Vibrational spectra of 1,3,5-hexatriene and methylated derivatives

The ground state vibrational spectra of E-hexatriene, Z-hexatriene, E,E-heptatriene, E,E,E-octatriene, and E-2,5-dimethyl-hexatriene are reported. For E- and Z-hexatriene complete vibrational assignments, and for heptatriene, octatriene, and dimethyl-hexatriene near-complete assignments are obtained. The effects of terminal and lateral methyl-substitution are compared.     

The excited electronic states of all-trans-1,3,5-hexatriene

Extensive configuration interaction calculations based on ab initio wavefunctions including diffuse basis functions are reported for all-trans-1,3,5-hexatriene. Using these results we have assigned the one-photon spectra of Gavin and Rice and the electron-impact spectra of Kuppermann, and we have confirmed the assignment of the two-photon spectra of El-Sayed. The valence 2 ¹A_g state is found to lie above the strongly allowed valence 1 ¹B_u state.     

Fluorescence of derivatives of all-trans-1,6-diphenyl-1,3,5-hexatriene

Fluorescence quantum yields and lifetimes have been measured for ring-substituted derivatives of all-trans-1,6-diphenyl-1,3,5-hexatnene in several non-polar solvents over a range of temperature. The fluorescence quantum yields were high in all non-polar solvents and showed small decreases with increasing temperature. The fluorescence lifetimes varied with the nature of the substituent group and showed a marked solvent dependence. The results are interpreted in terms of a model in which thermal repopulation of S₂ from S₁ proceeds at a rate which is comparable to radiative and non-radiative decay from S₁. It is shown that neglect of Franck-Condon packing strain effects leads to overestimates of the true S₂-S₁ energy differences. Significant S₂-S₀ emission can interfere with attempts to quantify enhancement of the S₁-S₀ transition by intensity borrowing from the S₂-S₀ transition, leading to serious overestimates of vibronic coupling matrix elements.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.     

The synthesis and facile thermal electrocyclization of 3-vinyl-1,3,5-hexatriene

3-Vinyl-1,3,5-hexatriene undergoes an extremely facile thermal electrocyclization to yield 2-vinyl-1,3-cyclohexadiene. Cyclization is at least thirty times faster than 3-ethyl-1,3,5-hexatriene, the saturated analog. Activation parameters indicate that this electrocyclization is one of the most facile yet observed for an acyclic 6π-electron system.     

A theoretical study of the structure and spectrum of 1,3,5-hexatriene

Ab initio STO-3G calculations are performed on tTt- and tCt-1,3,5-hexatriene, including a partial geometry optimization on the tTt-isomer. These results are compared with the conclusions of Traetteberg based on an analysis of electron diffraction data. Some significant discrepancies are found, particularly with respect to the relative lengths of the central and terminal double bonds. In addition, a CNDO/S spectral analysis is performed at several different geometries and is compared with experiment.     

Resolution of conformer-specific all-trans-1,6-diphenyl-1,3,5-hexatriene UV absorption spectra

All-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) exists in solution as a mixture of s-trans,s-trans and s-cis,s-trans conformers. The latter is higher in energy, and its contribution increases with increasing temperature. ttt-DPH UV absorption spectra broaden with increasing temperature and undergo blue shifts with decreasing polarizability. We describe here the resolution of two spectrothermal matrices of ttt-DPH UV absorption spectra into two conformer-specific components. The first matrix consists of DPH spectra measured in n-dodecane in the 283 to 374 K T range and the second of ttt-DPH absorption spectra measured in the even numbered n-alkanes (n-C(8)-n-C(16)) at temperatures selected to achieve isopolarizability (284-372 K). Principal component analysis (PCA) treatments showed that reasonable two-component systems are attained by compensation for T-induced broadening and shifting in the pure conformer spectra. The self-modeling (SM) method used to resolve the n-C(12) matrix is successfully tested on a simulated matrix closely mimicking ttt-DPH experimental spectra in n-C(12). Compensation for nonlinear effects yields robust two-component matrices from the experimental spectra. Their resolution into pure component spectra is based on the application of the Lawton and Sylvestre (LS) non-negativity criterion at the spectral onset to define the spectrum of the low energy s-trans-conformer and the optimum linearity van't Hoff (vH) plot criterion to find the spectrum of the higher-energy s-cis-conformer. Resolved spectra are somewhat sensitive to the choice of the spectral region in which the LS criterion is applied. The surprising result is that both resolutions lead to the conclusion that the molar fraction of the s-cis-conformer equals, or even exceeds, the molar fraction of the s-trans-conformer as the highest T's employed in our study are approached.     

Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene

Summary The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 1¹ B ₂ state 0.04 eV below the 2¹ A ₁ state. 16 excited states were studied incis-butadiene, using a CASPT2 optimized ground state geometry. The 1¹ B ₂ state was located at 5.58 eV, 0.46 eV below the 2¹ A ₁ state and 0.09 eV above the experimental value. No experimental assignments are available for the 15 other transitions. On leave from: Departmento de Quimica Física, Universidad de Valencia, Dr. Moliner 50, Burjassot, E-46100-Valencia, Spain     

Site selective cycloaddition reaction of 1 -methoxy- 1-trimethylsiloxy-1,3,5-hexatriene with dienophile

The Diels-Alder reaction of the title triene occurred at the site of C-3-C-6 rather than of intuitively favored C-1-C-4, as the result of the prevailing steric effect over the orbital effect.     

Matrix-isolation infrared and ultraviolet spectroscopic studies of less stable conformers of 1,3,5-hexatriene

The infrared spectra of the 3-trans and 3-cis isomers of 1,3,5-hexatriene in low-temperature Ar matrices deposited from a high-temperature nozzle or deposited under irradiation of the Hg 253.7 nm light show a number of new bands. Correspondingly, the ultraviolet spectra of the 3-trans isomer observed under similar experimental conditions show a new absorption at 276 nm. Most of these new infrared bands and the new ultraviolet absorption are attributable to less stable isomers which have the planar s-cis conformation [or gauche conformation(s) close to the planar s-cis] about either one or both of the two CC bonds.     

Solvent dependent conformational isomerism and ligand oxidation of novel Ru(II) cyclen complexes

The synthesis of cis-[Ru(II)(cyclen)(L)(x)](n+) (cyclen = 1,4,7,10-tetraazacyclododecane and L = 2,2'-bipyridine (bpy), phenanthroline (phen) or 4-cyanopyridinium (4-NCpyH(+))) is reported. The freshly prepared complexes are stable in aprotic solvents and cyclen undergoes oxidative dehydrogenation reaction at high pH. These compounds also present solvent dependent conformational isomerization.     

A theoretical study of the structure and spectrum of 1,3,5-hexatriene,Part II

The results of ab initio STO-3G and STO-4G calculations are reported on tTt- and tCt-1,3,5-hexatriene, including a STO-3G partial geometry optimization of all C/C bond lengths in both isomers. Results are also reported from a partial INDO geometry optimization on.the tTt-isomer, and complete optimizations on both isomers using empirical consistent force Meld programs. Significant discrepancies are found to exist between these calculated structures and those based on the electron diffraction analysis of Traetteberg. In addition, further results of a CNDO/S spectral analysis are presented and compared with experiment.     

Syntheses of copper(I)cis-1,3,5-tri-iminocyclohexane complexes

Copper(I) complexes of the ligand cis-1,3,5-tris(cinnamylideneamino)cyclohexane (L) have been prepared from a versatile precursor complex, [Cu(I)(L)NCMe]BF4, which incorporates a labile acetonitrile ligand that can be exchanged to give a range of new Cu(L)X complexes (where X = Cl, Br, NO2, SPh). 1H NMR spectra and X-ray structures of the Cl, Br and NO2 complexes show L coordinated in a symmetric fashion about the copper centre. The complexes have been further characterised using UV/Visible spectroscopy and cyclic voltammetry. CuLCl shows an electrochemically reversible Cu(I/II) redox couple at 0.51 V (vs. Ag/AgCl) while the CuLNO2 complex shows an analogous quasi-reversible wave at 0.41 V (vs. Ag/AgCl).     

A gas electron diffraction study of the conformational composition of 1,3,5-trimethyl-1,3,5-triazacyclohexane

The conformational composition of 1,3,5-trimethyl-1,3,5-triazacyclohexane was studied by gas electron diffraction and quantum-chemical calculations at the density functional theory (B3LYP) and MP2 levels. Conformers with the general chair, boat, and twist ring forms were considered possible. These structures differed in the arrangement of CH₃ groups in the axial (a) and equatorial (e) positions. A chair conformer with the axial orientation of one CH₃ group was found to satisfy the electron diffraction data. Its main structural parameters (mean values) were r _g(C-N) = 1.463(3) Å, r _g(C-H) = 1.117(5) Å, ∠(C-N-C) = 110.91(1)°, and ∠(N-C-N) = 111.1(1)°.     

NMR relaxation study of crosslinked cis-1,4-polybutadiene

Proton relaxation measurements have been used to investigate the effects of crosslinking on the segmental motion in cis-1,4-polybutadiene samples. The temperature dependence of proton spin–lattice relaxation time T₁ and spin–spin relaxation time T₂ at 60 and 24.3 MHz are reported in cis-1,4-polybutadiene (PB) samples with different crosslink density including uncrosslinked PB and samples with 140, 40, and 14 repeat units between crosslinks. In addition, spin-lattice relaxation times in rotating coordinate frame, T_1p, have also been determined. The relaxation data are interpreted in terms of the effects of crosslinks on segmental chain motions. Because of their sensitivity to low-frequency motion, T₂ data are of major interest. At temperatures well above the T₁ minimum the small T₂ temperature dependence resembles solidlike behavior reflecting the nonzero averaging of dipolar interactions due to anisotropic motion of the chain segments between crosslinks. The magnitude of T₂ at 60°C is found to be proportional to the average mass between crosslinks.