D. C. Conway

Ion‐molecule equilibria in mixtures of N2 and Ar

Equilibrium constants for the reactions 2N2+N2+=N4++N2 (I); N4++Ar=ArN2++N2 (II); ArN2++Ar=Ar2++N2 (III) have been determined mass spectrometrically. The reactions were studied as a function of E/P and temperature in a drift tube. It was found that ΔSI° = −16.2 ± 2.9 eu and ΔHI° = −24.4 ± 2.1 kcal/mole at 723°K. By extrapolation of enthalpy data to 0°K, the bond energies, D0(AB+), were found to be 25.9 ± 2.1, 26.2 ± 2.2, and 27.8 ± 2.2 kcal/mole for N2–N2+, Ar–N2+, and Ar–Ar+, respectively. When presumably more accurate literature data for Reaction (I) were used, the D0(AB+) for the same series were found to be 24.3, 24.5, and 26.3 kcal/mole. With literature data for Reaction (I) ΔSII° = 3.57 ± 0.06 eu, ΔHII° = −0.549 ± 0.026 kcal/mole, ΔSIII° = 1.87 ± 0.08 eu and ΔHIII° = 2.16 ± 0.03 kcal/mole at 298°K. Various models were used to determine the model dependence of the quantities derived from the equilibrium constant data. The ΔHn°, ΔSn° and D0(AB+) values are only weakly dependent on the assumed bond len...

Geometries of O4+, O4−, and N4+ by an Approximate SCF–MO Theory Which Considers Intermolecular Differential Overlap

We have found that the CNDO/2 (semiempirical SCF–MO) theory does not properly account for the long‐range repulsive interactions in O2–O2 (S = 2) and O2–O2+ (S = 32). This results in the O2–O2 interaction potential being zero at O–O separations of 2.3–3.5 A and the O2–O2+ bond energy being much too large. The CNDO/2 method has been modified by considering sp‐type integrals to account for charge–quadrupole and charge–atomic polarization interactions. In addition, differential overlap is considered and multicenter integrals are approximated for intermolecular interactions. This is a semiempirical method in which the adjustable constants are fixed by the quadrupole moments of O2 and N2 and the O4+ bond energy. The interaction energies of O4+, O4−, and N4+ are computed for structures of C2 symmetry as a function of θ and R. The equilibrium dissociation energies De (kcal) at Re (A) and θe are as follows: O4+, 11.66, 2.03, − 67°; O4−, 8.49, 2.08, − 71°; N4+, 34.7, 2.04, O°. The computed O2–O2 interaction potenti...

Study of the 2Ar+Ar+2=Ar+Ar+3 reaction

Equilibrium constants for the reaction 2Ar+Ar+2=Ar+3+Ar have been determined mass spectrometrically from 144 to 217 °K. Assuming Ar+3 to be linear, ΔH °200=−5.06±0.08 kcal/mole, ΔS °200=−20.3±0.4 cal/mole deg, and D0(Ar+2−Ar) =5.05±0.11 kcal/mole. The ΔS° is too negative for the ’’loose cluster’’ model in which it is assumed that the Ar+2 is freely rotating in the Ar+3 cluster. It is concluded that Ar+3 is probably linear. The rate constant for the formation of Ar+3 was computed and compared to experimental results at 77 and 298°K.

Determination of the Bond Energies for the Series O2–O2+ through O2–O10+

Equilibrium constants for the reactions O2 + O2n+ = O2n+2+, n = 1–8, have been determined mass spectrometrically. The fraction of the O4+–O12+ ions which dissociate in the spectrometer is computed theoretically. It is found that a signicant fraction of the O10+ and O12+ ions dissociate in this region. The equilibrium constants are corrected for this and other phenomena. Rotational entropies are computed for the theoretical structures of these complexes. Vibrational entropies are used to estimate weak mode vibrational frequencies. By extrapolation of the enthalpy data to 0°K, the bond energies are found to be D0(O2–O2+ = 10.53 ± 0.10, D0(O2–O4+) = 6.51 ± 0.07, D0(O2–O6+) = 2.56−0.06+0.16, D0(O2–O8+) = 2.43−0.09+0.06, and D0(O2–O10+) = 2.0−0.6+0.4kcal/mole. The De(O2–O2n+) for n = 3–5 obtained by correcting D0(O2–O2n+) for zero‐point energy differences are in reasonable agreement with the values computed by a classical electrostatic model. The vibrational entropy of O4+ at 298°K is found to be 9.93 e.u. Thi...

