Variable temperature ion trap studies of CH4++H2, HD and D2: negative temperature dependence and significant isotope effect

Quantum study of the CH3+ photodissociation in full-dimensional neural network potential energy surfaces

C H 3 + , a cornerstone intermediate in interstellar chemistry, has recently been detected for the first time by using the James Webb Space Telescope. The photodissociation of this ion is studied here. Accurate explicitly correlated multi-reference configuration interaction ab initio calculations are done, and full-dimensional potential energy surfaces are developed for the three lower electronic states, with a fundamental invariant neural network method. The photodissociation cross section is calculated using a full-dimensional quantum wave packet method in heliocentric Radau coordinates. The wave packet is represented in angular and radial grids, allowing us to reduce the number of points physically accessible, requiring to push up the spurious states appearing when evaluating the angular kinetic terms, through projection technique. The photodissociation spectra, when employed in astrochemical models to simulate the conditions of the Orion bar, result in a lesser destruction of CH3+ compared to that obtained when utilizing the recommended values in the kinetic database for astrochemistry.

High-resolution infrared action spectroscopy of the fundamental vibrational band of CN. J Mol Spectrosc 2020;374:111375. [PMID: 33162609 PMCID: PMC7116308 DOI: 10.1016/j.jms.2020.111375]

Rotational-vibrational transitions of the fundamental vibrational modes of the ¹²C¹⁴N⁺ and ¹²C¹⁵N⁺ cations have been observed for the first time using a cryogenic ion trap apparatus with an action spectroscopy scheme. The lines P(3) to R(3) of ¹²C¹⁴N⁺ and R(1) to R(3) of ¹²C¹⁵N⁺ have been measured, limited by the trap temperature of approximately 4 K and the restricted tuning range of the infrared laser. Spectroscopic parameters are presented for both isotopologues, with band origins at 2000.7587(1) and 1970.321(1) cm^-1, respectively, as well as an isotope independent fit combining the new and the literature data.

The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and its Relevance in Solar Geoengineering Models

SO₂ has been proposed in solar geoengineering as a precursor of H₂ SO₄ aerosol, a cooling agent active in the stratosphere to contrast climate change. Atmospheric ionization sources can ionize SO₂ into excited states of S O 2 · + , which quickly reacts with trace gases in the stratosphere. In this work we explore the reaction of H 2 D 2 with S O 2 · + excited by tunable synchrotron radiation, leading to H S O 2 + + H ( D S O 2 + + D ), where H contributes to O₃ depletion and OH formation. Density Functional Theory and Variational Transition State Theory have been used to investigate the dynamics of the title barrierless and exothermic reaction. The present results suggest that solar geoengineering models should test the reactivity of S O 2 · + with major trace gases in the stratosphere, such as H₂ since this is a relevant channel for the OH formation during the nighttime when there is not OH production by sunlight. OH oxides SO₂ , triggering the chemical reactions leading to H₂ SO₄ aerosol.

Dissociative recombination of CH4(+)

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

Transfer of a proton between H2 and O2

The proton affinities of hydrogen and oxygen are very similar. Therefore, it has been discussed that the proton transfer from the omnipresent H(3)(+) to molecular oxygen in the near thermoneutral reaction H(3)(+) + O(2) <--> O(2)H(+) + H(2) effectively binds the interstellar oxygen in O(2)H(+). In this work, the proton transfer reaction has been investigated in a low-temperature 22-pole ion trap from almost room temperature (280 K) down to the lowest possible temperature limited by freeze out of oxygen gas (about 40 K at a low pressure). The Arrhenius behaviour of the rate coefficient for the forward reaction shows that it is subject to an activation energy of E(A)/k=113 K. Thus, the forward reaction can proceed only in higher temperature molecular clouds. Applying laser-induced reactions to the given reaction (in the backward direction), a preliminary search for spectroscopic signatures of O(2)H(+) in the infrared was unsuccessful, whereas the forward reaction has been successfully used to probe the population of the lowest ortho and para levels of H(3)(+).

Variable-temperature rate coefficients for the electron transfer reaction N2+ + H2O measured with a coaxial molecular beam radio frequency ring electrode ion trap

The neutral molecule temperature dependence of the rate coefficient for the electron transfer reaction from H(2)O to N(2)(+) is determined using a coaxial molecular beam radio frequency ring electrode ion trap (CoMB-RET) method. The temperature of the N(2)(+) ions was maintained at 100 K, while the effusive water beam temperature was varied from 300 to 450 K. The result demonstrates the neutral molecule rotational/translational energy dependence on the rate coefficient of an ion-dipolar molecule reaction. It is found that the rate coefficient in the above temperature range follows the prediction of the simplest ion-dipole capture model. Use of different buffer gas collisional cooling in both the ion source and the RET reveals the effects of both translational and vibrational energy of the N(2)(+) ions.

