The Radical Cation of Cyclobutene and its Photoproduct. Preliminary communication

The hitherto unknown radical cation of cyclobutene ( 2 ) has been generated in a CFCl₃ matrix by γ rays at 77 K. The coupling constants, as determined from the ESR spectrum of 2 ⁺, are 2.80 and 1.11 mT for the four CH₂ and the two CH = protons, respectively. Photo-induced ring opening of 2 ⁺ yields a radical cation which exhibits the same ESR and ENDOR spectra as those observed upon direct ionization of s-trans-buta-1, 3-diene (s-trans -1 ). The radical cation s-trans -1 ⁺, should, therefore, be the final product of this conversion.     

Electronic spectra of α,β-unsaturated carbonyl compounds—II : An evaluation of increments characteristic of structural features of β-oxy-α,β-unsaturated ketones

By comparing UV spectra of β-alkoxy-α,β-unsaturated ketones of established steric structure, spectral constants characteristic of the cis/trans configuration change and s-cis/s-trans and O-s-cis/O-s-trans conformation changes have been evaluated. These are: Δλ^cis_trans = 0, Δλ^s-cis_s-trans = 8 nm and Δλ^O-s-trans_O-s-cis = 6 nm. A comparison of cis-s-cis enol ethers with the parent enols yielded the increment for the intramolecular (“chelating”) H-bond, Δγ_chel = 24 nm. The methanol-induced bathochromic shift has been found to depend strongly on s-cis/s-trans isomerism. The substituent increments have been shown to be dependent on the degree of substitution in the reference molecule. The results obtained have been summarized in a set of spectral increments complementing the basic system of Woodward and the Fiesers.     

Proton magnetic resonance study of rotational isomerism in vinylcycloalkanes

The vicinal coupling constant, J(12), between the vinyl CH and the ring CH protons in vinylcyclohexane was calculated from a ‘partial molecule’ six-spin system. The 100 and 270 MHz results are in good agreement; those at 60 MHz were, however, still inaccurate in this approximation. J(12) increases with increasing solvent polarity and decreasing temperature. The energy difference between the s-trans and gauche conformers in both C₂Cl₄ and perdeuterioacetone solvents is 879 ± 167 J mol^?1 (210±40 cal mole^?1). The s-trans conformer is the most stable, in contrast to the isoelectronic cyclohexylcarboxyaldehyde where the gauche rotamers are lower in energy.     

An ESR Study of the Radical Cations of Tetrathiafulvalene (TTF) and Electron Donors Containing the TTF Moiety

Hyperfine data and g factors are reported for the radical cations of tetrathiafulvalene (TTF; 1 ) and of its derivatives 2–13 . From the intense satellite spectra of 1 ⁺– 13 ⁺ not only the coupling constants of the ³³S isotopes in the TTF moiety could be determined, but also, in favourable cases, those of the ¹³C isotopes in the central double bond. The former values range from 0.370 ( 8 ⁺) to 0.470 mT ( 4 ⁺) and the latter from 0.255 ( 8 ⁺) to 0.360 mT ( 4 ⁺) in the radical cations of bis(ethylenedithio)-TTF ( 8 ⁺) and tetracyano-TTF ( 4 ⁺). The radical cation of TTF ( 1 ⁺) exhibits intermediate values, 0.425 for the ³³S and 0.285 mT for the ¹³C isotopes. The spin population in 1 ⁺– 13 ⁺ resides, to a large extent, in the central S₂C ? CS₂ part of the π-system. It tends to increase (decrease) by substitution with electron-accepting (donating) groups in the 2,3,6,7-positions of TTF.     

1H and 13C n.m.r. spectra of dichloro(trans-2-chlorovinyl)arsine

Proton and carbon magnetic resonance spectra of Lewisite or dichloro(trans-2-chlorovinyl)arsine have been measured and the results are compared with the n.m.r. spectral parameters of other trans-1,2-substituted ethylenes. The coupling constants can be rationalized by substituent electronegativity. The chemical shifts show an unusually large paramagnetic effect from the AsCl₂ group.     

Conformational analysis: XIV—A 1H n.m.r. conformational study of methyl substituted 2-oxo-1,3,2-dioxathians to confirm the predominance of chair forms in the trimethylene sulphite series

Seven isomeric 4,5,6-trimethyl-2-oxo-1,3,2-dioxathians, cis-4-trans-6-dimethyl-r-2-oxo-1,3,2-dioxathian and two isomeric 4,5,5,6-tetramethyl-2-oxo-1,3,2-dioxathians were prepared and their ¹H n.m.r. spectra analysed. The values of the vicinal coupling constants reported earlier for the cis-4-trans-6 compound were shown to be erroneous. In all cases the values of the vicinal coupling constants (and those of the chemical shifts) are indicative of a single chair conformation or a chair-chair equilibrium, in contrast to earlier reports on the significant contribution of twist forms but in agreement with an electron diffraction study. The chair form is ? 31 kJ mol^?1 thermochemically more stable than the twist form.     

