Photochemical Ring Opening of Oxirane Modeled by Constrained Density Functional Theory

Abstract

A constrained density functional theory/classical trajectory surface hopping study of the photochemical dissociation of oxirane (CH 2 ) 2 O is presented. The calculations confirm the Gomer–Noyes mechanism for the initial reaction and agree largely with experimental photolysis data including reaction yields. The calculated yields, however, depend both on temperature and its modeling. The timescales of the various reaction steps are well below 100 fs, similar to previous time-dependent density functional calculations. At variance with those, however, the present calculations obey Kasha’s rule, i.e., the photoreaction is initiated in the energetically lowest excited state.

Electronic Structure and Excited States of the Collision Reaction O(3P) + C2H4: A Multiconfigurational Perspective

“…The four geometries identified in the table can be found in Figure . For the CH 2 OCH 2 (oxirane) molecule, whereof we show the CAS­(12,12) active space in Figure , the MS-CASPT2 first singlet excited state S 1 is predicted at 7.18 eV, in good agreement with previous TD-DFT results and close to the experimental value of 7.24 eV . This state is obtained by a transition from the oxygen lone pair orbital 2p x to the virtual orbital 3p z ( 1 B 1 transition): this result is in disagreement with the earlier TD-DFT calculations (using different functionals) at the same geometry which predict a contribution from an s-like virtual orbital; however, this same result is in agreement with more recent cDFT calculations . The triplet states T 1 and T 2 are placed by the MS-CASPT2 at 6.65 and 7.76 eV, respectively, namely, 0.53 eV below and 0.58 eV above the singlet S 1 , respectively.…”

“…This difference is compatible with the El-Sayed rule, in the sense that S 0 and T 1 share the same electronic character in their electronic transitions and the same for S 1 and T 2 . The particular electronic properties of this transition state region have been reported in several theoretical studies on the same system reaction. ,,, According to the basic quantum multielectron theory, a singlet biradicaloid needs two determinants to be correctly represented, whereas a triplet biradicaloid needs only one determinant . Thus, in every structure, where the biradicaloid character becomes important, the singlet states systematically show a strong multireference character, whereas the triplets retain a singlet reference character. The last singlet configuration that has been optimized is the S 0 –S 1 conical intersection.…”

“…41 This state is obtained by a transition from the oxygen lone pair orbital 2p x to the virtual orbital 3p z ( 1 B 1 transition): this result is in disagreement with the earlier TD-DFT calculations (using different functionals) at the same geometry 12 which predict a contribution from an s-like virtual orbital; however, this same result is in agreement with more recent cDFT calculations. 43 The triplet states T 1 and T 2 are placed by the MS-CASPT2 at 6.65 and 7.76 eV, respectively, namely, 0.53 eV below and 0.58 eV above the singlet S 1 , respectively. The T 1 state is described by transitions involving the same orbitals of the S 1 state ( 3 B 1 transition), whereas the T 2 state is described by a transition from the π CC + 2p z orbital to the virtual orbital 3p x ( 3 A 1 transition).…”

“…The particular electronic properties of this transition state region have been reported in several theoretical studies on the same system reaction. 12,15,20,43 According to the basic quantum multielectron theory, a singlet biradicaloid needs two determinants to be correctly represented, whereas a triplet biradicaloid needs only one determinant. 47 Thus, in every structure, where the biradicaloid character becomes important, the singlet states systematically show a strong multireference character, whereas the triplets retain a singlet reference character.…”

The ΔSCF method for non-adiabatic dynamics of systems in the liquid phase

“…Experimental photolysis data of oxirane and the corresponding Gomer-Noyes mechanism were also confirmed with ΔSCF TFSSH simulations. 87 Discrepancies were assigned to the inaccurate energy dissipation description, which significantly impacted the dissociation yield.…”