Ion–molecule reactions in Ar at 296, 195, and 77 °K

Ion–molecule reactions have been studied at 296 and 195 °K in Ar gas which has been purified by passage through a Ti furnace. The rate constant k1 for the reaction, Ar+(2P3/2) + 2Ar → Ar+2 + Ar, was measured in a drift tube at pressures reduced to 0 °C (P0) of 1.414–3.00 torr. Ions effused from the drift tube and were counted with a TOF mass spectrometer. k1 was corrected for side reactions with impurities and found to be (2.3±0.2) ×10−31 and (3.0±0.2) ×10−31 cm6/sec at 296 and 195 °K, respectively. It appears that Ar+ in the 2P1/2 state does not form Ar+2 at these temperatures. An upper limit to the rate constant for the reaction Ar+(2P1/2) + Ar → Ar+(2P3/2) + Ar + energy, was observed to be 2×10−16 cm3/sec at 296 °K. At 77 °K both states of Ar+ appear to form Ar2+ with a rate constant of (5.2±0.1) ×10−31 cm6/sec. The rate of formation of Ar+3 and the equilibrium between Ar+3 and Ar+4 were also studied at 77 °K.

Stability of the NO+⋅N2 ion cluster

The equilibrium constant K for the reaction NO++2N2?NO+⋅N2+N2 has been determined mass spectrometrically from 178–273 °K at N2 pressures of 3–12 Torr. Small quantities of O2 were added to the gas stream to catalyze the charge exchange between N4+ and NO. By analysis of the K data it was found that ΔH°=−5.04±0.09 kcal/mole and ΔS°=−18.5±0.3 cal/mole °K at 298 °K. By extrapolation of the enthalpy data to 0 °K, the NO+–N2 bond energy was found to be 4.98±0.12 kcal/mole.

Abinitio and semiempirical SCF–MO computations for N+4; comparison of spin unrestricted and spin restricted methods

Various basis sets (all but one minimal) were used in spin unrestricted ab initio SCF–MO computations on N+4. Computed values of the N+4 dissociation energy, D (N2−N+2), ranged from 49–72 kcal/mole for linear N+4 at a fixed internuclear separation R of 2.00 A. Smaller dissociation energies were obtained by a spin restricted ab initio method in which the α‐ and β‐spin eigenvector matrices were forced to be the same. All dissociation energies are larger than the experimental bond energy, but the smallest value was obtained with the extended basis set by the spin restricted ab initio method. There De(N2−N+2) was found to be 32.7 kcal/mole at Re=1.90 A. Minimal basis set semiempirical computations were found to give results similar to those obtained by ab initio computations.

Ion‐molecule reaction rates in Ar at 296°K

The rate constant k1 for the reaction Ar+(2P3/2) + 2Ar→Ar2+ + Ar was found to be (2.07±0.20)×10−31 cm6/sec at 296°K. The reaction was studied in a drift tube at pressures reduced to 0°C (P0) of 1.414–3 torr. Ions effused from the drift tube and were counted with a TOF mass spectrometer. It was found that Ar+ in the 2P1/2 state does not form Ar2+ at room temperature. This fact appears to have been neglected in previous determinations of k1. Corrections were made for the collision‐induced transition Ar+(2P1/2) → Ar+(2P3/2), where k2 is the rate constant, and for side reactions with impurities. From experiments at P0=6 and 9 torr it was found that k2,Ar≤1.5×10−15 cc/sec. However, recent experiments indicate k2,Ar<4×10−16 cc/sec, so most of the transitions were caused by collisions with an impurity in the Ar.

Possible O8+–O12+ Structures Obtained by Use of Classical Electrostatic Theory

The interaction energy in the series O2–O6+ through 3O2–O6+ is approximated as the sum of the van der Waals interactions between the neutral species and the following electrostatic interactions: monopole–quadrupole, quadrupole–quadrupole, and (monopole+quadrupole)–induced dipole+induced dipole–induced dipole. Two planar O6+ structures and charge distributions obtained by the CDO–IBM method were used in the O8+ computations. The computed dissociation energy De(O2–O6+) is certainly too large for the O6+ structure with the larger charge on the central O2. The De values computed for the O6+ structure of smaller charge are as follows: De(O2–O6+) = 3.42, De(O2–O8+) = 3.26, and De(O2–O10+) = 3.12 kcal/mole. Although these values are larger than the experimental ΔH° data, they are in reasonable agreement considering the uncertainties in the van der Waals potential and the quadrupole moments and the lack of data concerning the higher moments and polarizabilities, e.g., the quadrupole polarizability. The O8+–O12+ h...

Possible O6+ Structures Obtained by a Semiempirical SCF–MO Method

A semiempirical SCF–MO method which considers intermolecular differential overlap has been used to compute the total dissociation energy (to 2O2 + O2+) for various planar structures of O6+. The five more stable structures with dissociation energies about two‐thirds the experimental value are all possible structures at the potential minimum. It is concluded that the spins of the five “1πg‐like” electrons are probably parallel in the O6+ ground state.