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The addition of silyl and germyl radicals to a series of substituted cyclopentenones and cyclohexenones has been investigated using EPR spectroscopy. At low temperatures the 3-position is the preferred site for addition unless it is substituted or sterically hindered in which case addition occurs at the carbonyl oxygen. The latter process is also favoured at higher temperatures but no evidence has been obtained for either addition at the 2-position or for an intramolecular migration following initial attack at the 3- or 2-position.

The synthesis and characterisation of a new phosphorylated nitrone belonging to the spiro[indoline-naphthoxazinic] family are described. The results of spin trapping experiments which indicate the ability of this photochromic nitrone to react with carbon-, oxygen- and sulfur-centred radicals are also reported.

Electron paramagnetic resonance has been used to characterise free radicals formed by the reaction of photochemically generated tert-butoxyl radicals with a range of methyl substituted cyclopentenones and cyclohexenones. The spectra have been interpreted using computer analysis and simulation and assigned to cyclic allylic and alkyl radicals formed by abstraction of a hydrogen atom. The 1-oxocyclohept-2-en-4-yl radical derived by abstraction of bromine from 4-bromocyclohept-2-enone exhibits a temperature dependent EPR spectrum attributed to ring inversion.

The addition of a variety of radicals to triphenylmethyl l, allyl 2, benzyl 3 and but-3-enyl (diethoxyphosphoryl)dithioformate 4 has been investigated by EPR spectroscopy. Reactions with the first substrate always led to displacement of the triphenylmethyl radical; release of the allylic or benzylic moieties from 2 and 3 was observed above 243 K under photolytic conditions, while at lower temperatures or in the dark the adducts resulting from thiophilic addition of the radicals were detected. The weak EPR spectra detected when reacting dithioformates 1–4 and methyl(diethoxyphosphono)dithioformate 5 with Grignard reagents are attributed to the radical anions of the starting compounds and to their ion pairs with magnesium cations.

Irradiation of a number of different sutures largely employed in the clinical practice with either high energy electrons or with g-rays followed by quenching with glycidyl methacrylate (GMA) conveniently led to derivatization through a radical-based process. The radicals involved were detected by means of ESR spectroscopy and were characterized on the basis of their ESR spectral parameters which were also found to be consistent with the hfs constants predicted by DFT calculations. Evidence of the GMA derivatization of the sutures was obtained via (13)C CP-MAS NMR spectroscopy, while its extent was evaluated gravimetrically.

The paramagnetic species detected by EPR spectroscopy upon electrochemical reduction of bis(2,4,7-tri-tert-butyl-phenyl)-1,3-diphosphaallene in THF solution has been unambiguously identified as the title radical, the identi-fication being supported by DFT calculations

Trimethylsilyl and triphenylsilylaryl thioketones have been synthesized by acid catalysed reaction of the corresponding ketones with hydrogen sulphide. Such silylated thioketones have been oxidized to give S-oxides and converted with diaryldiazomethanes into triaryl thiiranes; upon treatment with organolithium nucleophiles they undergo thiophilic addition. The ready desulphurization with triphenylphosphine of silylated thiirans to the corresponding silyl triaryl ethylenes is also reported.

The interpretation of the 1H n.m.r. spectrum of a meta-substituted biphenyl (3,3′,5,5′-tetrachlorobiphenyl) dissolved in the nematic phase of a liquid crystalline solvent showed unambiguously that in solution it has neither a planar structure nor free rotation but exists in a twisted conformation. A fairly accurate measurement of the twisting angle could also be obtained (34° 20′), the experimental error being similar to that reported in the electron diffraction studies of the vapour phase. The effects of the substituents and of the nematic solvent upon the conformation are discussed and the conclusion is reached that they are rather small and acting in opposite directions. Biphenyl is thus expected to have a similar dihedral angle in solution.

The rates for the prototropic shift of the NH hydrogen in the two isomeric triazoles [(1)= 1,2,4- and (2)= 1,2,3-triazole] have been measured at low temperature by 1H n.m.r. (100 and 300 MHz). Only the less symmetric (cs) of the two possible annular tautomers was found to be populated in the case of (1), whereas both (cs and c2v) were detected in (2). The relative proportions in the latter compound are dramatically dependent on temperature, concentration, and solvent. In either triazoles the kinetic process appears to be intramolecular and the mechanism corresponds to a 1,2 prototropic shift. The parameters of activation yielded similar ΔH‡(5.8 and 6.5 kcal mol–1) and quite negative ΔS‡(–29 and –19 cal mol–1 K–1) values, thus indicating a relatively low probability of attaining the ordered (three-membered) cyclic transition state.

