Pulsed-beam Fourier transform microwave spectroscopy was used to study propanethial S-oxide (1), an unstable species generated in a (Z)/(E) ratio of 98/2 by pyrolysis of 2-methyl-2-propyl-1'-propenyl vinyl sulfoxide at 350 degrees C in Ar or Ne/He gas flows. A fit of 25 transition frequencies to a Watson "A" reduced Hamiltonian gave the following rotational constants and centrifugal distortion constants for the normal isotopomer of (Z)-1a: A = 10 182.2558(3) MHz, B = 2209.5000(9) MHz, C = 1997.1734(8) MHz, Delta(JK) = -62.69(2) kHz, Delta(J) = 4.165(3) kHz, delta(J) = 0.3336(3) kHz and delta(K) = -31.7(4) kHz. Twelve transitions were fit in the same manner for (E)-1b to give A = 16231(86) MHz, B = 1823.6154(7) MHz, C = 1785.7215(7) MHz, Delta(JK) = -12.5(5) kHz, Delta(J) = 0.48(1) kHz, delta(J) = -0.060(8) kHz. Microwave spectra of six (Z) isotopomers were assigned and a partial substitution structure was derived: r(C=S) = 1.585(6) Angstrom, r(S-O) = 1.473(2) Angstrom, r(=CH-CH2) = 1.513(7) Angstrom, r(CH3-CH2) = 1.536(3) Angstrom, theta(CSO) = 113.8(2)degrees, theta(CCS) = 126.7(3)degrees, and phi(CCCS) = 118.4 degrees. The electric dipole moment components of the (Z) isomer along the a, b, and c principal axes were measured to be mu(a) = 2.59(2) D, mu(b) = 2.11(2) D, and mu(c) = 0.33(6) D, respectively, which gives a total electric dipole moment of 3.35(2) D.

The microwave spectra of CF3CH2OH and CF3CH2OD have been investigated from 5 to 26 GHz with a pulsed-nozzle Fourier-transform microwave spectrometer and from 26 to 42 GHz with an electric resonance optothermal spectrometer. Tunneling of the OH proton between the two isoenergetic gauche conformations splits the observed transitions into two tunneling components. An effective rotation-tunneling Hamiltonian is used to fit the aan both isotopomers to better than 5 and 13 kHz for the OH and OD forms, respectively. The tunneling splittings determined from the fits for the OH and OD isotopomers are 5868.6952(16) and 208.5037(42) MHz, respectively. A structural analysis using the moments of inertia of the OH and OD isotopomers determines that the hydroxyl hydrogen is directed toward the fluorine with a F...H separation of 2.561(1) Å and a dihedral angle of φ(CCOH)=68.97(6)°. The observed tunneling splittings are fit to a double-minimum potential, giving gauche–gauche tunneling barriers of 763 and 720 cm−1 and OH torsional fundamental frequencies of 364 and 271 cm−1 for CF3CH2OH and CF3CH2OD, respectively. The uncertainties shown in parentheses throughout the paper are one standard deviation.

Rotational constants (A0, B0, C0), centrifugal distortion constants (δj, δjk,δk, δj, δk) and the H, F nuclear spin-nuclear spin coupling constant DaaHF of the hydrogen-bonded complex (CH2)2S⋯HP were determined by analysis of 18 μa and μb transitions observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. An angular geometry in which the HF subunit is almost perpendicular to the plane of the (CH2)2S molecule and in which the S⋯H-F hydrogen bond deviates from linearity by ≈ 17° was established by an interpretation of the spectroscopic constants.

1. S. P. Sponseller and C. W. Gillies, 35th Symposium on Molecular Spectroscopy, Paper RC5, 1980, Ohio state University, Columbus, Ohio.

