An intraline of conical intersections for methylamine

In this article are considered the conical intersections (ci's) related to the N-H bond in the methylamine, CH₃NH₂, molecule. The novel feature that was revealed is that the two lowest states 1 A′ and 1 A″ are coupled by a line of cis located in 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 proven first by studying the singularities of the (angular) nonadiabatic coupling terms and then by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). A theoretical analysis indicated that the line has to be a finite closed line. We also calculated the Berry phase for a contour that surrounds this line and found it to be 3.127 rad, namely, a value reasonably close to π. The existence of such lines of cis-instead of isolated cis (as exhibited by other n -atomic (n>3) molecules such as HNCO or C ₂H₂)-may enhance significantly the transition rate from an upper adiabatic state to a lower one. There are also numerical advantages in such situations, that is, if such a line is properly placed in that plane (like in the present case) the wave-packet treatment of the nuclei can be carried out employing a single diabatic PES instead of having to consider two coupled PESs.