Henrik Grum Kjaergaard

Unimolecular reactions of organic peroxy radicals (RO2) play a central role in the atmospheric oxidation of volatile organic compounds (VOCs), yet experimental constraints on their reaction kinetics remain limited. Ring-opened RO2 formed in the OH-initiated oxidation of α-pinene undergoes rapid unimolecular reactions, including 1,5 H-shift, 1,6 H-shift, and endocyclization. However, previously reported experimental rate coefficients represent the overall unimolecular reaction rate from all channels and do not resolve branching among chemically distinct channels. As these channels lead to different oxidation products, channel-specific rate coefficients are needed to constrain product distributions under different atmospheric conditions. Here, we employ site-specific deuterium-labeled α-pinene isotopologues to selectively suppress individual H-shift channels and isolate the contributions of competing unimolecular channels. For each deuterated isotopologue, the overall unimolecular rate coefficient of the ring-opened RO2 was determined at 296 K using hydroxy nitrate (HN)-based kinetic analysis and compared with that of unlabeled α-pinene to derive channel-specific rate coefficients. We obtain 0.7−0.4+0.5 s–1 for the 1,6 H-shift,0.5−0.3+0.5 s–1 for the 1,5 H-shift, and 3.0−1.6+1.7 s–1 for endocyclization. These values are consistent with theoretical calculations and provide direct experimental constraints on individual channel rate coefficients in α-pinene-derived RO2 unimolecular chemistry. These channel-specific rate coefficients provide a quantitative basis for predicting oxidation product distributions under different atmospheric conditions and serve as experimental benchmarks for theoretical calculations of RO2 unimolecular chemistry.

Acyloxyl radicals (RCO2) are a common type of alkoxyl radical formed in the gas-phase oxidation of organic compounds in the atmosphere. Previous theoretical studies have predicted that the decomposition of RCO2 to an alkyl radical R and CO2 is a very fast reaction, suggesting it to be the exclusive fate for RCO2. In contrast, previous experimental work invokes intramolecular H-shift isomerization reactions of RCO2 in order to explain the formation of observed products. To resolve this apparent discrepancy, we investigate the electronic structure and energetics associated with the RCO2 decomposition reaction. Generally, reactions are studied on the electronic ground state of the reactant. However, using CH3CO2 as a model system, we show that it is not possible to accurately describe the CO2 loss reaction with only a single electronic state. Instead, we find that it is necessary to involve the three lowest-lying electronic states to describe the reactants and transition states accurately. Our state-averaged multireference calculations for the CO2 loss from CH3CO2 find a barrier of ∼7 kcal mol–1, corresponding to a rate coefficient of ∼108 s–1 at 298 K. Calculations for larger RCO2 test systems, where R is CH3CH2, (CH3)2CH, or (CH3)3C, show barriers that are 2–3 kcal mol–1 lower, corresponding to 1–2 orders-of-magnitude faster decomposition reactions. Thus, CO2 loss will likely dominate the fate of RCO2 but exceptionally fast H-shift reactions can be competitive. To support our theoretical calculations, we carry out OH oxidation experiments using glutaraldehyde as a model system and conclusively demonstrate that CO2 loss occurs for a variety of functionalized RCO2 with an empirically constrained lower-limit reaction rate coefficient of ∼107 s–1.

Water quality and the composition of minerals in the water impact coffee extraction and thus influence the sensory experience of coffee. This study investigated the effect of water hardness and alkalinity on the sensory properties of brewed coffee (Colombian Kachalu) using duo–trio screening and descriptive sensory analyses. Coffees brewed with ultrapure water (UPW), used as the reference, were compared with brews prepared from UPW containing additions of calcium (Ca ^{2 +} ), magnesium (Mg ^{2 +} ), and bicarbonate (HCO ₃ ⁻ ), spanning from the lower levels found within the “ideal brewing zone,” and up to higher experimental extremes designed to test the limits of their sensory impact. Across all sensory evaluations, water hardness alone produced no marked flavor changes, even when calcium was added at concentrations above those typically found in drinking water. In contrast, bicarbonate had a clear and pronounced effect on the sensory profile across the full tested range. Increasing bicarbonate levels, from brew‐zone concentrations and up to the experimental extreme of 300 mg/L, substantially reduced perceived acidity, fruitiness, and complexity, while increasing roasted and bitter notes. Small differences observed in samples containing both calcium and bicarbonate were attributable to reductions in effective bicarbonate concentration due to calcium carbonate (CaCO ₃ ) precipitation rather than to any direct flavor contribution from calcium. Overall, the findings indicate that, under the experimental conditions investigated, alkalinity, not hardness, was the primary determinant of sensory changes in brewed coffee. Maintaining appropriate bicarbonate levels is therefore essential for preserving desirable flavor attributes such as acidity and complexity, whereas variations in calcium within realistic ranges showed little sensory relevance. Practical Applications: This study shows that bicarbonate levels in water, rather than water hardness, were the main factor shaping the sensory profile of the Colombian Kachalu coffee used in this study. Even small increases in bicarbonate made the coffee less acidic and fruity and more flat or bitter. These findings suggest that controlling alkalinity may be more important than adjusting hardness when designing water for coffee brewing. Consumers, cafés, and equipment manufacturers can use these findings to optimize water composition and improve flavor consistency across different brewing environments.

