J Z. Gillies

A microwave spectral and ab initio investigation of O3H2O

Abstract Microwave spectra of O 3 H 2 O, O 3 H 2 18 O, O 3 HDO, and O 3 D 2 O have been observed with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. Two tunneling states, designated A 1 and A 2 , are found for the normal and dideuterated isotopic forms, while only one state is observed for the O 3 HDO isotope. The A 1 spectra of O 3 H 2 O, O 3 H 2 18 O, and O 3 D 2 O as well as the O 3 HDO spectrum were fit to a Watson asymmetric top Hamiltonian, giving A = 11 960.584(5), B = 4174.036(8), and C = 3265.173(8) in MHz for O 3 H 2 O. The A 2 states of O 3 H 2 O and O 3 D 2 O could not be fit to a Watson Hamiltonian. This result, when combined with the large frequency splittings and the observed nuclear spin statistics for the O 3 H 2 O and O 3 D 2 O isotopic forms, suggests that there is a low barrier to internal rotation of water. The tunneling motion may also involve ozone through a concerted internal rotation of both monomer subunits. Stark effect measurements of a - and c -type transitions for O 3 H 2 O give the electric dipole components μ a = 1.014(2) D and μ c = 0.522(6) D. The dipole and moment of inertia data indicate that the complex has C s symmetry with water and the unique oxygen of ozone lying in the symmetry plane. This plane bisects the OOO angle of ozone. The distance between the centers of mass of ozone and water is 2.957(2) A. From the microwave data and ab initio calculations at the MP 2 6–31 G (d, p) and MP 4( SDTQ ) 6–31 G (d, p) level of theory it is found that the terminal oxygens of ozone are tilted toward one of the nonequivalent hydrogen atoms in water. Furthermore, the calculations reveal that O 3 and H 2 O adopt an orientation within the complex that guarantees maximal stabilization by electric dipole-dipole attraction.

Microwave and infrared spectra of C2H4...HCCH : barrier to twofold internal rotation of C2H4

Abstract Microwave spectra of C 2 H 4 …HCCH, C 2 H 4 …DCCH, C 2 H 4 …DCCD, D 2 CCH 2 …HCCH, and trans-HDCCHD…HCCH have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. An α-type, Δ K α = 0 spectrum is observed, with a number of transitions being split into doublets due to tunneling arising from the hindered internal rotation of the ethylene and about its CC bond. For the normal species we find A = 25981 (33) MHz, B + C 3478.2560(13) MHz, and B  C = 89.45(18) MHz. The complex is shown to have a C 2v structure in which the HCCH unit hydrogen bonds to the ethylene π cloud, with the HCCH axis normal to the plane of the ethylene. The hydrogen bond length is found to be 2.78 A. Centrifugal-distortion analysis yields a weak-bond stretching force constant of 2.5 N/m (0.025 mdyn/A), corresponding to a stretching frequency of 56 cm −1 . Stark effect measurements determine the electric dipole moment of the complex to be 8.852 (21) × 10 −31 C m (0.2654(6) D). The observed tunneling-induced splittings yield an internal rotation barrier of 240 cm −1 . An infrared spectrum of the asymmetric acetylenic CH stretch in the complex has also been measured using an optothermal color-center laser spectrometer. The rotational lines are predissociation broadened, preventing the resolution of K structure. The observed band origin, ν 0 = 3271.61 cm −1 , is nearly identical to that found for the similar vibration in the acetylene dimer.

Peeling the Onion

There are more than 600 known species in the genus Allium. While some are little more than botanical curiosities, others are attractive ornamental plants of diverse size and hue (e.g., A. moly L., A. giganteum Regel, A. flavum L., A pulchellum, A. roseum) or economically important spices and vegetables (e.g., onion, garlic, leek, shallot, chive, and scallion, respectively A. cepa, A. sativum, A. porrum L.,A. ascalonicum auct., A schoenoprasum L., and A. fistulosum L.).1 The antibiotic, anticancer, antithrombotic, cholesterol-lowering, and other beneficial health effects associated with consumption of genus Allium plants are widely touted in the popular and scientific/medical press.23 Typical culinary usage of these plants involves cutting or crushing them so as to maximize flavor and aroma release. Cutting or crushing results in disruption of plant tissue with ensuing enzymatic and chemical reactions generating the actual flavorants and aroma compounds.4 The flavorants and aroma compounds probably serve as protective agents for the plant against attack by predators and infectious microorganisms.4 At the same time several insect pests, such as the leek moth or onion maggot, key in on these compounds to locate their next meal or egg-laying site.5