Mid-infrared signatures of hydroxyl containing water clusters: Infrared laser Stark spectroscopy of OH–H2O and OH(D2O)n (n = 1-3)

Abstract

Small water clusters containing a single hydroxyl radical are synthesized in liquid helium droplets. The OH-H2O and OH(D2O)n clusters (n = 1-3) are probed with infrared laser spectroscopy in the vicinity of the hydroxyl radical OH stretch vibration. Experimental band origins are qualitatively consistent with ab initio calculations of the global minimum structures; however, frequency shifts from isolated OH are significantly over-predicted by both B3LYP and MP2 methods. An effective Hamiltonian that accounts for partial quenching of electronic angular momentum is used to analyze Stark spectra of the OH-H2O and OH-D2O binary complexes, revealing a 3.70(5) D permanent electric dipole moment. Computations of the dipole moment are in good agreement with experiment when large-amplitude vibrational averaging is taken into account. Polarization spectroscopy is employed to characterize two vibrational bands assigned to OH(D2O)2, revealing two nearly isoenergetic cyclic isomers that differ in the orientation of the non-hydrogen-bonded deuterium atoms relative to the plane of the three oxygen atoms. The dipole moments for these clusters are determined to be approximately 2.5 and 1.8 D for "up-up" and "up-down" structures, respectively. Hydroxyl stretching bands of larger clusters containing three or more D2O molecules are observed shifted approximately 300 cm(-1) to the red of the isolated OH radical. Pressure dependence studies and ab initio calculations imply the presence of multiple cyclic isomers of OH(D2O)3.

Water Catalysis of the Reaction between Methanol and OH at 294 K and the Atmospheric Implications

“…On the other hand, it has been speculated that the presence of OH(H 2 O) n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of hydroxyl radical, but theoretical calculations predicted that these clusters are not abundant . Nevertheless, the OH(D 2 O) n =1,2 have been stabilized in He‐nanodroplets and characterized spectroscopically . Furthermore, CH 3 OH(H 2 O) complexes are also expected to exist in the troposphere (Supporting Information, Table S1).…”

“…We suggest that the reaction of OH(H 2 O) with CH 3 OH(H 2 O) could be responsible for the observed behavior, considering that these are the most abundant complexes of OH and CH 3 OH with water (Supporting Information, Table S1) and the fact that the dipolar moments of OH ( μ OH =1.67 D) and CH 3 OH ( μ =1.68 D) are enhanced upon complexation with one water molecule OH(H 2 O) ( μ =3.75 D) and CH 3 OH(H 2 O) ( μ =2.72 D) (Supporting Information, Table S2), which could induce longer range interactions and therefore an enhanced reaction cross‐section.…”

Infrared spectroscopy in superfluid helium droplets

“…Deuterated complexes were also studied to shed light on the speed of vibrational energy relaxation within the complex through observed linewidths. Pre-reactive complexes [156,160] and other hydroxyl-based species have additionally been studied recently [161][162][163].…”

“…than the energy of the thermally populated rotational states, so free rotation is replaced by libration of molecules around the electric field, which is described by motion in a parabolic potential [164,173,174]. The Stark spectra get more complex for open shell molecules, such as radicals [60,162,163,175,176]. One such example is that of hydroxyl radical (OH) which shows Ʌ-doubling of the Q(3/2) rotational branch [155].…”

“…Electric field study in helium droplets have also been employed for structure determination of molecular complexes, such as (HF) n [127], (H 2 O) m -(HCl) n [130], H 2 O clusters [187], and others [183,184,188]. VTMA was also instrumental in determination of the structure and spectral assignment of pre-reactive complexes [162,163,175,176,178].…”