Hydrogen bonding between water and volatile organic compounds (VOCs) plays a central role in atmospheric clustering and nucleation processes. We investigate the hydrogen-bonded complexes formed between water (H₂O) and three ethers: diethyl ether (DEE), tetrahydrofuran (THF), and ethyl vinyl ether (EVE) at room temperature in the gas phase using infrared spectroscopy and compare with previously reported spectra of H₂O·DME (dimethyl ether). Infrared spectra recorded at room temperature reveal distinct bound and free OH-stretching fundamentals of the H₂O unit in the monohydrated complexes. The observed red shifts of the bound OH-stretch establish a clear trend in hydrogen bond strength: H₂O·DEE ≈ H₂O·THF > H₂O·DME > H₂O·EVE, which is rationalized in terms of inductive effects, enhanced lone-pair availability, and π-delocalization effects that modulate the hydrogen-bond acceptor ability of the ether oxygen. We use reduced-dimensional anharmonic vibrational calculation to assign the measured spectra and determine the oscillator strengths of the OH-stretching transitions. In conjunction with measured band intensities, equilibrium constants (Gibbs energies) of complex formation are determined. These results provide room temperature thermodynamic characterization of H₂O·ether complexes and establish a robust combined experimental-theoretical framework for quantifying weak hydrogen bonds relevant to atmospheric chemistry.

Gas-phase autoxidation is an atmospheric chemistry reaction mechanism capable of transforming volatile organic compounds (VOCs) into highly oxygenated organic molecules (HOMs) that contribute to secondary organic aerosol (SOA) formation and growth. The key steps in this mechanism are intramolecular hydrogen shift (H-shift) reactions in organic peroxy radicals (RO₂). For acyclic saturated molecules, these H-shift reactions are generally sufficiently slow that they cannot compete with atmospheric bimolecular reactions with NO_x species, except for the 1,5 and 1,6 H-shifts, occurring via transition states (TS) of six- and seven-membered rings. Here, we report a surprisingly fast long-range H-shift reaction in a RO₂ formed in the photo-oxidation of a volatile diether. In 1,2-diethoxyethane (1,2-DEE), we observe experimentally a 1,8 H-shift reaction that occurs with a rate coefficient on the order of ∼1 s⁻¹ at 294 K – a rate that outcompetes all other RO₂ unimolecular chemistry in the system and will, under most atmospheric conditions, outcompete bimolecular processes as well. Theoretical calculations indicate that activation of the C–H bond by an α-oxyl substituent and weaker transannular strain in the 1,8 H-shift transition state, combined with inductive deactivation of C–H bonds by a β-oxyl group at the abstraction site of competing 1,5 and 1,6 H-shifts, enable the longer-span 1,8 H-shift to be competitive. Our findings broaden the recognized reactivity of functionalized RO₂ and highlight the potential for structurally diverse VOCs to undergo unexpected autoxidation pathways, producing HOMs at greater yield and with higher molecular complexity than previously anticipated.

Hydrogen shift reactions (H-shifts) are important unimolecular steps in the autoxidation of volatile organic compounds (VOCs). These potentially lead to the formation of highly oxygenated organic molecules (HOMs) that contribute to the formation of secondary organic aerosol (SOA). Although H-shift chemistry in hydrocarbon peroxy radicals (RO₂) is well-studied, the impact of incorporating a heteroatom, oxygen, nitrogen, sulfur, or phosphorus (O, N, S, or P), into the carbon backbone remains largely unexplored. We have used a multiconformer transition state theory approach to calculate H-shift rate coefficients in RO₂s that contain a heteroatom. We find that heteroatoms accelerate H-shift reactions, particularly when the heteroatom is positioned α with respect to the carbon from which the H atom is abstracted. For the 1,6 H-shift with the heteroatom β to the abstraction site, the rate coefficients are more similar to those in the hydrocarbon. For large H-shift spans, the variation in the H-shift rate coefficient with heteroatoms is large, and the reactions are faster than in hydrocarbons. For an ether, we find that the 1,8 H-shift is as fast as the 1,5 H-shift and both are competitive with bimolecular reactions under pristine atmospheric conditions. We explain the trends in selectivity and reactivity based on steric, inductive, and stereoelectronic effects. These findings highlight the importance of isomerization reactions in atmospheric chemistry, including those involving nonhydrocarbon peroxy radicals.

