R. D. Suenram

Pulsed beam Fourier transform microwave measurements on OCS and rare gas complexes of OCS with Ne, Ar, and Kr

A pulsed molecular beam Fourier transform microwave spectrometer, which has been recently constructed at NBS, was employed for measurements on several monomer and van der Waals species of OCS. The absorption–emission cell consists of a Fabry–Perot resonant cavity inside a high vacuum chamber. A pulsed nozzle is used to generate a supersonic molecular beam of a seeded inert gas. New spectra of Ne–OCS, Ar–OC34S, and for four Kr isotopes of Kr–OCS are reported, as well as structure analyses of each species. The nuclear electric quadrupole hyperfine structure of 83Kr–OCS has been resolved and yields eQqaa=1.601(7) MHz and eQqbb=−1.857(3) MHz.

New measurements of microwave transitions in the water dimer

New measurements of ten K=1 lines, including six Q type and four R type, were made on the completely protonated species of the water dimer. For some of these lines, as well as for some K=0 transitions known from the literature, Stark coefficients were determined, and these Stark coefficients provide a confirmation of the assignments. The new K=1 measurements show that the splitting associated with the (HF)2‐like tunneling motion decreases from about 19.5 GHz for K=0 to about 16.2 GHz for K=1. To understand the fact that K=1 lines are populated in our 1 K beam, we must assume, in accordance with the results of beam studies on other molecules, that levels of different nuclear spin modification relax separately. In an attempt to gain information on tunneling splittings other than that caused by the (HF)2‐like motion, we have made new measurements on 1–0 and 2–1 transitions with K=0 for several partially deuterated species, in which the (HF)2‐like motion cannot occur. Small splittings ranging from 4 to 145 MH...

A portable, pulsed-molecular-beam, Fourier-transform microwave spectrometer designed for chemical analysis

M. D. Harmony, K. A. Beran, D. M. Angst, and K. L. Ratzlaff [Rev. Sci. Instrum. 66, 5196 (1995)] recently published some design specifications for a smaller version of a Fourier transform microwave (FTMW) spectrometer. In that work they used a nozzle arrangement which pulsed the molecular beam perpendicular to the axis of the Fabry–Perot cavity. They found that even though the size of the vacuum chamber and Fabry–Perot cavity mirrors had been reduced, the overall sensitivity of the instrument was nearly the same as one with a conventional sized resonator. In an effort to establish FTMW spectroscopy as a viable new technique for analytical chemists, we have constructed a miniaturized version of our laboratory instrument for use as an analytical instrument. The vacuum chamber of the instrument is based on a commercially available, multiport 30 cm (12 in.) sphere. An integral end-flange mirror permits a coaxial nozzle injection of the molecular beam which greatly improves the sensitivity of the instrument. T...

Microwave spectrum, torsional barrier, and structure of BH3NH3

The microwave spectra of nine isotopic species of borane monoammoniate (11BH3NH3, 10BH3NH3, 11BH3ND3, 10BH3ND3, 11BD3NH3, 11BH3 15NH3, 10BH3 15NH3, 11BD2HNH3, 11BH3ND2H) have been observed. The rotational constants, centrifugal distortion constants, dipole moment, torsional barrier, and molecular geometry of borane monoammoniate were determined from these spectra. The rs structure is: BN=1.6576(16) A, BH=1.2160(17) A, NH=1.0140(20) A, ∠NBH=104.69(11), ∠BNH=110.28(14). The dipole moment is 5.216(17) D. The torsional barrier about the B–N bond, V3, is 2.047(9) kcal mol−1 for 11BH3ND2H and 2.008(4) kcal mol−1 for 11BD2HNH3.

Infrared and microwave investigations of interconversion tunneling in the acetylene dimer

A sub‐Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C–H stretching fundamental using an optothermal molecular‐beam color‐center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed‐nozzle Fourier transform microwave spectrometer. In the infrared spectrum, both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T‐shaped C2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three Ka=0 and three asymmetry‐doubled Ka=1 series. The transitions show a clear intensity alternation with Kc with two of the Ka=0 series missing every other line. In addition, the perpendicular band has the same ground‐state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T‐shaped complex with interconversion tunneling between f...

The Conformational Structures and Dipole Moments of Ethyl Sulfide in the Gas Phase

The pure rotational spectrum of ethyl sulfide has been measured from 12 to 21 GHz in a 1 K jet-cooled expansion using a Fourier-transform microwave (FTMW) spectrometer. Prominent features in the spectrum are assigned to transitions from three conformational isomers. Additional assignments of the 13C and 34S isotopomer spectra of these conformers effectively account for all of the remaining transitions in the spectrum. Accurate “heavy-atom” substitution structures are obtained via a Kraitchman analysis of 14 rotational parameter sets, permitting definitive identification of the molecular structures of the three conformers. Two of the structures designated as the gauche–gauche (GG) and trans–trans (TT) conformers have symmetric forms with C2 and C2v symmetries, respectively, and the third trans–gauche (TG) configuration is asymmetric. The components of the electric dipole moment along the principal inertial axes have been determined from Stark measurements and are consistent with these structural assignment...

Optothermal‐infrared and pulsed‐nozzle Fourier‐transform microwave spectroscopy of rare gas–CO2 complexes

Sub‐Doppler infrared spectra of Ne–CO2, Ar–CO2, and Kr–CO2 have been recorded near 3613 and 3715 cm−1, in the region of the 2ν02+ν3/ν1+ν3 Fermi diad of CO2, using an optothermal molecular‐beam color‐center laser spectrometer. In addition, pulsed‐nozzle Fourier‐transform microwave spectra are reported for the ground vibrational states of the complexes. The infrared and microwave spectra are consistent with T‐shaped complexes as shown originally by Steed, Dixon, and Klemperer for Ar–CO2.1 The infrared band origins for the Ar and Kr complexes are red shifted, from that of free CO2, by 1.09 and 0.95 cm−1 for Ar–CO2 and by 1.97 and 1.76 cm−1 for 84Kr–CO2. For Ne–CO2, blue shifts of 0.15 and 0.19 cm−1 are observed. The lower Fermi components are free of perturbations, whereas the upper components of Ar–CO2 and Kr–CO2 are perturbed. For Ar–CO2 the perturbation is strong, shifting the positions of the observed Q‐branch lines of the Ka =1←0 subband by as much as 500 MHz.

Water hydrogen bonding: The structure of the water–carbon monoxide complex

Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axi...

Microwave spectra of the (HF)2, (DF)2, HFDF, and DFHF hydrogen-bonded complexes

Abstract The microwave spectra of the tunneling-rotation bands of the hydrogen-bonded complexes (HF)2 and (DF)2 have been measured in the 50- to 126-GHz region. In addition, the pure rotation spectra of both the HFDF and DFHF molecules have been obtained. Transitions with K = 0 through K = 2 have been observed for all isotopic species. The hydrogen fluoride dimer is a very nonrigid molecular species. In order to fit the observed transitions adequately, empirical expressions for the energy levels were used, and each K subband was separately fitted. Constants obtained from a Pade approximant fitting of the microwave data of (HF)2 together with infrared ground state combination differences are given.

Ammonia dimer: Further structural studies

New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dime...