Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state

An approximation to the vibrational coupled-cluster method for CH-stretching of large molecules: application to naphthalene and anthracene

We propose an approximation to the vibrational coupled-cluster method (VCCM) to describe the CH-stretching region of the vibrational spectrum of large molecules. The vibrational modes of a molecule are divided into two sets: the target set and the bath set. The target set includes the CH stretches and the modes that are strongly coupled with the CH stretches and/or involve strong Fermi resonances with a CH stretch fundamental. The rest of the modes are in the bath set. First, the effective harmonic oscillator (EHO) approximation is invoked for the whole system to obtain the zeroth-order frequencies and modified potentials. The effects of interaction between the bath set and the target sets are included in the modified potential from the EHO calculation. The VCCM equations are constructed with the modified potential from the EHO calculations and for the target set only. The transition energies and intensities are calculated using such a truncated VCCM approximation. The proposed method is applied to calculate the IR spectra of naphthalene and anthracene. The results with three different criteria for selecting the modes in the target set are compared with the experimental IR spectra.

A local mode study of ring puckering effects in the infrared spectra of cyclopentane

We report and interpret recently recorded high-resolution infrared spectra for the fundamentals of the CH2 scissors and CH stretches of gas phase cyclopentane at −26.1 and −50 °C, respectively. We extend previous theoretical studies of this molecule, which is known to undergo barrierless pseudorotation due to ring puckering, by constructing local mode Hamiltonians of the stretching and scissor vibrations for which the frequencies, couplings, and linear dipoles are calculated as functions of the pseudorotation angle using B3LYP/6-311++(d,p) and MP2/cc-pVTZ levels of theory. Symmetrization ( D5 h) of the vibrational basis sets leads to simple vibration/pseudorotation Hamiltonians whose solutions lead to good agreement with the experiment at medium resolution, but which miss interesting line fractionation when compared to the high-resolution spectra. In contrast to the scissor motion, pseudorotation leads to significant state mixing of the CH stretches, which themselves are Fermi coupled to the scissor overtones.

UV-Vis, Raman spectroscopic and density functional theoretical studies on microsolvation 1, 2, 4-triazole-3-thione clusters

Photophysical and photochemical reactions of microsolvation clusters are attracting an increasing attention due to their wide applications in materials science and biology. In this paper, 1, 2, 4-triazole-3-thione (3TT) is investigated in solid, protic, and aprotic solvents using FT-Raman, resonance Raman and electronic absorption spectroscopic experiments. The structures of microsolvation clusters in solvents were confirmed by 488 nm Raman spectroscopy combining with density functional theory (DFT) calculation. Steady-state absorption and resonance Raman spectra of 3TT in different environments indicate that the intermolecular hydrogen bonding may reveal important insights in the photophysical and photochemical process. With the aid of DFT and time-dependent density functional theory (TDDFT) calculations, we assigned the observed Raman spectra to the microsolvation clusters in acetonitrile, water and methanol, and carried out preliminary investigations on spectrum shifts of UV and Raman spectra due to the hydrogen bonding with the solvent molecules. The intermolecular >NH···O and >=S···H hydrogen bonding interactions, which are the key constituents of stable thione structure of 3TT, revealed the obvious spectrum shifts of 3TT, including Raman and absorption shifts in CH₃CN, CH₃OH and H₂O. The hydrogen bond sites were further confirmed to be located on the functional group SCNH of 3TT with CH₃CN, H₂O and CH₃OH.

Spectroscopic Manifestations of Indirect Vibrational State Mixing: Novel Anharmonic Effects on a Prereactive H Atom Transfer Surface

The NH stretch region of the IR spectrum of methyl anthranilate is modeled in the S₁ state to understand the connection between the absence of this fundamental in the fluorescence-dip infrared spectra of Blodgett et al. [Phys. Chem. Chem. Phys. 2020, 22, 14077] and its relevance to the H atom dislocation that occurs upon electronic excitation. A set of coordinates are chosen that highlight the role of certain low-frequency modes. A Hamiltonian is developed in which a large-amplitude two-dimensional surface describing the H-bonded H atom is linearly and quadratically coupled to the remaining degrees of freedom which are treated at the harmonic level. The surface is calculated within the time-dependent density functional theory framework by using the B3LYP/6-311++(d, p) level of theory with dispersion. Our spectral results show that indirect couplings lead to massive intensity sharing over hundreds of wavenumbers. This sharing is predicted to be dramatically reduced upon deuteration. The spectral broadening mechanism is found to involve off-resonant doorway states that are themselves strongly coupled to states nearly degenerate with the NH stretch fundamental and represents a complementary mechanism to previous explanations based on Fermi resonance or the presence of Franck-Condon like combination bands with low-frequency motions. Consistent with the spectra predictions, time-dependent calculations show that if the NH stretch fundamental were excited with an ultrafast laser, it would decay within 40 fs. The competition between H atom dislocation and vibrational relaxation is discussed.

Excited-State Intramolecular Proton Transfer: A Short Introductory Review

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.

The missing NH stretch fundamental in S1 methyl anthranilate: IR-UV double resonance experiments and local mode theory

The infrared spectra of jet-cooled methyl anthranilate (MA) and the MA-H₂O complex are reported in both S₀ and S₁ 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 S₀ spectrum, in the S₁ 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.