n J ( 13 C, O 1 H) coupling constants of intramolecularly hydrogen‐bonded compoundsBorisov, E. V.; Zhang, W.; Bolvig, S.; Hansen, P. E.
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-Gpmid: N/A
nJ(13C,O1H) carbon–hydrogen couplings were measured for a broad series of intramolecularly hydrogen‐bonded compounds, some of which display tautomerism. A plot of Jobs(C‐3,OH)+Jobs(C‐1,OH) vs. δOH showed reasonable correlation both for compounds displaying tautomerism and for those with localized hydrogen bonds. Ketones and aldehydes fall on one line and esters on another line corresponding to a lower sum. The 4Jobs(C‐1,OH) coupling depends on orbital overlap between the hydrogen‐bonded hydrogen and the carbonyl oxygen. This coupling can therefore also be useful for monitoring twisting of the carbonyl group out of the plane of the hydrogen bond. An interesting finding is that for aromatic compounds 3Jobs(C‐2,OH)cis is larger than that for an olefin both having the same 1H OH chemical shift. A plot of Jobs(C‐3,OH) vs. Jobs(C‐1,OH) is very useful for monitoring tautomerism, and Jobs(C‐1′,OH) and Jobs(C‐1″,OH) are both well suited for estimating the mole fractions of tautomeric systems. © 1998 John Wiley & Sons, Ltd.
Analysis of substituent effects on 31 P NMR chemical shifts: PX 2 Y moleculesDransfeld, Alk; Ragué Schleyer, Paul von
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-Tpmid: N/A
Tricoordinate phosphorus NMR chemical shifts are computed (GIAO/6–311+G**//RMP2(fc)/6–31+G*) and analyzed for PX2Y molecules (X, Y=EHn: F, OH, NH2, CH3, BH2, BeH, Li and H=‘simple first row substituents’), the ‘normal’ δ(31P) relationship with the substituent electronegativity sum, ∑ EN (∑i=13 EN(Ei); downfield shift with increasing ∑ EN) is substantiated for experimentally important molecules by the ab initio results. The ‘inverse’ δ(31P)–∑ EN trend obtained for X=F is related to negative hyperconjugation (Lp(X)→σ* (P—X′) orbital interaction). The P(OH)2Y molecules display ‘normal’ ∑ EN–shift relationships when the electronegativity of Y is low and ‘inverse’ when the electronegativity of Y is high. For some PX2Y phosphanes with simple substituents, the δ(31P) values are well related to ∑ EN. The best correlation (cc: 0.955) between δ(31P) and ∑ EN is found for the set of monosubstituted phosphanes, PH2X. The δ(31P)–EN(Y) slope (in ppm per EN ‘unit’), ranges from 162 (X=BeH), 141 (X=H) and 98 (X=CH3 and X=BH2) to ‐105 (X=F). The δ(31P) values of the PX2Y molecules are representative for the complete set of PXYZ with simple substituents. Since there is no ‘simple, general relationship’ of δ(31P) for phosphanes with less or much more electronegative substituents, EN(E)<EN(P)≪EN(E), consequently, it does not exist in the complete set. © 1998 John Wiley & Sons, Ltd.
Ab initio IGLO studies of the conformational dependences of 13 C NMR chemical shifts and α‐through ε‐substituent effects in 1‐substituted pentanesBarfield, Michael
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-Hpmid: N/A
Ab initio IGLO (individual gauge for localized molecular orbital) methods of SCF‐MO theory were used to extend studies of the conformational dependences of isotropic 13C NMR chemical shifts to n‐hexane and three 1‐substituted pentanes XCH2CH2CH2CH2CH3 (X=CN, OH, F). Isotropic shifts were obtained as a function of the torsion angles φ1, φ2 and φ3 measured about the C1—C2, C2—C3 and C3—C4 bonds, respectively, with molecular structures optimized at the HF/6–31G* level. The calculated 13C chemical shifts and substituent effects, averaged over the torsional motions, compare favorably with the experimental data. The computed stereochemical dependencies of α‐ through ε‐effects are compared with experimental values in several series of bicyclic molecules which encompass a range of the three torsion angles. Emphasized here are δ‐ and ε‐effects, which are sensitive to all three dihedral angles. Inclusion of the third dihedral angle improves the results over those based on the substituted n‐butanes, especially for δ‐methylene carbons which can be shielded or deshielded depending on φ3. Analysis of the IGLO local bond contributions in sterically crowded conformations suggests a general tendency in which C—H bonds pointing toward and away from the substituent lead to deshielding and shielding, respectively. © 1998 John Wiley & Sons, Ltd.