Application of the temperature dependence of vicinal spin-spin coupling constants to the investigation of the configuration of substituents in saturated 5-membered rings

Temperature dependence of cis- and trans-vicinal spin-spin coupling constants of substituted cyclic 5-membered compounds is discussed. The temperature dependence of trans-vicinal coupling constants is shown to be essentially stronger than that of cis-vicinal coupling constants if the energies of the conformers corresponding to potential energy curve minima are different. The temperature dependence of 33 vicinal spin-spin coupling constants has been studied for di- and trisubstituted thiophanes with a known configuration of substituents. Experimentally determined changes of trans-vicinal coupling constants with temperature are markedly larger than those for cis-vicinal coupling constants, whose values, in most cases, are practically temperature independent. Differences in the temperature dependence of cis and trans-vicinal spin-spin coupling constants can be applied for the determination of the configuration of substituents in saturated 5-membered rings.     

Constantes de couplage de vinylallènes

In order to define the influence of an allenic type conjugation, the NMR spectra of vinylallene and of some of its methyl derivatives are analysed The signs of the coupling constants are also determined. These compounds are characterised by a planar s-trans structure of the two noncumulated double bonds. A discussion of the parameters, and especially of the methyl replacement effects, evidences the importance of the π type contributions. The theoretical calculations of the coupling constants by the methods of finite perturbation CNDO and INDO corroborate this interpretation.     

The Radical Anions of [2.2]Paracyclophane-1,9-diene and Some of its Derivatives. An Unexpected Conformational Interconversion

The radical anions of [2.2]paracyclophane-1,9-diene ( 2 ) and its 1,10,12,13,15,16-hexadeuterio derivative 2 -D₆, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-1,9-diene ( 3 ) and its 12,13,15,16-tetradeuterio derivative 3 -D₄, have been studied by ESR spectroscopy. The coupling constants for 2 ^?· at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D_2h symmetry. The ESR spectra of 2 ^?· are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D_2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D₂, i.e., the benzene rings in 2 ^?· are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2 -D₆^?· and 3 ^?·. In the case of 3 ^?·, the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2 ^?· and 3 ^?· gradually convert into the radical anions of [2.2]paracyclophane ( 1 ) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2 ^?· also occurs, with the formation of the radical anion of (E)-4,4′-dimethylstilbene ( 7 ). Both reactions of 2 ^?· must involve the transient radical anion of [2.2]paracyclophane-1-ene ( 4 ) as proved by the observation of the spectra of 1 ^?· and 7 ^?· with 4 as the starting material.     

trans‐Di­chloro­bis­(tri­phenyl­phosphine‐P)­platinum(II)

Two different crystals (A and B) were used to structurally characterize trans‐[PtCl₂(PPh₃)₂] and to study random and systematic errors in derived parameters. The compound is isomorphous with trans‐[PdCl₂(PPh₃)₂] and with one of the polymorphs of trans‐[PtMeCl(PPh₃)₂] reported previously. Half‐normal probability plot analyses based on A and B show realistic s.u.'s and negligible systematic errors. R.m.s. calculations give very good agreement between A and B, 0.0088 Å. Important geometrical parameters are Pt—P = 2.3163 (11) Å, Pt—Cl = 2.2997 (11) Å, P—Pt—Cl = 87.88 (4) and 92.12 (4)°. Half‐normal probability plots and r.m.s. calculations were also used to compare the title compound with the palladium analogue, showing small systematic differences between the compounds. The torsion angles around the Pt—P bond were found to be very similar to those reported for isomorphous complexes, as well as to the torsion angles around the Pt—As bond in trans‐[PtCl₂(AsPh₃)₂]. The NMR coupling constants for the title compound are similar to Pt—P coupling constants reported for analogous trans complexes.     

Studies on olefins. IX—proton NMR study of some CH2CR1R2 and CHR3CR1R2 alkenes

The proton NMR spectra of 2,3,4,4-tetramethyl-3-t-butylpent-1-ene rotarners have been completely assigned by low temperature NOE measurements. Chemical shifts and cis and trans allylic coupling constants are unambiguously determined. It is shown that other 2-substituted propenes can be assigned on the basis of the coupling constants, but not from the chemical shift data. In 1,2-disubstituited propenes, however, the transoid coupling constant falls in the range of the cisoid coupling constant values of the 2-substituted propenes. Coupling constants cannot, therefore, be used in the 1,2-disubstituted propene series as a criterion for determining structure.     

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho- and peri-interactions involving the carbonyl substituent

Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s-t)>²J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the C?O group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s-c)<³J(t,s-t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s-c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three-bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.     

Conformations of dibenzylideneacetone: An IR spectroscopic study

The conformational analysis of dibenzylideneacetone has been carried out using IR spectroscopy. Appearance of a triplet C=O band is attributed to the coexistence of three conformers viz s-cis, cis, s-cis, trans and nonplanar s-trans, trans in contrast to the earlier findings which showed the existence of only two conformers. The relative proportions of the conformers are in the order s-cis, trans> nonplanar s-trans, trans> s-cis, cis in less polar solvents and nonplanar s-trans, trans> s-cis, trans> s-cis, cis in more polar solvents.     

