Lindsay A. Summers

Conformational and UV photoelectron spectroscopy analysis of the chalcogenobispyridines

Abstract It is now possible to review the conformational and electronic behaviours of the isomers of chalcogenobispyridines and related compounds, so providing information on the link between the geometric and electronic properties, and providing some insight into their observed and anticipated physical and biological activities. Moreover, observations can now be made on the effect of the bridging atoms (O, S, Se, Te) and positional isomerism on conformation and electronic distributions; on the balance between resonance and inductive effects; and on the energetics of conformer interconversion. As a result of the non-rigid nature of the chalcogenobispyridines and related compounds, it is necessary to determine the preferred geometries to calculate the ionization energies (IEs). For the entire family of the chalcogenobispyridines, the minimum energy structures predict the pyridine ring planes to be nearly perpendicular, allowing maximum overlap of the chalcogen valence orbitals with the π system of only one of the pyridine rings. Rotational barrier height calculations do not support the fully conjugated planar forms on steric grounds, but suggest that a range of near-isoenergetic conformers surround the minimum structure. Ab initio methods (such as restricted Hartree-Fock (HF) methods) were found to assign incorrectly the UV photoelectron spectra of these pyridine-based compounds. While HF methods were shown to be reasonably successful for the prediction of the IEs of the π-type molecular orbitals (MOs), they failed to position the IEs correctly for the long-pair nitrogen MOs (n N ), so requiring the use of computer-intensive configuration interaction (CI) methods. Bands caused by distinct conformers could not be resolved in the UV photoelectron spectra of the chalcogenobispyridines. Conformational analyses predicted low energy pathways for concerted disrotatory motion, permitting a smooth transition between near isoenergetic rotamers. The calculated IEs of the most populated conformers were found to be within the instrumental resolution, so were difficult to resolve. This was supported by the production of ‘synthetic’ spectra. Within the composite-molecule model, correlations with the IEs of pyridine and the methylchalcogenopyridines assisted in the spectral assignment of the chalcogenobispyridines. For each congener, the first IE was assigned to ionization from an anti-bonding π−n x combination in which the contribution of pyridine π character was found to decrease down the group. The spectra recorded with different radiation sources were valuable in identifying bands associated primarily with the chalcogen character.

One- electron transfer properties and herbicidal activity of diquaternary salts of 2,4-di-(4-pyridyl)-1,3,5-triazines

Abstract Diquaternary salts of 2,4-di-(4-pyridyl)-1,3,5-triazines have been prepared. The salts can be regarded as reversible one electron transfer systems with redox potentials (E0) of about −0·44 V in the pH range 7·0–8·5. 2,4-Bis-(4-methyl-4-pyridinio)-1,3,5-triazinediium dihalides are effective post-emergent herbicides at application rates of 4–6 kg/hectare.

The photoelectron spectra of thiobispyridines

Abstract The He I and He II photoelectron spectra of the six isomers of thiobispyridine were recorded and interpreted in terms of a composite-molecule model in which the sequence of the first four ionization energies is: π6−nS

Conformational analysis and photoelectron spectra of selenobispyridines

Abstract The conformational behaviour of the six isomers of selenobispyridine has been investigated using the ab initio STO-3G * //STO-3G * model. The analysis reveals that the minimum energy conformers are not planar and that energy barriers between 23–90 kJ mol −1 restrict interconversion to planar structures, thereby preventing conjugation between the p electrons of the selenium atom and the π system of both pyridine rings. The He I and He II photoelectron spectra of the six isomers of selenobispyridine support this view and so are interpreted in terms of a composite-molecule model based on the “Morino” structure in which the sequence of the first five ionisation energies is: π 6 - n Se n N ≈ n N ′ 5 - δ n Se 4 - δ n Se . This sequence is consistent with the interpretation of the He I spectra of the oxy- and thiobispyridines, but is contrary to the STO-3G * //STO-3G * and the (6-31G ** /LWD)//STO-3G * model (the latter nevertheless gives a more plausible ionisation energy distribution). Furthermore, analysis of the intensity variations on going from He I to He II radiation suggests that the selenium lone-pairs are substantially delocalised within the π system of the pyridyl rings.