The 13C n.m.r. spectra of N-acylcarbazoles at low temperature (between –5 and –140 °C) display non-equivalence of the aromatic carbons owing to restricted rotation about the N–COR bond. The free energies of activation were measured and the differences between the various substituents interpreted as due to steric effects in the alkyl, and to electronic properties in the aryl derivatives. A quantitative lanthanide-induced shift (LIS) investigation allowed a measure of the angle between the plane of the RCO group and the aromatic ring for R = Me (25°) and R = But(75°).

The low-temperature 13C n.m.r. spectra (25.16 MHz) of thiophene-2- and -3-carbaldehyde [(1) and (2)] display line-broadening effects due to the exchange between their S,O-syn- and S,O-anti-conformers. In both cases the free energy of activation for rotation of the –CHO moiety was measured, the values being 10.15 and 8.5 kcal mol–1 for (1) and (2), respectively. In the case of thiophene-3-carbaldehyde (2) the signals of both conformers were detected, the ratios being 97:3 at –140 and 93:7 at –100 °C in favour of the S,O-anti-conformer. In thiophene-2-carbaldehyde (1) the minor conformer (S,O-anti) was present in too small an amount to be unambiguously detected by a conventional spectrometer, but spectra taken with a superconducting instrument (75.48 MHz) revealed ca. 1.5% of this conformer at –100 °C.

Evidence of the existence of correlated intramolecular orientation (gear effect) has been observed in Pri2NNO (1) and Pri2NNNPh (2) by 13C n.m.r. In the first case two conformers have been detected in a 92 :8 ratio at –120 °C. The corresponding activation parameters have been measured (ΔG‡ 9.7 kcal mol–1) and the structures of the two conformers assigned. Molecular mechanics calculations reproduce all the experimental observations and indicate, in addition, that this dynamic process is not synchronous but follows a two-step pathway. In the case of (2) the existence of the phenomenon could be detected through the interpretation of the selective broadening of one spectral line, and the free energy of activation could be measured (ΔG‡ 10.2 kcal mol–1) even though the spectrum of the less populated conformer was not directly detected.

10,10-Dimethylphenothia-silin and -germin have been prepared for the first time and their properties compared with those of the corresponding tin derivative and of 9,9-dimethyithioxanthen. The 13C n.m.r. spectra of these compounds suggest increasing strain in the heterocyclic framework on going down Group IVB. The oxidation rates of sulphur to sulphinyl group with perbenzoic acid have been measured in carbon tetrachloride. The first few bands in the photoelectron (HeI) spectra are assigned by comparison with the spectra of related compounds. The reactivity sequence and the ionization energy data indicate that, for geometrical changes along the series, the interaction between the sulphur lone pair and the benzene rings decreases on going towards the Sn derivative.

Restricted rotation about the N–N bond in unhindered, non-conjugated hydrazones (e.g. Me2N–NCH2) has been detected by 13C n.m.r. below –150°C. The results indicate that the NC plane is coplanar with the dynamically averaged plane of the sp3 nitrogen, in contrast with hindered hydrazones (containing the 2,2,6,6-tetramethylpiperidinyl ring, TMP) where the NC moiety adopts a perpendicular conformation. Effects due to the slowing of ring reversal were not observed in hydrazones containing the TMP ring, but were detected in other hindered derivatives such as TMP–NH2 and TMP–NO. A most interesting feature becomes apparent in two hydrazones with the 2,6-cis-dimethylpiperidinyl (DMP) ring, DMP–NCH2 and DMP–NCHPh. In the same molecule the NC moiety is coplanar with the averaged sp3 nitrogen plane when the methyls are axial, but perpendicular when the methyls are equatorial.

Conformational isomers due to the restricted rotation of the formyl group have been detected in some pyridinecarbaldehydes and non-equivalence has been observed in the symmetric pyridine-4-carbaldehyde, by means of 1H and 13C n.m.r. spectroscopy at low temperature. Assignment of the preferred conformers in pyridine-3-carbaldehyde has been achieved mainly by means of lanthanide shift effects. It has been shown that the carbonyl group shifts the 13C resonance of the syn-ortho-carbon upfield. This can be used as a general method for identifying rotational isomers in analogous derivatives. Barriers to internal rotation have been measured for all the derivatives examined by total line shape analysis of 1H or 13C spectral patterns. In one case both measurements were carried out and substantial agreement was obtained.

Rotational barriers about the N(1)–N(2) bond in a number of 1,1-dialkyl-3-aryltriazenes (R2N·N:NAr) have been measured by line-shape analysis of the n.m.r. signals. It has been observed that the barrier is only slightly affected by the bulkiness of the alkyl groups but is substantially lowered when the triazenes contain the piperidyl ring with two (DMP) or four (TMP) methyl groups in positions 2 and 6. This feature is explained in the DMP derivatives as being due to the repulsion of the two axial cis-methyl groups; this conformation has been confirmed by X-ray diffraction. In TMP derivatives, the lowering of the barrier can be explained by assuming a non-chair (twisted) conformation for the piperidyl ring in solution. 13C N.m.r. spectra also seem to support this conformation.

