Charles W. Gillies

Microwave spectrum, structure, and electric dipole moment of ArCH3OH

Abstract Microwave spectra of ArCH3OH, ArCD3OH, and Ar13CH3OH have been measured between 7 and 25 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Two tunneling states are observed which correlate to the A and E internal-rotor states of free methanol. For the lowest energy A state, a- and b-type spectra are assigned and fitted to an asymmetrical-top Hamiltonian, giving A = 25 468.821(4) MHz, B = 2084.42(2) MHz, C = 1928.46(2) MHz, ΔJ = 21.90(2) kHz, ΔJK = 371.7(1) kHz, δK = 474(10) kHz, δJ = 1.61(8) kHz, and hK = 10.1(8) kHz for ArCH3OH. The electric-dipole-moment components, μa and μb, are determined by Stark-effect measurements to be 1.079(1) and 1.069(5) D, respectively. The Stark-effect results and the absence of c-type transitions indicate that μc ∼ 0. The structure of the complex is found to be T-shaped with an Ar to CH3OH center-of-mass separation of 3.684(14) A. The inertial defect, Δ = −0.2347 u A 2 , is surprisingly small and suggests that the methanol unit is internally rotating against the Ar. This is in addition to the internal rotation of the CH3 group against the OH top. Finally, a number of transitions are observed which do not appear to fit an asymmetrical-top Hamiltonian. These are assigned to an E tunneling state of the complex.

Microwave spectra and electric dipole moments of X4Σ12−1 VO and NbO

Abstract Microwave spectra and electric dipole moments have been measured for the X 4 Σ 1 2 −1 states of VO and NbO using a pulsed-nozzle Fourier-transform microwave spectrometer with a laser-vaporization source. An analysis of the Stark effect of hyperfine components of the J = 3 2 ← 1 2 transition yields dipole moments of 3.355(14) and 3.498(7) D for VO and NbO, respectively. The dipole-moment measurements are compared with recent ab initio calculations.

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

Microwave spectrum, structure, and electric dipole moment of the Ar-formamide van der Waals complex

The microwave spectrum of the Ar–formamide van der Waals complex has been obtained using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The rotational constants of the complex are: A=10 725.7524(48) MHz, B=1771.0738(22) MHz, and C=1548.9974 (16) MHz. The complex is shown to be nonplanar with an inertial defect of −6.21 u A2. The Ar atom is located at 3.62 A from the center of mass of the formamide unit at Ar–O, Ar–N, and Ar–C distances of 3.55, 3.79, and 3.93 A, respectively. The shortest Ar–H distance is 3.25 A which is similar to that observed for Ar–vinyl cyanide (3.21 A). Stark effect and hyperfine analyses yield the following values for the electric dipole moment components and 14N quadrupole coupling constants for the complex: μa=0.922(1) D, μb=3.407(5) D, χaa=−1.164(7) MHz, χbb=1.906(5) MHz, and χcc=−0.742(6) MHz.

MICROWAVE DETECTION OF THE PRIMARY OZONIDE OF ETHYLENE IN THE GAS PHASE

Abstract The primary ozonide of ethylene has been observed and studied in the gas phase for the first time. A specially designed low-temperature absorption cell was employed in which the primary ozonide was prepared in situ by the low-temperature reaction of ozone with ethylene. An assignment of the rotational spectrum and electric dipole moment measurements have established the oxygen envelope conformation (C s symmetry) to the lowest-energy form for this elusive chemical species.

Structural effects in fluorinated cyclopropanes: A microwave study of cis-1,1,2,3-tetrafluorocyclopropane

Abstract Microwave and RFMDR spectra of cis - CHFCHFC F 2 , cis - 13 CHFCHFC F 2 , cis - CDFCDFC F 2 , and cis - 13 CDFCDFC F 2 have been measured between 26.5 and 40.0 GHz using an HP 8400C spectrometer. The a - and c -type transitions were assigned and fit to the quartic Watson Hamiltonian giving A = 3450.445(2) MHz, B = 2402.831(3) MHz, C = 2060.247(3) MHz, Δ J = 0.39(3) kHz, Δ JK = 0.26(1) kHz, δ K = 1.606(9) kHz, δ J = 0.059(1) kHz, and δ JK = −0.58(2) kHz for cis - CHFCHFC F 2 . A structure is derived from the moment of inertia data by fixing three parameters associated with the CF 2 group. The r S parameters for the CHFCHF segment of the molecule in the CHFCHFC F 2 isotopic frame are r (C 2 C 3 ) = 1.533(3) A , r (C 2,3 H) = 1.099(3) A , r (F 2 ⋯ F 3 ) = 2.775(2) A , and r (H 2 ⋯ H 3 ) = 2.622(2) A . Two algorithms describing the CC and CF bond distances are fitted to gas phase structural data for a series of fluorinated cyclopropane derivatives. A partial test of these algorithms is obtained from the structure of cis - CHFCHFC F 2 . The structural results are related to theoretical studies of fluorination effects in cyclopropane derivatives.

A reinvestigation of the microwave spectrum of methyl phosphonic difluoride

Abstract The microwave spectrum of methyl phosphonic difluoride, CH3POF2, was reinvestigated in the region of 26 500 to 36 000 MHz. Rigid rotor rotational constants (MHz) for the ground state were obtained from an improved data base and are A = 4495.814(7), B = 4271.851(5), and C = 4125.89(5). Previous microwave assignments of two vibrational satellites are shown to be erroneous. Three sets of excited vibrational state lines have been identified and fit well to rigid rotor theory. Relative intensity measurements permit tentative vibrational assignments of the states to first excitations of the CH3 torsion, CPO bend, and PF2 deformation by comparison to vibrational work. The three excited states exhibited no resolvable microwave transition splitting. The methyl torsional barrier is estimated to be 3.5 kcal/mol from the previously assigned v = 1 ← 0 torsional mode and the internal rotational parameter, F, which was calculated earlier from the structure.

Microwave spectra and molecular conformation of methoxy difluorophosphinoxide

Abstract Two closely spaced a -type low resolution microwave band series were observed for the normal, 13 C and d 3 isotopic forms of methoxy difluorophosphinoxide. The lower frequency series of the three isotopic species was found and assigned using a pulse beam Fabry—Perot cavity Fourier transform microwave spectrometer. The a - and b -type spectra of the three species were fitted to a Watson hamiltonian which gave A =4568.215(1) MHz, B =2500.471(9) MHz and C =2453.863(9) MHz for the normal species. Stark effect measurements of CH 3 OP(O)F 2 led to the determination of the electric dipole component μ a =2.699(2) D and μ b =1.522(2) D. Moments of inertia and electric dipole data show that the lowest energy conformation has C s symmetry. The data are most consistent with an assignment of the observed conformer to the trans configuration. The second a -type low resolution microwave band series not observed in the pulsed beam cavity spectrometer arises from a methoxy torsional excited state fo the trans conformer or a second higher energy conformation. The absence of A–E torsional splittings in the a - and b -type spectra gives a lower limit of ∼2500 cal mol −1 for the V 3 methyl internal rotation barrier.