Conformational analysis XX—13C NMR studies of saturated heterocycles 5—substituent effects on the 13C chemical shifts of methyl substituted 1,3-dithiolanes

The ¹³C NMR chemical shifts for 1,3-dithiolane and 13 methyl substituted derivatives are reported. Substituent effects are derived and compared with those for cyclopentanes and 1,3-dioxolanes. The magnitude and variety of the substituent effects are best explained with the aid of a half-chair conformation where the S-1? C-2? S-3 plane passes between C-4 and C-5.     

Tritium nuclear magnetic resonance spectroscopy. 14—analysis of tritiated methyl groups

The complete analysis of the tritium distribution in tritiated methyl groups in labelled compounds is achieved by a direct method for the first time, through ³H NMR spectroscopy aided by resolution enhancement.     

Unusual temperature effects on the non-equivalence of geminal methyl groups

The geminal anisochronism (Δδ) of the isopropyl methyl groups in the ¹H n.m.r. spectrum of N-[2-methyl-1-(1-naphthyl)propylidene]benzylamine showed unusually pronounced temperature and solvent effects. It is noteworthy that both accidental and dynamic equivalences were encountered. Slow rotation around the naphthyl–imino bond is responsible for the diastereotopic nature of the geminal methyl groups, and possible reasons for the temperature and solvent effects are considered. Surprisingly, the temperature effect was not reflected in the ¹³C anisochronism of the methyl carbon nuclei. These observations underline the need for caution in drawing firm conclusions from variable temperature studies on geminal non-equivalence.     

Studies on the biosynthesis of cholesterol—7 Rearrangement of methyl groups during enzymic cyclisation of squalene

The present state of knowledge of the biosynthesis of cholesterol is summarised. An experiment is described wherein a special application of the carbon isotope ¹³C permits a decision between possible modes of rearrangement during the enzymic cyclisation of squalene.     

Ionisation and appearance potentials in the evaluation of nonbonded interactions—IV: Conformational effects in methyl-substituted 1,3-oxathianes

The ionization and appearance potentials for the molecular and [M ? Me]⁺ ions of several stereoisomeric methyl-1,3-oxathianes were measured. The differences between the ionisation and/or appearance potentials are shown to correlate well with the conformational energy differences in question.     

The behaviour of stereoisomeric ions in the gas phase: 2—Negative and positive chemical ionization of cyclohexanehexacarboxylic methyl esters

The positive ion chemical ionization (CI (isobutane)), negative ion chemical ionization (NICI) electron attachment (CH₄, He) and NICI (OH^?) spectra of the title compounds have been studied in detail with the aid of deuterium-labelled derivatives. The obtained results show that under CI conditions the stereospecificity is retained. Interesting correlations with the condensed phase epimerization yields are emphasized.     

Stereochemical aspects of proton chemical shifts. VI—increments due to geminal hydroxyl,methyl substitution

In geminal methyl, hydroxyl substitution an axial hydroxyl, equatorial methyl arrangement has a downfield effect on a syn axial hydrogen atom which is larger by 0.3 ppm than an axial methyl, equatorial hydroxyl arrangement. It is proposed that these observations may constitute a basis for configurational assignment.     

Electron-impact studies. LXXXIV—Substituent effects in the negative ion mass spectra of aryl esters

Linear plots may be obtained for the fragmentations of aryl p-cyanobenzoates using the McLafferty approach, but the correlations are not as satisfactory as those observed¹ for aryl nitrobenzoates. Some satisfactory correlations of substituent effects are noted in the negative ion spectra of substituted benzyl-p-nitrobenzoates and substituted phenyl-(p-nitrophenyl)acetates. Pronounced rearrangement peaks are observed in the spectra of substituted phenyl-(p-nitrophenyl)acetates. Appearance potential studies suggest that the similarities in ρ values obtained¹ for the fragmentations of aryl m- and p-nitrobenzoate molecular anions are probably due to the correspondence between Hammett σ values and appearance potentials in the two series.     

Chemical shift non-equivalence of the isopropyl methyl groups of two 10-isopropoxyergoline-8β-methanol derivatives

The chemical shift non-equivalence values (about 1 ppm) of the isopropyl methyl groups in the ¹H and ¹³C NMR spectra of the compounds having structure 1 is interpreted as due to the existence of a highly preferred conformation.     

