Before we start, we need a working definition for MOPAC. The following description has been used many times to describe MOPAC: MOPAC is a general-purpose, semiempirical molecular orbital program for the study of chemical reactions involving molecules, ions, and linear polymers. It implements the semiempirical Hamiltonians MNDO, AM 1, MINDO/3, and MNDOPM3, and combir_es the calculations of vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants in a fully integrated program. Elements parameterized at the MNDO level include H, Li, Be, B, C, N, O, F, A1, Si, P, S, C1, Ge, Br, Sn, Hg, Pb, and I; at the PM3 level the elements H, C, N, O, F, A1, Si, P, S, C1, Br, and I are available. Within the electronic part of the calculation, molecular and localized orbitals, excited states up to sextets, chemical bond indices, charges, etc. are computed. Both intrinsic and dynamic reaction coordinates can be calculated. A transition-state location routine and two transition-state optimizing routines are available for studying chemical reactions.

MOPAC: A semiempirical molecular orbital program

The atmospheric chemistry of organic nitrates

To investigate the mechanical and other condensed phase properties of energetic materials using atomistic simulation techniques, the COMPASS force field has been expanded to include high-energy nitro functional groups In this paper, the parameterization and validation of COMPASS for nitrate esters (−ONO2) is presented The functional forms of this force field are of the consistent force field type The parameters were derived with an emphasis on the nonbonded parameters, which include a Lennard-Jones 9−6 function for the van der Waals (vdW) term and a Coulombic term for an electrostatic interaction To validate the force field, molecular mechanics calculations and molecular dynamics simulations have been made on a variety of molecules containing the nitrate ester functionality Excellent agreement has been obtained between the calculated and experimental values for molecular structures, vibrational frequencies, liquid densities, heats of vaporization, crystal structure, mechanical properties, and lattice

Molecular Modeling of Energetic Materials: The Parameterization and Validation of Nitrate Esters in the COMPASS Force Field

. We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene δ-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the δ-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12&plusmn3% with a large difference between the δ and β branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield ~ 15±3% from methacrolein) and methylvinylketone nitrates (modelled yield ~ 11±3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models.

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Isoprene photooxidation: new insights into the production of acids and organic nitrates

Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X 2A1, A 2B2, B 2B1, C 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the A 2B2, B 2B1, and C 2A2 states have all been found to be within 2 eV of the minimum of the X 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 A for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 A and 1.42 A and the inversion barrier is approximately 800 cm−1.

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

Microwave spectra for eight isotopic species of cis and trans nitrous acids are reported. The planarity of both isomers has been accurately established on the basis of the inertial defects. Accurate structures, electric dipole moments and quadrupole coupling constants at nitrogen have been determined for both isomers. The structure of cis nitrous acid is found to be: O—H = 0.982, N—O(H)= 1.392, N—O = 1.185 A, ∠ NOH = 104.0 and ∠ ONO = 113.6°, and for trans nitrous acid: O—H = 0.958, N—O(H)= 1.432, N—O = 1.170 A, ∠ NOH = 102.1 and ∠ ONO = 110.7°. Stark effect measurements yield the following values for the components of the dipole moment for cis nitrous acid: µa= 0.306, µb= 1.389 and the total dipole moment µ= 1.423±0.005 D. Previous investigations of the Stark effect in the trans isomer have been refined to give µa= 1.378, µb= 1.242 and µ= 1.855±0.016 D. Orientation of the dipole moment has been established for both isomers from isotopic measurements, despite a moderately large vibrational change in µa for trans DNO2. Quadrupole coupling constants in the principal inertial axis system have been determined for cis nitrous acid to be: χaa= 2.05, χbb=–5.83 and χcc= 3.78 MHz, and for trans nitrous acid: χaa= 1.73, χbb=–5.28 and χcc= 3.55 MHz.
 The structural parameters are markedly different in the two isomers and have been discussed, together with the torsional barrier and relative stability of the isomers, in terms of a cis interaction. The structures and charge properties of the nitrous acids are compared with a range of related compounds.

Microwave spectrum, structure, dipole moment and quadrupole coupling constants of cis and trans nitrous acids

The microwave spectra of the normal species and seven isotopic species of methyl nitrate have been measured. The detailed structure of the molecule, which has a planar heavy-atom skeleton, with the methyl group staggered with respect of the cis oxygen atom, is determined as : C—O′= 1.437 A, N—O′= 1.402 A, N—O (cis)= 1.205 A, N—O(trans)= 1.208 A, C—Hi(in plane)= 1.088A, C—H0(out-of-plane)= 1.095A, ∠O′CHi= 103.4°, ∠O′CH0= 110.4°, ∠H0CH0= 109.8°, ∠CO′N = 112.72°, ∠O′NO(cis)= 118.10°, ∠O′NO(trans)= 112.38°. The methyl group and nitro group are tilted away from each other; the former is inclined at 4.8° to the C—O′ bond direction, and the latter at 2.9° to the N—O′ direction. The components of the dipole moment have been determined, from Stark effect measurements, as µa= 3.07 D, µb= 0.235 D, µt= 3.08 D, and similar measurements on the trans-18O species have shown that the dipole moment in the normal species is oriented at 39.6° to the bridging N—O′ bond. The structure and bonding of methyl nitrate is discussed together with some related molecules; its structure relates to nitric acid as methyl formate does to formic acid.

Microwave spectrum, structure and dipole moment of methyl nitrate

The structure of N2O3 has been determined from the microwave spectrum of seven isotopic species. The detailed structure associated with the following planar configuration is: [graphic omitted] d(N2N3)= 1.864 A; d(N2O1)= 1.142 A; d(N3O4)= 1.202 A; d(N3O5)= 1.217 A; ∠O1N2N3= 105.1°∠O4N3N2= 112.7°∠O5N3N2= 117.5°. The torsional vibrational frequency has been measured as 124 ± 25 cm–1. The quadrupole coupling constants for the nitrosyl nitrogen are χaa=–2.34 MHz, χbb= 1.12 MHz, and χcc= 1.22 MHz. The dipole moment has been determined as |µa|= 2.05 D; |µb|= 0.54 D; and |µt|= 2.12 D inclined at an angle of 9.1° to the N—N bond. On the basis of these data, the bonding is discussed.

Microwave spectrum, structure, low frequency vibrations, dipole moment and quadrupole coupling constants of dinitrogen trioxide

The Microwave Spectrum, Structure, Dipole Moment, and Quadrupole Coupling Constants of trans-Nitrous Acid1a

The microwave spectra of five isotopic species of cyclopentadienyl nickel nitrosyl, C5H5NiNO, have been measured, from which the C5ν symmetry of the molecule is confirmed and the structural parameters found to be N—O = 1.165 ± 0.005 A, Ni—N = 1.626 ± 0.005 A, Ni—C = 2.11 ± 0.01 A, C—C = 1.43 ± 0.01 A and C—H = 1.09 A(assumed).

A. Peter Cox

Papers

Microwave spectrum, structure, dipole moment and quadrupole coupling constants of cis and trans nitrous acids

Microwave spectrum, structure and dipole moment of methyl nitrate

Microwave spectrum, structure, low frequency vibrations, dipole moment and quadrupole coupling constants of dinitrogen trioxide

The Microwave Spectrum, Structure, Dipole Moment, and Quadrupole Coupling Constants of trans-Nitrous Acid1a

Microwave spectrum and structure of cyclopentadienyl nickel nitrosyl