Mechanisms of glycine formation from aminoacetonitrile in space.

The potential energy surfaces for the formation of glycine (Gly) or its precursors, such as Gly radical cation and deprotonated Gly, starting from aminoacetonitrile (NH2CH2CN) or deprotonated aminoacetonitrile were determined by CBS-QB3 calculation. The reaction of NH2CH2CN + H2O+˙ + H2O gave mainly unwanted products other than Gly or its precursors. The reactions of ˙NHCH2CN + 2H2O, NH2˙CHCN + 2H2O, or NH2CH2CN + ˙OH + H2O, led to Gly or its precursors as one of the main products. The overall activation barriers were 229 kJ mol-1, 216 kJ mol-1, and 118 kJ mol-1, respectively. A barrierless pathway for the formation of Gly from the reaction of NH2CH2CN + ˙OH + H2O with a catalytic H2O was found, strongly suggesting that Gly can be formed thermally from NH2CH2CN and ˙OH on icy grain surfaces in the interstellar medium.

A Series of Quinolinol-Based Indium Luminophores: A Rational Design Approach for Manipulating Photophysical Properties.

An approach to the design of a series of quinolinol-based indium complexes that can exhibit different optical properties is proposed. Mono-incorporated (Inq1 and InMeq1), bis-incorporated (InMeq2), and tris-incorporated (Inq3 and InMeq3) indium quinolinate complexes have been prepared. These complexes have also been characterized by X-ray crystallography. The photophysical properties of these complexes have also been examined by a combination of experimental and theoretical techniques. The indium complexes with a single quinolinol ligand (Inq1 and InMeq1) showed higher quantum efficiency than those with two or three quinolinate ligands; in particular, InMeq1 exhibited the highest quantum yield [ΦPL = 59% in poly(methyl methacrylate) film]. The insights into the nature of these findings were obtained by the sequential synthesis of the quinolinol-based indium luminophores and a detailed investigation of their structural stability.

Infrared predissociation of ternary cluster cations: the solvent effects on the branching ratio.

Infrared (IR) predissociation of hydrogen-bonded ternary cluster ions such as aniline-water-ethanol (AWE(+)), aniline-water-isopropanol (AWP(+)), aniline-methanol-ethanol (AME(+)), aniline-water-pyrrole (AWPy(+)), and aniline-water-benzene (AWB(+)) was examined in the region of 2700-4000 cm(-1) to explore the key factors which determine the branching ratios in the concurrent unimolecular dissociation. The smaller solvent molecule was predominantly ejected when the binding energies of the two were not too different. On the other hand, when they were far off, the binding energy also acted significantly on the branching ratio. Besides, mode-selective IR predissociation was observed, while the selectivity was not quite distinct. The IR predissociation of ternary cluster ions bound via hydrogen bonding is considered to occur on a time scale much faster than intramolecular vibrational energy redistribution, which was proved by a statistical transition state theory.

Mechanism of Prebiotic Uracil Synthesis from Urea and HC3O+ in Space.

The potential energy surface for the formation of protonated uracil (UH+) from urea and HC3O+ was explored by performing quantum chemical complete basis set-QB3 calculations. A barrierless pathway was found for the formation of UH+, which was estimated to occur in the interstellar medium (ISM) much faster than the timescale of chemical revolution of typical dense interstellar clouds. Investigation of further reactions of UH+ formed through the obtained pathway led to the conclusion that uracil could be produced on icy grain surfaces but not in the gas phase of the ISM.

Formation of C4H4(•+) from the pyridine radical cation: a theoretical mechanistic and kinetic study.

The potential energy surface (PES) for the formation of C(4)H(4)(•+) from the pyridine radical cation by loss of HCN was determined from quantum chemical calculations using the G3//B3LYP method. The complete reaction pathways for the formation of the low-energy C(4)H(4)(•+) isomers, radical cations of methylenecyclopropene (MCP(•+)), vinylacteylene (VA(•+)), cyclobutadiene, and butatriene were obtained. Based on the PESs, a Rice-Ramsperger-Kassel-Marcus model calculation was performed to investigate the dissociation kinetics. The calculated dissociation rate constants agreed with the previous experimental data. It was predicted that a mixture of MCP(•+) and VA(•+) was formed by loss of HCN. The formation of MCP(•+) was more favored near the dissociation threshold and at high energies, whereas the formation of VA(•+) was more favored at the low energies corresponding to the ion lifetime of microseconds.

