Direct Determination of Hydrogen-Bonded Structures in Resonant and Tautomeric Reactions Using Ultrafast Electron Diffraction

Srinivasan, Ramesh; Feenstra, Jonathan S.; Park, Sang Tae; Xu, Shou‐Jun; Zewail, Ahmed H.

doi:10.1021/ja031927c

Cited by 111 publications

(128 citation statements)

References 31 publications

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“…In the ground state AA exhibits an asymmetric (C s ) structure (where the central H atom belongs to only one oxygen atom), and hence the geometry of the AA in the Franck−Condon region corresponds to an asymmetric structure where the H atom is closer to one of the oxygen atoms. 31 However, after the excitation, this asymmetry is broken as indicated by experimental and theoretical studies, 5,32−36 leading to a pseudoaromaticity. The 50 fs limit is very similar to the proton transfer rates of many related systems, e.g., o-hydroxybenzaldehyde, 1-hydroxy-2-acetonaphthone, methyl salicylate, 2-(2′-hydroxyphenyl)benzoxazole, and TINUVIN P. 37−40 With the time scale of τ 2 = 2.25 ps, the decrease of the ESA feature in the spectral range from 230 to 250 nm can be observed in the presented transients at 230 and 241 nm (confer Figure 3).…”

Section: ■ Results and Discussionmentioning

confidence: 98%

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

et al. 2014

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In photoinduced molecular reaction dynamics, the effects of electronic charge redistribution can lead to multiple pathways that are determined by the nature of the initial structures involved and the environment the molecule of interest is studied in. The β-diketones are a common example of this complexity. They show keto−enol tautomerism that is almost totally shifted toward the enolic form. However, compared to the gas phase, the photochemistry proceeds completely differently by virtue of the solvent environment for these compounds, which are used in commercial sunscreen agents due to a high absorption in the ultraviolet (UV) and fast deactivation processes. We disclose these dynamics by investigating three symmetrical β-diketones in various solvents. To observe these effects on an ultrafast time scale directly in the UV spectral region where the relevant electronic transitions take place, we have developed and employed femtosecond transient absorption with detection capability in the deep UV. Our studies confirm that electronic excitation of the chelated enol form does not lead to any ultrafast photochemistry other than proton transfer followed by rotamerization. The formation of the nonchelated conformers takes place on a picosecond time scale through a dark state, whereas the recovery to the stable chelated enol form is a comparably slow process. ■ INTRODUCTIONDerivatives of β-diketones are of great importance in diverse research fields by virtue of several remarkable chemical features that lead to a variety of applications. 1 For example, they are widely employed as chelating agents due to their binding affinity for transition metals, 1 or they are even contained in commercial sunscreen products 2−4 owing to the fast deactivation processes after ultraviolet (UV) irradiation. The most prominent property of β-diketones is their keto−enol tautomerism that is shifted almost totally toward the enolic form. 5−7 With structural similarities to relevant biomolecules and photochromic substances on the one hand, and the molecules' own versatility in combination with their structural simplicity on the other hand, small β-diketones are prototypical candidates for a systematic study of the photoinduced processes and the subsequent deactivation channels upon which the wide applicability of β-diketones is based.C o m p o u n d s l i k e β -d i k e t o n e s o f t y p e R−C(O)−CH 2 −C(O)−R, where R = H, CH 3 , have drawn continuous attention from chemists and physicists of different research areas because of the pronounced keto−enol tautomerism that is observable (and exploitable) in the gas 5,8 and the liquid phase 6,9 and even in isolated cryogenic matrices. 7,10 The reason for the stabilization of the enolic form is an intramolecular H-bond, coupled with a π-electronic delocalization over the O−C−C−C−O pseudocycle (see Scheme 1). The two simplest and smallest structures exhibiting the central six membered ring closed by the intramolecular Hbond in the chelated enol (CE) form are the compounds malonaldehyde (M...

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Section: ■ Results and Discussionmentioning

confidence: 98%

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

et al. 2014

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“…Nevertheless, electron diffraction results are contradictory [2,6,7], predicting a C s or a C 2v structure. A recent ultra fast electron diffraction study of Zewail et al [8] has demonstrated that, at least at the short timescale (~100 ps), the AA structure is asymmetrical with a d OO distance of 259 pm, 8 pm larger than the previous estimation [6]. The proton transfer barrier has been estimated at 6.6 kcal·mol -1 from the measured tunnelling splitting (21.6 cm -1 ) for MA [9][10][11], and besides, many theoretical studies have focussed on the description of this tunnelling process, trying to reproduce the measured splitting and to estimate the barrier height.…”

Section: Introductionmentioning

confidence: 99%

Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices

Trivella¹,

Coussan²,

Chiavassa³

et al. 2006

Low Temperature Physics

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Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of malonaldehyde and acetylacetone are compared through theoretical and experimental data. Ground-state geometries, energies, and vibrational frequencies are calculated with the B3LYP/6-311++G(2d,2p) model chemistry. The electronic delocalisation as well as the cis/trans rotamer properties are analysed. The hydrogen bond strength of the chelated forms can be estimated by the energy difference between chelated and non-chelated forms, and its enhancement due to methyl-induced electron release is estimated at 1.7 kcal·mol -1 . UV-and IR-induced reactivity of molecules isolated in nitrogen matrices is studied by means of FT-IR spectrometry. Interconversion between rotamers is the main process observed for both molecules, only some among the seven non-chelated forms being created.PACS: 36.20.Ng, 31.15.Ar

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“…Similarly, the formation of an intramolecular hydrogen bond in the protonated species is responsible for a significant enhancement of the intrinsic basicity of some acid anhydrides [8]. Finally, IHB's are crucial in many ultra-fast hydrogen transfer phenomena through the use of pulsed lasers [9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

et al. 2005

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High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono-and dithioderivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono-and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σ * NX antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH · · · O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the π * NN antibondig orbital, and which is significantly perturbed when the IHB is formed.

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Direct Determination of Hydrogen-Bonded Structures in Resonant and Tautomeric Reactions Using Ultrafast Electron Diffraction

Cited by 111 publications

References 31 publications

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices

The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

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