The dissociative photoionization processes of methyl hydroperoxide (CH 3 OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH 3 OOH + ions dissociate into CH 2 OOH + , HCO + , CH 3 + , and H 3 O + ions in the 11.4–14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH 2 OOH + . An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH 2 OOH + ion, and a 74.2 ± 2.6 kJ mol –1 mixed experimental-theoretical 0 K heat of formation for the CH 2 OOH radical. The proton affinity of the Criegee intermediate, CH 2 OO, was also obtained from the heat of formation of CH 2 OOH + (792.8 ± 0.9 kJ mol –1 ) to be 847.7 ± 1.1 kJ mol –1 , reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes were used to model the higher-energy fragmentation. Supported by Born–Oppenheimer molecular dynamics simulations, we found that the HCO + fragment ion is produced through a roaming transition state followed by a low barrier. H 3 O + is formed in a consecutive process from the CH 2 OOH + fragment ion, while direct C–O fission of the molecular ion leads to the methyl cation.
more »
« less
A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study
The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00–10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH 3 OH) n H + . However, the stable dimer parent ion (CH 3 OH) 2 + is readily detected below the dissociation threshold to yield the dominant CH 3 OH 2 + fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH 3 OH) 2 + parent ion dissociates via five parallel channels at low internal energies. The loss of both CH 2 OH and CH 3 O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH 3 (H)OH + ⋯CH 2 OH ion, the precursor to the CH 2 OH-, H 2 O-, and CH 3 -loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH 3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH 3 OH 2 + , which explains the dominance of the protonated methanol fragment ion in the mass spectrum.
more »
« less
- Award ID(s):
- 1665464
- NSF-PAR ID:
- 10382702
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 24
- Issue:
- 3
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 1437 to 1446
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)A combined experimental and theoretical study is presented on the collision-induced dissociation (CID) of 9-methylguanine–1-methylcytosine base-pair radical cation (abbreviated as [9MG·1MC]˙ + ) and its monohydrate ([9MG·1MC]˙ + ·H 2 O) with Xe and Ar gases. Product ion mass spectra were measured as a function of collision energy using guided-ion beam tandem mass spectrometry, from which cross sections and threshold energies for various dissociation pathways were determined. Electronic structure calculations were performed at the DFT, RI-MP2 and DLPNO-CCSD(T) levels of theory to identify product structures and map out reaction potential energy surfaces. [9MG·1MC]˙ + has two structures: a conventional structure 9MG˙ + ·1MC (population 87%) consisting of hydrogen-bonded 9-methylguanine radical cation and neutral 1-methylcytosine, and a proton-transferred structure [9MG − H]˙·[1MC + H] + (less stable, population 13%) formed by intra-base-pair proton transfer from the N1 of 9MG˙ + to the N3 of 1MC within 9MG˙ + ·1MC. The two structures have similar dissociation energies but can be distinguished in that 9MG˙ + ·1MC dissociates into 9MG˙ + and 1MC whereas [9MG – H]˙·[1MC + H] + dissociates into neutral [9MG – H]˙ radical and protonated [1MC + H] + . An intriguing finding is that, in both Xe- and Ar-induced CID of [9MG·1MC]˙ + , product ions were overwhelmingly dominated by [1MC + H] + , which is contrary to product distributions predicted using a statistical reaction model. Monohydration of [9MG·1MC]˙ + reversed the populations of the conventional structure (43%) vs. the proton-transferred structure (57%) and induced new reactions upon collisional activation, of which intra-base-pair hydrogen transfer produced [9MG + H] + and the reaction of the water ligand with a methyl group in [9MG·1MC]˙ + led to methanol elimination from [9MG·1MC]˙ + ·H 2 O.more » « less
-
null (Ed.)We investigated the collision-induced dissociation (CID) reactions of a protonated Hoogsteen 9-methylguanine–1-methylcytosine base pair (HG-[9MG·1MC + H] + ), which aims to address the mystery of the literature reported “anomaly” in product ion distributions and compare the kinetics of a Hoogsteen base pair with its Watson-Crick isomer WC-[9MG·1MC + H] + (reported recently by Sun et al. ; Phys. Chem. Chem. Phys. , 2020, 22 , 24986). Product ion cross sections and branching ratios were measured as a function of center-of-mass collision energy using guided-ion beam tandem mass spectrometry, from which base-pair dissociation energies were determined. Product structures and energetics were assessed using various theories, of which the composite DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97XD/6-311++G(d,p) was