In a recent theoretical study Yurchenko et al. (1) assigned the near ultraviolet band system of methylene, that had been observed by Herzberg and Johns (2), as the c1 A 1 ← ( ã1 A 1 / b1 B 1 ) band system. This assignment required that the c1 A 1 state be predissociated in order to account for the disappearance of the band system above 31000 cm -1 . This note reports an ab initio investigation of the dissociation path of the c1 A 1 state and supports that conjecture.
The potential energy surfaces were determined by ab initio calculations using the program MOLPRO (3). The orbital basis set was of a triple-ζ quality derived from Dunning's triple-ζ contraction (4) of Huzinaga's primitive Gaussian functions (5) and they are designated (10s 6 p/5s 3 p) for C and (5s/3s) for H. To this set were added polarization functions with the following exponents: α d (C) = 3.00, 0.75, 0.1875, and α f (C) = 1.60, 0.40 with α p (H) = 3.00, 0.75, 0.1875, and α d (H) = 2.00, 0.50. In order to represent the Rydberg behaviour of the excited states, diffuse functions on carbon with exponents (6) α s (C) = 0.04812, 0.01669, 0.005867, α p (C) = 0.03389, 0.01253, 0.004648, α d (C) = 0.09375, 0.02344, 0.005860 were employed. Six Cartesian d-like and ten Cartesian f -like functions were used. The calculations were performed in C 2v or C s symmetry as appropriate. The CH 2 molecule was placed in the yzplane and in C 2v the z-axis was chosen to be the C 2 axis. The valence internally contracted multireference configuration interaction (MRCI) (7) method was employed for the calculation of the bending and asymmetric stretching potential energy curves. Complete active space self consistent field (CASSCF) (8) calculations were used to define the orbitals for MRCI. In the CASSCF and MRCI calculations the reference wave function was a complete expansion in both the orbitals that correlate with atomic valence orbitals together with those that were necessary to describe Rydberg states when they occur. The carbon 1s orbital was not correlated, but was optimized at the CASSCF level. The calculation of the bending potential curves was performed in C 2v symmetry. Six electrons were distributed among the 10 orbitals in the active space. The active space, (2a 1 , 3a 1 , 4a 1 , 5a 1 , 6a 1 | 1b 1 , 2b 1 | 1b 2 , 2b 2 , 3b 2 ), included valence orbitals, two a 1 orbitals of 3s and 3 p z Rydberg character, one b 1 orbital of 3 p y Rydberg character, and one b 2 orbital of 3 p x Rydberg character. The CASSCF state-averaged calculations included five, four, two, and two roots in A 1 , B 1 , B 2 , and A 2 symmetry, respectively. The calculation of the asymmetric stretching potential curves was performed in C s symmetry. The active orbital space, (2a to 7a | 1a ), included valence orbitals and one a orbital of 3s Rydberg character. The CASSCF calculations included three roots in A and two roots in A symmetry.
The ã1 A 1 state is found to have an equilibrium C-H bond length of 1.1088 A ❛