The S(0), T(1), and S(1) potential energy surfaces for the HCOOH dissociation and isomerization processes have been mapped with different ab initio methods. The wavelength-dependent mechanism for the HCOOH dissociation was elucidated through the computed potential energy surfaces and the surface crossing points. The HCOOH molecules in S(1) by excitation at 248 nm mainly decay to the ground state via the S(0) and S(1) vibronic interaction, followed by molecular eliminations in the ground state. The S(1) direct dissociation to HCO((2)A') + OH((2)Pi) is the dominant pathway upon photoexcitation at 240-210 nm. Meanwhile, there is a slight probability that the system relaxes to the ground state via the S(0) and S(1) vibronic interaction at these wavelengths. After irradiation of HCOOH at 193 nm, the S(1) direct dissociation into HCO((2)A') + OH((2)Pi) is energetically the most favorable pathway. In view of high IC efficiency at the S(0)/S(1) conical crossing, the S(1) --> S(0) internal conversion via the S(0)/S(1) point can occur with considerable efficiency. In addition, the S(1) isomerization probably plays a dominant role in the partially conformational memory of the HCOOH photodissociation, which has been discussed in detail.

The complete-active-space self-consistent field (CASSCF) and density functional theory (DFT) approaches have been used to study the mechanistic details of Norrish type I and II reactions of aromatic carbonyl compounds, with butyrophenone (PhCOCH 2 CH 2 CH 3 ) as a representative. A minimum energy crossing point was found to exist among three potential energy surfaces (S 1 , T 1 , and T 2 ), and the three-surface crossing allows the T 2 state to act as a relay that enables the intersystem crossing (ISC) from S 1 to T 1 to occur with a high efficiency for PhCOCH 2 CH 2 CH 3 . Once the molecule is in the T 1 state, the 1,5-H shift reaction is the predominant reaction pathway and yields a triplet 1,4-biradical of PhC(OH)CH 2 CH 2 CH 2 as an intermediate species. Since the formation of excited triplet products is energetically improbable, the subsequent decomposition, cyclicization, and disproportionation of the 1,4-biradical proceed after intersystem crossing from the triplet to singlet state. The singlet 1,4-biradical was found to have three isomers, which determine to a certain extent the branching ratios of the subsequent reactions. The study given here provides new insights into the S 1 relaxation dynamics of aromatic carbonyl compounds and their subsequent reaction mechanisms.

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The potential energy surfaces for N‐methlyformamide(HCONHCH3) dissociation into HCO + NHCH3 and CONHCH3 + H in the ground and lowest triplet states have been characterized with DFT, MP2 and CASSCF methods with the cc‐pVDZ basis set, while the S1 potential energy surfaces for the dissociation reactions were determined by the CASSCF/cc‐pVDZ optimizations followed by MRSDCI single‐point calculations. The mechanism of the HCONHCH3 photodissociation has been elucidated on the basis of the computed potential energy surfaces and surface crossing points.