Experimental NMR Chemical Shifts Research Articles

Semi-Synthesis of Triterpene A-ring Derivatives from Oleanolic and Maslinic Acids. Theoretical and experimental 13C chemical shifts

Oleanolic and maslinic acids were isolated from solid waste from olive oil and several derivatives were semi-synthesised using typical reaction procedures. Experimental and theoretical (GIAO, B3LYP/6-31G*//MM+) 13C NMR chemical shifts for 21 compounds are given.

Synthesis and molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)2-1,12-C2B10H10, in the gaseous phase, determined by electron diffraction and ab initio calculations; geometrical consequences of three-dimensional aromaticity in carbaboranes 1,12-X2-1,12-C2B10H10 †

The molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)2-1,12-C2B10H10, prepared by means of an improved synthesis, has been determined by gas-phase electron diffraction restrained by ab initio calculations. The carbaborane core, shown by calculations at the MP2(fc)/6-31G* level to be very close to D5d symmetry, gave good agreement between theoretical and experimental 11B NMR chemical shifts. A model of the entire molecule in overall C2 symmetry led to an experimental geometry (RG = 0.077), in good agreement with the theoretical findings. The substituents do not distort the cage significantly. The well determined parameters, the C–B, B(2)–B(3), and B(2)–B(7) distances, 170.6(4), 177.5(3) and 176.5(9) pm respectively (ra), are consistent with the analogous parameters established experimentally for other 1,12-disubstituted 1,12-dicarbadodecaboranes. Whereas the C–B and B–B distances are found to be relatively constant in the MP2(fc)/6-31G* geometries of a series of carbaboranes 1,12-X2-1,12-C2B10H10 (in addition to SH, X = H, Li, BeH, BH2, CH3, SiH3, NH2, OH, F and Cl) the C(1)⋯C(12) distances and B(2)–C(1)–C(12) angles are appreciably sensitive to the nature of X, in a similar manner to the para-disubstituted benzene derivatives, 2 × B(2)–C(1)–C(12) being viewed as an analogue of the ipso angle in the latter.

Experimental and theoretical investigations into the stereoselectivities of peracid promoted epoxidations of substituted norbornenes and norbornadienes

MCPBA promoted diastereoselective epoxidations of functionalized norbornenes and norbornadienes have been performed. The experimental results thereby obtained have been evaluated by using semiempirical (AM1) quantum chemical modeling. The AM1 computational results are consistent with experiment and correctly predict the major product of all MCPBA promoted epoxidations studied herein. Product characterization was assisted via the results of X-ray crystallographic structural analysis in the case of 10a and by comparison of calculated vs. experimental 13C NMR chemical shifts of the cyclopropyl carbon atoms in 12a and 12b.

3-Bromo-2,5-dihydrofuran-2-one and 4-Bromo-2,5-dihydrofuran-2-one

Although 3-bromo-2,5-dihydrofuran-2-one and 4-bromo-2,5-dihydrofuran-2-one were first reported in 1894, considerable ambiguity still exists about the correct structure assignment of the two compounds. The present article gives a brief overview on the varying assignments of constitution and describes a novel method for the formation of 3-bromo-2,5-dihydrofuran-2-one. A comparison is made of the experimental 13C NMR chemical shifts with values predicted by increment calculations and experimental 1JC-C coupling constants are given for both compounds. Secure structural assignments are now available for both isomers.

Benzocyclynes adhere to Hückel's rule by the ring current criterion in experiment ( 1H NMR) and theory (NICS)

The experimental 1H NMR chemical shift values of six benzocyclynes (benzodehydroannulenes) have been assigned and, in combination with nucleus-independent chemical shift (NICS) calculations, shown to indicate diatropic behavior for 4 n+2, paratropic behavior for 4 n π systems.

Ab Initio/IGLO/GIAO-MP2 Study of Hypercoordinate Square-Pyramidal Carbocations

The structures of hypercoordinate square-pyramidal carbocations were calculated at the correlated MP2/6-31G* level. The 13C NMR chemical shifts of the cations were also calculated using IGLO and GIAO-MP2 methods. The IGLO calculated data show only a reasonable correlation with the experimental 13C NMR chemical shifts. The correlated GIAO-MP2 calculated 13C NMR shifts, however, showed significant improvements over the SCF IGLO calculated chemical shifts. It was also found that among the C8H9+ isomers, the bishomo square-pyramidal cation 7 is only 0.2 kcal/mol more stable than the trishomocyclopropenium-type ion 8 at the MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE level. Almost a calculated 1:2 equilibrium mixture of ions 7 and 8 seems to best represent the experimental NMR spectrum of C8H9+ at −80 °C. The structures and 13C NMR chemical shifts for the elusive (CH)5+ 1 and its monomethyl-substituted analogues 2 and 3 were also computed.