It has been shown that molecules containing hindered piperidyl rings can have planar or perpendicular conformations depending on the functional group attached to nitrogen. In addition, we have shown that when the group is CRNPh (amidines) the molecule may switch from a planar to a perpendicular conformation depending on the steric hindrance generated by alkyl substituents. Also, the position of the 2,6-cis-methyls in the dimethylpiperidyl ring (axial or equatorial) depends on the overall molecular conformation (planar or perpendicular). Finally, in at least one case we demonstrated that the dynamic motion usually observed is rotation at the NC bond, as previously assumed, in that only at lower temperature could a second motion (ring reversal or nitrogen inversion) be detected.

The low-temperature n.m.r. spectra at the 13C frequency (25.16 MHz) showed the non-equivalence of the ortho- and meta-carbons in a number of substituted N-alkylanilines. Line-shape analysis, or measurement at the coalescence temperature, yielded the free energy of activation for the rotational process about the C–N bond. The ΔG‡ values were found to be linearly related to the Hammett constants for a variety of para-substituted derivatives. The barriers were found also to decrease with an increase in the size of the alkyl substituents bonded to the nitrogen atom.

Product isotope effects (p.i.e.), given by the product ratio RH : RD obtained in 1 : 1 MeOH–MeOD, have been determined for cleavages of some substituted benzyltrimethylsilanes by methanolic sodium methoxide; values (with estimated uncertainties of ±20%) given in the sequence of decreasing ease of cleavage, are: (X or XY in XC6H4– or XYC6H3CH2–SiMe3)p-NO2, 2-Me-4-NO2, or o-NO2, 10 (25 °C); p-COPh, 7 (50 °C); p-SO2Ph, 2.9 (50 °C); 2-Me-6-NO2, 10 (25 °C); p-CN, 2.0 (50 °C); 3,5-Cl2, 1.3 (50 °C); m-NO2, 1.3 (25 °C); m-CN, m-CF3, or H, 1.2 (50 °C). These values are believed to represent the kinetic isotope effects for interactions of the carbanions R– with MeOH and MeOD; as expected, except when there is unusual steric hindrance, they show an overall increase with increasing reactivity of the RSiMe3 compound, and thus with the pKa of the carbon acid RH, but remain effectively constant over a large pKa range before increasing steeply over a small pKa range. The p.i.e. values (at 50 °C) for the compounds Ph2CHSiMe3 and Ph3CSiMe3 have been redetermined, and found to be ca. 1.3 ± 0.2, consistent with the values for substituted benzyl compounds of comparable reactivity. The value for the triphenyl compound is incompatible with a reported value of the kinetic isotope effect for proton abstraction from triphenylmethane by sodium methoxide in methanol.

Low-temperature 13C n.m.r. spectra allowed the observation of two different groups of signals, corresponding to the O,O-anti- and O,O-syn-rotamers, of a number of 2-acylfurans with the following alkyl groups bonded to carbonyl: Me, Et, Pri, But, pentan-3-yl. The ratios of these rotamers in dimethyl ether were thus obtained, and the barriers for the syn-anti interconversion were found to decrease with increasing bulkiness of the alkyl groups. Although separate 13C signals could not be detected for the rotamers of the corresponding 2-acylthiophenes, line-broadening effects nonetheless allowed measurement of the interconversion barriers. Lanthanide-induced shift experiments carried out on the latter derivatives (at both 1H and 13C frequencies) showed that in all the 2-acylthiophenes investigated the S,O-syn rotamers are predominant.

1 H and 13C n.m.r. spectroscopy at low temperature allowed the detection of restricted rotation about the N–N bond in sulphinylhydrazines. In the case of the di-isopropyl derivative the existence of two conformers due to a ‘correlated orientation’(gear effect) was also discovered. The trend of the measured ΔG‡ values and the interpretation of the chemical shifts suggested conformational assignments which were supported by an X-ray diffraction investigation of the solid.

The conformational analysis of three substituted 2,2′-bithienyl derivatives has been accomplished by means of the liquid crystal n.m.r. spectroscopy. The interpretation of the spectral data allows the possibility of free rotation as well as the existence of a single planar or twisted conformation to be ruled out. Models of an equilibrium between the S-trans(= 78%) and S-cis(= 22%) rotamers were in agreement with the experimental data. The results also agree with previous e.s.r. experiments and with the predictions of theoretical calculations.