Structure and Reactivity of Organic Intermediates as Revealed by Time‐Resolved Infrared Spectroscopy

Toscano, John P.

doi:10.1002/9780470133576.ch2

Cited by 28 publications

(9 citation statements)

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“…The calculated structure of the π,π* triplet m -dimethylamino phenyloxenium indicates a C–O bond length of 1.216 Å, with the electronic density delocalized into the benzene ring rather than in the C–O bond. For an aromatic ketone (e.g., such as triplet benzophenone), a C–O bond length extending up to 1.32 Å has an n,π* triplet state character and the triplet electronic density is mainly located in the CO group. − For the second species with a main Raman band at 1656 cm –1 , we tentatively assigned this species to the m -dimethylamino phenyloxenium radical cation which would be generated by the hydrogen abstraction of the triplet oxenium ion from the surrounding water molecules. Figure (right) indicates that the computationally predicted Raman spectrum of the m -dimethylamino phenyloxenium radical cation is consistent with the experimental Raman spectrum obtained at 10 μs, although the noise in the spectrum at this time delay prevents a clear assignment.…”

Section: Resultsmentioning

confidence: 99%

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Albright

Hanway

et al. 2015

J. Am. Chem. Soc.

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Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR 3 ), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S 1 over 800 fs are followed by bond heterolysis in ∼1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ∼50 ps followed by intersystem crossing in ∼300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR 3 experiments, permitting a more clear assignment and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ∼4 K and observing a triplet species by cryogenic EPR spectroscopy. Disciplines Materials Chemistry | Other Chemistry | Physical Chemistry CommentsReprinted (adapted) with permission from J. Am. Chem. Soc., 2015, 137 (32) ABSTRACT: Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phe...

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Section: Resultsmentioning

confidence: 99%

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Albright

Hanway

et al. 2015

J. Am. Chem. Soc.

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“…For benzophenone itself, this is demonstrated by timeresolved IR-experiments, 55 where the C-O stretching vibration (m C=O ) of the 3 np* (T 1 ) state appears at lower frequencies relative to the corresponding ground state band. In agreement with this, we have found also a lowering of 38 cm −1 in the m C=O vibration frequency of the IR-spectrum of the ground state of 1 (1650 cm −1 ) in respect to that of the T 1 state (1488 cm −1 , see vibrational splitting of the phosphorescence spectrum, Fig.…”

Section: Methodsmentioning

confidence: 92%

Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

Zarkadis

Georgakilas

Perdikomatis

et al. 2005

Photochem Photobiol Sci

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The photochemistry of three structurally very similar triphenylmethylsilanes 1, 2, 3 [p-X-C(6)H(4)-CPh(2)-SiMe(3): X = PhCO, 1; H, ; Ph(OCH(2)CH(2)O)C, 3] is described by means of 248 and 308 nm nanosecond laser flash photolysis (ns-LFP), femtosecond LFP, EPR spectroscopy, emission spectroscopy (fluorescence, phosphorescence), ns-pulse radiolysis (ns-PR), photoproduct analysis studies in MeCN, and X-ray crystallographic analysis of the two key-compounds 1 and 2. The photochemical behavior of 1, 2 and 3 is discussed and compared with that of a fourth one, 4, bearing on the p-position an amino group (X = Me(2)N) and whose detailed photochemistry we reported earlier (J. Org. Chem., 2000, 65, 4274-4280). Silane 1 undergoes on irradiation with 248 and 308 nm laser light a fast photodissociation of the C-Si bond giving the p-(benzoyl)triphenylmethyl radical (1*) with a rate constant of k(diss)= 3 x 10(7) s(-1). The formation of 1* is a one-quantum process and takes place via the carbonyl triplet excited state with high quantum yield (Phi(rad)= 0.9); the intervention of the triplet state is clearly demonstrated through the phosphorescence spectrum and quenching experiments with ferrocene (k(q)= 9.3 x 10(9) M(-1) s(-1)), Et(3)N (1.1 x 10(9) M(-1) s(-1)), and styrene (3.1 x 10(9) M(-1) s(-1)) giving quenching rate constants very similar to those of benzophenone. For comparative reasons radical 1* was generated independently from p-(benzoyl)triphenylmethyl bromide via pulse radiolysis in THF and its absorption coefficient at lambda(max)= 340 nm was determined ([epsilon]= 27770 M(-1) cm(-1)). We found thus that the p-PhCO-derivative 1 behaves similar to the p-Me(2)N one (the latter giving the p-(dimethylamino)triphenylmethyl radical with Phi(rad)= 0.9), irrespective of their completely different ground state electronic properties. In contrast, compounds 2, 3 that bear only the aromatic chromophore give by laser or lamp irradiation both, (i) radical products [Ph(3)C* and p-Ph(OCH(2)CH(2)O)C-C(6)H(4)-C(*)Ph(2), respectively] after dissociation of the central C-Si bond (Phi(rad)= 0.16), and (ii) persistent photo-Fries rearrangement products (of the type of 5-methylidene-6-trimethylsilyl-1,3-cyclohexadiene) absorbing at 300-450 nm and arising from a 1,3-shift of the SiMe(3) group from the benzylic to the ortho-position of the aromatic ring (Phi approximately 0.85 for 2). Using fs-LFP on 2 we showed that the S(1) state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph(3)C* through C-Si bond dissociation. In a second step and within the next 10 ps trityl radicals either escape from the solvent cage (the quantum yield of Ph(3)C* formation Phi(rad)= 0.16 was measured with ns-LFP), or undergo in-cage recombination to photo-Fries products. Thus, singlet excited states (S(1)) of the aromatic organosilanes (2, 3) prefer photo-Fries rearrangement products, while triplet excited states (1, 4) favor free radicals. Both reactions proceed via a common primary photodissociation step (C-Si bond homolysis) and diffe...

