Adeel Jamal

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block—the aromatic benzene molecule—has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C2H + H2CCHCHCH2 → C6H6 + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

An experimental and theoretical study on the formation of 2-methylnaphthalene (C11H10/C11H3D7) in the reactions of the para-tolyl (C7H7) and para-tolyl-d7 (C7D7) with vinylacetylene (C4H4).

We present for the very first time single collision experimental evidence that a methyl-substituted polycyclic aromatic hydrocarbon (PAH)-2-methylnaphthalene-can be formed without an entrance barrier via indirect scattering dynamics through a bimolecular collision of two non-PAH reactants: the para-tolyl radical and vinylacetylene. Theory shows that this reaction is initiated by the addition of the para-tolyl radical to either the terminal acetylene carbon (C(4)) or a vinyl carbon (C(1)) leading eventually to two distinct radical intermediates. Importantly, addition at C(1) was found to be barrierless via a van der Waals complex implying this mechanism can play a key role in forming methyl substituted PAHs in low temperature extreme environments such as the interstellar medium and hydrocarbon-rich atmospheres of planets and their moons in the outer Solar System. Both reaction pathways involve a sequence of isomerizations via hydrogen transfer, ring closure, ring-opening and final hydrogen dissociation through tight exit transition states to form 2-methylnaphthalene in an overall exoergic process. Less favorable pathways leading to monocyclic products are also found. Our studies predict that reactions of substituted aromatic radicals can mechanistically deliver odd-numbered PAHs which are formed in significant quantities in the combustion of fossil fuels.

Reactions of C2H with 1- and 2-butynes: an ab initio/RRKM study of the reaction mechanism and product branching ratios.

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C(6)H(7) potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C(2)H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C(6)H(6)) + H and ethynylallene (C(5)H(4)) + CH(3) are predicted to be the major products of the C(2)H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C(2)H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C(2)H addition to 1-butyne, whereas ethynylallene + CH(3) are preferred for the terminal C(2)H addition. A relatively minor product favored at higher collision energies is diacetylene + C(2)H(5). Three other acyclic C(6)H(6) isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C(6)H(6) species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C(6)H(6) isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C(2)H + 2-butyne reaction, the calculations predict the C(5)H(4) isomer methyldiacetylene + CH(3) to be the dominant product, whereas very minor products include the C(6)H(6) isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.

Formation of 6-methyl-1,4-dihydronaphthalene in the reaction of the p-tolyl radical with 1,3-butadiene under single-collision conditions.

Crossed molecular beam reactions of p-tolyl (C7H7) plus 1,3-butadiene (C4H6), p-tolyl (C7H7) plus 1,3-butadiene-d6 (C4D6), and p-tolyl-d7 (C7D7) plus 1,3-butadiene (C4H6) were carried out under single-collision conditions at collision energies of about 55 kJ mol(-1). 6-Methyl-1,4-dihydronaphthalene was identified as the major reaction product formed at fractions of about 94% with the monocyclic isomer (trans-1-p-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by barrierless addition of the p-tolyl radical to the terminal carbon atom of the 1,3-butadiene via a van der Waals complex. The collision complex isomerizes via cyclization to a bicyclic intermediate, which then ejects a hydrogen atom from the bridging carbon to form 6-methyl-1,4-dihydronaphthalene through a tight exit transition state located about 27 kJ mol(-1) above the separated products. This is the dominant channel under the present experimental conditions. Alternatively, the collision complex can also undergo hydrogen ejection to form trans-1-p-tolyl-1,3-butadiene; this is a minor contributor to the present experiment. The de facto barrierless formation of a methyl-substituted aromatic hydrocarbons by dehydrogenation via a single event represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated analogues in combustion flames and the interstellar medium.

A crossed beams and ab initio investigation on the formation of cyanodiacetylene in the reaction of cyano radicals with diacetylene

The crossed molecular beams reaction of ground state cyano radicals (CN) with diacetylene (HCCCCH) was studied in the laboratory under single collision conditions. Combining the derived center-of-mass translational energy and angular distributions with novel electronic structure calculations, we show that the linear cyanodiacetylene molecule (HCCCCCN) is the sole reaction product. Our study provided no substantiation of two alternative products which have been suggested previously: cyanoacetylene (HCCCN), speculated to be synthesized via the exchange of the ethynyl by the cyano group, and the 1,3-butadiynyl radical (HCCCC), thought to be formed via hydrogen abstraction. The unambiguous identification of cyanodiacetylene formed in an exoergic, barrierless bimolecular reaction of the cyano radical with diacetylene strongly suggests that cyanodiacetylene can be also synthesized via this process in the interstellar medium (cold molecular clouds) and in hydrocarbon-rich atmospheres of planets and their moons such as Titan.

Separation mechanism of chiral impurities, ephedrine and pseudoephedrine, found in amphetamine-type substances using achiral modifiers in the gas phase

A new mechanism is proposed that describes the gas-phase separation of chiral molecules found in amphetamine-type substances (ATS) by the use of high-resolution ion mobility spectrometry (IMS). Straight-chain achiral alcohols of increasing carbon chain length, from methanol to n-octanol, are used as drift gas modifiers in IMS to highlight the mechanism proposed for gas-phase separations of these chiral molecules. The results suggest the possibility of using these achiral modifiers to separate the chiral molecules (R,S) and (S,R)-ephedrine and (S,S) and (R,R)-pseudoephedrine which contain an internal hydroxyl group at the first chiral center and an amino group at the other chiral center. Ionization was achieved with an electrospray source, the ions were introduced into an IMS with a resolving power of 80, and the resulting ion clusters were characterized with a coupled quadrupole mass spectrometer detector. A complementary computational study conducted at the density functional B3LYP/6-31g level of theory for the electronic structure of the analyte–modifier clusters was also performed, and showed either “bridged” or “independent” binding. The combined experimental and simulation data support the proposed mechanism for gas-phase chiral separations using achiral modifiers in the gas phase, thus enhancing the potential to conduct fast chiral separations with relative ease and efficiency.

