(X)OVER-TILDE(3)B(1), (A)OVER-TILDE(1)A(1), (B)OVER-TILDE(1)B(1), AND(C)OVER-TILDE(1)SIGMA(+)(G) ELECTRONIC STATES OF NH2+

Ab initio molecular electronic structure theory has been employed in o rder to investigate systematically the (X) over tilde B-3(1), (a) over tilde (1)A(1), (b) over tilde B-1(1), and (c) over tilde (1) Sigma(g) (+) states of NH2+, with emphasis placed on the (b) over tilde B-1(1) and (c) over tilde (1) Sigma(g)(+) states. The self-consistent-field ( SCF), configuration interaction with single and double excitations (CI SD), complete active space (CAS) SCF, and CASSCF second-order configur ation interaction (SOCI) wave functions with nine basis sets, the larg est being a triple-xi basis set with three sets of polarization functi ons and two additional sets of higher angular momentum and diffuse fun ctions [TZ3P(2f,2d)+2diff], were used to determine equilibrium geometr ies, harmonic vibrational frequencies, infrared (IR) intensities, and dipole moments. The ground, first, and second excited states are confi rmed to be bent, while the third excited state is predicted to be line ar. The bond angles of NH2+ are shown to be larger than those of the c orresponding isoelectronic CH2 molecule. At the highest level of theor y, TZ3P(2f,2d)+2diff CASSCF-SOCI, the triplet-singlet splitting is pre dicted to be 29.4 kcal/mol (1.28 eV, 10 300 cm(-1)), which is in good agreement with the experimental observation of 30.1 kcal/mol (1.305 eV , 10 530 cm(-1)). With the same method, the second excited state ((b) over tilde B-1(1)) lies 43.7 kcal/mol (1.89 eV, 15 300 cm(-1)) above t he ground state, which is significantly lower than the experimentally proposed value of 2.54 eV. The third excited state ((c) over tilde (1) Sigma(g)(+)) is predicted to lie 77.0 kcal/mol (3.34 eV, 26 900 cm(-1 )) above the ground state. The equilibrium geometry of this (c) over t ilde 1 Sigma(g)(+) state is determined to be r(e) 1.030 Angstrom at th e TCSCF-CISD level with the largest basis set. Since the IR intensitie s of all active vibrational modes are predicted to be substantial, IR spectroscopic studies of the four states are feasible. However, of the six fundamentals experimentally assigned to date, two appear to be in correct. The energy separations among the four lowest-lying states of NH2+ are found to be larger than the corresponding states of CH2.