Knowledge Bank

The high resolution infrared spectrum of pyrazine and naphthalene were measured in a molecular beam in the vicinity of the C-H stretching transitions. In the case of pyrazine, two vibrational bands were examined, the $\nu 9$ band from $3015-3019cm-1$ and the $\nu 15$ band from $3065-3073cm-1$. The rotational structure in the $\nu 9$ band is well modeled by an asymmetric top/rigid rotor Hamiltonian with no significant perturbations. The rotational structure in the $\nu 15$ band reveals that this C-H stretch is coupled to one other vibrational mode in the molecule. The mode coupling is manifested in the spectrum as two overlapping vibrational bands. Each of these two bands are well modeled by an asymmetric top/rigid rotor Hamiltonian. The lack of any angular momentum dependence on the coupling indicates that the vibrations are coupled by an anharmonic mechanism. The magnitude of the coupling matrix element was determined to be $0.36cm-1$. The rotational structure in the spectrum of naphthalene from $3063-3067cm-1$ reveals that except for several local perturbations, the spectrum is well modeled by an asymmetric top/rigid rotor Hamiltonian. The local perturbations include transitions that are split into doublets as well as transitions that have been shifted form their expected positions. The magnitude of the average coupling matrix element for the doublets was determined to be $0.0016cm-1$. A comparison between the vibrational mode coupling in pyrazine and naphthalene indicated that mode coupling does not correlate with the density of states in the two molecules.