Theoretical investigation of the mechanism and product branching ratios of the reactions of cyano radical with 1- and 2-butyne and 1,2-butadiene.

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311g(d,p) calculations of the C(5)H(6)N potential energy surface have been performed to investigate the reaction mechanism of cyano radical (CN) with C(4)H(6) isomers 1- and 2-butyne and 1,2-butadiene. They were followed by RRKM calculations of the reaction rate constants and product branching ratios under single-collision conditions in the 0-5 kcal/mol collision energy range. With the assumption of equal probabilities of the barrierless terminal and central addition of the cyano radical to 1-butyne, 2-cyano-1,3-butadiene + H, and cyanoallene + CH(3) are predicted to be the major reaction products with a branching ratio of ∼2:1. The terminal CN addition to C(1) favors the formation of cyanoallene + CH(3), whereas the central CN addition to C(2) enhances the formation of 2-cyano-1,3-butadiene + H. For the CN + 2-butyne reaction, the dominant product is calculated to be 1-cyano-prop-1-yne + CH(3), and the CH(3) loss occurs directly from the initial adduct formed by the barrierless CN addition to either of the two acetylenic carbon atoms. A small amount of the H loss product, 3-cyano-1,2-butadiene (1-cyano-1-methylallene), can be also formed as was observed in earlier crossed molecular beam experiments. Three different products are predicted for the CN + 1,2-butadiene reaction, which also occurs without entrance barriers. If various initial complexes formed by the CN addition to C(1), C(2), C(3), or to the C═C double bonds in 1,2-butadiene are produced in the entrance channel with equal probabilities, the dominating product (70-60%) is 2-cyano-1,3-butadiene + H, and the other significant products include 1-cyano-prop-3-yne + CH(3) (19-25%) favored by the initial CN addition to C(1) and cyanoallene + CH(3) (11-15%) preferred for the CN addition to C(3). The H abstraction HCN + C(4)H(5) products may also be formed either from the initial CN addition adducts through a CN roaming mechanism or via certain trajectories directly from the initial reactants, but their yield is not expected to be significant, at least at low temperatures. The energetics, mechanisms, and product branching ratios of the cyano radical reactions with various C(4)H(6) isomers and their analogous isoelectronic C(2)H + C(4)H(6) reactions have been summarized and compared.

A Combined Experimental and Theoretical Study on the Gas‐Phase Synthesis of Toluene under Single Collision Conditions

physical-organic chemistry communities. Toluene portrays the simplest representative of an alkyl-substituted benzene molecule with the methyl group enhancing the reactivity toward radical and electrophile aromatic substitution compared to benzene. It is further considered as a crucial building block to form methyl-substituted polycyclic aromatic hydrocarbons (PAHs) such as 1and 2-methylnaphthalene. In combustion flames, the formation of phenylacetylene 2, styrene 3, and biphenyl 4 (Scheme 1) can be rationalized through phenyl addition. Atomic hydrogen is eliminated upon reaction of the phenyl radical with acetylene (C2H2), [14] ethylene (C2H4), [15] and benzene (C6H6). [16] The analogous reaction of the phenyl radical with methane (CH4) does not lead to toluene, but solely to benzene plus a methyl radical (CH3) through hydrogen abstraction. [17] Consequently, the formation routes of toluene in combustion flames and in the interstellar medium have not been unraveled to date. Here, we show that the reaction of the ethynyl radical (CCH; C2H) with isoprene (2-methyl-1,3-butadiene; C5H8) presents a facile, barrierless route to the toluene molecule in a single collision event in the gas phase through the reaction of two acyclic precursor molecules. This reaction is also of interest to the physical-organic chemistry community since it represents a benchmark system to untangle the formation of a methylsubstituted aromatic molecule through radical substitution reactions involving successive isomerizations through cyclization and hydrogen shifts. Reactive scattering signal from the reaction of [D1]ethynyl radical C2D(X S) with isoprene (C5H8; X A’) was monitored at mass-to-charge ratios (m/z) of 93 (C7H7D ), 92 (C7H6D /C7H8 ), and 91 (C7H5D /C7H7 ). The signal atm/ z= 93 originates from the C7H7D product(s) formed through atomic hydrogen loss, whereas ion counts at m/z= 92 could have two contributions: an atomic deuterium loss connected to the formation of C7H8 products or dissociative ionization of C7H7D products in the electron impact ionizer of the detector. The signal at m/z= 91 may depict two contributions from dissociative electron impact ionization of C7H7D and/or C7H6D product molecules. However, all data could be fit with the product mass combinations of 93 amu (C7H7D) plus 1 amu (H). This finding suggests the existence of a [D1]ethynyl (C2D, 26 amu) versus atomic hydrogen replacement channel and the gas-phase synthesis of a molecule with the molecular formula C7H7D (Figure 1). Considering that the [D1]ethynyl reactant does not have a hydrogen atom, the hydrogen atom is emitted from the isoprene molecule. The laboratory angular distribution (Figure 2) extends at least 408 within the scattering plane and peaks close to the center-of-mass angle at 41.1 1.28. These results suggest indirect scattering dynamics through C7H8D complex(es). Scheme 1. Toluene 1, phenylacetylene 2, styrene 3, and biphenyl 4.