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.

Protonation energies of 1-5-ring polycyclic aromatic nitrogen heterocyclics: comparing experiment and theory.

Polycyclic nitrogen heterocyclic compounds (PANHs) can be protonated in the gas phase in mass spectrometry, in solution in acidic and biological environments, and if present, in interstellar clouds. Intrinsic molecular effects on PANH basicities can be observed by their gas phase protonation thermochemistry. We determined the gas phase basicities/proton affinities (GBs/PAs) of prototype one-nitrogen, 3-5-ring PANH compounds of increasing sizes and polarizabilities by kinetic bracketing, using proton transfer reactions to reference bases. The experimental proton affinities increase from 1-ring (pyridine, 222.2); to 2-ring (quinoline, 227.8); to 3-5-ring compounds, 227-234 kcal mol(-1). We also calculated the GB/PA values at the M06-2X/6-311+G**//B3LYP/6-31g* level. The computed PAs agree, within the experimental uncertainty, with the experimental values anchored to the upper range of the NIST GB/PA database. Specifically, the computed PAs are smaller than the experimental values by 1.4 ± 0.9 kcal/mol for nonaromatic nitrogen reference bases and for 1-5-ring PANHs, independently of the number of rings, aromaticity, and molecular size. Therefore, a useful method to calculate proton affinities of PANH compounds can use M06-2X/6-311+G**//B3LYP/6-31g* computational PAs + 1.4 ± 0.9 kcal mol(-1). The agreement with experiment supports the NIST database within this accuracy, in the upper range up to 235 kcal mol(-1), even though there are no direct absolute experimental anchor points in this range. For astrochemical applications, the measured PAs allow calculating the energies of the (PANH)(+•) + H2 → (PANH)H(+) + H(•) reactions that may convert the radical ions to less reactive 11-electron ions. The reactions are endothermic or nearly thermoneutral for the 3-5-ring ions and would be very slow at low temperatures, allowing reactive (PANH)(+•) radical ions to persist in interstellar clouds.

Structural, theoretical and spectroscopic studies of the dichloride hexahydrate cube [Cl2(H2O)6]2-.

The structure of the dichloride hexahydrate cube, [Cl(2)(H(2)O)(6)](2-), as a salt with the tris(diisopropylamino)cyclopropenium cation, [C(3)(N(i)Pr(2))(3)](+), has been determined by low-temperature X-ray and neutron-diffraction studies. H atoms not involved in O-H···Cl bonding are disordered over two 0.5 occupancy sites around the O(6) ring. Calculations of the dianionic cube in the gas phase show remarkably good agreement with the solid-state structures with the exception of short O-H bond distances around the O(6) ring that suggests the involvement of a dynamic process. The cluster was also characterised by single-crystal infrared spectroscopy, and vibrational wavenumbers were found to be in good agreement with hydrogen bonding distances. Dibromide and difluoride hexahydrates were also studied theoretically, and O···O distances were found to decrease in the order difluoride > dichloride > dibromide > (H(2)O)(6) and as O···O···O angles increased towards an almost planar ring in (H(2)O)(6). NMR spectra of a chloroform solution of the hydrated salt at -25 °C is consistent with cluster formation.

First-principle calculations on the microscopic 57Fe electric-field-gradient tensor of ferrous chloride tetrahydrate: a prototypical Mössbauer species.

Since the pioneering work on the theoretical description of Mössbauer quadrupole line intensities for a single-crystal to elucidate information on the electric-field-gradient (EFG) tensor, ferrous chloride tetrahydrate, FeCl(2)·4H(2)O, has represented a prototypical Mössbauer species. In addition, this species also typifies a so-called ambiguous, low symmetry, iron center where traditionally only macroscopic tensors have been assumed available for determination. Recent experiments on FeCl(2)·4H(2)O have successfully determined physically meaningful spin-Hamiltonian parameters for the (57)Fe microscopic (local) EFG and mean-squared displacement tensors. This paper reports a density functional theory investigation that finds good agreement with experiment. Assuming a gas-phase scaffold cluster approach to describe the crystalline geometry, the sign of the EFG was determined to be positive, in agreement with earlier magnetically perturbed experiments, and the EFG asymmetry parameter η was calculated to be 0.21-0.23 depending on the density functional used, which is in excellent accord with experiment at 0.25(2). By virtue of theoretical and experimental agreement, this work indicates that simultaneous electric-field-gradient-mean-square-displacement Mössbauer determinations can resolve the apparent ambiguity associated with monoclinic, 2/m or 1 Laue class, sites provided there is sufficient anisotropy in the Lamb-Mössbauer recoilless fraction.

On the electron affinity of nitromethane (CH3NO2).

A high-level systematic computational study is presented on an accurate value for the adiabatic valence electron affinity of nitromethane, CH(3)NO(2), to resolve literature disagreements in theoretical and experimental reported values. Density functional methods with triple-zeta quality basis sets gave good fortuitous agreement to early experimental determinations, while single-reference wave function based methods employing up to CCSD(T) gave poor or fortuitous agreement depending on the experimental reference value. DFT methods in general cannot accurately describe electron attachment from the result of unphysical self-interaction. It is found that multireference methods with aug-cc-pVTZ or similar basis sets are required to converge to an experimental value. Our highest level of theory 3S-MCQDPT2 and 7S-MCQDPT2 calculations with an aug-cc-pVTZ quality basis description yield values of 0.188 and 0.176 eV (0.170 eV with polynomial extrapolation), in excellent agreement with the most recent experimental value of 0.172 +/- 0.006 eV. CCSD(T)//aug-cc-pVTZ provides a fortuitously reasonable description. The isolated dipole-bound anion binding energy is tentatively calculated to be 7-8 meV.

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.