adopted as the best-performing method for constructing a reaction potential energy surface. The statistical Rice–Ramsperger–Kassel–Marcus theory was found to provide a useful framework for rationalizing the dominating abundance of [1MC + H] + over [9MG + H] + in the fragment ions of HG-[9MG·1MC + H] + . The kinetics analysis proved the necessity for incorporating into kinetics modeling not only the static properties of reaction minima and transition states but more importantly, the kinetics of individual base-pair conformers that have formed in collisional activation. The analysis also pinpointed the origin of the statistical kinetics of HG-[9MG·1MC + H] + vs. the non-statistical behavior of WC-[9MG·1MC + H] + in terms of their distinctively different intra-base-pair hydrogen-bonds and consequently the absence of proton transfer between the N1 position of 9MG and the N3′ of 1MC in the Hoogsteen base pair. Finally, the Hoogsteen base pair was examined in the presence of a water ligand, i.e. , HG-[9MG·1MC + H] + ·H 2 O. Besides the same type of base-pair dissociation as detected in dry HG-[9MG·1MC + H] + , secondary methanol elimination was observed via the S N 2 reaction of water with nucleobase methyl groups.more » « less
-
more » « less
Abstract We report a collision‐induced dissociation (CID) based gas phase rearrangement study using quadrupole time‐of‐flight mass spectrometry coupled with liquid chromatography on a novel endothelin and angiotensin II receptor antagonist, sparsentan. We performed tandem mass spectrometry to identify precursor and fragment ion relationships and assigned structures for major fragment ions. We propose a benzyl migration mechanism based on bond length measurements in density functional theory (B3LYP/6‐31+G*) optimized geometries of protonated sparsentan and its
m/z 547 fragment. Protonated sparsentan undergoes loss of ethanol, which yields a resonance‐stabilized benzylic cation withm/z 547, which further fragments intom/z 353 via benzyl migration, where the benzylic cation migrates to one of the nucleophilic nitrogen atoms followed by proton transfer from the sulfonamide nitrogen to a carbonyl oxygen, resulting in a neutral loss of mass 194. Further fragmentation ofm/z 353 results inm/z 258, which undergoes radical and neutral loss to yieldm/z 193 and 194, respectively. The proposed mechanism of generation ofm/z 353 was confirmed by CID of deuterated sparsentan. Considering the importance of gas phase rearrangements of organic molecules in structural identifications as well as the novelty of the molecule, these findings will be helpful for future studies to predict gas phase benzyl migration in sparsentan analogs and for degradation product and metabolite identification of sparsentan and its analogs using LC–MS. -
null (Ed.)A guided-ion beam tandem mass spectrometric study was performed on collision-induced dissociation (CID) of a protonated 9-methylguanine–1-methylcytosine Watson–Crick base pair (designated as WC-[9MG·1MC + H] + ), from which dissociation pathways and dissociation energies were determined. Electronic structure calculations at the DFT, RI-MP2 and DLPNO-CCSD(T) levels of theory were used to identify product structures and delineate reaction mechanisms. Intra-base-pair proton transfer (PT) of WC-[9MG·1MC + H] + results in conventional base-pair conformations that consist of hydrogen-bonded [9MG + H] + and 1MC and proton-transferred conformations that are formed by PT from the N1 of [9MG + H] + to the N3′ of 1MC. Two types of conformers were distinguished by CID in which the conventional conformers produced [9MG + H] + product ions whereas the proton-transferred conformers produced [1MC + H] + . The conventional conformers have a higher population (99.8%) and lower dissociation energy than the proton-transferred counterparts. However, in contrast to what was expected from the statistical dissociation of the equilibrium base-pair conformational ensemble, the CID product ions of WC-[9MG·1MC + H] + were dominated by [1MC + H] + rather than [9MG + H] + . This finding, alongside the non-statistical CID reported for deprotonated guanine–cytosine (Lu et al. ; PCCP , 2016, 18 , 32222) and guanine–cytosine radical cation (Sun et al. ; PCCP , 2020, 22 , 14875), reinforces that non-statistical dissociation is a distinctive feature of singly-charged Watson–Crick guanine–cytosine base pairs. It implies that intra-base-pair PT facilitates the formation of proton-transferred conformers in these systems and the ensuing conformers have loose transition states for dissociation. The monohydrate of WC-[9MG·1MC + H] + preserves non-statistical CID kinetics and introduces collision-induced methanol elimination via the reaction of the water ligand with a methyl group.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d1cp05155e