Dynamics of intramolecular hydrogen exchange in cyclic organohydroborate–zirconocene complexesChow, Albert; Liu, Fu‐Chen; Fraenkel, Gideon; Shore, Sheldon
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-Epmid: N/A
ABSTRACT: Proton and 11B NMR investigations of two zirconocene boracycloalkane derivatives A and B, as a function of temperature, identified two hydrogens bridged between boron and zirconium and a third attached only to zirconium. Observation of the 11B decoupled proton NMR lineshape of A and B (toluene‐d8 solution) revealed a fast intramolecular mutual exchange of the two bridged hydrogens and a slower process which exchanges each of the latter with the single hydrogen bonded to zirconium alone. Comparison of the observed 11B decoupled proton NMR spectra with calculated lineshapes which take account of the exchange processes yielded the corresponding rates and activation parameters. Proton NMR data for A and B (with 11B coupled) indicate spin coupling between 11B and the bridge hydrogens of ca. 55 Hz, subject to averaging by 11B nuclear electric quadrupole induced relaxation; its rate increases with decreasing temperature. Thus, the coupling is averaged at the lower temperatures due to 11B relaxation and on warming becomes obscured as a result of hydrogen exchange. Analysis of these latter lineshapes taking account of exchange dynamics, 11B relaxation and the 11B proton coupling constant provided dynamic parameters closely similar to those obtained from the 11B decoupled proton NMR data. At elevated temperature (50°C), broadening of the Cp proton resonance indicates that these protons undergo slow exchange with the three hydrido hydrogens. © 1998 John Wiley & Sons, Ltd.
7 Li solid‐state NMR of lithium amides: structural information from 7 Li quadrupolar coupling constantsHartung, Michael; Günther, Harald; Amoureux, Jean‐Paul; Fernandéz, Christian
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-Dpmid: N/A
Lithium‐7 quadrupolar coupling constants, χ(7Li), were measured for a number of microcrystalline lithium amides by solid‐state 7Li magic angle spinning (MAS) NMR. A linear correlation between χ(7Li) and the structural N—Li—N angle known from x‐ray diffraction work was found: χ(7Li)=(4.1±0.5)∢N—Li—N ‐(110.8±69.0). On this basis, structural information for new systems becomes available. For the polymer material (H2C(CH2)3NLi)n a ladder structure is proposed. The π‐flip of the five‐membered metallacycle formed by the coordination of tetramethylethylenediamine (TMEDA) with lithium in ((H2C(CH2)3NLi)2·TMEDA)2 was detected by 13C cross‐polarization/MAS DNMR spectroscopy. From the analysis of the temperature‐dependent lineshape of the methyl signals, a barrier of ΔG≠(293)=66 kJ mol‐1 was derived for this rotational process in the solid, which compares favourably with data derived for related compounds. © 1998 John Wiley & Sons, Ltd.
Isolation and identification by 2D NMR of two new complex saponins from Michrosechium helleriLeón, Ismael; Enríquez, Raul G.; McLean, Stewart; Reynolds, William F.; Yu, Margaret
doi: 10.1002/(SICI)1097-458X(199806)36:13 3.0.CO;2-9pmid: N/A
Two complex saponins, amole F and G, were characterized and their spectra were assigned using only 1D and 2D 1H and NMR methods. Amole F and G have seven and six monosaccharides, respectively, linked to the triterpene aglycone bayogenin. In addition to standard 2D methods, a series of TOCSY spectra with different mixing times and a high‐resolution coupled HSQC spectrum were particularly useful for assigning the monosaccharide units. It is concluded that saponins of this complexity are approaching the limit of structural complexity that can be solved by NMR alone, although the limit might be pushed further by access to ultra‐high field NMR spectrometers. © 1998 John Wiley & Sons, Ltd.