The rotational spectra of six isotopomers of trimethylamine-alane have been observed by pulsed beam Fourier transform microwave spectroscopy. Hyperfine analyses of the symmetric top isotopomers, (CH3)(3)NAlH3, (CH3)(3)NAlD3, and (CH3)(3)(NAlH3)-N-15, determined the spectroscopic constants, B-0, D-J, D-JK, eQq(Al-27), and eQq(N-14). For (CH3)(3)NAlH3, B-0 = 2745.113(1) MHz, D-J = 643(24) Hz, D-JK = 1372(250) Hz, eQq(Al-27) = 25.032(6) MHz, and eQq(N-14) = -3.777(5) MHz. Spectral assignments have also been made for the asymmetric top isotopomers, [(CH3)(2)(CH3)-C-13]NAlH3, (CH3)(3)NAlH2D, and(CH3)(3)NAlHD2. Substitution structural parameters of r(NC) 1.487(2) Angstrom and theta(AlNC) = 109.9(2)degrees are obtained from the microwave moments of inertia. With the assumption of r(AlN) = 2.063(7) Angstrom from an electron diffraction structure reported previously (Almenningen, A.; Gundersen, G.; Haugen, T.; Haaland, A. Acta Chem. Scand. 1972, 26, 3928) microwave substitution coordinates for the aluminum hydrogens give r(HAl) = 1.585(2) Angstrom and theta(HAlN) = 98.1(6)degrees. The structure of (CH3)(3)NAlH3 is compared to the related complexes, H3AlNH3, H3NBH3, and (CH3)(3)NBH3. In the same series, bond orders calculated from measured values of eQq(Al-27) and eQq(B-11) are compared to Al-N and B-N dative bond dissociation energies.

The weakly bound dimer H3P...Cl2 has been isolated in a pulsed jet by keeping the highly reactive components H3P and Cl2 separate until the point at which they expand into the evacuated Fabry–Pérot cavity of a pulsed-nozzle Fourier-transform microwave spectrometer. The rotational spectra of the three symmetric-top isotopomers H3P...35Cl 35Cl, H3P...35Cl 37Cl, and H3P...37Cl 35Cl detected in this way have been analyzed to give the ground-state spectroscopic constants B0, DJ, DJK, χ1(Cl), and χ2(Cl). Comparison of the dimer properties determined from the spectroscopic constants of H3P...Cl2 with those of the hydrogen-bonded dimer H3P...HCl indicates that charge transfer is not predominant in the interaction of PH3 and Cl2. Some parallels in the properties of B...Cl2 and B...HCl are discussed for B=PH3 and HF.

Ground-state rotational spectra of the isotopomers C2H6…HC14N and C2H6…DC14N of a weakly bound dimer formed by ethane and hydrogen cyanide have been analysed to give the spectroscopic constants B0, DJ, DJK and χ(14N) in each case. The dimer is shown to have a C3v equilibrium geometry, with HCN lying along the molecular symmetry axis and its H atom forming a hydrogen bond to a terminal face of a methyl group of ethane. The propensity of a CH3 group to act as a proton acceptor in hydrogen bond formation is discussed.

Durch coaxiales Mischen eines gepulsten CH3NC/Ar‐Strahls mit einem konti‐nuierlichen HF/Ar‐Strahl wird der Komplex CH3NC …︁ HF gebildet. Da die beiden Komponenten bis zur Expansion im Fouriertransform‐Mikrowellen‐spektrometer getrennt sind, kann man ihre große Reaktivitat wirksam unterdrücken. Das Rotationsspektrum des Komplexes entspricht dem eines symmetrischen Kreisels, d.h. eine lineare CNC …︁ HF‐Anordnung ist im C3v‐ symmetrischen Molekiil vorhanden.

Through coaxial mixing of a pulsed jet of CH3NC/Ar with a continuous stream of HF/Ar, the complex CH3NCċHF is formed. Since the two components are separate until their expansion in the Fourier‐transform microwave spectrometer, their high reactivity can be suppressed. The rotation spectrum of the complex is characteristic of that of a symmetric top, that is, the C3v symmetric complex contains a linear CNCċHF arrangement.

