Impact of methyl rotor in the excited state level mixing of doubly hydrogen-bonded complexes of 2-pyridone

We have presented in this paper the laser-induced fluorescence excitation and resolved fluorescence spectra of five 1:1 hydrogen-bonded complexes of 2-pyridone (2PY) with formic acid (FA), acetic acid (AA), propanoic acid (PA), formamide (FM), and acetamide (AM). The resolved fluorescence spectra, measured following excitation to different single vibronic levels of the dimers indicate that the intermolecular hydrogen bond vibrations undergo mixing with a number of intramolecular modes of the 2PY moiety in the excited state. A comparison of the emission spectral features of these dimers clearly indicates that the methyl groups belonging to the AA and AM moieties spectacularly accelerate the vibrational energy redistribution (IVR) in the 2PY moiety. On the other hand, although the molecular size of PA is bigger than AA, the spectral features of the 2PY−PA dimer bear signatures of a slower IVR rate compared to those of 2PY−AA. We propose that hyperconjugation of the methyl group with the cyclic hydrogen-bonded network involving AA and AM is responsible for the observed phenomenon

2-hydroxypyridine ↔ 2-pyridone tautomerization: catalytic influence of formic acid

A 1:1 hydrogen-bonded complex between 2-pyridone and formic acid has been characterized using laser-induced-fluorescence excitation and dispersed fluorescence spectroscopy in a supersonic jet expansion. Under the same expansion condition, the fluorescence signal of the tautomeric form of the complex (2-hydroxypyridine···formic acid) is absent, although both the bare tautomeric molecules exhibit well-resolved laser-induced-fluorescence spectra. Quantum chemistry calculation at the DFT/B3LYP/6-311++G** level predicts that in the ground electronic state the activation barrier for tautomerization from hydroxy to keto form in bare molecules is very large (∼34 kcal/mol). However, the process turns out to be nearly barrierless when assisted by formic acid, and double proton transfer occurs via a concerted mechanism

Impact of OH Radical-Initiated H₂CO₃ Degradation in the Earth’s Atmosphere via Proton-Coupled Electron Transfer Mechanism

The decomposition of isolated carbonic acid (H₂CO₃) molecule into CO₂ and H₂O (H₂CO₃ → CO₂ + H₂O) is prevented by a large activation barrier (>35 kcal/mol). Nevertheless, it is surprising that the detection of the H₂CO₃ molecule has not been possible yet, and the hunt for the free H₂CO₃ molecule has become challenging not only in the Earth’s atmosphere but also on Mars. In view of this fact, we report here the high levels of quantum chemistry calculations investigating both the energetics and kinetics of the OH radical-initiated H₂CO₃ degradation reaction to interpret the loss of the H₂CO₃ molecule in the Earth’s atmosphere. It is seen from our study that proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) are the two mechanisms by which the OH radical initiates the degradation of the H₂CO₃ molecule. Moreover, the PCET mechanism is potentially the important one, as the effective barrier, defined as the difference between the zero point vibrational energy (ZPE) corrected energy of the transition state and the total energy of the isolated starting reactants in terms of bimolecular encounters, for the PCET mechanism at the CCSD­(T)/6-311++G­(3df,3pd) level of theory is ∼3 to 4 kcal/mol lower than the effective barrier height associated with the HAT mechanism. The CCSD­(T)/6-311++G­(3df,3pd) level predicted effective barrier heights for the degradations of the two most stable conformers of H₂CO₃ molecule via the PCET mechanism are only ∼2.7 and 4.3 kcal/mol. A comparative reaction rate analysis at the CCSD­(T)/6-311++G­(3df,3pd) level of theory has also been carried out to explore the potential impact of the OH radical-initiated H₂CO₃ degradation relative to that from water (H₂O), formic acid (FA), acetic acid (AA) and sulfuric acid (SA) assisted H₂CO₃ → CO₂ + H₂O decomposition reactions in both the Earth’s troposphere and stratosphere. The comparison of the reaction rates reveals that, although the atmospheric concentration of the OH radical is substantially lower than the concentrations of the H₂O, FA, AA in the Earth’s atmosphere, nevertheless, the OH radical-initiated H₂CO₃ degradation reaction has significant impact, especially toward the loss of the H₂CO₃ molecule in the Earth’s atmosphere. In clean environments, which exist in greater numbers in comparison to the polluted environments of the Earth’s atmosphere, the impact of the OH radical-initiated H₂CO₃ degradation reaction is seen to be comparable to that from a competing pathway which utilizes hydrogen bonded molecules such as H₂O, FA or AA to catalyze the H₂CO₃ decomposition. Similarly, in the polluted environments, and especially in the Earth’s troposphere, although the reactions rates for the OH radical-initiated H₂CO₃ degradation and FA-assisted H₂CO₃ decomposition are comparable within a factor of ∼15, nevertheless, the AA-assisted H₂CO₃ decomposition reaction is appeared to be the dominant channel. This follows only because of slightly greater catalytic efficiency of the AA over FA upon the H₂CO₃ → CO₂ + H₂O decomposition reaction. In contrary, although the catalytic efficiencies of SA, FA, and AA upon the H₂CO₃ decomposition reaction are similar to each other and the concentrations of both the SA and OH radical in the Earth’s atmosphere are more-or-less equal to each other, but nevertheless, the SA-assisted H₂CO₃ decomposition reaction cannot compete with the OH radical-initiated H₂CO₃ degradation reaction

