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Electronic spectra often exhibit vibronic structure when vibrational and electronic transitions occur in concert. Theory reveals (1) that orbital symmetry considerations determine specific roles played by the nuclear degrees of freedom and (2) that the vibrational excitation is often highly regiospecific, that is, attributable to an identifiable subset of atoms within the molecule. Spectra obtained from a chromium(III) complex involving a macrocyclic ligand and two axially disposed butadiynide groups nicely illustrate many of the concepts involved. [ABSTRACT FROM AUTHOR]

Tetrapyrrole-based pigments play a crucial role in photosynthesis as principal light absorbers in light-harvesting chemical systems. As such, accurate theoretical descriptions of the electronic absorption spectra of these pigments will aid in the proper description and understanding of the overall photophysics of photosynthesis. In this work, time-dependent density functional theory (TD-DFT) at the CAM-B3LYP/6-31G* level of theory is employed to produce the theoretical absorption spectra of several tetrapyrrole-based pigments. However, the application of TD-DFT to large systems with several hundreds of atoms can become computationally prohibitive. Therefore, in this study, TD-DFT calculations with reduced orbital spaces (ROSs) that exclude portions of occupied and virtual orbitals are pursued as a viable, computationally cost-effective alternative to conventional TD-DFT calculations. The effects of reducing orbital space size on theoretical spectra are qualitatively and quantitatively described, and both conventional and ROS results are benchmarked against experimental absorption spectra of various tetrapyrrole-based pigments. The orbital reduction approach is also applied to a large natural pigment assembly that comprises the principal light-absorbing component of the reaction center in purple bacteria. Overall, we find that TD-DFT calculations with proper and judicious orbital space reductions can adequately reproduce conventional, full orbital space, TD-DFT results of all pigments studied in this work. [ABSTRACT FROM AUTHOR]

Transient electronic and vibrational absorption spectroscopy unravel the mechanisms and dynamics of bimolecular reactions of CN radicals with acetone in deuterated chloroform solutions. The CN radicals are produced by ultrafast ultraviolet photolysis of dissolved ICN. Two reactive forms of CN radicals are distinguished by their electronic absorption bands: "free" (uncomplexed) CN radicals, and "solvated" CN radicals that are complexed with solvent molecules. The lifetimes of the free CN radicals are limited to a few picoseconds following their photolytic production because of geminate recombination to ICN and INC, complexation with CDCl3 molecules, and reaction with acetone. The acetone reaction occurs with a rate coefficient of (8.0 ± 0.5) × 1010 M-1 s-1 and transient vibrational spectra in the C-N and C-O stretching regions reveal that both the nascent HCN and 2-oxopropyl (CH3C(O)CH2) radical products are vibrationally excited. The rate coefficient for the reaction of solvated CN with acetone is 40 times slower than for free CN, with a rate coefficient of (2.0 ± 0.9) × 109 M-1 s-1 obtained from the rise in the HCN product v1(C-N stretch) IR absorption band. Evidence is also presented for CN complexes with acetone that are more strongly bound than the CN-CDCl3 complexes because of CN interactions with the carbonyl group. The rates of reactions of these more strongly associated radicals are slower still. [ABSTRACT FROM AUTHOR]

The electronic absorption spectrumof 9,9-dimethyl-2,7-dinitrofluoreneradical anion in HMPA displays both a NIR intervalence charge transferand a visible excited state mixed valence transition. These transitionscontain a similar vibronic progression resulting from molecular orbitalsthat are common to both transitions. Vibrational frequency and intensitydata are acquired from the resonance Raman spectrum and used to calculatea best fit for the absorption spectrum. The normal coordinate distortionsare analyzed in terms of the electronic changes for both transitionsto explain their similarity. The Raman scattering intensity decreasesat lower excitation wavelength as a result of Raman de-enhancementcaused by interference between neighboring excited states. [ABSTRACT FROM AUTHOR]

A combinationof ultrafast transient electronic absorption spectroscopy(TEAS) and transient vibrational absorption spectroscopy (TVAS) isused to investigate whether photoinduced N–H bond fission,mediated by a dissociative 1πσ* state, is activein aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visibleand IR spectral signatures of the adeninyl radical (Ade[-H]), formedas a result of N–H bond fission, TEAS and TVAS are performedon Ade in D2O under basic conditions (pD = 12.5), whichforms Ade[-H]−anions via deprotonation at the N7or N9 sites of Ade’s 7H and 9H tautomers. At 220 nm we observeone-photon detachment of an electron from Ade[-H]−, which generates solvated electrons (eaq–) together with Ade[-H] radicals,with clear signatures in both TEAS and TVAS. Additional wavelengthdependent TEAS measurements between 240–260 nm identify a thresholdof 4.9 ± 0.1 eV (∼250 nm) for this photodetachment processin D2O. Analogous TEAS experiments on aqueous Ade at pD= 7.4 generate a similar photoproduct signal together with eaq–afterexcitation at 266 and 220 nm. These eaq–are born from ionization ofAde, together with Ade+cations, which are indistinguishablefrom Ade[-H] radicals in TEAS. Ade+and Ade[-H] are foundto have different signatures in TVAS and we verify that the pD = 7.4photoproduct signal observed in TEAS following 220 nm excitation issolely due to Ade+cations. Based on these observations,we conclude that: (i) N–H bond fission in aqueous Ade is inactiveat wavelengths ≥220 nm; and (ii) if such a channel exists inaqueous solution, its threshold is strongly blue-shifted relativeto the onset of the same process in gas phase 9H-Ade (≤233nm). In addition, we extract excited state lifetimes and vibrationalcooling dynamics for 9H-Ade and Ade[-H]−. In both cases, excited state lifetimes of <500fs are identified, while vibrational cooling occurs within a timeframe of 4–5 ps. In contrast, 7H-Ade is confirmed to have alonger excited state lifetime of ∼5–10 ps through bothTEAS and TVAS. [ABSTRACT FROM AUTHOR]

A detailed mechanistic investigationof the early stages of thePaternò–Büchi reaction following 267 nm excitationof benzaldehyde in cyclohexene has been completed using ultrafast,broadband transient UV–visible and IR absorption spectroscopies.Absorption due to electronically excited triplet state benzaldehydedecays on a 80 ps time scale via reaction with cyclohexene. The growthand subsequent decay of the biradical intermediate produced followingC–O bond formation is followed by transient vibrational spectroscopy.The biradical decays by ring closure to an oxetane or by dissociating,reforming the ground state reactants. Detailed kinetic analysis allowedderivation of quantum yields and rate constants for these competingbiradical decay processes, ϕoxetane= 0.53, ϕdiss= 0.47, koxetane= 0.27 ±0.09 ns–1and kdiss=0.24 ± 0.09 ns–1. This study provides a strikingillustration of the ways in which contemporary ultrafast transientabsorption spectroscopy methods can be used to dissect the mechanismand kinetics of a classic photoreaction. [ABSTRACT FROM AUTHOR]