Formation of C7H7(+) from benzyl chloride and chlorotoluene molecular ions: a theoretical study.

The potential energy surface (PES) for the formation of C7H7(+) from benzyl chloride and chlorotoluene ions was obtained by quantum chemical calculations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level. On the basis of the PES, the RRKM model calculations were carried out to predict the rate constants of the dissociations of the molecular ions of o-, m-, and p-chlorotoluene, all of which agreed well with previous experimental results. The kinetic analysis showed that the benzylium ion was the predominant product in the dissociations of the four isomeric molecular ions, below the thresholds of the formation of tolylium ions.

Dissociation mechanism of the 3-hydroxypyridine radical cation: A computational study.

The potential energy surface (PES) for the losses of CO and HCN from the 3-hydroxypyridine radical cation (3-HP(+•)) and for the isomerizations to 2- and 4-HP(+•) was determined from G3//B3LYP calculations. The overall activation energies are similar for six dissociation channels to form the radical cations of CH(2)=CHCH=N=CH, 1-H, 2-H, and 3-H pyrrole by the loss of CO and to form the radical cations of CH(2)=CHCH=C=O and furan by the loss of HCN. A kinetic analysis based on the obtained PES shows that the acyclic C(4)H(5)N(+•) and C(4)H(4)O(+•) ions are the main products formed by the losses of CO and HCN, respectively.

Long-lived excited electronic states of the dichloroethylene cation isomers probed by charge exchange ionization.

Charge exchange technique has been used to detect the presence of long-lived excited electronic states of trans-, cis-, and 1,1-C(2)H(2)Cl(2)(+.). The B states of the three cations which are formed by removal of an electron from an in-plane chlorine nonbonding orbital of the corresponding neutrals have been found to have long lifetimes (tens of microseconds or longer). Whether the A states formed by removal of an electron from the other in-plane chlorine nonbonding orbitals are long-lived also can not be determined by the present experiments. Cations in the excited electronic states above the B? states were not detected because of their prompt dissociation following intramolecular relaxation or radiative decay.

Discovery of long-lived excited electronic states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations.

Charge exchange technique has been used to detect the presence of long-lived excited electronic states of some monosubstituted ethene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (DeltaE < or = 0), or the exoergicity rule. The A2A' states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations have been found to have long lifetimes (tens of microsecond or longer) while all the excited electronic states of vinylfluoride cation have been found to undergo rapid dissociation or nonradiative relaxation to the ground state. The long-lived states found are those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these are the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or the in-plane pi orbital of the triple bond C[triple bound]N. The present findings are very similar to those for the monosubstituted benzene cations reported previously.

Mechanisms of prebiotic adenine synthesis from HCN by oligomerization in the gas phase.

We explored the potential energy surfaces for adenine synthesis by oligomerizations of HCN or HNC from CBS-QB3 calculations. The pathways have been obtained for the formation of the covalently bound HCN dimer, trimer, tetramer, and pentamer (adenine) by sequential additions of HCN or HNC. The activation energies of the individual oligomerization stages are a few hundred kilojoules per mole, which prevent efficient adenine synthesis in interstellar space or in the atmosphere of Titan. On the other hand, when the oligomerizations start from HCNH(+), the activation energies of sequential HCN or HNC additions are significantly reduced. Kinetic analyses results suggest that adenine synthesis by proton-catalyzed oligomerizations cannot occur efficiently in interstellar space or in the atmosphere of Titan, even though some oligomerization stages can occur under the latter condition.

Why do the abundances of ions generated by MALDI look thermally determined?

