Ab initio large-amplitude quantum-tunneling dynamics in vinyl radical: a vibrationally adiabatic approach

Abstract

Large-amplitude tunneling in vinyl radical over a C2v planar transition state involves CCH bending excitation coupled to all other internal coordinates, resulting in a significant dependence of barrier height and shape on vibrational degrees of freedom at the zero-point level. An ab initio potential surface for vinyl radical has been calculated at the CCSD(T) level (AVnZ; n=2, 3, 4, 5) for vibrationally adiabatic 1D motion along the planar CCH bending tunneling coordinate, extrapolated to the complete basis set (CBS) limit and corrected for anharmonic zero-point effects. The polyatomic reduced moment of inertia is calculated explicitly as a function of tunneling coordinate, with eigenvalues and tunneling splittings obtained from numerical solution of the resulting 1D Schrödinger equation. Linear scaling of the CBS potential to match predicted and observed tunneling splittings empirically yields an adiabatic barrier height of DeltaEadiab=1696(20) cm(-1) which, when corrected for zero-point energy contributions, translates into an effective barrier of DeltaEeff=1602(20) cm(-1) consistent with estimates (DeltaE=1580(100) cm(-1)) by Tanaka and coworkers [J. Chem. Phys., 2004, 120, 3604-3618]. These zero-point-corrected potential surfaces are used to predict tunneling dynamics in vibrationally excited states of vinyl radical, providing strong support for previous jet-cooled high-resolution infrared studies [Dong et al., J. Phys. Chem. A, 2006, 110, 3059-3070] in the symmetric CH2 stretch mode.

The ionization energy of the vinyl radical: a Mexican standoff with a happy ending

“…Experimental and theoretical studies on the tunneling splitting found that the minima are connected by a straight Y-shaped C2v transition state, [2] ‡ , along the CCH bending mode. The electronic energy of this transition state has been reported to be about 0.21 eV above that of the global minimum in various experimental 8,9 and computational [10][11][12][13][14] works. After years of quantum chemical research, a consensus has emerged that, similarly to the ethyl cation, 15 the vinyl cation also has a non-classical C2v global minimum with a proton bridging the two carbon atoms, [3] + .…”

“…Calculations have established a single, doubly degenerate bent Y-shaped minimum for the neutral vinyl radical [1]. 10,11 The singly occupied HOMO has some C-C anti-bonding character, and the C=C bond contracts upon ionization to the 1 The neutral isomerization coordinate between the degenerate minima 10,11 and the ionic pathway between the classical and non-classical isomers 21,22 have both been studied computationally. We have found that the latter strongly depends on the level of theory used.…”

Instanton theory of ground-state tunneling splittings with general paths

“…Tunneling splittings in symmetric double-well systems such as, for example, proton transfer in malonaldehyde 3,4 and the vinyl radical 5 , or the collective migration of hydrogen atoms in ammonia 6 and hydronium 7 , have been studied extensively. The splittings serve as benchmarks for studying the interactions at play, while the models of interactions serve as a testing ground for dynamical calculation methods of tunneling splittings [8][9][10][11][12][13][14][15][16][17][18][19] . Studies of tunneling splitting patterns in water clusters have particularly come into focus, as the interactions and the rearrangements involved are important for the understanding of liquid water from first principles 20,21 .…”

Insight from Singlet into Triplet Excited‐State Hydrogen Bonding Dynamics in Solution

“…It has also been demonstrated that, for the T 1 (np à ) state of benzophenone, the carbonyl group has a single-bond character, while for the T 1 (pp à ) state of 4-phenylbenzophenone the carbonyl group retains a double-bond character [82][83][84][85]. Moreover, the time-resolved infrared absorption spectra of fluorenone and its carbonyl 18 O-substituted analogues have also been studied to determine the nature of the T 1 state of fluorenone in different solvents [85]. The stretching frequencies of the T 1 state in both polar and nonpolar solvents were found to be located in the double-bond region, indicating that the T 1 state of fluorenone is of pp à character [85].…”

“…In addition to non-specific dielectric interactions between solute and solvent, site-specific intermolecular hydrogen bonding interaction between hydrogen donor and acceptor molecules is another important type of solute-solvent interaction and is central to the understanding of the microscopic structure and function in many molecular systems [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Moreover, the dynamical behaviour of intermolecular hydrogen bonds in electronic excited states plays an important role in determining the rates of many chemical, physical and biochemical processes that occur in hydrogen-bonded surroundings [30][31][32][33][34][35].…”