Chemical dynamics simulations of collision induced dissociation of deprotonated glycolaldehyde
Introduction
In the formose or Butlerov reaction, formaldehyde oligomerizes to form carbohydrates and this reaction is considered to be the source of prebiotic sugar formation on early Earth [1]. The first step of this reaction involves the formation of glycolaldehyde from two formaldehyde molecules.2HCHO→CH2OHCHO
In solution, this reaction has been proposed to occur between a deprotonated formaldehyde (HCO− anion formed under basic conditions) and a neutral formaldehyde molecule [2]. Studies have shown that the proposed mechanism was not entirely correct and further experimental and computational investigations of the formose reaction have been reported [[3], [4], [5], [6], [7], [8], [9]].
Glycolaldehyde (GA) is the simplest possible sugar which is also known as diose. GA has been detected in the atmosphere as a product of photochemical oxidation of volatile organic molecules such as ethene and isoprene [[10], [11], [12]] in addition to direct emission from biomass burning [13,14]. Decay of GA in the troposphere occurs mainly via OH radical reaction and photochemical decomposition. Photolysis reaction of GA has been investigated in solution [15] and in the gas phase [16,17]. Porterfield et al. [18], have studied the pyrolysis of GA and products such as H, CO, HCHO, CH2CO, etc. were identified using photoionization mass spectrometry. More than two hundred organic molecules have been detected in the interstellar medium and GA is the only sugar among them [[19], [20], [21], [22], [23]]. GA combines with propenal resulting in ribose, a central component of RNA, and hence GA is considered to be a precursor of RNA [24]. Even though GA has been detected in space, no well established mechanism for the gas phase formation of GA is available in the literature. For instance, formation of protonated GA from association reaction between formaldehyde and protonated formaldehyde in the interstellar medium was considered [7] but later this reaction was shown to be inefficient [8,9]. Electronic structure theory studies have shown that metal ion catalyzed and hydrogen bond mediated dimerization of formaldehyde to form glycolaldehyde are nearly barrier-less processes [25].
Relevant to the formose reaction, gas phase fragmentation of deprotonated GA, C2H3 was investigated by Uggerud and co-workers [26]. From a retro-synthetic point of view, dissociation of the conjugate base of GA forming HCO− + HCHO products was investigated using tandem mass spectrometry under collision induced dissociation (CID) conditions [27]. Experiments were carried out for a range of collision energies (0.2–20 eV) and the HCO− + HCHO products were not identified as primary decomposition products of deprotonated GA. Signal at m/z = 31, corresponding to the reaction C2H3O2- → CO + CH3O−, was the dominant peak in the mass spectrum. Other than this work, there are no studies in the literature reporting the fragmentation chemistry of the deprotonated GA. In the present work, we have investigated the dissociation chemistry of deprotonated GA using detailed electronic structure theory calculations and classical direct chemical dynamics simulations [28,29] to model the collision induced dissociation experiments by Uggerud and co-workers [26]. The simulations were performed by on-the-fly integration of classical trajectories using potentials and gradients computed from density functional theory. The trajectory initial conditions were selected to mimic single collision conditions in the gas phase. A variety of decomposition products including HCO− + HCHO were identified. Simulation results were compared with previously reported CID experiments and detailed atomic level dissociation mechanisms are presented. The article is organized as follows. Computational Methodology is presented in the next Section followed by a detailed Results and Discussion. The article is summarized in the last Section. Supporting Information containing optimized coordinates of stationary points, reaction schemes, etc. is provided.
Section snippets
Computational methods
Electronic structure calculations and the direct classical trajectory integrations were performed using the density functional B3LYP method utilizing the 6-31+G∗ basis set. This level of theory was selected because of the accuracy [26] and the limited computational costs associated with the method. A comparison of stationary point energies computed using B3LYP/6-31+G∗ method and higher level methods is presented in the Supporting Information. Deprotonation of GA (CH2OHCHO) can occur at three
Potential energy surface
It can be seen from Table 1 of Supporting Information that the stationary point energies on the dissociation energy profile of deprotonated GA calculated using B3LYP/6-31+G∗ theory are not much different from those computed using higher level theories. Fig. 2 shows the potential energy profile computed using B3LYP/6-31+G∗ theory and a brief discussion is given here. The reaction of interest (deprotonated GA → HCO− + HCHO) occurs from the 1O isomer and a direct reaction path from the lowest
Discussion
CID experiments of deprotonated GA showed that the uncatalyzed, gas phase formose reaction HCO− + HCHO → HCOCH2O− is an unlikely event [26]. Electronic structure calculations show that the lowest energy isomer of deprotonated GA (1C) does not have a direct reaction pathway to form the retro-formose products. Consistently, direct dynamics simulations of 1C dissociation showed formation of HCO− + HCHO only in a small number of trajectories. On the other hand, the slightly higher energy 1O isomer
Summary
Classical chemical dynamics simulations modeling previously reported collision induced dissociation experimental study of deprotonated glycolaldehyde are reported. The experimental study was performed to understand the gas phase reaction between formyl anion and formaldehyde in the interstellar media to form carbohydrates. The study concluded [26] that the efficiency for this reaction is low and the results of the present simulations are in qualitative agreement with this work. Deprotonation of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Funding from Department of Science and Technology, India, through grant number CRG/2019/000454 is acknowledged. The lead author of the paper expresses his sincere gratitude to Prof. William L. Hase for his guidance and support during the author’s stay in Prof. Hase’s research group and thereafter.
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