Unusually large deuterium isotope effects on one-bond 13C, 1H coupling constants in trans-stilbene challenged

The reported apparent large decrease in ¹J(C-α, H-α) by 1.75±0.20 Hz on replacement of one of the olefinic protons by deuterium in trans-stilbene is due to improper first-order analysis of the ¹H-coupled ¹³C spectrum of the parent compound. Consequently, the implied conformational difference between trans-stilbene and α-deuterio-trans-stilbene, which was used to explain the result, is not substantiated.     

Solvent effects on one-bond B-Li coupling constants in boryllithium compounds

EOM-CCSD 11B-7Li coupling constants and B chemical shifts have been computed for Li-diazaborole and its complexes with one H2O or FLi molecule. B-Li coupling constants for a model compound H(2)BLi and its complexes with up to 4 H2O or FLi molecules have also been obtained in an attempt to resolve discrepancies between the computed values of these properties for isolated Li-diazaborole and experimentally determined values for boryllithium in a THF solution. The presence of solvent molecules increases the ion-pair character of the B-Li bond, with the result that 1J(B-Li) decreases systematically as the basicity and the number of solvent molecules increases. In the presence of even a single solvent molecule, the boron chemical shift for Li-diazaborole increases, and approaches the experimental value. The computed results emphasize the role of the solvent in determining these NMR properties.     

Electric field effects on one-bond indirect spin-spin coupling constants and possible biomolecular perspectives

Electric field (EF) induced changes of one-bond indirect spin-spin coupling constants are investigated on a wide range of molecules including peptide models. EFs were both externally applied and internally calculated without external EF application by the hybrid density functional theory method. Reliable agreement with experimental data has been obtained for calculated one-bond J-couplings. The role of the EF sign and direction, internal and induced components, hydrogen bonding, internuclear distance and hyperconjugative interactions on the one-bond J-coupling vs EF interconnection is analyzed. A linear dependence of 1J on EF projection along the bond is obtained, if the bound atoms possess different enough electron densities and an EF determined by the electronic polarization exists along the bond. Accentuating the 1JNH couplings as possible EF sensitive parameters, a systematic study is done in two sets of molecules with a large variation of the native internal EF value. The most EF affected component of the 1JNH coupling constant is the spin-dipole term of Ramsey's formulation; however, in the total J-coupling formation, the EF influence on the Fermi contact term is the most significant. The induced EF projection along the bond is 6.7 times weaker in magnitude than the simulated external uniform field. The absolute EF dependence of the one-bond J-coupling involves only the internal field, which is the sum of the induced field (if the external field exists) and the internuclear field determined by the native polarization. That linear and universal dependence joins the corresponding couplings in a diverse set of molecules under various electrostatic conditions. Many types of the one-bond J-couplings can be potentially measured in biomolecules, and the study of their relation with the electrostatic properties at the corresponding sites opens a new avenue to the full exploitation of the NMR measurable parameters with novel and exciting applications.     

The stereochemical dependence of one-bond carbon-carbon coupling constants

Spin-spin coupling constants for carbon-carbon single bonds vary with the orientation of lone pairs on adjacent nitrogen atoms and of adjacent carbonyl groups.     

Quantum effects on hydrogen isotope adsorption on single-wall carbon nanohorns

H(2) and D(2) adsorption on single-wall carbon nanohorns (SWNHs) have been measured at 77 K, and the experimental data were compared with grand canonical Monte Carlo simulations for adsorption of these hydrogen isotopes on a model SWNH. Quantum effects were included in the simulations through the Feynman-Hibbs effective potential. The simulation predictions show good agreement with the experimental results and suggest that the hydrogen isotope adsorption at 77 K can be successfully explained with the use of the effective potential. According to the simulations, the hydrogen isotopes are preferentially adsorbed in the cone part of the SWNH with a strong potential field, and quantum effects cause the density of adsorbed H(2) inside the SWNH to be 8-26% smaller than that of D(2). The difference between H(2) and D(2) adsorption increases as pressure decreases because the quantum spreading of H(2), which is wider than that of D(2), is fairly effective at the narrow conical part of the SWNH model. These facts indicate that quantum effects on hydrogen adsorption depend on pore structures and are very important even at 77 K.     

Deuterium isotope effect in electron impact desorption of hydrogen on tungsten

The deuterium isotope effect of electron impact desorption cross sections has been measured for two adsorption states of hydrogen on tungsten with widely differing cross sections. The results of these measurements are in good agreement with values calculated according to the theory of this process proposed earlier.     

Deuterium isotope effects in solutions of 2,4-pentanedione

Proton and deuterium n.m.r. measurements in acetylacetone and 3,3-d₂-acetylacetone are reported. Deuterium-isotope effects on chemical shifts and keto–enol equilibria as a function of concentration in the two solvents triethylamine and pyrrole are determined. A strong solvent concentration dependence for d-isotope effects is observed in triethylamine; in pyrrole no dependence is obtained. The results are interpreted in terms of an equilibrium between the symmetrical (C_2v) and asymmetrical (C_s) enolic forms in acetylacetone. The usefulness of ²H n.m.r. for the study of exchange processes is demonstrated.     

