Three isotopic species of a weakly bound T-shaped π complex formed between acetylene and HCl have been observed through the assignment of their rotational spectra by the use of a pulsed, Fourier-transform spectrometer with a high pressure gas being pulsed into an evacuated Fabry–Perot cavity. The spectra clearly show the complex to be a nearly prolate asymmetric top (κ = −0.9898) with the HCl on the a-inertial axis and perpendicular to acetylene and with the hydrogen atom pointing towards the middle of the acetylene triple bond. The chlorine is situated, on average, 3.699 Å from the edge of the acetylene molecule. The following molecular constants have been obtained for the C2H2, H35Cl isotopic species: The rotational constants are A = 36 084(838), B = 2481.065(6), and C = 2308.602(6) MHz; the nuclear quadrupole coupling constants are χaa = −54.342(4), χbb = 26.862(5), and χcc = 27.480(9) MHz, and the rotational centrifugal distortion constants are DJ = 7.9(4) and DJK = 497(8) kHz.

The rotational spectrum and molecular structure of the furan-HCl complex J. Chem. Phys. 78, 3545 (1983); 10.1063/1.445178The rotational spectrum and molecular structure of the ethylene-HF complex J. Chem. Phys. 76, 4857 (1982); 10.1063/1.442804The rotational spectrum and molecular structure of the acetylene-HCl dimer J. Chem. Phys. 75, 625 (1981); 10.1063/1.442079Microwave rotational spectrum, molecular geometry, and intermolecular interaction potential of the hydrogen bonded dimer OC-HCl Three isotopic species of a hydrogen bonded dimer formed between ethylene and HCI have been observed through assignment of their rotational spectra using a pulsed. Fourier transform microwave spectrometer in which a gas mixture is pulsed into an evacuated Fabry-Perot cavity. The spectra unambiguously reveal a nonplanar. near prolate. asymmetric top (K = -0.9880) in its ground vibrational state. The equilibrium structure has C 2V symmetry with the HCI on the C 2 symmetry axis of the dimer. perpendicular to the carbon-carbon bonding axis with the hydrogen atom located between the chlorine and the ethylene molecule. pointing midway between the carbon atoms. The chlorine atom is situated. on average. 3.724 A from the edge of the ethylene molecule. The following spectroscopic constants have been obtained for the ethylene-H"CI dimer: the rotational constants are A = 25 457(349). B = 2308.143(3). and C = 2167.970(3) in units of MHz; the chlorine nucleus quadrupole coupling constants are Xaa = -54.076(4). Xbb = 27.091(6). and X" = 26.985(10) in units of MHz. and the centrifugal distortion constants are D J = 7.2(2) and DJK = 282(5) in units of kHz. 2126

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is observed due to a simpler reaction pathway than for It (eq 2).Formation of IC would have to be accomplished by energetically milder means than electron impact, such as field ionization31 or charge transfer.32 Plans have been made to study the photodissociation of the parent cation of 1,2-diphenylcyclopentene, which cannot undergo cis-trans isomerism.33 ConclusionThe photodissociation of the stilbene radical cation was studied to determine if light-induced fragmentation could be analogous to the neutral photochemical reactions, cis-trans isomerization, and electrocyclic ring closure. The results show that the trans (31) P. , 259 (1965). radical cation is formed predominantly from electron impact of either cis-or trans-stilbene at low electron energies. The lightinduced fragmentation of the trans cation suggests a reaction pathway in which the trans cation isomerizes to the cis cation, which undergoes a ring closure in the excited state prior to fragmentation. The formation of the (M -2)' cation is best explained as occurring via the initially formed excited state rather than after internal conversion to the ground state. The formation of ( M -2)' from this state can be rationalized by a Woodward-Hoffmann analysis for electrocyclic ring closure. Abstract:The rotational spectra of several isotopic species of a dimer formed in the gas phase between cyclopropane and hydrogen chloride have been observed by using the technique of Fourier transform microwave spectroscopy conducted in a Fabry-Perot cavity fed by a pulsed nozzle source of molecular dimers. Rotational constants, centrifugal distortion constants, and CI nuclear quadrupole coupling constants are reported respectively for the three isotopic species (V, H3%l ) , (V, H3'C1), and (V, D3SCl) as follows:(5). These spectroscopic constants are interpreted in favor of the edge-on form of the dimer of C , symmetry in which the HCI molecule lies in the plane of the ring along the C2 axis and forms a hydrogen bond to the midpoint of one of the C-C bonds. Some quantitative details of the dimer geometry and the weak binding are determined.

Four isotopic species of a hydrogen-bonded dimer formed between cyclopropane and HF have been observed through assignment of their rotational spectra using a pulsed, Fourier-transform microwave spectrometer in which a gas mixture is pulsed into an evacuated Fabry–Perot cavity. The geometry of the dimer has unambiguously been shown to have HF perpendicular to the cyclopropane edge and in the plane of the ring. HF, on average, lies on the C2 symmetry axis of the dimer which is additionally the a principal axis. Hyperfine structure due to the spin–spin interaction in HF and DF was resolved as well as the hyperfine structure due to the nuclear quadrupole moment of deuterium in the cyclopropane–DF species. The following spectroscopic constants have been obtained for the cyclopropane–HF species: The rotational constants are A = 19 948(60), B = 2753.706(2), and C = 2539.115(2) MHz, the HF spin–spin constant is 121(43) kHz, and the centrifugal distortion constants are DJ = 5.08(27) and DJK = 43.5(36) kHz.