Water dimer is a classical example of an OH···O hydrogen bound complex and plays a critical role in the atmosphere. The bound OH-stretching (OH_b) fundamental transition displays the characteristic wavenumber redshift and intensity enhancement, observed repeatedly under many different experimental conditions. In contrast, the first OH_b-stretching overtone is predicted to be several orders of magnitude weaker in intensity. Recent full-dimensional ro-vibrational calculations have revised earlier theoretical predictions, suggesting that the OH_b-stretching first overtone may exhibit an intensity comparable to that of other observed OH-stretching transitions at these higher wavenumbers. Using jet expansion cavity ring-down spectroscopy in the predicted spectral region, we successfully observed this previously elusive transition. The observed transition wavenumber, intensity, and band shape show excellent agreement with the new theoretical predictions. The overlap of many closely lying ro-vibrational transitions, even at the cold temperature of the jet, gives rise to a relatively unstructured broad band. We determine the experimental OH_b-stretching first overtone vibrational band origin to be 7049 cm^-1 and estimate the lifetime to be about 10 ps.

Hydroperoxyaldehydes are a class of compounds frequently formed in the atmosphere from oxidation of emitted volatile organic compounds. One recently detected example is hydroperoxymethyl thioformate (HPMTF), which is formed in large quantities from the widely emitted dimethyl sulfide. Understanding the atmospheric fate of HPMTF, including the contribution of photolysis, is crucial for quantifying its impacts on important atmospheric outcomes such as sulfate production. We have used a nuclear ensemble approach to calculate the electronic absorption cross section of three test molecules, HPMTF, and selected hydroperoxyaldehydes. We use the calculated absorption spectra to estimate the atmospheric photolysis rates of the hydroperoxyaldehydes. We implement the calculated absorption cross section of HPMTF into a global atmospheric model and find that photolysis is only a minor loss path for HPMTF. We show that photolysis rates obtained using structure-activity relationship approaches based on the functional groups of molecules are unreliable, because these approaches ignore effects from heteroatoms. For hydroperoxyaldehydes with oxygen or nitrogen in the α-position to the carbonyl group, we find that photolysis is insignificant, due to strong heteroatom effects.

The formation of a peroxide accretion product (ROOR) has recently been shown to be a significant channel in self- and cross-reactions of peroxy radicals (RO₂) in the gas-phase. Here, we examine the formation of this accretion product in the self- and cross-reactions of RO₂ derived from the OH-initiated oxidation of propene, cis-2-butene, and methylpropene in the presence of ethene. We measure the formation rate coefficient of the various accretion products in each system relative to the formation rate coefficient of the ethene-derived ROOR, which was measured in our previous work. We find that the accretion product forms in all of the studied self- and cross-reactions. The measured ROOR formation rate coefficient for the self-reaction decreases by approximately an order of magnitude with increasing substitution, with average rate coefficients of 4.7 × 10⁻¹³ cm³ molec⁻¹ s⁻¹ for primary hydroxy peroxy radicals, 2.7 × 10⁻¹⁴ cm³ molec⁻¹ s⁻¹ for secondary hydroxy peroxy radicals, and 8.0 × 10⁻¹⁶ cm³ molec⁻¹ s⁻¹ for the tertiary hydroxy peroxy radical. The cross-reaction rate coefficients of secondary and tertiary peroxy radicals with primary peroxy radicals are both higher than the corresponsing self-reactions, and also decrease with increasing radical substitution. We estimate the branching fraction to the formation of the ROOR for these peroxy radical self- and cross-reactions and find that branching fractions range from 0.03–0.33, with self- and cross-reactions of primary peroxy radicals having the highest branching fractions. Finally, we compare the reaction and ROOR formation rate coefficients of self- and cross-reactions of small RO₂, and determine that the arithmetic mean of self-reaction rate coefficients provides a suitable method for estimating cross-reaction rate coefficients.