Studies of infrared multiphoton dissociation rates of protonated aliphatic alcohol dimers

Abstract

The rates of photodissociation induced by absorption of infrared radiation of protonated dimers of 2-propanol and its deuterated analogues are reported. Protonated alcohol dimer ions created in a quadrupole ion store (QUISTOR) were irradiated in isolation and as a function of parent gas pressure and added collision gas (N2). The relative weights and nature of ionic photoproducts were determined by mass spectrometry. The effect upon photolysis of collisions both prior to and during irradiation were examined. Photodissociation rates were defined by σD, which is the measured photodissociation rate constant normalized to the laser intensity. Nascent dimer ions which can possess up to 130 kJ mol−1 internal energy dissociated readily with σD of 15.8 × 10−2 cm2 extrapolated to collision-free conditions. Photodissociation products from nascent dimer ions were produced via two major reaction channels. As the number of collisions suffered by the nascent dimer ion prior to irradiation was increased, the value of σD diminished and a single reaction channel became more prevalent. After some 270 collisions the ions appear to be collisionally relaxed prior to photolysis. The value of σD of 13.1 × 10−21 cm2 extrapolated to collision-free conditions was obtained. The products from photodissociation of relaxed ions were obtained from a single reaction channel. An increase in σD values obtained with added nitrogen collision gas indicated a blockage or "bottleneck" in multiphoton absorption which is attributed to rotational population depletion. Three deuterated analogues of 2-propanol showed similar behaviour to that of 2-propanol with the exception that deuteron-bound dimers formed from 2-propanol (OD) and perdeutero-2-propanol failed to show rotational population depletion. The apparent lack of a "bottleneck" in multiphoton absorption by deuteron bound dimers is attributed to the diminished spacings between rotational levels of these species compared with the rotational level spacings for proton-bound dimers. Proton-bound ethanol dimers were selected for the examination of temperature effects upon photodissociation rates. While values for σD, extrapolated to collision-free conditions, of 15.6 × 10−21 cm2 at 321 K (in agreement with the 2-propanol results obtained solely at this temperature) and 8.55 × 10−21 cm2 at 293 K were obtained, the precise interpretation of such a marked temperature effect is not immediately obvious.