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Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV–UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA–H 2 O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S 0 –S 1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm −1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NH⋯OC H-bond, which shifts the 3 nπ* state well above the 1 ππ* S 1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck–Condon activity due to three in-plane vibrations that modulate the distance between the NH 2 and CO 2 Me groups, ν 33 (421 cm −1 ), ν 34 (366 cm −1 ), and ν 36 (179 cm −1 ). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NH⋯OC H-bond. This leads to a shortening of the NH⋯OC H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the H⋯O distance compared to full H-atom transfer. As a result, the excited state process near the S 1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.
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The missing NH stretch fundamental in S 1 methyl anthranilate: IR-UV double resonance experiments and local mode theory
The infrared spectra of jet-cooled methyl anthranilate (MA) and the MA–H 2 O complex are reported in both S 0 and S 1 states, recorded using fluorescence-dip infrared (FDIR) spectroscopy under jet-cooled conditions. Using a combination of local mode CH stretch modeling and scaled harmonic vibrational character, a near-complete assignment of the infrared spectra is possible over the 1400–3700 cm −1 region. While the NH stretch fundamentals are easily observed in the S 0 spectrum, in the S 1 state, the hydrogen bonded NH stretch shift is not readily apparent. Scaled harmonic calculations predict this fundamental at just below 2900 cm −1 with an intensity around 400 km mol −1 . However, the experimental spectrum shows no evidence of this transition. A local mode theory is developed in which the NH stretch vibration is treated adiabatically. Minimizing the energy of the corresponding stretch state with one quantum of excitation leads to a dislocation of the H atom where there is equal sharing between N and O atoms. The sharing occurs as a result of significant molecular arrangement due to strong coupling of this NH stretch to other internal degrees of freedom and in particular to the contiguous HNC bend. A two-dimensional model of the coupling between the NH stretch and this bend highlights important nonlinear effects that are not captured by low order vibrational perturbation theory. In particular, the model predicts a dramatic dilution of the NH stretch oscillator strength over many transitions spread over more than 1000 cm −1 , making it difficult to observe experimentally.
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- Award ID(s):
- 1764148
- PAR ID:
- 10174003
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 25
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 14077 to 14087
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP01916J