In a previous study (J. Mass Spectrom. 48, 299-305, 2013), we observed that the abundance of each ion in a matrix-assisted laser desorption ionization (MALDI) spectrum looked thermally determined. To find out the explanation for the phenomenon, we estimated the ionization efficiency and the reaction quotient (QA) for the autoprotolysis of matrix, M + M → [M + H](+) + [M - H](-), from the temperature-controlled laser desorption ionization spectra of α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB). We also evaluated the equilibrium constants (KA) for the autoprotolysis at various temperatures by quantum chemical calculation. Primary ion formation via various thermal models followed by autoprotolysis-recombination was compatible with the observations. The upper limit of the effective temperature of the plume where autoprotolysis-recombination occurs was estimated by equating QA with the calculated equilibrium constant. Figure ᅟ

Tunneling in the loss of hydrogen chloride from isopropyl chloride cation.

The unimolecular dissociation of isopropyl chloride cation has been investigated using mass-analyzed ion kinetic energy spectrometry. The C3H6*+ ion was the only product ion in the metastable dissociation. The kinetic energy release distribution for the HCl loss was determined. Ab initio molecular orbital calculations were performed at the MP2/6-311++G(d,p) level together with single point energy calculations at the QCSID(T)/6-311++G(2d,2p) level. The calculations show that the molecular ion rearranges to an ion-dipole complex prior to loss of HCl via a transition state containing a four-membered ring. The rearrangement involves H atom transfer. On the basis of the potential energy surface obtained for the loss of HCl and Cl*, the rate constants were calculated by transition-state statistical theories with considering tunneling effect. From the calculated result, it is proposed that the observed HCl loss would occur via tunneling through the barrier for isomerization to the ion-dipole complex, CH3CHCH2*+...HCl.

Dissociation of toluene cation: a new potential energy surface.

The potential energy surface (PES) for the formation of tropylium and benzylium ions from toluene cation (1) has been explored theoretically. Quantum chemical calculations at the B3LYP/6-311++G and G3//B3LYP levels were performed. A pathway to form o-isotoluene (5-methylene-1,3-cyclohexadiene) cation (5) from 1 was found. The isomerization occurs by two consecutive 1,2-H shifts from CH(3) to the ortho position of the aromatic ring via a distonic benzenium cation (2), which is also an intermediate in the well-known isomerization of 1 to cycloheptatriene cation (4). Since the barrier for the formation of 2 is the highest in the two isomerization pathways, 1, 4, and 5 are interconvertible energetically prior to dissociation. The benzylium ion can be produced via 5 as well as from 1 and the tropylium ion via 4. Rice-Ramsperger-Kassel-Marcus model calculations were carried out based on the obtained PES. The result agrees with previous experimental observations. From a theoretical analysis of kinetics of the isomerizations and dissociations, we suggest that 5 plays an important role in the formation of C(7)H(7)(+) from 1.

Collision-induced dissociation of the phenylsilane molecular ion.

The high-energy collision-induced dissociation of the phenylsilane molecular ion generated by electron ionization has been investigated using tandem mass spectrometry (MS/MS). It was observed that the dissociation of the molecular ion (M(+*)) occurs mainly via [M-H](+), [M-2H](+*), and [M-3H](+), followed by two consecutive losses of C(2)H(2). The structures of the precursors for the [M-CH(3)](+), [M-SiH](+), and [M-SiH(2)](+*) ions are proposed. The data suggest that the molecular ion undergoes rearrangements to several isomers prior to dissociation, including the ion containing a five-membered carbon ring. Reaction mechanisms are proposed for the dissociations via the isomeric molecular ions.

Reliability of estimation of recombination energies of molecular radical cations by charge exchange and a test of the exoergicity rule.

Charge exchange occurring in a collision cell of a mass spectrometer was used previously to find the very long-lived excited electronic states of molecular cations generated by electron ionization. The underlying criterion in the method, called the exoergicity rule, was that only electronically exoergic and resonant processes have large cross sections at low collision energy. The validity of this rule has been checked in this work by estimating the recombination energies of some molecular ions based on the presence/absence of charge-exchange signals from gases with different ionization energies, and comparing these estimates with the literature values. It is concluded that a recombination energy can be estimated to within an upper limit uncertainty of 0.1 eV with this technique, which validates the exoergicity rule proposed previously.