Importance and Impact of International Agreements or Protocols to Protect Ozone and Beyond

Synchronized control of both global warming and ozone depletion is prerequisite for the sustainable stability of our bio-diversity. Here we uncover the importance of Paris Agreement and Kyoto Protocol and the new impact of Montreal Protocol to protect stratospheric ozone by considering the prototype H₂O + O(¹D) → 2OH insertion/addition reaction as the representative reaction of potent GHG molecules with steady state O(¹D). While CO₂ via its continuous regeneration CO₂ + O(¹D) → CO₂ + O(³P) reaction in stratosphere is involved in ozone hole formation, the geoengineering via nonstop CaCO₃ injections in stratosphere in future would also be responsible for ozone hole formation. Moreover, the classical nature of single potential wells of single step exothermic barrierless reactions governed by quantum mechanics de-energizes the energized adducts or products analogous to energy conservation of a falling mass from some height under gravity and plays decisive role in the extent of pressure independency of rate constants

DISPERSED FLOURESCENCE SPECTROSCOPY OF JET-COLLED pp-AMINOTOLUENE

Author Institution: Department of Chemistry, Indian Institute of Technology Kanpur, UP 208016, India; Department of Physical Chemistry, Indian Association for the Cultivation of Science,; Jadavpur, Calcutta 700032, IndiaLarge amplitude hindered internal rotation of a methyl group and umbrella inversion of amine have been extensively investigated over the past several decades because of their important chemical significances. In $p$-aminotoluene ($p$-AT) both the groups are present and they can interact with each other via the aromatic ring. Such interaction has been studied earlier by Yan and Spangler by measuring the fluorescence excitation spectra, and subsequently by Tan and Pratt using rotationally resolved electronic spectroscopy. In this work, we show evidence of coupling between the two groups in the excited electronic state (S$_1$) of the molecule by measuring the dispersed fluorescence spectra following excitations to single vibronic levels, and by comparing the spectra with those of $p$-aminofluorobenzene

OVERTONE SPECTROSCOPY OF PEROXYACETIC ACID AND PEROXYFORMIC ACID : INFLUENCE OF INTRAMOLECULAR HYDROGEN BONDING

Author Institution: Department of Chemistry and Biochemistry, University of California-San Diego; 9500 Gilman Drive, La Jolla, California 93093-0314.The absorption of solar radiation by hydrogen-bonded (H-bonded) complex, particularly those containing water, is important in atmospheric chemistry. However, because of their low concentration, intermolecular hydrogen bonded complexes of atmospheric interest are difficult to study in the gas phase. Consequently, our initial efforts have been directed towards investigating the spectroscopy of molecules with internal hydrogen bonds. In this talk, we present the vapor phase vibrational overtone spectra of peroxyacetic acid (PAA) and peroxyformic acid (PFA), two molecules of atmospheric importance, and discuss the effect of intramolecular hydrogen-bonding on their OH stretching overtone transition strength and band positions. A comparison of the results of PAA and PFA with those of other intramolecular H-bonded and non-H-bonded molecules provides a useful gauge of the extent of hydrogen bonding in these peroxyacids

Autocatalytic Isomerizations of the Two Most Stable Conformers of Carbonic Acid in Vapor Phase: Double Hydrogen Transfer in Carbonic Acid Homodimers

The <i>cis–cis</i> [(<i>cc</i>)] and <i>cis–trans</i> [(<i>ct</i>)] conformers of carbonic acid (H₂CO₃) are known as the two most stable conformers based on the different orientations of two OH functional groups present in the molecule. To explain the interconversion of the (<i>cc</i>)-conformer to its (<i>ct</i>)-conformer, the rotation of one of the two indistinguishable OH functional groups present in the (<i>cc</i>)-conformer has been shown until now as the effective isomerization mechanism. Moreover, the (<i>ct</i>)-conformer, which is slightly energetically disfavored over the (<i>cc</i>)-conformer, has been considered as the starting point for the decomposition of H₂CO₃ into CO₂ and H₂O molecules. Experimentally, on the other hand, the infrared (IR) and Raman spectroscopy of the crystalline H₂CO₃ polymorphs suggest that the most possible basic building blocks of H₂CO₃ polymorphs consist of only and exclusively the (<i>cc</i>)-conformers. However, the sublimations of these crystalline H₂CO₃ polymorphs result both the (<i>cc</i>)- and (<i>ct</i>)-conformers in the vapor phase with the (<i>cc</i>)-conformer being the major species. In this article, we first report the high level ab initio calculations investigating the energetics of the autocatlytic isomerization mechanism between the two most stable conformers of carbonic acid in the vapor phase. The calculations have been performed at the MP2 level of theory in conjunction with aug-cc-pVDZ, aug-cc-pVTZ, and 6-311++G­(3df,3pd) basis sets. The results of the present study specifically and strongly suggest that double hydrogen transfer within the eight-membered cyclic doubly hydrogen-bonded (H-bonded) ring interface of the H₂CO₃ homodimer formed between two (<i>cc</i>)-conformers is ultimately the starting mechanism for the isomerization of the (<i>cc</i>)-conformer to its (<i>ct</i>)-conformer, especially, during the sublimation of the H₂CO₃ polymorphs, which result in the vapor phase concentration of the (<i>cc</i>)-conformer at the highest levels