Lowest excited triplet state of 2,5-dimethyl-1,3,5-hexatriene: resonance Raman spectra and quantum chemical calculations

Abstract

Lowest excited triplet state of 2,5-dimethyl-1,3,5-hexatriene : resonance Raman spectra and quantum chemical calculations Negri, F.; Orlandi, G.; Brouwer, A.M.; Langkilde, F.W.; M?ller, S.; Wilbrandt, R. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Theoretical and Raman spectroscopic studies are presented of the ground and lowest excited triplet states of (E)-and (Z)-2,S-dimethyl-l,3,S-hexatriene and their 3,4-dideuterio and 3-deuterio isotopomers. The TI potential energy surface is calculated from SCF-LCAO-MDCI theory. Energy minima and equilibrium geometries are determined in the TI state. Frequencies and normal modes of vibrations are calculated for the minima of the TI and S, states. Energies of higher triplet levels are computed and oscillator strengths for the TI -T, transitions are determined. The displacements in equilibrium geometries between the TI and the T, level corresponding to the strongest TI -T, transition are calculated and are used to estimate the intensities of the resonance Raman spectra of the TI state under the assumption of a predominant Franck4hndon scattering mechanism. The influence of the ground-state conformation around C-C single bonds on TI resonance Raman spectra is considered in detail for the two isomers. It is found that for the E isomers the tEt and for the Z isomers the tEc forms are the predominant ones in the TI state. The 2 forms are at considerable higher energy than the E forms in the TI state due to nonbonded interaction. A good agreement is found between theoretically calculated and experimental spectra. The results are compared with previously published data for 1,3,S-hexatriene. Iatroductioawe considered the transient resonance Raman (TRR) spectra of 1,3,5-hexatriene (HT) in its TI state with the purpose of obtaining information about the geometry of the TI intermediate and about the mechanism of the cis-trans photoisomerization in TI. The experimental TRR spectra, which have been found to be identical in experiments on solutions of either of the two ground-state isomers, were interpreted by quantum mechanical calculations of equilibrium geometries and vibrational frequencies at the trans (E), cis ( Z ) , and centrally twisted (P) geometries in TI and in the T, state responsible for the TRR activity. On the basii ...

Triplet State Z/E-Photoisomerizations of Polyenes: A Comparison of ab Initio and Density Functional Methods

“…The T 1 PESs at the UHF level also do not agree well with those calculated at the PUMP4(SDTQ)//UMP2 level (Figures and ). For example, the drop in energy by rotation of the central CC bond is 5.2 kcal/mol, when the PES with regard to this parameter should be flat according to both more advanced calculations and experiments. 4l,, Spin projection of the UHF wave function, as performed during the UMP4(SDTQ) calculation, leads to even more spurious results. In this case, the S 0 −T 1 gap is for some geometries below 10 kcal/mol, whereas the correct energies are in the range 40−50 kcal/mol.…”

“…However, the uncertainty with regard to the 0−0 gaps is at least 0.5 kcal/mol. Furthermore, in both HT and 2,5-dimethyl-1,3,5-hexatriene the minima at E and twisted geometries are roughly isoenergetic (within 1 kcal/mol), and these minima are separated by a barrier sufficiently low to allow equilibration during the T 1 lifetime . Thus, the PES for twisting around the central double bond in HT should be rather flat.…”

Potential energy surfaces of short polyenes in the state T1: analysis of time resolved resonance Raman spectra

“…A number of methylated hexauienes, 2-methyl-hexatriene, 3methylhexatriene and 2,5-dimethyl-hexatriene in their cis and tram forms, have been studied in the same way [3,4]. The TI RR spectra of these compounds have been found to be due to planar isomers and it was deduced that tram and perpendicular forms are roughly at the same energy, as in hexauiene.…”

Influence of steric effects on the excited triplet-state lifetime of 2,2'-dinitrostilbene and 2,2',4,4',6,6'-hexanitrostilbene in acetonitrile solution

“…Steric hindrance to molecular planarity may also be present in the excited triplet state, and this will affect the excited triplet state's potential energy surfaces, as recently noted for methyl substitution of 1,3,5-hexatriene. 18 If steric distortion of the excited triplet-state (Ti) potential energy surface results in the closer approach of T[ to the ground state (S0), as the molecule is twisted about the double bond, we would expect to see a reduction in triplet state lifetime relative to that of transstilbene or 4,4'-DNS.119 •20 It should also be noted that the excited triplet state of 2,4-dinitrostilbene21 and other nitrated compounds12•22•23 is possibly involved in shock-induced chemistry. HNS is used widely as a thermally stable energetic compound, and studies on its excitedstate chemistry are therefore of particular interest.…”