Vibrational dynamics of hydrogen-bonded HCl-diethyl ether complexes

We studied the relaxation of the H–Cl stretch vibration of hydrogen-bonded HCl-diethyl ether [HCl–(CH₃CH₂)₂O] complexes at room temperature using femtosecond mid-infrared pump-probe spectroscopy. The lifetime of the H–Cl stretch vibration is determined to be 0.9±0.2 ps. The relaxation is found to occur via an intermediate state which causes a transient blue-shift of the H–Cl stretch frequency. This blue-shift indicates that the low-frequency (CH₃CH₂)₂O···H–Cl hydrogen bond is the main accepting mode of the vibrational energy. The excited population of this hydrogen-bond mode decays with a time constant of 3.1±0.5 ps to the ground state.