Electronic Spectroscopy and Photodissociation Dynamics of the 1-Hydroxyethyl Radical CH₃CHOH

Abstract

The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19,400-37,000 cm(-1) (515-270 nm) were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of the transition to the Rydberg 3s state, the lowest excited state, is estimated at 19,600 +/- 100 cm(-1). The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3pz state, identified by 2 + 2 REMPI, lies at 32,360 +/- 70 cm(-1), and a vibrational progression in the C-O stretch is assigned. When exciting CH3CHOH near the onset of the unstructured absorption to the 3s state, only one peak is observed in the center-of-mass (c.m.) translational energy (Et) distribution obtained by monitoring H photofragments. The measured recoil anisotropy parameter beta = -0.7 +/- 0.1 is typical of a perpendicular transition. The O-H bond energy is determined to be D0 = 1.1 eV +/- 0.1 eV. At excitation energies >31,200 cm(-1) (3.87 eV) a second, low Et peak appears in the c.m. Et distribution with beta approximately 0. Its relative intensity increases with excitation energy, but its beta value does not change. In contrast, the beta value of the higher Et peak becomes monotonically less negative at higher excitation energies, decreasing to -0.2 +/- 0.1 at 35,460 cm(-1). By comparison of the TOF distributions of the isotopologs CH3CHOH, CH3CHOD, and CD3CHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy. It appears that separate conical intersections lead to the observed channels, and the dynamics at the conical intersection and the exit channel deposit much of the available energy into internal energy of the products.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…A second, slow component apparent in the H atom velocity distributions measured following excitation at   320 nm has been attributed to the onset of rival C-H bond fission yielding vinyl alcohol (enol) partner fragments. 210 The methoxy (CH3O) radical is an isomer of hydroxymethyl. C-O bond fission is the dominant outcome following photoexcitation of CH3O radicals in the wavelength range 282    267 nm 211 and we have not found any reports of C-H bond fission following UV photoexcitation of CH3O.…”

Multistate, multichannel coupled diabatic state representations of adiabatic states coupled by conical intersections. CH2OH photodissociation

“…Experimental and computational studies of ground and excited state photodissociation of CH 2 OH have been reported, [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] as have studies of CH 3 CHOH. 17,18 Computational studies 10 complementing experimental studies 15 of the ground state photodissociation CH 2 OH(X 2 A) + hv → CH 2 O(X 1 A 1 ) + H have been reported. The computational work employed two high quality adiabatic potential energy surfaces (PESs), developed in that study 10 denoted KRBRK below.…”

Electronically excited and ionized states of the CH2CH2OH radical: A theoretical study

“…However, such adiabatic excitation may not be easily accessible from the ground state of the neutral because of the extensive nuclei rearrangement required for CH 2 CH 2 OH → CH 3 CHOH isomerization. The Rydberg states of CH 3 CHOH are known to couple efficiently with the ground state, 3 and thus fast dissociation on the ground state PES may follow isomerization on the Rydberg state to CH 3 CHOH and give rise to H photofragments by fission of the OH and CH bonds. 3…”