A Novel Intraline of Conical intersections for Methylamine: A Theoretical Study

In this article the study of conical intersections (ci) related to the N-H bond in the methylamine, CH3NH2, molecule is extended. In a previous publication (Levi et al., J Chem Phys 2008, 128, 244302) we reported on a novel feature associated with the intersection of the two lowest states (1)A' and (1)A of the methylamine. We established the existence of a finite (closed) line of ci located in the HC-NHH plane-a line that is formed by moving a single hydrogen on that plane while fixing the (six) other atoms. The validity of this line was proved by studying the singularities of the (angular) nonadiabatic coupling terms (NACT)-a study that was later supported by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). This situation led to two additional interesting features: (i) Along any (open) contour in the above plane that intersects this line is formed a narrow, spiky NACT for which the area under it is similar to pi/2; (ii) In case of a closed contour the corresponding topological (Berry) phase is zero (and not an integer multiple of pi as is usually the case). In the current article we present the theory to support these findings. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 109: 2482-2489, 2009