Robert G. A. R. Maclagan

Some ways of looking at compensatory kosmotropes and different water environments

Hydration of macromolecular structures determines biological activity. Stabilizing solutes are kosmotropic (increase order of water) rather than chaotropic (decrease order). Preferential hydration of surfaces is a thermodynamic consequence of the solution behavior of kosmotropic solutes, but inconsistencies imply interactions such as the hydration of specific sites within macromolecules. Thermodynamic measures require bulk pure solutes; here simpler measures of the effects on bulk water, water at surfaces and hydration water of probes have been applied to solutes including natural stabilizers, analogues and example chaotropes. Changes in the near-infrared spectra, water proton NMR chemical shifts and relaxation times measure changes in the bulk liquid; HPLC-column retention of solutes indicate interactions with hydration water at different surfaces, and fluorescence probes detect effects on functional group hydration water. Ab initio calculations and Monte-Carlo simulations of the solutes in water measure the energetics of the solute-water interactions, the dipole moments of these molecules, their charge distributions and the effect of the solute molecules on the structure of water. The rankings of the test solutes by these measures are not consistent. Thus, stabilizing solutes are not interchangeable in biological systems and the intracellular replacement of one by another could affect the integration of cell metabolism.

Ab initio calculations related to the formation of propynal and propadienone in interstellar clouds

Abstract The proposal that propynal, CHCCHO, observed in interstellar clouds is formed by the ion molecule reacion of C 2 H 3 + and CO has been investigated in a G2 study. The calculations show that propadienone CH 2 CCO could be formed by this reaction but not propynal. At the G2 level of theory, propadienone is calculated to be lower in energy than propynal.

Termolecular ion-molecule reactions in titan's atmosphere. II : The structure of the association adducts of HCNH+ with C2H2 and C2H4

The ion—molecule reactivity of the products formed in the association reactions of HCNH+ with C2H2 (C3H4N+) and C2H4 (C3H6N+) has been investigated to provide information on the structures of the adducts thus formed. The C3H4N+ and C3H6N+ adducts were formed in the reaction flow tube of a flowing afterglow sourced-selected ion flow tube (FA-SIFT) and their reactivity with a neutral molecular “probe” examined. The reactivity of possible known structural isomers for the C3H4N+ and C3H6N+ ions was investigated in both the FA-SIFT and an ion cyclotron resonance spectrometer (ICR). Ab initio investigations of the potential energy surfaces for both structures at the G2(MP2) level have also been performed and structures corresponding to local minima on both surfaces have been identified and evaluated. The results of these experimental and theoretical studies show that at room temperature, the C3H4N+ adduct ion contains two isomers; a less reactive one that is likely to be a four-membered cyclic covalent isomer (∼70%) and a faster reacting component that is probably an electrostatic complex (∼30%). The C3H6N+ adduct ion formed from HCNH+ + C2H4 at room temperature is a single isomer that is likely to be the four-membered covalently bound cyclic CH2CH2CHNH+ species.

Protonated Polycyclic Aromatic Nitrogen Heterocyclics: Proton Affinities, Polarizabilities, and Atomic and Ring Charges of 1–5-Ring Ions

Calculated proton affinities, polarizabilities, and some ionization energies and atomic and ring NBO charges are reported for 31 polycyclic aromatic nitrogen heterocyclics (PANHs) with 1-5 rings, calculated on the on the M06-2X/6-311+g**//B3LYP/6-31g* level of theory. The calculated proton affinities from 226 to 241 kcal mol(-1) for 3-5-ring compounds, predict well the relative experimental values. The proton affinities increase with increasing molecular size and show a linear correlation with polarizabilities. Linear geometry and nitrogen located in the central ring also favor increased proton affinity. These trends estimate a PA > 241 kcal mol(-1) for an infinite linear chain, end-ring-N PANH molecule, and >261 kcal mol(-1) for an edge-N-doped graphene sheet, making it a superbase. NBO analysis shows that from pyridineH(+) to large 5-ring ions, the N-H nitrogen carries a constant q(N) = -0.46 ± 0.1 charge, and the N-H hydrogen a constant q(H) = 0.43 ± 0.01 positive charge, similar to the q(H) in NH4(+). Overall, the NH group is nearly electrically neutral, and a nearly full positive charge is distributed on the aromatic hydrocarbon rings of the ions. When the nitrogen is in a central ring, that ring is negative, and the positive ionic charge is delocalized toward the end rings. When the nitrogen is in an end ring, the ionic charge is distributed more evenly. Increasing proton affinities with increasing polarizability result not from increasing charge transfer from the proton to the aromatic rings, but from increasing delocalization of the transferred charge in the aromatic hydrocarbon rings of the ions. In two-nitrogen compounds, interactions between the ring nitrogens decrease the proton affinities, but this effect decreases in larger ions.