Temperature variable ion trap studies of C3Hn+ with H2 and HD

Hydrogenation and deuteration of C3+, C3H+, C3H2+ in collisions with H2 and HD has been studied from room temperature down to 10 K using a 22-pole ion trap. Although exothermic, hydrogenation of C3+ is rather slow at room temperature but becomes faster with decreasing temperature. In addition to the increasing lifetime of the collision complex this behavior may be caused by the floppy structure of C3+ and the freezing of soft bending modes below 50 K. For C3(+) + HD it has been shown that production of C3D+ is slightly favored over C3H+ formation. The controversy over which products are really formed in C3H(+) + H2 collisions and deuterated variants has a long history. Previous and new ion trap results prove that formation of C3H2(+) + H is not endothermic but rather fast, in contradiction to erroneous conclusions from flow tube experiments and ab initio calculations. In addition the reaction shows a complicated isotope dependence, most probably caused by the influence of zero point energies in entrance and exit transition states. For example hydrogen abstraction with HD is faster than with H2 while radiative association is slower. The most surprising result has been obtained for C3H(+) + HD. Here C3HD+ formation is over one hundred times faster than C3H2+. In addition to the details of the potential energy surface it may be that in this case an H-HD exchange reaction takes place via an open-chain propargyl cation intermediate (H2CCCH+). Reactions of C3H2+ and C3H3+ with H2 are very slow but, due to the unique sensitivity of the trapping technique, significant rate coefficients have been determined. The presented results are of fundamental importance for understanding the energetics, structures and reaction dynamics of the deuterated variant of the C3Hn+ collision system. They indicate that the previous quantum chemical calculations are not accurate enough for understanding the low energy behavior of the Cn,Hm+ reaction systems. The laboratory experiments are of essential relevance for the carbon chemistry of dense interstellar clouds, both for formation of small hydrocarbons and deuterium fractionation.

The effect of the argon carrier gas in the multiphoton dissociation-ionization of tetracene

The multiphoton dissociation-ionization of tetracene at 355 nm using 6.5 nanosecond laser pulses, with and without argon as a carrier gas (CG), has been studied and compared. Ion fragments were analyzed in a time-of-flight mass spectrometer and separated according to their mass-to-charge ratio (m/z). The results show that the dynamic of photodissociation at approximately 10(10) W cm(-2) intensities is strongly influenced by the CG. The suppression of fragmentation channels primarily those relating to the formation of the CH(m) (+) (m = 2, 4), C(2)H(4) (+) and C(5)H(4) (+2) ions. CH(5) (+) and CH(6) (+) were observed which have not been reported before in photodissociation tetracene experiments.

A remeasurement of the products for electron recombination of N2H+ using a new technique: no significant NH+N production

A remeasurement of the product distribution from dissociative electron-ion recombination (DR) of N2H+ has been made using a new technique. The technique employs electron impact to ionize the neutral products prior to detection by a quadrupole mass analyzer. Two experimental approaches, both using pulsed gas techniques, isolate and quantify the DR products. In one approach, an electron-attaching gas is pulsed into a flowing afterglow to transiently quench DR. Results from this approach give an upper limit of 5% for the NH+N product channel. In the second approach, the reagent gas N2 is pulsed. The absolute percentages of products were monitored versus initial N2 concentration. Results from this approach also give an upper limit of 5% for NH+N production. This establishes that N2+H is the dominant channel, being at least between 95 and 100%, and that there is no significant NH production contrary to a recent storage ring measurement that yielded 64% NH+N and 36% N2+H. Possible reasons for this dramatic difference are discussed.

Ion-molecule reactions in 4He droplets: flying nano-cryo-reactors

Ion-molecule reactions are studied inside large (approximately equal to 10(4) atoms) very cold (0.37 K) superfluid (4)He droplets by mass spectrometric detection of the product ions. He+ ions initially formed inside the droplets by electron impact ionization undergo charge transfer with either embedded D(2), N(2), or CH(4). For D(2) this charge transfer process was studied in detail by varying the pickup pressure. For either N(2) or CH(4) the reagent ions were formed by this charge transfer and the reaction pathways of the secondary reactions N(2) (+)+D(2), CH(4) (+)+D(2), and CH(3) (+)+D(2) each with an additionally embedded D(2) molecule were also determined from the pickup pressure dependencies. In several cases, notably He.N(2) (+) and CH(3)D(2) (+) reaction intermediates are observed. The analysis is facilitated by the tendency for molecular ion products to appear without (or with only very few) attached He atoms whereas the atomic ion products usually appear in the mass spectra with several attached He atoms, e.g., He(m).D+ ions with up to m=19.

Computational Study of the Ion−Molecule Reactions Involving Fluxional Cations: CH4+ + H2 → CH5+ + H and Isotope Effect

Potential energy surfaces for the reactions of CH4+ with H2, HD, and D2 have been calculated using high-level ab initio methods, including coupled cluster theory, complete active space self-consistent field, and multireference configuration interaction. The energies are extrapolated to the complete basis set limit using the basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6). The CH4+ + H2 reaction produces CH5+ and H exclusively. Three types of reaction mechanisms have been found, namely, complex-forming abstraction, scrambling, and S(N)2 displacement. The abstraction occurs via a very minor barrier and it is dominant. The other two mechanisms are negligible because of the significant barriers involved. Quantum phase space theory and variational transition state theory are used to calculate the rate coefficients as a function of temperatures in the range of 5-1000 K. The theoretical rate coefficients are compared with the available experimental data and the discrepancy is discussed. The significance of isotope effect, tunneling effect, and nuclear spin effect is investigated. The title reaction is predicted to be slightly exothermic with DeltaHr = -12.7 +/- 5.2 kJ/mol at 0 K.