomers CHX = CHX for the series X = Cl, Br, I, CO2C2H5 was previously attributed to the inhibition of contributing resonance structures by bulky substituents in cis isomers. The present work, which includes the determination of 13C chemical shifts of cis and trans isomers with nonequivalent substituents, CHX = CHY, indicates that the previous interpretation must be modified considerably and that the tendency toward charge separation and reduced bond order in the C = C π bond of cis isomers may be the major factor in these differential shieldings. An application of the Pople–Karplus equations for 13C shielding is consistent with this view.

The effect of geometric isomerism on the carbon-13 chemical shift of diiodoethylene and related compounds☆

Abstract The origin of the large difference between the 13C chemical shifts of cis and trans diiodoethylene and related compounds was investigated. The involvement of significant medium effects was ruled out by results with dilute solutions of 13C-enriched materials. Experiments with the analogous diethyl maleate and fumarate in a variety of solvents including hydrogen-bonding acids and data on some other symmetrical disubstituted ethylenes cast doubt on an earlier interpretation based on the steric inhibition of resonance in cis isomers. An application of the Pople-Karplus formalism in the manner previously used for unsymmetrical ethylenes does not lead to a satisfactory interpretation if the mean ΔE is assumed to be independent of geometrical isomerism.

Isotope effects on order in nematic solutions. Deuterium quadrupole coupling constants for CDH2I

In a previous publication [J. Chem. Phys. 66, 4226 (1977)] it was noted that dipolar coupling constants obtained from the NMR spectra of mixtures containing 13CH3I and 13CDH2I dissolved in nematic solvents revealed the presence of an isotope effect. In order to investigate this phenomenon quantitatively we have carried out vibrational averaging of the dipolar couplings, and derived the order parameters, Sαβ, for the two molecules. We also derived the quadrupole coupling constant, QD, for deuterium in 13CDH2I from the splitting observed in the deuterium spectrum and values of Sαβ obtained from vibrationally corrected dipolar couplings. The average value of QD was found to be 160.7±1.7 KHz if the asymmetry parameter, η, is assumed zero, and 153.9±1.6 KHz or 168.2±1.8 KHz for values of η=−0.04 and +0.04, respectively.

NMR of oriented CDH2I and symmetry related isotope effects

Measurements of the orientation parameters of a 13C enriched sample of CDH2I in nematic solvent employing 1H, 2H, and 13C NMR are reported. These parameters denoted as SCH, SCD, SHH, and SHD, obey the following relation which assumes mirror plane symmetry: SCD=−2cosα (SHH+2SHD)−2SCH, rather than the following relations which assume C3 rotational symmetry: SCD=SCH=−2cosα SHH=−2cosαSHD, where α equals the HCH or HCD bond angle. Consequently it is concluded that isotope effects on the orientation parameters, such as SCH≠SCD, reported earlier [J. B. Wooten, A. L. Beyerlein, J. Jacobus, G. B. Savitsky, J. Chem. Phys. 65, 2476 (1976)] result from a change in symmetry upon deuteration. The value of α=112.67° that best describes the reported data on CDH2I and CH3I in nematic phase differs from the gas phase microwave value 111.6° suggesting bond angle distortions in the nematic phase. Thus the preferable method of obtaining SCD for quadrupole coupling constant determinations of the CD bond is from the 13CD dipola...

The vibrational and asymmetry corrections to quadrupole coupling constants determined from 13C satellites in 2 NMR spectra of benzene-d1 and acetonitrile-d1

The vibrational corrections have been made to the quadrupole coupling constants of benzene-d1 and acetonitrile-d1 previously determined by data from the 13C satellites in nematic oriented phase. A significant isotope effect on such corrections has been evidenced. The vibrational corrections together with asymmetry corrections result in values for the quadrupole coupling constants which are in good agreement with values obtained by other methods.

A novel deuteration technique and its application to NMR of substituted benzenes

Abstract A novel deuteration technique which will assist in the analysis of proton magnetic resonance spectra at the same time simplifying synthetic procedures has been developed. It consists of deuterating a parent hydrocarbon in several selected patterns leaving only one or two protons in the molecule at a time. Subsequently, a molecule of interest is synthesized from the deuterated compound under conditions which avoid deuterium exchange. Only one such synthetic route has to be devised and is applied to each specifically deuterated compound. The resulting spectra in each case are superpositions of a few simple spectra consisting of singlets and AB quartets leading to direct experimental determination of chemical shifts and important coupling constants. An application to the analysis of the spectrum of nitrobenzene has been made and is discussed.

Deuteron quadrupole coupling constant of deuterochloroform determined from direct 13C2H couplings in 2H NMR spectra

Abstract The deuteron quadrupole coupling constant, Q D , of 13 CDCl 3 was measured in three different nematic solvents employing the quadrupolar and the direct dipolar 13 C 2 H splittings in 2 H NMR. The values of Q D found in Phase IV and EBBA were 158.9 ± 1 and 157.8 ± 1 kHz, respectively, while the value of Q D in ZLI-1167 was found to be 170.5 ± 1 kHz. These results are discussed and compared with literature values obtained in various media in liquid and solid states.

Solvent dependence of the deuteron quadrupole coupling constant of CDCl3 determined by 2H spin‐lattice relaxation and Raman line shape studies

The deuterium quadrupole coupling constant in deuterochloroform was determined in an inert solvent (C6H12) and in polar solvents (neat liquid, C6H6, and THF), by combining NMR relaxation data with Raman line shape analysis of the ν1 band (2253 cm−1) of deuterochloroform. The QD value in a 20 mol % solution of CDCl3 in C6H12 was found to be (186±4 kHz), significantly higher than any previously reported value obtained in liquid crystals. A substantial lowering of the quadrupole coupling constant was observed in polar solvents.

Isotope effects on the carbon-hydrogen bond orientation of methyl iodide in nematic phase

Abstract Deuterium substitution which results in no change in molecular symmetry is demonstrated to produce negligible change in the orientation parameter (SCD) of methyl iodide dissolved in nematic phase. Previous indirect estimates of quadrupolar coupling constants (QD), which have assumed SCD = SCH, are high in relation to those obtained by a direct method. This discrepancy is attributed to geometric distortion caused by dissolution of the probe molecule in nematic phase and not to the inequality of orientation parameters.

13CH satellite spectra of partially deuterated monosubstituted benzenes

13CH coupling constants were determined in partially deuterated mono-ubstituted benzenes (C6H5X, X = NH2, NO2, F, Cl, Br, I) from the satellites of the PMR spectra. The coupling constants in the CH unit para to the substituent (JCHp) were found to be relatively insensitive to substitution. The coupling constants in the CH units meta and ortho to the substituent (JCHm, JCHo) showed a considerable spread of values and could be related to Hammett substituent parameters in reasonable qualitative agreement with theory. The assumption of additivity of 13CH coupling constants in polysubstituted benzenes was examined in terms of previously published results.