Relocalization in Floppy Free Radicals:  The OCNO and OCCHO Isoelectronic Series

Abstract

A survey of the XCNY and XCCHY radicals with X and Y = CH(2), NH, and O has been carried out by ab initio QCISD/6-311G(d,p) calculations to assess the impact of low-lying excited electronic states on the molecular dynamics. Multiple canonical structures may be drawn for each of these structural formulas, with the principal competition for most stable configuration between a (2)A' form with four electrons in a" orbitals and a (2)A" form with five a" electrons. Other low-lying configurations may include a 5a" state with nominally pentavalent nitrogen and a 6a" state. Optimized geometries and harmonic frequencies were evaluated for the lowest-energy minima on the potential energy surfaces. Localized unpaired electron density causes the 4a" state to be the most stable for (NH)CCHO and OCCHY, whereas allylic resonance stabilization favors the 5a" state for all other radicals in the set. For five of the 18 molecules studied, secondary minima (excluding conformers) are found within 30 kJ mol(-)(1) of the most stable state at the QCISD/6-311G(d,p) level, suggesting that photolysis or pyrolysis of parent compounds may result in multiple isomers of the resulting reactive intermediates. Predicted equilibrium geometries, approximate thermochemical quantities for dissociation of the central bond, and selected spectroscopic parameters are presented for all 18 structural formulas. Convergence tests were also performed for the glyoxallyl radical (OCCHO) to resolve discrepancies between single- and multireference post-SCF results. These tests find that extension of the MCSCF methods to include sigma bonding orbitals or virtual-orbital CI brings MCSCF relative energies into agreement with results from standard single-reference CI and CC methods. Relative configurational energies evaluated at Hartree-Fock levels routinely differ from post-SCF values by 30 kJ mol(-1) or more.

Ab Initio Study of 1,4-Pentadienyl Electrocyclic Reactions

“…The discrepancies of 30−40 kJ/mol between UHF and CCSD(T) relative energies seen in Table are typical of those seen in earlier work; , the largest here are 40−50 kJ/mol. The ROHF values are generally in better agreement with CCSD(T); the largest discrepancies are the set of activation energies, 41−48 kJ/mol lower at the CCSD(T) level than at ROHF for the electrocyclic reactions and 37 kJ/mol higher for the methene torsion.…”

“…Our ab initio studies of similar radicals, including C 4 H 5 , , C 3 H 3 O, and the OCNO isoelectronic series, reveal that relative energies among structural isomers are sometimes quite poorly predicted by a variety of methods, including unrestricted and restricted Hartree−Fock. Most striking among these findings was the range of predicted relative energies for cyclobuten-3-yl relative to 2-butyn-1-yl, from 12 kJ/mol to over 80 kJ/mol, even among post-SCF techniques.…”

Solving the vibrational Schrödinger equation on an arbitrary multidimensional potential energy surface by the finite element method

“…Chiang and Laane [88] derived the G-matrix element functions for the kinetic energy operator based on the vector method [37,89]: (2) cos(2φ 1 ) + cos(2φ 2 ) + G (4) cos(4φ 1 ) + cos(4φ 2 ) + G (c) cos(2φ 1 ) cos(2φ 2 ) + G (s) sin(2φ 1 ) sin(2φ 2 ). (40) Since φ 1 and φ 2 are equivalent, G 11 and G 22 are identical functions.…”

“…Of relevance to our research are the family of free radicals involving coupling between the unpaired electron(s) and a conjugated π -system, particularly in unsaturated carbon-chain or monocyclic hydrocarbon intermediates. In these cases, the structure of the radical intermediate may differ substantially from that of the parent compound, and the redistribution of electron spin along the conjugated π -system may lead to shifts from the anticipated location of the principal reactive site [1][2][3][4][5]. The PES for motion along this coordinate of spin "relocalization" may feature extensive flat regions, multiple minima, and strongly coupled vibrational modes.…”

A matrix isolation study of the Cs symmetric OCNO(X 2A″) radical

“…More recently, Cooksy has investigated the mechanism of the lowest energy dissociation pathway in addition to the overall thermodynamics. 5 He found that rather than following the 2 A 00 surface to dissociate, the molecule may alter its symmetry to more closely resemble resonance structure B from Fig. 1.…”