The structure and vibrational frequencies of NH2NO

Abstract The geometry of NH 2 NO has been optimised at the HF level of theory with basis sets ranging from STO-3G to 6-311G ★★ . The lowest-energy structure is not planar as previously predicted. The geometry of NH 2 NO was optimised at the MP2/6-31G ★★ level of theory. The geometry of the planar structure was also optimised with various basis sets. The optimised structures and HF energies are reported for two confonners of NH 2 NO resulting from rotation about the NN bond. Harmonic vibrational frequencies for the four conformers are reported and scaled frequencies for the lowest energy structure given.

The proton affinity and selected ion/molecule reactions of diacetylene

Abstract The proton affinity (PA) of C 4 H 2 has been determined by measuring the forward and reverse rate coefficients for proton transfer, from selected ion flow tube measurements at 300 K, as 741 ± 4 kJ mol −1 . Ab initio calculations at the MP4SDQ/6-311G**//HF/6-311G**// level of theory give a proton affinity for C 4 H 2 of 734 kJ mol −1 . The PA of BrCN was measured as 747 ± 2 kJ mol −1 . These experimental PAs are relative PAs on a scale in which the PA of CH 3 NO 2 is 750 kJ mol −1 . Rate coefficients and branching ratios are reported for CH 3 OH + 2 , H 2 CN + , CH 3 NO 2 H + , HCOOH + 2 , HBrCN + and C 2 H 5 IH + with C 4 H 2 and C 2 H 5 I, BrCN, CH 3 NO 2 with C 4 H + 3 . Rate coefficients for proton transfer in the system BrCNH + (CH 3 NO 2 , BrCN)CH 3 NO 2 H + were also measured.

Transport coefficients for NO+ ions in helium gas: a test of the NO+He interaction potential

Abstract Transport cross-sections for the collision of positive nitrogen oxide ions with helium atoms have been computed from a theoretical NO + He interaction potential. These cross-sections have been used in a kinetic theory of diatomic ion motion in atomic gases to determine the mobility and diffusion coefficients parallel and perpendicular to an external electric field. Comparison of the calculated mobilities with experimental data shows that theory and experiment agree within their mutual uncertainties.

Interaction potentials and mobility calculations for the HeO+ system

Valence bond, SCF, and MP4SDQ calculations are reported for three low lying states of the HeO+ molecular ion; 4Σ(4S), 2Π(2D), and 2Π(2P). Together with the two‐temperature theory of ion transport, these interaction potentials have been used to calculate the drift velocity and reduced mobility of O+ in helium as a function of the electric field to gas number density ratio. The calculated HeO+(4Σ) interaction potentials adequately describe the mobility of ground state O+ in helium, however, the O+(2D) mobility calculated using the 2Π(2D) interaction potential does not match the experimental mobility measurements for the metastable O+* ion which have been reported as the O+(2D) state. An interaction potential is reported for HeO+[2Π(2P)] which will reproduce the experimental mobility of O+*.

Real‐time measurement of peroxyacetyl nitrate using selected ion flow tube mass spectrometry

The on-line detection of gaseous peroxyacetyl nitrate (PAN) using selected ion flow tube mass spectrometry (SIFT-MS) has been investigated using a synthetic sample of PAN in air at a humidity of approximately 30%. Using the H(3)O(+) reagent ion, signals due to PAN at m/z 122, 77 and 95 have been identified. These correspond to protonated PAN, protonated peractetic acid and its water cluster, respectively. These products and their energetics have been probed through quantum mechanical calculations. The rate coefficient of H(3)O(+) has been estimated to be 4.5 x 10(-9) cm(3) s(-1), leading to a PAN sensitivity of 138 cps/ppbv. This gives a limit of detection of 20 pptv in 10 s using the [M+H](+) ion of PAN at m/z 122.

ION-MOLECULE ASSOCIATION OF H3O+ AND C2H2 : INTERSTELLAR CH3CHO

The C2H2· H3O+ product of the ion–molecule association reaction between H3O+ and C2H2 is found to consist of a ca. 50 : 50 mixture of two isomeric ions. These two isomeric ions are identified in a selected ion flow tube, by their different proton transfer behaviour with the neutral reagents C2H5Br, 4-fluorotoluene, CH3OH and benzene, as protonated vinyl alcohol, CH2CHOH2+ and either protonated acetaldehyde, CH3CHOH+ or the electrostatic complex H3O+· C2H2. These conclusions are supported by Gaussian G2 level calculations based on ab initio molecular orbital theory, which are applied to calculate the proton affinities of CH3CHO, CH2CHOH, oxirane and acyclic CH2OCH2. Reaction rate coefficients and product ratios are also reported for the reactions of specific C2H5O+ isomers, viz: CH3CHOH+, CH3OCH2+ and [graphic omitted] with CH3OH, 4-fluorotoluene and C6H6. The implications of the current results to the interstellar synthesis of CH3CHO are discussed briefly.