Conformational preference of substituted naphthalenes: 1—Aldehyde derivatives of naphthalene

Long-range proton-proton coupling constants between the aldehyde proton and the ring protons were used to deduce the conformational preference of 1-naphthaldehyde, 2-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, 2-methoxy-1-naphthaldehyde and 1-bromo-2-naphthaldehyde. Molecular orbital calculations at the CNDO/2 and INDO level were used to calculate the stability and the long-range proton-proton couplings of different conformers of 1- and 2-naphthaldehyde. The Z conformer was found to be more stable in 1-naphthaldehyde and the E conformer was more stable in 2-naphthaldehyde.     

Conformational and energetic properties of the ammonia dimer—comparison of post-Hartree—Fock and density functional methods

The equilibrium structure of the ammonia dimer has been investigated with density functional and MP2 calculations. We used Slater- and Becke-exchange functionals combined with correlation functionals as recommended by Vosko-Wilk-Nusair, by Perdew, and by Lee-Yang-Parr, respectively. The potential energy surfaces was investigated. The asymmetric cyclic “microwave” structure could be identified as a minimum. Optimization of the intermolecular parameters showed that this structure has nearly the same energy as the centrosymmetric cyclic structure. Full optimization transformed the asymmetric cyclic structure into the linear structure. The interaction energies in the dimer were corrected for the basis set superposition error using the Boys-Bernardi counterpoise method and the a priori chemical Hamiltonian approach, respectively. © 1996 by John Wiley & Sons, Inc.     

Studies in negative ion mass spectrometry. V—A Comparison of positive a positive and negative ion mass spectra of copper(II) complexes of bidentate schiff base ligands

70 eV positive and negative ion mass spectra of a series of copper(II) Schiff base complexes have been obtained consecutively under the same ion source conditions. The characteristic feature of the negative ion spectra is their extreme simplicity relative to the corresponding positive ion spectra, the only ions present in significant abundance being the molecular anions and ligand ions. The influence of substituents (R) on positive and negative ion fragmentation patterns is discussed. Metastable peaks have been obtained in all cases for the transition [Cu(Ligand)₂]^? → [Ligand]^?.     

pH-independent large-volume sample stacking of positive or negative analytes in capillary electrophoresis

In capillary electrophoresis, the short optical path length associated with on-column UV detection imposes an inherent detection problem. Detection limits can be improved using sample stacking. Recently, large-volume sample stacking (LVSS) without polarity switching was demonstrated to improve detection limits of charged analytes by more than 100-fold. However, this technique requires suppression of the electroosmotic flow (EOF) during the run. This necessitates working at a low pH, which limits using pH to optimize selectivity. We demonstrate that LVSS can be performed at any buffer pH (4.0-10.0) if the zwitterionic surfactant Rewoteric AM CAS U is used to suppress the EOF. Sensitivity enhancements of up to 85-fold are achieved with migration time, corrected area, and peak height reproducibility of 0.8-1.6%, 1.3-3.7%, and 0.8-4.9%, respectively. Further, it is possible to stack either positively or negatively charged analytes using zwitterionic surfactants to suppress the EOF.     

Electron-impact studies. LXXXVII—ortho effects in the negative ion mass spectra of salicylates and anthranilates

The molecular anions of the methyl and phenyl esters of 4-nitrosalicylic acid and 5-nitroanthranilic acid eliminate ROH (R ? Me or Ph) by an ortho rearrangement. The molecular anions of phenyl 3-hydroxy-5-nitrobenzoate and phenyl 3-amino-5-nitrobenzoate lose PhOH to a lesser extent, and a study of the pressure dependence of these eliminations indicates the operation of simultaneous two-stage cleavage reactions. The competitive ortho effects observed when the nitro group is adjacent to either the ester or hydroxyl functions are described.     

