Propargyl Radical:  Ab Initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated HCCCH₂

Abstract

The propargyl radical has twelve fundamental vibrational modes, gamma(vib)(HCCCH2) = 5a1 [symbol: see text] 3b1 [symbol: see text] 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC=:C-:CH2 matrix frequencies (cm(-1)) and polarizations are a1 modes--3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b1 modes--686.6 +/- 0.4, 483.6 +/- 0.5; b2 modes--1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC=:C-:CH2 2 X (2)B1. From an analysis of the vibrational spectra, the small electric dipole moment, mu(D)(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol(-1), we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC=:C-:CH2.

Anharmonic force field and vibrational dynamics of CH2F2 up to 5000 cm−1 studied by Fourier transform infrared spectroscopy and state-of-the-art ab initio calculations

“…Recently, a number of works have demonstrated that ANO basis sets coupled to the CCSD(T) treatment of electronic correlation provide excellent results for level positions. 32,33,46,48,57,58 While excitations from the K shell molecular orbitals were not considered at the CCSD(T)/ANO2 level (i.e., frozen core), core correlations effects were tested for the Dunning's basis sets, by correlating all the electrons and using the core-valence aug-cc-pCVnZ (n = T or Q) functions. 55 In an explanatory run, the anharmonic force field of CH 2 F 2 was evaluated using either the frozen core CCSD(T)/aug-cc-pVTZ or the fully correlated CCSD(T)/aug-cc-pCVTZ levels.…”

Propargyl + O2 Reaction in Helium Droplets: Entrance Channel Barrier or Not?

“…6,7,16 For propargyl, quantum chemistry (CCSD(T)/ANO) predicts that 35% of the electron spin density resides on the terminal acetylenic carbon while the remaining 65% is on the methylenic carbon. 22 The resonance stabilization energy associated with this electron delocalization has been computed to be ≈11 kcal·mol −1 . 22 Because this stabilization energy is lost upon C−O bond formation, the C 3 H 3 −OO (propargyl peroxy) radical is less strongly bound than typical alkylperoxy radicals, 23 and unlike alkyl + O 2 , the propargyl + O 2 reaction is predicted to proceed over a small entrance channel barrier (≈1−4 kcal·mol −1 ).…”

“…22 The resonance stabilization energy associated with this electron delocalization has been computed to be ≈11 kcal·mol −1 . 22 Because this stabilization energy is lost upon C−O bond formation, the C 3 H 3 −OO (propargyl peroxy) radical is less strongly bound than typical alkylperoxy radicals, 23 and unlike alkyl + O 2 , the propargyl + O 2 reaction is predicted to proceed over a small entrance channel barrier (≈1−4 kcal·mol −1 ). 23 Hence, the propargyl radical is less reactive with O 2 and is therefore oxidized less effectively in comparison to nonresonantly stabilized radicals, leading to high concentrations in combustion environments.…”

Gas-phase infrared spectrum of methyl nitrate

“…Going to the ANO set that is the same size as cc-pVQZ (ANO2), the situation improves yet further [10]. Indeed, such calculations were instrumental in our assignment of the infrared spectrum of matrix-isolated cis-cis HOONO [11], and the propargyl radical [12].…”