Crystal structure, Raman and resonance Raman spectroscopic studies of the dimethyl diquaternary salt of 2,2′-dicyano-4,4′-bipyridine and its radical monocation

Abstract The crystal structure of the dimethyl diquaternary salt of 2,2′-dicyano-4,4′-bipyridine is reported. Crystal data: C 14 H 12 Br 2 N 4 , M r = 396.1, a = 7.1934(9), b = 10.192(1), c = 10.715(1) A, β = 103.89(1)°, V = 762.61 A 3 , monoclinic, space group P 2 1 / n , Z = 2, D m = 1.70, D c = 1.73 g cm −3 , Mo K α, λ = 0.71069 A, μ = 56.24 cm −1 , F (000) = 388, T = 293 ± 1 K, R = 0.028, R w = 0.029 for 1156 ( I > 2.50σ(I)) reflections. The cation straddles a crystallographic centre of symmetry with the two pyridine rings being coplanar. The bromide ion is in a general position. Bond lengths and angles are normal. Raman spectra of the parent dication and resonance Raman spectra of the coloured radical monocation are also reported. The CN stretching frequency for the radical monocation is only weakly enhanced and is displaced only 12 cm −1 from the corresponding feature for the parent dication.

Conformational and photoelectron analysis of isomers of the methylchalcogenopyridines

Abstract The He I/He II photoelectron spectra of 2-, 3-, and 4-methylthio- and methylselenyl-pyridine are reported and interpreted in terms of a composite-molecule model based on ab initio calculations. Results from STO-3G∗ calculations at STO-3G∗ optimised geometries suggest that, for the 2- and 4-isomers, the planar conformer is more stable than the non-planar conformer, although bands due to minor conformers are evident in some of the spectra. The He I/He II band ratios are shown to be useful in locating those MOs with significant S or Se p AO contributions, thereby indicating the chalcogen content in the bonding and anti-bonding X-π orbitals. Based on spectral features, He I/He II ratios and in conjunction with ab initio calculations, the preferred sequence for the four lowest ionisation energies for all isomers of the methylchalcogenopyridines is: π - n x < n N∼ π < π + n x, where X represents the chalcogen atom.

Bridged diquaternary salts of di-2-pyridyl ketone and their radical cations: crystal structure, raman and resonance raman studies

Abstract Raman spectra are reported for two bridged diquaternary salts of di-2-pyridyl ketone. The crystal structure of one parent dication is also reported. Crystal data: C13H12Br2N2O, Mr = 372.05, a = 9.369(1), b = 11.036(4), c = 7.115(1) A, α = 97.60(2), β = 90.12(1), γ = 67.79(1)°, V = 674.20 A3, triclinic, space group P 1 ; Z = 2, Dm = 1.83, Dx = 1.832 g cm−3, MoKα, λ = 0.71069, A, μ = 63.54 cm−1, F(000) = 364, T = 292 ± 1 K, R = 0.033, Rw = 0.032 for 1330 (I &> 2.5σ) reflections. Resonance Raman spectra of the coloured radical cations obtained from the diquaternary salts by one-electron reduction are also reported. Changes in the frequency of the carbonyl stretching vibration of the parent dications are correlated with changes in the conformation of the conjugated system. Changes in the Raman spectrum of a parent dication between the solid state and aqueous solution appear to arise from a modification of molecular geometry in solution. The decrease in frequency of the carbonyl stretching mode on radical formation is attributed to a decrease in the carbonyl bond order, due to delocalisation of the odd electron.

Conformations of 2,2′-bipyridine in acidic media

Abstract The conformational behaviour of 2,2′-bipyridine in acidic media is investigated by Raman spectroscopy. Monoprotonated and diprotonated 2,2′-bipyridine in solution are shown to exist in similar conformations to those determined in the solid state.

Resonance Raman studies of charge-transfer complexes of bipyridinium dications

Abstract Resonance enhancement was exhibited by a range of bipyridinium—iodide charge-transfer complexes. The excitation profiles measured for each species, however, did not correspond closely with the visible absorption spectra. This was attributed to interference effects in the intensity measurements caused by the existence of a second, lower-wavelength absorption band in the visible spectrum. In one complex, enhancement of non-totally symmetric modes could be established, indicating that B term resonance enhancement was occurring as the result of vibronic coupling to another excited electronic state. The position of the Raman excitation profile maximum (and the higher-wavelength absorption maximum) was correlated with the half-wave potentials of the acceptors, showing the increase in wavelength (and therefore decrease in energy) of the charge-transfer transition with increasing acceptor ability. The effect of varying the extent of charge transfer with an acceptor was studied by Raman spectroscopy. Small frequency changes in the spectra could be correlated with differences in the extent of charge transfer.

MASS SPECTRAL FRAGMENTATION PATTERN OF 2,2′‐BIPYRIDYLS. PART VII. 2,2′‐DITHIODIPYRIDINE

The mass spectral fragmentation of 2,2′ -dithiodipyridine involves loss of the elements S, SH and S2 from the molecular ion in addition to rupture of the central bonds. Molecular rearrangements accompany the disintegration.