The hydrogen-bonded dimer CH3NC...HCl has been isolated in a pulsed jet by keeping the reactive components CH3NC and HCl separate until the point at which they expand into the evacuated Fabry-Perot cavity of a pulsed nozzle Fourier-transform microwave spectrometer. The rotational spectra of the three symmetric-top isotopomers CH3NC...(HCl)-Cl-35, CH3NC...(HCl)-Cl-37, and CH3NC...(DCl)Cl-35 detected in this way have been analyzed to give the ground-state spectroscopic constants B0, D(J), D(JK), and chi(Cl). Interpretation of the spectroscopic constants in terms of simple models leads to r(C...Cl) = 3.404 (4) angstrom and k(sigma) = 11.45 (5) N m-1 for this species of C3v symmetry. The propensity of the isocyano carbon atom in CH3NC to act as a proton acceptor in forming a dimer CH3NC...HX is compared with that of the cyano nitrogen atom in CH3CN.

The ν1 (NH stretch) fundamental and ν1 + ν7 − ν7 hot band of HNCCN+ have been observed with the difference-frequency laser system in the hollow cathode discharge. The ion was observed in a dry-ice cooled discharge of (CN)2 and H2, modulated at 10 kHz. The constants obtained for the fundamental were ν0 = 3448.2701(1) cm−1, B″ = 4438.22(8) MHz, D″ = 602(25) Hz, B′ = 4430.37(9) MHz, and D′ = 598(26) Hz. For the hot band we obtained ν0 = 3448.2281(1) cm−1, B″ = 4456.22(9) MHz, D″ = 739(61) Hz, B′ = 4448.30(10) MHz, and D′ = 741(66) Hz. The values for the fundamental band agree extremely well with the theory.

A hydrogen-bonded dimer formed by thiirane with hydrogen bromide has been detected and characterized in the gas phase by means of its ground-state rotational spectrum. The spectrum was detected within ca. 1-mu-s of the creation of the nascent dimer by using a fast mixing nozzle in conjunction with a pulsed-nozzle, FT microwave spectrometer. With the former device the reactive components thiirane and hydrogen bromide remain separate until they expand into the Fabry-Perot cavity of the latter. Rotational constants A(o), B(o) and C(o), centrifugal distortion constants DELTA-J, DELTA-JK and delta-J, Br nuclear quadrupole coupling constants chi-aa, chi-bb and chi-ab, and Br spin-rotation coupling constants have been determined for each of the isotopomers (CH2)2S...(HBr)-Br-79 and (CH2)2S...(HBr)-Br-81. Interpretation of the various spectroscopic constants leads to the conclusion that the dimer has C(s) symmetry, with HBr lying in the plane perpendicular to the thiirane heavy-atom plane and with a pyramidal arrangement at S. There is some evidence of a bent hydrogen bond. The angle phi almost-equal-to 80-degrees made by the S...H line with the thiirane plane is discussed in terms of some simple rules for predicting angular geometries of hydrogen-bonded dimers and a non-bonding electron-pair model for thiirane.

The J=1 â†� 0 rotational transitions of the four isotopomers H314N11BF3, H314N10BF3, H315N11BF3 and H315N10BF3 of the archetypal donor–acceptor complex formed between ammonia and boron trifluoride have been detected with a pulsed-nozzle, Fourier-transform microwave spectrometer; by using a fast-mixing nozzle, H3NBF3 has been isolated in collisionless expansion before condensation can occur, thereby allowing the first detection of this elusive species in the gas phase.

The gas phase C2D absorption spectrum has been studied in the 2800 cm-1 region using a high resolution difference-frequency laser spectrometer. The C2D molecules were produced in a hollow cathode discharge in a mixture of C2H2 or C2D2 (approximately 20 mTorr) and D2 (approximately 400 mTorr). Two bands were identified and rotationally analyzed. One band at 2796 cm-1 was found to be a 2-PI-2-SIGMA+ transition with the lower state being the ground state. The upper level of this band is assigned to the (110) state in the ground electronic state, approximately X 2-SIGMA+. The other band at 2851 cm-1 is a 2-SIGMA+ -2-SIGMA+ transition from the (001) state in approximately X2-SIGMA+ to a vibronic 2-SIGMA+ state which was found to be perturbed. Accurate molecular constants for the upper levels involved were determined from least-squares fit. These constants as well as the observed transition frequencies are reported.