Deuterium isotope effects on 13C chemical shifts of nitromalonamide

The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH...O hydrogen bond as well as a NH...O-N=O hydrogen bond.     

13C NMR: Substituent effects on one-bond 15N?13C coupling constants in anilines and anilinium ions

¹J(¹⁵N¹³C) values obtained from FT ¹³C NMR spectra were measured for a number of ¹⁵N-enriched aniline derivatives and are found to exhibit varying degrees of dependence on the nature of the ring substituent. Theoretical calculations of ¹J(¹⁵N¹³C) values for representative members of the systems examined were made using INDO parameters and a ‘sum-over-states’ perturbation approach. The calculated coupling constants are generally in fair agreement with experimental values when the integral products S_N²(o)S_C²(o) and (r^?3)_N(r^?3)_C have values of 34.437 au^?6 and 2.770 au^?6, respectively.     

13C-detected IPAP-INADEQUATE for simultaneous measurement of one-bond and long-range scalar or residual dipolar coupling constants

The sensitivity of cryoprobes, which are rapidly becoming available, means that the measurement of coupling constants involving 13C, 13C pairs at the natural abundance of 13C can now, in principle, be done by using tens rather then hundreds of milligrams of compounds. However, a robust method that would yield reliable values of small long-range carbon--carbon coupling constants is still missing. In this Communication, we describe a novel 13C-detected incredible natural-abundance double-quantum transfer experiment (INADEQUATE) experiment for simultaneous correlation of one-bond and long-range 13C- 13C pairs and the measurement of both types of coupling constants in 13C natural abundance samples. This method yields accurate values of one-bond and long-range coupling constants by manipulation of pure phase in-phase (IP) and antiphase (AP) doublets, and is referred to as 13C-detected IPAP-INADEQUATE. It is illustrated by the measurement of interglycosidic (3)J(CCOC) coupling constants in a disaccharide molecule providing important information about the conformation of the glycosidic linkage. Owing to the simplicity of INADEQUATE spectra the carbon-carbon coupling constants are particularly suitable for studies of partially oriented molecules through the measurement of carbon-carbon residual dipolar couplings (RDCs). An example of this approach is presented. We expect the method to find a variety of applications in the conformational analysis of small molecules, determination of diastereoisomers and enantiomers, and studies of molecules in aligned media.     

Deuterium isotope effect on charge-transfer fluorescence

Quantum yields and decay times of fluorescence of charge-transfer complexes of tetracyanoethylene (an electron acceptor) with protonated and deuterated aromatic hydrocarbon donors were measured. The deuterium isotope effect on radiationless transition (identified as the

internal conversion) was observed. This observation is taken as evidence of the dominant role of intramolecular within the donor and/or the acceptor molecule) vibrations in radiationless transitions from excited charge-transfer states of molecular complexes.     

Computer-assisted determination of carbon connectivity patterns based on natural abundance one-bond 13C13C coupling constants: terpenes

With the help of the interactive computer program CABSA the carbon connectivity pattern of representative known terpenes was established by measuring the one-bond ¹³C¹³C coupling constants in the presence of the signals of molecules containing one ¹³C isotope.     

Long-range tin-tin coupling constants: II. Two-bond coupling via carbon

Tin-119 and carbon-13 NMR data for a total of 34 compounds containing the grouping Sn-C-Sn (C is either sp³- or sp²-hybridised) are presented and discussed. In organotin derivatives of alkanes, ²J(Sn-C-Sn) can only be correlated with ¹J(Sn-C₂) if a sign change for the former coupling is assumed. In most of the compounds of this type studied, ¹J(Sn-CH₃) is, due to rehybridisation and in contrast to the usual situation, larger than ¹J(Sn-C²); the same is true in some cases for distannylakenes, the behaviour of which is complicated by changes in the torsional angle about the carbon-carbon double bond. Thus correlation of ²J(Sn-C-Sn) with other spectral parameters is not possible in these cases. The total tin chemical shift range for compounds MeⁿSn(CH₂MME₃)_4-n (M  C, Si, Ge, Sn; n  0–4) is 140 ppm. Incorporation of a ditin fragment in a six-membered ring causes a downfield tin shift of 30 ppm.     

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions

This work describes the first experimental studies of deuterium kinetic isotope effects (KIEs) for the gas-phase E2 reactions of microsolvated systems. The reactions of F(-)(H(2)O)(n) and OH(-)(H(2)O)(n), where n = 0, 1, with (CH(3))(3)CX (X = Cl, Br), as well as the deuterated analogs of the ionic and neutral reactants, were studied utilizing the flowing afterglow-selected ion flow tube technique. The E2 reactivity is found to decrease with solvation. Small, normal kinetic isotope effects are observed for the deuteration of the alkyl halide, while moderately inverse kinetic isotope effects are observed for the deuteration of the solvent. Minimal clustering of the product ions is observed, but there are intriguing differences in the nature and extent of the clustering process. Electronic structure calculations of the transition states provide qualitative insight into these microsolvated E2 reactions.     

Rare gas effects on hyperfine coupling constants of BO, AlO, and GaO

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.