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.     

A theoretical study of the structure and vibrational spectrum of trifluoromethanol

Ab initio molecular orbital calculations have been applied to trifluoromethanol. The potential barrier for rotation has been computed. The internal coordinate valence force field is presented and harmonic vibrational frequencies of all the normal modes are reported. Theoretical results are in good agreement with the few observed frequencies.     

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     

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.     

A molecular orbital study comparing the effects of heteroatom substitution in trans-1,3-butadiene,all-trans-1,3,5-hexatriene,and benzene

To gain more understanding of the nature of the perturbation that results from heteroatom substitution in conjugated polyenes and in an aromatic ring environment, we have carried out calculations ontrans-H₂C=X(H)-CH=CH₂, all-trans-H₂C=CH-X(H)=CH-CH=CH₂, and with X = C, N, Si, and P, using the 6-31G^*(5D) basis set with full geometry optimization. Linear relationships are found between (a) the C-X and C-C bond lengths, (b) the total overlap population in the C-C, C-N, C-Si, and C-P bonds and the bond length, and (c) the total atomic charge on the N, Si, and P atoms and the corresponding C atom in the various structures, and the electronegativity of N, Si, P and C. Whereas Si is more strongly bonded in the diene and triene compared to the aromatic ring, P, like N, appears to be bonded equally well in all three structural environments.     

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.     

A theoretical CNDO CI study of the electronic spectrum and structure of a spiropyran

The electronic structure and spectrum of a model compound of a spiropyran were investigated using an all valence-electrons CNDO CI method. The π → π^* electronic excitations are localized on a given half of the molecule. The photochromic process is discussed on the basis of charge densities and bond orders.     

Experimental and theoretical investigation of the first overtone spectrum of 1,3,5-trinitrotoluene

Vibrational overtone spectroscopy is a powerful tool for studying intramolecular and intermolecular interactions. We report on a combined experimental and modeling study of the C-H stretch first overtone of bulk 1,3,5-trinitrotoluene (TNT) and TNT on fumed-silica powder. We recorded the overtone spectra by laser photoacoustic spectroscopy and compared them with those predicted with the harmonically coupled anharmonic oscillator model in the 5600-6600 cm(-1) region. The model systems included single molecules and hybrid quantum and molecular mechanical (QM:MM) clusters to account for the effects of intermolecular interactions on the observed spectra. We performed the hybrid QM:MM calculations at the HF/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) levels of theory and with the universal force field (UFF) to account for van der Waals and electrostatic effects from surrounding molecules. Overtone spectra calculated from the MP2 level of theory, using a HF/3-21+G* calculation to assign molecular charges in the MM layer, and the Merz-Singh-Kollman population analysis for assigning partial charge in the QM layer and determining the transition dipole moment agreed best with the experimental data.     

A theoretical study of the vibronic structure in the electronic spectrum of HNO+

The results of an ab initio study of the vibronic and rotational structure in the ²Π state of HNO⁺ are presented. It is shown that the absorption spectrum at 7200 Å observed by Herzberg could be caused by the transition from the ground to the first excited electronic state of HNO⁺     

Solvent dependent conformational relaxation of cis-1,3,5-hexatriene

Ultrafast transient absorption spectroscopy was used to study the conformational relaxation dynamics of 1,3,5-cis-hexatriene (Z-HT) produced in the photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) in methanol and n-propanol solvents. The results are compared with earlier investigations performed using cyclohexane and hexadecane solvents [Anderson, N. A.; Pullen, S. H.; Walker II, L. A.; Shiang, J. J.; Sension, R. J.; J. Phys. Chem. A 1998, 102, 10588-10598.]. The conformational relaxation between hot cZc-HT, cZt-HT, and tZt-HT, where the labels c and t designate cis and trans configurations about the single bonds, is much faster in alcohol solvents than in alkane solvents. The hot Z-HT produced in the photochemical ring-opening reaction evolves from the conformationally strained cZc-HT form to the more stable cZt-HT form on a time scale of 2 ps in alcohols compared with 6 ps in alkanes. The overall decay of the internal vibrational temperature of the hot Z-HT is faster in alcohols (5-6 ps) than alkanes (12-20 ps) and is weakly dependent on the specific alcohol or alkane solvent. A small population of cZt-HT (5-10%) is trapped as the solute equilibrates with the surrounding solvent following UV excitation of CHD or direct UV excitation of Z-HT. The influence of solvent on conformational relaxation of Z-HT was investigated further by probing the temperature dependence of the decay of this thermally equilibrated cZt-HT population. The apparent barrier for the cZt --> tZt conformational isomerization is lower in alcohols (17.4 kJ/mol) than in alkanes (23.5 kJ/mol). However the equilibrium Arrhenius prefactor (A(h)) is an order of magnitude smaller for alcohols (ca. 4 x 10(12)) than alkanes (ca. 6 x 10(13)) resulting in an absolute rate of decay that is faster in the alkane than in the alcohol solvents. These results are discussed in the context of transition state theory and Kramers' theory for condensed phase reaction dynamics.     

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.     

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.     

Oligothiophene catenanes and knots. Part II. Mono and dications. A theoretical study

Mono- and dications of catenanes and knots containing 16, 22, and 28 thiophene units have been studied at the BHandHLYP/3-21G* level of theory. The polaron localization and relaxation energies of monoionized molecules increase with dihedral angle between thiophene fragments, being higher for catenanes and knots compared to linear oligomers. Monoionization of catenanes results in the polaron localization at one macrocycle leaving another one intact. In all diionized oligomers, polaron pairs were found to be more stable than corresponding bipolarons. The energy difference between bipolaron and polaron pairs increases with the number of repeating units in oligomers for all studied molecular architectures. Singlet polaron pairs are more stable than triplet ones. The energy difference between triplet and singlet states does not exceed 7-8 kcal/mol and decreases with the number of thiophene units in oligomers. Two different singlet minima were found for diionized catenanes. In the first one (the most stable), each macrocycle loses one electron, and in the other one, the polaron pairs are located at one macrocycle, leaving another intact. The energy difference between two minima decreases with the number of repeating units in catenanes.     

A theoretical study of the structure of tricarbonatodioxouranate

The results of a study on the ground states of tricarbonato complexes of dioxouranate using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to uranium in the formal oxidation states VI and V, [UO(2)(CO(3))(3)](4)(-) and [UO(2)(CO(3))(3)],(5)(-) have been fully optimized in D(3)(h)() symmetry at second-order perturbation theory (MBPT2) level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. The uranyl fragment has also been optimized at CASSCF/CASPT2, to obtain an estimate of the MBPT2 error. Finally, the effect of distorting the D(3)(h)() symmetry to C(3) has been investigated. This study shows that only minor geometrical rearrangements occur in the one-electron reduction of [UO(2)(CO(3))(3)](4)(-) to [UO(2)(CO(3))(3)],(5)(-) confirming the reversibility of this reduction.     

A theoretical study of the structure of pentadienyllithium

Conclusions MNDO quantum-chemical calculations indicated a mechanism for the rapid rearrangement of PDL with migration of lithium along the hydrocarbon skeleton of the ligand.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2132–2134, September, 1986.