Ab initio calculation of NMR properties (shielding and electric field gradient) of 33S in sulfur compounds

Chemical shieldings of the sulfur nucleus have been calculated with the GIAO method for a wide range of organic and inorganic sulfur compounds, using the 6–311++G(2d,2p) basis set. The resulting data, including calculated electric field gradients, are compared with other theoretical results and experimental 33S NMR chemical shifts and line widths; with a few accountable exceptions (mostly charged species), calculated shieldings are well correlated with experimental data. The resulting relationship, joined to calculated electric field gradients at the nuclei, can be employed for predicting the chemical shift and line width of hitherto undetected signals. There is no general relationship between 33S shielding and electronic charge at sulfur.

A Theoretical Computation of the Aromaticity of (Benzene)Cr(CO)3 Compared to Benzene Using the Exaltation of Magnetic Susceptibility Criterion and a Comparison of Calculated and Experimental NMR Chemical Shifts in These Compounds

A theoretical calculation of the aromaticity of benzene relative to (benzene)Cr(CO)3 (1) based on the exaltation of magnetic susceptibility criterion was carried out using ab initio MO theory. As others have also found, benzene exhibits a diamagnetic susceptibility exaltation, Λcalc = −15.1 ppm cgs, Λexp = −13.7 ppm cgs, and is aromatic. In contrast, (benzene)Cr(CO)3 (1) has a positive susceptibility exaltation, Λcalc = 12.3 ppm cgs, characteristic of an antiaromatic compound. The validity of susceptibility exaltation as an aromaticity indicator for organometallic compounds was also tested for (cyclobutadiene)Fe(CO)3 (16), which proved to be aromatic (Λcalc = −6.10 ppm cgs). The validity of the calculations was further supported by a comparison of the calculated isotropic susceptibility χav of 1 (−109.3 ppm cgs) with an experimental result (−113 ± 22 ppm cgs). The related NMR calculations for 1 reproduce very well the 13C solid state results of Waugh and also the experimental isotropic upfield shift of ca. 2 ppm seen in the 1H NMR spectra of complex 1 relative to benzene. Contrary to the usual assumptions, the in-plane shieldings of the complexed benzene ring are more important than the perpendicular (ring current) counterparts. As expected, the present theoretical study reproduces very well the experimental geometries, energies, and harmonic frequencies of the purely organic compounds, but there is also very good agreement in the calculated properties of the organometallic compounds, where such data are available for comparison. The present study is based on GIAO, CSGT, and IGAIM NMR calculations performed on the optimized geometry of the most stable conformation at the B3LYP/6-311+G** level for the 12 organic and organometallic compounds needed directly or indirectly for the “group increment” magnetic susceptibility exaltation determinations. The organometallic structures include Cr(CO)6 (10), (ethylene)Cr(CO)5 (11), (1,3-butadiene)Cr(CO)4 (12), (benzene)Cr(CO)3 (1), Fe(CO)5 (13), (ethylene)Fe(CO)4 (14), (1,3-butadiene)Fe(CO)3 (15), and (cyclobutadiene)Fe(CO)3 (16).

Comparison of the 129Xe NMR Chemical Shift with Simulation in Zeolite Y

We report quantitative agreement between Monte Carlo simulation and the experimental NMR chemical shift of 129Xe adsorbed in the supercages of zeolite Y. This agreement supports previous assertions, originating from Ripmeester and Fraissard, that the Xe shift can in principle provide a sensitive measure of the structure imposed on Xe by the three-dimensional potential field provided by the crystal structure of the zeolite. Up to a loading of 7 Xe/cage at 300 K, we verify that the linear rise of shift with loading is due solely to Xe−Xe interaction. We also find excellent agreement at 144 K between simulation and the nonlinear experimental data of Cheung et al. The nonlinear dependence of shift on loading arises both from repulsive Xe−Xe and from repulsive Xe−O interactions. Finally we verify that the effect of temperature on the shift at zero loading can be related to the change of Xe−O pair correlation function at short separations.

HNMR Chemical Shift Information on the Conformation of the Glycosidic Bond in Disaccharides

ABSTRACT Experimental 1H NMR chemical shifts were related to results obtained from theoretical calculations, in this case the HSEA approach, in order to obtain information about conformational equilibria about the glycosidic bond of disaccharides. Assuming the existence of the exo-anomeric effect, it was shown that the sign of torsion angle Ψ can be obtained tentatively from the 1H N M R chemical shifts.