An NMR,ESR, ENDOR and TRIPLE resonance investigation of a cation radical formed from 1,2,4,5-tetramethoxybenzene

Several methods have been established for preparing cation radicals from 1,2,4,5-tetramethoxybenzene that allow highly resolved ESR spectra to be recorded. Precise values of the hyperfine coupling constants for the aromatic and methoxy protons have been obtained; the values are 0.2268±0.0004 and 0.0863±0.0002 mT, respectively, with dichloromethane as solvent. No temperature dependence is evident. TRIPLE resonance experiments showed that both coupling constants have the same sign. NMR experiments provided contact shift and line broadening measurements; these proved that both the above constants are positive and led to a value of 3.1 (±0.3)×10⁸M^?1 s^?1 at 23°C for the rate constant for electron exchange between the cation radical and the parent compound.     

Conformational study of phosphoserine in aqueous solution. I—13C n.m.r. results

¹³C chemical shifts and coupling constants of phosphoserine in aqueous solutions were studied as a function of pH values. Carboxyl and α-amino titration shifts agree with those observed in amino acids. The analysis of the coupling constants indicates that for rotation about the (C)? C? O? (P) axis the trans conformer predominates for all pH values. The fractional population of the gauche conformer reaches a maximum at pH=8.     

Application of the Seth-Paul-Van Duyse equation—II : Transmission of polar effects by the CHCH group

The use of the improved Seth-Paul-Van Duyse equation (SPVDE) provides a new statistically significant method for determination of transmissive factors of polar effects for various bridge groups. The method was applied to the CO stretching frequencies of s-cis and s-trans conformers of 98 R₁CHCHCOR₂ compounds. The transmissive factors for CHCH group in both conformations s-cis and s-trans have been calculated with an accuracy about of one order higher than found by using the methods described earlier. The applicability of the improved SPVDE to the 137 CO stretching frequencies of various R₁CHCHCOR₂ compounds has been proved.     

A study of 5-membered ring conformations by the temperature dependence of the vicinal spin–spin coupling constants. Conformations of cis- and trans-isomers of 4-benzoylamino-3-acetoxythiophane

Conformational equilibrium in cis- and trans-4-benzoylamino-3-acetoxythiophanes has been analysed on the basis of the temperature dependence of the vicinal spin–spin coupling constants. The conformations of the compounds have been established. It has been shown that in problems of this kind the real accuracy of the experiments corresponds to a wide range of spin-spin coupling constant values and differences between the conformer energies. To narrow the range, additional data have been used, such as the temperature dependence of other vicinal coupling constants in the same molecule, angular dependence of the vicinal constants, etc.     

The crystal structure and conformation of N-[2,2-dimethyl-3-(2-methylpropenyl)- cyclopropylcarbonyl]-5-methylisoxazolin-3-one

The title compound has been synthesized and its crystal structure was determined by X-ray crystallographic method. The crystal is triclinic, space group P-1, with unit cell dimensions a = 6.146 (5), b = 8.473(6), c = 14.383(5) Å; α = 77.47 (4), β = 82.84 (4), γ = 69.00 (8)° and Z = 2. The results obtained reveal that the molecule of the title compound keeps a long conjugative system involving CC double bond, cyclopropane ring, carbonyl group and isoxazolinone ring and adopts a low energy conformation including s-trans of CC double bond, s-cis of carbonyl group with respect to the three-membered ring and like-s-trans of carbonyl group with the carbonyl group within the heterocyclic moiety.     

Conformational studies on 2-fluoro-1,2-disubstituted ethanes by NMR spectroscopy. Influence of electronegativity on vicinal proton–proton and fluorine–proton coupling constants

The analysis of the ABKX spectra of thirteen compounds of the series RC(H-K)(F-X)C(H-A)(H-B)X gave the four vicinal proton-proton and fluorine-proton coupling constants. These coupling constants of conformationally mobile structures were used (i) to calculate the populations of the rotational states of the ? CHF? CH₂? bond, (ii) to calculate the vicinal trans proton-proton J(HH)^t and gauche and trans fluorine-proton coupling constants J(FH)^g and J(FH)^t and (iii) to give the unambiguous assignment of protons H-A and H-B. The dependence of the gauche and trans coupling constants with substituent electronegativity is explored. The results extend known correlations towards smaller electronegativity values. More quantitatively, the results and those in the literature, excluding those where deformations of torsional or bond angles occur, give a good fit of the data: a linear fit for J(HH)^t = 15.0-0.77 Σ(ΔE), an exponential fit for J(FH)^g = 15.35 exp [-0.266 Σ (ΔE)] and a linear fit for J(FH)^t = 65.75 - 7.52 Σ (ΔE), where Σ (ΔE) is the sum of the electronegativity difference between hydrogen and the six atoms or groups on the CH? CF fragment.