Seams near seams: the Jahn-Teller effect in the 1E" state of N3+

The (1)E(") electronic state of cyclic N(3) (+) arising from the singly excited electron configuration e('3)e(") is studied using multireference configuration interaction wave functions and a quadratic Jahn-Teller Hamiltonian determined from those calculations. It is shown that these two states have both a symmetry-required seam of conical intersections at D(3h) geometries and three proximal symmetry equivalent C(2v) seams, located on a circle with radius rho(0) from the D(3h) intersection. rho(0), a function of Q(s), the breathing mode, is quite small but only attains a value of zero at Q(s) (crit)=1.252 A, resulting in a confluence or intersection node of the three C(2v) seams with the D(3h) seam. At this point only, g= parallelg(Q(s)) parallel, the norm of half the energy difference gradient, the linear Jahn-Teller term, vanishes and the intersection is of the Renner-Teller type. The close proximity of the previously unreported C(2v) seams to the D(3h) seam over the range of Q(s) considered is a consequence of the small values of g, compared to the quadratic Jahn-Teller term. The present analysis has important implications in the study of Jahn-Teller effects in ring systems and provides insight into a recent report that characterized this D(3h) seam as a Renner-Teller or glancing intersection.

Potential energy surfaces and Jahn-Teller effect on CH4⋯NO complexes

The potential energy surface of the CH(4)...NO van der Waals complexes was explored at the RCCSD(T)/aug-cc-pVTZ level including the full counterpoise correction to the basis set superposition error. The Jahn-Teller distortion of the C(3v) configurations for the CH bonded and CH(3) face complexes was analyzed. From this distortion, two A(') and A(") adiabatic surfaces were considered. The estimated zero point energy of C(s) configurations is above the barrier of the C(3v) ones. Therefore, the CH(3) face complexes are dynamic Jahn-Teller systems. The D(0) (140 cm(-1) for A(") state and 100 cm(-1) for A(')) values obtained are in good agreement with the experimental values (103+/-2 cm(-1)) recently reported.

Quadratic Description of Conical Intersections: Characterization of Critical Points on the Extended Seam

In this paper, we present a practical approach for the characterization of critical points on conical intersection seams as either local minima or saddle points using second-derivative technology. The utility of this methodology is illustrated by the analysis of seven S0/S1 (2Ag/1Ag) conical intersection points involved in the photochemistry of butadiene. The characterization of critical points on the crossing seam requires second derivatives computed in curvilinear coordinates. Using such coordinates, we can represent the branching space and the intersection space to second order. Although these curvilinear coordinates are conceptually important, they also give rise to two additional practical applications. First, such coordinates yield information on the nature of vibrational modes that are stimulated following radiationless decay at a crossing point. Second, the second-order force field is directly comparable to experimental spectroscopic data for Jahn-Teller systems. We will illustrate the latter idea for the cyclopentadienyl radical.

Photodissociation of methane: Exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.