Unimolecular Dissociation of Formyl Halides HXCO → CO + HX (X= F, Cl):  An Ab Initio Direct Classical Trajectory Study

Abstract

The dynamics of the unimolecular dissociation of formyl halides, HXCO (X = F, Cl), have been studied by classical trajectory calculations at the MP2/6-31G(d,p) level of theory and have been compared to similar calculations for H2CO. The calculated transition states are planar with elongated C−X and H−X bonds. Trajectories were started from the transition state with 12 kcal/mol of energy above the potential barrier, corresponding to the zero-point energy plus 2.25 and 2.87 kcal/mol of excess energy in the transition vector of formyl fluoride and formyl chloride, respectively. The CO fragments are produced rotationally hot with 〈J〉 = 40−50 but vibrationally cold with only 10−20% in v = 1. The HX fragment shows significant vibrational excitation with 〈v〉 = 1.0 for HF and 〈v〉 = 2.3 for HCl, compared to 〈v〉 = 1.16 for H2. The average rotational quantum number for HCl, 〈J〉 = 12.8, is considerably higher than for HF, 〈J〉 = 9.5, or H2, 〈J〉 = 3.3. Product translation receives about 42% of the available energy in formyl fluoride and about 31% in formyl chloride compared to 70% for formaldehyde. The results show good qualitative agreement with the available experimental energy partitioning for HFCO; HF is observed to be vibrationally hot, but the distribution has not been measured. No experiments are available for HClCO dissociation.

Theoretical study of structures of the X2CO and XYCO molecules (X and Y = H, F, or Cl) in the ground and lowest excited triplet electronic states

“…Deviations of the geometric parameters (a) of the Cl 2 CO molecule in the S 0 (1, 2) and T 1(3,4) states and the energy parameters (b), viz., the inversion barriers(1,2) and the energies of the electronic transition T 1 ←S 0(3,4), calculated by the MP2(1, 3)and B3LYP (2, 4) methods with different AO basis sets from the corresponding values estimated at the CCSD(T)/cc pV(T+d)Z level. See also the caption forFig.…”

Ab initio MRD-CI study of excited states of formyl chloride HClCO and photofragmentation along Cl–C cleavage

“…while all other geometry parameters were kept frozen at the equilibrium values obtained by Anand and Schlegel [19] at the QCISD/6-311G(d,p) level. The results obtained for the three lowest singlet A 0 and A 00 excited states are displayed in Fig.…”

“…The equilibrium geometry of formyl chloride has been determined both experimentally [47][48][49] and theoretically [19][20][21][50][51][52]. However, as a starting point of our investigation of excited states of formyl chloride we fully optimised the geometry at the QCISD/6-311G(d,p) (quadratic configuration interaction with single-and double-excitations [53]) level of theory using the Gaussian 98 [54] program package.…”

“…Libuda et al [18] measured the UV absorption cross sections and rate constants for the reactions of formyl chloride with Cl and OH. Anand and Schlegel [19] studied the unimolecular dissociation of formyl halides (X = F, Cl) by an ab initio direct classical trajectory study. Francisco et al [20] carried out ab initio studies of dissociation pathways on the ground state potential energy surface for HFCO and HClCO whereas Fang and Liu [21] investigated dissociation pathways on the ground-(S0) and the first two low lying excited-states (S1 and T1) of formyl chloride employing ab initio calculations.…”

Ab Initio Molecular Dynamics with Born-Oppenheimer and Extended Lagrangian Methods Using Atom Centered Basis Functions

“…With updating, the step size needs to be only slightly smaller to maintain the same energy conservation as without updating. We have used the Hessian-based predictor-corrector method (with and without updating) in studies of H2CO → H2 + CO, F + C2H4 → C2H3F, C2H2O2 (glyoxal) → H2 + 2 CO & H2CO + CO, C2N4H2 (s-tetrazine) → N2 + 2 HCN and HXCO → HX + CO. 31,[37][38][39][40][41][42][43][44][45][46] Collins has developed a novel method for growing potential energy surfaces for dynamics by using trajectories to determine where additional electronic structure calculations are needed. [47][48][49][50][51][52][53] An initial approximation to the potential energy surface is constructed with a modest number of energy, gradient and Hessian calculations along the reaction path.…”