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“…Insight into calculated anharmonic vibrational spectra also has the potential to provide a deeper understanding in a wide range of experimental techniques, including vibrational circular dichroism (VCD), ultrafast 2-dimensional infrared spectroscopy (2D-IR), and the assignment of infrared spectra for lowest triplet electronically excited states. For example, anharmonic effects are expected to be significantly more pronounced in the latter . Consequently, it remains desirable to have accurate calculations of vibrational frequencies incorporating anharmonicity from first principles to ensure their continued widespread applicability with increasing molecular size.…”

Section: Introductionmentioning

confidence: 99%

“…For example, anharmonic effects are expected to be significantly more pronounced in the latter. 11 Consequently, it remains desirable to have accurate calculations of vibrational frequencies incorporating anharmonicity from first principles to ensure their continued widespread applicability with increasing molecular size. Recently, hybrid schemes that combine accurate methods such as coupled-cluster theory for the harmonic force field with DFT for the anharmonic part have been demonstrated to give close agreement with experiment.…”

Section: ■ Introductionmentioning

confidence: 99%

Investigating the Calculation of Anharmonic Vibrational Frequencies Using Force Fields Derived from Density Functional Theory

2012

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The calculation of anharmonic vibrational frequencies for a set of small molecules has been examined to explore the merit of applying such computationally expensive approaches for large molecules with density functional theory. The performance of different hybrid and gradient-corrected exchange-correlation functionals has been assessed for the calculation of anharmonic vibrational frequencies using second-order vibrational perturbation theory with two-and four-mode couplings and compared to the recently developed transition optimized shifted Hermite method. A range of exchange-correlation functionals (B3LYP, BLYP, EDF1, EDF2, B97-1, B97-2, HCTH-93, HCTH-120, HCTH-147, and HCTH-407) have been evaluated with reference to a large experimental data set comprising 88 species and 655 modes as well as a smaller set of shifts in frequency because of anharmonicity derived from experimental data. The anharmonic frequencies calculated using hybrid functionals provide the best agreement with experiment, and are not significantly improved by frequency scaling factors, indicating an absence of significant systematic error. For the molecules studied, the B97-1 and B97-2 functionals give the closest overall agreement with experiment, although the improvement over the best case for pure harmonic frequencies is modest. Predictions of the experimental anharmonic shifts are closest for the B3LYP and EDF2 functionals, with B97-1 performing well because of a good description of the harmonic force field. Investigations using modified hybrid functionals with increased fractions of Hartree−Fock exchange indicate that approximately 20% Hartree−Fock exchange is optimal.

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Structure and Reactivity of Organic Intermediates as Revealed by Time‐Resolved Infrared Spectroscopy

Cited by 28 publications

References 299 publications

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

Investigating the Calculation of Anharmonic Vibrational Frequencies Using Force Fields Derived from Density Functional Theory

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