Ab Initio Calculation of the Photoelectron Spectra of the Hydroxycarbene Diradicals



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Ab Initio Calculation of the Photoelectron Spectra of the Hydroxycarbene Diradicals
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Ab Initio Calculation of the Photoelectron Spectra of the Hydroxycarbene Diradicals
Lucas Koziol,† Vadim A. Mozhayskiy,† Bastiaan J. Braams,‡ Joel M. Bowman,‡ and
Anna I. Krylov*,†
Department of Chemistry, UniVersity of Southe
Califo
ia, Los Angeles, Califo
ia 90089-0482,
C. L. Emerson Center for Scientific Computation, Department of Chemistry, Emory UniVersity,
Atlanta, Georgia 30322
ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: May 19, 2009
Photoelectron spectra of the cis and trans isomers of HCOH were computed using vibrational wave functions
calculated by diagonalizing the Watson Hamiltonian, including up to four mode couplings. The full-dimensional
CCSD(T)/cc-pVTZ potential energy surfaces were employed in the calculation. Photoionization induces
significant changes in equilibrium structures, which results in long progressions in the ν5, ν4, and ν3 modes.
The two isomers show progressions in different modes, which leads to qualitatively distinguishable spectra.
The spectra were also calculated in the double harmonic parallel-mode (i.e, neglecting Duschinsky rotation)
approximation. Calculating displacements along the normal coordinates of the cation state was found to give
a better approximation to the vibrational configuration interaction spectrum; this is due to the effects of
Duschinsky rotations on the vibrational wave functions.
I. Introduction
Hydroxycarbene, HCOH, is a high-energy diradicaloid isomer
of formaldehyde. It is believed to play a role in formaldehyde
photochemistry and its “roaming hydrogen” dynamics, the
interstellar medium, and reactions of carbon atom with water.1-5
HCOH production is a major channel in the photodissociation
of hydroxymethyl radical, CH2OH, in the 3p Rydberg state.6
Reisler and co-workers determined the heat of formation of the
deuterated isotope HCOD to be 24 ( 2 kcal/mol.7 Recently, its
synthesis and spectroscopic characterization were reported by
Schreiner et al.,8 who isolated the trans-HCOH and HCOD in
argon matrix at 11 K and identified several infrared (IR) band
origins. The experiment was supported by variational calculations
of the anharmonic energies using the CCSD(T)/cc-pVQZ
quartic force field. In an independent study, the vibrational levels
and IR intensities for the ground states of neutral cis- and trans-
HCOH were reported.9 The calculated lines and intensities
matched the experimental data of Schreiner et al. closely. It
was found that anharmonicities were crucial for correctly
describing IR intensities as well as energies. The harmonic
approximation described the lowest fundamental frequencies
accurately, although it overestimated the stretching modes by
approximately 200 cm-1 in both isomers. Several combination/
overtone bands acquired intensity in the low-energy region
(0-3000 cm-1) and complicated the spectrum.
The cation HCOH+ has also been studied. Berkowitz10 and
also Burgers11 observed the species by mass spectroscopy in
the dissociative photoionization of methanol. Near the dissociation
threshold of hydrogen elimination, HCOH+, rather than
H2CO+, was the dominant product.10 Radom and co-workers
characterized trans-HCOH+, formaldehyde cation, and the
transition state using molecular orbital theory.12 They were the
first to suggest that HCOH+ is the most stable isomer of ionized
formaldehyde. The following year, McLafferty and co-workers13
performed collision-activated mass spectroscopy experiments
and were able to infer the stability of a product in the correct
energy range, which they attributed to HCOH+. The heat of
formation, based on careful comparison between theoretical
calculations and experimental data (reverse activation energy
and isotope effects were found to be crucial in analysis of
appearance energy experiments) was established by Radom and
co-workers.14 The energy difference between the formaldehyde
cation and HCOH+ was found to be poorly reproduced by
perturbation theory (MP2) due to convergence issues in the
perturbative series. Finally, Wiest15 characterized energy and
structure for both cis and trans isomers in a DFT study of the
methanol radical cation surface.
Neutral HCOH tunnels effectively through the barrier to
formaldehyde.8 The calculated rate constant for the forward
reaction is almost an order of magnitude higher than for the
reverse reaction,16 as would be expected from energetics.
Whereas on the neutral surface, HCOH is much higher than
formaldehyde, the energy gap between HCOH+ and H2CO+ is
much smaller (1811 cm-1; see Figure 1). Thus, it might be easier
to observe HCOH isomer in the ionized rather than neutral state.
From the electronic structure point of view, HCOH is an
example of substituted carbenes, diradical species playing an
important role in organic chemistry.17 Spectroscopically, prototypical
substituted carbenes have been studied by Reid and
co-workers.18-21 Using high-resolution spectroscopy, they characterized
the singlet-triplet gaps, spin-orbit couplings, and
mode-specific dynamics of several triatomic carbenes.18-21
Halogen substitution reduces diradical character, resulting in
the singlet ground state. The OH group has a similar effect: the
ground state of hydroxycarbene is a singlet, and the singlet-triplet
gap is about 1 eV.2-5,22,23

 


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