Ortho,Effects in organic molecules on electron impact: X—unusual ortho effects of the methyl group in o-picolinotoluidide

The mass spectrum of o-picolinotoluidide gives rise to three major fragments at m/z 184, m/z 169 and m/z 168, corresponding to the loss of CO from the molecular ion followed by the loss of ?H₂ and ?H₃ by independent pathways. It has been shown that the ortho methyl group and the nitrogen of the pyridine ring in the 2-position are involved in the formation of these three major fragments observed in the mass spectrum of o-picolinotoluidide. The mass spectrum of 2-(o-toluidino) pyridine, the molecular ion of which can resemble the [M? CO]⁺ ion in o-picolinotoluidide, also shows loss of CH₃ and NH₂ radicals from the molecular ions. Based on these observations coupled with the high resolution data, the mass analysed ion kinetic energy spectrometry and high voltage scans of these fragments in both the compounds, two mechanistic pathways have been proposed for the formation of these ions in o-picolinotoluidide.     

Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles

The electron-deficient pentaarylborole 1-(2′,4′,6′-tris(trifluoromethyl)phenyl)-2,3,4,5-tetraphenylborole (1) has been synthesised with the long-term aim of developing borole-based optoelectronic materials. The bulky 2,4,6-tris(trifluoromethyl)phenyl (FMes) group on the boron atom of 1 significantly improves (>600 times) its air stability relative to its mesityl analogue. Moreover, 1 shows good thermal stability without undergoing the dimerisation or isomerisation reactions reported for some other boroles. A triarylborole analogue (2), belonging to a new class of borole with the 3- and 4-positions of the BC₄ ring linked by a –(CH₂)₃– group, has also been synthesised to elucidate the influence of carbon-bonded substituents on the stability of boroles. Both boroles were prepared through the reaction of Li[FMesBF₃] and divinyldilithium reagents, a new and general method for borole syntheses. Compound 2 was found to isomerise through a [1,3]-H shift and double-bond rearrangement to an s-trans-butadienylborane species under highly basic (NaOH) conditions. The increased steric crowding at the boron centre through incorporation of the FMes group does not preclude binding of Lewis bases to either 1 or 2, as demonstrated by their fully reversible binding of pyridine. Interestingly, 1 exhibits a blue-shifted absorption spectrum, as compared with its mesityl analogue, a result contrary to previous understanding of the influence of substituent electronics on the absorption spectra of boroles. Most importantly, these boroles exhibit much greater air-stability than previously reported analogues without sacrificing the strong electron-accepting ability that makes boroles so attractive; indeed, 1 and 2 have very low reduction potentials of –1.52 and –1.69 eV vs. Fc/Fc⁺, respectively.     

PMR investigation of organic cations—I: Trithienyl methyl carbonium ions

PMR spectral parameters are reported for the isomeric trithienyl carbonium ions. The low field shifts, relative to the corresponding carbinols, are discussed in relation to the delocalization of the positive charge into the thiophene ring bonded in position 2 or 3. A linear correlation is found between chemical shifts and π-charge densities and the proportionality constant is higher than that previously found for benzene derivatives. Vicinal coupling constants in these systems are satisfactorily in agreement with the corresponding calculated π-bond order.     

Conformational aspects of glutathione tripeptide: electron density topological & natural bond orbital analyses

Glutathione tripeptide (γ-glutamyl-cysteinyl-glycine) is a flexible molecule and its conformational energy landscape is strongly influenced by forming intramolecular hydrogen bond, its charge and the environment. This study employs DFT-B3LYP method with the 6-31+G (d,p) basis set to carry out conformational analysis of neutral, zwitterionic, cationic, and anionic forms of glutathione. In analyzing the structural characteristics of these structures, intramolecular hydrogen bonds were identified and characterized in details by topological parameters such as electron density ρ(r) and Laplacian of electron density $ \nabla^{2} $ ρ(r) from Bader’s atom in molecules theory. Charge transfer energies based on natural bond orbital analysis are also considered to interpret these intramolecular hydrogen bonds. Our results show that these hydrogen bonds are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the stabilization energy of hydrogen bond increases. Furthermore, the back bone and side chain (Ramachandran map) orientations of various ionic forms of glutathione have been studied and conformation of each constitution of glutathione tripeptide (i.e., Glu, Cys, and Gly moieties) was determined. In most species side chain conformation were found to be hindered gauche–gauche orientation by intramolecular hydrogen bonds.